In-plane anisotropic nanocolumnar films for advanced functional applications

In-plane anisotropic nanocolumnar films for advanced functional applications Julián Parra Barranco Instituto de Ciencia de Materiales de Sevilla (US-CSIC) Universidad de Sevilla PhD thesis, 2015 iii In-plane anisotropic nanocolumnar films for advanced functional applications A dissertation submitted for the degree of Philosophiæ Doctor (PhD) Memoria presentada para optar al grado de Doctor por la Universidad de Sevilla Seville, April 2015 Julián Parra Barranco Directed by: Tutor: Pr. José Antonio Odriozola Gordón Catedrático Universidad de Sevilla Dpt. Química Inorgánica (Universidad de Sevilla) Dr. Ángel Barranco Quero Científico Titular CSIC Instituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla) Pr. Agustín Rodríguez González-Elipe Profesor de Investigación CSIC Instituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla) iv v Index Chapter 1. Introduction 1 1.1 Thin films deposition techniques 5 1.1.1 Physical Vapour Deposition 5 1.2 Glancing Angle Deposition (GLAD) 6 1.2.1 Geometry of the deposition process and shadowing effects 6 1.3 Microstructure of GLAD thin films 10 1.3.1 Surface roughness and porosity 11 1.3.2 Separation and association of nanocolumns: bundling effects 13 1.4 Plasma Assisted GLAD. (PA-GLAD). 16 1.5 GLAD thin films on flexibles substrates 17 1.6 GLAD properties and applications 19 1.6.1 Electrical Properties and applications 19 1.6.2 Optical Properties and applications 21 1.6.2.1 Optical anisotropy in columnar glancing angle deposition 21 1.6.2.2 Plasmonic. Applications 23 1.6.3 Wetting Properties and applications 25 1.7 Objectives. 28 vi REFERENCES 30 Chapter 2. Resumen en español 35 2.1 Microestructura mediante deposición en ángulo rasante. Anisotropía estructural 38 2.2 Modificación de la microestructura GLAD 42 2.3 Propiedades y aplicaciones de nanoestructuras orientadas obtenidas mediante GLAD 43 2.4 Metodología experimental. Reactor de deposición (GLAD-PVD) 44 2.5 Objetivos 48 2.6 Películas delgadas de ITO con conductividad anisotrópica en el plano fabricadas por deposición a ángulo rasante asistida por plasma. 49 2.7 Anisotropía óptica de láminas de ITO crecidas por ángulo rasante asistido por plasma para el grabado de información. 56 2.8 Rejillas ajustables mediante curvado de sustratos poliméricos 63 2.9 Mojado anisotrópico de capas de TiO2 sobre sustratos PDMS controlado mediante actuación mecánica. 68 vii Chapter 3. Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles 77 3.1 Introduction 79 3.2 Experimental set up 81 3.2.1 Thin films characterization 83 3.2.2 Electrical measurements 84 3.2.3 Electrochemical deposition of gold 84 3.3 Results 85 3.3.1 Morphology of GLAD and GLAD plasma assisted ITO thin films 85 3.3.2 Chemistry, crystalline structure and porosity 95 3.3.3 Electrical conductivity 99 3.3.4 Surface anisotropy and fabrication of dichroic thin films 101 3.4 Discussion 104 3.5 Conclusions 107 REFERENCES 108 viii Chapter 4 Highly birefringent Indium Tin Oxide thin films grown by Plasma Assisted Glancing Angle Deposition for advanced labeling applications 117 4.1 Introduction 119 4.2 Experimental set up 121 4.3 Results and discussion 125 4.3.1 Microstructure of GLAD and PA-GLAD thin films 125 4.3.2 Birefringence of PA-GLAD thin films 128 4.3.3 Optical anisotropy by rotation along polar angles 130 4.3.4 PA-GLAD thin films as retarders 133 4.3.5 Optical encoding 136 4.4 Conclusions 140 REFERENCES 141 Chapter 5. Bending induced self-organized switchable gratings on polymeric substrates 5.1. Introduction 145 5.2 Experimental set up 146 5.3. Results and discussion 147 5.3.1. Bending induce formation of parallel grooves 147 5.3.2 Switchable light diffraction 152 5.3.3. Fabrication of optical patterns 157 ix 5.4 Conclusions 161 5.5 Supporting Information 163 REFERENCES 165 Chapter 6 Anisotropic wetting in glancing angle deposited TiO2 coatings on flexible PDMS substrates 169 6.1. Introduction 171 6.2 Experimental set up 174 6.3. Results and discussion 175 6.3.1. Microstructural characterization of the self-patterned TiO2/PDMS surface 175 6.3.2 Wetting behavior of the TiO2/PDMS surface 181 6.3.3. Selective wetting TiO2/PDMS devices by mechanical deformation 188 6.4 Conclusions 198 REFERENCES 200 Chapter 7 General Conclusions 201 Introduction 7 Figure 1.2. (Top) Scheme of an electron beam physical vapour deposition in glancing angle configuration. The design of the nanostructure of the thin film is achieved by adjusting the angles (α) and (φ) during deposition. (down).- Scheme of columnar growth mechanism in a GLAD configuration. In the first stages of growth (a) atoms condense and form nuclei that produce a shadowing effect in different regions (b). The final film is composed by columns which grow from nuclei and are inclined in the direction of the vapour source (c). The main geometrical parameters have been indicated: the zenithal angle of the vapour incidence (α) and the angle of the tilted columns (φ). 1 Glancing Angle Deposition (GLAD) 8 In general, the low kinetic energy of the vapour species when landing onto the surface (the energy of evaporated atoms is around 0.2 eV) ensures that no kinetic energy-induced processes are likely to take place and that the competitive growth of tilted nanostructures is directly dependent on the arrival angle of vapour species26. The relation between the tilt angle of the nanocolumns and the zenithal angle of deposition α , is of the outmost importance for the control of the film properties27–29 and was roughly described by means of the heuristic tangent rule deduced from experimental data by Nieuwenhuizen and Haanstra 23. This rule describes the relation between both angles (α, β)28 by a direct tangent relation. More recently, Tait et al.30 proposed the so-called cosine rule, which attempts to describe the relation between both angles using cosine relationships. None of the two relations succeed in accounting the experimental tilting angles and, in general, fail for most deposition conditions. Based on recently published results, Álvarez et al.31,32, have succeeded in accounting for the dependence between α and β by means of new concept that takes into account the interaction between the deposition particles and the surface of the deposited material.(next section and Chapter 3) Figure 1.3 shows some examples of thin films prepared by GLAD. Different microstructures can be obtained by changing the deposition angle (α). Moreover, ziz-zag or helicoidally structures (i.e., sculptured thin films) can be developed by changing during deposition the orientation of the substrate with respect to the direction of arrival of the vapour flux (sculptured layers)33. Introduction 9 Figure 1.3 Examples of nanostructured thin films prepared by GLAD-PVD. The thin film a) shows a columnar structure grown at different oblique angles, and b) the same kind of deposition, but rotating the sample 180º at regular time intervals. Samples c) and d) are grown by rotating the sample at a constant speed, c) spirals obtained at low speed while isolated columns are formed at high rotation speed 33. In the present work we will develop microstructures obtained by changing the zenithal angle (α) during the evaporation of single inorganic oxides such as SiO2, TiO2 and ITO. Other experimental parameters, e.g., the use of a plasma during deposition, are also incorporated during the deposition process as discussed in the next chapters. 1 Microstructure of GLAD thin films 10 1.3 Microstructure of GLAD thin films. Nowadays, the development of new material systems is intended to manufacture devices with predicted and controlled properties as required for specific technological applications. Unlike isotropic materials that have identical properties in all directions, the properties of anisotropic materials change with the direction along the object. Anisotropic properties can be a result of differences in the structure or composition along given directions. In nature, there are many examples of anisotropic structures, e.g., the cellular structure of the skin, leafs of trees or wood which present anisotropy due to their asymmetric structure along given directions, etc. A typical example of anisotropic material is the calcite mineral that presents optical anisotropy because of the difference chemical composition along given directions. Such a type of anisotropy is quite common in the natural world where birefringence and double refractive properties are quite widespread. Figure 1.4 Piece of wood showing structural anisotropy in the x and y axis. (left). Piece of calcite showing an optic effect (Birefringence) due to composition anisotropy in two perpendicular directions(right). Processed anisotropic items have found many applications in fields such as Computer Graphics, Geology and Geophysics, Medicine, Microfabrication, Chemistry, Physics and Material Science and engineering. In this latter case, there Introduction 11 are a large number of materials that have been developed to cope with specific functions for optical, magnetic, microfluidics, electro-optical, construction, aeronautics and many other applications34–38. For instance, honeycomb structures are employed to fabricate the inner pieces of planes or trains with high compressive and tensile strengths in a preferential direction. This structural anisotropy complies with the conditions of a light material and a high consistency under high pressure which are required in aeronautic applications37,39. Our group have employed glancing angle deposition (GLAD) thin films as template to control the growth of anisotropic gold nanoparticles with outstanding properties that are used for encoding of information and advanced anticounterfeiting applications40. The motivation of the present work is the systematic development of anisotropic oxide materials by GLAD where the anisotropy of their optical and electrical properties provide new functions as invisible labelling, data encoding, plasmonics or wetting. 1.3.1 Surface roughness and porosity. The previous sections have implicitly assumed that the development of tilted and/or sculptured nanostructures is the most typical morphological characteristic of GLAD thin films. Herein, it is discussed about other relevant microstructural features such as roughness, preferential association of the nanocolumns (i.e., phenomenon usually denoted as bundling) or porosity, features that confer to the GLAD films some of the unique properties required for a large variety of applications. Owing to the importance of bundling within the context of the present work, we will dedicate a specific section to present some basic issues associated with its development. The termination of the nanocolumnar features at the film surface generates a peculiar topography characterized by a high surface roughness. Surface analysis by atomic force microscopy (AFM) of GLAD evaporated thin films reveals that the RMS surface roughness increases with the deposition angle α and with the thickness of the films 41–43. This situation is schematically described in Figure 1.5. 1 Microstructure of GLAD thin films 12 Figure 1.5. AFM images of TiO2 GLAD thin films with a similar thickness of 300 nm prepared at deposition angles of 60º 70º, 80º and 85º. For comparison normal SEM images of the surface of these films taken with the same scale (small squares) are included as inset.43. . For many applications (e.g., sensors, electrochemistry, catalysis, electrochromism, antireflective layers, etc.) the porous character of the GLAD thin films is a key feature for the fabrication of final devices. An important consequence of the high porosity of GLAD thin films of oxides or other transparent dielectric materials is that, according to the effective medium approximation44, their optical constants vary by condensation of air or water in their pores. Surprisingly, this effect has been generally overlooked in numerous works using GLAD thin films as antireflective coatings and multilayers for solar cells or other related environmental applications45,46. García-García L. et al.43 have showed that percentage pore volume of TiO2 GLAD thin films increases with the deposition angle (α), at the same time the meso Introduction 13 and micro porous increases with the deposition angle but in different aspect ratio. This aspect result essential for the properties of thin films and consequently the applications such as have been studied in several works.40,47 1.3.2 Separation and association of nanocolumns: bundling effects. A first assessment of the nanocolumnar arrangement in GLAD thin films might suggest a random distribution of nanocolumns within the film. However, some experimental evidences by AFM and SEM analysis of surface and film morphologies contradict this simple view (Figure 1.5)43,48,49 For example, in evaporated GLAD thin films, well defined correlation lengths and intercolumnar distances have been determined by SEM and AFM. These correlations are dependent on the deposition conditions and can be formally described with a power law scaling approach49. For example, by analyzing the surface SEM micrographs of a series of GLAD thin films Krause et al.48 identified some repetitive correlation distances between the surface voids separating the nanocolumns that were dependent on deposition parameters. Similarly, by using AFM, Mukherjee et al.50 were able to determine the period of the surface roughness features in different GLAD thin materials and correlate the obtained values with specific growth exponents. To confirm the existence of possible correlations between nanocolumns in the interior of the films and not merely at their surface, previous works of our groups have proposed the use of a bulk technique such as GISAXS10. By this method it is possible to determine both the tilting orientation of the nanocolumns and the correlation distances existing among them. Figure 1.6 shows a series of GISAXS patterns corresponding to TiO2 thin films that were prepared by e-beam GLAD at different zenithal angles. While the asymmetric shape of the patterns clearly sustains a tilted orientation of the nanocolumns, the position of the maxima in each pattern provides rather accurate values of correlation distances in the system10,43. It is also interesting that these patterns depict a well-defined 1 Microstructure of GLAD thin films 14 maximum suggesting the existence of correlation distances varying between 30 and 200 nm, progressively increasing with film thickness and the deposition angle. Figure 1.6 GISAXS and SEM analysis of tilted nanostructures prepared by GLAD.-TiO2 thin films prepared by evaporation at the zenithal angles of 60º, 70º ,80º, 85º (a,b,c,d) respectively (ref. [48]). The GISAXS spectra are asymmetric and depict well defined maxima that can be associated with the correlation distances existing in the system. Introduction 15 In early studies of evaporated GLAD thin film materials23,24,51, it was soon recognized that nanocolumns may associate in the form of bundles, i.e., laterally connected associations of nanocolumns arranged in a direction perpendicular to the vapour flux. Later on, bundling association has been reported for GLAD thin films prepared by either evaporation or MS (Magnetron Sputtering)52,53. Although the development of this microstructural arrangement has been mainly reported for the case of metal thin films23,24,51,53, it can be also found for oxides where it has been used as a template structure to develop new composite thin films40,47 Surprisingly, except for some detailed discussions in early reviews24, the bundling phenomenon has not conceited much attention in recent investigations on GLAD thin films. The growth association of nanocolumns in the form of bundles cannot be explained by traditional ballistic processes neither described by the heuristic tangent rule exposed in the previous section. However, some hints about its understanding can be deduced within the context of the so called surface trapping mechanism developed in our group31. According to it vapour species passing a few angstroms away from the surface may bend its trajectory and be deposited at nearby locations (see figure 1.7(a)). Figure 1.7 a) Illustration of the surface trapping mechanism: unlike typical ballistic models, where vapour species follow straight trajectories, the surface trapping mechanism introduces the possibility that trajectories may bend within an effective shadowing region. b) Illustration of the lateral sticking process, implicit in the trapping mechanism. 1 Microstructure of GLAD thin films 16 This trapping mechanism not only accounts for the tilting angle of the nanoclumns, but also for the formation of bundles. Figure 1.7(b) illustrates that trapping processes may also occur for vapour species passing laterally near a growing nanocolumn. A consequence of such preferential side trapping would be a faster lateral growth of the nanostructures in the direction perpendicular to the vapour flux, ultimately provoking their collapse along this direction In the present work we will show that the bundling phenomenon is a versatile and powerful tool to get anisotropic optical and electrical properties that can find extremely interesting applications in plasmonic, data encoding invisible labelling and anisotropic wetting. 1.4. Plasma Assisted GLAD Tuning the nanostructure of GLAD thin films may give rise to new nanostructures with singular surface roughness, organization of nanocolumnar structure or porosity and a large number of experimental strategies are used for this purpose. A first approach is to grow the films on nanostructured substrates where the key issue is to control the shadow casted by a series of patterned hillocks, protrusions or other well-defined features of the surface 54,55. Another classical way to induce changes in the morphology is to modify the deposition speed, working pressure or substrate temperature56,57. A less common approach is to use plasma to modify the microstructure of the deposited GLAD thin films. An example of this possibility is reported in Figure 1.8 showing the development of nanopillar structures by plasma etching58. Introduction 23 In general, the planar birefringence can be written: Δn = ns – ny. (1) Figure 1.12 shows values of the planar birefringence (Δn) for several GLAD inorganic oxides as a function of the deposition angle (α). Figure 1.12. Planar birefringence values measured for several materials, showing the complex 𝛼 -dependence of the birefringence2,29 . 1.6.2.2 Plasmonics. Applications At certain incident conditions, when the incident light frequency matches the intrinsic frequency of the free electrons, a resonant absorption occurs for metals with a high concentration of free electrons. This phenomenon is called surface plasmon resonance (SPR) or, in the case of nanometric metallic structures, localized surface plasmon resonance (LSPR). 1 GLAD thin films properties and applications 24 Figure 1.13. Schematic showing surface plasmon resonance (SPR) for a metallic sphere. The peculiar morphologies and microstructures generated by GLAD have been widely used to grow metallic nanoparticles and nanostructures, either selfsustained or embedded in a host matrix formed by a transparent thin film prepared by this method. Specially, SiO2 GLAD thin films have been used for this latter purpose due their high transmittance in the visible region. For example, anisotropic SiO2 nanocolumnar films have been used as templates for the fabrication of dichroic Ag films by direct DC sputtering deposition of silver on their surface47 Laser posttreatment silver melting has been used for optical encryption of information at micron level. Recently, we have developed in our group a host-guest strategy based on the formation of anisotropic Au nanodiscs inside the intercolumnar space of a bundled SiO2 GLAD thin film40 . These composite materials have been applied for optical encryption of information after laser treatment ( Figure 1.14a), so that the distinct colours of the untreated zones are due to the different anisotropies of the gold nanoparticles depending on the characteristics of the SiO2 host layer. This laser treatment Introduction 25 removes the dichroic character of the gold nanoparticles in selected zones with control at the micron scale (( Figure 1.14b)). Figure 1.14. a) Pictures taken with x- and y-polarized lights for a series of dichroic Au/SiO2 samples deposited at α= 60, 70, 80 and 85º. Selected zones of these samples were treated with laser at different powers: b) Standard bar code written with the laser onto a Au/SiO2-60º sample and monitored with x- and y-polarized lights40 1.6.3 Wetting Properties and applications Mimicking the nature to obtain superhydrophobic/superhydrophilic, adhesive/anti-adhesive and, lately, omniphobic and wetting-anisotropic surfaces has been a topic of constant interest in Nanotechnology during the last two decades, with converging efforts from academy and industry towards the search of new surface functionalities82,83. 1 GLAD thin films properties and applications 26 The wetting angle of a drop on a flat surface is determined by the Young Law and is the result of the balance among the cohesive forces acting at the contact line between the drop, the solid surface and the air or gas environment84. GLAD techniques have been extensively utilized to control the wetting properties of materials because they provide a fine control over both surface chemistry and roughness. Important achievements have been the fabrication of surfaces with singular adhesives properties85, hydrophobicity86–88 superhydrophobicity89–91, superhydrophilicity92 or superolephobicity93,94. Figure 1.15a,b showe an example of a droplet wetting anisotropy on a patterned surface59,95. The droplet presents a spherical shape when is deposited on smooth surface. However, when a patterned surface is created, the droplet adopts an elongated, parallel-sided shape. In most occasions, the formation and control of droplet wetting anisotropy is the first step to develop anisotropic wetting devices for wetting and microfluidics applications such as the one shown in Figure 1.15c. This example illustrates the use of GLAD to fabricate anisotropic films with unidirectional adhesive and wetting properties38. It shows the use of glancing angle technique for the fabrication of Parylene (PPX) nanorods depicting a pin-release droplet ratchet mechanism derived from their singular nanoscale microstructure. These nanofilms exhibited a difference of 80 μN in the droplet retention forces between the pin and release directions, this latter characterized by a microscale smooth surface on which microliter droplets are preferentially transported. Introduction 27 Figure 1.15. Top view optical micrograph of 2 mL water droplet on the smooth surface (a) and patterned surface (b), revealing an elongated, parallel-sided shape; C) Time-lapse of droplet motion in the coated and uncoated half pipes. Water drops moved axially on the half-pipe coated with the PPX nanofilm, whereas those on the uncoated half-pipe merely vibrated randomly59,95. In the present work dealing with anisotropic properties of GLAD thin films, we have focussed on the development of anisotropic-wetting applications. The interest on this topic stems from the industrial need of materials capable of inducing a preferentially oriented liquid spreading or rendering asymmetrical adhesive surfaces 1 GLAD thin films properties and applications 28 1.7 Objetives The main objective of this thesis is the control, enhance and manipulation of anisotropic microstructural properties of thin films prepared at glancing angle deposition to develop advanced functional applications. From this general objective, more concrete goals are also within the intention of this thesis, namely:  To optimize the plasma assisted at glancing angle deposition process.  To develop and synthesis porous ITO thin films by plasma assisted at glancing angle deposition with optimal optical transparency and electrical conductivity. To study the microstructural influence of plasma assisted during the growth.  To characterize and study of the electrical and optical properties of ITO thin films at glancing angle deposition derivated of their structural anisotropy.  To fabricate electronic and optical devices for plasmonic and encoding date applications using the anisotropic microstructure of ITO GLAD thin films.  To study and optimize thin films at glancing angle deposition on flexible polymers. To study the microstructural changes and how it affect, the properties under mechanic deformations.  To study the thin film-polymer interactions and reversibility after mechanic deformations.  To optimize the optical and wetting properties generated in the nanocomposite before, during and after mechanic stress.  To fabricate optical and wetting devices for inviable labeling and wetting control with the nanocomposites generated. Introduction 29 1.16 Thesis scheme. Figure 1.16. 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R.; Barranco, A. J. Mater. Chem. C 2014, 2 (32), 6561. (68) Okamoto, K.; Itoh, K. Jpn. J. Appl. Phys. Part 1 Regul. Pap. Short Notes Rev. Pap. 2005, 44 (3), 1382–1388. Introducción 39 Durante el intervalo inicial del crecimiento de una película, se condensan átomos procedentes de la sublimación en el sustrato y forman islas separadas individualmente o “núcleos”. (Véase en Capítulo 1, los modelos de crecimiento. Trapping). Cuando el sustrato está inclinado con respecto a la dirección del vapor, este incide a ángulos oblicuos de forma que los núcleos proyectan un cono de sombra sobre el sustrato, impidiendo la deposición en esa zona, evitando la coalescencia de los núcleos y la formación de una capa continua. Los núcleos iniciales crecen mediante captura de nuevos átomos evaporados, lo que origina la formación de columnas inclinadas que crecen en la dirección de aporte del vapor12– 14 Mediante esta técnica se han llegado a producir nanoestructuras columnares de gran complejidad, como nanoestructuras periódicas, perforadas o de porosidad gradual para el uso en multitud de aplicaciones que van desde sensores hasta filtros ópticos, dispositivos microfluídicos, magnéticas y catalíticos5,7,8,15–19. Ejemplos de microestructuras de capas GLAD con carácter “escultural” se presentan en la Fig.2.2 2 Resumen en español 40 Fig.2.2: Ejemplos de películas delgadas nanoestructuradas de carácter escultural preparadas mediante GLAD. La película delgada a) muestra una estructura columnar crecida a un ángulo oblicuo y la b) ese mismo tipo de deposición girando la muestra 180º según su eje azimutal a intervalos de tiempo regulares. Las muestras c) y d) se crecieron girando a una velocidad constante. Para una velocidad lenta (c) se obtienen espirales mientras que a velocidades altas (d) se forman columnas perpendiculares. Los efectos de sombra que dan lugar al crecimiento de materiales nanoestructurados formados por columnas inclinadas(Ver figura2.1a) respecto al sustrato son una de las causas de que las capas GLAD presenten anisotropía en sus propiedades físico-químicas. En materiales anisotrópicos sus propiedades son dependientes de la dirección de análisis. En concreto, mientras que los materiales isotrópicos presentan propiedades equivalentes en todas direcciones, las propiedades de los materiales anisotrópicos dependen de la dirección en la que se realiza el análisis de dicha propiedad. La anisotropía en los materiales puede ser impuesta por una estructura direccionada según sus ejes de crecimiento o por una composición química distinta dentro de una estructura determinada, la cual puede ser direccionada o no. Por Introducción 41 ejemplo, minerales, como la calcita presentan propiedades ópticamente anisotrópicas debido a que su composición química cambia con la dirección de crecimiento. Otros ejemplos de anisotropía estructural que aparecen en la naturaleza pueden ser la estructura celular de la piel, o la estructura de las hojas o la propia madera.20,21 Figura 2.3. Lámina de madera donde se muestra la anisotropía estructural entre los ejes x e y.(izqda.) Pieza de calcita relevando un efecto óptico (Birrefringencia) atribuido a la distinta organización química en ejes perpendiculares a su crecimiento(drcha.). Como se ha comentado anteriormente, las láminas delgadas mediante GLAD presentan una microestructura singular en forma de nanocolumnas inclinadas. Su porosidad, rugosidad, espesor, conectividad entre columnas etc. son clave para entender las propiedades y propiciar aplicaciones dependientes del tipo de crecimiento.(Véase Capitulo 1). Por ello uno de los objetivos que persigue este trabajo es profundizar en el conocimiento de la anisotropía estructural generada por la agrupación de nanocolumnas GLAD para obtener nuevas propiedades y aplicaciones de estas nanoestructuras. Diversos trabajos sobre la preparación de láminas delgadas mediante GLAD12,13,20 , han mostrado que las nanocolumnas pueden asociarse lateralmente en forma de agregados en la dirección perpendicular a la llegada de material al sustrato. Este fenómeno es conocido como “bundling”, habiéndose estudiado sobre todo en el caso de películas metálicas12,13,20–22. Su aplicación a óxidos inorgánicos ha 2 Resumen en español 42 sido mucho más restringida hasta el momento, desconociéndose sus posibles modificaciones y cómo estas afectan a las propiedades finales de las capas5,19 Una de las motivaciones de este trabajo es desarrollar nanoestructuras orientadas de óxidos inorgánicos mediante la técnica GLAD para desarrollar propiedades ópticas y eléctricas anisotrópicas específicas que permitan su aprovechamiento en diversos campos de aplicación como la codificación de información, plasmónica, mojado superficial controlado y microfluídica. 2.2 Modificación de la microestructura GLAD En las últimas décadas, el uso de plasmas a baja temperatura ha tenido una alta repercusión en ciencia de materiales. La deposición química desde fase vapor asistida por plasma (PECVD) y los procesos de tratamiento superficial empleando plasmas son técnicas bien establecidas en campos como la microelectrónica, optoelectrónica, recubrimientos protectores, tecnología de polímeros, etc. Así mismo, en los últimos años, el plasma está siendo usado como técnica para la modificación de las propiedades de una superficie para ampliar sus rangos de aplicación23–25 En esta memoria se muestra por primera vez el efecto de un plasma sobre los crecimientos a ángulo rasante de películas evaporadas (plasma-GLAD) con el objeto de controlar y modificar la microestructura y porosidad de las mismas. Como efecto de esta interacción, se han puesto de manifiesto cambios importantes a nivel nanométrico de la microestructura de las capas y, en consecuencia, de sus propiedades y posibilidades de aplicación. Para lograr la modificación controlada de nonaestructuras y su modo de agrupación, otra aproximación abordada en el presente trabajo ha sido cuando las capas se depositan sobre sustratos flexibles y son sometidas a esfuerzos mecánicos controlados. Se ha puesto de manifiesto que los cambios que se inducen con las tensiones externas sobre la microestructura de las capas generan otro nuevo campo de propiedades y aplicaciones donde la anisotropía estructural juega un papel Introducción 43 esencial. En concreto, las aplicaciones investigadas en este caso caen dentro de los campos de la óptica y la microfluídica. 2.3 Propiedades y aplicaciones de nanoestructuras orientadas obtenidas mediante GLAD Las propiedades de los recubrimientos preparados mediante GLAD van a depender en gran medida de la naturaleza química del material y del desarrollo de propiedades anisotrópicas asociadas a su microestructura. Un ejemplo de la bibliografía sobre propiedades eléctricas se refiere a recubrimientos metálicos de cobre o níquel, cuya resistividad varía en función de la dirección de la medida26,27 Otro ejemplo clásico, es la variación de las propiedades ópticas de capas transparentes en función de la microestructura columnar, tal y como se presenta en la Figura 2.4. En ella se muestra la diferencia entre los valores de índice de refracción para las polarizaciones perpendicular y paralela al eje de anisotropía (Birrefringencia) que poseen una serie de recubrimientos GLAD de distinta naturaleza química.( Ver con más detalle sección 1.3 del Capítulo 1) Figura2.4. Variación de birrefringencia medidas para distintos óxidos inorgánicos mostrando el grado de dependencia de esta propiedad con el ángulo de deposición ( 𝛼 )2 2 Resumen en español 44 En el resumen bibliográfico que acompaña a la introducción de esta tesis se comentan los resultados más relevantes sobre la influencia de la anisotropía estructural sobre las propiedades de estos materiales, principalmente eléctricas, ópticas y de mojado. 2.4 Metodología experimental. Reactor de deposición (GLAD-PVD) Para la fabricación de capas finas nanoestructuradas la cámara de deposición empleada contiene un evaporador por bombardeo electrónico “Multihearth electron beam source”, EV M-5, suministrada por la empresa Advanced Products &Technologies GmbH (AP&T). La fuente de electrones es un filamento espiral de wolframio que opera a un determinado potencial fijo y se calienta mediante el efecto Joule por el paso de una corriente eléctrica. El sistema experimental empleado consta de 4 crisoles y un sistema que permite el cambio de uno al otro mediante un motor. La fuente proporciona una potencia máxima de 4 kW, suministrando una corriente variable al filamento y generando un potencial de aceleración para los electrones. La Figura 2.5 muestra un esquema general del sistema de deposición por evaporación electrónica. La cámara tiene forma cilíndrica, 55 cm de diámetro y 70 cm de altura y está unida a un sistema de vacío.(Ver más Capítulo 1) Introducción 45 Fig.2.5 . Esquema general del sistema de deposición por evaporación electrónica. El portamuestras se coloca en la parte superior de la cámara, pudiéndose cubrir numerosos sustratos a la vez. El portamuestras está diseñado para poder depositar a varios ángulos de evaporación simultáneamente: 65°, 70°, 80°, 85° y 90°. Una foto y un esquema del portasustratos se muestran en la Figura 2.6 Fig.2.6. Esquema geométrico (izquierda) y Fotografía (derecha) del portasustratos. 2 Resumen en español 46 2.4.1 Plasma GLAD. (PA-GLAD) Para esta investigación se ha puesto a punto un nuevo sistema experimental que permite asistir la deposición GLAD mediante un plasma. Para ello se ha acoplado un reactor de plasma comercial (Micro-Slam 32) a una cámara de evaporación electrónica. Se trata de una fuente de plasma de microondas a 2.45 GHz con un potencia máxima de 800 W. La fuente puede trabajar en condiciones de “Resonancia Ciclotrónica Electrónica” (ECR), lo que permite obtener plasmas a presiones relativamente bajas (rango 10-4 Torr), y densidades electrónicas altas en una región bien confinada28. El principal reto de esta modificación a la técnica GLAD es conseguir acoplar un plasma a las condiciones de deposición en ángulo rasante, sin que estas cambien. Para la fabricación de las películas se ha trabajado a una potencia de 460 W. Se ha adaptado un sistema de varios controladores de flujo másico a la cámara de GLAD para poder realizar experimentos con mezclas controladas de gases (Ar + O2). Para poder emplear este nuevo sistema experimental es necesario sustituir el portamuestras original de la cámara por otro que permita un contacto directo del plasma con los distintos substratos. Por esta razón las distintos experimentos realizados bajo condiciones de plasma se han llevado a cabo a un ángulo de deposición constante de 80º. Introducción 47 Fig.2.7.(arriba) Fotografía del sistema experimental Plasma asistido PVD. (abajo) Esquema del sistema experimental. 2 Resumen en español 48 2.5 Objetivos En esta memoria se incluyen los resultados más relevantes de la fabricación de películas delgadas mediante la técnica de evaporación electrónica en ángulo rasante (GLAD), así como en procesos asistidos por plasma y la fabricación de este tipo de láminas sobre sustrato flexible. De forma general, la fabricación de este tipo de películas ha perseguido en este trabajo la consecución de los siguientes objetivos: 1. Controlar y mejorar la anisotropía de la microestructura de las capas GLAD durante su crecimiento. 2. Obtención de películas delgadas crecidas a ángulo rasante sobre sustrato flexible. Análisis de los cambios estructurales en las láminas inducidos tras deformaciones mecánicas 3. Obtención y desarrollo de dispositivos para aplicaciones electro ópticas, ópticas y microfluídicas donde los cambios en la anisotropía estructural es el elemento controlante de las mismas. Películas delgadas de ITO con conductividad anisotrópica en el plano fabricadas por deposición a ángulo rasante asistida por plasma. 55 anisotrópico que se asocia a la agregación de nanocolumnas a lo largo de la dirección de apilamiento (bundling). Gracias a esta nanoestructura orientada estas capas propician la electrodeposición de partículas alargadas de oro, las cuales presentan un comportamiento dicroico bajo acción de luz linealmente polarizada en direcciones perpendiculares. 2 Resumen en español 56 2.7 Anisotropía óptica de láminas de ITO crecidas por ángulo rasante asistido por plasma para el grabado de información. En este capítulo se desglosa el comportamiento óptico que presentan las capas de ITO fabricadas y caracterizadas en el capítulo anterior. Concretamente, este capítulo se centra en el estudio óptico de las capas en ausencia de plasma y de películas crecidas en presencia de un plasma de O2, ambas depositadas a un mismo ángulo de deposición (α=80º). Se muestra cómo el crecimiento asistido por plasma influye en propiedades ópticas tales como la birrefringencia en el plano obteniéndose valores no descritos anteriormente en la bibliografía. En la parte final de este capítulo se propone una aplicación directa para codificación de información tanto a nivel macroscópico como en la microescala cuando estas capas de alta birrefringencia son expuestas a la luz polarizada o polarizadores cruzados. Tal y como se mostró en la figura 2.8 del anterior capítulo, el crecimiento de ITO mediante plasma asistido genera el apilamiento de nanocolumnas a lo largo del eje perpendicular a la dirección del crecimiento, (bundling). Este fenómeno se pone de manifiesto de manera más evidente, cuando se obtienen dos secciones transversales a lo largo del eje del bundling y perpendicular al bundling (growing) en un microscopio de haz iónico focalizado (FIB) observándose que en la película asistida por plasma de oxígeno las distancias intercolumnares son diferentes en ambas direcciones, tal y como muestra la Figura 2.12. Anisotropía óptica de láminas de ITO crecidas por ángulo rasante asistido por plasma para el grabado de información. 57 Figure 2.12. Micrografías FIB en sección transversal de películas de ITO crecidas al mismo ángulo de deposición (80º) en ausencia de plasma (GLAD (a, c)) y en presencia de plasma de O2(PA-GLAD ITO) (b, d)) Ambas nanoestructuras se analizaron en dos orientaciones perpendiculares(a, b) respecto del bundling(c, d). Esta anisotropía estructural produce que el camino óptico recorrido por la luz sea diferente actuando por tanto como una lámina retardadora (Ver más capítulo 4) Esto se traduce en una diferencia de índices de refracción entre los dos ejes perpendiculares entre sí, en este caso entre el eje de asociación de columnas (bundling) y el perpendicular a este (growing). A este fenómeno se le denomina birrefringencia en el plano, y los valores de índice de refracción y de porosidad obtenidos para las nanoestructuras analizadas se muestran en la tabla 2.2. 2 Resumen en español 58 Se puede observar que para la película PA-GLAD la birrefringencia en el plano es de 0,2, valor que está por encima de los recogidos hasta ahora en bibliografía2,32 Para ello se ha realizado un exhaustivo estudio del comportamiento de estas láminas cuando se iluminan con luz polarizada y polarizadores cruzados para la rotación azimutal y polar.(Ver esquema capítulo 4). Los cambios más interesantes se observaron al analizar las películas entre polarizadores cruzados girando en el plano azimutal (plano perpendicular al crecimiento de las columnas) tal y como se observa en la figura 2.13 d. Para la película PA-GLAD aparece un máximo de transmitancia cuando la muestra se gira 45º respecto de los polarizadores cruzados, coincidiendo con el máximo desalineamiento del eje del bundling respecto a los ejes de polarización de los polarizadores cruzados tal y como muestra la figura 2.13 b. Para esta misma rotación la muestra isotrópica (ausencia de plasma) presenta un comportamiento totalmente diferente.(Ver más Capítulo 4) Anisotropía óptica de láminas de ITO crecidas por ángulo rasante asistido por plasma para el grabado de información. 59 Figure 2.13: Espectros de transmitancia a través de polarizadores cruzados para la películas de ITO en ausencia de plasma (a) y asistida con plasma (b) para una rotación azimutal (c) Equema rotación azimutal. Imágenes de láminas de ITO crecidas en ausencia de plasma (d) y asistida por plasma (e) entre dos polarizadores cruzados para la rotación azimutal. 2 Resumen en español 60 La combinación de la respuesta óptica de las láminas de ITO crecidas por ángulo rasante cuando se interponen polarizadores cruzados, con la posibilidad de crecer estas capas en distintas zonas de las muestra mediante máscaras permite el desarrollo de dispositivos de encriptación óptica, codificación de información o etiquetado invisible. En este capítulo se muestran varios ejemplos, a nivel macroscópico y microscópico de encriptación óptica empleando estos materiales nanoestructurados y orientados. Procesos tan sencillos como usar tinta nos permite replicar la huella dactilar humana en una sola etapa de procesado, tal y como se observa en la Figura 2.14. En la misma se muestra cómo alineando el eje óptico (bundling) con los polarizadores cruzados la información acuñada (huella) en la superficie no es visible (figura 2.14a), mientras que con un simple giro de 45º (azimutal) se observa con detalle una imagen de la huella dactilar encriptada (figura 2.14 b). (Véase capítulo 4) El efecto de birrefringencia fue tan evidente, que este tipo de encriptación se pudo llevar a cabo a nivel microscópico. Para ello se realizaron una serie de grabados por fotolitografía, tal y como muestran las figuras 2.14c-f. Las imágenes con un polarizador revelan que la información contenida en los grabados es totalmente invisible (figura2.14 c,e), mientras que con dos polarizadores cruzados se pudieron observar los motivos replicados por fotolitografía en dimensiones del orden de las decenas de micras. Anisotropía óptica de láminas de ITO crecidas por ángulo rasante asistido por plasma para el grabado de información. 61 Figure 2.14: a,b) Imágenes de huella dactilar encriptada en una lámina de ITO crecida mediante PA-GLAD entre polarizadores cruzados. a) eje óptico (bundling) alineado con los polarizadores cruzados. b) eje óptico desalineado respecto a los polarizadores cruzados. cf) Imágenes de microscopía óptica de grabados por fotolitografía sobre las capas de PAGLAD, con un polarizador (.c,e) y entre polarizadores cruzados (d,f.). Micrografías SEM de la película de ITO asistida con plasma tras la retirada de la tinta y del fotoresistor.(insets). Imperfección en la muestra, la cual verifica el fenómeno descrito (elipse roja) 2 Resumen en español 62 2.7.1 Conclusiones En este trabajo se ha desarrollado una sencilla metodología para crecer películas delgadas de ITO con alta anisotropía estructural y birrefringencia óptica asistiendo mediante plasma durante el crecimiento. Esta anisotropía estructural está compuesta por columnas que presentan diferentes distancias intercolumnares a lo largo del eje de agregamiento (bundling) y el perpendicular a este (growing). Los valores obtenidos de birrefringencia para la película asistida por plasma muestra valores superiores a los registrados en la bibliografía para óxidos obtenidos por GLAD, (Δn=0,2). Estos valores se han extraído, tras la modelización de la respuesta óptica de las muestras a luz polarizada o polarizadores cruzados, obteniendo valores de índice de refracción diferentes y perpendiculares entre sí (1.48 y 1.68 para las direcciones de growing and bundling). El proceso de fabricación es totalmente compatible con el uso de máscaras o técnicas photolitográficas que permiten la fabricación directa de grabados dicroicos. Por tanto, esta metodología abre la puerta a procesos de encriptación donde el grabado solo podría ser legible con polarizadores en las configuraciones adecuadas. Rejillas ajustables mediante curvado de sustratos poliméricos 63 2.8 Rejillas ajustables mediante curvado de sustratos poliméricos Tanto en el capítulo que se comenta a continuación como en el siguiente, se estudian materiales nanoestructurados GLAD preparados sobre substrato flexible. Este tipo de materiales abre la puerta al desarrollo de propiedades vinculadas con la anisotropía de las capas usando sustratos más interesantes desde el punto de visto económico e industrial. El objetivo inicial de este trabajo perseguía comprobar que el desarrollo estructural de las capas GLAD era similar cuando crecían sobre polímeros, analizando su adhesión frente a deformación mecánica. Estos objetivos iniciales fueron superados, ya que estos sistemas han permitido desarrollar nuevas funcionalidades ópticas y fluídicas asociadas a la anisotropía estructural inducida en los sistemas preparados cuando estos se someten a deformaciones mecánicas. En esa línea, los resultados de este capítulo permiten el desarrollo de dispositivos con aplicaciones potenciales para el etiquetado invisible, transductores optomecánicos o ventanas inteligentes7,33–36. Para este trabajo se han preparado películas GLAD de SiO2 crecidas sobre polidimetilsiloxano (PDMS). Sobre estos sustratos poliméricos, la distribución de los nanocolumnas antes del proceso de deformación mecánica se ha caracterizado mediante microscopia electrónica de barrido, observándose geometrías similares a las obtenidas sobre sustrato rígido.(Véase Capítulo 5) Tras su curvatura mecánica, las estructuras híbridas mostraron la formación de rupturas paralelas entre sí y a distancias controladas por la nanoestructura de la película depositada pero independiente del espesor y del ángulo de curvatura, tal y como se muestra en la Figura 2.15 (Ver más capítulo 5) . En este trabajo se ha podido poner de manifiesto que la anisotropía microestructural en el plano que presentan las películas de SiO2 genera grietas paralelas separadas por un espaciado diferente según el sistema se deforma según ejes perpendiculares entre sí. Concretamente se puede observar que las distancias entre grietas en la dirección del bundling (eje x) son menores que en el eje perpendicular a este (growing-eje y). 2 Resumen en español 64 Además, independientemente del tipo de deformación (cóncavo o convexo), las distancias entre grietas se mantienen constantes, siendo esta característica un hecho singular respecto al comportamiento de otros materiales referenciados en la bibliografía donde las distancias y el patrón de grietas dependen del tipo de doblado37,38 Figure 2.15. a) Micrografías FESEM de dos láminas G-SiO2/PDMS en posición plana después de doblarlo a lo largo del eje x (izq) y del eje y (dcha).b) Micrografías FESEM de las películas G-SiO2/PDMS en posición cóncava durante su observación. c) Micrografías FESEM de las películas G-SiO2/PDMS en posición convexa durante su observación. Todos los ejemplos corresponden a una película de 300 nm de espesor crecida a un ángulo de deposición 85º. Mojado anisotrópico de capas de TiO2 sobre sustratos PDMS controlado mediante actuación mecánica. 71 Una vez comprobado el tipo de estructuras anisotrópicas generadas en el nanocomposite G-TiO2/PDMS tras su deformación y teniendo en cuenta los resultados de anisotropía óptica obtenidos en el capítulo anterior, se desarrolló un estudio amplio sobre el deslizamiento de gotas de líquidos de distinta naturaleza polar (agua y diiodometano), con el fin de estudiar su comportamiento frente a distintas configuraciones de deformación mecánica. A su vez, estos resultados se compararon con los obtenidos por una lámina de PDMS tal y como muestra la figura 2.20. En este caso, las gotas depositadas sobre la lámina de PDMS presentaron un deslizamiento equivalente tanto para la dirección normal como la paralela a la tensión/deformación aplicada. En comparación, el comportamiento de los líquidos sobre la superficie con la capa nanoestructurada se vieron muy afectados según el ángulo de deslizamiento, especialmente en el caso de gotas de diiodometano, presentando un comportamiento de deslizamiento anisotrópico. Este comportamiento singular se atribuye a la diferente micro y nano estructura de la superficie según el eje sobre el que se aplique la tensión y se produzca la deformación correspondiente, así como al efecto de su comportamiento diferente en las configuraciones cóncava, convexa o estirada. (Estudio detallado en el capítulo 6) 2 Resumen en español 72 Figura 2.20. Estudio de deslizamiento de una gota de agua (izqda) y diiodometano (dcha) sobre una lámina de PDMS (arriba) y 85ºG-TiO2/PDMS (abajo). Una apreciación visual de los efectos anteriores resulta también muy ilustrativa de la existencia de fenómenos de mojado anisotrópico sobre las superficies deformadas. Así, sobre una superficie inclinada no deformada de PDMS, una gota de agua se mantiene anclada incluso para inclinaciones muy considerables y sólo se desprende de la superficie cuando se coloca en posición vertical y los efectos gravitatorios exceden a las tensiones superficiales. Sin embargo, para una posición vertical, el movimiento o deslizamiento de una gota de agua resulta restringido sobre la superficie de la lámina G60ºTiO2/PDMS cuando se curva en posición convexa con el eje de doblado ajustado a la dirección del growing. Por el contrario, cuando la lámina se deforma en la dirección del bundling, la gota de agua abandona la superficie. Los distintos comportamientos se pueden Mojado anisotrópico de capas de TiO2 sobre sustratos PDMS controlado mediante actuación mecánica. 73 observar en la conjunto de imágenes que muestra la figura 2.21. La explicación de los mismos hay que buscarlos en la distinta nanoestructuración superficial inducida por el doblado y la exposición de la gota de agua a zonas de PDMS de las grietas en el caso de que el doblado deje zonas del sustrato expuestas. (Ver más Capítulo 6.) Figura2.21 Deslizamiento selectivo de agua por deformación mecánica de la superficie de G-60ºTiO2/PDMS en función del dirección de deformación. La gota de agua se ancla sobre la superficie de G-60ºTiO2/PDMS cuando se curva en la dirección perpendicular al bundling(growing) mientras que se desprende cuando la lámina se dobla según la dirección del bundling 2 Resumen en español 74 2.9.1 Conclusiones Se ha demostrado que mediante la manipulación manual de sustratos de PDMS recubiertos de nanocolumnas inclinadas de TiO2, el control y transporte selectivo/guiado de gotas de líquidos es posible sin la necesidad de generar superficies superhidrofóbicas debido a la inducción de una doble rugosidad. La rugosidad generada se debe a la distribución nanocolumnar que ofrece este tipo de crecimiento más el patrón de grietas y arrugas (wrinkling) que se generan al deformar la lámina del sustrato flexible con la capa nanoestructurada sobre su superficie. La retención/deslizamiento controlados de gotas de agua es posible deformando las láminas G-TiO2/PDMS en posición cóncava y convexa. Mientras, estirando las láminas de G-TiO2/PDMS, se exhibe un comportamiento omnifóbico haciendo posible el control de líquidos no polares sobre la superficie. Además, se han encontrado dos niveles de anisotropía de mojado: 1) el mojado según la dirección del bundling (eje x) de la estructura de TiO2 es mayor que en la dirección perpendicular a este, 2) el deslizamiento sobre la dirección paralela a la tensión aplicada externamente es mejor que sobre la normal a este, especialmente para líquidos no polares. En conclusión, las láminas de TiO2/PDMS han resultado materiales eficaces para el control dinámico del mojado de líquidos con grandes perspectivas de uso para el desarrollo de dispositivos microfluídicos selectivos o filtros de separación agua/aceite bajo manipulación manual o mecánica. References 75 REFERENCES (1) Ohring, M. In Materials Science of Thin Films (Second Edition); Ohring, M., Ed.; Academic Press: San Diego, 2002; pp 1–56. (2) Hawkeye, M. M.; Taschuk, M. T.; Brett, M. J. Glancing angle deposition of thin films: engineering the nanoscale; 2014. (3) Murray, W. A.; Barnes, W. L. Adv. Mater. 2007, 19 (22), 3771–3782. (4) Beaudry, A. L.; LaForge, J. M.; Tucker, R. T.; Li, P.; Taschuk, M. T.; Brett, M. J. Cryst. Growth Des. 2013, 13 (1), 212–219. (5) Sanchez-Valencia, J. R.; Toudert, J.; Borras, A.; Barranco, A.; Lahoz, R.; de la Fuente, G. F.; Frutos, F.; Gonzalez-Elipe, A. R. Adv. Mater. 2011, 23 (7), 848–853. 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A 2013, 1 (9). + Chapter 3 Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles Introduction 79 3.1 Introduction Such as was commented in Chapter 1, Indium tin oxide (ITO) presents excellent conductivity, high optical transmission and the relatively easy up-scaling of manufacturing processes, being a key transparent electronic material.1 Only its cost and the relative scarcity of indium have prompted the search for other alternative transparent and conductive (TCO) films.2 ITO thin films have been prepared by a large variety of procedures including wet (i.e., sol-gel, spray, dip-coating, etc.3,4) or dry, (i.e., vacuum and plasma) methods of deposition, these latter including e-beam evaporation or magnetron sputtering.5–8 Recently, porous nanostructured ITO thin films and related stacked nanostructures have been prepared by deposition at glancing angles (GLAD thin films)(Chapter 1).9–13 By this manufacturing process, the substrates form an oblique angle with respect to the flux direction of the deposition material usually generated by e-beam or magnetron sputtering.14–16 Metals,17,18 oxides,19–22 semiconductors23,24 and molecular materials25 have been prepared in the form of thin films by using this methodology. Their microstructure consists of tilted nanocolumns which, extending from the interface with the substrate up to the surface, confer the films outstanding optical, adsorption or magnetic properties.26–32GLAD-ITO nanostructured layers have been also stacked in the form of 1D photonic crystals33,34 that depict singular Bragg reflection and conductivity performances.35,36 On the other hand, branched ITO nanostructures have been also prepared by GLAD evaporation under conditions inducing a vacuum liquid-solid (VLS) growth.37–41 In these systems the electrical conductivity is expected to be higher along the direction of the specific elements in their nanostructure. In the present Chapter, has been developed an experimental strategy consisting of plasma assisting the growth of e-beam GLAD ITO thin films, which is explained in the next section. 3 Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles 80 In literature, GLAD thin films have been grown under the effect of a beam of accelerated ions.42Once formed, compact ITO thin films have been also subjected to plasma etching to induce the growth of nanopillar structures.12 However, to our knowledge, there are no previous essays where a plasma is used to assist the growth of e-beam GLAD thin films during deposition. Besides describing the experimental procedure,in this work is presented a characterization of the GLAD-ITO films prepared both by direct e-beam evaporation or by assisting the deposition with Ar or O2 plasmas supplied with a low pressure plasma source. Scanning electron microscopy (SEM), topographic atomic force and electrical scanning microscopy (AFM), gracing incidence small angle X-ray scattering (GISAXS), besides sheet and in-depth conductivity measurements have been utilized for the characterization analysis of the samples. It will be shown that, besides changes in the nanocolumns orientation, an outstanding effect of plasma assisting the film growth is the bundling association of the individual nanocolumns14,43 that define a preferential direction rendering anisotropic in-plane conductivity44. A preliminary discussion about this anisotropic bundling association of nanocolumns under the action of a plasma has been carried out within the frame of the trapping mechanism concept45,46recently proposed by us to account for the tilting angle of nanocolumns beyond the heuristic tangent and cosine rules.47 To illustrate the possibilities of application of this type of anisotropy in microstructure and electrical properties, is proposed the preparation of dichroic thin films formed by the electrochemical deposition of gold nanostructures on the anisotropic ITO films. In Chapter 1 has been shown that anisotropic gold or silver nanostructures presenting a dichroic plasmon resonance behavior can be prepared by different chemical or physical methods making use of a template effect of the bundles developed in GLAD SiO2 thin films.48,49 In the present chapter, contrary to the electrochemical deposition of isotropic and spherical gold nanoparticles found on conventional ITO films,50,51a Results and discussion 87 When the growing films were exposed to plasma at a similar pressure of 10- 4 torr, the morphology of the films experienced significant changes that affected the shape, size, agglomeration and tilting angle of the nanocolumns. Figure 3.2 shows a series of normal and cross section SEM micrographs corresponding to ITO thin films prepared at a zenithal angle of 80º in the presence of several types of plasmas. As explained in the experimental section other evaporation angles were not possible because of experimental restrictions. An outstanding characteristic of the microstructure of these plasma assisted GLAD thin films is that the nanocolumns are not isolated but associate in the form of bundles14,43,48,49,59 that stretch along a direction perpendicular to the incoming flux of evaporated material. In the first chapter was explained that bundling association of nanocolumns in GLAD thin films is a common phenomenon reported for a large variety of materials.14,43,59 . 3 Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles 88 Figure 3.3. Normal (top) and cross section (bottom) SEM micrographs of ITO thin films prepared by GLAD at a zenithal angle of 80º while assisting their growth with different types of plasma at an overall pressure of 10-4 torr. The insets in the normal SEM micrographs correspond to the FFT diagrams obtained from the normal SEM micrographs in each case (see text). Results and discussion 89 Another important characteristic of the plasma assisted GLAD-ITO thin films is that the tilting angle β stays around 30 º, varying only slightly with the type of plasma gas (see Table 3.1). A different growing mechanism for the plasma assisted GLAD-ITO thin films is clearly evidenced in Figure 3.4 showing a cross section micrograph of a stacked bilayer of ITO films sequentially deposited in the absence (bottom) and in the presence (top) of oxygen plasma. Figure 3.4. SEM cross section micrograph of a ITO bilayer prepared by GLAD, where the bottom layer has been prepared in the absence of plasma and the top layer by assisting the growth with a plasma of oxygen. 3 Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles 90 Table 3.1. Tilting angle of nanocolumns in the GLAD ITO films grown by assisting the thin film growth with different plasmas. In Chapter 1 was demonstrated that GISAXS analysis of GLAD thin films can be a very powerful technique to retrieve information about possible anisotropy in the bulk and to determine the existence of specific correlation distances between the nanocolumnar features.53,54 Figure 3.5 shows a series of GISAXS patterns of ITO thin films deposited at different zenithal angles in the absence of plasma and for the plasma assisted films deposited at α=80º. Except for the pattern of the ITO compact thin film (i.e., a commercial ITO plate), the patterns recorded for the GLAD films are characterized by an asymmetric shape indicative of a tilted orientation of the nanocolumns with respect to the film surface.53,54 Moreover, the development in all cases of well-defined maxima proves the existence of specific correlations distances between the basic microstructural units of the films (i.e., the existence of a repetitive distance that, on average, separate these microstructural units). Results and discussion 91 Figure 3. 5. GISAXS patterns of ITO thin films. (left) Thin films deposited at different zenithal angles α in the absence of plasma: 60º (a), 70º (b), 80º (c), 85º (d), compact ITO thin films (e). (right) Thin films deposited at α=80º under the assistance of the following plasmas: Ar (a), O2 (b), Ar:O2 (100:5) (c), Ar:O2 (100:15) (d). The color scale is indicative of the scattering intensity. Figure 3.6 shows the values of the grain sizes estimated from the Bearing plots of the AFM images that are higher than GISAXS correlation distances (see Table 3.2). Similar differences were found in a previous work for GLAD TiO2 thin films reported in previous work of the group. 3 Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles 92 Figure 3.6.- AFM images (top) of the surface of the ITO thin films, 80º GLAD (a) 80º-O2 plasma (b), 80º-Ar plasma (c), 80º-ArO2 100:5 plasma (b), 80º-ArO2 100:15 plasma (d).Bearing plots (down) determined from the surface roughness estimated of the AFM images of the films. Average grain sizes are approximated to by the height values at 50% of the plot height. Results and discussion 93 Table 3.2 summarizes the values of these correlation distances for the examined films and show that the average separation between nanostructures in the O2 plasma assisted films is slightly larger than in the evaporated films or in the films grown in the presence of a plasma of Ar. Since the GISAXS values are averaged through the whole thickness of the films and the nanocolum width usually increases with this parameter60,61 it is expected that the surface periodicity deduced from an AFM analysis of roughness defined by the nanocolumn termination differs from the GISAXS correlation distances. Table 3.2 Correlation distances determined from GISAXS patterns and surface periodicity determined from the AFM images of the same samples. Figures 3,7 shows additional differences in surface topography between the GLAD and GLAD plasma assisted thin films can be also deduced from a detailed analysis of their normal SEM micrographs showing that at the surface of the plasma assisted films the nanocolumns are interconnected in the form of extended bundles. 3 Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles 94 Figure 3.7.- SEM images of the surface of the ITO thin films, 80º GLAD (top) 80º-O2 plasma (down). The plotted areas in the right side images highlight the surface zones that are interconnected in the two cases. This visual assessment is confirmed by the 2D Fourier transform (FFT) plots of these images reported as insets in Figures 3.2 and 3.3 that depict an asymmetric shape for the plasma GLAD thin films, but a symmetric rounded shape for the 80º thin films deposited in the absence of plasma. This difference sustains that in the plasma assisted films the surface grains are associated in the form of bundles along a preferential direction perpendicular to the incoming material flux. A similar conclusion can be gained by looking to the topographic AFM profiles along these two perpendicular directions. The comparison between the line profiles of the 80º GLAD and the 80º GLAD O2 plasma assisted films along two perpendicular directions clearly confirms the existence of a preferential bundling association direction of nanocolumns in the latter case is showed in the figure 3.8 For the O2 plasma sample, the roughness variation amplitude is higher in the direction perpendicular to the vapor flux than in the parallel direction, while is 500 nm 500 nm 500 nm 500 nm Results and discussion 95 rather similar irrespective of the chosen direction for the sample prepared in the absence of plasma. Figure 3.8.- Left) AFM images of the surface of the ITO thin films, 80º GLAD (top) and 80º- O2 plasma (down). Right) Line profiles measured for these samples along a perpendicular (a) and parallel (b) directions to the material flux as indicated in the AFM images. 3.3.2 Chemistry, crystalline structure and porosity E-beam evaporation of complex materials like ITO may induce changes in stoichiometry due to the preferential evaporation of one of the components. The “as-prepared” ITO thin films deposited in the absence of plasma were amorphous when examined by X-ray diffraction and presented a dark coloration indicating a certain lack of oxygen with respect to the stoichiometric mixed oxide. The plasma assisted films, although also amorphous, were less dark when their growth was assisted with a plasma of oxygen (or mixtures oxygen plus argon). As evidenced by SEM analysis in figure 3.9, after annealing in O2 at 350ºC for 6 3 Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles 96 hours all the GLAD ITO films became transparent and crystalline and kept a microstructure similar to that of the original samples Figure 3.9- Normal SEM micrographs after calcinations treatment of ITO thin films prepared by GLAD at the indicated zenithal angles from 60º to 85º and ITO thin films prepared by GLAD at a zenithal angle of 80º while assisting the growth of the films with different types of plasma. The X-ray diagrams of the annealed samples reported in Figure 3.10 are characterized by well defined peaks attributed to ITO (see for comparison the equivalent diagram for the commercial ITO film) and, in the case O2 or mixtures of O2/Ar plasma assisted films, some small peaks due to SnO2. This minority segregation of tin oxide did not significantly affect the conductivity or optical properties of the films (see below) and no further discussion will be carried out here on this question. Results and discussion 103 Figure 3.14. Normal (left) and cross section (right) SEM micrographs of 80º- GLAD (top) and 80º-O2 Plasma (down)ITO thin films after the electrochemical deposition of gold. The enlarged views included as insets in the normal SEM micrographs show the different shape of the gold aggregated formed on the surface of these two types of thin films. The different distribution of gold in the two kinds of ITO films yields two types of surface plasmon resonances. Figure 3.13 shows the UV-vis spectra recorded with linearly polarized light for two azimuthal orientations of the films defined along the bundling direction in the plasma assisted GLAD films (0º polarization) or the perpendicular to it (90º polarization). For these two orientations, the spectra of the evaporated ITO films depict a similar absorption band at 535 nm attributed to the surface plasmon resonance of the electrochemically deposited gold nanoparticles.50,51,64,65 Conversely, in the plasma assisted film, azimuthally turning the sample leads to significant changes in the absorption spectra. At 0º, the spectrum is characterized by a well-defined 3 Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles 104 resonance peak at 520 nm, while at 90º it depicts a less resolved structure where two brGLAD and little resolved bands at 580 nm and 700 nm can be seen. This change renders a clear variation in the color of the sample as evidenced by the photographs shown in the Figure. A similar behavior has been reported in previous studies on anisotropic gold nanoparticles embedded in GLAD SiO2 thin films.49 This dichroic behavior can be attributed to a different plasmon resonance response when aligning the polarization vector of the light along the longest or shortest dimensions of the gold nanostructures.66 Figure 3.13. UV-vis transmittance spectra recorded with linearly polarized light 0º (dashed line) and 90º (full line) for gold electrochemically deposited on a 80º GLAD ITO (left) and 80º-O2 Plasma ITO (right). The images clearly show the different aspect of the films examined with the two orientations of the polarized light. 3.4 Discussion Such as have been comented in the Chapter 1 , the bundling association of nanocolumns has been neither sufficiently explained nor exploited to get new complex nanostructures. In general, it is more common in metals17 than in oxides13,19,20 and therefore it seems associated with the different mobility and/or trapping probability of the species impinging onto the growing films. Results and discussion 105 In a recent discussion about the heuristic “tangent rule”45 relating the tilting angle of the nanocolumns (β) with the zenithal evaporation angle (α). R. alvarez et all have proposed that this dependence can be explained by assuming that the deposition particles flying close to the surface can be trapped before impinging onto it. Such a trapping mechanism would cast an effective shadowing behind the growing nanostructures that would depend on an effective “trapping parameter” different for each material. Using a Monte Carlo simulation model, it has been demonstrated demonstrated that β decreases for higher trapping probabilities, having a value of approximately 0.3 for GLAD ITO. The decrease in the tilting angle found here for the plasma assisted GLAD films would imply an increase in this parameter to a value close to one. Although the reasons accounting for this increase in the surface trapping probability in the presence of a plasma still require an specific investigation, we can anticipate that the electrical field associated to the sheath formed on the surface of the growing film exposed to the plasma67 must be a critical factor enhancing the trapping probability of the deposition particles. Besides a change in the nanocolumnar orientation, the morphological analysis of the plasma assisted films are characterized by an enhancement in the bundling association of the nanocolumns. The hypothesis of an enhancement of the trapping probability under the action of the plasma can also explain the development of bundling since this phenomenon would not only occur at the nanocolumn tips but also at their sides, whereby favoring their lateral growth and association such as have been explained in chapter 1. Similarly to other GLAD metal oxides prepared at room temperature where only pure ballistic deposition processes are involved1, the “as-deposited” ITO samples were amorphous. To get transparency in the visible, absorption in the NIR and electrical conductivity, the films were annealed in air at 350ºC, a treatment that produces their crystallization and complete oxidation. The crystalline GLAD ITO films presented sheet and in-depth conductivities that differ from those of dense ITO films, a difference that has been related to their microstructure formed by tilted and separated nanocolumns. 3 Anisotropic In-plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma Assisted ITO Thin Films e-Beam Evaporated at Oblique Angles 106 Distinct in-depth and sheet electrical conductivities have been reported for a wide variety of thin film materials68,69 characterized by a different arrangement of the microstructural or molecular units in-depth or laterally. The additional in-plane anisotropy found in the plasma assisted GLAD ITO thin films is an outstanding new feature that we have linked with the association of nanocolumns in the form of bundles and the definition of separated conductivity path-ways along them. The prospects of use of this feature are enormous and in the present work we have exploited a first possibility consisting of the electrochemical fabrication of dichroic films based on the surface plasmon resonance response of gold nanostructures. The different shape of the deposited aggregates found on the GLAD and GLAD plasma assisted ITO films must be related with local variations in the electrical potential at the surface. On GLAD thin films the homogenous distribution of equally sized and round nanoparticles formed on the tips of the nanocolumns (cf. Figure 3.12) must be associated with an enhancement of the electrical field at these sit Conclusions 107 3.5 Conclusions The previous results and discussion have shown that the microstructure of GLAD ITO thin films can be modified by exposing the growing films to a plasma. The observed decrease in the tilting angle and the bundling association of nanocolumns have been attributed to a higher trapping probability of the evaporated particles along their trajectory in the vicinity of the film. Both the GLAD and the plasma assisted GLAD ITO films present different in-depth and sheet conductivities as expected for their porous and nanocolumnar structure. In addition, the anisotropic in-plane conductivity behavior found in the plasma assisted samples has been associated with the definition of preferential conductivity tracks along the bundling direction of nanocolumns. The surface anisotropy of these samples has been utilized in the present work for the electrochemical deposition of elongated gold nanoparticles. The obtained composite films present a dichroic behavior when examined with polarized light. 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Due to the anisotropy of the tilted nanocolumnar morphology, these two rotations are not equivalent. They correspond to a polar around the bundling axis or polar Y (green schemes) and polar around the growing direction or polar X (red scheme). These three different rotations were analyzed with two orthogonal light polarizations, s and p. 4 Highly birefringent Indium Tin Oxide thin films grown by Plasma Assisted Glancing Angle Deposition for advanced labeling applications 124 Simulation of optical response towards polarized light Simulation of the optical response of the ITO thin films was done using WVASE software by considering a uniaxial birefringent model consisting on two layers, one with a high refractive index characteristic of bulk ITO and a second layer with a low refractive index. The optical response of each layer was accounted for by two refractive indices, one in the direction of bundling (nb) and the second along the incoming direction of the flux material during deposition (ng). According to this description, each layer was considered as a uniaxial birefringent medium, with the optical axis oriented along the normal to the bundling direction. In this model, birefringence was interpreted as a difference in the percentage of porosity between the bundling direction and its perpendicular. Porosity was simulated according to Bruggeman effective medium approximation, considering bulk ITO as compact media and the void fraction filled with air. To properly describe the experimental data, each layer was characterized by different absorption coefficients and dispersion values according to the Cauchy theory. The optical axis (THETA angle in WVASE) was fixed to 60º off surface normal, coinciding with the experimental tilting of the nanocolumns in the films. To determinate the orientation of the projection of the optical axis on the samples surface we used a PHI angle of 90º (growing direction) and 0º (bundling direction) Unfortunately, the spectra taken under polar rotation for the PAGLAD films could not be simulated due to software restrictions unable to select the axis coordinates for polar rotations of anisotropic materials. Fingerprint encoding. Experimental Set up 125 A fingerprint was transferred to a glass substrate by using ink. Then a PA-GLAD layer of ITO was deposited and the sample was immersed in ethanol for 20 min to dissolve the ink and release the ITO thin film deposited on it. As a result, the areas previously covered with ink had no thin film material deposited, while the others remain intact. ITO micropattern by Photolithography. Micropatterning by photolithography was carried out in a clean room by spin coating (4,000 rpm/min, acceleration of 6,000 rpm/s² during 60 s) the photoresin 51813 onto PA-GLAD ITO thin films. The samples were annealed during 2 minutes at 115⁰C in air and then exposed during 6.5 seconds to UV light (λ=250nm) using a previously aligned mask. The non UV-exposed resin was then removed by immersing the samples in the developer MF319 during 35 seconds and then rinsing with water. Finally, the non-covered regions of the samples were etched in a solution of H2O:HCl (1:1) during 1.30 min. The resin rests were cleaned with acetone 4.3 Results and discussion. 4.3.1 Microstructure of GLAD and PA-GLAD thin films In the chapter 3 has been demonstrated that the microstructure of GLAD thin films can be modified by assisting their growth with a plasma. Under these conditions it occurs the aggregation or budnling of the nanocolumns in the direction perpendicular to the material flux. Concretely, under oxygen plasma conditions, ITO thin films present an outstanding anisotropic 4 Highly birefringent Indium Tin Oxide thin films grown by Plasma Assisted Glancing Angle Deposition for advanced labeling applications 126 microstructure where bundling association is enhanced in comparison with traditional GLAD ITO thin films. Cross section FIB micrographs of GLAD and PA-GLAD ITO thin films are presented in Figure 4.2 a)-d). Both samples were imaged with SEM, (GLAD (a,c) and PA-GLAD (b,d)) for two different orientations to better visualize the differences between growing (a,b) and bundling (c,d) directions. For the GLAD case (a, c), the intercolumnar distances are small and very similar for the two orientations. By contrast, the PA-GLAD anisotropic samples present a much higher intercolumnar distance along the growing axis (b) than along the bundling direction (d). The micrograph taken along the growing direction (Figure 4.2 b) shows a well-defined tilted nanocolumnar morphology with large intercolumnar distances of around 30- 40 nm, while images along the bundling direction shows a continuous material in the form of a “wall”. Figures 4.2 e-h) show surface SEM views (e,f) and AFM topography images (g,h) of both samples. The topography of the GLAD sample (e) appears as a random granular distribution over the surface. This topography corresponds to the tips of the independent nanocolumns characteristic of the isotropic case. The bundling and growing directions are displayed in the figure along the (x) and (y) lines. The linear profiles along these two directions extracted from the AFM images depict a very similar contour for the GLAD case. By contrast, the topography of the PA-GLAD sample presents a granular appearance highly elongated along the bundling direction. Consequently, the line profile along the bundling direction depicts very broad features separated around 40 nm, in good agreement with the observations by SEM. Results and discussion 127 Figure 4.2. Cross section SEM micrographs obtained by FIB for GLAD (isotropic, a,c) and PA-GLAD (anisotropic, b,d) ITO thin films. Both samples were analyzed under two perpendicular orientations along the growing (a, b) and bundling (c, d) directions. Top view SEM micrographs of GLAD (e) and PA-GLAD ITO (f). AFM images of the ITO thin film surfaces of GLAD (g) PA-GLAD (h). Right) Line profiles measured for these samples along a bundling(x) growing direction(y) 4 Highly birefringent Indium Tin Oxide thin films grown by Plasma Assisted Glancing Angle Deposition for advanced labeling applications 128 4.3.2 Birrefringence of PA-GLAD thin films In chapter 3 we showed that the bundling anisotropy obtained in oxygen PA-GLAD samples is responsible for an in-plane anisotropic conductivity. It is likely that bundling aggregation of nanocolumns also affects the optical properties of the sample. Figure. 4.3 shows the optical transmittance spectra recorded at different azimuthal orientations of the GLAD and PA-GLAD ITO samples with respect to the polarization plane of light. The transmittance of GLAD ITO thin film (top) is independent on the polarization of light, a behavior that is typical for isotropic thin films. However, the structurally anisotropic PAGLAD thin film (bottom) depicts different absorption spectra according to its azimuthal orientation. A lower/higher transmittance was measured when the polarized light was aligned along the growing/bundling directions. This behavior is consistent with the microstructural anisotropy observed in Figure 4.2 and can be accounted for by the following terms: when the bundling direction is aligned/misaligned with respect to the polarization plane of light, the effective refractive index and consequently the dispersion coefficient have higher/lower values, a feature that leads to an decrease/increase in the transmittance at low wavelengths. Results and discussion 129 Figure 4.3. Transmittance spectra recorded with polarized light while azimuthally rotating the GLAD (top) and the PA-GLAD ITO (bottom) thin films (full lines). Simulation of experimental spectra using the WASE software (dash lines) To understand the optical properties of GLAD and PA-GLAD thin films we proceed to the simulation of experimental spectra using the WASE software (see Experimental Section). The simulated transmittance spectra of 4 Highly birefringent Indium Tin Oxide thin films grown by Plasma Assisted Glancing Angle Deposition for advanced labeling applications 130 the azimuthally rotated GLAD and PA-GLAD films are shown in Figure 2 a) and b), respectively. The good agreement between the experimental and simulated curves proves the reliability of the assumptions of the optical model. Table 4.1 shows the refraction indices values deduced from this analysis along the fast and slow axis, the difference between them (birefringence) and the porosities estimated along the bundling and growing directions. It is apparent from this table that while the GLAD sample presents an isotropic behavior with a porosity of 27% (calculated with respect to the bulk ITO), the PA-GLAD sample is anisotropic and presents a birefringence of 0.2 (λ=550 nm) and porosities along bundling and growing axes of 41% and 63%, respectively. To our knowledge, this is by far the highest birefringence obtained for metal oxide GLAD thin films at fixed angles17 or by means of the SBD27 technique, where maximum birefringence values of Δn=0.08 for ITO, or 0.16 for TiO2 have been reported. 4.3.3 Optical anisotropy by rotating along polar angles The alignment of the nanocolumns with respect to the polarization plane of light is expected to affect the thin film optical properties when rotating around the polar axis such as explained in Figure 4.1 Moreover, the tilted and aggregated nanocolumnar structure opens the way to anisotropic Results and discussion 131 optical responses when rotating along the polar axis, such as we have shown in previous works22. The transmittance spectra with polarized light of the GLAD thin films rotated around the polar axis X and Y depict an almost isotropic behavior and very little change in transmittance. Only a little difference in transmittance is found for p/s polarizations, an effect generally observed for any interface between two materials of different refraction index, even for the fused silica/ air interface such as showed Figure 4.3 Figure 4. 3“p” and “s” polarized transmittance spectra for X and Y- polar rotations of GLAD ITO sample. The polar rotation was performed for positive and negative angles from -60º to 60º. 4 Highly birefringent Indium Tin Oxide thin films grown by Plasma Assisted Glancing Angle Deposition for advanced labeling applications 132 Polar rotation of PA-GLAD ITO thin films leads to substantial changes in transmittance. The samples present an isotropic behavior when using either “p” or “s” polarized light and were rotated around the X-axis, showing similar transmittance values for negative and positive polar rotation angles such as shown in Figure 4.4. By contrast, these films present an anisotropic angular behavior when rotated around the Y-axis (c.f.Figure 4.4 a,c In this case, transmittance of “p” polarized light (a) is significantly different for positive and negative rotations. These differences are even higher for “s” polarized light (d). This distinct behavior can be related with the tilted alignment of the columns and the birefringence associated to the bundling both contributing to induce different refractive indices when the nanocolumns are aligned along the incoming direction of light or in another direction. For-30º polar rotation around Y-Axis, the polarization plane of light coincides with the tilting direction of nanocolumns in the PA-GLAD (scheme in Figure 4.1). Meanwhile, the rotation around X-Axis yields similar spectra for positive and negative angles, and therefore do not produce any difference for positive and negative angles. The spectra at -60º to 60º agree with the alignment and misalignment of the tilted nanocolumns for X and Y polar rotations. Results and discussion 139 The giant birefringence (Δn=0.2) of the PA-GLAD thin films sustains their behavior as quarter waveplates if their thickness is adjusted to ca. 500 nm. To make apparent their high birefringence, the optical response of PAGLAD ITO films was compared with a set of samples prepared by traditional GLAD at 80º of ITO, TiO2 and SiO2, as well as vertical columns of ITO obtained by rotating samples during deposition (all the samples have a similar thickness of around 500 nm). It is quite clear in the pictures in Figure 4.6 e), taken with the samples located in between crossed polarizers at 45º, that the anisotropic PA-GLAD ITO film is the only sample which transmits a high intensity of light. Although a detailed study of the birefringence obtained for different materials prepared by traditional GLAD is out of the scope of this work21,23, the comparison shown in Figure 4e) further confirms that the Plasma Assisted GLAD is a very efficient technique to enhance the birefringence of GLAD thin films. it 4 Highly birefringent Indium Tin Oxide thin films grown by Plasma Assisted Glancing Angle Deposition for advanced labeling applications 140 4.4 Conclusions In this work we have developed a methodology to grow highly anisotropic and birefringent ITO thin films by assisting the deposition with oxygen plasma. Besides its relative simplicity, the method is quite reliable and scalable to large areas. The films present a highly anisotropic structure composed by tilted columns with very different columnar separation along the bundling and growing directions. Simulating the optical response of the samples with polarized light and azimuthal rotation with the software WVASE has evidenced a complex birefringent behavior due to the bundling aggregation of nanocolumns. The refractive indexes deduced by modelling had values of 1.48 and 1.68 along the growing and bundling directions, respectively. The resulting birefringence of 0.2 is the highest obtained by GLAD of metallic oxides. In addition, the nanocolumnar tilting orientation and bundling association of nanocolumns give rise to a complex birefringent response when turning the samples around both their polar and azimuthal axis, specially when the rotation modifies the orientation of the nanocolumns with respect to the polarization plane of light (i.e., around the y-AXIS). The deposition process turned out to be fully compatible with the use of masks or photolitographyc techniques and permits the direct fabrication of highly dichroic patterns. This combined methodology opens a new methodology for encryption where the patterned code would only be readable with two polarizers oriented according to a correct geometrical arrangement. . References 141 REFERENCES (1) Granqvist, C. G.; Hultåker, A. Thin Solid Films 2002, 411 (1), 1–5. (2) Schubert, M. F.; Kim, J. K.; Chhajed, S.; Schubert, E. F. Ellison, M. J., Ed.; 2007; pp 667403–667403 – 7. (3) Kim, J. K.; Chhajed, S.; Schubert, M. F.; Schubert, E. F.; Fischer, A. J.; Crawford, M. H.; Cho, J.; Kim, H.; Sone, C. Adv. Mater. 2008, 20 (4), 801– 804. (4) Poxson, D. J.; Kuo, M.-L.; Mont, F. W.; Kim, Y.-S.; Yan, X.; Welser, R. E.; Sood, A. K.; Cho, J.; Lin, S.-Y.; Schubert, E. F. MRS Bull. 2011, 36 (06), 434–438. (5) Yan, X.; Mont, F. W.; Poxson, D. J.; Cho, J.; Schubert, E. F.; Kim, M.-H.; Sone, C. J. Appl. Phys. 2011, 109 (10). (6) Sunde, T. O. L.; Garskaite, E.; Otter, B.; Fossheim, H. E.; Sæterli, R.; Holmestad, R.; Einarsrud, M.-A.; Grande, T. J. Mater. 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R.; Toudert, J.; Borras, A.; Barranco, A.; Lahoz, R.; de la Fuente, G. F.; Frutos, F.; Gonzalez-Elipe, A. R. Adv. Mater. 2011, 23 (7), 848–853. (24) Parra-Barranco, J.; Oliva-Ramirez, M.; Gonzalez-Garcia, L.; Alcaire, M.; Macias-Montero, M.; Borras, A.; Frutos, F.; Gonzalez-Elipe, A. R.; Barranco, A. ACS Appl. Mater. Interfaces 2014, 6 (15), 11924–11931. (25) Hodgkinson, I.; Wu, Q. H.; Collett, S. Appl. Opt. 2001, 40 (4), 452–457. (26) Drzaic, P. S. J. Appl. Phys. 1986, 60 (6), 2142–2148. (27) Hodgkinson, I.; Wu, Q. hong. Appl. Opt. 1999, 38 (16), 3621–3625. Chapter 5 Bending induced selforganized switchable gratings on polymeric substrates Introduction 145 5.1. Introduction After prolonged immersion in water the skin expands producing the universally experienced pruney fingers. Similar surface instabilities are ubiquitous in nature and arise whenever a stiff film coats a compliant substrate. The field of surface instabilities in engineered thin films is in continuous expansion since the pioneered publications by Whiteshides and co-workers on hierarchical surface patterning1-3 inspiring a rich variety of applications such as optical gratings2, nanofluidic manipulation,4-6controlled cell adhesion surfaces,7 and others.8,9 In addition, surface wrinkling is the basis of novel successful methodologies of thin film and coating properties characterization.10-13 Glancing angle deposition (GLAD) of evaporated layers is a wellestablished methodology for the fabrication of tilted oriented nanocolumnar thin films.14-17 By this method the substrate surface is place at a glancing angle with respect to the evaporation source and this particular film microstructure is the result of shadowing effects during the film growth produced by the arriving of a vapor flux at a glancing angle (θ ≥ 60º) with respect to the substrate surface such as have been explained in Chapter 1.14 In some materials like SiO2, the tilted nanocolumns tend to aggregate along a direction perpendicular to the incoming vapor flux forming strongly anisotropic surfaces.18 This process known as bundling19 has been recently utilized for the fabrication of tunable dichroic optical structures.20,21 Herein we studied the process of self-structuring of transparent GLAD SiO2 films deposited on PDMS when subjected to manual bending. The results demonstrate that the properties of the patterns generated by this extremely simply process are directly determined by the film nanostructure. Moreover, it is shown that the characteristics of this process differ from those reported for wrinkled surface structures and cracks patterns generated on PDMS by combining deposition/surface oxidation and uniaxial strains.1-9 A thorough characterization of the system microstructure and its phenomenological optical behavior sustain the 5 Bending induced self-organized switchable gratings on polymeric substrates 146 ample possibilities open by the proposed methodology for the fabrication of foldable optical devices. 5.2 Experimental set up For this work, GLAD SiO2 and TiO2 thin films were prepared by electron evaporation as reported elsewhere21-23 at the glancing angles θ= 60º, 70º, 80º, and 85º. SiO2 and TiO pellets were used as targets for the depositions of SiO2 and TiO2 thin films, respectively. The evaporations were carried out at a pressure of 10-4 torrs of oxygen to obtain stoichiometric films. A calibrated quartz crystal monitor was used to control the individual layer thicknesses for each type of films. The temperature of the sample holder during the deposition of the SiO2 film was measured to be only 2-4 degrees higher than room temperature. PDMS foils and Si(100) were used as substrates, the latter for the control of the evaporation process and thin film characteristics. The nanocolumnar films were studied without any additional treatment after deposition.The PDMS foils with a thickness of 1.5 mm were prepared by mixing the Sylgard 184 (DOW) two part silicon elastomer and degassing and curing the mixture at 80 ºC during 30 minutes. After this process, the foils were cut in pieces of 2 x 2 cm2. FESEM characterization was carried out with a Hitachi S4800 microscope. Planar views of thin films on Si(100) and PDMS were measured as deposited. The cross-sections of films on PDMS were measured after coating the films with a thin gold layer to reduce charging effects during the characterization. The UV-Vis transmission of the films was measured with a Cary 100 spectrophotometer from Varian. Diffracted light pattern intensities were measured with a calibrated visible-light photometer PMMA2100 from Solar Light Co. The curvature (κ) of the coated foils during the diffraction measurements is taken as the inverse of the radius of the circle that most closely approximates the curve in the position of the bending axis illuminated with the laser. The values were Experimental Set up 147 calculated from pictures of the bent film sections in the sample holder during the measurements. 5.3. Results and discussion 5.3.1. Bending induce formation of parallel grooves. In general, cracking of thin films is synonym of irreversible damage, delamination and/or device failure. Nevertheless, under certain well controlled conditions, a tensile stress applied to a film supported on a compliant substrate can induce the development of a regular pattern perpendicular to the resulting strain.24,25 For example, patterns of cracks produced by uniaxial straining of plasma surface oxidized PDMS foils have been used for the fabrication of protein matrices26 and nanofluidic channels.27 A simpler way of applying stress to a PDMS foil is by bending. We have found that when this elastomeric polymer coated with a relatively rigid silicon dioxide layer is bent, irregular cracks are formed to release the accumulated mechanical stress. Figure S5.1 a-c in the Supporting Information (SI) shows a series of micrographs of PDMS foils bent after being coated with continuous SiO2 thin films. These experiments have been conducted with SiO2/PDMS systems fabricated by surface oxidation with an oxygen plasma28 (cf. Figure S5.1a), plasma-enhanced chemical vapor deposition29 (cf. Figure S5.1b) or electron evaporation of silica at normal incidence (cf. Figure S5.1c), respectively. In the three cases, the slight bending of the coated foils produce the formation of a series of irregular cracks whose number irreversibly increases with additional bending operations. This accumulated cracking is neither controllable nor reproducible and typically yields local delamination after a small number of bending events. As it will be shown, bending a PDMS foil coated with a nanocolumnar SiO2 thin film prepared by physical vapor deposition at glancing angles renders quite different results. 5 Bending induced self-organized switchable gratings on polymeric substrates 148 The room temperature GLAD deposition of SiO2 on PDMS foils (hereafter called G-SiO2/PDMS) yields anisotropic nanostructured thin films characterized by a tilted columnar microstructure as shown in Figure 5.1a. The figure shows as example the SEM images of a film deposited simultaneously on Si(100) and PDMS foil substrates. Apart from the charge effects that render difficult the observation of films on dielectric substrates such as PDMS by FESEM, the images demonstrate a similar nanocolumnar tilted arrangement in both cases. The determined tilted angles of the nanocolumns are ~55º ±2º and ~53º ±2º for the film deposited on Si(100) and PDMS, respectively. Thus, it can be concluded that the PDMS substrate does not modify the properties of the glancing angle SiO2 nanocolumnar thin films. In the first chapter was explained that the nanocolumns tilting angle and porosity of oxide thin films deposited by GLAD increase gradually with the deposition angle while, simultaneously, the number of nanocolumns per unit area decreases.17,23 Another characteristic microstructural effect sometimes encountered in this type of thin films is the preferential association of the nanocolumns in a direction perpendicular to the arrival flux of material.19-21 This association is usually designed with the term bundling and it is known to enhance the surface anisotropy of the system. Clearly, in the G-SiO2 thin film such bundling association of nanocolumns (see the example in Figure 5.1a) originates a strong film in-plane anisotropy.17,19