Organic molecules adsorbed on inorganic substrates are the topics of interest in this thesis. Interfaces of this kind are found in dye sensitized solar cells that convert solar energy to electricity, a promising environmentally friendly energy source which might provide a route to replace fossil fuels. Another field where these interfaces play a role is in molecular electronics, an approach to solve the down scaling in the ever increasing hunt for miniaturized electronic devices. The motivation for this work lies among other in these applications and surface science is a suitable approach to investigate the electronic and morphologic properties of the interfaces as it provides detailed knowledge on an atomic level. Phthalocyanines are the organic molecules investigated and the inorganic substrates range from wide band gap via narrow band gap semiconductors to metals. Photoelectron and X-ray spectroscopy experiments are performed to shed light on the electronic properties of the adsorbed molecules and the substrate, as well as the chemical interaction between adsorbate and substrate at the interface. The ordering of the adsorbate at the interface is important as ordered molecular thin films may have other properties than amorphous films due to the anisotropic electronic properties of the organic molecules; this is investigated using scanning tunneling microscopy. We find that the phthalocyanines are affected by adsorption when the substrate is TiO2 or Ag, where charge transfer from the molecule occurs or an interface state is formed respectively. The molecules are adsorbed flat on these surfaces giving a large contact area and a relatively strong bond. On Ag, ordered structures appear with different symmetry depending on initial coverage. The reactivity of the TiO2 surface is not ideal in the solar cell application and by modifying the surface with a thin organic layer, the negative influence on the adsorbed phthalocyanine is reduced. ZnO is not as reactive as TiO2, thanks maybe to the upright adsorption mode of the phthalocyanines. The semiconductor InSb is less reactive leading to self-assembled molecular structures on the (001) surface, either homogenously distributed in a one monolayer thick film or in strands along the reconstruction rows. InAs on the other hand has a larger influence on the adsorbed molecules resulting in a metallic film upon thermal treatment. / QC 20100812

Quasicrystals is a new kind of material that have several interesting aspects to it. The unusual atomic structure entails many anomalous and unique physical properties, for example, high hardness, and extremely low electrical and thermal conductivity. In thin films quasicrystals would enable new functional materials with a combination of attractive properties.In this work, AlCuFe and Nanoflex steel, materials that are known to form quasicrystals in bulk, have been deposited as thin films on Si and Al2O3 substrates using DC magnetron sputtering. These thin films were heat treated, and the formation and growth of different phases, among other approximant and quasicrystalline phases, were studied using mainly in-situ X-ray diffraction.During the project several problems with the formation of quasicrystals were encountered, and it is proposed how to overcome these problems, or even how to make use of them. Finally, the quasicrystalline phase was realized, although it was not completely pure. In the end some suggestions for future work is presented.

Thermistor material can be used to fabricate un-cooled IR detectors their figure of merit is the Temperature Coefficient of Resistance (TCR). Ge dots in Si can act as a thermistor material and they have a theoretical TCR higher than for SiGe layers but they suffer from intermixing of Si into the Ge dots. Ge dots were grown on unstrained or strained Si layers and relaxed or strained SiGe layers at temperatures of 550 and 600°C by reduced pressure chemical vapor deposition (RPCVD). Both single and multilayer structures where grown and characterized. To achieve a strong signal in a thermal detector a uniform shape and size distribution of the dots is desired. In this thesis work, an endeavor has been to grow uniform Ge dots with small standard deviation of their size. Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) have been used to characterize the size and shape distribution of Ge dots. Ge contents measured with Raman spectroscopy are higher at lower growth temperatures. Simulation of TCR for the most uniform sample grown at 600°C give 4.43%/K compared to 3.85%/K for samples grown at 650°C in a previous thesis work. Strained surfaces increases dot sizes and make dots align in crosshatched pattern resulting in smaller density, this effect increases with increasing strain. Strain from buried layers of Ge dots in a multilayer structure make dots align vertically. This alignment of Ge dots was very sensitive to the thickness of the Si barrier layer. The diameter of dots increase for each period in a multilayer structure. When dots are capped by a Si layer at the temperature of 600°C intermixing of Si into the Ge dot occurs and the dot height decrease.

<p>Quasicrystals is a new kind of material that have several interesting aspects to it. The unusual atomic structure entails many anomalous and unique physical properties, for example, high hardness, and extremely low electrical and thermal conductivity. In thin films quasicrystals would enable new functional materials with a combination of attractive properties.In this work, AlCuFe and Nanoflex steel, materials that are known to form quasicrystals in bulk, have been deposited as thin films on Si and Al2O3 substrates using DC magnetron sputtering. These thin films were heat treated, and the formation and growth of different phases, among other approximant and quasicrystalline phases, were studied using mainly in-situ X-ray diffraction.During the project several problems with the formation of quasicrystals were encountered, and it is proposed how to overcome these problems, or even how to make use of them. Finally, the quasicrystalline phase was realized, although it was not completely pure. In the end some suggestions for future work is presented.</p>

O estudo de sistemas híbridos compostos por interfaces orgânico/inorgânico, ou sólido/líquido, tem apresentado crescente interesse nas áreas de eletroquímica e nanotecnologia. Além de objetos de pesquisas básicas, estes sistemas apresentam um potencial para inúmeras aplicações, dentre elas: línguas eletrônicas, \\emph{self-assembled monolayers} (SAMs), dispositivos fotovoltáicos, baterias alternativas, carreadores de drogas, entre outras. Em especial, o uso de superfícies de ouro em pesquisas ocorre principalmente por causa da natureza inerte deste material, permitindo explorar uma ampla quantidade de potenciais eletrostáticos que induzem a eletrólise ou reações eletroquímicas em outras superfícies. O estudo de voltametria cíclica em SAMs formadas por cadeias polipeptídicas funcionalizadas com ferroceno vem sendo realizado durante as duas últimas décadas. Recentemente, iniciou-se uma controvertida discussão a respeito do mecanismo de transporte eletrônico entre o centro oxidativo e o eletrodo destes sistemas. Alguns grupos argumentam a favor de tunelamento eletrônico do centro oxidativo, enquanto outros grupos têm proposto uma interação de pares \\emph{elétron-buraco} dos grupos amida dos peptídeos. Além disto, interfaces com sistemas primitivos, como por exemplo a água, ainda são temas de pesquisa correntes de diversos grupos de pesquisa, devido à complexidade dos resultados experimentais reportados. De forma a contribuir com estas discussões correntes na literatura, selecionamos dois problemas distintos utilizando a superfície Au(111) como base comum: i) estudo do mecanismo de transferência de carga de um peptídeo funcionalizado com ferroceno; ii) estudo das propriedades eletrônicas e estruturais da água interagindo com NaCl. Para realizarmos a investigação das propriedades eletrônicas, empregamos a Teoria do Funcional da Densidade no esquema de Kohn-Sham (KS). Para analisar as propriedades dinâmicas e estruturais, foi utilizada também a técnica de dinâmica molecular clássica (MD). A partir de diversos modelos da interação do peptídeo sobre Au(111), investigamos as densidades de estados, cargas de Löwdin e funções de onda de KS. Notamos a presença de estados eletrônicos localizados tanto sobre o ferroceno quanto sobre o ouro, ambos sempre próximos da região da energia de Fermi, em todos os casos propostos. Estes resultados sugerem um tunelamento eletrônico entre sítio do ferroceno e a superfície Au(111) como o mecanismo de transferência eletrônica. Para o caso do sal dissociado em água, investigamos e discutimos a estrutura eletrônica em diversas situações e configurações. Além disto, realizamos um estudo MD, onde observamos que o ordenamento das moléculas de água é bastante sensível à presença da superfície Au(111). Os resultados obtidos apresentam uma visão ampla dos comportamentos eletrônicos e dinâmicos de sistemas envolvendo a superficie Au(111) que discutem questões correntes na literatura. / The study of hybrid interfaces, e.g. organic/inorganic or solid/liquid, have been showing an increasing interest in electrochemistry and nanotechnology. Within this subject, there are basic and applied studies, such as electronic tongues, self-assembled monolayers (SAMs), photovoltaic devices, alternative batteries, drug carriers and others. In special, the preference for gold surface occurs due to its inert nature, allowing the exploration of a wide range of electrostatic potentials which induces electrolysis and chemical reactions in other surfaces. The Cyclic Voltammetry study in Peptide-SAMs modified by ferrocene has been investigated in the literature. In recent years, a controversy on the charge transfer mechanism in biological materials started: at one hand, there are arguments in favor of an electronic tunneling process from the oxidative center to the eletrode; on the other hand, some authors suggest electronic hopping from the amide groups of the peptides, generating an electron-hole pair that ``walks\'\' from the ferrocene to the eletrode. Furthermore, systems with primitive interfaces, such as water, are also subject of current research due to the complexity of the experimental results reported in the literature. Within this scenario, we selected two distinct problems using the surface Au(111): i) the study of charge transfer mechanism using a peptide modified by ferrocene; ii) the study of electronic and structural properties of water interacting with NaCl. In order to obtain the electronic properties, we employed the Density Functional Theory in the Kohn-Sham (KS) scheme. For the structural and dynamics properties, we also used classical molecular dynamics (MD). Based on different models for the ferrocene-peptide/Au(111) interface, we investigate the density of states, Löwdin charges and KS wavefunctions. We notice the presence of localized electronic states on the ferrocene and gold which are close to the Fermi energy in all studied cases. These results suggest an electronic tunneling from the ferrocene site to the surface Au(111) as the mechanism for the charge transfer. In the case of salt dissociated in water, we investigated the electronic properties in several different configurations. Furthermore, in a MD perspective, the orientation of the water molecules presented a high sensitivity for the Au(111) interface. These results represent a wide view of the electronic and dynamic behavior of systems using the surface Au(111) as a common subject.

Orientadores: Richard Landers, Abner de Siervo / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-14T22:49:38Z (GMT). No. of bitstreams: 1 Pancotti_Alexandre_D.pdf: 18876572 bytes, checksum: 607e084dcdd143737b311eaaf705dc73 (MD5) Previous issue date: 2009 / Resumo: Este trabalho apresenta um estudo sobre a estrutura atômica da superfície de dois materiais com potencial para suportar nano partículas metálicas num catalisador modelo, o primeiro, Óxido de Cromo é um suporte usado comercialmente, e o segundo, SrTiO3(100) (STO) tem a característica interessante de induzir o crescimento de nano partículas de alguns metais com formatos definidos pelo tipo de tratamento térmico que o suporte recebeu. O Óxido de cromo estava na forma de filmes ordenados crescidos epitaxialmente sobre um cristal de Pd(111) enquanto que o SrTiO3 consistia de cristal dopado com Nb cortado segundo a face (100). Os filmes de óxido de cromo sobre Pd(111) foram crescidos "in-situ" na câmara de análises pela deposição de cromo metálico numa atmosfera de oxigênio (1,0.10-6 mBar) sobre o substrato aquecido (623K) o que produziu filmes com boa cristalinidade e estequiometria. Foram estudados filmes com duas estruturas diferentes: óxido de cromo com 3.5 Å de espessura que apresentava uma reconstrução tipo p(2x2) e um filme mais espesso com 12.0 Å que mostrava uma reconstrução (V3xV3)R30o, ambos determinados por LEED (Low Energy Electron Diffraction). A composição e a estrutura atômica foram determinadas por XPS (X-Ray Photoelectron Spectroscopy) e XPD (X-Ray Photoelectron Diffraction), respectivamente. Titanato de Estrôncio, SrTiO3 normalmente é um material isolante, mas a adição de Nb como dopante o transformou em um material suficientemente condutor para permitir o uso das mesmas técnicas para o estudo da estrutura atômica e composição de sua superfície. Nestes estudos foram utilizados como fontes de excitação radiação Síncrotron de 700 eV de energia da linha SGM do Laboratório Nacional de Luz Síncrotron e fontes convencionais de raios-X (Al, Mg) Ka. Para o modelamento teórico das estruturas superficiais empregou se o pacote MSCD [1] juntamente com um algoritmo genético [2] para acelerar a procura dos parâmetros estruturais. Determinou se que o filme mais fino de óxido de cromo correspondia a CrO com uma reconstrução p(2x2). A primeira e a segunda distâncias interplanares foram determinadas como sendo iguais a 0,16 Å e 1,92 Å, respectivamente. O filme mais grosso foi estudado em duas situações, como crescido e após tratamento térmico a 973K. Nas duas situações o filme se apresentou como a-Cr2O3(0001), mas com terminações diferentes. Antes do aquecimento do filme os resultados de XPD mostraram que a superfície é terminada por uma camada de "O" com a primeira distância interplanar expandida de 9,5% em relação ao seu valor no volume. Depois do aquecimento, a superfície é terminada em uma dupla camada formada por átomos de Cr, com a primeira distância interplanar reduzida de 68% em relação ao seu valor no volume. Os resultados para a superfície do STO mostraram regiões de SrO e TiO2. Em ambos os casos todas as camadas de cátions relaxam para dentro e os átomos da segunda camada relaxam para fora resultando na corrugação das superfícies do TiO2e SrO. Estes resultados se comparam muito bem com os resultados encontrados por LEED. Usando o algoritmo genético foi possível determinar que 30% da superfície do cristal é recoberta por ilhas de SrO(100) / Abstract: This thesis presents a study of the atomic structure of the surface of two materials with potential as supports for metallic nanoparticles in model catalysts. The first is Chromium Oxide that is used as a support for commercial catalysts, and the second is SrTiO3(100) (STO), which has the very interesting characteristic of inducing the growth of nanoparticles of some metals with different shapes depending on the type of heat treatment of the support. The Chromium Oxide used in this study was in the form of ordered films grown epitaxially on a Pd(111) crystal, while the SrTiO3 consisted of a bulk crystal doped with Nb cleaved along the (100) face. The epitaxial Chromium Oxide films were grown in situ in a surface analysis chamber by evaporating metallic Cr under 1.0x10-6 mBar of O2 pressure on to the substrate heated to 623K. The films as grown showed clear LEED (Low Energy Electron Diffraction) patterns and constant stoichiometry. Two different reconstructions were studied: p(2x2), that was present for thin films of about 3.5 Å, and (V3xV3)R30o, which is characteristic of thicker films (above 12.0 Å). The composition and detailed surface structures were determined by XPS (X-Ray Photo electron Spectroscopy) and XPD (X-Ray Photoelectron Diffraction). SrTiO3 is an insulator, but doping with Nb makes it sufficiently conducting so that it is possible to use LEED, XPS and XPD without charging problems inherent to insulating samples. Two types of radiation were used for exciting the samples: synchrotron radiation (700eV) from the SGM beam line at the Brazilian National Synchrotron Radiation Light Laboratory and conventional X-rays from Al and Mg anodes. To simulate the surface structures the MSCD package [1] was used. To accelerate the optimization of the structural parameters a genetic algorithm [2] was used in conjunction with the MSCD package. The thinner Chromium Oxide film was shown to consist of CrO with a p(2x2) reconstruction, having as first and second interlayer distances 0,16 Å and 1,92 Å respectively. The thicker Chromium Oxide film was studied as grown and after annealing at 973K. In both cases the structure was determined to be a-Cr2O3(0001), but with different surface termination. XPD revealed that the film as grown was terminated by an O monolayer, with the first interlayer distance expanded by 9.5% relative to bulk values. After annealing the film was shown to terminate in two atomic layers of Cr, who¿s interlayer distance was reduced by 68% relative to the bulk. Results for the surface of STO showed regions covered by SrO and TiO2. In both cases the cation layers relaxed inwards and the atoms of the second layer outwards, resulting in corrugated surfaces. These results compare very well with results obtained by LEED. By using the genetic algorithm [2], it was possible to show that 30% of the surface was covered by SrO(100) islands / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Understanding the “incomprehensible” world of materials is the biggest challenge to the materials science community. To access the properties of the materials and to utilize them for positive changes in the world are of great interest. Often scientists use approximate theories to get legitimate answers to the problems. Density functional theory (DFT) has emerged as one of the successful and powerful predictive methods in this regard. The accuracy of DFT relies on the approximate form of the exchange-correlation (EXC) functional. The most complicated form of this functional can be as accurate as more complicated and computationally robust method like Quantum Monte Carlo (QMC), Random Phase Approximation (RPA). Two newest meta-GGAs, SCAN and SCAN+rVV10 are among those functionals. Instantaneous charge fluctuation between any two objects gives rise to the van der Waals (vdW) interactions (often termed as dispersion interactions). It is a purely correlation effect of the interacting electrons and thus non-local in nature. Despite its small magnitude it plays a very important role in many systems such as weakly bound rare-gas dimers, molecular crystals, and molecule-surface interaction. The traditional semi-local functionals can not describe the non-local of vdW interactions; only short- and intermediate-range of the vdW are accounted for in these functionals. In this thesis we investigate the effect of the weak vdW interactions in surface properties, rare-gas dimers and how it can be captured seamlessly within the semi-local density functional approximation. We have used summed-up vdW series within the spherical-shell approximation to develop a new vdW correction to the meta-GGA-MS2 functional. This method has been utilized to calculate binding energy and equilibrium binding distance of different homo- and hetero- dimers and we found that this method systematically improves the MGGA-MS2 results with a very good agreement with the experimental data. The binding energy curves are plotted using this MGGA-MS2, MGGA-MS2-vdW and two other popular vdW-corrected functionals PBE-D2, vdW-DF2. From these plots it is clear that our summed-up vdW series captures the long-range part of the binding energy curve via C6, C8, and, C10 coefficients. The clean metallic surface properties such as surface energy, work functions are important and often play a crucial role in many catalytic reactions. The weak dispersion interactions present between the surfaces has significant effect on these properties. We used LDA, PBE, PBSEsol, SCAN and SCAN+rVV10 to compute the clean metallic surface properties. The SCAN+rVV10 seamlessly captures different ranges of the vdW interactions at the surface and predicts very accurate values of surface energy (σ), and work function (Φ) and interlayer relaxations (δ%). Our conclusion is adding non-local vdW correction to a good semilocal density functional such as SCAN is necessary in order to predict the weak attractive vdW forces at the metallic surface. The SCAN+rVV10 has also been employed to study the hydrogen evolution reaction (HER) on 1T-MoS2. We have chosen as a descriptor differential Gibbs free energy (ΔGH to understand the underlying mechanism of this catalytic reaction. Density functional theory calculations agree with the experimental findings. In the case of layered materials like 1TMoS2, vdW interactions play an important role in hydrogen binding, that SCAN+rVV10 calculation was able to describe precisely. We have also used SCAN and SCAN+rVV10 functionals to understand bonding of CO on (111) metal surfaces, where many approximations to DFT fail to predict correct adsorption site and adsorption energy. In this case SCAN and SCAN+rVV10 do not show systematic improvements compared to LDA or PBE, rather, both SCAN and SCAN+rVV10 overbind CO more compared to PBE but less compared to the LDA. This overbinding of CO is associated with the incorrect charge transfer from metal to molecule and presumably comes from the density-driven self-interaction error of the functionals. In this thesis we assessed different semi-local functionals to inivestigate molecule surface systems of π-conjugated molecules (thiophene, pyridine) adsorbed on Cu(111), Cu(110), Cu(100) surfaces. We find the binding mechanism of these molecules on the metallic surface is mediated by short and intermediate range vdW interactions. Calculated values of binding energies and adsorbed geometries imply that this kind of adsorption falls in the weak chemisorption regime. Structural phase transitions due to applied pressure are very important in materials science. However, pressure induced structural phase transition in early lanthanide elements such as Ce are considered as abnormal first order phase transition. The Ce α-to-γ isostructural phase transition is one of them. The volume collapse and change of magnetic properties associated with this transition are mediated by the localized f -electron. Semi-local density functionals like LDA, GGA delocalize this f -electron due to the inherent self-interaction error (SIE) of these functionals. We have tested the SCAN functional for this particular problem, and, it was found that the spin-orbit coupling calculations with SCAN not only predicts the correct magnetic ordering of the two phases, but also gives a correct minima for the high-pressure α-Ce phase and a shoulder for the low-pressure γ-Ce phase. / Physics

The work of this thesis comprises extensive Noncontact Atomic Force Microscopy (NC-AFM) characterization of clean metal-oxide (YSZ(100)/(111) and MgO(100)) and graphitic (HOPG) supports as templates for the novel, photochemically induced nucleation of cobalt oxide nanostructures, particularly Cobalt (III) Oxide. The nanostructure-support surfaces were also studied by X-ray Photoelectron Spectroscopy (XPS) to verify the nature of the supported cobalt oxide and to corroborate the surface topographic and phase NC-AFM data. Heteroepitaxial growth of Co2O3 nanostructures proves to exhibit a variety of different growth modes based on the structure of the support surface. On this basis, single-crystal support surfaces ranging from nonpolar to polar and atomically flat to highly defective and reactive were chosen, again, yielding numerous substrate-nanostructure interactions that could be probed by high-performance surface science techniques.

This Thesis concerns the development of fullerene-like (FL) carbon nitride (CNx) thin films and the discovery of phosphorus-carbide (CPx) compounds. The work dedicated to CPx include first-principles theoretical simulations of the growth and properties of FL-CPx structures. I have employed DC magnetron sputtering methods to synthesize both CNx and CPx thin films. The deposition conditions for CPx films were chosen on the basis of the theoretical results as well as from the experience from the deposition of FL-CNx thin films. The characterization of the CPx films is divided into three main directions: structural characterization by transmission electron microscopyand scanning electron microscopy, analysis of the amount of elements and chemical bonds presentin the structure by X-ray photoelectron spectroscopy and Auger spectroscopy, and mechanicalproperty analysis by nanoindentation. The CPx films exhibit a short range orderedstructure with FL characteristics for substrate temperature of 300 °C and for a phosphorus content of 10-15 at.%, which isconsistent with the theoretical findings. These films also displayed the best mechanical properties in terms of hardness and resiliency, which are better than those of the corresponding FL-CNx films. For the FL-CNx thin film material, I find that the surface water adsorption is lower compared to commercial computer hard disk top coatings. Following that line the dangling bonds in FL-CNx coatings have been investigated  by electron spin resonance (ESR). The absence of ESR signal for FL-CNx indicates very low density of dangling bonds in the material, which explains the low water adsorption tendency. The potential for using highly elastic FL-CNx coatings in an automotive valve-train environment has also been investigated. CNx coatings of different nitrogen content were investigated using microscopy, wear testing, nanoindentation testing, and in an experimental cam-tappet testing rig. The FL-CNx coating with the higher value of hardness/elastic modulus showed greater durability in cam-tappet wear testing.

Metastable (Ti,Si)N alloy and TiN/SiNx multilayer thin solid films as well as SiNx/TiN surfaces have been explored. Cubic Ti1-xSixN (0≤x≤0.14) films deposited onto cemented carbide (WC-Co) substrates by arc evaporation exhibited a competitive columnar growth mode where the structure transforms to a feather-like nanostructure with increasing Si content as revealed by x-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy revealed the presence of Ti-N and Si-N bonding, but no amorphous Si3N4. Band structure calculations showed that phase separation of NaClstructure Ti1-xSixN solid solution into cubic SiN and TiN phases is energetically favorable. The metastable microstructure, however, was maintained for the Ti0.86Si0.14N film annealed at 900°C, while recrystallization in the cubic state took place at 1100°C annealing during 2h. The Si content influenced the film hardness close to linearly, by combination of solid-solution hardening in the cubic state and defect hardening. For x=0 and x=0.14, nanoindentation gave a hardness of 29.9±3.4 GPa and 44.7±1.9 GPa, respectively. The hardness was retained during annealing at 900°C. Nanostructured materials, e.g., nanocomposites and nanolaminates, are defined by internal interfaces, of which the nature is still under debate. In this work two-phase model systems were explored by depositing SiNx/TiN nanolaminate films, including superlattices containing cubic SiNx, by dual target reactive magnetron sputtering. It is demonstrated that the interfacial phase of SiNx onto TiN(001) and TiN(111) can be crystalline, and even epitaxial with complex surface reconstructions. Using in situ structural analyses combined with ab initio calculations, it is found that SiNx layers grow epitaxially, giving rise to strong interfacial bonding, on both TiN(001) and TiN(111) surfaces. In addition, TiN overlayers grow epitaxially on SiNx/TiN(001) bilayers in nanolaminate structures. These results provide insight into the development of design rules for novel nanostructured materials. / Report code: LiU-TEK-LIC-2006:51.