Maria Manuela Carvalho Proença Development of nanoplasmonic thin films for gaseous molecules detection

Universidade do MinhoEscola de Ciências

Dezembro de 2017

Manuela

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Developm

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Maria Manuela Carvalho Proença Development of nanoplasmonic thin films for gaseous molecules detection Dissertação de Mestrado Mestrado em Biofísica e Bionanossistemas Trabalho efetuado sob a orientação da Professora Doutora Ana Paula Fernandes Monteiro Sampaio Carvalho e do Doutor Joel Nuno Pinto Borges Dezembro de 2017

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DECLARAÇÃO Nome: Maria Manuela Carvalho Proença Endereço eletrónico: manuelaproenca12@gmail.com / pg28883@uminho.pt Contacto: 00351 916070301 Número de identificação civil: 14625467 8 ZY3 Título da dissertação: Development of nanoplasmonic thin films for gaseous molecules detection Orientadores: Professora Doutora Ana Paula Sampaio e Doutor Joel Nuno Pinto Borges Ano de conclusão: 2017 Designação do mestrado: Mestrado em Biofísica e Bionanossistemas É AUTORIZADA A REPRODUÇÃO INTEGRAL DESTA DISSERTAÇÃO APENAS PARA EFEITOS DE INVESTIGAÇÃO, MEDIANTE DECLARAÇÃO ESCRITA DO INTERESSADO, QUE A TAL SE COMPROMETE. Universidade do Minho, __/__/____ Assinatura:

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Agradecimentos Esta dissertação não é apenas resultado de um empenho individual, mas sim de um conjunto de esforços que a tornaram possível. Desta forma, quero agradecer a todos os que de uma maneira ou de outra me ajudaram e apoiaram na concretização deste trabalho. Agradeço aos meus orientadores, Joel Borges e Professora Paula Sampaio, bem como ao coordenador do grupo, o Professor Filipe Vaz, pela oportunidade que me deram de poder trabalhar neste projeto, apoio, confiança, incentivo e disponibilidade sempre demonstrada. Rui, ao Marco e ao Diogo pela disponibilidade demonstrada em me acompanharem no laboratório, bem como a todos os elementos, investigadores e alunos, que integram o grupo de investigação por todo o seu apoio, partilha de conhecimento e companheirismo. Ao departamento de Física da Universidade do Minho pelas condições de trabalho proporcionadas. A todos os colaboradores e parceiros do grupo de investigação, pelas caracterizações e análises realizadas no âmbito deste trabalho. Agradeço também à Fundação para a Ciência e Tecnologia (FCT) o financiamento no âmbito do Projeto 9471 - Reforçar a Investigação, o Desenvolvimento Tecnológico e a Inovação (Projeto 9471-RIDTI) comparticipado pelo Fundo Comunitário Europeu FEDER. O mencionado Projeto tem designação de "NANOSENSING" e referência PTDC/FIS-NA/1154/2014. Aos meus pais e irmão pelo apoio e carinho incondicional, acreditando sempre no meu esforço e empenho. Por fim a todos os meus amigos pela amizade, auxílio e momentos de partilha e descontração. Muito obrigada por tudo!

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Resumo Filmes finos nanocompósitos, constituídos por nanopartículas nobres incorporadas numa matriz de oxido metálico, têm despertado considerável interesse na deteção ótica de moléculas gasosas. A sensibilidade dos filmes pode ser otimizada de acordo com o efeito de ressonância de plasmões de superfície localizados (LSPR) revelado por estes materiais, que depende fortemente da composição, distribuição, tamanho e forma das nanopartículas, bem como do meio dielétrico que as rodeia. Neste trabalho, foram preparados filmes finos com diferentes concentrações de Au e/ou Ag incorporados numa matriz de CuO, com o objetivo de encontrar plataformas nanoplasmónicas capazes de detetar a presença de moléculas gasosas (ex. CO) através de desvios da banda de LSPR. Os sistemas de filmes finos (Au:CuO, Ag:CuO e Au:Ag-CuO) foram depositados por pulverização catódica reativa em magnetrão e, posteriormente, submetidos a tratamentos térmicos (temperaturas de 300 a 700 ºC), para promover a formação das nanopartículas de Au e/ou Ag. A composição, microestrutura e a resposta ótica dos filmes foram estudadas em função da concentração de metal e da temperatura do tratamento térmico e correlacionadas com o comportamento plasmónico. Quanto ao sistema Au:CuO, o efeito LSPR aparece após temperaturas de tratamento térmico de 300 ºC, ou superiores, com bandas de LSPR a tornarem-se progressivamente mais estreitas; comportamento associado ao crescimento das nanopartículas de Au e diferentes distribuições de tamanhos. No caso do sistema de Ag:CuO, apenas foi possível observar bandas LSPR para temperaturas de tratamento térmico de 500 ºC, e superiores. Para o sistema Au-Ag:CuO, um único pico de LSPR apareceu para temperaturas de tratamento térmico até aos 500 ºC. Para temperaturas superiores, foram observados dois picos LSPR, provavelmente devido à presença de nanopartículas de Au e Ag, embora a formação de nanopartículas bimetálicas de Au-Ag não possa ser descartada. A espectroscopia fotoeletrónica de raio X, com e sem a presença do gás CO, indicou que a superfície dos filmes finos nanoplasmónicos de Au:CuO está contaminada com uma camada sub-nanométrica (~0,6 nm) de hidrocarbonetos, onde algum CO pode ser quimissorvido. Contudo, parte das moléculas de CO expostas ao filme parecem ser fisissorvidas na superfície, o que é importante para a aplicação alvo. Para além disso, através de uma posterior aplicação de tratamentos de plasma com Ar, a camada de hidrocarbonetos pode ser removida e as nanopartículas de Au emergir à superfície. Durante este trabalho, um sistema ótico portátil foi também desenvolvido para realizar os ensaios de deteção molecular, seguindo os desvios da banda de LSPR.

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Abstract Nanocomposite thin films, containing noble nanoparticles embedded in an oxide matrix have been a subject of considerable interest for optical gas sensing, due to their localized surface plasmon resonance (LSPR) properties. The sensitivity of the films can be tailored according to the LSPR phenomenon revealed by these materials, which is strongly dependent on the composition, distribution, size and shape of the nanoparticles, and dielectric medium surrounding them. In the present work, thin films were prepared with different amounts of Au and/or Ag embedded in a CuO matrix, aiming to find nanoplasmonic platforms capable of detecting the presence of gaseous molecules (e.g. CO) through LSPR band shifts. The thin films systems (Au:CuO, Ag:CuO and Au:Ag.CuO) were deposited by reactive DC magnetron sputtering and then submitted to annealing treatments (temperatures from 300 to 700 ºC) to promote the Au and/or Ag nanoparticles formation. The composition, microstructure and optical response of the thin films were studied as a function of the metal concentration and annealing temperature and correlated with the LSPR behaviour. Regarding the Au:CuO system, a LSPR effect appeared after annealing temperatures of 300 ºC, or higher, with bands becoming progressively narrower; behaviour associated to the Au nanoparticles growth and different size distributions. In the case of the Ag:CuO system, LSPR bands were observed only for temperatures of 500 ºC and above. For the Au-Ag:CuO system, a unique LSPR peak appeared for annealing temperatures up to 500 ºC. For higher temperatures, two faint LSPR peaks were observed, probably due to the presence of both Ag and Au nanoparticles, although the formation of bimetallic Au-Ag nanoparticles was also suggested. X-Ray Photoelectron Spectroscopy, without and with the presence of CO gas, indicated that the surface of the Au:CuO nanoplasmonic thin film is contaminated with a sub-nanometric layer (~0.6 nm) of hydrocarbons, where some CO might be chemisorbed. However, part of the CO molecules exposed to the film seems to be physisorbed at the surface, which is important for the targeted application. Furthermore, through a subsequent application of Ar plasma treatments, the hydrocarbons layer may be removed, and the Au nanoparticles emerge at the surface. During this work, a portable optical system was designed and built in the laboratory to detect the presence of gas molecules, in particular CO, by following the shifts of the LSPR band in transmittance.

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Index Agradecimentos ........................................................................................................................ iii Resumo ..................................................................................................................................... v Abstract ................................................................................................................................... vii Index ........................................................................................................................................ ix Index of figures ......................................................................................................................... xi Index of tables ......................................................................................................................... xv Abbreviations and Acronyms .................................................................................................... xvi Chapter 1- Introduction ............................................................................................................. 1 1.1 Contribution of Nanotechnology ...................................................................................... 1 1.2 What is a chemical sensor?............................................................................................. 3 1.2.1 Monitoring our Environment .................................................................................... 3 1.2.2 Chemical sensors: definitions and classification ....................................................... 4 1.3 Optical Chemical sensors ............................................................................................... 7 1.3.1 Surface Plasmons ................................................................................................... 7 1.3.1.1 Surface Plasmon Resonance phenomenon ........................................................ 7 1.3.1.2 Localized Surface Plasmon Resonance ............................................................ 10 1.3.2 Noble metal nanoparticles embedded in metal oxides with LSPR effect for gas sensing ...................................................................................................................................... 14 Chapter 2- Production of thin films with LSPR effect ................................................................ 19 2.1 Reactive Magnetron Sputtering for incorporation of gold and/or silver in a copper oxide matrix ................................................................................................................................ 19 2.1.1 Reactive Magnetron Sputtering .............................................................................. 19 2.1.2 Experimental details for thin films production ......................................................... 22 2.1.2.1 Cleaning and activation of substrate surface by plasma treatment .................... 22 2.1.2.2 Thin films deposition by Reactive DC Magnetron Sputtering ............................. 22 2.2 Formation of gold and silver nanoparticles inside a CuO matrix ..................................... 25

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2.2.1 General description of nanoparticles formation....................................................... 25 2.2.2 Thermal annealing process on thin films ................................................................ 25 Chapter 3- Analysis and characterization techniques for the study of the microstructure, optical response and gas sensing properties of the thin films. Experimental details. ............................ 27 3.1 Chemical composition, structure and morphology of thin films ...................................... 27 3.1.1 Rutherford Backscattering Spectrometry for chemical analysis ............................... 27 3.1.2 X-Ray Diffraction to structure analysis .................................................................... 28 3.1.3 Scanning Electron microscope for morphological features ...................................... 31 3.2 X-Ray Photoelectron Spectroscopy for surface analysis .................................................. 32 3.3 Optical spectrometry to measure the optical response of the thin films .......................... 33 3.4 Optical system for gas detection ................................................................................... 35 Chapter 4- Results and Discussion .......................................................................................... 37 4.1 Morphology and deposition rate of the as-deposited films .............................................. 37 4.2 Composition of the thin films for the different systems produced ................................... 40 4.3 Crystalline and Morphological evolution of the thin films with annealing treatments ........ 44 4.3.1 Structural evolution of the Au:CuO system ............................................................. 46 4.3.2 Structural evolution of the Ag:CuO system ............................................................. 51 4.3.3 Structural evolution of the Au-Ag:CuO system ......................................................... 57 4.4 Optical response .......................................................................................................... 62 4.5 Surface analysis of the Au:CuO film for CO gas detection .............................................. 66 4.5.1 Surface analysis by XPS ........................................................................................ 66 4.5.1.1 Surface analysis of the nanoplasmonic films by XPS ........................................ 67 4.5.1.2 High resolution XPS spectra of the C 1s line before, during and after sample exposure to CO ........................................................................................................... 73 4.5.2 Surface functionalization with plasma etching ........................................................ 74 Chapter 5- Conclusions and future perspectives ...................................................................... 77 References ............................................................................................................................. 81

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Index of figures Figure 1- Number of scientific publications that include the word “nanosensors” between the years 1995 and 2016............................................................…………………………………………………....1 Figure 2- Chemical structure of Carboxyhemoglobin.....................................................................3 Figure 3- Typical structure of a gas sensor system……………………………………………………………...5 Figure 4- Schematic diagrams of electromagnetic fields propagation due to surface plasmon resonance, a surface plasmon polariton (SPP)...................................…………………………………...8 Figure 5- Illustration of the two main SPR configurations based on (a) a Kretschmann prism coupler and (b) a grating coupler................................................………………………………………….………...9 Figure 6 – Schematic diagram illustrating a localized surface plasmon, when the incident light interacts with metal nanoparticles.............................................................................................11 Figure 7 – Photograph of the south rose window of Notre Dame Cathedral in Paris, France, where specific colours are scattering by different types of nanoparticles.......………….……………………….12 Figure 8 - Diagram illustrating how the nanostructure shape effects the LSPR absorption band. Here, the LSPR band of the Ag nanoparticles blue-shift after an oxidation/ depletion of vertices. .....................................……………………………………………………………………………………..……..13 Figure 9 – Schematic illustration of the sensing principle of SPR (left) and LSPR (right). The graphs below show the calculated shift of bands upon adsorption of 10 nm (green) and 20 nm (red) thick molecular film......................................................................…………………....……..……………….14 Figure 10- UV-visible absorption of the Cu:CuO core:shell nanoparticles array under air and under various CO gas flow from 1.6 up to 16 L/h (a) and schematic of the experimental setup for flow cell (b).......................................................................……………………………………………....…….16 Figure 11- Schematic diagram of high-resolution spectroscopy apparatus........................………17 Figure 12- Schematic of a conventional sputtering process, in which the target is connected to a negative voltage supply (cathode) while the substrate holder along with the chamber maintained at a ground potential acts as the anode……………………………………………………………….…………….20 Figure 13- Schematic showing the magnetic field and basic components of a magnetron sputtering technique…………………………………………………………………………………………………………….....21 Figure 14- a) Schematic of the components of the reactive magnetron sputtering chamber used for the thin films deposition and b) Schematic illustration of the magnetron and the copper target with

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60 pellets of gold placed symmetrically in the preferential erosion zone of the target......................................................................................................................................23 Figure 15- Schematic representation of the principal processes of nanoparticles formation by aggregation.............................................................................................................................25 Figure 16- Schematic representation of the thermal annealing steps used to promote the growth of Au/Ag nanoparticles in a CuO matrix. (I) heating ramp programmed to increase de temperature 5 ºC/min to the chosen temperature; (II) isothermal period at chosen temperature for 5 hours and (III) a free cooling phase...........................................................................................................26 Figure 17-Schematic of the: (a) characteristic face centred cubic structure of gold and silver and (b) monoclinic structure of CuO. Copper atoms are those of pink colour and oxygen atoms are the blue ones................................................................................................................................29 Figure 18- Schematic of the X-ray diffractometer: (a) to the Bragg’s Law application between interplanar space and (b) to grazing incidence diffraction geometry...........................................................................30 Figure 19- Diagram showing the various components of a scanning electron microscope.............32 Figure 20- Basic schematic of the new optical system for gas detection.......................................35 Figure 21- Cross-section SEM micrographs and deposition rate of the as-deposited films as a function of the total area of pellets incorporated in the Cu target for (a) Au:CuO, (b) Ag:CuO and (c) Au-Ag:CuO samples…………………………………………………………………………………………………..38 Figure 22- Atomic concentration of Au (black) and Ag (red) on the as-deposited samples as a function of the total area of pellets incorporated in the Cu target for the Au:CuO, Ag:CuO and Au-Ag:CuO as-deposited samples………………………………………………………………………………….40 Figure 23- Atomic concentration (at.%) of the different elements present in the as-deposited CuO matrix (solid lines) and in the CuO matrix with annealing at 700 ºC (dash lines) (a)) and in the as-deposited samples of Au:CuO (b)), Ag:CuO (c)) and Au-Ag:CuO (d)) systems deposited with a pellets are of 900 mm2, obtained by the RBS data analysed with the code IBA DataFurnace NDF v9.6i ..42 Figure 24- Crystalline and morphological evolution of the CuO matrix as a function of the thermal treatment. X-ray diffractograms of the CuO matrix are represented in a) and the SEM micrographs of the cross-section and the surfaces through secondary electron detector in b) and c), respectively……………………………………………………………………………………………………………..44 Figure 25- XRD diffractograms of the Au:CuO films for different annealing treatments with different Au contents: a) CAu=15.6 at.% (pellets’ area: 720 mm2); b) CAu=15.0 at.% (pellets’ area: 960 mm2) and c) CAu=16.8 at.% (pellets’ area: 1200 mm2).........................................................................46

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Figure 26- SEM micrographs of the Au:CuO system observed in cross-section (to the left), using secondary (SE) and backscattered (BE) electron detectors, and in top-view (to the right) through backscattered electron detector, at different temperatures, for all atomic concentrations of Au: a) CAu=15.6 at.% ; b) CAu=15.0 at.% and c) CAu=16.8 at.% .................................................................48 Figure 27- Feret diameter histograms of the nanoparticles present in SEM micrographs’ surface and the respective nanoparticles aspect ratio histograms, as insets of the former, for all Au compositions at different annealing temperatures......................................................................50 Figure 28- XRD diffractograms of the Ag:CuO films for different annealing treatments with different Ag contents: a) CAg=18.7 at.% ; b) CAg=17.7 at.% and c) CAg=29.5 at.%...........................................52 Figure 29- SEM micrographs of the surface of the Ag:CuO, at different temperatures, for all atomic concentrations of silver: a) CAg=18.7 at.% ; b) CAg=17.7 at.% and c) CAg=29.5at.%.............53 Figure 30- SEM micrographs of the Ag:CuO system observed in cross-section (to the left), using secondary (SE) and backscattered (BE) electron detectors, and in top-view (to the right) through backscattered electron detector, at different temperatures, for all atomic concentrations of Ag: a) CAg=18.7 at.% ; b) CAg=17.7 at.% and c) CAg=29.5 at.% .................................................................55 Figure 31 - Feret diameter histograms of the nanoparticles present in SEM micrographs’ surface and the respective nanoparticles aspect ratio histograms, as insets of the former, for all Ag compositions at different annealing temperatures......................................................................56 Figure 32- XRD diffractograms of the Au-Ag:CuO films for different annealing treatments with different Au and Ag contents: a) CAu=7 at.% and CAg=8.5 at.%; b) CAu=6.7 at.% and CAg=8 at.% and c) CAu=7.8 at.% and CAg=9.1 at.%....................................................................................................58 Figure 33- SEM micrographs of the Au-Ag:CuO system observed in cross-section (to the left), using secondary (SE) and backscattered (BE) electron detectors and in top-view (to the right) through backscattered electron detector, at different temperatures, for all atomic concentrations of Au and Ag: a) CAu=7 at.% and CAg=8.5 at.%; b) CAu=6.7 at.% and CAg=8 at.% and c) CAu=7.8 at.% and CAg=9.1 at.%.........................................................................................................................................59 Figure 34 - Feret diameter histograms of the nanoparticles present in SEM micrographs’ surface and the respective nanoparticles aspect ratio histograms, as insets of the former, for all Au-Ag compositions at different annealing temperatures......................................................................61 Figure 35- Transmittance spectra of the substrate (fused silica), of the CuO film without heat treatment (as-deposited) and of the CuO film with heat treatment between 300 and 700 ºC.........63

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Figure 36- Transmittance spectra of the Au:CuO (a), Ag:CuO (b) and Au-Ag:CuO (c) films with different contents of Au and/or Ag and subjected to different annealing treatments……..…………..64 Figure 37 - High resolution XPS spectra of a) C 1s, b) O 1s, c) Au 4f and Cu 3p and d) Cu 2p3/2 lines, taken from the nanoplasmonic Au:CuO sample annealed at 700 °C..................................68 Figure 38- Determination of the hydrocarbons’ overlayer thickness from the ARXPS results..........71 Figure 39- HRXPS spectra of the C 1s line taken from the Au:CuO thin film, a) before, b) during and c) after the exposure to CO…………………………………………………………………….………………74 Figure 40- Transmittance spectra for different plasma treatments of the nanoplasmonic Au:CuO film (annealed at 700 ºC)..........................................................................................................75 Figure 41- SEM micrographs of the Au:CuO film’s surface before a plasma treatment (a) and after 3 minutes of plasma treatment (b)............................................................................................76

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Index of tables Table 1- Total area of metallic pellets and target potential for each deposition of the Au:CuO, Ag:CuO and Au-Ag:CuO thin films…………………………………………………………………………………………….24 Table 2- Chemical composition and simulated thickness, performed by RBS, of the different systems for the as-deposited thin films and CuO matrix annealed at 700ºC………………………….43 Table 3- Values of magnitudes used to determine sensitivity factors…………………………………….72

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Abbreviations and Acronyms AFD- Average Feret Diameter ARXPS- Angle Resolved X-Ray Photoelectron Spectroscopy BCC- Body centred cubic BE - Backscattered Electrons DC - Direct Current FCC- Face Centred Cubic LSPR- Localized Surface Plasmon Resonance PVD- Physical Vapour Deposition RBS- Rutherford Backscattering Spectrometry SE - Secondary Electrons SEM- Scanning Electron Microscopy SPP- Surface Plasmon Polariton SPR- Surface Plasmon Resonance SPs- Surface plasmons T-LSPR- Transmittance- Localized Surface Plasmon Resonance UV-Vis-IR - Ultraviolet-Visible-Infrared XPS- X-Ray Photoelectron Spectroscopy

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Chapter 1- Introduction 1.1 Contribution of Nanotechnology In this technological era in which we live, human beings are creating new devices, machines and systems to make our lives easier, more enjoyable and more comfortable. These new “gadgets”, increasingly, have to be incorporated in cheaper, faster, smaller and more complex “technological organisms” [1]. In the last decades, the word micro has been part of the common journey that humanity initiated. But now, we are just beginning to observe even smaller things. Things that go beyond the micro, things that belong to the nanometer scale, to the nano reign, which means a milliardth (10-9). For many, the formal initiation of nanotechnology occurred when, in 1959, the Nobel laureate Richard Feynman stated that there was “plenty of room at the bottom”, speculating on the possibility and potential of nanosized materials [2]. Today, we can already find nanotechnological applications on diverse areas such as materials and manufacturing [3], nanoelectronics [4], medicine and healthcare [5], energy [6], biotechnology [7], information technology [8], and national security [9,10]. The number of scientific publications with the word nano has increased exponentially since the twentieth century, predicting that nanotechnology will be the next industrial revolution. Besides, not only the number of science papers with the word nano has increased, but also the number of scientific publications related to nanosensors, as it plotted in Figure 1, according to the databases of ISI Web of KnowledgeSM and ScopusTM.

Figure 1- Number of scientific publications that include the word “nanosensors” between the years 1995 and 2016 (data extracted from ScopusTM database).

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After observing this plot, it is easy to understand and find an explanation to this trend, since the nanotechnology introduced many advantages in the sensors’ field: an obvious decrease in the sensors’ dimensions; or at least the sensing films; an increase of surface area; a shorter diffusion time; consequently a shorter response time, and the possibility of personalized materials with tailored and new properties not present in the original bulk materials [1,11]. This has opened the door to new and real applications for sensing. Sensors can be used to detect very different things, such as bacteria [12], viruses [13], biological markers [14], explosives [15], magnetic fields [16], infrared radiation [17] and traces of pollutants [18]. Metal Oxide nanomaterials have a key role in the successful implementation of chemical sensor technology due to their unique combination of redox chemistry, electrical, semiconductor, dielectric and optical properties [19]. The addition of a noble metal, like gold (Au) or silver (Ag), in the form of nanoparticles, into a metal oxide matrix, have been a subject of considerable interest over the past few years, constituting an effective design for an optical gas sensing system due to their localized surface plasmon resonance (LSPR) properties [20–22]. However, as with most of sensing research, work is still needed to improve the sensitivity, stability and selectivity in respect to the large variety of target gases. Furthermore, another important challenge is the development of low-cost techniques for controlled and reproductive fabrication of these novel sensing elements.

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1.2 What is a chemical sensor? 1.2.1 Monitoring our Environment The release of anthropogenic toxic pollutants into the atmosphere resulting from industrialization of society, introduction of motorized vehicles motivated by a tremendous increase of the population, contributed to the growing of air pollution problem. Common hazardous compounds such as carbon monoxide (CO), nitric oxides (NOx), sulfur oxides (SOx), hydrocarbons, and particulate matter (both solid and liquid) are dispersed throughout the atmosphere in progressively higher concentrations, being the major concern for environmental air pollution. Moreover, when these gases are concentrated, serious health problems can occur quickly, which should be taken quite seriously [23–25]. Carbon monoxide (CO) is a tasteless, odourless and colourless gas formed by hydrocarbon combustion, which consists of one carbon atom and one oxygen atom, connected by a triple bond (|C≡O|). Beyond fires, the most common sources of CO emission into the atmosphere are mainly motor vehicle exhaust gases, gas powered engines and paints including methylene chloride. Although its atmospheric concentration is generally below 0.2 ppm, CO gas is heavier than air and thus can accumulate quickly near the ground even in well-ventilated confined areas. A major problem occurs when CO causes damage to human body, since it binds irreversibly to the iron centre of haemoglobin, the oxygen transport molecule in blood, forming carboxyhemoglobin (COHb), represented in figure 2, and resulting in impaired oxygen transport and utilization. This behaviour originates a reduction in cellular respiration that may result in brain and heart damages, or even death [26,27].

Figure 2- Chemical structure of Carboxyhemoglobin (figure adapted from [28]).

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Various studies have shown that CO poisoning is the most common type of fatal air poisoning in many countries [29]. In UK, unintentional poisoning demonstrates both seasonal and regional variation, and it is most common during winter months in cold climates due to poorly functioning heating systems and fireplaces. Motor vehicles operating in poorly ventilated areas, such as in closed garages, are the main causes of poisoning too [30]. There are many CO exposure limits set by government organizations. In January 2005, the European Commission, lists a maximum allowable short term limit of 10 ppm for 8 h of exposure, therefore, any CO reading over 10 ppm should be investigated and acted upon [31]. The human olfactory system recognizes many odours; however, some gaseous compounds are odourless. For this reason, it is important the development of sensors to monitor the environment, in terms of pollution, health and safety, to detect which substances are present and in what quantity, in real time. Gas sensor systems, sometimes considered artificial noses, are devices able to detect the presence of gas molecules and at the same time are a great topic for generating revenue, especially for massive volume applications in the industrial, consumer and motor vehicle sectors [32]. The world market for chemical sensor is projected to reach 27.83 milliard € by the year 2020 with a growing demand that will be fuelled by new applications for established sensor products, product innovation and falling cost of high performance sensors. Moreover, Europe was considered to be the largest contributor to the global gas sensors market in 2012, accounting for around 30% share of the overall market. The development of environmentally friendly and low cost gas sensors with near single molecule sensitivity will have a large relevance for several industrial sectors, namely for house and industry detection that will contribute for the position of the European companies on this sector [33]. 1.2.2 Chemical sensors: definitions and classification Although this topic is widely discussed, there is still no commonly accepted definition of the notion “chemical sensor” in the literature. According to the IUPAC (International Union of Pure and Applied Chemistry), “a chemical sensor is a device that transforms chemical information, ranging from the concentration of a specific sample component to total composition analysis, into an analytically useful signal. The chemical information, mentioned above, may originate from a chemical reaction of the analyte or from a physical property of the system investigated”. Gas sensors are obviously included in the chemical sensors’ class [34].

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Chemical sensors are traditionally comprised of two basic components: a chemical (molecular) recognition system (receptor) that transforms a chemical information into a form of energy, and a transducer that converts this energy into a useful analytical signal, such as electrical, thermal or optical output signals (figure 3). The receptor part may be based upon different principles. If the receptor reacts with the analyte by a chemical reaction, the sensor is defined as “chemical sensor”, if no chemical reaction takes place and the effect of the interaction between the analyte and the receptor is the change of some of its physical properties (mass, temperature, electrical, magnetic, optical properties), the sensor is said to be a “physical sensor”. In some cases, the interaction involves biochemical reactions mediated by enzymes, immunosystems, tissues, organelles or whole cells to detect chemical compounds, and in this case the sensors are termed biosensors.

Figure 3- Typical structure of a gas sensor system. There is a variety of sensor devices that depend on different chemical or physical phenomena and can be classified according to the method of functioning into the following groups: - Optical sensors, following absorbance, reflectance, luminescence, fluorescence, refractive index, optothermal effect and light scattering [35]; - Electrochemical sensors, such as voltammetric and potentiometric devices, chemically sensitized field effect transistor (CHEMFET) and potentiometric solid electrolyte gas sensors [36];

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- Electrical sensors related to changes in conductivity, impedance and capacitance [37]; - Mass sensitive sensors, such as piezoelectric devices and those based on surface acoustic waves [38]; - Magnetics sensors based on the change of paramagnetic gas properties [39]; - Thermometric sensors based on the measurement of reaction heat [40]; - Other sensors related to emission or absorption of radiation [41]. Despite of such differences, there is a broad agreement about characteristics of sensors. Sensors should be small; automatic; respond quickly; have autonomous power (batteries), and operate continuously or at least in repeated cycles; have low consumption of energy; be reversible (restoration of initial properties) with respect to the component being monitored; have low detection limit or a high sensitivity, (i.e. low concentration values should be detected), and be cheap. Furthermore, in the gas sensing field there is one important parameter that is called selectivity, which estimates the sensitivity of a device to substances other than the one whose measurement is required. The sensors should respond exclusively to one analyte, or at least be specific to a group of analytes [19,32]. Among the several transducing platforms, optical sensors for gas species’ recognition have attracted a lot of interest in the last decades since they may find several advantages over the electrical-based processes [42–44]. In fact, optical sensors are known to be highly electromagnetic noise independent and fire resistant, having the capability to be implemented in optical fiber networks, which allows fast and easy responses and in-situ measurements with a compact, flexible and environmental robust setup. Moreover, variations in intensity, frequency, polarization and phase of the transmitted/reflected light can be studied, improving the device performance by lowering the cross sensitivity between different gases. The principles of optical chemical sensors are based in classical spectroscopy and there are two main categories, the direct optical sensor and the reagent-mediated sensors. In the first one, the analyte is detected directly by monitoring some intrinsic optical property, like a specific absorption band of a gas molecule. In the second case, the analyte is detected after its interaction with a sensitive material by absorbance (employs variation in optical absorption, transmission or reflection after the interaction of the light with the active materials in the presence of a target molecule), luminescence (based on photo-luminesce or chemo-luminescence properties of the

Introduction

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sensing material that are monitored and related to the target gas concentration) or surface plasmon resonance (SPR). SPR based gas sensor detect local refractive index changes, such as those induced by gas species interacting near the surface [45]. The present work will focus on the Localized SPR based sensors, effect that will be explained in the following subchapter. 1.3 Optical Chemical sensors 1.3.1 Surface Plasmons The field of plasmonics has attracted the attention of chemists, physicists, biologists and material scientists for extensive use in areas such as electronics, optical sensing, biomedicine, data storage and light generation [46]. The discovery of this area began in 1902, when Wood described a pattern of anomalous dark and light bands in the reflected light in the moment that he shone polarized light onto a metal-backed diffraction grating [47]. But, only in the late 1960s’ the phenomenon was interpreted, when Otto and Kretschmann reported the optical excitation of surface plasmons [48,49]. Surface plasmons (SPs) are coherent oscillations of free electrons excited by electromagnetic radiation at the boundaries between a metal and a dielectric. They may be categorized into two classes: propagating surface plasmons, also known as surface plasmon polaritons, which can be excited on the metallic films, and localized surface plasmons that are excited on metallic nanoparticles or nanostructures [50–52]. More about these phenomena will be explained on this chapter, where the differences between them, the advantages and disadvantages as well as their mains applications in optical sensing are highlighted. 1.3.1.1 Surface Plasmon Resonance phenomenon Since the first application of SPR phenomenon for gas detection and biosensing in 1982 by Nylander and Lieberg, the SPR sensing technology has been widely used for the detection of biological and chemical analytes [53,54]. SPR is the phenomenon of excitation of coherent charge density oscillations that may exist at the interface of two materials that possess a negative real and small positive imaginary dielectric constant, for instance, a metal and a dielectric. This excitation is caused when the wave vector of the incident light becomes equal to that of the SPs modes supported by the interface.

Chapter 1

8

In surface plasmons polaritons, the plasmons propagate in the x- and y- directions, parallel to the plane of the interface, for distances on the order of tens to hundreds of microns, and decay evanescently in the z-direction, perpendicular to the direction of propagation of the surface plasma wave, as illustrated in figure 4, with a decay length on the order of 200 nm [50,55].

Figure 4- Schematic diagrams of electromagnetic fields propagation due to surface plasmon resonance, a surface plasmon polariton (SPP). Reprinted image with permission of Journal of Annual Review of Physical Chemistry [50]. The SPR effect is highly dependent on the refractive indices of both materials, the metal and the dielectric media, and any change in either of these leads to a change in the resonance condition [56]. Due to this property of SPs, the SPR effect is generally used in sensing applications, where the interaction between the metallic surface and a molecular layer of interest leads to a change in the refractive index of the dielectric medium and hence to the resonance condition which can be detected in three interrogation modes: angle resolved [57], wavelength shift [58] and imaging [59]. The first one measures the reflectivity of light from the metal surface as a function of either angle of incidence (at constant wavelength), whereas the second one measures the reflectivity of light as a function of either wavelength (at constant angle of incidence). The third mode maps the reflectivity of the surface as a function of position using light of both constant wavelength and incident angle to interrogate a two-dimensional region of the sample.

Introduction

9

A SPR optical sensor is generally composed by an optical system (light source, and an optical structure to excite the surface plasmon wave), a transducing medium and an electronic system to generate and to process the electronic signal. Since the propagation length of the SPP (surface plasmon polariton) is limited, it is essential the use of optical structures and advanced configurations [51,55,60]. There are two main configurations, which are illustrated in the figure 5.

Figure 5- Illustration of the two main SPR configurations based on (a) a Kretschmann prism coupler and (b) a grating coupler. Reprinted image with permission of Journal of Biophotonics [60]. The Kretschmann configuration is the most widely used geometry in SPR sensors. This configuration requires a prism coupler (dielectric material) where a light wave is completely reflected at the interface between a prism and a thin metal layer and excites a SPP at the outer boundary of the metal. All the main interrogation modes described above have been experimented using a prism coupler [53,61,62], however the “angle resolved mode” demonstrated to have the best resolution [63]. The use of grating couplers for sensors were early introduced in 1983 [64]. When a metal–dielectric interface is periodically interrupted, the incident radiation is diffracted, forming a series of beams directed away from the surface at a variety of angles. The use of grating couplers involve more complex mathematical modelling than for prism couplers and because of this its use is more difficult. [65].

Chapter 1

10

Over the past two decades, SPR spectroscopy has dominated commercial instrumentation. Most of the systems are based on the prism coupling design, such as BIAcore series, Autolab SPR, Plasmonic, Spreeta, Nanofilm, Multiskop, etc [55,60]. Although SPR sensors present high resolution, label-free and real-time response, many challenges remain ahead. The development of advanced recognition elements is needed to make an ideal recognition surface that will not be susceptible to interferers, since a nonspecific adsorption results in high background signals. The improvement of detection limits is also another challenge, since sensitivity limit is not sufficient for detecting low concentrations of low molecular weight analytes [55,60,66,67]. For practical applications, further improvement is necessary and localized surface plasmons provides possible solutions. 1.3.1.2 Localized Surface Plasmon Resonance When the SPR effect is observed in nanostructures with more confined dimensions, such as nanoparticles, it is denominated as Localized Surface Plasmon Resonance (LSPR). LSPR is an optical phenomenon generated by interactions between the incident light and surface electrons in a conduction band of a noble metal nanostructure that causes a coherent localized plasmon oscillations of free electrons (figure 6). If the nanoparticles material is Au or Ag the resonance absorption occurs within the ultraviolet-visible (UV-Vis) region [46,50,68]. In LSPR, the light interacts with particles much smaller than the incident wavelength and the decay length is about 1 order of magnitude shorter than that of the SPR (for SPR is about 200 nm, figure 4), both decaying exponentially [59,69,70]. The shorter field decay length for LSPR increases sensitivity to refractive index changes on the surface, since the sensitivity to interference is reduced. Hence, although SPR spectroscopy offers much higher sensitivity to changes in the bulk refractive index than LSPR spectroscopy, the response of two techniques becomes comparable when measuring short-range variations in the refractive index due to a molecular adsorption layer. These properties make LSPR more sensitive to molecular adsorption and less sensitive to bulk effects, which causes errors in experimental data [50,71].

Introduction

11

Figure 6 – Schematic diagram illustrating a localized surface plasmon, when the incident light interacts with metal nanoparticles. Reprinted image with permission of Journal of Biosensors [72]. The oscillation of conduction electrons leads to an accumulation of polarization charges on the surface of a nanoparticle. When only dipole oscillations contribute to extinction (absorption plus scattering), E (λ), Mie’s solution to Maxwell’s equation can be used to obtain the spectrum for well separated nanoparticles [73,74]. The equation (1) reveals the dependence of extinction on the size, shape, density and local environment of the nanostructure: E λ = 4πΝR εh ⁄λ n [ εi εr+χεh +εi

] (1) Where 𝜀h is the dielectric constant of the host surrounding medium, N is the area density of the nanostructure, R is the radius of the nanostructure, 𝜒 is the term for the shape of the nanoparticle, and 𝜀i and 𝜀r are the imaginary and the real parts of the nanostructure’s dielectric function [75,76]. Palladium (Pd), platinum (Pt), copper (Cu), gold (Au) and silver (Ag) are some of the several elements that have been shown to support localized surface plasmons [77]. Both Au and Ag nanostructures exhibit LSPR effect in the visible range of the spectrum due to the energy levels of d-d transitions, making these materials suitable for various applications involving colour [78]. These properties have been apparent for centuries, since metallic nanoparticles are visible in the vibrant hues of the great stained-glass windows of the world, such as the stained-glass windows of Notre

Chapter 1

12

Dame Cathedral (as illustrated in figure 7), where the nanoparticles scatter specific colours depending on the particles’ size and geometry when the windows are illuminated [79]. For sensing applications, Au and Ag nanostructures are also ubiquitous in the literature due to their high values of refractive index sensitivity [77]. Although Ag nanostructures have a higher sensitivity than Au nanostructures, and present sharpest and strongest bands among all metals, the Au nanostructures are more frequently selected for sensing applications, due to their lower toxicity, inert nature (less prone to oxidation) and stability [80,81]. The shape and size of metallic nanoparticles, as well as the distance between them, have been experimentally shown to contribute to LSPR spectral properties due to changes in surface polarization. The ability to change these parameters and study the effect on the LSPR is an important experimental challenge [81,82].

Figure 7 – Photograph of the south rose window of Notre Dame Cathedral in Paris, France, where specific colours are scattering by different types of nanoparticles [79]. In figure 8 is presented the effect that the Ag nanostructures’ shape can have on the extinction wavelength maximum (𝜆max ). For this experiment, silver triangles were prepared on an Indium Tin Oxide (ITO) substrate and then were selectively oxidized (first at the bottom edges, then at the triangular tips, and finally from the top face), allowing to directly correlate the LSPR response to morphological changes. Through the figure it is possible to understand that LSPR band of the Ag nanoparticles shifts towards shorter wavelengths after oxidation, presenting a LSPR band near the infra-red region where it has more vertices [83].

Introduction

13

The size of nanoparticles is also important. Link et al., studied the size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles and they discovered that when the diameter of Au nanoparticles is increased from 10 nm to 100 nm, the LSPR band shifts from 520 nm to 580 nm [84].

Figure 8 - Diagram illustrating how the nanostructure shape affects the LSPR absorption band. Here, the LSPR band of the Ag nanoparticles blue-shift after an oxidation/ depletion of vertices. Reprinted image with permission of Journal of Nano letters [83]. Apart from the advantages of LSPR already mentioned above, LSPR presents also a simpler instrumentation necessary to obtain the signal. LSPR instruments are typically built in-house and are designed to handle application. For example, for sensing is normally used ultraviolet-visible-infrared (UV-Vis-IR) spectroscopy. Moreover, they can be little, light and cheap, consisting in three basic components: a light source, a detector and the sample. Hence, these characteristics allows the construction of LSPR nanosensors, able to detect chemical or biomolecules, using basic optical instrumentation [46]. As it can be seen in figure 9, while the main SPR instrument uses a prism coupled and a monochromatic light with a range of incident angles, the LSPR are excited by direct illumination independent of the angle. Hence, for the first one, the plasmon excitation corresponds to the minimum in the graph of the reflectivity and for the LSPR the plasmon excitation corresponds to the maximum observed in absorbance [85,86]. After changes of the refractive index in the vicinity

Chapter 1

14

of the nanoparticles caused by target molecules, the coupling condition changes and leads to a shift of the peak maximum [52,72].

Figure 9 – Schematic illustration of the sensing principle of SPR (left) and LSPR (right). The graphs show the calculated band shifts upon adsorption of 10 nm (green) and 20 nm (red) thick molecular film. Reprinted image with permission of Journal of Sensing and Bio-sensing Research [52]. 1.3.2 Noble metal nanoparticles embedded in metal oxides with LSPR effect for gas sensing Plasmonic sensing that utilizes spectral responses of metal nanoparticles to the refractive index of the surrounding dielectric medium has been explored for various areas, including biomedical assays [87], disease diagnostics [88], food [89] and environmental safety [90]. However, these applications have been predominantly focused on biological detection, since to obtain a spectral response of metal nanoparticles is required a large change in surrounding refractive index which can be easily obtained from the binding event of bulky biomolecules [74,91,92]. Hence, contrariwise, a limited number of studies have been conducted on small

Introduction

15

molecules such as the gas molecules. Gas sensing based on LSPR has been known to be challenging because gas molecules generally exhibit low adsorption to metal surface and, when occurs, induces small refractive index changes [93]. Several strategies were explored to improve sensing capabilities of LSPR based gas sensors, such as the utilization of polymers [94], the design of molecules [95] or the utilization of natural molecules [96] to adhere to the surface on metallic nanoparticles and, at the same time, capture the gas molecules. However, for metal nanoparticles embedded in a metal oxide matrix, the strategies might be different to build a sensitive film. Metal oxide nanostructures, such as CuO, TiO2, ZnO and SnO2 in the form of thin films have been widely used in sensing applications. One of the most traditional methods that makes use of metal oxide nanostructures is the conductometric method. It is basically an electrical process, which allows the detection of considerable amounts of gases, through changes in the electrical resistance, by redox reactions between target gas molecules and a “sensitive” oxide surface. When O2 molecules are adsorbed on the surface of a metal oxide thin film, they trap the electrons in the form of ions, and when metal oxide is exposed to CO gas, CO is oxidized by O- and releases electrons to the surface, operating at elevated temperatures [19,97,98]. In general, metal oxides are optically transparent in the visible wavelengths, are often catalytically active and have a wide band gap, being used as electrical insulators or semiconductors in a variety of electrical devices. Furthermore, metal oxides are resistant to high temperatures and are relatively easy to produce [19,99–102]. To use these oxides for LSPR optical sensors, one needs to add a “beacon” to the material, in the form of plasmonic nanoparticles [19,103]. Since the discovery of Masatake Haruta [104], in 1987, Au nanoparticles are considered an active catalytic material, something unthinkable until date because gold electrons are not easily transferred due to its electronic structure. His work initiated more investigations on Au catalyst nanoparticles and many studies proceeded on the location of the CO oxidation catalysed by gold, and the adsorption of gases (especially the adsorption of oxygen) on supporting oxide metal materials [105–107]. Like the conductometric method, oxygen is directly adsorbed, reacting with adsorbed CO to form CO2. Hence, Au and Ag nanoparticles additives act as catalysts to modify the surface chemical reactions of metal oxide semiconductors toward sensing gases, thus improving the sensing process. Some studies were reported for optical detection of CO by LSPR effect, using Au:Co3O4 [108], Au:CuO [20], Au:YSZ [43] and Au:CeO2 [109]. In these works, the researchers used the

Chapter 1

16

catalysis mechanism to promote an easier chemisorption of CO molecules on the surface of the film, consequently improving their detection by LSPR effect. In this case, the exposure of Au nanoparticles at the surface is required. However, these approaches needed a rise in temperature in the range of 150-800 ºC, being necessary the use of an oven in the flow cell, becoming the detection more expensive and slower. Furthermore, at these temperatures and especially in oxidizing or reducing environments, sensor reliability becomes an important issue. Recently, Ghodselahi et al, [110] presented a work where thin films composed by an hexagonal array of Cu:CuO core:shell nanoparticles deposited on a-C:H thin films were prepared by co-deposition using Radio Frequency- Sputtering (RF-Sputtering) and Plasma-enhanced chemical vapor deposition (PECVD). In this research, it was detected a low flow rate of CO gas at room temperature and the results indicated that the surface of nanoparticles did not have chemical reaction with the CO molecules. Moreover, the physical interaction between the CO molecules and the nanoparticles increased the LSPR absorbance and caused a red shift in LSPR wavelength (as illustrated in figure 10 (a)). A simple schematic for the experimental flow cell setup is given in figure 10 (b).

Figure 10- UV-visible absorption of the Cu:CuO core:shell nanoparticles array under air and under various CO gas flow from 1.6 up to 16 L/h (a) and schematic of the experimental flow cell setup (b). Reprinted images with permission of Journal of Physical Chemistry C [110]. The use of high-resolution spectroscopy is another approach that has been developed by some researcher groups [93,111]. Kreno’s group developed a high-resolution UV-Vis transmission spectrometer which tracks LSPR peak position with a low noise level. Furthermore, they used a very porous metal-organic framework (MOF) material thin film that allowed the increase of contact

Introduction

17

area between the material and the target gas by preconcentrating the gas inside the pores, without any chemical interaction. The equipment used is shown in a schematic representation (figure 11). To measure the extinction spectra, the apparatus was mainly composed by a white light from a tungsten halogen lamp, a stainless-steel flow cell for the sample which allows dosing of gas or liquid phase analytes and a photodiode array spectrometer. Beyond this, two gas cylinders with H2 and CO2 were used with two mass-flow controllers (MFC) to regulate the gas flow rate. Inline moisture traps were installed to trap any residual moisture from the cylinders and direct-acting solenoid valves to the gas streams were diverted to the sample or to separate exhaust lines and finally, the outlet of the flow cell was connected to an exhaust line or pump and a stainless steel capillary for using a residual gas analyser. In the right side of figure 11, a schematic of the differentially pumped mass spectrometry setup is shown.

Figure 11- Schematic diagram of high-resolution spectroscopy apparatus. Reprinted image with permission of Journal of American Chemical Society [111] Previous studies reported a great diversity of oxide metal matrices used in gas sensing applications. In this work, the first approach consisted in the application of CuO matrix. Typically, CuO thin films exhibit p-type semiconducting nature and semi-transparency in the visible region, with a direct optical band gap energy in the range of 1.4 to 2.3 eV and a refractive index of about 1.3 to 2.3 [112–115]. However, these fundamental optical properties seem to depend on the deposition method and conditions, namely the temperature and thickness. It is known that when

Chapter 1

18

the temperature and thickness of the thin film increases, there is a reduction in the bandgap and consequently a reduction in transmittance [115,116]. Due to this properties, CuO could be used for several applications in catalysis [117], solar cells [118], electronics [119], among other. Furthermore, CuO thin films are promising material in gas sensor applications [120,121], since CuO exhibits high sensitivity for CO [110]. Therefore, its optical properties and previous studies were determinant for the choice of CuO as a matrix for CO sensing.

19

Chapter 2- Production of thin films with LSPR effect In this chapter, the production of nanocomposite thin films by Reactive DC Magnetron Sputtering and post-deposition thermal annealing will be reported. As it was described above, thin films composed by metallic noble nanoparticles embedded in a CuO matrix have already demonstrated a huge potential for gas detection with LSPR effect [110,122]. In these films, it is possible to adjust the LSPR absorption band through a change on metal concentration, size, shape and distribution of nanoparticles [103,123]. With the rise of the annealing temperature, the noble metal diffuses and coalesces, creating nanoparticles embedded in a host matrix, whose size depends on the thermal treatment temperature [124,125]. Hence, it is possible to observe the evolution of LSRP absorption band as a function of the annealing temperature. 2.1 Reactive Magnetron Sputtering for incorporation of gold and/or silver in a copper oxide matrix 2.1.1 Reactive Magnetron Sputtering One of the most used processes to produce thin films and coatings is the physical vapour deposition (PVD), which encloses a variety of environmentally friendly vacuum deposition methods. Generally, these methods involve individual or little groups of atoms which are removed from a target, are vaporized and transported in vacuum to a solid surface (substrate) where they are condensed to form a thin film. Although different PVD technologies use the same process, they differ in the methods used to remove and deposit material, since some material can be evaporated by using plasmas, ions or electrons beams or LASERs (light amplification by stimulated emission of radiation) [126,127]. Among the PVD methods, sputtering is one of the most popular technique, both at industrial and academic level, due to its versatility, since theoretically any material can be vaporized, offering good adhesions to the substrate and easy control of the deposition parameters [128–130]. In the basic sputtering process, a target (or cathode) plate is bombarded with high velocity positive ions generated by a plasma, situated in front of the target. This plasma is produced when an electric potential is applied between the cathode and the anode (substrate holder or even

Chapter 2

20

chamber walls) and the electrons collide with an inert gas, usually Argon (Ar), creating positively charged ions [131,132]. The bombardment causes the ejection (sputtering) of atoms from the target, which may then condense as a thin film on solid substrates, such as polymers, steel, glass, silicon wafers, etc, [133]. Apart from the neutral atoms, charged species and secondary electrons are also emitted from the target surface, and these electrons play an important role on maintaining the plasma [129]. A schematic of a conventional sputtering process is given in figure 12, in which are represented the plasma and its constituents, the target with negative voltage, the substrate holder along with the chamber at a ground potential, acting as an anode. The sputtering yield, number of atoms ejected from the target surface per incident ion, depends on the voltage and current (sputter power) at which sputtering takes place, on the target material composition, binding energy, characteristics of the incident ion and the experimental geometry [132,134].

Figure 12- Schematic of a conventional sputtering process, in which the target is connected to a negative voltage supply (cathode) while the substrate holder, along with the chamber is maintained at a ground potential, acting as the anode. Despite of the many advantages, the process is limited by low deposition rates and low ionisation efficiencies in the plasma [129,130]. Hence, to increase the sputter yield, magnetrons

Production of thin films with LSPR effect

21

(that use strong magnetic fields) are often employed in sputtering sources to confine charged plasma particles close to the surface of the target, as represented in figure 13. The introduction of a magnetic field parallel to the target surface can constrain secondary electron motion to the vicinity of the target. This trapping of the electrons causes an increase of the probability of an ionising electron–atom collision occurring, and consequently an increased ion bombardment of the target, giving higher sputtering rates and, therefore, higher deposition rates at the substrate [135–137]. As disadvantage, an annular erosion profile may appear on the surface of the target in the highest magnetic flux zones as the material is depleted due to sputtering, leading to an inefficient utilization of the target [135].

Figure 13- Schematic showing the magnetic field and basic components of a magnetron sputtering technique. Apart from the inert gas, in a reactive sputter deposition it is possible to add a reactive gas such as oxygen (O2) or nitrogen (N2) to the sputtering process, in order to form a compound between sputtered metal atoms and reactive gas molecules, such as oxides and nitrites. The addition of the reactive gas considerably changes the behaviour of the sputtering process, influencing the deposition rate as well as the composition of the film [138].

Chapter 2

22

2.1.2 Experimental details for thin films production 2.1.2.1 Cleaning and activation of substrate surface by plasma treatment For the depositions, two different types of substrates were used, namely SiO2 (fused silica), for optical spectra measurements, and Silicon wafers (Boron doped, p type, <100> orientation, 525 µm thick), for chemical and (micro)structural characterization purposes. Initially, and to clean and activate the surface of the substrates, a plasma treatment was applied. The plasma surface technology is used to treat different surfaces of materials by improving their adhesion to the deposited films. Electrons, ions and radicals are generated in a plasma and their interactions with substrates causes cleaning, etching and activation of the surfaces. The purpose of surface cleaning is to remove contaminants such as the natural contaminants that come from exposures of surfaces to the ambient atmosphere, mostly containing oxygen, carbon and hydrogen species, while the aim of surface activation is to generate surface functional groups and radical sites that increase the adhesion [139–141]. The plasma treatment was made in a Low-Pressure Plasma Cleaner by Diener Electronic (model Zepto Model) (Laboratório de Revestimentos Funcionais III no Centro de Física do Campus de Azurém da Universidade do Minho). This equipment works in primary vacuum conditions and it is constituted by a rotary vane pump by Pfeiffer, model Duo 1.3, and a HF power supply. The substrates were “immersed” into two distinct plasmas, using a 40 kHz RF generator with a power of 100 W; firstly, with O2 (80 Pa for 5 min), for cleaning, and then with Ar (80 Pa for 15 min), for surface activation. 2.1.2.2 Thin films deposition by Reactive DC Magnetron Sputtering In a second step, Au:CuO, Ag:CuO and Au-Ag:CuO thin films were deposited by reactive Direct Current (DC) magnetron sputtering (Laboratório de Revestimentos Funcionais III no Centro de Física do Campus de Azurém da Universidade do Minho). A schematic of this system is represented in figure 14 a). The substrates were placed in a hexagonal substrate holder that rotates at 16 rpm, in the centre of the deposition chamber, at 7 cm from the target. The magnetron is refrigerated with water and supplied by a DC power supply (Hüttinger Elektronik, model PFG 2500DC). The vacuum system coupled to the deposition chamber is constituted by a primary rotary pump (AEG, model AMME 80ZCA4), and by a turbomolecular pump (Adixen/Alcatel, model ATP 400), responsible for

Production of thin films with LSPR effect

23

the secondary vacuum. The pressure inside the chamber is measured by a pressure sensor of Pfeiffer vacuum, model Compact FullRange TM Gauge PKR251, connected to a Balzers Single Gauge digital display. The deposition system is linked to an acquisition system by a digital multimeter (Agilent Technologies, model 34970A). The adjustment of pressure for process (Ar) and reactive (O2) gases are made using Mass Flow Controllers (MFCs), powered by a Bronkhorst High-Tech model F201CV-500-AAD-33-V (EL-FLOW). By doing so, it is possible to control the deposition parameters, namely the target potential, the working pressure inside the chamber, the flow of process gas, among others.

Figure 14- a) Schematic of the components of the reactive magnetron sputtering chamber used for the thin films deposition and b) Schematic illustration of the magnetron and the copper target with 60 pellets of gold placed symmetrically in the preferential erosion zone of the target. After a previous optimization of the deposition conditions (working and reactive gas partial pressures, target power density, etc.) it was found that the deposition rate of CuO was relatively high (about 50 nm/min.). Furthermore, taking into account that a transparent CuO matrix is required to detect the LSPR band (in transmittance mode), a compromise between the deposition parameters and the number of pellets was established, which led to the production of thin films with deposition times of only 60 s, though with a relatively high number of pellets. For the depositions, Au and/or Ag pellets (each one with a surface area of 16 mm2 and 0.5 mm thick) were placed symmetrically, in the preferential erosion zone of a rectangular Cu target (200×100×6 mm3, 99.99% purity). For each system, three distinct compositions (45, 60,

Chapter 2

24

75 pellets) were used to vary the concentration of each nanoplasmonic metal in the produced films, in order to study their influence on the optical response (LSPR band). The total area of metallic pellets placed onto the Cu target for each deposition is listed in table 1. In addition, a pure CuO thin film was also deposited as matrix reference. The illustration of the relative positions of 60 pellets of gold placed symmetrically in the preferential erosion zone of the Cu target, is given in figure 14 b). The target was sputtered in a gas atmosphere composed of Ar (15 sccm) and O2 (9 sccm), maintaining a constant working pressure of 3.5×10-3 mbar. The base pressure of the system was around 4.8×10-6 mbar and the target potential was limited to 500 V. The values for the target current density measured at each deposition are presented in table 1. It is observed a small decrease as the total area of pellets increases, with exception to the system containing 75 silver pellets, that presents a slight increase. This behaviour suggests that the incorporation of Au and/or Ag on the target slightly affects the discharge conditions and hence the plasma characteristics. Furthermore, higher values of target current density are noticeable for the Ag: CuO samples, compared to the other samples. Table 1- Total area and number of metallic pellets and target current density for each deposition of the Au:CuO, Ag:CuO and Au-Ag:CuO thin films.

System Au pellets Ag pellets Total area of metallic pellets (mm2) Target current density (mA/cm2) Au:CuO 45 - 720 3.25 60 - 960 3.00 75 - 1200 2.50 Ag:CuO - 45 720 4.15 - 60 960 3.45 - 75 1200 3.55 Au-Ag:CuO 22.5 22.5 720 3.30 29.5 29.5 960 3.15 37.5 37.5 1200 2.70

Production of thin films with LSPR effect

25

2.2 Formation of gold and silver nanoparticles inside a CuO matrix 2.2.1 General description of nanoparticles formation An important step to find the LSPR effect in Au/Ag:CuO thin films is the formation of the noble metal nanoparticles into the CuO dielectric matrix. This mechanism of formation of Au and/or Ag nanoparticles requires energy, and may involve several steps [142–144], which are schematically represented in figure 15.

Figure 15- Schematic representation of the principal processes of nanoparticles formation by aggregation. Nucleation is the first step that involve the formation of clusters randomly distributed by the matrix, creating a stable nucleus (nucleation centres) when they reach a critical size. The nucleus grows due to the aggregation of noble metal atoms dispersed in the matrix, forming a nanoparticle, being this phase called by Normal Growth. Lastly, the nanoparticles may increase the size by two distinct mechanisms: by particle diffusion/coalescence, and that happens when two nanoparticles come into contact, forming aggregate coalesced nanoparticle, or by the Ostwald ripening process, which takes place when the individual metal atoms of a nanoparticles join another metal particle by diffusion [145,146]. In this process favoured by thermal heating, the size distribution and the average size depend of the temperature and the time of thermal treatment used. 2.2.2 Thermal annealing process on thin films After the depositions, all samples were subjected to annealing treatments that were carried out in an in-air furnace, with a Termolab controller, in order to promote the necessary changes on the films’ nanostructure and to tailor their optical properties, especially the LSPR band.

Chapter 2

26

Temperatures ranging from 300 to 700 °C (with temperature intervals of 100 ºC) were chosen for the annealing, since J.F.Pierson et al [147] proved that Cu2O and Cu4O3 are partially oxidised into CuO when they are annealed in a furnace at 300 ºC in air and, contrary, a thermal treatment in air of CuO films does not modify its structure [148]. For the annealing treatments, the controller was programmed according to the graph of figure 16.

Figure 16- Schematic representation of the thermal annealing steps used to promote the growth of Au/Ag nanoparticles in a CuO matrix. (I) heating ramp programmed to increase de temperature 5 ºC/min to the chosen temperature; (II) isothermal period at chosen temperature for 5 hours and (III) a free cooling phase. After putting the samples in the furnace, the controller was programmed with: (I) A heating ramp of 5 °C/min from the room temperature to the chosen temperature; (II) An isothermal period of 5 hours for all chosen temperatures; (III) A free cooling phase, in which the samples cooled down freely inside the furnace, before reaching room temperature. All samples subjected to this thermal treatment will be characterized regarding their (micro)structural and optical properties. It will be possible to study the matrix crystallization, the aggregation and the grain size of nanoparticles, the morphology of these thin films and the evolution of LSPR band with the increase of temperature through the techniques that will be enunciated in the next chapter.

27

Chapter 3- Analysis and characterization techniques for the study of the microstructure, optical response and gas sensing properties of the thin films. Experimental details. 3.1 Chemical composition, structure and morphology of thin films After the production of the Au/Ag:CuO thin films, reported in the previous chapter, it is crucial to proceed to the materials characterization. The chemical, structural and morphological analysis of thin films produced allows to understand the characteristic evolution of thin films for different preparation conditions and, at the same time, evaluate how they affect the optical response of nanoplasmonic thin films. Thus, the study of these properties is fundamental, not only for the optimization of the deposition processes and annealing treatments but also for the perception of the LSPR band evolution. 3.1.1 Rutherford Backscattering Spectrometry for chemical analysis The study of chemical composition of thin films is fundamental not only to determine the microstructural alterations, as the noble metal concentration in the different compositions, but also to conclude how these differences in chemical composition influence the optical properties of nanoplasmonic thin films. Rutherford Backscattering Spectrometry (RBS) is a widely used method for the study of the composition of surface layers of solids. It allows to determine the chemical composition of materials, and also to analyse the composition profile in-depth up to few micrometres, based on the measurement of the energy of the scattered probing particles after an elastic collision with atoms in a sample [149]. E. Rutherford discovered the nucleus of the atom in 1911, when he first used the backscattering of alpha particles from a gold film to determine its atomic structure [150,151]. In RBS, a monoenergetic ion beam, usually of helium or hydrogen, produced in a linear accelerator, bombards the sample, and the ions scattered from the sample are detected in an energy-sensitive detector. The energies of the scattered ions depend on the mass of the atoms and the depth inside the sample where they are located. The number of detected ions at a given energy is directly related to the concentration of the atomic species. Hence, through a RBS analysis it is

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possible to obtain information about the type, concentration and depth distribution of atoms in a thin film [151–153]. Furthermore, RBS is a non-destructive method, since the material composition is essentially unaltered. However as disadvantage, the RBS has a poor capacity for the analysis of elements with low atomic weight [154]. The RBS analysis of the as-deposited samples were realized with a Van de Graaff accelerator in a small chamber with three detectors (Centro de Ciências e Tecnologias Nucleares do Instituto Superior Técnico da Universidade de Lisboa). One of them, a standard detector, was placed at 140 º, while the other two pin-diode detectors, located symmetrically to each other, were placed both at a 165 º scattering angle respective to the beam direction. Spectra were collected using 2.0 MeV 4He+, and 1.45 MeV 1H+ beams at normal incidence. Normal incidence was used ( = 0 º) in the experiments. The RBS data were analysed with the code IBA DataFurnace NDF v9.6i [155]. 3.1.2 X-Ray Diffraction to structure analysis Through the different compositions obtained by the production process, it was possible to form thin films with different microstructures. Hence, a structural and morphological analysis is fundamental to understand possible alterations in the nanoplasmonic thin films’ properties. The nanostructure is one of the most important characteristics in this type of materials, being one of the factors that determine the optical properties of thin films. Materials can be called amorphous when the atoms of the deposited material are in an unorganized state (without an ordered structure) or can be crystalline when their atoms are arranged in repeatable 3-dimensional arrays. This 3-dimensional array that repeats over space is designed by unit cell and can be cubic, hexagonal, or a few other types with different geometries, or in other words, with different lattice parameters [156,157]. In the case of gold and silver, they typically present a face centred cubic structure (fcc) as represented in figure 17 (a), while copper oxide (CuO) has a monoclinic structure, as represented in figure 17 (b) [158].

Analysis and characterization techniques. Experimental details.

29

Figure 17- Schematic of the: (a) characteristic face centred cubic structure of gold and silver and (b) monoclinic structure of CuO (figure adapted from [159]). Copper atoms are those of pink colour and oxygen atoms are the blue ones. The crystal plane of a material may be defined by three non-collinear atoms represented by the Miller indices (h,k,l) that identify the various possible planes by integer numbers. In the case of cubic structures, the lattice parameter (a) and the Miller indices can be used to determine the interplanar spacing (d), and the correlation between them is defined in equation 2 [160]. d = a√ + + (2) The X-Ray diffraction (XRD) is a widely used characterization technique for studying crystal structures. This technique is not destructive, and it is very used to characterize thin films, since it allows to identify and quantify the crystalline phase composition of a material. In addition, it is also possible to analyse the preferential orientation of crystals, the grain size and the lattice parameter of a crystal structure. As represented in figure 18 a), the principle of XRD technique is based on the interference of X-rays reflected in the sample. The photons of this radiation collide elastically with the target material atoms, being the reflected radiation of the same angle of the incident radiation (θ) [161,162]. In a crystalline material, the electromagnetic radiation can hit the atoms on two parallel lattice planes, one closer and another one further away, in which the beam will travel an extra

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distance (shown as l in figure 18 a)) than the other. Hence, after diffraction off of the atoms, the travel difference between the two beams is given by 2 x l= 2 × dsenθ [160].

Figure 18- Schematic of the X-ray diffractometer: (a) to the Bragg’s Law application between interplanar spacing d (figure adapted from [163]) and (b) to grazing incidence diffraction geometry (figure adapted from [164]) . When the extra distance that one beam travels (2𝑙) is exactly equal or multiple of one full wavelength ( ), the reflected beams of the different planes interfere constructively according to the following equation (3), which translates the Bragg’s Law equation [157]. d = nλs n θ (3) From the values obtained for 2, the interplanar distance is determined and comparing the values with crystallographic databases it is possible to identify crystal planes. In obtained diffractograms, the peaks’ position is related to the material nature and its crystalline structure, the peaks’ intensity is directly related to the volume fraction of the material phases and the peaks’ shape is related to the dimensions, deformations and heterogeneities of the crystalline domains [160,162]. To analyse thin films is convenient to use the technique of X-ray diffraction at grazing incidence (figure 18 b)) in order to minimize the contribution related to the substrate. In the grazing incidence experiments, the incidence angle ( i) is fixed at a small angle and the angle between the incident beam and the diffracted beam (2) is varied, moving only the detector arm. Thus, the incident beam goes over a long way on the film’s surface, reinforcing its diffraction pattern, while the signal from the substrate is reduced due to the small angle of incidence. Contrary to the conventional diffraction geometry (Bragg Brentano configuration), which observes planes parallel

Analysis and characterization techniques. Experimental details.

31

to the surface of the sample, in the case of grazing incidence the crystal planes inclined to the surface of the sample are observed, whose normal is the bisector of the angle formed by the incident and the diffracted beam. [165,166]. The crystalline structure of the as-deposited and annealed Au/Ag:CuO thin films was investigated by XRD (Instituto Pedro Nunes, Coimbra), using a Philips X-pert diffractometer with Co-K radiation, operating in a grazing incidence mode at an angle of = 2°. The diffractograms were recorded between 2θ angles from 30° to 100°, with a scanning step size of 0.025°. 3.1.3 Scanning Electron microscope for morphological features The scanning electron microscope (SEM) was marketed for the first time in 1965 after the research of Charles W. Oatley. It is an extremely useful tool for the analysis of new materials, such as thin films, because it offers a better resolution than the optical microscope (in order of 2 to 5 nm) [167]. This technique gives information about microstructure, dimensions and impurity or defects of a sample through its interaction with an electron beam of high energy [160,162]. In an ultra-vacuum column, electrons are emitted from tungsten thermionic filaments and they are generated and accelerated by an electron gun with energies in the range 0.1-30 keV towards the sample [168]. The electron beam is focused by a series of condenser and objective lenses onto the sample, where it interacts with a depth that can reach 1 m [169]. The components of a scanning electron microscope are represented in figure 19. When the specimen is bombarded by primary electrons, many events can occur, such as the emission of secondary and backscattered electrons and X-rays. The two signals most often used to form SEM images are secondary electrons (for topographic images, etc.) and backscattered electrons (for chemical weight contrast). When electrons interact with positively charged nucleus, they are scattered at large angles and these elastically scattered electrons are called backscattered electrons. They are used for SEM imaging, principally to detect contrast between areas with different chemical compositions since elements with high atomic number backscatter electrons more strongly than elements with low atomic number, appearing brighter in the image. By the other hand, the secondary electrons result of the inelastic scattering interaction between the primary electrons and the orbital shell electrons of the sample that are ejected and laterally detected to form an image of the sample’s surface. Through this detection it is possible to observe the surface and cross-section of the films, to study the topography, the type of film growth and to determinate their thickness [131,170,171].

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Figure 19- Diagram showing the various components of a scanning electron microscope (figure adapted from [172]). The morphology and thickness of the films were studied by SEM, using a Dual Beam SEM/FIB FEI Helios 600i instrument, through secondary electron and circular backscatter detectors (Instituto FEMTO-ST da Universidade de Borgonha, em Besançon, França). Then, the surface micrographs were analysed using MatLab software, by calculating Feret diameter and aspect ratio of the contrasted nanoparticles. 3.2 X-Ray Photoelectron Spectroscopy for surface analysis X-Ray Photoelectron Spectroscopy (XPS) is a surface sensitive technique that operates under ultra-high-vacuum. It is commonly used for the compositions and oxidation state analysis, as well as to perform chemical analysis, since it gives an exact quantitative information of the surface of the sample [173]. XPS is based on the concept of photoelectric effect. When a high energy X-ray beam irradiates a sample, electrons are emitted from valence and core levels of the atoms constituting the sample, resulting into ionization. The kinetic energy of ejected photoelectrons can be calculated using a suitable electrostatic or electromagnetic analyser and thus the spectrum of the sample is registered as a function of number of electrons of one energy ejected per unit time. Through the

Analysis and characterization techniques. Experimental details.

33

recorded kinetic energy of ejected photoelectrons (EK), the binding energy (EB) for the specific atomic orbital of an electron can be measured with respect to the Fermi energy level in solids by the following equation: 𝐄𝐁 = 𝐡𝛖 − 𝛟 − 𝐄𝐊 (4) Where h is the Planck’s constant, υ is the frequency of incident X-rays and ϕ represents the surface work function, which is the minimum energy required to extract an electron from the surface of a solid [174]. For a given element, the binding energy of an electron does not only depend upon the energy level of emission but also upon the oxidation state and bounding of the atom or the local environment to that atom. A change in either of the two factors results in slight variations in the location of corresponding photoelectron peaks and consequently a shift of the peak for that atom in the spectrum. This ability of the XPS technique to discriminate between different oxidation states and chemical environments is one of its major strengths [175,176]. In XPS technique, the average depth of analysis is approximately 1-10 nm because emitted photoelectrons lose kinetic energy as they travel through the sample [177]. In this work, XPS analysis was performed on a recently upgraded XSAM 800 Kratos UHV system [178] equipped with a non-monochromatic dual anode X-ray source (Mg/Al K lines), and a spectrometer based on the hemispherical energy analyser with the main path radius of 127 mm. Spectra were taken in fixed analyser transmission (FAT) mode with the pass energy of 40 eV (survey spectra) and 20 eV (high resolution spectra). The binding energy axis was calibrated using the acquired high resolution spectra of Au 4f and Cu 2p lines from previously sputter cleaned Au and Cu samples, by taking Au 4f7/2 and Cu 2p3/2 lines at 83.96 eV and 932.61 eV, respectively. To get closer insight into the surface structure of the films, XPS spectra were taken for different emitting angles i.e. using the approach known as Angle Resolved XPS (ARXPS) (Departamento de Física da Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa). 3.3 Optical spectrometry to measure the optical response of the thin films The interaction of a solid material with electromagnetic radiation gives a response that is strongly related to the chemical composition, chemical bonds and microstructures characteristic

Chapter 3

34

of the material. The study of the optical properties allows to obtain fundamental information about intrinsic properties of the material, such as the refractive index, extinction coefficient, bandgap, among others. Apart from this characteristics, the optical studies on thin films gain a great importance since the LSPR effect has an optical consequence at the level of transmission/absorption spectrum, being possible to discover the plasmon frequency of these materials through spectrophotometry [179]. Absorption spectrometry in the different regions of electromagnetic spectrum has been, since a long time, an important tool for the analyst to study and understand some optical properties of materials. In this technique, the light passes through the sample to be analysed and some of the light is absorbed by the sample. In materials such as thin films, the incident light on surface may be absorbed (A), transmitted (T) or reflected (R) through the material due to the interaction of the light beam with the structure. The principle of energy conservation relates the intensity of light reflected, absorbed and transmitted through a surface by the following expression: 𝑅+𝐴+𝑇=1 (5) When the light beam propagates through the material (transmitted light), it takes a deviation in the direction at the same time the speed of propagation decreases. The ratio between the speed of light in the vacuum (c) and the speed of light in a material (ʋ) is defined as the refractive index of a material (n) [180,181]: n = cʋ (6) The evaluation of the optical response of the thin films, and the LSPR band was performed in transmittance mode (Laboratório de Revestimentos Funcionais III no Centro de Física do Campus de Azurém da Universidade do Minho). The spectral range measured was between 300 and 900 nm, using a Shimadzu UV2450-PC UV-Vis spectrophotometer.

Analysis and characterization techniques. Experimental details.

35

3.4 Optical system for gas detection Once demonstrated the ability to produce plasmonic Au/Ag:CuO thin films with different properties, and proved the existence of a plasmon peak in these films that allows to follow the gas sensing response, it is necessary to test the behaviour towards reactive gases. To fulfil this goal a new equipment was designed and built in the laboratory (Universidade do Minho). The optical system for gas sensing is composed by two main parts (illustrated in figure 20), the optical components and the vacuum gas injection circuit. The optical system is composed by a tungsten light source, two optical fibre cables, a cell holder, a detector and a computer. The spectrometer makes transmittance measurements in milliseconds, being able to follow the spectra, quickly for a chosen time. The other part of the system is related with the vacuum system and introduction and the outlet of the testing gases. The cell holder is coupled to a vacuum pump that will produce a “primary” vacuum (~ 0.2 mbar). The gas can be injected through the other side of the cell holder, or in this case, the cell flow. In addition, the new optical system can perform spectroscopic measurements in both gas and liquid solutions.

Figure 20- Basic schematic of the new optical system for gas detection. Under these circumstances, it is believed that all the conditions are ready to make a portable system, easy to measure, fast, cheap and light, able to follow the shifts of the LSPR band in the transmittance spectra and the response time of the sensor, due to an interaction of the gas with a surface of the film.

37

Chapter 4- Results and Discussion 4.1 Morphology and deposition rate of the as-deposited films The deposition process by reactive magnetron sputtering depends of many factors that can influence the deposition kinetic and consequently the characteristics of the thin films produced. Thus, it is needed to understand the influence of the pellets’ area (Au and/or Ag) exposed to the sputtering process, in the deposition rate of the thin films produced and consequent growth morphology. The evolution of the growth (deposition) rate of the as-deposited films as a function of the total area of pellets in each system (Au:CuO, Ag:CuO and Au-Ag:CuO) is presented in figure 21. The deposition rate was calculated from the quotient between the thickness (estimated by SEM micrographs, also displayed in figure 21 and the deposition time (1 min), therefore it is numerical equal to the thickness if the units are in nm/min. Based on the SEM micrographs present for each deposition, it is possible to observe that the morphology of the as-deposited films depended on the total area of the pellets and varied according to the system considered. For the Au:CuO samples (figure 21 a)) a dense and compact morphology with, apparently, very low surface roughness is observed for all compositions of gold pellets. On the other hand, the Ag:CuO thin films (figure 21 b)) present a little compacted and porous microstructure, where is notorious the presence of granular aggregates for all depositions. The microstructure of Ag:CuO films becomes more compact and dense for the intermediate area of pellets, however a granular voided growth of the film is observed with the increase of the silver pellets for a total area of 1200 mm2. Moreover, the microstructure of the latter becomes more disordered and the surface seems to be extremely rough. This can be attributed to an increase of the amount of Ag aggregates into the film during the early stages of its growth, as also previously reported in other related works but in another matrix (TiO2) [182,183]. In what concerns the Au-Ag:CuO system (represented in figure 21 c)), this presents a dense and compact morphology, which is more similar to the Au:CuO films, regardless the gold and silver content of the target.

Chapter 4

38

Figure 21- Cross-section SEM micrographs and deposition rate of the as-deposited films as a function of the total area of pellets incorporated in the Cu target for (a) Au:CuO, (b) Ag:CuO and (c) Au-Ag:CuO samples.

Results and Discussion

39

For the different systems, the deposition rate takes different behaviors with the increase of the total area of metallic pellets. In the case of Au:CuO system, the deposition rate decreases a little with the increase of the amount of gold pellets, in a linear way, which is in agreement with the small decrease observed in the current applied to the target (Table 1), assuming the values of 36 nm/min, 34 nm/min and 31 nm/min when the total area of Au pellets is, respectively, 720 mm2, 960 mm2 and 1200 mm2. Regarding the Ag:CuO system, there is a decrease of the deposition rate from 51 nm/min to 44 nm/min when the amounts of Ag pellets increase to the intermediate composition, again in agreement with the decrease of the target current density (table 1). This is followed by a notorious growth in the deposition rate to 88 nm/min when the pellets’ area increases to 1200 mm2, which cannot be related solely with the target current, but also with microstructural features, as it will be explained below. For the Au-Ag/CuO films, an increase in the total area of pellets from 720 mm2 to 960 mm2 led to a smooth increment of the deposition rate from 41 nm/min to 46 nm/min. When the pellets’ area was 1200 mm2 the deposition rate was again reduced to 34 nm/min, which might be a consequence of the target current evolution (table 1). In general, the deposition rate seems to agree with the target current density results. Furthermore, the values of deposition rate of the Au:CuO system are generally lower than the ones of Ag:CuO and Au-Ag:CuO systems, fact also observed in the results of the target current density. The former behavior is directly related with the larger sputtering yield of Ag (2.522) comparatively to the sputtering yield of Au (1.608). The sputtering yields were obtained assuming 500 eV argon ions [184]. On the other hand, the accentuated growth of the deposition rate observed when the pellets’ area increases to 1200 mm2 for the system Ag:CuO is not only attributed to the higher sputtering yield of Ag in comparison to Au [185,186], but also to the morphological differences between the films of the various systems. According to the SEM micrograph, the film deposited with the highest number of silver pellets is more porous, and consequently have a lower density, the deposition rate measured may have been influenced by this change of morphology and not by an increase of the amount of material deposited. Furthermore, the porous structure observed in the Ag:CuO thin films is not verified in the Au-Ag:CuO system, which indicates that the presence of gold in the system influences the growth of Ag atoms and consequently the growth of the film, becoming more dense and not so porous as the Ag:CuO films.

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4.2 Composition of the thin films for the different systems produced The chemical composition of the thin films is an important factor in the microstructure and crystalline phases formed. Specifically, the chemical composition of the thin films produced is expected to be directly related to the pellets’ area placed in the preferential erosion zone of the target and with the sputtering yield of the materials involved. The atomic concentration (at.%), performed by RBS, of the plasmonic metal (Au and/or Ag) present in the different as-deposited samples produced (Au:CuO, Ag:CuO and Au-Ag:CuO), is displayed in figure 22, as a function of the total area of pellets incorporated in the Cu target. In a first analysis is possible to verify that all the systems considered have the same behaviour with the increase of the total area of pellets from 720 mm2 to 960 mm2, decreasing very smoothly, but increasing with different trends when the total area of pellets rise to 1200 mm2.

Figure 22- Atomic concentration of Au (black) and Ag (red) of the as-deposited samples as a function of the total area of pellets incorporated in the Cu target, for the Au:CuO, Ag:CuO and Au-Ag:CuO as-deposited samples. In particular, for the Au:CuO system, the increase of the total area of gold pellets from 720 mm2 to 960 mm2 resulted in a little reduction of the atomic concentration of Au in the films, from 15.6 at.% to 15.0 at.% and an increase of the total pellet’s area to 1200 mm2, led to a growth of

Results and Discussion

41

atomic concentration of Au to 16.8 at.%. A similar trend was observed for the Ag:CuO thin films, where Ag contents of 18.7 at.%, 17.7 at.% and 29.5 at.% were obtained for a total area of silver pellets of 720 mm2, 960 mm2 and 1200 mm2, respectively. This means that, for the same area of pellets, precisely for 720 mm2 and 960 mm2, the concentration of Au is about 16% lower than that of Ag. Once again, this behavior is attributed to the higher sputtering yield of Ag in comparison to Au [185–187], leading to an enhancement of the metal content sputtered from the target during the deposition of the Ag:CuO films. However, the increase of total area of Ag pellets to 1200 mm2, caused a more pronounced increase of the Ag contents than in the case of Au. This higher increase of Ag content can also be directly correlated with the morphology observed for this sample, which is much more porous than the others due to the presence of a higher amount of plasmonic metal (figure 21). Regarding the Au-Ag:CuO system, atomic concentrations of Au and Ag of 7.0 at.% and 8.5 at.%, respectively, were estimated for a total area of pellets of 720 mm2, decreasing for 6.7 and 8.0 at.% when the total pellets’ area increase to 960 mm2, and increasing again to atomic concentrations of Au and Ag of 7.8 at.% and 9.1 at.%, respectively, for the highest pellets’ area. As happened between the other systems, the concentration of Au is about 16% lower than that of Ag, but in this case for all conditions (i.e. total area of pellets of gold and silver). It is thus showed that the increase of the gold and silver pellet’s area do not significantly change the metal concentration in the CuO matrix, except for the highest concentration of the Ag:CuO system. Figure 23 represents the concentration profiles of the chemical elements, determined by RBS, for the studied films with intermediate pellets’ area. In figure 23 a) the as-deposited CuO matrix (solid lines) and the CuO matrix with thermal treatment at 700 ºC (dash lines) are represented, while in figure 23 b)-d) the three as-deposited systems are characterized. According to the RBS profile analysis, all the as-deposited samples present a chemical composition roughly constant across their thickness, fact also verified for the remaining concentrations of all the systems (not represented here). The chemical compositions and the corresponding RBS simulated thicknesses, of the various systems for the as-deposited thin films and for the CuO matrix annealed at 700 ºC, are also shown in table 2. The elemental concentration analysis of the as-deposited films revealed that the matrix is not fully CuO stoichiometric, since the atomic ratio CO/Ccu is always different from 1. However, as soon as the films are subjected to thermal annealing it seems that the CuO matrix

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becomes stoichiometric, as can be observed by the corresponding RBS profile (figure 23 a)). When the films are subject to thermal treatment in air, the chemical composition may change in relation to the as-deposited films [148], mainly due to oxygen incorporation.

Figure 23- Atomic concentration (at.%) of the different elements present in the as-deposited CuO matrix (solid lines) and in the CuO matrix with annealing at 700 ºC (dash lines) (a)) and in the as-deposited samples of Au:CuO (b)), Ag:CuO (c)) and Au-Ag:CuO (d)) systems deposited with a pellets are of 900 mm2, obtained by the RBS data analysed with the code IBA DataFurnace NDF v9.6i [155]. The simulated thickness values (by RBS) are also presented in table 2, and they seem to agree qualitatively with the average thickness of the as-deposited films measured by SEM, especially for the Au:CuO system. The highest deviations are found for the system containing Ag. For this estimation, some assumptions were taken into account, such as the bulk densities of Au, Ag and CuO compounds, which is usually not the case of thin films, which reveal commonly much lower values of density when compared to bulk materials due to porosity and roughness effects [188]. The proximity of the thickness values (estimated by RBS and measured by SEM) suggests

Results and Discussion

43

that the density of the Au:CuO films might be not too different from that of the bulk Au, and CuO. This might be also supported by the micrographs of the films, which exhibits a dense and compacted microstructure in figure 21. Nevertheless, since Ag is usually associated to more porous structures, the differences between the simulated thickness and experiment results are much higher. Table 2- Chemical composition and simulated thickness, performed by RBS, of the different systems for the as-deposited thin films and CuO matrix annealed at 700ºC.

On the other hand, when the CuO matrix is annealed at 700 ºC, the simulated thickness increase from 47.2 to 52.6 nm, which indicates that the matrix undergoes alteration at the microstructural level. This evolution was also confirmed by SEM analysis, which gave thickness values varying from 36 nm (as-dep) to 62 nm (700 C). At the same time, the matrix becomes stoichiometric (O/Cu= 1), which means that oxygen is being incorporated in the film which justifies the thickness increase.

Samples Pellets’ area (mm2) Cu at.% O at.% O/Cu Au at.% Ag at.% Simulated Thickness by RBS (nm) Thickness by SEM (nm) CuO as-deposited -- 55.6 44 0.8 -- -- 47 36 CuO annealed -- 50.0 50 1.0 -- -- 53 62

As-deposited samp

les

Au:CuO 720 47.9 37 0.8 15.6 -- 33 36 960 41.3 44 1.1 15.0 -- 33 34 1200 30.4 53 1.7 16.8 -- 32 31 Ag:CuO 720 43.6 38 0.9 -- 18.7 43 51 960 36.8 46 1.2 -- 17.7 40 44 1200 40.3 30 0.7 -- 29.5 38 88 Au-Ag:CuO 720 37.3 47 1.3 7.0 8.5 38 41 960 41.1 44 1.1 6.7 8.0 35 46 1200 27.1 56 2.1 7.8 9.1 39 34

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4.3 Crystalline and Morphological evolution of the thin films with annealing treatments The structure and morphology of a thin film is determined not only by the chemical composition, but also by the deposition parameters used in the preparation process, such as the discharge conditions, the deposition rate, gas pressures, working temperature, among others. In this case, the different microstructural characteristics are defined not only by the mentioned criteria, but also by the different thermal treatments performed to the various thin film systems. As mentioned, to promote structural and morphological changes with the aim of tuning the LSPR band, the thin films produced were subjected to thermal treatments between 300 °C and 700 °C. It is intended that the energy supplied during the annealing favours the formation of metallic nanoparticles with different dimensions and distributions throughout the matrix, promoting significant changes in the films’ microstructure and consequently in the optical response of the materials. To understand the influence of the annealing on the structure and morphology of the matrix, the X-ray diffractograms of the CuO matrix are displayed in figure 24 a), as well as the SEM micrographs of the cross-section (figure 24 b)) and surface (figure 24 c)), as a function of the annealing temperature.

Figure 24- Crystalline and morphological evolution of the CuO matrix as a function of the thermal treatment. X-ray diffractograms of the CuO matrix are represented in a) and the SEM micrographs of the cross-section and corresponding surfaces through secondary electron detector in b) and c), respectively.

Results and Discussion

45

As it can be observed in figure 24 a), the as-deposited film revealed a cubic structure of the Cu2O phase (Cu atoms positioned in face centred cubic (fcc) and O atoms in body centred cubic (bcc) sublattices) [ICDD card No. 34-1354], characterized by diffraction peaks located at 2θ = 43.3º and 49.9º of the (111) and (200) planes, respectively. However, an annealing temperature of 300 ºC was enough to promote the oxidation of Cu2O into CuO, being noticeable the crystallization of CuO in the monoclinic structure. The most intense diffraction peaks at 2θ = 37.9º, 41.5º, 45.6º, 78.3º and 81.2º can be indexed to the (110), (111), (200), (220) and (022) planes of monoclinic CuO structure [ICDD card No. 72-0629], respectively. These results seem to be in agreement with the literature, where it has been proven that Cu2O is partially oxidised into CuO when it is annealed in a furnace at 300 ºC in air [147]. They also agree with the RBS results, which showed the formation of a stoichiometric CuO matrix (O/Cu = 1) after the annealing treatment. The XRD results seem to correlate with the cross-section SEM micrographs in figure 24 b), in which the films reveal a thickness growth with the raise of annealing temperature, from 36 nm (as-deposited) to 62 nm, when the annealing temperature reaches 700 ºC. This increase in films thickness is most likely related to porosity effects, becoming less dense due to phase transformation (from Cu2O to CuO) and crystallization of the matrix [148]. As seen in figure 24 c), the as-deposited film is smooth and uniform. By correlating these structural results with RBS analysis which revealed that the as-deposited matrix has atomic ratio CO/Ccu different from 0.5 (stoichiometric conditions for Cu2O), it is possible to claim that the matrix is not only composed by Cu2O crystals, but also by atoms in an unorganized state (amorphous state). On the other hand, a progressive crystallization of the matrix is observed when the films are annealed between 300 and 700 ºC due to oxygen incorporation and grain growth. After annealing at 300 ºC, the film becomes relatively porous and the crystalline domains of CuO (grain size) become more granular with the increase of the annealing temperature. The same behaviour is described in literature by Rydosz [121] and Yiming [148] . In following sections, the microstructural analysis is performed on the three nanoplasmonic thin film systems in study. The X-ray diffractograms obtained as a function of the thermal treatment temperature are presented, where it will be possible to analyse the crystalline phases developed during thermal annealing, as well as the direct observation, in some cases, of the nanoparticles dispersed in the matrix, using chemical weight contrast (in SEM).

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4.3.1 Structural evolution of the Au:CuO system The X-ray diffractograms obtained for the films ascribed to the Au:CuO system, as a function of the annealing temperature, are plotted in figure 25.

Figure 25- XRD diffractograms of the Au:CuO films for different annealing treatments with different Au contents: a) CAu=15.6 at.% (pellets’ area: 720 mm2); b) CAu=15.0 at.% (pellets’ area: 960 mm2) and c) CAu=16.8 at.% (pellets’ area: 1200 mm2). According to the results, all as-deposited samples of the Au:CuO system reveal a quasi-amorphous structure, regardless the composition. Nevertheless, one might distinguish extremely broad and almost negligible XRD peaks, which might indicate an early crystallization of Au and/or Cu2O phases. Through these results, it is possible to claim that the presence of Au in the film has an important influence on the structure of the matrix, since the as-deposited films do not present well-defined Cu2O phases, as observed in the film deposited without Au (see figure 24 a). As the annealing temperature increases, a progressive crystallization of the Au and CuO phases takes place for the Au:CuO thin films series. Since the composition of the three sets of films is not very different, the evolution of the XRD peaks is roughly similar. The Au crystallization in its most common structure, face centred cubic [ICDD card No. 04-0784], starts for a thermal annealing of 300 ºC, for every concentration of Au. The diffraction peaks at 2θ = 44.6º, 52.0º, 76.6º, 93.3º and 98.9º can be indexed to the (111), (200), (220), (311) and (222) planes of Au. As expected from previous works [123,182], the diffraction peaks became narrower and more intense with the increase of the annealing temperature. This behaviour might be associated to

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crystallization and grain growth processes, possibly due to the diffusion and coalescence of Au atoms/clusters into larger nanoparticles [123,183,189]. The other diffraction peaks located at 2θ= 37.9º, 41.5º, 45.6º and 78.3º correspond to the (110), (111), (200) and (022) planes of the monoclinic structure of CuO [ICDD card No. 72-0629], respectively, and they are found in the samples with 15.0 and 15.6 at% of Au. For the sample with the highest Au content (16.8 at.%), only the CuO (111) and (200) peaks are observed in the XRD diffractogram. In this case the increase of the concentration of gold in the films might be inhibiting the grain growth of CuO crystals. The XRD results seem to be in full agreement with the SEM micrographs of the samples, which are displayed in figure 26. The cross-section micrographs (to the left) were taken using secondary (above) and backscattered (below) electron detectors (SE and BE, respectively), while micrographs of the surface (to the right) were taken through backscattered electrons (BE) detector, in order to study the morphological changes, including chemical phase contrast of the nanocomposites, induced by heat-treatment, for all atomic concentrations of Au: CAu=15.6 at.% (figure 26 a)), CAu=15.0 at.% (figure 26 b)) and CAu=16.8 at.% (figure 26 c)). As expected, SEM micrographs reveal a similar morphology for all contents of Au, occurring two major morphological modifications when the samples are subjected to thermal annealing at different temperatures. The first one is related with the structural and morphological changes that arise due to the heat-treatment, while the second one is associated with the formation of Au nanoparticles throughout the film. As aforementioned, the as-deposited films of the Au:CuO system present a dense and compacted microstructure, being the presence of Au in the matrix imperceptible, even with chemical weight contrast through the backscattered electron detectors. With the increase of the annealing temperature, cross-section SEM micrographs show that the film thickness seems to increase, while the dense microstructures seem to gradually disappear. The small increase of the films’ thickness is most likely related to porosity and roughness increase, due to Au diffusion inside the matrix and crystallization mechanisms [190,191]. In fact, the observed increase of the films’ thickness with the increment of the annealing temperature is followed by the coarsening, or even coalescence, of Au nanoparticles. This is particularly evident from the micrographs of the surface of the films (to the right), where the presence of Au nanoparticles, contrasted against the CuO matrix, is noticeable.

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Figure 26- SEM micrographs of the Au:CuO system observed in cross-section (to the left), using secondary (SE) and backscattered (BE) electron detectors, and in top-view (to the right) through backscattered electron detector, at different temperatures, for all atomic concentrations of Au: a) CAu=15.6 at.% ; b) CAu=15.0 at.% and c) CAu=16.8 at.% . In a first analysis of the surface micrographs, not only the nanoparticles’ growth is possible

to be verified with the increase of temperature (from top to bottom), but also different nanoparticles’ distributions according to the different contents of gold, associated to different preparation conditions, from the left to the right. The biggest nanoparticles present for all temperatures belong to the highest atomic concentration of Au (figure 26 c)), while slightly smallest nanoparticles correspond to lower Au atomic concentrations (figure 26 a) and b)). After annealing at 300 °C, the surface micrographs of all Au contents show very small Au aggregates in the films, although they are almost negligible for the two compositions with less contents of Au. However, for the temperature of 400 °C the presence of Au aggregates is clearly perceptible for all the compositions.

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By increasing the annealing temperature to 500 and 600 °C, the Au domains seem to undergo significant coalescence, resulting in more inter-connected Au assemblies. This behaviour is consistent with earlier reported works, where heat-treatments were used to promote the growth of Au nanoparticles [189,192,193]. Moreover, it seems that the films annealed with temperatures lower than 700 °C correspond to intermediate stages of Au recrystallization and that the annealing time or temperature was insufficient to promote the formation of more defined shapes. However, for temperature of 700 °C, individual Au nanoparticles with approximately spherical shapes are formed, displaying different sizes and with a homogeneous distribution, but probably with only one nanoparticle per film thickness (see cross-sectional images). The nanoparticles present in the surface of SEM micrographs (figure 26) were also analysed in detail using MatLab software, in order to study their size, shape and distribution in the films’ surface, as a function of the annealing temperature. This analysis is of paramount importance to the application envisaged for these nanocomposite films, which depends on the interaction between the surface with gas molecules and posterior measurement of the LSPR shifts induce by the adsorbed analytes. Figure 27 shows the size distribution of nanoparticles (Feret diameters) histograms, and the respective nanoparticles aspect ratio histograms, as insets of the former. The number of nanoparticles present in the surface of the nanocomposites, and the respective Average Feret Diameter (AFD), for all sets of Au:CuO films at different annealing temperatures are also shown. The as-deposited samples are not present in this study because, as it was seen above, through the SEM analysis (figure 26), these samples do not present any measurable Au nanoparticles. The increase of nanoparticles AFD, with higher Au concentrations (from left to right) is confirmed in figure 27, as previously seen in SEM micrographs. For all compositions, as the annealing temperature increases, the Au nanoparticles distributions are becoming more dispersed to higher Feret diameters. Furthermore, the aspect ratio of the Au nanoparticles distribution is approaching 1, which means that the nanoparticles are becoming almost circular (presumably spherical) with the increase of the temperature. This behaviour confirms the Au diffusion and crystallization processes seen in figure 26, which can also be proved by the decrease in the number of nanoparticles analysed after an annealing at 700 ºC [190,191].

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Figure 27- Feret diameter histograms of the nanoparticles present in the films’ surface and the respective nanoparticles aspect ratio histograms, as insets of the former, for all Au compositions at different annealing temperatures.

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For the films annealed at 300 ºC (figure 27 a) and c)), the size distribution is very narrow, with more than 80% (CAu=15.6 at.%) and 70% (CAu=16.8 at.%) of the nanoparticles having sizes below 20 nm. Regarding the Au nanoparticles’ aspect ratio, it is distributed between 1 and 3, which means that the samples annealed at 300 ºC have both circular and non-circular (irregular) nanoparticles. With the increase of the annealing temperature to 400 ºC and 500 ºC the number of nanoparticles present in the films’ surface increases (less for the sample with highest Au concentration at 500 ºC, since they are bigger). The nanoparticles’ size is also becoming bigger and increasingly heterogeneous as the annealing temperature increases, which means that have both small and large nanoparticles. At 500 ºC, 70% (CAu=15.6 at.%), 40% (CAu=15 at.%) and 20% (CAu=16.8 at.%) of the nanoparticles have sizes below 20 nm. The broadening of the mentioned size distribution is accompanied by a narrow aspect ratio distribution for the samples with lower Au concentration, with more than 50% of the nanoparticles having an aspect ratio between 1 and 1.3. Furthermore, for the sample with the highest Au concentration, presents a broader aspect ratio distribution, with only 25% of the nanoparticles having an aspect ratio between 1 and 1.3, which means that Au aggregates have shapes very different from spherical, undergoing coalescence and diffusion processes to promote Au nanoparticles growth [189,192,193]. For the samples annealed at 600 ºC, the number of nanoparticles present in the surface continues to increase and the nanoparticles are becoming spherical. For temperature of 700 ºC, spherical Au nanoparticles are formed, having more than 60% an aspect ratio between 1 and 1.3. The number of nanoparticles present in the films’ surface decreases, but the nanoparticles AFD increases, which might be due to the process of coalescence, with an even broader size distribution. 4.3.2 Structural evolution of the Ag:CuO system The X-Ray diffractograms obtained for the Ag:CuO films with different silver concentrations, as a function of the annealing temperature, are presented in figure 28. As expected, the Ag crystalized in the faced centred cubic structure [ICDD card No. 87-0597]. The as-deposited samples showed very broad and faint peaks, typical of quasi-amorphous structures, similarly to what was observed in the Au:CuO system. Once again, these results show that the addition of a noble metal to the thin film has an important influence on the crystallization of the matrix, since the as-deposited films do not present a clear Cu2O phase such as the case of the pure matrix (figure 24).

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Figure 28- XRD diffractograms of the Ag:CuO films for different annealing treatments with different Ag contents: a) CAg=18.7 at.% ; b) CAg=17.7 at.% and c) CAg=29.5 at.%. An annealing temperature of 300 ºC was enough to promote the crystallization of Ag in its typical face centered cubic structure for all compositions, characterized by the diffraction peaks located at 44.7º, 51.9º, 76.5º and 93.1º and indexed to the (111), (200), (220), and (311) crystal planes, respectively. For all Ag contents, the intensity of the peaks increases and they become narrower with the annealing temperature. This behaviour might be associated to crystallization and grain growth processes, possibly due to the diffusion and coalescence of Ag atoms/clusters into larger nanoparticles [182,194,195]. Nevertheless, when the annealing temperature rises from 600 to 700 ºC, the intensity of the Ag peaks decreases for all compositions of silver, fact that will be explained below. Following a similar trend, the peaks belonging to the CuO structure also increased in intensity, but here up to 700 ºC. They are located at 2θ= 41.5° and 45.6º, corresponding to the (111) and (200) planes of the monoclinic structure of CuO [ICDD card No. 72-0629], respectively. For the sample with the lowest Ag content, figure 28 b), also the CuO (100) peak appears at 2θ= 37.9º. The SEM micrographs of the surface of the Ag:CuO system for all atomic concentrations, CAg=18.7 at.% (figure 29 a)), CAg=17.7 at.% (figure 29 b)) and CAg=29.5 at.% (figure 29 c)), are displayed in figure 29.

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Figure 29- SEM micrographs of the surface of the Ag:CuO, at different temperatures, for all atomic concentrations of silver: a) CAg=18.7 at.% ; b) CAg=17.7 at.% and c) CAg=29.5 at.% . The SEM micrographs, figure 29, show that the films’ surface present large Ag aggregates from 400 ºC, which appear with more frequency for higher temperatures. In particular, when these samples are annealed at 600 °C, the large Ag structures take a parallelepiped or U shaped with about 700 nm in height, as illustrated by the cross-section SEM micrographs of the sample with silver content of CAg=18.7 at.% (figure 29 a) at 600 ºC). Furthermore, the presence of smaller

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aggregates (Ag nanoparticles) in films’ surface are also visible from 600 ºC. However, for 600 and 700 ºC, both large Ag structures and nanoparticles seem to suffer sublimation, since some “holes” are observed, which has already been reported in the literature [196,197], especially for higher annealing temperatures. This Ag behaviour is in agreement with XRD diffractograms presented in figure 28, in which the intensity of the Ag peaks decreases for all compositions of Ag at 700 ºC. In order to obtain a LSPR effect, it is known that the nanoparticles’ size must be much smaller than the incident wavelength. Therefore the “homogeneous” parts of the surface micrographs presented above, i.e. without the large Ag structures, was studied in more detail and are shown below (figure 30). The cross-section micrographs (to the left) were taken using secondary (above) and backscattered (below) electron detectors, while micrographs of the surface (to the right) were taken through backscattered electrons detector, to study the morphological changes, including chemical phase contrast of the nanocomposite, induced by heat-treatment for all atomic concentrations of silver, CAg=18.7 at.% (figure 30 a)), CAg=17.7 at.% (figure 30 b)) and CAg=29.5 at.% (figure 30 c)). As aforementioned, the as-deposited films of the Ag:CuO system present a little compacted and porous microstructure. With the increase of annealing temperature (from top to the bottom), a progressive crystallization of the Ag nanoparticles is observed in the top-view micrographs. At 300 ºC, Ag nanoparticles start to form and become bigger, evenly distributed throughout the film, being possible to see some aggregates through the surface micrographs. When the film is treated at 400 ºC, the Ag nanoparticles seem to diffuse for the surface of the substrate, for the samples with lower (figure 30 a) and b)) contents of silver, being almost imperceptible the presence of nanoparticles in the corresponding surface micrographs, since Ag also started to form large aggregates at the surface, as seen above (figure 29 at 400 ºC). However, the same behaviour seems not to occur to the film with the highest Ag content because its surface present already well-defined nanoparticles. By increasing the annealing temperature to 500 ºC, the diffusion of silver is promoted towards the surface, and consequently the formation of Ag nanoparticles start to be observed on the surface of the film for the sample with 18.7 at.% of Ag.

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Figure 30- SEM micrographs of the Ag:CuO system observed in cross-section (to the left), using secondary (SE) and backscattered (BE) electron detectors, and in top-view (to the right) through backscattered electron detector, at different temperatures, for all atomic concentrations of Ag: a) CAg=18.7 at.% ; b) CAg=17.7 at.% and c) CAg=29.5 at.% . Similarly to the Au:CuO system, the nanoparticles present in the surface of the thin films observed in the SEM micrographs (figure 30) were analysed using Matlab software as a function of the annealing temperature. Figure 31 shows the nanoparticles Feret diameter histograms, and the respective nanoparticles aspect ratio histograms, as insets of the former. The number of nanoparticles present in the surface of the nanocomposites, and the respective Average Feret Diameter (AFD), for all sets of Ag:CuO films at different annealing temperatures are also shown. At this point, it is also important to reinforce the fact that the large Ag aggregates are not included for these statistics.

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Figure 31 - Feret diameter histograms of the nanoparticles present in SEM micrographs’ surface and the respective nanoparticles aspect ratio histograms, as insets of the former, for all Ag compositions at different annealing temperatures.

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In general, the nanoparticles AFD do not present a linear trend with increasing annealing temperature, since their size do not vary much. For almost all films annealed between 300 and 500 ºC, the distribution of Ag nanoparticles’ size is narrow, where more than about 60% of the nanoparticles have sizes below 20 nm. For the films annealed at 300 ºC, 65% (CAg=18.7 at.%), 70% (CAg=17.7 at.%) and 30% (CAg=29.5 at.%) of the nanoparticles having an aspect ratio between 1 and 1.3. While for annealed films at 400 and 500 ºC, the size distribution is broader, with 56% (CAg=18.7 at.%), 69% (CAg=17.7 at.%) and 79% (CAg=29.5 at.%) of the Ag nanoparticles have an aspect ratio between 1 and 1.5 at 500 ºC. This behaviour means that samples annealed at 300 ºC have nanostructures almost spherical, while samples annealed at 400 and 500 ºC have both spherical and non-spherical nanostructures, which begin to undergo coalescence in order to promote Ag nanoparticles’ growth. With the increase of temperature to 600 and 700 ºC, the number of

nanoparticles present in the films’ surface increases, as well as it is noticeable a broader size distribution. However, about 60% of the nanoparticles have sizes below 20 nm. Most of the Ag nanoparticles of the annealed films at 600 and 700 ºC, have a circular shape, since more than 50% of the nanoparticles have an aspect ratio between 1 and 1.5. 4.3.3 Structural evolution of the Au-Ag:CuO system The XRD diffractograms obtained for the bimetallic system, Au-Ag:CuO, as a function of the annealing temperature are plotted in figure 32. It is possible to observe that the XRD diffractograms present characteristics of both monometallic systems, Au:CuO and Ag:CuO, which are represented in figure 26 and 29, respectively. All as-deposited samples of the Au-Ag:CuO system reveal a quasi-amorphous structure, since all XRD diffractograms present a broad and faint peak, almost negligible. This behaviour is similar to the monometallic systems, Au:CuO and Ag:CuO. Once again, the peaks belonging to CuO structure increase in intensity with the increment of temperature, which is an indicator of matrix crystallization. They are located at 2θ= 37.7º, 41.5º and 45.6º, corresponding to the (100), (111) and (200) planes of the monoclinic structure of CuO [ICDD card No. 72-0629], respectively. For the sample with the highest Au and Ag contents, represented in figure 32 c), the CuO (100) peak does not appear.

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Figure 32- XRD diffractograms of the Au-Ag:CuO films for different annealing treatments with different Au/Ag contents: a) CAu=7.0 at.% and CAg=8.5 at.%; b) CAu=6.7 at.% and CAg=8.0 at.% and c) CAu=7.8 at.% and CAg=9.1 at.% . Diffraction peaks indexed to the (111), (200), (220) and (311) planes, located at 2θ= 44.6º, 52.2º, 76.7º and 93.5º, respectively, are present for all compositions of Au-Ag:CuO. However, it is not possible to clearly distinguish if these crystal planes correspond to Au (COD-04-0784), Ag (COD-87-0597) or bimetallic Ag-Au (COD-1509205) phases, since the diffraction peaks position of the bimetallic Ag-Au phase is very close to the ones of Au and Ag (all materials present fcc structures with very similar lattice parameters). Therefore, for now, it is assumed that all metallic phases might co-exist in the films. The peaks became narrower and more intense with the increase of the annealing temperature. As mentioned above, this behaviour might be associated to crystallization and grain growth processes, due to the diffusion and coalescence of Au and Ag atoms/clusters into larger nanoparticles. On the other hand, as in the case of the Ag:CuO system, when the films are annealed at 700 ºC, the intensity of the Au-Ag peaks decreases, probably due to Ag sublimation. On the other hand, the large aggregates observed for Ag:CuO (figure 29) no longer appear in the bimetallic Au-Ag:CuO system or, at least, are less frequent and much more reduced in size, as seen below in figure 33. The SEM micrographs of the Au-Ag:CuO system are displayed in a similar sequence as the monometallic systems (figure 33).

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Figure 33- SEM micrographs of the Au-Ag:CuO system observed in cross-section (to the left), using secondary (SE) and backscattered (BE) electron detectors and in top-view (to the right) through backscattered electron detector, at different temperatures, for all atomic concentrations of Au and Ag: a) CAu=7 at.% and CAg=8.5 at.%; b) CAu=6.7 at.% and CAg=8 at.% and c) CAu=7.8 at.% and CAg=9.1 at.%. SEM analysis were not conclusive about the formation of bimetallic Ag-Au nanoparticles by diffusion of the Ag and Au due to a thermal annealing. However, the SEM micrographs of the Au-Ag:CuO films present behaviors similar to those mentioned above corresponding to the monometallic systems. For all Au-Ag:CuO compositions, the as-deposited films revealed a dense and compacted microstructure, such as in the case of the Au:CuO system, being the presence of Au and Ag in the quasi-amorphous matrix imperceptible even through the backscattered electron detectors. Nevertheless, with the increase of the annealing temperature, cross sectional SEM micrographs show that the film’s thickness increases, which might be related to porosity and roughness increase, due to the presence of Ag in the bimetallic films.

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In fact, when the films are annealed at 300 ºC it is noticeable the presence of some aggregates for all compositions, as evidenced in the surface micrographs, with a roughly uniform distribution throughout the film. The number of nanoparticles seem to increase from the left to the right, which is in concordance with the SEM micrographs of the Au:CuO films, presented in figure 27. Furthermore, large nanostructures with about 100 nm in size, appear in the surface of the films, as it is possible to verify in the cross-section micrographs of the films with lower contents of Au-Ag, figure 33 a) and c). These nanostructures are in agreement with the nanostructures present in the Ag:CuO films of the figure 29. By the increase of temperature to 400 ºC and 500 ºC, the nanoparticles seem to undergo significant coalescence, resulting in more inter-connected assemblies to promote the growth of the nanoparticles. The large nanostructures observed in the surface micrographs are now bigger than the nanostructures present at 300 ºC. At 600 ºC, the films present on the one hand, spherical nanoparticles, possibly due to their recrystallization, displaying different sizes and, on the other hand, large nanostructures that start to suffer sublimation and create some “holes” in the surface of the films (figure 33 b) and c)), as it was also observed in the Ag:CuO system (figure 29). For the temperature of 700 ºC, the matrix seems to become more crystalline and porous with the spherical nanoparticles homogeneous dispersed. In all compositions it is also possible to verify the existence of “holes”, more than at 600 ºC due to a more intense sublimation of the large nanostructures containing Ag, which are present in the surface of the films. These results suggest some separation between Au-rich and Ag-rich regions throughout the film, being the spherical nanoparticles attributed to the presence of Au in the samples, or even bimetallic Au-Ag, since pure Ag leads to the formation of large nanostructures and consequent sublimation of these ones, as seen above in this work. Figure 34 shows the nanoparticles Feret diameter histograms and the respective nanoparticles aspect ratio histograms, as insets of the former, as well as the number of nanoparticles present in the SEM micrographs’ surface, and the respective AFD, for all Au-Ag compositions at different annealing temperatures.

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Figure 34 - Feret diameter histograms of the nanoparticles present in SEM micrographs’ surface and the respective nanoparticles aspect ratio histograms, as insets of the former, for all Au-Ag compositions at different annealing temperatures. For the sample annealed at 300 ºC, the Au-Ag nanoparticles’ size distribution is very narrow, with more than 70% of the nanoparticles having sizes below 20 nm. The corresponding

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aspect ratio histograms show that the nanoparticles for all compositions have a broad Au, Ag nanoparticles’ aspect ratio, with about 56% (CAu=7 at.% and CAg=8.5 at.%), 68% (CAu=6.7 at.% and CAg=8 at.%) and 75% (CAu=7.8 at.% and CAg=9.1 at.%.) of the nanoparticles having an aspect ratio between 1 and 1.5, which means that both spherical and non-spherical nanoparticles are formed, having the highest Au, Ag concentration more spherical nanoparticles. With the increase of the annealing temperature to 400 and 500 ºC, the nanoparticles AFD increase for all samples (less for the sample with Au-Ag concentration of CAu=7.8 at.% and CAg=9.1 at.%. at 400 ºC), while size distributions become broader. At 500 ºC, 68% (CAu=7 at.% and CAg=8.5 at.%), 44% (CAu=6.7 at.% and CAg=8 at.%) and 60% (CAu=7.8 at.% and CAg=9.1 at.%.) of the nanoparticles have sizes below 20 nm. For these samples, most of the nanoparticles having an aspect ratio between 1 and 1.5, meaning that they are almost circular. For the annealing at 600 ºC, the nanoparticles AFD increase from left to right, but quickly decrease for the samples annealed at 700 ºC. This behaviour is also confirmed by the broadening of the nanoparticles size that is higher for the samples annealed at 600 ºC. For both temperatures, the nanoparticles size distribution becomes broader from the left to the right, since at 600 ºC, 78% (CAu=7 at.% and CAg=8.5 at.%), 41% (CAu=6.7 at.% and CAg=8 at.%) and 35% (CAu=7.8 at.% and CAg=9.1 at.%.) of the nanoparticles have sizes below 20 nm , while at 700 ºC, 64% (CAu=7 at.% and CAg=8.5 at.%), 60% (CAu=6.7 at.% and CAg=8 at.%) and 37% (CAu=7.8 at.% and CAg=9.1 at.%.) of the nanoparticles have sizes below 20 nm, being the nanoparticles of the highest Au-Ag contents with the broader distribution. About 70% of the Au, Ag nanoparticles have an aspect ratio between 1 to 1.5, which means that with the increase of temperature, they become more spherical. 4.4 Optical response The optical transmittance measurements of the CuO matrix, with and without thermal treatment, deposited on fused SiO2 substrates are shown in figure 35. When the substrate is coated with the pure matrix film (black line), the transmittance decreases for about half of the substrate transmittance, decreasing faster to about 5% in the range of wavelengths between 300 nm and 350 nm. With a thermal treatment at 300 ºC, the CuO matrix becomes opaquer than the as-deposited film between 400 and 900 nm and with higher transmittance between 300 and 400 nm, which might be due to different crystalline structures, has already been seen above. Furthermore, for higher annealing temperatures the films do not show major changes in their transmittance profiles. These results seem to be in agreement with the literature [190].

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Figure 35- Transmittance spectra of the substrate (fused silica), of the CuO film without heat treatment (as-deposited) and of the CuO film with heat treatment between 300 and 700 ºC. The presence of plasmonic metals (Au and/or Ag) in the CuO matrix, with different compositions, and subsequent annealing at different temperatures, promoted the nanoparticles formation, with different sizes and distributions, essential factors that influence the LSPR band. The influence of the annealing temperature on the optical response of the Au:CuO, Ag:CuO and Au-Ag:CuO films with different contents of Au and/or Ag was determined via optical transmittance analysis, whose spectra are displayed in figure 36. As expected, the transmittance profiles of all as-deposited samples of the Au:CuO (figure 36 a)), Ag:CuO (figure 36 b)) and Au-Ag:CuO (figure 36 c)) systems do not exhibit a LSPR band in the transmittance spectra (T-LSPR). These results agree with the XRD and SEM analysis, which showed a quasi-amorphous structure of the films and suggested the presence of Au and Ag nanoparticles still homogenously dispersed inside the matrix in the form of low-size clusters. However, as soon as these films are subjected to thermal treatment, their optical response dramatically changes, as a consequence of the formation and growth of plasmonic nanoparticles dispersed throughout the CuO matrix. In fact, according to the previous analysis, different microstructures were achieved by varying the composition and temperature of the annealing treatment applied to the films, being expected different optical responses.

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Figure 36- Transmittance spectra of the Au:CuO (a), Ag:CuO (b) and Au-Ag:CuO (c) films with different contents of Au and/or Ag and subjected to different annealing treatments.

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For the Au:CuO system, the heat-treatment (between 300 and 700 ºC) promotes an accentuated decrease of the transmittance for all spectral range, with the appearance of LSPR absorption bands located between 650 and 840 nm. This phenomena is due to the formation of Au nanoparticles, that begin to be observed, as it can be seen in figure 27 [122]. All gold compositions present a similar transmittance as a function of each temperature, since the composition between the samples is not so different, as previously discussed. The T-LSPR band red-shifts when the films are annealed between 300 and 500 ºC, from about 720 nm at 300 ºC to about 840 nm at 500 ºC. However, they suffer blue shifting after annealing at 600 and 700 ºC, to 760 nm at 600 ºC, and to 720 nm at 700ºC for the films with Au contents lower than 16 at.% (figure 36 a1) and a2), respectively). In the case of the film with the highest content of Au (figure 36 a3)), at 700ºC of annealing, the LSPR absorption band even presents a greater shift for smaller wavelengths, being positioned at about 650 nm. These different behaviours are certainly related with different effects, principally with the presence of Au nanoparticles with different size distributions. Firstly, between 300 and 500 ºC, the matrix progressively starts to crystalize, increasing the refractive index, and consequently the LSPR band are red-shifting. In this range of temperatures, the Au nanoparticles seem also to be in intermediate phases of its coalescence and this also might explain why the LSPR band is broad. However, between 600 and 700 ºC, the matrix is becoming less dense and more porous with the annealing temperature (see figure 24 and 27), which means that its refractive index is probably decreasing [174], and thus shifting the LSPR peak to lower wavelengths [198]. Moreover, the T-LSPR band becomes sharper and this effect might be attributed to the gradual refinement of the Au nanoparticles distribution, which become sharper and gradually more homogenous in terms of shapes (becoming more spherical, figure 27) [189,198]. Since the film with higher Au content has a higher number of nanoparticles at 700 ºC than the other compositions, this presents a greater shift of its LSPR band for smaller wavelengths than the other compositions. Regarding the Ag:CuO system (figure 36 b)), for all different compositions of Ag, T-LSPR bands seem to only appear when the films are annealed at temperatures of 500 ºC or higher. With the increase of annealing temperature, the T-LSPR bands of all silver compositions red shift from about 387 nm, at 500 ºC, to 393 nm, at 600 ºC, and subsequently to 400 nm when the annealing temperature rises at 700 ºC. These small redshifts might be due to the crystallization of the matrix, that increase the refractive index (although there is an increase of films’ porosity that decrease the

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refractive index), which are then followed by a narrowing of the LSPR absorption band with the increase of annealing temperature, probably related with the narrowing of the Ag size distribution. In a first analysis of the Au-Ag:CuO system (figure 36 c)), as in the monometallic Au:CuO and Ag:CuO systems, all compositions of Au-Ag seem to have similar trends for the evolution of the transmittance spectra with annealing temperature. This is certainly related with the similar atomic concentration of Au and Ag found for these sets of films. A T-LSPR band is visible at 300 ºC, but only for the highest concentrations of Au/Ag, positioned at about 500 nm. For the films annealed at 400 ºC, the LSPR band is located close to 800 nm for all the films, analogous to the film annealed at 400 ºC in the Au:CuO system (figure 36 a)). This means that the Ag presence in the films effectively do not contribute to LSPR absorption, since in the Ag:CuO system did not also appear any LSPR band at 400 ºC. However at 500 ºC, a LSPR peak appear at about 500 nm for the samples with lowest concentrations of Au/Ag. Two T-LSPR peaks are also observed in films annealed at 600 ºC (~ 450 and 700 nm) and 700 ºC (~460 nm and 650 nm), which suggest the presence of separate phases of Ag and Au nanoparticles in these films, since their positions correspond to the LSPR positions of Ag and Au observed in the monometallic systems (figure 36 a) figure 36 b)). However, at 600 ºC the first peak tends to disappear for the highest Au-Ag concentration, becoming the second one more defined and pronounced. For 700 ºC, the transmittance spectra seem also to be affected by the Ag sublimation and the second peak becomes peripheral These results obtained at 600 and 700 ºC seem to agree with SEM micrographs, which suggested the appearance of Au-rich and Ag-rich regions throughout the film, nevertheless the formation of bimetallic Au-Ag nanoparticles cannot be disregarded. 4.5 Surface analysis of the Au:CuO film for CO gas detection 4.5.1 Surface analysis by XPS The potentiality of the thin films produced in this work to be applied as gas sensors was demonstrated by the appearance of well-defined LSPR bands. It is now expected that they will be capable of detecting the presence of a determined gas through shifts of the LSPR band. Before carrying out tests with the CO gas, it is needed to know in detail the bonding character of the films’ surface, which will be in direct contact with the analyte. In this way, and since the Au:CuO was the one presenting the most pronounced T-LSPR behaviour, a sample of this system annealed at 700

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°C was analysed through XPS. Furthermore, ARXPS technique was also employed to evaluate the chemical composition of the surface as well as the nanoparticles location in surface (if they are embedded in the matrix or air-exposed at the surface). After the surface composition of the film was determined in vacuum conditions, adsorption processes of CO gaseous molecules on the surface of the nanoplasmonic Au:CuO thin film were analysed by High-Resolution X-Ray Photoelectron Spectroscopy (HRXPS). 4.5.1.1 Surface analysis of the nanoplasmonic films by XPS Detailed surface analysis by XPS was performed in the Au:CuO sample annealed at 700 °C, which appears to have promising optical properties. The results are displayed in figure 37. In the survey spectrum of this sample (not shown), the main photoelectron and Auger lines of Cu, O and Au could be clearly observed. The presence of C was also detected, which is believed to be the result of organic impurities adsorbed at the sample’s surface due to the air exposure. The position of the C 1s was used as reference for the binding energy values, by taking into account that its main contribution, attributed to adventitious C (saturated hydrocarbons, C-C and C-H bonds), being situated at 284.8 eV [199]. After this correction, discrepancies still remain between the expected and actual line positions for different elements up to about 0.3 eV, slightly above the experimental uncertainty. This effect of non-uniform surface charging is caused by the multiphase characteristic of the sample surface. Chemical bonds of surface atoms were studied by fitting the high-resolution spectra of the dominant photoelectron lines of carbon, oxygen, copper and gold, shown in figure 37. Unless otherwise stated, line fitting was performed using pseudo-Voigt profiles (a product of a Gaussian and a Lorentzian functions with intensity ratio of 70:30 (usually denoted as GL(30)). In addition, the copper Cu L3M45M45 line was also taken in order to determine the modified Auger parameter attributed to this line and Cu 2p3/2 photoelectron line. Its value, ’ = 1851.3 eV, perfectly fits to the result obtained from the reference CuO sample [200]. Although the modified Auger parameter of metallic copper is only slightly different (1851.2 eV [200]), this phase can be readily excluded according to the Cu 2p3/2 line shape (see figure 37 d)). High resolution spectrum of the C 1s line, shown in figure 37 a), was fitted assuming three contributions. As expected, the dominant contribution is attributed to saturated hydrocarbons (C1

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at 284.8 eV, 84.3%), whilst the additional ones are due to C-OH and/or C-O-C bonds (C2 at 286.3 eV, 12.4%), and eventually C=O bonds (C3 at 287.8 eV, 3.3%) [201].

Figure 37 - High resolution XPS spectra of a) C 1s, b) O 1s, c) Au 4f and Cu 3p and d) Cu 2p3/2 lines, taken from the nanoplasmonic Au:CuO sample annealed at 700 °C. The O 1s line, presented in figure 37 b), can also be ascribed to three contributions O1-3, positioned at 529.8 eV (51.6%), 531.5 eV (27.7%) and 532.7 eV (20.7%), respectively. High resolution spectrum of this line taken from the reference CuO sample reveals the presence of two peaks, at 529.7 eV and 531.0 eV, attributed to perfect and defective (eventually hydroxylated) CuO [200]. The intensity ratio of these two peaks is around 2:1. Bearing this in mind, it seems likely that the contributions O1 and O2 in figure 37 b) correspond to CuO. The 0.5 eV shift of O2 towards higher binding energies, may be a result of several effects, such as the fact that the type of defects may be somewhat arbitrary or that C=O bonds (possibly observed in C 1s line) are present [201]. Finally, this shift could be also interpreted as the presence of smaller amounts of Cu(OH)2 at the surface [200]. Contribution O3 can be readily attributed to C-OH bonds [201], which were already identified from the C 1s line fitting.

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The main photoelectron line of Au, Au 4f7/2, is situated closely to the Cu 3p1/2 line, as can be seen from figure 37 c). To properly quantify the Au 4f line intensity, fitting of both lines had to be performed in order to resolve them properly. The relative intensities of the spin-orbit peaks were fixed to the theoretical values, i.e., 4:3 for Au 4f7/2 line, and 2:1 for Cu 3p1/2 line. The line Au 4f7/2 is situated at 84.0 eV, which fits perfectly to metallic gold [202]. The shape of the Cu 2p3/2 line, shown in figure 37 d), is characteristic for Cu in Cu(II) oxidation state – it has highly pronounced shake up satellite at about 942 eV. However, differentiating Cu(OH)2 and CuO contributions is not straight forward. Although modified Auger parameter favours CuO contribution, smaller amounts of the hydroxyl phase (with ’ = 1850.9 eV) cannot be excluded. The presence of Cu(OH)2 can be suspected from the O 1s line fitting as well. Due to the complex shape of the Cu 2p3/2 line, it is fitted to the sum of peaks having well defined constraints (relative positions and intensities, and FWHMs). The fit in figure 37 d), made using the constraints recommended in [200] for CuO, has only two fitting parameters: the position and the intensity of the Peak 1. Attempts to consider this line as the superposition of CuO and Cu(OH)2 contributions did not improve the quality of the fit. Therefore, it appears that CuO phase is the dominant copper phase at the surface, although smaller amounts of copper hydroxide cannot be excluded. From this basic analysis of the XPS spectra, it is already clear that the sample contains a thin layer of hydrocarbons on the top, covering the CuO matrix in which Au nanoparticles are distributed. Even if this layer is very thin, it is expected to influence and as well mediate the chemi- and/ or physisorption mechanisms occurring at the surface of the nanoplasmonic films. Since this is a key feature to take into account if one wants to use the films as a LSPR sensor, a detailed analysis of the surface structure of the sample was performed by ARXPS, using the same XPS lines. If an element ‘A’ is situated only in the thin surface layer, such as carbon in the present case, its intensity (IA) , measured at the emitting angle γ, should be described with very good approximation by IA(γ) = IA0·(1 – exp(-d/(LA·cos γ))), where IA0 is the line intensity in the case of the infinitely thick layer, LA is the effective attenuation length of the corresponding photoelectron line for the layer material, and d is the layer thickness [203]. Therefore, the intensity IA will be increasing with the emitting angle γ. In the case of an element ‘B’ situated below the layer, its intensity will be attenuated to a very good approximation according to the Beer-Lambert law IB(γ) = IB0·exp(-d/(LB·cos γ)), where IB0 is the line intensity for the zero thickness and LB is the effective attenuation

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length of the corresponding photoelectron line in the layer material. Clearly, the intensity IB will be decreasing with γ [203]. High resolution XPS spectra were taken for 0°, 40°, 60° and 70° emitting angles. The C 1s line suffered an enhancement with the emitting angle, while the intensities of the other lines decreased. This behaviour is a confirmation that carbon originates from the impurity layer covering the thin film. Changing the emitting angle also influenced the shape of the O 1s line contribution. The O3 line increases with the emitting angle, while the intensity ratio of O1 and O2 lines decreases. This confirms the previous considerations that O3 line results from surface impurities, while O2 line corresponds to surface defects of CuO. Evolution of line intensities with the emitting angle reveals that the lines of Cu, Au, and O1 and O2 contributions of oxygen originate from the region below the surface contamination layer. According to the previous analysis, the logarithm of the intensity of these lines vs. (cos γ)-1 should follow a linear dependence with the slope equal to -d/L. If the attenuation length (L) of these lines is known, the slopes can provide information about the hydrocarbon layer’s thickness. These dependences for Cu 2p3/2 and Au 4f lines, as well as the sum of the O1 and O2 contributions of the O 1s line, are presented in figure 38, together with the corresponding linear fits. Effective attenuation lengths (LC) were calculated using the NIST database [204] and by modelling the contamination layer of saturated hydrocarbons as a paraffin. The LC values are 3.578 nm, 1.261 nm and 2.404 nm for Au 4f, Cu 2p3/2 and O 1s lines, respectively. From the experimentally obtained slopes, the estimated thickness of the contamination layer (dC) is 0.53 nm (Cu 2p3/2 line), 0.62 nm (O 1s line) and 1.0 nm (Au 4f line). It was obtained a rather good agreement for the copper and oxygen lines, whilst the apparent thickness of the hydrocarbon layer appears to be significantly higher when using Au line. This significant discrepancy is probably caused by the exact position of the gold nanoparticles: if the gold nanoparticles are buried inside the CuO matrix, the slope should be dC/LC + dCuO/LCuO, where index ‘C’ corresponds to the

contamination layer and ‘CuO’ to the copper oxide overlayer covering the Au nanoparticles. Assuming that dC ≈ 0.58 nm, from the calculated LCuO = 1.715 nm, the obtained value is dCuO ≈ 0.2 nm. Therefore, according to ARXPS study, the surface of the Au:CuO sample, annealed at 700 ºC, is covered by a layer of saturated hydrocarbons with thickness of bout 0.53-0.62 nm, whilst the gold nanoparticles closest to the surface are covered with a single atomic layer of CuO.

Results and Discussion

71 Figure 38- Determination of the hydrocarbons’ overlayer thickness from the ARXPS results for a) O 1s line, b) Cu 2p3/2 line and c) Au 4f line.

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The molecules of the gas that should be detected by LSPR will be adsorbed on this layer, therefore, effective ways to reduce or completely remove the “contamination” layer are also an important task, which can be performed by applying a plasma treatment with Ar, or other atmospheres such as O2 [205]. Still, as already discussed, the sensitivity of the sensor will be strongly affected by the amount of Au nanoparticles near the surface. From the known thickness of the contamination layer, true intensities of the Cu, O and Au lines can be obtained by multiplying the measured values with corresponding factors of the form exp(d/Lc). The corrected intensities can then be used to perform quantitative surface composition analysis of the film. Relative concentrations of Cu and O in CuO matrix can be calculated from the corrected intensities of the Cu 2p3/2 and O 1s (i.e. contributions O1 and O2) lines and the appropriate sensitivity factors defined as the product of a photoionization differential cross-section σ, transmission function and the matrix dependent quantitative effective attenuation length LQ [203]. For that purpose, photoionization cross sections of Scofield [206] were used, the asymmetry parameters were taken from [207], whilst quantitative attenuation lengths LQ were calculated using the NIST database [204]. The transmission function of the spectrometer was experimentally determined to be proportional to E -0.77, with E being kinetic energy of photoelectrons [178]. All the values necessary for the quantification are summarized in Table 3. By following the approach in [203], relative atomic concentration of oxygen and copper appear to be 53.5% and 46.5%, respectively. The result is in good accordance with the expectations of a stoichiometric CuO matrix and the small excess of oxygen is probably related to the defective and/or hydrated oxide surface. Table 3- Values of magnitudes used to determine sensitivity factors

Relative concentration of Au cannot be applied using the equivalent approach, since the sample is laterally non-uniform i.e. Au forms another phase, embedded in the CuO matrix. However, it is possible to determine the fraction of surface covered by gold nanoparticles, a. The

Line E (eV) σ (arb.un.) β LQ (nm) Matrix Au 4f 1170 17.47 1.006 1.380 Au O 1s 724 2.85 2 1.281 CuO Cu 2p3/2 322 15.87 1.344 0.731 CuO

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intensity of gold signal is directly proportional to this fraction and to the concentration of gold atoms (NAu) composing the nanoparticles. Similarly, the intensity of oxygen and copper lines is directly proportional to (1 – a)·NCuO (NCuO is the concentration of CuO). The quantification of Au 4f, assuming it originates from pure gold, was calculated in the similar way as for copper and oxygen in CuO matrix, using the data from [202,206,207] given in Table 3. Assuming that NCuO and NAu correspond to bulk concentrations of pure CuO and Au respectively, the fraction of surface covered by Au is a ≈ 6.1%. The relative atomic concentration of these three elements at the surface is then 7.0% of gold, 43.2% of copper and 49.8% of oxygen. 4.5.1.2 High resolution XPS spectra of the C 1s line before, during and after sample exposure to CO In order to scan de sensing response, the adsorption processes of CO molecules on the surface of the nanoplasmonic Au:CuO thin film were characterized by High Resolution XPS (HRXPS) [178]. The HRXPS spectra were taken before, during, and after exposure of the thin film to 1×10-6 Pa of CO (the base pressure of the system was about 1×10 -7 Pa [178]). Initially, the sample was found to be covered by a thin layer of hydrocarbons, which are typical contaminants of surfaces exposed to air, corresponding to about 2-3 atomic monolayers, as discussed in the previous section. Consequently, this ultra-thin layer may play an important role in the eventual CO adsorption taking place on the thin film’s surface, and thus it was carefully taken into account. HRXPS spectrum of the C 1s line taken from the Au:CuO sample is shown in figure 39 a). Two contributions can be identified at 284.8 eV (88%) and 286.8 eV (12%), attributed to saturated hydrocarbons (C-C and C-H bonds) and C-OH groups [201]. The latter was also confirmed by the fitting of the O 1s line. The C 1s line taken during the sample exposure to CO is presented in figure 39 b), where an additional contribution at about 288.5 eV can be observed, which can be readily attributed to the presence of CO [201]. This contribution is still present in the spectrum of this line after pumping out CO, although its relative contribution is reduced from 7.2% to 4.7%, as shown in figure 39 c). This indicates that beyond the sensing positive response, part of the CO is probably chemisorbed at the surface, which can be explained by the fact that this molecule possesses some chemical reactivity. At the same time, different dangling bonds on the irregular structure of hydrocarbon impurities can be expected, which may represent adsorption sites for CO molecules.

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Figure 39- HRXPS spectra of the C 1s line taken from the Au:CuO thin film, a) before, b) during and c) after the exposure to CO. In terms of its sensing potentialities, HRXPS clearly detected adsorbed CO species at the surface of Au:CuO. After the CO was pumped out, the corresponding contribution was reduced, but did not disappear, revealing that only part of the CO is physisorbed on the surface, while the rest seems to be chemisorbed. Therefore, the optimization of sensing capabilities of the nanocomposite thin films, the physisorbed and chemisorbed contributions must be carefully separated. The chemisorption contribution should be reduced in order to tailor the sensing response of the films, which is being tested and optimized by surface modification, namely with plasma etching and activation. 4.5.2 Surface functionalization with plasma etching As mentioned above, some CO molecules seem to have been chemisorbed at the surface of the Au:CuO film, namely in the hydrocarbon sites. Thus, it is important to remove the hydrocarbons layer from the surface in order to evaluate the interaction between the contaminant-free surface with the CO. Ideally the CO should be only physiosorbed in the film, in order to ensure the reuse of the sensor. The films optimization is a necessary process and involves i) the cleaning of the surface, by removing the hydrocarbon contaminants, and ii) bring the nanoparticles closer to the surface, or even partially expose them, so that the gas molecules adsorption occurs in the proximity of the nanoparticles and the LSPR film becomes more sensitive to the gas presence. To know if the hydrocarbons layer was removed and to study the nanoparticles proximity to the film’s surface, the nanoplasmonic thin films of Au:CuO (annealed at 700 °C) was subjected to a plasma treatment with argon at different times up to a maximum of 1 hour, in a Low-Pressure Plasma Cleaner by Diener Electronic (model Zepto Model), intercalating with transmittance

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measurements of the LSPR band of the film. It is expected that argon etching will remove, in a first step, the hydrocarbons layer present at the surface of the film, and then erode the film. The transmittance spectra taken at different times of consecutive plasma treatment are presented in figure 40.

Figure 40- Transmittance spectra for different plasma treatments of the nanoplasmonic Au:CuO film (annealed at 700 ºC). For the first plasma treatments, till the 60 seconds, the resonance peak was slightly shifted for smaller wavelengths, from 710 to 700 nm, probably due to removal of the hydrocarbons layer at the film’s surface. For a plasma treatment of 2 minutes (120 seconds), a higher shift of the LSPR band is observed from 700 to 630 nm. At this time of plasma treatment, a superficial film erosion is suggested, since the presence of air causes a decrease of the refractive index of the dielectric medium surrounding the Au nanoparticles, and consequently a decrease of the LSPR band wavelength [188]. Furthermore, an increase of the transmittance is also observed which indicates that films suffering erosion and becoming thinner and thus less opaque. When the films are subjected to a plasma treatment for 5 minutes (300 seconds), the LSPR band suffer again a blue shift from 630 to 530 nm and becomes narrower, which suggest that the nanoparticles are emerging at the surface of the film. The transmittance considerably increases and the band shape changes. With the continuum increase of plasma treatment time to

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1 hour, the transmittance seems to stabilize, although the LSPR band continues blue shifting and the transmittance increase a little. In general, the film transmittance increases due to film erosion and the LSPR peak has a blue shift and becomes narrower when the nanoparticles are emerging at the surface. SEM analysis of the film’s surface were carried out in order to study the morphological alteration that the thin film suffers when is subjected to plasma treatment of 3 minutes. SEM micrographs are present in figure 41 for the sample before a plasma treatment (figure 41 a)) and after 3 minutes of plasma treatment (figure 41 b)).

Figure 41- SEM micrographs of the Au:CuO film’s surface before a plasma treatment (a) and after 3 minutes of plasma treatment (b). As transmittance spectra suggest, figure 40, a plasma treatment for 3 minutes is sufficient to promote some modification of the film’s surface and this behaviour can be proved through SEM micrographs. In a first analysis, a large increase in the number of nanoparticles in the surface of the film with plasma treatment is observed against the surface before plasma treatment, passing from 9% of area covered by nanoparticles before the treatment, to 22% after the treatment, which confirms the emerging of nanoparticles to the top of surface after a plasma treatment of 3 minutes.

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Chapter 5- Conclusions and future perspectives The preparation of nanoplasmonic thin films composed by gold (Au) and/or silver (Ag) nanoparticles dispersed in a copper oxide (CuO) matrix was possible in this work. By magnetron sputtering, a set of different Au:CuO, Ag:CuO and Au-Ag:CuO systems of thin films were deposited, using a Cu target with 45, 60 and 75 small pellets of Au and/or Ag (pellets’ area: 720, 960 and 1200 mm2). After a deposition of 1 min, with a target potential limited to 500 V, the different systems were annealed at various temperatures, ranging from 300 to 700 ºC, in order to promote the growth of nanoparticles. In general, the atomic concentration of the noble metal (Au and Ag) in the as-deposited films did not change significantly with the increase of the Au and Ag pellets’ area, although the sample deposited with the highest amount of Ag pellets had increased considerably its atomic concentration. These results induced similar microstructures and optical responses for each concentration obtained, as a function of annealing temperature. Furthermore, Ag presented an atomic concentration about 16% higher than Au, due to its higher sputtering yield. The CuO matrix became stoichiometric after thermal annealing in air. The structural and morphological changes, induced by the thermal treatment, affected the optical properties of the films, particularly their LSPR response. Due to the formation of Au nanoparticles from the temperature of 300 ºC, LSPR bands are clearly observed in the transmittance spectra of the Au:CuO. However, the samples annealed at 600 and 700 ºC showed the most promising properties to be employed in nanoplasmonic applications such as those involving gas sensing devices. For these samples, the T-LSPR band become narrower with the broadening of Au nanoparticles distribution and with the homogeneity of the Au nanoparticles’ shapes, which become more spherical. Although the Ag:CuO system showed Ag aggregates in nanoparticles at 300 and 400 ºC in SEM micrographs, the LSPR bands were only observed for temperatures of 500 ºC and above. From 400 ºC large Ag structures are observed, however they do not contribute for the LSPR effect, while for temperatures of 500 ºC and above, many nanoparticles appear and have adequate sizes to LSPR effect occur. Furthermore, some “holes” appear for 700 ºC, which was related with Ag sublimation phenomenon. Similar properties to those mentioned above for the Au and Ag:CuO systems were observed for the Au-Ag:CuO system. The presence of spherical nanoparticles, attributed to the presence of

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Au in the samples, and bigger nanoparticles with some “holes”, as in the Ag:CuO system was possible to be observed. However, the presence of Ag large structures was not visible here. Regarding the optical properties, a unique LSPR peak was visible for the samples annealed from 300 ºC to 500 ºC and two LSPR peaks for the annealed films at 600 and 700 ºC. Thus, it is possible to conclude that there was a formation of Au and Ag nanoparticles throughout the film, but also the formation of bimetallic Au-Ag nanoparticles. Both series of samples annealed samples at 400 and 600 ºC showed the most promising properties to be employed in nanoplasmonic applications. The presence of a hydrocarbons overlayer (~0.6 nm thick), was confirmed for the Au:CuO sample annealed at 700ºC, whereas gold nanoparticles appear to be embedded in the matrix, but very close to the surface, and covered by a 0.2 nm thick CuO layer. Furthermore, the estimated surface coverage of gold nanoparticles closest to the surface is about 6%. Through the interaction of the CO gaseous molecules with the Au:CuO surface, it was possible to conclude that some CO molecules are chemisorbed on the Au:CuO surface, maybe in the hydrocarbons layer, and that part of the CO molecules are physisorbed at the surface, which is important for sensing. Furthermore, through Ar plasma treatments, it was shown that hydrocarbons layer may be removed, and the nanoparticles emerge at the surface. In the course of this project many interesting results were found, however many more questions arose. Further research is needed to make a specific gas sensor, and a future development of this work should be focused on the following goals: - Performing optical transmittance spectra (T-LSPR) in the presence of various gaseous atmospheres, in the new equipment, to study shifts in T-LSPR band due to its presence; - Optimization of the LSPR band of the studied systems, both to reduce the amount of gold and silver used, as well as to decrease the annealing temperatures, which allows to reduce both the processing costs and the production costs of the films; - Production of thin films more porous, with different architectures (GLAD system), to increase the contact surface between the film and the gaseous molecules;

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- Structural characterization of the thin films by HR-TEM, for a better resolution of the films’ structure, and by AFM to study the films’ rugosity, relating it with the surface sensitivity to a gas. - Understanding the importance of the hydrocarbons layer on the surface of a sensing platform, since the sensitivity of the sensor will be closely related to the kind of adsorption (chemi- or physisorption) that the gas can establish with the surface of the hydrocarbons layer; - Evaluation of the sensitivity and selectivity of the system for the CO gas and application of recognition elements (chemical or biological) to increase the selectivity.

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