Remote plasma assisted fabrication of functional organic and hybrid thin films and supported nanostructures

Remote plasma assisted fabrication of functional organic and hybrid thin films and supported nanostructures María Alcaire Martín Supervisors: Ángel Barranco Quero & Ana Isabel Borrás Martos Instituto de Ciencia de Materiales de Sevilla (US-CSIC) Universidad de Sevilla A thesis submitted for the degree of PhD 2015 INDEX ii 1 INTRODUCTION 1.1 Introduction to plasma 4 1.2 Introduction to Plasma Enhance Chemical Vapour Deposition (PECVD) and Plasma Polymerization 8 1.3 Remote Plasma Assisted Vacuum Deposition (RPAVD) 13 1.4 Fundamentals of 1D organic nanowires 33 1.5 Introduction to the growth and optical properties of ZnO 40 1.6 Objectives 44 REFERENCES 47 2 RESUMEN EN ESPAÑOL 2.1 Introduction 58 2.2 Láminas delgadas orgánicas conformales para aplicaciones en electrónica molecular 60 2.2.1 Experimental 62 2.2.2 Resultados y discusión 62 2.3 Mojado y retardo en la congelación de recubrimientos de adamanatano: de la estrucutura en lámina delgada a redes 3D 62 2.3.1 Experimental 62 2.3.2 Resultados y discusión 62 2.4 Emisión multicolor y láser en nanocomposites de DCM y adamantanto 62 2.4.1 Experimental 62 2.4.2 Resultados y discusión 62 2.5 Tratamientos con plasma débil de nanohilos orgánicos: una vía para la fabricación de heteroestructuras orgánicos 1D y la síntesis de patrones de diseño de nanoestructuras inorgánicas 1D 62 2.5.1 Experimental 62 2.5.2 Caracterización de heteroestructuras orgánicas 1D 62 2.5.3 Resultados y discusión 62 2.6 Láminas delgadas porosas y estructuras 1D de óxido de cinc mediante procesado por plasma remoto de ftalocianina de cinc 62 2.6.1 Experimental 62 2.6.2 Resultados 62 REFERENCIAS 62 iii 3 CONFORMAL DIELECTRIC ORGANIC THIN FILMS FOR MOLECULAR ELECTRONICS 3.1 Introduction 112 3.2 Experimental Setup 113 3.3 Results and discussion 114 3.4 Conclusions 123 REFERENCES 125 Supporting Information 128 4 WETTING AND ANTI-FREEZING PROPERTIES OF ADAMANTANE COATINGS: FROM THIN FILMS TO 3D NETWORKS 4.1 Introduction 130 4.2 Experimental Set Up 137 4.3 Results and discussion 139 4.3.1 Temperature effects on wetting 139 4.3.2 Water condensation and freezing on smooth flat adamantane RPAVD 142 4.3.3 Water condensation and freezing on adamantane RPAVD grown on rough surfaces.. 153 4.3.4 Water condensation and freezing on adamantane RPAVD grown on highly hydrophobic 1D nanostructures 158 4.3.5 Water condensation and freezing on adamantane RPAVD grown on 3D nanonetworks 163 4.3.6 Condensation and freezing on adamantane RPAVD on metal substrates effects.... 170 4.4 Conclusions 172 REFERENCES 174 5 MULTICOLORED EMISSION AND LASING IN DCM-ADAMANTANE PLASMA NANOCOMPOSITES 5.1 Introduction 180 5.2 Experimental Setup 182 5.2.1 Materials 182 5.2.2 Plasma assisted vacuum deposition procedure 183 5.2.3 Thin film characterization 185 5.3 Results and discussion 189 5.3.1 Thin film composition and microstructure 189 5.3.2 Optical properties 192 5.3.3 Steady state and time resolved luminescence 195 5.3.4 ASE experiments 200 5.3.5 DFB laser experiments 202 iv 5.4 Conclusions 206 REFERENCES 208 6 SOFT PLASMA PROCESSING OF ORGANIC NANOWIRES: A ROUTE FOR THE FABRICATION OF 1D ORGANIC HETEROSTRUCTURES AND THE TEMPLATE SYNTHESIS OF INORGANIC 1D NANOSTRUCTURES 6.1 Introduction 212 6.2 Experimental Setup 213 6.2.1 Organic Nanowires formation by PVD 214 6.2.2 Soft Plasma treatments 215 6.2.3 Secondary Organic Nanowires formation by PVD 215 6.2.4 Characterization of 1D organic heterostructures 215 6.3 Results and discussion 216 6.3.1 Heterostructured hierarchical NWs formed by oxygen plasma 216 6.3.2 Metal-Organic Hybrid NWs formed by SOFT plasma processing 222 6.3.3 From Hybrid to metal and oxide NWs 227 6.4 Conclusions 229 REFERENCES 231 7 HIGHLY POROUS ZNO THIN FILMS AND 1D NANOSTRUCTURES BY REMOTE PLASMA PROCESSING OF ZN-PHTHALOCYANINE 7.1 Introduction 236 7.2 Experimental Setup 237 7.2.1 Deposition of ZnPc thin films and ONWs 239 7.2.2 Deposition of Au by sputtering 240 7.2.3 Soft etching by ECR-MW oxygen plasma 241 7.2.4 Characterization methods 241 7.3 Results and discussion 242 7.3.1 Organic thin films and single crystal nanowires by Physical Vapor Deposition of ZnPc 242 7.3.2 Oxygen plasma etching of ZnPc thin films 245 7.3.3 Oxygen plasma etching of ZnPc thin films 258 7.4 Conclusions 264 REFERENCES 266 8 GENERAL CONCLUSIONS 269 v A 1 ADAMANTANE RPAVD: FROM THIN FILMS TO 3D NETWORKS A1.1 Introduction 276 A1.2 Experimental Setup 278 A1.3 Results and discussion 281 A1.3.1 Characterization of RPAVD thin films 282 A1.3.2 Adamantane RPAVD in the reactor: effects of pressure, power of MW plasma source and growth rate on 298 A1.3.3 Effect of substrate microstructure on ADAMANTANE RPAVD thin film synthesis 305 A1.4 Conclusions 317 REFERENCES 318 A2 OXYGEN OPTICAL SENSING WITH NANOSTRUCTURED ZNO THIN FILMS A2.1 Introduction 322 A2.2 Experimental Setup 323 A2.3 Results and discussion 325 1 Introduction 1.1 Introduction to plasma 8 frequency of the electromagnetic field, i.e., the gyromotion of the free electrons is in resonance with microwave, the system is in electron cyclotron resonance condition. The main advantages of ECR plasma are the low operating pressures and the strong microwave coupling due to the magnetic confinement that generates higher electron temperatures. On the other hand, this magnetic confinement allows the use of ECR plasmas in remote configuration with respect to the reactor for the synthesis of thin films. This avoids the damage induced to sensitive substrates by energy ions characteristics of discharge plasmas.14, 16 In addition, remote plasma configurations decrease the fragmentation processes (strength discharge decreases as coupling region distances) in the synthesis region as that produced in a plasma enhanced chemical vapour deposition (PECVD) reactor. For this reason, the work developed in the present Thesis has been carried out in a remote plasma reactor (described in the next section). This reactor design is also employed for polymers superficial treatments.17, 18 1.2 Introduction to Plasma Enhance Chemical Vapour Deposition (PECVD) and Plasma Polymerization 1.2.1 Plasma enhance chemical vapour deposition (PECVD) Chemical vapour deposition (CVD) process is a vacuum technique for the deposition of thin films. It consists of the reaction among one or several gaseous o vaporized compounds to produce a solid product onto the substrates. This solid product is a new material fabricated from the original reagents, as opposed to physical vapour deposition (PVD) process, which a vapour of a previously synthesized material is condensed. The CVD reactions are produced by the activation of the precursor gases in order to raise the reaction rates, given that it is so low under normal conditions (pressure and temperature). The kinetics and reaction mechanisms are difficult to predict because the CVD reactions are highly complex. In view of this, the working procedure involves Introduction 1 9 previous empirical evidences and analysis to achieve the desired final material. Once it is analysed, the deposition technique facilitates to control the composition from the appropriate proportion of reactives/reagents to the desired specific properties of the resulting final material. The different activation methods of the reaction define the different CVD techniques: thermal activation, plasma or discharge activation, photon activation.15 In this thesis, we have developed new materials by plasma-enhance CVD (PECVD). In a PECVD process, an intense electric field produces a discharge that activates the molecules into the reactor, leading them to more reactive states. Consequently, the deposition rate is increased noticeably at given temperature, which enables depositions at lower temperatures than other synthesis procedures. This makes the technique compatible with precursors of low melting point being a property predominantly important for applications. The PECVD technique was initially developed in microelectronics (synthesis of conductors, semiconductors and insulators) and subsequently was spread to other fields, such as optics, ceramics and protective coatings and sensors.15 In PECVD, the typical operating pressures are between 10-4 and atmospheric pressure. The supply of energy is generally used to generate and maintain the concentration of active species because lifetimes of plasma species are very short.15 PECVD technique using microwave (MW) power source to excite and sustain plasma involve some advantages over plasmas generated by other kind of discharges (RF or DC). MW plasmas achieve higher ionization degrees, lower and broad range of operating pressures and not require the use of electrodes in the reactor. The discharge is produced by an electromagnetic wave excited at a frequency of 2.45 GHz, which is generated by a MW power source and guided by a waveguide into the reactor.14, 15 In spite of its benefits, PECVD shows two main disadvantages, impurities in the end material and damage by plasma radiation. Both, the 1.2 Introduction to Plasma Enhance Chemical Vapour Deposition (PECVD) and Plasma Polymerization 10 impurities in the thin film derived from the decomposition of reactive gases in radicals (-NH,-OH) and plasma radiation contribute to the degradation of the deposited material properties.15 1.2.2 Plasma polymerization Plasma polymerization is a well-established technique of synthesis that involves the fabrication of a polymeric material from an electric discharge in an organic gas at atmospheric or low pressure, typically in the form of thin film.19 In a few cases, inorganic gases and compounds may also form organic plasma polymers, reason why the formation of amino acids in the primeval atmosphere of Earth is explained.20-22 As affirmed by A. Grill, “Plasma polymerization is essentially a PECVD process resulting in the deposition of an organic polymer”14. Plasma polymers establish a new type of material, different from conventional polymers and inorganic materials. In contrast with conventional polymers, they do not exhibit regularly repeating ensembles. The mechanism of plasma polymerization differs strongly from known conventional chemical polymerization mechanisms. Their monomer chains are shorter and randomly rearranged causing a high degree of crosslinking. Therefore, plasma polymerization is a complex process in which different chemical reactions take place between plasma and surface species, as well as among each one19 Figure 1.2). Historically, the plasma polymerization process mechanisms has been analysed and modelled20, 23-30 in many instances depending on the monomers and other parameters of synthesis. 14, 19, 20, 24, 31 During the process of low-pressure glow discharge plasmas, the following plasma-induced deposition situations can be distinguished:20  Radical and ionic chain growth polymerization mechanism. Plasma-produced radical fragments, radical-sites at solid surfaces or gaseous monomers can initiate a chemical process as Introduction 1 11 classical chain-growth polymerization by continuous addition of monomers. Similar ionic chain growth mechanism may also occur in the plasma. But low pressure hinders chain propagation.  Ion-molecule reactions, based on charge transfer between atoms or dissociative charge transfer between molecules.  Fragmentation-(Poly) Recombination (plasma polymerization). Plasma induces the fragmentation of the original monomers to atoms and small fragments by a steady bombardment of the monomer molecules and the polymeric material surface with high-energy particles and UV irradiation. It entails the following mechanisms:  Monomer fragmentation-poly recombination  Monomer conversion to polymer-forming intermediates partially–defined structural elements  Co-monomer fragmentation-recombination  Radical chain growth copolymerization  Graft copolymers. Chemical grafting onto radical sites or functional groups of plasma polymers or plasmaexposed polymer surfaces H-abstraction, double bond formation, radical formation, crosslinking or degradation, loss in supramolecular structure and oxidation via peroxide formation are the resulting products in the course of a plasma polymer deposition. The obtained materials present a disordered structure formed by poly-recombination of these fragments (radical) and atoms that depends on the intensity and energy of the species bombarding the growing film.19, 20, 32 Moreover, the multiple poly-recombination processes produce enthalpy dissipation, which can lead to local thermo-chemical processes contributing to end plasma polymers. Endothermic dehydrogenation of monomers and plasma 1.2 Introduction to Plasma Enhance Chemical Vapour Deposition (PECVD) and Plasma Polymerization 12 polymer is an example of these reactions that lead to the formation of double and triple bonds. On the other hand, radicals trapped in the deposited plasma polymer can react with oxygen when it is exposed to the ambient.20 Plasma polymer analyses have proved its structural and compositional irregularity,33-45 with only a few structural elements derived from the monomer in the end deposited material. In general, aromatics rings are less represented as expected in plasma polymer,20, 33, 34, 37 while non–aromatic double and triple bonds33, 34 and high concentrations of CH3-groups by hydrogenation of broken chains,36, 46-49 are observed.20 Plasma polymers do not manifest a sharp melting point. On the contrary, a broad regime of softening begins to certain temperature. The films sometimes become darker during heating. 20, 40 Figure 1.2. Processes involved in plasma polymerization – Scheme of energy distribution to collision and radiation processes during plasma polymerization deposition.20 Introduction 1 13 Finally, the relevance of plasma polymers lies in their applications as coatings of solids with corrosion-inhibition, abrasion-resistance, chemical, permeability and optical properties.19 1.3 Remote Plasma Assisted Vacuum Deposition (RPAVD) As already mentioned, in a conventional plasma polymerization process, the vapour pressure of an organic precursor is dosed into a plasma discharge for the synthesis of a solid thin film. During this complex process, the breaking of the starting monomer takes place, as well as different chemical reactions between the resulting species (reactive and stable groups), fragmentation and reorganization processes.14, 16, 19, 50, 51 The resulting materials are highly crosslinked organic films formed by the plasma fragmentation and recombination of the precursor molecules. Generally, crystalline and supramolecular structures are absent in plasma polymers, while the formation of linear molecules and oligomers are most likely20. Otherwise, plasma polymers that retain original bond structures or functional groups of starting precursor groups have also aroused interest for researchers since films can present the combination of plasma polymer properties with monomer functionalities. Experimentally, these approaches have consisted in the use of higher operating pressures, the restriction of exposure to the plasma by pulse discharges and other approaches.11, 20, 52, 53 In last few years in our laboratory, a synthesis technique called remote plasma assisted vacuum deposition (RPAVD) has been developed for the deposition of functional thin film as dye containing plasma polymers.4-10, 54 The scheme of the RPAVD reactor is showed in Figure 1.3. The process combines a remote plasma discharge with vacuum deposition of an organic solid monomer on substrates placed in the afterglow, see Figure 1.3. During RPAVD process, a 1.3 Remote Plasma Assisted Vacuum Deposition (RPAVD) 14 plasma polymer matrix retaining a substantial amount of whole functional precursor molecules are grown on the substrates. In contrast to conventional plasma polymerization, these precursor molecules have not been completely fragmented by the discharge due to a fine regulation of the interaction between the low power microwave plasma and the precursor molecules sublimated in the afterglow region. This is achieved by the simultaneous adjustment of different deposition parameters, namely MW power, the total pressure, the sublimation rate and the geometrical arrangement used for the deposition. From previous results, the distance between the plasma and the growing film (z in Figure 1.3) is one of the main operating parameters of RPAVD process, given that it controls the interaction between plasma discharge and precursors molecules.6-8, 11 This distance, limiting the plasma-precursor interaction, makes largely versatile the deposition process, inasmuch as gradually regulates the film properties. Only modifying it, the method provides a wide range of films, from a highly crosslinked plasma polymer without the retention of original precursor monomers (deposition at the glow discharge), to plasma films containing a controllable amount of active dye molecules (deposition in afterglow).6-8, 11 Increasing more the distance z, the films are seemed to those prepared by vacuum deposition (without plasma). Furthermore, the influence of the deposition rate was also investigated.11 In general, we have founded a relationship among the operating parameters (the distance between the plasma and the growing film, pressure and deposition rate). Thus, increasing the distance is equivalent to reduce the power and to increase the deposition rate. In brief, the main objective of the use of RPAVD process is to fabricate thin films that provide less cross-linked structures than plasma polymerization technique and more highly functionalized materials for many applications by the incorporation of functional groups of the monomers. Hence, the method has Introduction 1 15 been used for applications as photonic layers with tailored luminescent and optical properties,8-10, 54 and photonic gas sensors.6, 7, 11 1.3.1 Experimental system The experimental setup used for the deposition has been described repeatedly.6, 55 A scheme of the system is shown in Figure 1.3. It consists of a chamber with two zones, the plasma zone and the deposition zone. In the plasma zone, a discharge is produced by MW applied to the plasma chamber through a flat Pyrex window, and is sustained under ECR conditions thanks to a set of magnets surrounding the chamber. In the deposition zone, there is a substrate in a remote configuration with respect to the plasma zone and two different Knudsen cells (evaporators) to sublimate the dyes. Additionally, an external and heatable glass ampoule is connected to the deposition chamber in order to supply a regulated vapour pressure of an additional precursor inside the reactor. The system is pumped up to 10-6 mbar base pressure before the deposition. The distance between the sample-holder and the plasma zone, z, (a critical parameter for the control of the process as already mentioned) has been measured according to the Figure 1.3. The pressure of the system was controlled by an automatically regulated pressure controller connected to a capacitance pressure gauge (Baratron). Gases were dosed to the plasma chamber using a calibrated mass flow controller (MKS). The growth rate was controlled by a quartz crystal microbalance located in the same plane than the substrates. The deposition and/or treatment experiments have been carried out by using Ar and O2 as the plasma gas. The microwave power range was 30-270 W and the gas pressure during the discharge 10-1-10–3 mbar. The reflected power measured was less than 10 W. It was minimized by achieving a good coupling between the rectangular and the circular waveguides. This ECR system was characterized previously by two diagnostics methods, a Langmuir probe and an electrostatic energy 1.3 Remote Plasma Assisted Vacuum Deposition (RPAVD) 16 analyzer.56 Later, the plasma characterization by optical emission spectroscopy (OES) showed the fragmentation of the evaporated dye molecules by interaction of the dye molecules with the electrons of the plasma. During the plasma deposition of Rh6G with Ar as the plasma gas, C2, CN, and CHx species were detected in the plasma. C2 species can be considered as an intermediate species of the formation and/or etching of carbon materials with plasmas of Ar. The detection of C2 together with CN and CHx indicated that the dye molecules coming from the Knudsen cell are effectively activated and a part of them even fragmented by interaction with the plasma. In general, an increase in the pressure of the plasma gas produces a decrease in the kinetic energy of the plasma electrons.6 Measurements of the electron density as a function of pressure showed a maximum in the electron density at approximately 7 × 10–3 Torr. The ion flux was measured as a function of the working pressure in the centre of the chamber. It was higher under low pressures, decreasing rapidly as the pressure or the distance, z, increased. These measurements also showed that the ion energy distribution function (IEDF) shifts to lower energies for higher pressures.56 Plasma polymerization occurs while the dye is being sublimated. The deposition process was controlled by a quartz crystal monitor placed besides the substrate in the deposition zone. Typical growing rates between ∼2–5 nm min–1 were achieved by controlling the heating of the Knudsen cells. These relatively low growth rates were selected to efficiently control the deposition of thin films of ∼50–500 nm.6 Introduction 1 17 Figure 1.3. RPAVD reactor – Scheme of the main components of a remote plasma assisted vacuum deposition reactor for the fabrication of organic thin films and nanostructures. 1.3 Remote Plasma Assisted Vacuum Deposition (RPAVD) 24 The dye containing thin films were also studied by spectroscopic ellipsometry.7, 10, 11 The analysis of perylene thin films demonstrated that the refractive index can be adjusted as a function of the plasma conditions using a single dye.7 In addition, these studies evidence that films are transparent in the region corresponding to the fluorescence emission and at higher wavelengths because the extinction coefficient, k, decreases for wavelength longer than the value for the maxima absorption. On the other hand, the analysis of copolymerization between two precursors (perylene and adamantane) indicated that the refractive index, n, of the films can be also adjusted in function of the composition.10 Figure 1.8 shows the differences among the refractive index measurements for RPAVD films deposited from adamanatane, perylene and different concentrations of perylene-adamantane. Later ellipsometry studies with 3HF also suggest that the optical properties can be finely adjusted within a wide range for the suitable integration of the films in photonic structures.11 Figure 1.8. Refractive index for a set of perylene−adamantane (PA) – The refractive index have been determined by variable-angle spectroscopic ellipsometer (VASE) remote plasma deposited films prepared from: lowest to highest concentration of peryleneadamantane (PA-0.5, PA-1.5, PA-3), only adamantane (RPD-A) and only perylene (RPD-P).10 500 600 700 800 900 1000 1100 1200 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 n RPP Perylene RPP Adamantane PA-0.5 PA-1.5 PA-3 Wavelength (nm) Introduction 1 25 Figure 1.9. Photoluminescence of perylene thin films – On top, the graphic shows normalized photoluminescence spectra of a set of perylene RPAVD thin films grown under different experimental parameters (i.e., distance between sample-holder and the plasma zone, and MW power source). The spectrum of a sublimated sample is included as a reference. An image of these samples illuminated with a UV lamp (λ=365 nm) in a dark room is shown below.7 Works using perylene and 3HF dyes as sole organic precursors have confirmed the growth of luminescent polymeric thin films by RPAVD. The control of the interaction between sublimated perylene dye and plasma species during the preparation of the films is possible by varying experimental conditions. Therefore, we can tailor the dispersion degree of the dye in the film matrix selecting the experimental conditions.7, 11 These distinct types of films presented different properties. As an illustration the thin films containing 1.3 Remote Plasma Assisted Vacuum Deposition (RPAVD) 26 dispersed perylene molecules (lower dye-plasma interaction) did not disperse the light and showed blue light luminescent emission (Figure 1.9). While, the luminescent emission shifts to longer wavelengths, i.e., orange luminescent emission of light, as the percentage of nanoagreggates increases7 (greater dyeplasma interaction). 1.3.2.4 Sensor response Immediate applications of the perylene dye containing thin films are the use of them as nanometric long pass optical filters, due to the complete light absorption at wavelengths lower than 470 nm (Figure 1.7).7 Moreover, thin films with a high concentration of perylene molecules within the matrix formed a by molecular fragments of the dye were proposed for optical nitrogen dioxide sensing because of the changes in colour and fluorescence emission observed when they are exposed to NO2. These films depicts a good choice for the development of integrated photonic NO2 gas sensors such as optical dosimeter for the remote monitoring of the degradation of high explosive materials7, 10, 61. Some limits were found in the detection sensitivity (<15 min of exposure to a NO2/air flow of 0.5 ppm). The aforementioned modification of the perylene RPAVD method by adding adamantane molecule during the process also brought improvements in the sensitivity of the films. Plasma polymerized adamantane films are transparent in the visible range and NO2-permeable. For this reason, adamantane molecules were selected as precursor of the polymeric matrix for this sensing function. Thus, a set of photonic sensors films with different sensitivities were fabricated by adjusting the optically active molecule of perylene concentration within the transparent polymeric adamantane matrix films.10 These materials provide the possibility to adapt the sensitivity to required environmental necessities. Figure 1.10 shows the complete process from the sensing film deposition at wafer scale to the real implementation of Introduction 1 27 these thin films in a photonic sensor prototype for measuring the pollution in a traffic tunnel. Recently, a new kind of photonic organic thin film was synthesized from a flavonoid compound, 3-hydroxyflavone (3HF), by RPAVD technique. These films were implemented as active components in photonic structures used for the fabrication of devices such as UV sensors, UV-to-visible wavelength shifters and UV.9 Figure 1.10. Process for the fabrication and implementation of a photonic system on a chip based on perylene dye for sensor application – The picture gathers step by step the process from de fabrication of the sensor to its implementation in real conditions measurements: (a) wafer scale synthesis of sensing photonics chips from RPVD technique, (b) chips encapsulation at wafer scale, (c) prototype chip, (d) integration of the chip in in NO2 detection platform, (e-f) the system installed in a real scenario (urban tunnel).60 1.3 Remote Plasma Assisted Vacuum Deposition (RPAVD) 28 Polymeric thin films from 3HF by RPAVD have resulted transparent in the visible range (at wavelengths higher than 390 nm), absorbent in the UV and shown an emission at the visible range (green emitter).11 Figure 1.11 shows a 3HF RPAVD film and quartz substrate without (a) and under (b) UV illumination. 3HF molecules in solution and under UV illumination show two types of fluorescence emission, blue and green emissions. The blue emission was assigned to the normal form of 3HF. Large Stoke shifted emission in the green was associated to the tautomer generated through an excited state intermolecular proton transfer mechanism (ESIPT), in which, when the molecules are in the excited state, one hydroxyl proton is transferred to a neighbouring carbonyl group. It is a very efficient and non-radiative energy dissipating mechanism9, 11, 62-65 and substantially sensitive to the chemical environment surrounding the 3-HF molecules, particularly to the formation probability of H bonds with nearby molecules or groups. Therefore, the relative intensity of the blue and green emission bands can be controlled by the hydrogen bond donor ability of the environment.62, 64 Fluorescence time resolved analyses of RPAVD films indicated that 3HF molecules were trapped in indifferent chemical environments. In some cases, green emission was partially quenched. This also modifies the efficiency of the ESIPT process. These two factors affect the quantum yield of the trapped dye molecules. According to this interpretation, the fluorescence efficiency of the films could be further enhanced by the fine adjustment of the chemical structures forming the plasma polymer host matrix. Moreover, the emission is noticeable even under ambient light conditions due to the high light absorption of these nanometric films and the large Stokes shift (i.e., absence of reabsorption).11 The large Stokes shift (180 nm) of the green emission of the samples illuminated in the UV region (350 nm) and the lack of fluorescence under visible irradiation made these films good candidates as wavelength shifters and UV photodetectors. This application has been Introduction 1 29 successfully proved in films that remain stable the fluorescence under UV irradiation of 156 µWcm-2 after pre-irradiation treatment, and were capable to detect very low UV light intensities below 10 µWcm-2 (close to the detection limits of some commercial detectors), see Figure 1.11. Figure 1.11. Photoluminescence under UV illumination of 3HF thin films- (a,b) Images of both 3HF RPAVD thin film and the substrate used in the deposition (as reference) before and after illuminating with a UV lamp (λ=365 nm). (c) Emission spectra as a function of the UV irradiance. The graphic on the right shows the stability of the emission intensity at 530 nm for the samples when they are illuminated during 10 min with different UV intensities.11 1.3 Remote Plasma Assisted Vacuum Deposition (RPAVD) 30 1.3.2.5 Lithographic processing Additionally, separated studies demonstrate the possibility of processing plasma polymeric films using shadow masks, either during the deposition process or in an after-etching process.7 In this perspective, the method enables the selective deposition of the surfaces accordingly the implementation in photonics devices9 and encryption applications for labelling. Figure 1.12 (b, c) shows the direct observation of the fluorescence light paths in two photonic structures incorporating the 3HF films: a Fabry–Perot resonator (b) and a ring resonator (c).9 The Fabry-Perot structure is based on a silicon nitride multimode strip waveguide with a width of 10 μ m and a height of 300 nm. The lower cladding is a 5 μm-thick SiO2 layer grown on Si. The planar F–P resonator consists of Si3N4/air pairs separated 90 nm with a cavity height of 140 nm. The dye layer was selectively deposited through a mask, only onto the resonant microcavities. The waveguide and resonator structure are designed to transmit the 3HF emission up to the output, even if there are several transmission modes. Figure 1.12 (b) shows a fluorescence microscopy image of the 3HF active area covering the resonator structure. The fluorescence light is selectively transmitted through the wave guide and the transmission modes of the fluorescence emission are modulated by the F–P resonant cavity at the edge of the output waveguide. The second photonic structure consist of a disk resonator and a Si3N4 strip forming a high refractive index waveguide and a Si3N4 strip forming a high refractive index waveguide. The waveguide cross section was 600 nm and its height was ≈ 80 nm, a topology that only permits the propagation of the fundamental transverse electric (TE) mode. The lower cladding was a layer of SiO2 with a thickness of 5 μ m on a Si substrate. The upper cladding was also SiO2. A window through this SiO2 upper cladding was open over the resonant disk and nearby waveguide to enable that the active 3HF film is selectively Introduction 1 31 Figure 1.12. Processing and photonics implementation of RPAVD films –Fluorescence microscopy images taken of: (a) 3HF films deposited and exposed to oxygen plasma (sample at the centre) through masks designing a squared pattern and two lines of a waveguide structure, (b) surface view of a Fabry - Perot resonator coated with 3HF RPAVD film. (c) surface view of a resonator disc of 120 µm diameter coated with 3HF RPAVD film. It is remarkable the possibility of observing the emission at the device edges which were not coated 1.3 Remote Plasma Assisted Vacuum Deposition (RPAVD) 32 deposited onto this zone. The design and dimension of this photonic structure is such that the waveguide can couple the green fluorescence emitted by the active 3HF film. The green light is collected and transmitted up to the waveguide output in the chip edge. This preferential distribution of the fluorescence light supports the conclusion that the field intensity is higher in the disk and is transmitted through the waveguide out of the 3HF-coated region up to the border of the structure, as shown in Figure 1.12 (c). 1.3.2.6 Wafer scale and sensitive substrates Other relevant achievements have emerged in the development of RPAVD technique. The process enables wafer-scale fabrication for industrial application such as microelectronics and optoelectronics10 (Figure 1.13). An example of the application of RPAVD films at wafer scale has been introduced in previously section. Perylene-adamantane RPAVD thin films have been implemented in a sensor photonic system of NO2. Finally, the deposition of these films on sensitive substrates by RPAVD9 has yielded an improvement in plasma deposition process. This technique avoids the common UV irradiation and ion bombardment damages of the surfaces produces by plasma processes that involve a disadvantage for these applications11. Figure 1.13 shows some examples of the luminescent RPAVD films coating sensitive substrates such as paper, polycarbonate and adhesive tape. Thus, the process involves soft plasmas providing the opportunity to use a wide range of substrates which are not available in other plasma techniques of deposition. Introduction 1 33 Figure 1.13. Wafer scale and sensitive substrates in RPAVD films – Images of perylene RPAVD films at wafer scale and 3HF RPAVD films coating several sensitive substrates, such as polycarbonate, PMMA, adhesive tape or a five euro bill. 1.4 Fundamentals of 1D organic nanowires One-dimensional (1D) nanomaterials of π conjugated organic molecules in electronic and optoelectronic devices at nanoscale become of great interest in nanoscience and nanotechnology. 1D nanostructures from inorganic materials have much thoroughly studied on inorganic materials. However, organic materials, less researched up today, show many advantages such as unlimited choices of molecular structures for property optimization, or high flexibility and low cost of materials fabrication, or high photoconductivity, or nonlinear optical effects. In fact, these materials have already used in the form of thin film as field-effect transistor (FETs), light-emitting diodes (LEDs) and photovoltaic 1.4 Fundamentals of 1D organic nanowires 40 Figure 1.17. Fluorescence microscopy of hybrid core@shell NWs – MePTCDI@ZnO system micrograph recorded by using an excitation wavelength of 550 nm.102 In this thesis, the use of organic nanowires is addressed in different ways. On the one hand, from organic a metallorganic nanowires, the synthesis and characterization of hierarchical (branched) and hybrid Metal-NPs/organic supported NWs by plasma processing is developed (Chapters 7, 8 and annex 1). And, on the other hand, in order to take a step from the film to the nanowire and due to their interesting properties, nanowires are also used as degradable scaffolds or templates for the fabrication of RPAVD supported nanostructured materials. The combination of plasma polymers grown by RPAVD and these organic nanotemplates are presented in chapters 3, 4, 5 and 8. 1.5 Introduction to the growth and optical properties of ZnO ZnO shows an ultraviolet luminescence band due to a wide band gap (~3.37 eV at room temperature) and high exciton energy (60meV)103. For these optical properties, it is used in the realization of electroluminescent diodes or room temperature UV lasers104. Besides the UV emission, the Introduction 1 41 photoluminescence spectra at room temperature of ZnO present other visible bands due to intrinsic and extrinsic defects105. These surface defects of ZnO are chemically sensitive to several gases, making it suitable for sensing aplications103, 106. In this connection, nanostructured materials (two examples are included in Figure 1.18(a)) introduce a higher surface-to-volume exposition, thus greater chemical sensitivity in ZnO nanostructures is expected104. Indeed, the fabrication and structural characterization of different ZnO nanostructures (nanoparticles, nanoribbons, nanowires, nanorods) by several fabrication methods have been extensively studied up to now103, 104. Figure 1.18. ZnO rods and photoluminescence spectra at different temperatures- (a) Representative scanning electron microscopy images of ZnO nanorods. (b) PL spectra at 7, 60, 140, 210 and 300 K as a function of the wavelength of ZnO nanorods107. 1.5 Introduction to the growth and optical properties of ZnO 42 Variable-temperature photoluminescence measurements can provide information about the structural and optical properties of ZnO. On the one hand, the number of observed bound-exciton peaks in ZnO single crystal is higher than ZnO nanostructures. Moreover, low-temperature (4-10 K) photoluminescence measurements of different structures exhibit several bound peaks, donoracceptor pair transitions and longitudinal optical phonon replicas, whereas in all ZnO samples (single crystals, films and nanostructures) if the temperature is increased to room temperature, only free-exciton emission is observed103. In Figure 1.19, an example of photoluminescence spectra at different temperatures of ZnO faceted rods is shown. In addition, as aforementioned, room temperature photoluminescence spectra reveal different peaks in the visible spectral region which have been attributed to the defect emission. Blue-green, green, yellow and orange-red defect emissions in ZnO structures have been reported. The origin of these defects are have been tried to identify by the combination of photoluminescence with other techniques, such as electron paramagnetic resonance (EPR) spectroscopy, positron annihilation spectroscopy (PAS) or X- ray photoelectron spectroscopy (XPS), but it remains unresolved. For instance, the identification of green emission, which is the most common, is not completely clear, it could stem from oxygen or Zn vacancy or interstitials and antisite oxygen, but it has been evidenced experimentally that the defect responsible are located at the surface. While, the yellow emission is ascribed to transitions involving interstitial oxygen and the responsible defect are not located at the surface. And the orange-red emissions were assigned to oxygen interstitial or oxygen-rich samples for different nanostructured analysed. However, in general, all of the defects emissions have been founded for both nanostructures and thin films103. The two main strategies for the fabrication of ZnO nanostructures are: spontaneous growth and template based synthesis. In the case of the method Introduction 1 43 without templates, such as thermal evaporation, metal-organic vapour phase epitaxy, laser ablation, hydrothermal synthesis, metal catalysts like gold are often used for a aligned growth on a selective area103, 108. These seed layers used as substrates in the fabrication of ZnO influence in its structure. ZnO nanostructures grown by vapour deposition method depends basically on the temperatures of the source and the substrates, the distance between both, the heating rate the gas flow rate and the starting precursor109. Therefore, the growth parameters during the deposition related to crystalline quality affect the optical properties of nanostructured ZnO materials. Thus, the defects can be deleted by varying the fabrication conditions104 or by post-fabrication treatment110. As examples, hydrogen plasma enhancing UV-to-defect emission intensity ratio for ZnO nanorods,109, 111 and reduced defect yellow emission by annealing in hydrogen/argon mixture112 has been reported. In Figure 1.19, the photoluminescence spectra obtained for columnar ZnO films grown by O2 plasma deposition at different substrate temperatures. The position of peak at 378 nm, which was attributed to the recombination of free excitons and the others to phonon replicas, and the visible emissions vary with the experimental conditions during the deposition. The fabrication of ZnO from an organic compound, zinc phthalocyanine (ZnPc), is addressed in the present thesis, as well as their optical properties and sensing aplications. 1.5 Introduction to the growth and optical properties of ZnO 44 Figure 1.19. ZnO columnar films gown by PECVD at different temperatures photoluminescence spectra at room temperature - Films present two photoluminescence bands, a narrow one in the UV range (around 380 nm) and a broad one in the visible range (500-700 nm range).102, 104 1.6 Objectives The main objective of this thesis is the development of novel multifunctional thin films and supported nanostructures by using remote plasma processes. Accordingly the project will address the following specific Scientific Objectives: • To develop the synthetic methodology for the deposition of multifunctional thin films by plasma assisted vacuum deposition of solids precursors. The properties of the films will be determined by the nature of the precursor molecules and by the interaction of them with a remote plasma discharge during the deposition. Introduction 1 45 • To optimize the conformallity of the remote plasma deposited thin films. The conformal deposition is a requisite for the application of the films in optical and electrical devices. For this, the deposition on different type of nanostructured substrates will be studied. • To develop and optimize optical thin films by the remote plasma assisted deposition of adamantane. For this it will be critical to study the experimental parameters that determine the optical properties as UV-Vis transparency and the control of the refractive index. • To study the wetting properties of the organic thin films surfaces as a function of the temperature. • To study the deposition of thin films by combining the adamantane synthesis with a second precursor consisting in a highly luminescent dye. For this it will be necessary to control the refractive index of the composite films and the optimization of their luminescent properties. • To study the amplified spontaneous emission properties of the luminescent films in order to select the best candidates for lasing emission. For this type of application it will be necessary to adjust very precisely the optical and luminescent properties of the films. • To fabricate a 1D distributed feedback laser incorporating the optimized luminescent thin films and characterization of the lasing emission. • To study the dielectrical properties of remote plasma deposited adamantane functional films and the optimization of their dielectric breakdown field. The films will be studied in humid and saline environments to obtain films compatible with the operation in such conditions. • To study the electrical properties of remote plasma polymerized adamantane films coating graphene, supported organic nanowires and bridge structures formed by organic nanowires connecting metallic microelectrodes. 1.6 Objectives 46 • To develop of a general template for the synthesis of supported metal and metal oxide nanowires by remote plasma processing of 1D supported nanowires. • To fabrication ZnO supported nanostructured surfaces by remote plasma deposition and by remote plasma processing of 1D organic nanofibers and study their optical, luminescent and sensing properties. Introduction 1 47 REFERENCES 1. Atkinson, H. V., Structural and Functional Materials. Material Science and Engineering 2002, II. 2. Banerjee, S. T. A. K., Functional materials preparation, processing and applications. Elsevier: Amsterdam; Boston; Paris, 2012. 3. Zhong Lin Wang, Z. C. K., Functional and Smart Materials Structural Evolution and Structure Analysis. Plenum Publishing Corp: New York, 1998. 4. Barranco, A.; Bielmann, M.; Widmer, R.; Groening, P., Plasma polymerization of rhodamine 6G thin films. Advanced Engineering Materials 2005, 7, 396-400. 5. Barranco, A.; Groening, P., Fluorescent plasma nanocomposite thin films containing nonaggregated rhodamine 6G laser dye molecules. Langmuir 2006, 22, 6719-6722. 6. Barranco, A.; Aparicio, F.; Yanguas-Gil, A.; Groening, P.; Cotrino, J.; Gonzalez-Elipe, A. R., Optically active thin films deposited by plasma polymerization of dye molecules. Chemical Vapor Deposition 2007, 13, 319- 325. 7. Blaszczyk-Lezak, I.; Aparicio, F. J.; Borras, A.; Barranco, A.; Alvarez- Herrero, A.; Fernandez-Rodriguez, M.; Gonzalez-Elipe, A. R., Optically Active Luminescent Perylene Thin Films Deposited by Plasma Polymerization. Journal of Physical Chemistry C 2009, 113, 431-438. 8. Aparicio, F. J.; Borras, A.; Blaszczyk-Lezak, I.; Groning, P.; Alvarez- Herrero, A.; Fernandez-Rodriguez, M.; Gonzalez-Elipe, A. R.; Barranco, A., Luminescent and Optical Properties of Nanocomposite Thin Films Deposited by Remote Plasma Polymerization of Rhodamine 6G. Plasma Processes and Polymers 2009, 6, 17-26. 9. Aparicio, F. J.; Holgado, M.; Borras, A.; Blaszczyk-Lezak, I.; Griol, A.; Barrios, C. A.; Casquel, R.; Sanza, F. J.; Sohlstrom, H.; Antelius, M.; Gonzalez- Elipe, A. R.; Barranco, A., Transparent Nanometric Organic Luminescent Films as UV-Active Components in Photonic Structures. Advanced Materials 2011, 23, 761-765. 10. Aparicio, F. J.; Blaszczyk-Lezak, I.; Sanchez-Valencia, J. R.; Alcaire, M.; Gonzalez, J. C.; Serra, C.; Gonzalez-Elipe, A. R.; Barranco, A., Plasma Deposition of Perylene-Adamantane Nanocomposite Thin Films for NO2 Room- Temperature Optical Sensing. Journal of Physical Chemistry C 2012, 116, 8731- 8740. 11. Aparicio, F. J.; Alcaire, M.; Borras, A.; Gonzalez, J. C.; Lopez-Arbeloa, F.; Blaszczyk-Lezak, I.; Gonzalez-Elipe, A. R.; Barranco, A., Luminescent 3- hydroxyflavone nanocomposites with a tuneable refractive index for photonics REFERENCES 48 and UV detection by plasma assisted vacuum deposition. Journal of Materials Chemistry C 2014, 2, 6561-6573. 12. Fridman, A., Plasma Chemistry. Cambridge University Press: New York, 2012. 13. Langmuir, I., Oscillations in ionized gases. Proceedings of the National Academy of Sciences of the United States of America 1928, 14, 627-637. 14. Grill, A., Cold Plasma Materials Fabrication: From Fundamentals to Applications. IEEE Press: Piscataway, 1993. 15. Albella Martín, J. M., Láminas delgadas y recubrimientos. Preparación, propiedades y aplicaciones. Consejo Superior de Investigaciones Científicas Madrid, 2003. 16. Denes, F. S.; Manolache, S., Macromolecular plasma-chemistry: an emerging field of polymer science. Progress in Polymer Science 2004, 29, 815- 885. 17. Foerch, R.; McIntyre, N. S.; Hunter, D. H., Modification of polymer surfaces by 2-step plasma sensitized reactions. Journal of Polymer Science Part a-Polymer Chemistry 1990, 28, 803-809. 18. Foerch, R.; Hunter, D. H., Remote nitrogen plasma treatment of polymers-polyethylene,nylon 6,6, poly(ethylene vinyl alcohol), and poly(ethylene-terephthalate). Journal of Polymer Science Part a-Polymer Chemistry 1992, 30, 279-286. 19. Biederman, H., Plasma polymer films. Imperial College Press: London, 2004. 20. Friedrich, J., Mechanisms of Plasma Polymerization - Reviewed from a Chemical Point of View. Plasma Processes and Polymers 2011, 8, 783-802. 21. Miller, S. L., A production of amino acids under possible primitive Earth conditions Science 1953, 117, 528-529. 22. Miller, S. L.; Urey, H. C., Organic compound synthesis on the primitive Earth. Science 1959, 130, 245-251. 23. Neiswend, D. D., Polymerization of Benzene in a Radiofrequency Discharge. Advances in Chemistry Series 1969, 338. 24. Yasuda, H., Plasma polymerization. Academic Press: Orlando, 1985. 25. Yasuda, H., Glow-Discharge Polymerization. Macromolecular Reviews Part D-Journal of Polymer Science 1981, 16, 199-293. 26. Westwood, A. R., Glow discharge polymerization.1.Rates and Mechanisms of polymer formation. European Polymer Journal 1971, 7, 363-&. 27. Kobayash.H; Bell, A. T.; Shen, M., Foramation of an amorphous podwer during polymerization of ethylene in a radio-frequency discharge. Journal of Applied Polymer Science 1973, 17, 885-892. 28. Kobayash.H; Bell, A. T.; Shen, M., Plasma polymerization of saturated and unsaturated-hydrocarbons. Macromolecules 1974, 7, 277-283. Introduction 1 49 29. Tibbitt, J. M.; Jensen, R.; Bell, A. T.; Shen, M., Model for kinetics of plasma polymerization. Macromolecules 1977, 10, 647-653. 30. Tibbitt, J. M.; Shen, M.; Bell, A. T., Structural characterization of plasma-polymerized hydrocarbons. Journal of Macromolecular Science- Chemistry 1976, A 10, 1623-1648. 31. R. d’Agostino, P. F., F. Fracassi, Plasma Processing of Polymers Kluwer Academic: Dordrecht, 1996. 32. Hudis, M., Surface crosslinking of polyethylene using a hydrogen glowdischarge. Journal of Applied Polymer Science 1972, 16, 2397-&. 33. Jesch, K., Bloor, J. E. Kronick,P.L, Structure and physical properties of glow discharge polymers.I. Polymers from hydrocarbons. Journal of Polymer Science Part a-1-Polymer Chemistry 1966, 4, 1487-&. 34. Kronick, P. L. J., K. F.Bloor,J.E., Spectra of glow-discharge polymers from aromatic compounds. Journal of Polymer Science Part a-1-Polymer Chemistry 1969, 7, 767. 35. M.Tibbitt, J. M. B., A. T.Shen, Dielectric relaxations in plasmapolymerized hydrocarbons and fluorocarbons. Journal of Macromolecular Science-Chemistry 1976, A 10, 519-533. 36. Friedrich, J.; Kuhn, G.; Mix, R.; Fritz, A.; Schonhals, A., Polymer surface modification with monofunctional groups of variable types and densities. Journal of Adhesion Science and Technology 2003, 17, 1591-1617. 37. J.Fredrich, G. K., R.Mix , I.Retzko, V.Gerstung, St.Weidner, R.D.Schulze,W.Unger In Polyimides and other high temperature polymers:Synthesis, characterization, and applications, Polyimides and other high temperature polymers:Synthesis, characterization, and applications, Utrech, 2003; K.L.Mittal, Ed. VSP: Utrech; pp 359-388. 38. Chowdhury, F. U. Z.; Bhuiyan, A. H., Dielectric properties of plasmapolymerized diphenyl thin films. Thin Solid Films 2000, 370, 78-84. 39. Stundzia, V.; Biederman, H.; Slavinska, D.; Nedbal, J.; Hlidek, P.; Poskus, A.; Mackus, P. K.; Howson, R. P., Dielectric relaxation in hard, plasmapolymerized C : H films. Journal of Physics D-Applied Physics 2000, 33, 719- 724. 40. K. G.Thompson, L. F. M., Plasma polymerization of vinyl monomers.1. design,construction and operatin of indcutively coupled plasma generator and preliminary studies with 9 monomers. Journal of Applied Polymer Science 1972, 16, 2291-&. 41. Haupt, M.; Barz, J.; Oehr, C., Creation and recombination of free radicals in fluorocarbon plasma polymers: An electron spin resonance study. Plasma Processes and Polymers 2008, 5, 33-43. 42. Wrobel, A. M. K., J.Grebowicz, J.Kryszewski, M., Thermaldecomposition of plasma-polymerized organo-silicon thin-films. Journal of Macromolecular Science-Chemistry 1982, A17, 433-452. 2 Resumen en español 2.1 Introduction 58 2.1 Introduction La presente tesis se ha desarrollado en el grupo de investigación Nanotecnología de Superficies del Instituto de Ciencia de Materiales de Sevilla. En ella se estudian nuevos materiales obtenidos mediante una técnica de síntesis de láminas delgadas orgánicas funcionales basada en el empleo de plasmas remotos de microondas, se desarrollan nuevos métodos de nanoestructuración de estos materiales. Los materiales estudiados en esta tesis presentan diversas propiedades. Así, a lo largo de la memoria se presentan láminas transparentes, dieléctricas, con propiedades particulares de mojado y retardadoras en la congelación, luminiscentes, con emisión láser y sensoras a gases, entre otras propiedades. Empleando esta técnica desarrollada en nuestro laboratorio en los últimos años, se han fabricado nuevas láminas orgánicas, constituidas por una matriz polimérica de alto grado de entrecruzamiento, que incorpora de forma controlada una determinada cantidad de moléculas de colorante embebidas en la matriz. El proceso combina una descarga de plasma con la deposición en vacío de monómeros de sólidos orgánicos sobre sustratos situados en configuración remota respecto del plasma. La Figura 2.1 muestra un esquema del reactor de síntesis. La descarga de plasma fragmenta las moléculas precursoras. De manera controlada mediante el ajuste simultáneo y controlado de diferentes parámetros de deposición, es decir, MW de potencia, la presión total, la velocidad de sublimación y la disposición geométrica utilizada para la deposición se obtiene una fragmentación parcial de dichas moléculas. La distancia entre el plasma y la película en crecimiento (z en la Figura 2.1) es uno de los principales parámetros experimentales ajustables del proceso.1-4 Esta distancia limita la interacción plasma-precursor y ofrece versatilidad al proceso de deposición puesto que regula gradualmente las propiedades de la película. El método de síntesis Resumen en español 2 59 proporciona un a amplia gama de películas sólo modificando la distancia z, desde polímeros de plasma con una lato grado de entrecruzamiento sin la retención del monómero original (deposición en la descarga de plasma) hasta laminas que contienen una cantidad de moléculas activas controlable (deposición en remoto). En el límite, si la distancia z es lo suficientemente grande, el proceso de síntesis se asemeja a una deposición en vacío sin plasma. El control de otros parámetros experimentales como la tasa de crecimiento, la presión o la potencia de la fuente de plasma microondas influyen de forma equivalente a la distancia sobre el método en algunos casos. Además del método de deposición asistido por plasma mencionado, en esta tesis se explora el uso de tratamientos con plasma controlados de baja intensidad se ha aplicado también a la fabricación de nanoestructuras orgánicas híbridas. Se ha establecido este protocolo de síntesis como obtención de sustratos soporte nanoestructurados para el crecimiento de materiales nanoestructurados orgánicos mediante la técnica de deposición por plasma remoto. La nanoestructuración de materiales es un tema de actualidad debido a las nuevas propiedades que adquieren los materiales al confinar sus dimensiones en la nanoescala. Dentro de la amplia variedad de nanoestructuras, se presenta la síntesis de materiales 1D, es decir los que presentan solo una de sus dimensiones por encima de 100 nm. Por último, se ha aplicado también el uso de postratamientos de plasma a láminas delgadas y nanoestructuras 1D orgánicas por el método de deposición de vacío (PVD) que modifican la estructura en la nanoescala y la química del material mediante el mismo montaje experimental. 2.1 Introduction 60 Figura 2.1. Reactor de preparación de muestras por deposición de vapor a vacío asistida por plasma (RPAVD) – Esquema de los principapels componentes del reactor RPAVD utilizado en esta tesis para la fabricación de láminas delgadas orgánicas y nanoestructuras. 2.2 Láminas delgadas orgánicas conformales para aplicaciones en electrónica molecular Los nanomateriales basados en grafeno y la electrónica molecular son temas de extrema importancia en la nanotecnología actual debido a las posibilidades que ofrecen en el desarrollo de nanodispositivos. Para alcanzar las los objetivos de esta revolución tecnológica, estas nuevas nanoestructuras conductoras y semiconductoras tienen que combinarse con elementos dieléctricos mediante técnicas de última generación con el fin de producir 2.5 Tratamientos con plasma débil de nanohilos orgánicos: una vía para la fabricación de heteroestructuras orgánicos 1D y la síntesis de patrones de diseño de nanoestructuras inorgánicas 1D El estudio de nanoestructuras 1D fabricadas a partir de pequeñas moléculas orgánicas ha despertado un gran interés durante los últimos años debido a su aplicación en fotónica, fotovoltaica, fotocatálisis, microelectrónica y nanosensores. Este campo emergente de nanohilos orgánicos (ONWs) se ha ido estableciendo gracias al esfuerzo realizado durante los últimos años tanto en la fabricación como en el análisis de sus propiedades.28-38 En paralelo con su homólogo inorgánico, la fabricación de nanocables orgánicos heteroestructurados está ganando cada vez más atractivo debido a su uso en los ámbitos de las aplicaciones mencionadas anteriormente, así como en el estudio de sistemas modelo para heterouniones n-p 1D.39 Este trabajo presenta una ruta sin precedentes para la síntesis de nanohilos orgánicos heteroestructurados. El método consiste en la combinación de dos procesos de vacío establecidos: la síntesis de nanohilos orgánicos monocristalinos (ONWs) por deposisción física de vapor (PVD)5-7, 40, 41 y el desbastado mediante por plasma de oxígeno.42, 43 Dos ejemplos que demuestran las posibilidades de esta metodología se presentan en este apartado. Por un lado, se presenta la fabricación de dos tipos diferentes de nanoestructuras orgánicas mediante el control de la intensidad del tratamiento de plasma: heterostructured nanohilos (NWs) jerárquicos y nanohilos (NWs) híbridos metal-orgánicos. Por otra parte, atendiendo a los estudios previos mediante plasma débil de aire sobre el procesado de nanoscristales coloidales para la eliminación de los ligandos orgánicos conservando las propiedades dependientes del tamaño de nanocristal,44 exponemos en este trabajo la descripción de los primeros resultados de un método modelo para la síntesis de nanoestructuras 1D 2.5 Tratamientos con plasma débil de nanohilos orgánicos 82 inorgánicos mediante el procesamiento de nanohilos orgánicos. 2.5.1 Experimental El método de síntesis de heteroestructuras orgánicas hace utiliza un reactor de plasma frío cuyo diseño ha sido descrito en la Sección Introducción, concretamente un plasma remoto de microondas (MW) de resonancia ciclotrónica (ECR) que permite tratamientos de las nanoestructuras orgánicas a potencias más bajas y el control sobre los parámetros experimentales críticos del proceso (distancia a la descarga de plasma, presión del gas, aplicación de voltaje al portamuestras, etc.). La fabricación de loss diferentes tipos de nanoestructuras orgánicas, heteroestructuras NWs jerárquicas y NWs híbridos metal-orgánicos, y los tratamientos de plasma débil se producen en el mismo reactor remoto de deposición al vacío asistido por plasma (RPAVD). El proceso de fabricación en todos los casos se realiza en tres etapas (Figura 2.12): i) la síntesis de NWs orgánicos o metal-orgánicos por PVD, ii) formación de nuevos centros de nucleación mediante tratamientos de plasma de baja intensidad, iii) la síntesis de NWs secundarias a partir de estos nuevos núcleos. 2.5.1.1 Formación de nanohilos orgánicos mediante PVD Los precursores orgánicos utilizados en la fabricación de hetroestructuras han sido: perileno-diimida (2,9-dimetil-anthra [2,1-def: 6,5,10- d'e'f '] diisoquinoline 1,3, 8,10-tetrona (Me-PTCDI de Sensient Imaging Technologies.), octaethyl porfirina de paladio (PdOEP de Frontier Sci.), y ftalocianina de cobalto (CoPc de Aldrich). Resumen en español 2 83 Figura 2.12. Mecanismo de formación heteroestructuras orgánicos 1D – Diferentes etapas del método de fabricación de dos tipos de nanoestructuras orgánicas heteroestructuradas desarrollado a partir de la combinación de los procesos de PVD y de tratamientos con plasma de oxígeno. Los modelos de crecimiento de NWs jerárquicas (a) y metal-orgánicas (b) se representan en esta imagen mediante tres etapas de formación. La sublimación de las moléculas para la deposiciónde los nanohilos se realizó mediante una celda de 10 cm3 OLED Knudsen (Lesker) colocada en la cámara de alto vacío (sin plasma). El sistema se bombeó hasta la presión base de 5 · 10-6 mbar antes de la deposición. La distancia entre el sustrato y la fuente de sublimación fue de 8 cm. Durante los experimentos, el argón se dosificó usando un controlador de flujo de masa calibrado. La presión en el sistema durante la deposición se fijó en 0.020 mbar. La tasa de crecimiento fue controlada por una microbalanza de cristal de cuarzo situada en el mismo plano que los sustratos. El crecimiento de los nanohilos heteroestructurados se ejecutó combinando dos células Knudsen diferentes. Las tasas de deposición fueron entre 0.30 y 0.50 Å · s-1 considerando una densidad en la balanza de 0.5 g · cm-3. Las ONWs se fabricaron simultáneamente en diferentes sustratos: ITO, Si (100), sílice fundida y rejillas TEM de oro previamente recubiertas con nanopartículas de plata por 2.5 Tratamientos con plasma débil de nanohilos orgánicos 84 pulverización catódica (DC-sputtering) y películas delgadas TiO2 depositadas mediante técnicas de vacío. La temperatura de los sustratos temperatura (TS) para la formación ONWs depende de la molécula orgánica: a) para el Me- PTCDI la temperatura del sustrato óptima TS fue de 175 ± 5 ° C; b) para el TS PdOEP fue 140 ± 5 ° C y c) para el TS COPC fue de 260 ± 5 ° C. 2.5.1.2 Pos-tratamientos con plasma de baja intensidad El reactor de deposición está equipado con un sistema de microondas a distancia de plasma (2,45 GHz) que permite el procesamiento de plasma de baja intensidad in situ. Todos los experimentos recogidos en este trabajo se realizaron in situ bajo las siguientes condiciones: 150 W en la fuente de microondas, la composición del gas de plasma fue de 80% de oxígeno - 20% de argón, la presión total en la cámara de 0,020 mbar, y la configuración remota, es decir, sustratos se situaron alejados del plasma, concretamente a los 5 cm. 2.5.1.3 Nanohilos orgánicos secundarios fabricados por PVD Los tratamientos de plasma inducen cambios en la rugosidad de la superficie de ONWs a lo largo de su estructura, incluso agregación de átomos de metal para las ONWs crecidas a partir de moléculas que contienen metales. Dichas modificaciones de la superficie a su vez forman nuevos centros de nucleación a lo largo de la estructura de las NWs. Por tanto, las nanoestructuras secundarias 1D fabricadas mediante PVD crecen a partir de estos centros de nucleación, siguiendo el mismo procedimiento que las ONWs primarias. 2.5.2 Caracterización de heteroestructuras orgánicas 1D La caracterización SEM se realizó en un Hitachi S-4800 mientras que la caracterización TEM y tomografía de electrones HAADF-STEM se realizó en un microscopio electrónico de transmisión de campo de emisión FEI Tecnai F20 operando a 200 kV. Los datos se recopilaron inclinando la muestra sobre un solo Resumen en español 2 91 A diferencia del procesado de NWs por plasma a temperatura ambiente, el tratamiento a temperaturas más altas modifica dramáticamente la microestructura de las ONWs. En la figura 2.16 se comprueba cómo después del tratamiento con plasma a 150º C las nanopartículas de diámetro medio disminuyen mientras que su número aumenta bruscamente. Este resultado se puede interpretar suponiendo que el efecto combinado de plasma y la temperatura aumenta la movilidad de las moléculas que producen una nanoestructura 1D altamente porosa. Además, la superficie de los nanohilos presenta una rugosidad pronunciada y pequeños canales que comunican con sus núcleos. El elevado número de nanopartículas en la superficie de las NWs verificaría la hipótesis de una cierta difusión de moléculas hacia la superficie. Figura 2.16. Formación de nanopartículas de metal en ONWs por tratamiento con plasma de oxígeno a temperaturas superiores a la temperatura ambiente - (a,d) Micrografías STEM a diferentes aumentos de NWs híbridas Pd-PN / PdOEP-NWs formadas mediante un tratamiento con plasma de oxígeno a 150 ºC de temperatura de PdOEP NWs. (e)Vista de proyección Voxel y (f) ortho-sección vertical de la NW. (d) (diámetro de ~ 90 nm) que muestra la nanoestructura altamente porosa. 2.5 Tratamientos con plasma débil de nanohilos orgánicos 92 De una manera similar que para la formación de NWs jerárquicas partiendo de la superficie modificada de las NWs primarias mediante tratamientos de plasma de oxígeno, las nanopartículas situados en la superficie de las ONWs también pueden actuar como centros de nucleación para el crecimiento de ONWs secundarias siguiendo el procedimiento descrito previamente. 2.5.3.3 De NWs híbridas a NWs metálicas y de óxidos En último lugar, la metodología de plasma desarrollada en este trabajo nos ofrece el uso de estas estructuras como un modelo para la fabricación de nanoestructuras 1D metálicas y de óxido soportadas en distinto s sustratos. La figura 2.17 muestra un ejemplo. La nanoestructura 1D de la figura es el resultado de un tratamiento de 10 minutos de plasma sobre PdOEP NWs a 150 ºC de temperatura. Los espectros XPS (no mostrados) ratifican que después de este tratamiento los componentes orgánicos se eliminan por completo y que nanopartículas de paladio parcialmente oxidadas se han aglomerado en forma de una nanoestructura inorgánica altamente porosa manteniendo la memoria de la ONW original. Es interesante destacar la homogeneidad del procesamiento de plasma de las nanoestructuras 1D a lo largo de toda su longitud. La aplicación de este procedimiento a otras moléculas orgánicas de metal abre claramente una nueva ruta para la formación de nanoestructuras 1D inorgánicos altamente porosas, como se discutirá en el próximo apartado. El método presenta varias ventajas. Una de ellas consiste en la posibilidad de combinar las diferentes moléculas metal orgánicas en el mismo nanohilo para obtener heteroestructuras inorgánicos 1D de dos elementos diferentes, gran área de deposición y bajo consumo de energía. Por otra parte, los dos procesos involucrados en la metodología propuesta, PVD para el crecimiento ONWs y el tratamiento con plasma, se realizan en vacío que es Resumen en español 2 93 totalmente compatible con la fabricación de dispositivos fotónicos y microeléctrónicos.1, 50 Figura 2.17. Nanohilo de óxido metálico soportado – Micrografía STEM de un nanohilo de paladio obtenido tras el tratamiento de plasma prolongado (unos 8 minutos) a temperatura suave (150ºC). 2.6 Láminas delgadas porosas y estructuras 1D de óxido de cinc mediante procesado por plasma remoto de ftalocianina de cinc En este apartado se presenta la fabricación de láminas de alta porosidad de ZnO y nanoestructuras 1D mediante un protocolo combinado de vacío y desbastado de plasma inspirado en la metodología desarrollada en la sección anterior. Como precursor sólido se utiliza ftalocianina de cinc (ZnPc) para formar un óxido de cinc (ZnO). En primer lugar, la ZnPc se sublima a baja presión de argón, y dependiendo de la temperatura del sustrato y la microestructura, crecen películas policristalinas o nanohilos monocristalinos de ZnPc. Estos materiales de partida se someten entonces a un tratamiento oxidante 3 Conformal dielectric organic thin films for molecular electronics ABSTRACT Graphene based nanomaterials and molecular electronics are the paramount topics of the present nanotechnology due to the extraordinary possibilities that open for the development of nanodevices. To achieve the promises of these technological revolutions these novels conducting and semiconducting nanostructures have to be combined with state-of-the-art dielectric elements in order to produce complex architectures. Advanced dielectric materials are essential to encapsulate nanodevices for protection and packaging. Herein, we present a technique to produce transparent and robust dielectric organic thin films that can be directly deposited in delicate organic structures as organic nanowires or graphene. The films are completely conformal at the nanometer scale even on high aspect-ratio nanoobjects. Besides the films are homogeneous, crack-free and stable to the atmosphere and their dielectrics properties are unaffected by corrosive saline environments. 4 Wetting and anti-freezing properties of adamantane coatings: from thin films to 3D networks ABSTRACT Here we first analyze the water wetting properties of adamantane RPAVD thin films and 1D nanostructures from room temperature to -10 ºC in the micro and macroscale. During this study, significant freezing delays times are detected for RPAVD coatings respect to the substrates. We attempt to explain this phenomenon from the study of experimental variables such as the size of water droplet and the thickness of RPAVD film. The property of confomallity in RPAVD composites enables the fabrication of adamantane RPAVD films with different roughness. The roughness of the adamantane RPAVD coatings shows different behaviour on the wetting properties and the freezing delay times. We present a deep study by SEM, ESEM and WCA measures on the flat, rough and nanostructured adamantane RPAVD thin films, 5 Multicolored emission and lasing in DCM- Adamantane plasma nanocomposites ABSTRACT We study the synthesis of DCM laser dye containing luminescent nanocomposites in combination with adamantane. These films will be fabricated by plasma assisted vacuum deposition. The experimental parameter that determined the concentration of DCM in the films and their optical properties will be evaluated. The objective is to control the films refractive indexes and their luminescent emission in order to prepare active layers for laser devices. We will demonstrate that in this system the thin film deposition by remote plasma polymerization technique can yield optical gain materials. The ASE experiments in DCM dye/Adamantane nanocomposite waveguides will show the improvement of the nanocomposites properties as compared to the neat DCM films fabricated by the same technique. Moreover, 1D DFB laser emission will be probed and characterized in some of the nanocomposites films studied, opening up the use of remote plasma polymerization deposition technique for integrated photonics and optoelectronics. 6 Soft Plasma processing of Organic Nanowires: a route for the fabrication of 1D organic heterostructures and the template synthesis of inorganic 1D nanostructures ABSTRACT Hierarchical (branched) and hybrid Metal-NPs/organic supported NWs are fabricated through controlled plasma processing of metalloporphyrin, metallophthalocyanine and perylene nanowires. The procedure is also applied for the development of a general template route for the synthesis of supported metal and metal oxide nanowires. 6.1 Introduction 212 6.1 Introduction The emerging field of organic nanowires (ONWs) is settling down thanks to the effort carried out during the last years in both the fabrication and the study of their properties.1-11 The outstanding performance of 1D organic nanostructures based on small molecules has been proved in photonics, photovoltaic, photocatalysis, microelectronic and nanosensoring applications.1-11 Parallel with the inorganic counterpart, the fabrication of heterostructured organic nanowires is increasingly gaining interest because of their use in the aforementioned fields of applications as well as in the study of model systems for 1D n-p heterojuctions.12 Within this framework in this work we report about an unprecedented route for the synthesis of heterostructured organic nanowires which consist of the combination of two well established vacuum processes, namely the synthesis of single crystal supported organic nanowires by physical vapour deposition13-16 and oxygen plasma etching.17, 18 The possibilities of this methodology are illustrated with the fabrication of two different types of organic nanostructures: heterostructured hierarchical NWs and metal-organic hybrid NWs. Ozin et al.19 have previously reported on the mild air plasma processing of colloidal nanocrystals aiming at removing the organic ligands while preserving the size-dependent properties of the nanocrystals. In the present chapter we go a step forward in the use of plasmas by describing the first results of an original template method for the synthesis of inorganic 1D nanostructures by processing organic nanowires. We also demonstrate that by controlling the intensity of the plasma treatment, the method can be used to fabricate hierarchical 1D organic heterostructures formed by two or more interconnected ONWs. This has been possible by using a cold plasma reactor, concretely a remote electron cyclotron resonance (ECR) microwave plasma that permits treatments of the organic Soft Plasma processing of Organic Nanowires 6 213 nanostructures at lower powers and a much better control over critical experimental parameters of the process (distance to the plasma discharge, pressure of gas, bias application on the sample holder, etc.). 20, 21 6.2 Experimental Setup Both different types of organic nanostructures, heterostructured hierarchical NWs and metal-organic hybrid NWs, have been grown in a remote plasma assisted vacuum deposition reactor (see Chapter 1). The fabrication process in every case is performed in three stages (Figure 6.1): i) synthesis of organic or metal-organic NWs by PVD (developed in Chapter 1), ii) formation of new nucleation centres by soft plasma processing, iii) synthesis of secondary NWs from new nuclei. Figure 6.1. Mechanism of 1D organic heterostructures formation - Different types of organic heterostructured nanostructures stages developed combining PVD and oxygen plasma etching process. Hierarchical (a) and Metal-organic (b) NWs growth model are represented in the image by their three stages of formation. 6.2 Experimental Setup 214 6.2.1 Organic Nanowires formation by PVD The procedure development has been fully described in previous references.13, 14 Perylene-diimide (2,9-dimethyl-anthra[2,1-def:6,5,10- d′e′f′]diisoquinoline 1,3,8,10-tetrone (Me-PTCDI) from Sensient Imaging Technologies.), Pd Octaethyl porphyrin (PdOEP from Frontier Sci.), and Co phthalocyanine (CoPc from Aldrich) were used as purchased (See Figure 6.2 (a) for their molecular structures). The sublimation of the molecules for the nanowires deposition was carried out using a 10 cm3 OLED Knudsen cell (Lesker) placed in the highvacuum chamber (without plasma). The deposition system is shown in chapter 1. The system was pumped up to 5·10 -6 mbar base pressure before the deposition. The distance between the substrate and the sublimation source was 8 cm. During the experiments argon was dosed using a calibrated mass flow controller. The pressure in the system during deposition was fixed at 0.020 mbar by a VAT controller. The growth rate was controlled by a quartz crystal microbalance located in the same plane as the substrates. The growth of the heterostructured nanowires was carried out combining two different Knudsen cells. The deposition rates were between 0.30 and 0.50 Å·s-1 considering a density in the balance of 0.5 g·cm-3. ONWs were grown simultaneously on different substrates: ITO, Si(100), fused silica and Au TEM grids previously coated with silver nanoparticles by dc-sputtering and TiO2 thin films deposited by vacuum methodologies. Substrates temperature (TS) for the ONWs formation depended on the organic molecule: a) for the Me-PTCDI the optima substrate temperature TS was 175 ± 5°C; b) for the PdOEP TS was 140 ± 5°C and c) for the CoPc TS was 260 ± 5°C. Soft Plasma processing of Organic Nanowires 6 221 Figure 6.6. Fluorescence images and emission spectra of hierarchical structures – (a) Fluorescence microscope images (a-c) and emission (d) of perylene/PdOEP NWs recorded at different excitation wavelengths as labelled. The emission peaks or bands for each component are marked. The spectra show that the two component materials, Pd porphyrin and perylene (MePTCDI), emit independently when excited at λ =350 nm and λ = 490 nm correspondingly. The obtained spectra show that the contribution of the perylene emission grows as the excitation wavelength increases while that of the Pd porphyrine decreases, in agreement with the fact that the excitation of this latter vanishes at the longest wavelengths. This result supports that for 6.3 Results and discussion 222 components sensitive to different gases or agents (e.g. perylene for NO2 and porphyrin for oxygen)22-24 the system would act as an optical multisensor by following the corresponding enhancement/quenching of the different light emissions. 6.3.2 Metal-Organic Hybrid NWs formed by SOFT plasma processing Another possibility of this methodology is the formation of hybrid metal-organic nanowires. The origin of the inorganic material is the formation of metal nanoparticles by aggregation of the metal atoms resulting from the plasma decomposition of the outerlayers of the primary metal organic molecular structure. During the oxygen plasma treatment, the organic part of the molecules becomes etched by reacting with the excited species of oxygen and the volatile fragments and products evacuated by the vacuum system17, 18. As a consequence, metal atoms remain at the NW forming nanoparticles whose size and density depend on the experimental conditions and metal atom within the molecule. The examples gathered in Figure 6.7 and Figure 6.9 refer to plasma treated PdOEP and CoPc NWs at different temperatures conditions. They illustrate the possibilities of the method by showing Co-NPs/CoPc NW and Pd-NPs/PdOEP NWs with different NPs size distributions. 6.3.2.1 Nps formed by treatments at RT on NWs surfaces The bright spots in the STEM micrographs in Figure 6.7 clearly shows the formation of metal NPs at the surface of the ONWs treated with an oxygen plasma at room temperature. Soft Plasma processing of Organic Nanowires 6 223 Figure 6.7. Formation of metal NPs on ONWs by oxygen plasma treatment at RT – (a-b) STEM micrographs at different magnifications of hybrid Co-NPs/CoPc-NWs formed by oxygen plasma treatment at room temperature of CoPc NWs. Co NPs mean diameter ca. 2 nm (c-e) 3D HAADF-STEM tomography reconstruction of a single Pd- NPs/PdOEP-NW. The micrographs show the vertical orthoslices through the 3D reconstruction at the surface ((c) and (e)) and the interior of the wire. A close inspection of the NWs in Figure 6.7 (a) indicates that the formation of the NPs occurs all along the NWs length. Figure 6.8 shows the NPs size histograms for different molecule composition and plasma treatments. The NPs size has been estimated from a statistical analysis of SEM micrographs. NPs mean diameter as low as 1.6 nm can be achieved by these treatment. 6.3 Results and discussion 224 Figure 6.8. Size histograms for NPs – Co (a) and Pb (b-c) nanoparticles formed at: a) room temperature after 1 minutes; b) room temperature after 5 minutes and c) 150ºC after 5 minutes. The mean diameter for the Co NPs in a) is about 2.5 nm; for the Pd NPs in b) about 2.3 nm and c) 1.6 nm. Furthermore, the 3D reconstruction by the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) tomography technique25-27 has allowed us to fully describe the internal microstructure of these hybrid nanostructures. The snapshots in Figure 6.7 (c)-(e) (see also Videos 1 and 2) show that the Pd NPs have been formed at the surface of the ONW (Figure 6.7 (c) and (e)) while the inner ONW structure remains untouched (Figure 6.7 (d)). This analysis also demonstrates that the longer the plasma treatment duration, the higher the number of nanoparticles (compare Figure 6.7 0 1 2 3 4 5 diameter (nm) Co NPs @RT 1.5 2.0 2.5 3.0 3.5 diameter (nm) Pd NPs @RT 1.0 1.5 2.0 2.5 diameter (nm) Pd NPs @150 ؛C Soft Plasma processing of Organic Nanowires 6 225 (b) corresponding to plasma treatment of 1minute and (c) resulting from a plasma treatment of 5 minutes). 6.3.2.2 Nps formed by treatments at temperatures above RT on NWs surfaces Unlike the plasma processing at room temperature, the treatment at higher temperatures strongly modifies the ONWs microstructure. Figure 6.9 and Video 6.3 shows that after the plasma treatment at 150º C the nanoparticles mean diameter decreases while their number abruptly increases (Figure 6.8). This result can be interpreted by assuming that the combined effect of plasma and temperature enhances the mobility of the molecules producing a highly porous 1D nanostructure. Moreover, the surface of the nanowires presents a pronounced roughness and small channels communicating with their core. Clear examples of this new type of nanostructure are reported in Figure 6.9 (a) and (d) where the NWs appear to be hollow. The vertical orthoslice in (f) further demonstrates the internal porosity induced by the plasma processing at mild temperatures. The high number of nanoparticles at the surface of the NWs would support the hypothesis of a certain diffusion of molecules towards the surface. 6.3.2.3 NPs as nucleation centres allowing the growth of secondary NWs In a similar way than for the formation of hierarchical NWs from the modified surface of the primary NWs by oxygen plasma treatments, the nanoparticles located at the surface of the ONWs can also act as nucleation centres for the growth of secondary ONWs following the procedure described in section 3.1.3. This effect can be observed in Figure 6.10. 6.3 Results and discussion 226 Figure 6.9. Formation of metal NPs on ONWs by oxygen plasma treatment at temperatures above RT – (a-d) STEM micrographs at different magnifications of hybrid Pd-NPs/PdOEP-NWs formed by oxygen plasma treatment at 150 ºC temperature of PdOEP NWs. See also the 3D reconstruction of a Pd-NP/PdOEP-NW in the Video 6.3. (e) Voxel projection view and (f) vertical orthoslice of the NW in (d) (diameter  90 nm) showing the highly porous nanostructure. Soft Plasma processing of Organic Nanowires 6 227 Figure 6.10. Metal-Organic hybrid NW – TEM micrograph showing perylene NWs growing from the Co NPs formed at RT on the surface of a CoPc NW. 6.3.3 From Hybrid to metal and oxide NWs Finally, another choice of the plasma methodology developed here consists of the use of these structures as a template for the fabrication of supported 1D metal and oxide nanostructures. Figure 6.11 illustrates an example of the realization of this idea. The reported 1D nanostructure is the result of the plasma processing during 10 minutes of PdOEP NWs at 150 ºC. XPS spectra (not shown) exhibit that after this treatment the organic components are completely removed and partially oxidized palladium nanoparticles agglomerate in the form of a highly porous inorganic nanostructure keeping memory of the original ONW. It is interesting 6.3 Results and discussion 228 to emphasize the homogeneity of the plasma processing of the 1D supported nanostructures along their entire lengths. The application of this procedure to other metal organic molecules clearly opens a new route for the formation of highly porous inorganic 1D nanostructures, as will be discussed in the next chapter. Figure 6.11. Supported metal oxide NWSTEM micrograph of a Pd nanowire obtained after prolonged plasma processing (about 8 min) at mild temperature (150ºC). Soft Plasma processing of Organic Nanowires 6 229 The method presents several advantages. One of them consists in the possibility of combining different metal organic molecules in the same nanowire to get inorganic 1D inorganic heterostructures of two different elements, large area deposition and low energy consumption. Furthermore, the two step processes involved in the proposed methodology, PVD for ONWs growth and the plasma treatment, are made in a vacuum environment that is fully compatible with the fabrication of photonic and microelectronic devices.28, 29 6.4 Conclusions In summary, herein we have discussed the effect of the soft plasma processing of organic nanowires formed by the stacking of small molecules. Plasma treatments of pure organic NWs induce the formation of nucleation sites along the NW length that act as centres for the growth of secondary ONWs in a hierarchical configuration. The treatment of NWs fabricated from metal organic molecules results in the formation of hybrid metalorganic NWs whose microstructure can be controlled by the substrate temperature and the plasma treatment duration. Furthermore, under certain conditions, the procedure works as a template process for the development of inorganic NWs constituted by the metal NPs (or the corresponding metal oxide NPs) corresponding to the metallic cation of the original organic molecules. Different fields of applications will likely profit from these supported hybrid nanowires (e.g., applications in catalysis, for non-volatile memory devices, SERS, enhanced fluorescence and electroluminescence, etc.).1, 22, 30, 31 On the other hand, although in this work we have processed supported ONWs fabricated by a PVD method, we believe that similar results will be reproducible on ONWs synthesised by other established methodologies. 1, 22, 30, 31 Finally, it is important to remark the large margin for experimental conditions provided by the cold plasmas for the fabrication of these 6.3 Conclusions 230 new organic and hybrid nanostructures. This type of plasmas permits a wide range of possibilities from mild treatments with effects barely visible under the electron beam of the SEM or TEM instruments to the much stronger conditions enabling a new type of template synthesis for the formation of inorganic NWs. 270 This chapter summarize in a list the main achievements and conclusions of the work developed throughout the present Thesis:  A deposition technique based on ECR remote plasma has been used to grow organic thin films and nanostructures integrating functional molecules. The technique called remote plasma assisted vapour deposition (RPAVD) employs solid organic precursors.  Transparent thin films have been obtained by the plasma assisted vacuum deposition of adamantane. The films are completely transparent in the visible and in the UV regions at wavelengths higher than 300 nm. The deposition process is fully compatible with plasma etching processes and wafer scale integration.  The films present very high dielectric breakdown field in the range 4.5- 7.3 MV/cm and can be conformally deposited on delicate substrates as graphene monolayers and organic nanowires to fabricate devices. The deposition process does not damage these substrates.  The results probe the adamantane films are ideal candidates for their integration as versatile dielectric element in organic electronic devices. For example, the films can be used as insulator elements for conducting organic nanowires protecting such structures against oxidation and reduction in saline environments.  Adamantane thin films deposited on flat substrates are partially hydrophobic (WCA about 85º) at room and sub-zero temperatures.  The WCAs of the adamantane thin films does no depend on the thin film thickness.  The freezing delay time of flat adamantane thin films strongly depends on the thickness. Thicker films yield longer delay times. A freezing delay times as long as 54 minutes has been achieved for a 1 um thick General conclusions 8 271 film. The freezing delay time of adamantane thin films decreases as the water droplet volume increases.  Hydroxilation of the adamantane surface by functionalization with a soft oxygen plasma treatment reduces the freezing delay time.  Adamantane layers delay the freezing on nanostructured SiO2 thin films (up to 90 times with respect to the reference) while the deposition of adamantane on 1D supported nanostructures provides highly hydrophobic surfaces with freezing delay time of about 40 minutes. The deposition of adamantane on dual scale rough substrates produces water repellent surfaces (WCA ca. 180º) showing freezing times above 2 hours.  Three dimensional nanonetworks can be fabricated by combining growth of ONWs by physical vapor deposition, soft plasma etching of the nanowires and conformal deposition of adamantane by RPAVD.  Ultra-water-repelent surfaces are formed by controlling the adamantane thickness deposited on the organic nanonetwork. The organic nanonetworks delay the freezing time up to 80 minutes.  Anti-freezing properties of the adamantane can be applied also to metallic substrates. The degree of coverage of these substrates and thickness of the adamantane controls the anti-freezing behaviour, the thicker the film the higher the coverage of the metal and therefore the longer the freezing time delay.  Nanocomposite films have been fabricated from the simultaneous copolymerization of adamantane and DCM laser dye. The films are thermally and chemically stable at temperatures higher than the corresponding to the sublimation of the precursors.  Luminescent thin films have been obtained by the remote plasma assisted vacuum deposition of adamantane and DCM laser dye. The 272 optical properties of the films can be controlled as a function of the dilution of the dye in the organic matrix. Thus, films with refractive indexes in the range 1.56-1.67 have been deposited.  The films present a multicolor luminescent emission that is a function of the dilution of the dye in the range λem=507-607 nm. The intensity of the luminescent emission is also a function of the dye dilution.  DCM/Adamantane composite films show improved stability and mechanical properties against photoinduced sublimation as compared to pure DCM plasma films. Moreover, ASE experiments have shown positive optical gain, with a threshold 450 kW/cm2, in the nanocomposite material, while no ASE was detected in neat DCM waveguides.  Distributed feedback laser emission was obtained when the remote plasma polymerization composite dye thin film was deposited on a 1D corrugated silica substrate  Plasma treatments of pure organic nanowires induce the formation of nucleation sites along the NW length that act as centres for the growth of secondary organic nanowires in a hierarchical configuration.  The treatment of nanowires fabricated from metal organic molecules results in the formation of hybrid metalorganic nanowires whose microstructure can be controlled by the substrate temperature and the plasma treatment duration.  Under certain conditions, the procedure works as a template process for the development of inorganic nanowires constituted by the metal nanoparticles or the corresponding metal oxide nanoparticles corresponding to the metallic cation of the original organic molecules. General conclusions 8 273  A versatile protocol for the formation of highly porous ZnO thin films and nanowires by oxygen plasma treatment under different conditions of Zn-phthalocyanine thin films and nanowires has been demostrated.  The ZnPc conversion to ZnO is preferentially dominated by the geometry of the plasma and the substrate temperature during the experiments.  Highly porous ZnO thin films with surface coverages as low as 55% can be produced by the aforementioned method. Such a reduced density yields an extremely low refractive index (n0 = 1.05) for an optical thickness of t0 = 135 nm. As far as we know, this is one of the lowest refractive index reported for ZnO and opens the possibility of applying these layers as antireflective coatings and in graded index multilayer systems.  Fluorescence emission of the thin films is dominated by the exciton in the UV region.  The exciton peak can be monitored to follow the amount of oxygen in gas phase.  1D nanostructure formed by a ZnPc core surrounded by ZnO nanoparticles with sizes in the range between 1.1 and 3.5 nm have been synthesized by plasma treatments. A 1 Adamantane RPAVD: from thin films to 3D networks Flat thin films NWs Nanofabric 3um 2 um 4 um Nanostructured films 3 um PLASMA ABSTRACT Protective organic composite in the form of thin films completely transparent in the UV-visible region were synthesized by plasma assisted vacuum deposition method (RPAVD) from adamantane as precursor. We control the optical, compositional and microstructural properties of these films by imposing different experimental conditions: varying geometry of the substrates with respect to the plasma, pressure in the reactor, plasma source power and growth rate. Once these properties were defined, the synthesis of RPAVD materials has been extended to the nanoscale due to its interest in several applications. Adamantane RPAVD nanostructures (nanostructured films, nanowires and nanofabric) have been achieved taking the advantage of conformality property of RPAVD composites using nanostructured substrates. A1.1 Introduction 276 A1.1 Introduction Adamantane (C10H16) is a molecular hydrocarbon compound whose structure is relatively similar to the diamond. It has a tricyclic rigid ring structure as shown in Figure A1.1 (a).1 This molecule has a high vapour pressure even at room temperature and can be easily sublimated. In recent years, we have developed remote plasma polymerized thin films of dye molecules incorporating adamantane.2-6 These previous investigations have revealed RPAVD adamantane as an optimum host matrix for the development of photonically active composite intended for the development of sensors materials due to their chemical stability and transparency in the UV- region3. In the present Thesis, the characteristics and the applications of RPAVD films fabricated from adamantane without any other molecule added have been studied in-depth. The experimental device has been designed to control the variable reactor parameters (geometry of the substrates with respect to the plasma, reactor pressure, MW power of plasma source, etc.). In this Annex, we have selected a wide range of growth conditions in order to found the differences among the fabricated adamantane RPAVD thin films. As a result of this analysis, it is evidenced that adamantane RPAVD nanocomposites provide new possibilities what are beyond in advance properties. Thus, the reactions produced between plasma (in this case, argon plasma) and precursor during the deposition can influence in the ultimate thin films. On another side of the process, we have also performed a qualitative analysis of the deposition system by mass spectrometry. The objective was to identify the produced compounds into the reactor by dosing adamantane vapour to argon plasma under different experimental conditions. Finally, the last part of this annex is focused on the synthesis of RPAVD nanostructures. The main property allowing the fabrication of these nanostructures is the conformal growth RPAVD materials on the substrates. Adamantane RPAVD: from thin films to 3D networks A 1 277 Thus, we have achieved the growth of adamantane RPAVD nanostructured films as well as nanowires and as a last resort, nanofabrics, using organic nanostructures as substrates. Some advantages and applications of the adamantane RPAVD nanostructures have been displayed throughout this Thesis. The conformality is a common property of all RPAVD materials. We add some cases of nanostructures by RPAVD using other precursors. These results involve an important accomplishment in RPAVD since they entail the combination of properties of RPAVD films and nanostructures. Figure A1.1.Adamantane precursor and RPAVD thin films on regular substrates – Images of (a) adamantane solid precursor, (b) adamantane RPAVD films on fused silica and silicon (100) substrates (below) and fused silica and silicon (100) substrates as references (above). (a) (b) A1.2 Experimental Setup 278 A1.2 Experimental Setup *Thin film preparation The plasma deposition process is carried out in a remote configuration by sublimating adamantane (Aldrich) in the downstream region of an inert (Ar) microwave ECR plasma. The characteristics of the plasma reactor have been described in Chapter 1. The vapour pressure of adamantane is continuously dosing into the chamber by heating the external adamantane container. The temperature of dosing line is held at 65 ºC while the container of adamantane is heating to 32, 37, 40, 43 ºC according to the desired thin film deposition rate on the substrates, 0.1, 0.2, 0.3 and 0.4 A·s-1, respectively. The thickness of the films (range 90-500 nm) and the evaporation rate were monitored by using a quartz crystal monitor placed beside the sample holder in the deposition region. Pressure conditions are established among 10-1-10-3 Torr and MW power of plasma source was varied in the range 90-270 W. The thin films have been deposited on polished fused silica plates (1 mm thick), silicon (100) substrates and gold (100 nm) coated silicon wafer (100) (Aldrich). The fabrication of the films was carried out at two different distances between sample holder and plasma source. In addition, there are also two possible positions on the sample holder respect to the plasma source for the substrates where the film is synthesized, in front of the plasma, and back to the plasma. In summary, we studied four cases corresponding to two different distances between the sample holder and the plasma source (4 cm; 17 cm) in combination with two positions on the sample holder for the substrates (front and back). Figure A1.2 shows a diagram with the four deposition geometries analysed in the present Annex. The four types of samples fabricated in the four selected geometries are denominated samples A, B, C, D. The experimental protocol of samples A, B, C and D has been chosen to enable a wide variety of Adamantane RPAVD: from thin films to 3D networks A 1 279 different interactions of the sublimated adamantane molecules with the plasma species (from lowest and highest interaction) during the preparation of plasma polymeric adamantane thin films. The nanostructures adamantane RPAVD were fabricated under geometry C conditions at pressure of 10-2 Torr and 150 W for the MW source. We used organic nanowires grown by PVD as support for RPAVD adamantne. The fabrication of these organic nanostructured substrates has been described in Chapter 1. Figure A1.2. Location of substrates with respect to the sample holder and the plasma source - (a) the sample holder is located 17 cm from the plasma source while the substrates are back to the plasma source, (b) the sample holder is located 17 cm from the plasma source while the substrates are situated in front of the plasma source, (c) sample holder is located 4 cm from the plasma source while the substrates are back to the plasma, (d) sample holder, 4 cm from the plasma; the substrates, in front of the plasma. A1.3 Results and discussion 286 100 200 300 400 500 600 700 800 900 1000 0 10 20 30 40 50 60 70 80 90 100 Sample B Sample A Sample D Transmittance (%) Wavelength (nm) Sample C Fused silica substrate (a) 200 300 400 500 600 700 800 900 0 10 20 30 40 50 60 70 80 90 100 Sample type D Transmittance (%) Wavelength (nm) 0.3 cm 2.0 cm Sample type C (b) Figure A1.4. UV-visible spectra of transparent adamantane RPAVD thin films at different gemoetries – UV-visible transmittance spectra of adamantane RPAVD thin films grown simultaneously in: (a) the four different chamber geometries showed in Figure A1.2, (b) at topgeometry (C-D) varying slightly the distance plasma-substrate. Adamantane RPAVD: from thin films to 3D networks A 1 287 Figure A1.5. UV-visible spectra of adamantane RPAVD thin films at different temperatures and geometries - UV-visible spectra of adamantane RPAVD thin films above RT temperature at top-geometry (C-D) and RT and 90ºC. The type C of adamantane RPAVD samples is a suitable candidate overlooking optimal conditions in producing thin films for optical applications due to their topographic and optical characteristics at RT: homogeneity, low grain size, low roughness, controlled growth rate and transparency. The present Thesis shows several applications of the adamantane thin films fabricated at the geometry of sample C. However, we will continue evaluating the characterization of adamantane thin films grown under different geometrical conditions in the next sections in order to establish the differences among them. 200 300 400 500 600 700 800 900 0 10 20 30 40 50 60 70 80 90 100 RT T=90C Sample D Transmittance (%) Wavelength (nm) Sample C A1.3 Results and discussion 288 *Chemical composition of thin films The surface composition of adamantane thin films types C and D have been determined by X-ray Photoelectron Spectroscopy (XPS) deposited on Si (100) under mentioned conditions. The survey XPS spectra of the samples C and D have been measured (spectra not shown). The polymerized adamantane films are composed of C, O and N atoms. The results are summarized in Table A1.2 where the atomic percentages are presented. It is observed lower percentage of carbon in the sample deposited from direct exposure to plasma (D) than in the sample deposited back to the discharge (C). Both samples contain oxygen even if this element is not present in the adamantane monomer, and no oxygen flow is added to the discharge. A higher amount of oxygen is noticed for samples D in comparison with the film growth under geometry D (C: 8.3%; D: 14.6%). Oxygen enrichment is a common phenomenon in most plasma polymerized organic materials. Its origin can be post deposition reactions of the samples exposed to the air or direct incorporation of residual oxygen from the reactor during the deposition.3, 9-12 The incorporation of nitrogen into the sample occurs similarly. Table A1.2. Relative atomic concentration of oxygen and carbon on adamantane RPAVD grown at different geometries by XPS. – C1s,O1s and N1s relative surface concentration on samples grown at geometries C and D. Sample C1s (%) O1s (%) N1s (%) C 90.1 8.3 1.6 D 76.7 1.6 8.7 Adamantane RPAVD: from thin films to 3D networks A 1 289 Figure A1.6. XPS spectra of C1s signal of adamantane RPAVD thin films grown at different geometries – Fitted C1s signal of adamanatane into different components for a sample type C (a) and D (b). Figure A1.6 shows fitted C1s spectra XPS corresponding to the survey of the samples C and D. The fitting procedure enables a semiquantitative evaluation of the relative concentration of carbon functional groups in the surface of the films. C1s signals obtained can be divided into several components due to different types of carbon-carbon, carbon-oxygen and hydrogen bonds in the thin film. The position of the individual components has been taken from previous works of plasma polymerization.13 Both spectra are characterized by an intense peak at around 285 eV due to C-C and C-H bonds. It also presented a defined shoulder at 288 eV which is typical of the presence of C-O and C-N bond structures in the film. The relative intensity of the shoulder is 295 290 285 280 275 0 10000 20000 30000 40000 295 290 285 280 275 0 10000 20000 30000 40000 Intensity (a.u.) C1s (a) Sample C Binding Energy (eV) Intensity (a.u.) C1s (b) Sample D A1.3 Results and discussion 290 significantly smaller for the sample C which shows an additional slight peak at around 281 eV. Thus, the C1s spectrum of sample C is more symmetric, confirming the relatively small oxygen content in this sample. While the relative high intensity of the shoulder at 288 eV in the sample D indicates that the concentration of C-O and C-N groups is rather abundant in this case. The FTIR and RAMAN spectroscopy examination of the adamantane thin films provide important information about the chemical bonds and functional groups constituting the film structure. Hence, the polymeric character of the films was completed by these analyses. Figure A1.7 shows the normalized FTIR spectra of an adamantane plasma polymerized films. To record these spectra, it is necessary a thickness in the range at least of 100 nm to have enough sensitivity. Assignment of the bands in the FT-IR spectra has been done according to the bibliography.1, 6, 12, 14, 15 Spectra recorded for sublimated adamantane molecules and remote plasma polymerized adamantane film have been reported previously in some works on perylene plasma polymer. 3 Both spectra are dominated by two strong bands, at around 2900 cm-1, that are attributed to C-H vibrations, but with an appreciable broadening for the plasma polymer. Thus, these vibrational features in plasma deposited sample have a tendency to a single broad band at the same time that the plasma-molecules interaction has been higher during the deposition (see Figure A1.7 (a)). Another band at around 1500 cm-1 due to C-H vibrations is present in all cases. In addition, other less-intense bands due to C=C, –C=O and –OH stretching vibration are observed at about 1400, 1700 and 3400 cm-1. Such bands due to – C=O and –OH groups respond to the aforementioned phenomenon in plasma deposition processes since the residual oxygen and/or water molecules in the chamber that become plasma activated and react with the fragments of the monomer and post deposition reactions of trapped radicals and O2. In agreement with the XPS results, these contributions are greater in the sample D. Adamantane RPAVD: from thin films to 3D networks A 1 291 Table A1.3. Raman spectroscopy peak positions (cm–1) – Raman peaks assignations for the adamantane RPAVD samples.15 In Figure A1.7 (b), visible RAMAN spectra show the comparison among adamantane power and remote plasma polymeric films deposited at geometries C and D. In general, an amorphous carbon can have any mixture of sp3, sp2, and even sp1 sites, with the possible presence of up to 60 at.% hydrogen.16 The Raman spectra of carbons show several common features in the 1000–2000 cm-1 region. Two basics common features are observed around 1350 and 1600 cm-1 for the visible excitation, called G and D peaks. The G and D peaks are due to sp2 sites only. The G peak is due to the bond stretching of all pairs of sp2 atoms in both rings and chains. The D peak is due to the breathing modes of sp2 atoms in rings.1 7 However, it was demonstrated the indirect influence of the sp3 content on increasing G position. The experimental visible Raman spectra on amorphous carbons is interpreted using a phenomenological three-stage model.16 The most intense band in the spectrum of the adamantane powder around 1230 cm-1 does not appear in adamantane polymerized samples C and D. The peaks G and D are presented in both samples, but the bands are much broadened for the sample D than C. This band broadening has also been observed in FTIR spectra. The spectrum shape of sample D indicates a greater Position (cm-1) Group Vibration mode 3400 O-H Stretching 2940 C-H Stretching, aliphatic groups 2860 1466 CH2 CH2 Stretching, aliphatic groups Scissors 1700 C=O Stretching in aliphatic groups 1450-1600 C=C Stretching in aromatic ring system A1.3 Results and discussion 292 bond disorder than in sample C. The band from adamantane powder at 1440 cm- 1 is observed in the sample C, while it disappears for sample D. Figure A1.7. FTIR and visible RAMAN spectroscopy of adamantane RPAVD thin film at different geometries - (a) FTIR spectra for samples A,B;C ;D. (b) visible RAMAN spectra of samples C and D 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 Adamantane powder Sample D Intensity (a.u.) Raman shift (cm-1) Sample C (b) 4000 3000 2000 1000 0 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 Sample B Sample C Absorbance (a.u.) Wavenumber (cm-1) Sample D (a) Adamantane RPAVD: from thin films to 3D networks A 1 293 The XPS, FTIR and RAMAN analyses of the polymeric thin film are congruent with a cross-linked structure resulting from the fragmentation of the adamantane molecules during the film growth. The cross-linking is apparently greater in the case of the samples growth under geometries more exposed to the plasma. This solid polymeric character of the thin films provides thermal and chemical stability in air and insolubility in organic solvents. Despite the high cross-linking degree observed in sample C, plasma is softer and less effective than in sample D. Thus, position C allows the use of sensitive substrates as paper, polymers or organics nanostructures. A1.3.1.2 Effect of Pressure, Power of MW plasma source and growth rate on Adamantane RPAVD thin film synthesis The RPAVD reactor supplies a very wide range of experimental possible conditions for the growth of the polymeric material, as discussed in the experimental setup section. Previously, it has been proved that the geometry of sample holder with respect to the plasma influences the growth and the resulting materials. Others experimental conditions such as pressure, MW power and growth rate can potentially affect the cross-linking degree of the polymeric thin film and consequently the material properties. A study of a set of samples in geometries close to C at different growth conditions selected in the reactor and thickness is presented in Table A1.4. The objective of this study was to achieve the conditions of the polymerization degree for adamantane thin films providing transparency in UV-range, chemical and thermal stability and insolubility in solvents. A1.3 Results and discussion 294 Sample Sample-plasma distance (cm) p (Torr) P (W) Growth rate (A/s) Thickness (nm) RPDA-1 1.8 10-2 150 0.2 82 RPDA-2 1.8 10-2 240 0.3 102 RPDA-3 1,8 10-3 240 0.3 116 RPDA-4 0.3 10-2 150 0.2 164 RPDA-5 0.3 10-3 240 0.2 165 RPDA-6 1.8 10-2 240 0.4 290 RPDA-7 3.8 10-2 150 0.2 395 Table A1.4. Experimental growth conditions of a set of samples of adamantane RPAVD films at geometry type C – The experimental parameters of growth are sample-plasma distance, pressure, MW source power, growth rate and the thickness of the films. In Figure A1.8 (a) are plotted UV-visible transmission spectra of the set of adamantane RPAVD samples. Fused silica spectrum is included in the figure as reference. The spectra define a very flat high transmittance in the spectral region extending from 300 to 900 nm for samples of up to thickness at around 200 nm while a shorter transparent spectral region is observed for thicker samples (400-900 nm). The transmittance decreases in the RPAVD thin films showing a broad absorption (at about 270 nm) that is not noticeable in fused silica substrate spectrum. Such absorption in the UV region can be attributed to the presence of conjugated unsaturated bonds in the film because of adamantane are not completely fragmented.3 This partial fragmentation could confirm that the plasma in RPAVD is soft. It represents an advantage in the use of sensitive substrates that are not damaged for the plasma. Adamantane RPAVD: from thin films to 3D networks A 1 295 Figure A1.8. Spectroscopy of RPAVD thin film deposited under different experimental parameters at geometry type C - (a) UV-visible transmittance and (b) RAMAN spectra of samples which growth conditions are described in Table A1.4. The mesurement of fused silica in (a) and adamantane power in (b) have been included as references respectively. 1200 1300 1400 1500 1600 1700 0,0 0,4 0,8 1,2 1,6 2,0 2,4 2,8 3,2 3,6 RPDA-7 RPDA-6 RPDA-5 RPDA-4 Intensity (a.u) Raman Shift (cm-1) Adamantane powder RPDA-3 (b) 100 200 300 400 500 600 700 800 900 1000 0 10 20 30 40 50 60 70 80 90 100 Transmittance (%) Wavelength (nm) Fused silica RPDA-1 RPDA-2 RPDA-3 RPDA-4 RPDA-5 RPDA-6 RPDA-7 (a) A1.3 Results and discussion 302 Figure A1.11 (a) shows the mass spectrum of adamantane plasma (withouth argon) obtained at 10-3 Torr (lowest pressure) and 150 W. There were not any signals under others experimental conditions. Figure A1.11 (b,d) presents RPAVD at different powers and pressures. Greater mass fragments predominate at higher pressures, while lower mass fragments increase at lower pressures. Ion mass spectra of adamanatane show low intensity, especially at higher pressures. Particularly, at 150 W and pressure close to 10-2 Torr, the intensity is almost zero, see Figure A1.11(c). H2+ H2O+ Ar++ m/z=27 m/z=28 m/z=29 O2+ m/z=39 Ar++ m/z=41 CO2+ m/z=53 m/z=55 m/z=57 m/z=67 m/z=77 m/z=79 m/z=80 m/z=93 m/z=94 m/z=95 m/z=107 m/z=121 m/z=135 ADA+ m/z=137 0 2000 4000 6000 Adamantane plasma (without Ar) 10-2 Torr Intensity (au) (a) H2+ H2O+ Ar++ m/z=27 m/z=28 m/z=29 O2+ m/z=39 Ar++ m/z=41 CO2+ m/z=53 m/z=55 m/z=57 m/z=67 m/z=77 m/z=79 m/z=80 m/z=93 m/z=94 m/z=95 m/z=107 m/z=121 m/z=135 ADA+ m/z=137 -- 0 200 400 600 10-3 Torr 10-2 Torr 10-1 Torr Intensity (au) 90 W (b) H2+ H2O+ Ar++ m/z=27 m/z=28 m/z=29 O2+ m/z=39 Ar++ m/z=41 CO2+ m/z=53 m/z=55 m/z=57 m/z=67 m/z=77 m/z=79 m/z=80 m/z=93 m/z=94 m/z=95 m/z=107 m/z=121 m/z=135 ADA+ m/z=137 0 10 20 30 40 1·10-3 Torr 5·10-3 Torr 1·10-2 Torr 5·10-2 Torr 1·10-1 Torr Intensity (au) 150 W (c) H2+ H2O+ Ar++ m/z=27 m/z=28 m/z=29 O2+ m/z=39 Ar++ m/z=41 CO2+ m/z=53 m/z=55 m/z=57 m/z=67 m/z=77 m/z=79 m/z=80 m/z=93 m/z=94 m/z=95 m/z=107 m/z=121 m/z=135 ADA+ m/z=137 0 10 20 30 40 50 60 10-3 Torr 10-2 Torr 10-1 Torr Intensity (au) 270 W (d) Figure A1.11. SIMS mass spectrometry in the RPAVD reactor under different adamantane RPAVD experimental conditions – Mass spectroscopy of: adamantane plasma (a), RPAVD conditions at 90 W (b), at 150 W (c) and 270 W (d) MW power source for several pressures values. Adamantane RPAVD: from thin films to 3D networks A 1 303 Regarding energy measurements of argon (Ar+) and adamantane (ADA+), the increment of growth rate of adamantane shifts argon peak to higher energies. Greater energies of ions argon and adamantane are observed for lowest pressures. The intensity of ADA+ signal is lower than Ar+, and becomes absent at higher pressures, mainly at 10-1 Torr (Figure A1.12). It is worth noting that intermediate values of pressure and MW power (about 10-2 Torr, 150 W) show minimum energies. Figure A1.12. Ar + and adamantane positive ion energies under different experimental conditions – Spectrometry of Ar + and positive adamantane ion energies at pressure range of 10 10-1-10-3 Torr and MW power source values of 90-270 W. 012345678910 11 12 13 14 15 0 5000 10000 15000 20000 ADA+, 90 W Intensity (au) Ion Energy (eV) 012345678910 11 12 13 14 15 0 100000 200000 300000 400000 Ar+, 90 W Intensity (au) ion Energy (eV) 012345678910 11 12 13 14 15 0 5000 10000 15000 20000 25000 ADA+, 150 W Intensity (a.u.) Ion Energy (eV) 1·10-3 Torr 5·10-3 Torr 1·10-2 Torr 5·10-2 Torr 1·10-1 Torr 012345678910 11 12 13 14 15 0 100000 200000 300000 400000 500000 600000 700000 800000 Ar+, 150 W Intensity (a.u.) Ion Energy (eV) 012345678910 11 12 13 14 15 0 2000 4000 6000 8000 10000 12000 Intensity (a.u.) Ion energy (eV) ADA+, 270 W 012345678910 11 12 13 14 15 0 200000 400000 600000 800000 Ar+, 270 W Intensity (a.u.) Ion energy (eV) A1.3 Results and discussion 304 Figure A1.13. Mass spectrometry of adamantane RPAVD conditions at different growth rate – Mass spectra (a) and Ar + (b) and adamantane positive (c) ion energies at growth rate of 0.1, 0.2, 0.3 and 0.4 A/s (10- 2 Torr, 150 W) 020 40 60 80 100 120 140 0 200 400 600 800 1000 1200 1400 Intensity (au) m/z (amu) 0.1A/s 0.2A/s 0.3A/s 0.4A/s (a) 012345678910 11 12 13 14 15 0 100000 200000 300000 400000 500000 0.1A/s 0.2A/s 0.3A/s 0.4A/s Intensity (au) Ion energy (eV) Ar+ (b) 012345678910 11 12 13 14 15 0 10000 20000 30000 40000 50000 0.1A/s 0.2A/s 0.3A/s 0.4A/s Intensity (a.u.) Ion Energy (eV) ADA+ (c) Adamantane RPAVD: from thin films to 3D networks A 1 305 Mass spectra under fixed pressure and plasma power conditions (10-2 Torr, 150 W) at different growth rate of material only are obtained for the greatest growth rate, 0,4 A/s, Figure A1.13(a). Ar+ shape spectrum of energy changes when growth rate of adamantane raises. Figure A1.13(c) ADA+ energy shifts towards higher energies with increasing the adamantane growth rate, Figure A1.13(b). A1.3.3 Effect of substrate microstructure on ADAMANTANE RPAVD thin film synthesis Adamantane RPAVD thin films on Si (100) and quartz substrates (flat substrates without microstructure) are very stable, insoluble in water, ethanol and toluene solvents, thermally stable up to 250°C and have flat, continuous, regular and homogeneous microstructures, free of cracks and micro aggregates that are responsible for light dispersion (Figure A1.14). These properties are general for RPAVD thin film and were presented in Chapter 1. Taking step forward in the analysis of the material, the microstructure of the RPAVD films is quite different when the deposition takes place on substrates that are not flat in the microscale. In previous works, it has previously reported conformal plasma polymerization of 3-hydroxyflavone with adamantane on polystyrene opals18, 19. The topmost layer of the opal acted as a template for the growth of the polymeric nanostructured film with a perpendicular preferential direction of expansion and a low lateral growth. A similar behaviour is depicted by adamantane RPAVD film grown on ZnO nanorods.20 The preferential growth of the polymeric material is on top of these nanostructures (Figure A1.15 (a) and (b)). The images further illustrate that the RPAVD film is covering entirely the nanorods reaching even their bases (Figure A1.5 (c) and (d)). A1.3 Results and discussion 306 Figure A1.14. SEM micrography of surface and edge of RPAVD adamantane on flat substrate of Si (100) – Smooth grown of RPAVD adamantane film on flat substrate has been grown in position C. Figure A1.15. Adamantane RPAVD on nanorods of oxides - SEM (a) and TEM (b-d) micrographs of adamantane RPAVD on Ag@ZnO nanorods. Adamantane RPAVD: from thin films to 3D networks A 1 307 Figure A1.16. Adamantane RPAVD on carbon nanotubes - Surface view (a) and (b) cross section SEM micrographs of adamantane RPAVD on carbon nanotubes. On the other hand, Figure A1.16 shows an adamantane RPAVD material deposited on carbon nanotubes. RPAVD adamantane have partially deposited on the outermost surface of the nanotubes (Figure A1.16 (b.1,b.2)) while the innermost areas of the polymeric material has not reached (Figure A1.16.b.3). Thus, the distance among the nanostructured features is playing an essential role in the growth of RPAVD adamantane. In the same way, adamantane RPAVD materials on other not flat substrate were grown. Figure A1.17 (a),(b) and (c) show SEM micrographs of adamantane plasma polymerized thin films deposited on ITO, gold, and NaCl substrates. The images confirm that the RPAVD films reproduce the shape of the surface microstructure of the substrates where the thin film has been deposited. In addition, the film fills the void space among nanostructures composing the substrates. Nevertheless, the covering presents a strong dependency with: the size of substrates nanostructures, the distance among them A1.3 Results and discussion 308 and their height. In summary, when the nanostructured features are separated hundred nanometers and have a height of the order of microns, the adamantane RPAVD thin film reaches the base. Hence, the materials are conformed to the surface of the substrates. On the other side, if the nanostructures have a separation of tens of nanometers and a height of tens of microns such as carbon nanotubes in Figure A1.16, the coating is partial and does not reach the substrate. This behaviour of the material has been also tested by AFM analysis technique. Figure A1.17. Adamantane RPAVD on rough surfaces of ITO, gold and NaCl - SEM micrographs of conformed and self-supported adamantane RPAVD on substrates of ITO (a), NaCl crystals (b) and Au (c), grown in position of the sample C. An image of adamantane RPAVD self-supported is shown in (d). Adamantane RPAVD: from thin films to 3D networks A 1 309 Moreover, the polymeric film can be easily separated from the substrate. This sets a way to obtain self-supported thin film of adamanatane RPAVD with a desired microstructure. Figure A1.17(c) illustrates adamantane polymeric film removing from the gold substrate and Figure A1.17 (d) an adamantane RPAVD completely removed from the NaCl crystal substrate by dissolving in water and transferred to a TEM grid. The observed characteristics of adamantane RPAVD nanostructuration in the present section are applicable to all polymeric materials deposited by RPAVD. A1.3.3.1 1D nanostructures of adamantane RPAVD In the course of this Thesis we have contemplated the possibility of achieving the synthesis of adamantane RPAVD 1D nanostructures due to the importance of nanostructures such as nanowires in photonics, photovoltaic, photocatalysis, microelectronic and nanosensing applications.21-25 Previous section demonstrated that adamantane RPAVD films are conformal to the substrate surface under certain conditions of separation among its micro/nanostructured motifs. Based on this property, the combination of two methodologies was proposed for the synthesis of RPAVD nanostructures: organic nanowires (ONWs) by PVD and organic composite thin films by Remote Plasma Activated Vacuum Deposition (RPAVD). The procedure consists of two steps. First, we deposit organic nanowires by physical vapour deposition, as was detailed in the Chapter 1. Secondly, these ONWs are covered with an adamantane coating grown by remote plasma assisted vapour deposition (RPAVD). By this way, we achieve supported organic core@shell structures by vacuum methodologies. SEM and STEM micrographs in Figure A1.18 show an example. Perylenediimide (2,9-dimethyl-anthra[2,1-def:6,5,10-d′e′f′] diisoquinoline 1,3,8,10-tetrone (Me-PTCDI) (from Sensient Imaging A1.3 Results and discussion 310 Technologies) ONWs by PVD are forming the core of these heterostructures. It is worth to notice that organic shell also covers the substrate even high NWs density (as shown in Figure A1.18 (b)). The studies of adamantane RPAVD nanowires included in Chapters 3-4 have demonstrated that dielectric or freezing properties of adamantane RPAVD thin films remains in the adamantane RPAVD shells of ONWs. Thus, the adamantane RPAVD ONWs keep their chemical and optical properties. In addition, the procedure enables the fabrication of organic hollow 1D RPAVD nanostructures by vacuum heating post-treatments. Organic core@shell nanostructures such as those of Figure A1.18 were heated until temperatures above 250 ºC in vacuum (10-5 mbar). This process leads to the sublimation of the organic inner part of structure fabricated by PVD. Figure A1.18. 1D nanostructures core@shell - SEM (a,b) and STEM (c) images of organic core@shell fabricated from Me-PTCDI ONWs as core and adamantane RPAVD coatings as shell. Adamantane RPAVD: from thin films to 3D networks A 1 311 Figure A1.19. Hollow adamantane nanowires – (a) Growth sketch, (b) SEM micrographs and (c) fluorescent emission of hollow adamantane nanostructures. 350 400 450 500 550 600 650 1.6x106 1.8x106 2.0x106 2.2x106 2.4x106 2.6x106 2.8x106 3.0x106 ex= 350 nm Photoluminiscence (a.u.) Wavelength (nm) 600 650 700 750 800 850 900 0.0 4.0x105 8.0x105 1.2x106 1.6x106 2.0x106 2.4x106 2.8x106 Photoluminiscence (a.u.) Wavelength (nm) ex=540 a) b) c) i) Original NWs by PVD ii) RPAVD of adamantane iii) Resistive heating