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This thesis deals with electrical conductivity in thin films. Classical and quantum size effects in conductivity are discussed including some experimental evidence of quantum size effects. The component conductivity along the applied electric field of a thin film in a transverse magnetic field is developed in a density matrix method.

This research work is aimed at understanding the electronic properties of Bi(110) nanostructures. This study chiefly uses Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS) and Non Contact Atomic Force Microscope (NCAFM) to investigate the geometric and electronic structure of Bi(110) islands on highly oriented pyrolytic graphite (HOPG) substrate. STM measurements are the primary focus of the thesis which involves imaging the bismuth islands and study of its atomic structure. STM images of the Bi(110) islands reveal a ‘wedding cake’ profile of the bismuth islands that show paired layers on top of a base. I(V) (Current vs voltage) data was acquired via STS techniques and its first derivative was compared to DFT calculations. The comparison implied the presence of a dead wetting layer which was present only underneath the bismuth islands. We observed bilayer damped oscillations in the surface energy that were responsible for the stability of paired layers in Bi(110) islands. Interesting Moiré pattern arising out of misorientation between the substrate and the overlayer are also observed in STM images on some bismuth islands. Bright features pertaining to enhanced LDOS (local density of states) were observed on the perimeter of the bismuth islands and stripes in the STM images and STS dI/dV maps which appear at energies around the Fermi level. The bright features which we termed as ‘bright beaches (BB)’ are also observed on grain boundaries and defects that suggest that they are related to termination of the chain of bismuth atoms. The Bi(110) islands and stripes were observed to form preferred widths with a well defined periodicity. This peculiar phenomenon was attributed to a lateral quantum size effect (QSE) that results from a Fermi wave vector with appropriate shifts in Fermi energy. The widths of the islands prefer to adjust themselves at the nodes of this in-plane Fermi wavelength. NaCl deposited on a HOPG substrate forms cross shaped islands which were used as spacers to limit the interaction between the bismuth films and the underlying HOPG substrate. The NaCl islands are transparent to the flow of tunneling current and allow STS measurements. The LDOS of Bi/HOPG was very similar to the LDOS of Bi deposited on NaCl/HOPG which suggests that the wetting layer underneath the bismuth islands plays an important role in decoupling the film from the underlying substrate.

Low-dimensional magnetic systems are of interest due to several new effects and modifications that occur at sizes below the average domain grain boundary within the bulk material. Molecule-like magnetite (Fe3O4) nanoparticles, with sizes ranging from one to two nm were synthesized and characterized in order to investigate new properties arising from quantum size effects. These small systems will provide opportunities to investigate magnetism of zero-dimension systems. A zero-dimensional object is usually called a quantum dot or artificial atom because its electronic states are few and sharply separated in energy, resembling those within an atom. Since the surface to volume ratio is the highest for zero-dimensional systems, most of the changes to magnetic behavior will be observed in ultra-fine magnetic particles. Chemically functional magnetic nanoparticles, comprised of a Fe3O4 magnetite core encased in a thin aliphatic carboxylate, have been prepared by sequential high temperature decomposition of organometallic compounds in a coordinating solvent. In this work, aliphatic carboxylic acid chain length, reaction temperature and duration were varied to produce small core diameters. In order to correlate size effects with changes in particle formation, it is important to have a through understanding of the structural components. This includes studies of the core size, surface effects, decomposition, electronic properties and magnetic behavior. Quantum size effects were observed in the (Fe3O4)X(carboxylate)Y monolayer protected clusters (MPCs) when the average core diameter was ≤ 2.0 nm, evidenced by a blue shifted absorbance band maxima, suggesting the onset of quantum confinement. These (Fe3O4)X(carboxylate)Y MPCs also posses a complex interplay between surface and finite size effects, which govern the magnetic properties of these zero-dimensional systems. These MPCs are all superparamagnetic above their blocking temperatures with total magnetic anisotropy values greater than the bulk value due to an increase in surface and magnetocrystalline anisotropy. A non-linear decrease in saturation magnetization (MS) [Bohr Magneton] per cluster) as a function of the reciprocal of core radius have been attributed to surface effects such as a magnetically inactive layer or an increase in spin disorder as core diameter decreases. The reduced core dimensions of these MPCs make them ideal candidates for further investigation of quantum magnetic systems.

Orientador: Italo Odone Mazali / Dissertação ( mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-13T04:18:17Z (GMT). No. of bitstreams: 1 Correa_DeleonNascimento_M.pdf: 2416044 bytes, checksum: 1853938c864df0a03a91908eb0f6353b (MD5) Previous issue date: 2009 / Resumo: Este trabalho reporta o estudo e o desenvolvimento da metodologia de síntese e de caracterização de nanopartículas isoladas e nanopartículas heteroestruturadas caroço@casca (NCC) envolvendo os óxidos semicondutores (TiO2, CeO2 e SnO2) impregnados em suporte poroso funcional (vidro poroso Vycor ¿ PVG). Empregou-se a metodologia de Ciclos de Impregnação-Decomposição (CID) alternados de compostos metalorgânicos, a partir da técnica de decomposição de precursores metalogânicos (MOD). A metodologia CID prosseguiu com a impregnação dos compostos metalorgânicos di-(propóxido)-di-(2-etilhexanoato) de titânio (IV) [Ti(OnPr)2(hex)2], 2-etilhexanoato de cério (III) [Ce(hex)2] e 2-etilhexanoato de estanho (II) [Sn(hex)2] no PVG em condições controladas. Os estudos das curvas de ganho de massa cumulativo em função de cada CID evidenciaram que para 3 CID os sistemas responderam com um ganho cumulativo de massa de 17,5% (PVG/3SnO2), 4,3% (PVG/3CeO2) e 2,5% (PVG/3TiO2) com a concentração inicial dos precursores de partida de 0,75 molL. O efeito de confinamento quântico foi descrito pelo Modelo da Aproximação da Massa Efetiva (MAME), observado experimentalmente na borda de absorção dos espectros de refletância difusa, DRS, e pelo Modelo de Confinamento de Fônons (MCF), no deslocamento dos modos vibracionais nos espectros Raman. O tamanho de cristalito para a amostra PVG/3TiO2 por TEM e Raman/MCF foi de 4,7 e 4,9 nm, respectivamente, mostrando boa aproximação. O sistema PVG/xTiO2 apresentou variações sistemáticas (blue shift) da energia da banda proibida (Eg*) do TiO2 (óxido de titânio anatásio) nos espectros DRS, evidenciando que Eg* é uma função direta do tamanho de partícula (Eg* = f(2r)) e da metodologia CID. O raio de Bohr, aB, descrito na literatura para a aplicabilidade do MAME ao sistema PVG/xTiO2 não reproduziu a função Eg* = f(2r) de acordo com os resultados TEM. A Partir dos espectros Raman e DRS e os dados teóricos MCF, realizou-se a determinação empírica do aB de 6,4 nm para os cristalitos de TiO2 impregnados no PVG, constituindo nova metodologia para determinação do tamanho de cristalito das amostras PVG/xTiO2. A aplicabilidade do MAME ao sistema PVG/xCeO2 não ofereceu sucesso, pois os cristalitos de CeO2 sofrem acoplamento elétrons-fônons sofrendo um red shift da borda de absorção do espectro DRS. A média de tamanho de cristalito obtida por TEM e estimado por espectroscopia Raman e associado ao MCF está em torno de 5,0 nm para amostras de PVG/5CeO2 0,75-1,0 molL do precursor Ce(hex)3. Os resultados obtidos por DRS para o sistema PVG/xSnO2 demonstraram que o efeito de confinamento quântico ocorre apenas para precursores de concentração abaixo de 0,25 molL. A média de tamanho encontrado para as imagens TEM das amostras PVG/1SnO2 0,10 molL e PVG/1SnO2 0,25 molL é de 3,5 e 5,8 nm e a associação DRS/MAME 3,8 e 4,6 nm, respectivamente. Sobre a obtenção das NCC, as amostras PVG/xTiO2@yCeO2 e PVG/xCeO2@TiO2 (x = 3, 5 e 7 e y = 3, 5 e 7) demonstraram mudança da inclinação da reta de ganho de massa cumulativo após a alternância dos precursores Ti(OnPr)2(hex)2 e Ce(hex)3 A partir das estimativas das Eg* para as amostras PVG/3TiO2@xCeO2 (x = 1, 2 e 3) comparadas com as amostras PVG@xCeO2, relacionou-se tais energias com a formação das NCC. A NCC PVG/3TiO2@3CeO2, 0,75 molL apresentou tamanho de cristalito de 6,9 nm, constituindo um caroço de TiO2 de até 4,7 nm (Raman/MCF, TEM e DRS/MAME) e uma casca de CeO2 inseridos pelos 3 CID do precursor de cério (PVG/3CeO2 constitui 4,1 nm pelo MCF) nucleando sobre o caroço PVG/3TiO2 corroborando com os dados descritos pelo ganho de massa cumulativo com a mudança da inclinação da reta. Observou-se que o sistema PVG/5CeO2@3TiO2 constituiu uma borda de absorção em torno 3,23 eV, sendo uma evidência qualitativa do recobrimento e a formação de NCC PVG/5CeO2@3TiO2, pois, se as nanopartículas PVG/5CeO2 não estivessem sido encapadas ver-se-ia uma borda de absorção correspondendo a PVG/5CeO2 em torno de 3,17 eV. O sistema PVG/xCeO2@yTiO2 (x = 3, 5 e 7 e y = 3, 5 e 7) foi estudado por espectroscopia Raman. Os resultados mostraram deslocamentos sistemáticos do modo vibracional Eg do TiO2 dependentes da espessura da casca e a estabilização da banda T2g do CeO2 no caroço. Espectros Raman do sistema PVG/xTiO2@yCeO2 (x = 3, 5 e 7 e y = 3, 5 e 7) demonstraram a formação de bandas muitos deslocadas, quando o CeO2 se encontra na casca / Abstract: This work reports the development of a synthesis and characterization methodology for isolated nanoparticles and core-shell heterostructures nanoparticles (CSN), involving the semiconducting oxides (TiO2, CeO2 and SnO2) impregnated into a functional porous support (porous Vycor glass - PVG). The alternated impregnation¿decomposition cycle (ICD) methodology was applied from metallo-organic precursors by the used metalloorganic decomposition (MOD) technique. The ICD methodology used Ti (IV) di-(n-propoxy)-di-(2-ethylhexanoate) [Ti(OnPr)2(hex)2],.Ce(III) 2- ethylhexanoate [Ce(hex)3] and Sn(II) 2-ethylhexanoate [Sn(hex)2] impregnation into PVG in controlled conditions. The studies of the cumulative mass gain curves as functions of each ICD evidenced that, for 3 ICD, the systems had cumulative mass gains of 17.5% (PVG/3SnO2), 4.3% (PVG/3CeO2) e 2.5% (PVG/3TiO2) with initial precursor concentrations of 0.75 mol L. The quantum size effect was described by the effective mass approximation model (EMAM), observed experimentally in the absorption edge of the diffuse reflectance spectra (DRS), and by the phonon confinement model (PCM), in the vibrational modes of the Raman shift. The PVG/3TiO2 sample crystallite size was determined by TEM and Raman/PCM to be 4.7 and 4.9 nm, respectively, showing a good approach. The PVG/xTiO2 system showed systematic blue shift variations in the band gap energies (Eg*) in DRS spectra, showing that Eg* is a particle size function (Eg* = f(2r)) and ICD methodology. The Bohr radius (aB), described in literature for the EMAM application, did not describe the Eg* = f(2r) function for the PVG/xTiO2 system, in concordance with TEM data. From Raman and DRS spectra associated with PCM data, followed by the empirical aB determination for TiO2 (anatase titanium oxide) crystallites impregnated in PVG found to be 6.4 nm, constituting important methodology for crystallite size determination in PVG/xTiO2 samples. The EMAM on the PVG/xCeO2 system was not successful, the CeO2 crystallites suffers a red-shift in the DRS absorption edge as a result of effects arising from electron¿phonon coupling. The average crystallite size from TEM data and estimated by Raman spectroscopy associated with PCM are found to be around 5.0 nm for PVG/5CeO2 (0.75-1.0 mol L precursor concentration) samples. The DRS results for the PVG/xSnO2 system demonstrates that the quantum size effects occurs only below 0.25 molL precursors concentrations. The PVG/1SnO2 0.10 mol L and PVG/1SnO2 0.25 mol L average size found from TEM images were 3.5 and 5.8 nm, respectively, and the DRS/EMAM showed 3.8 and 4.6 nm, respectively. The PVG/xTiO2@yCeO2 and PVG/xCeO2@TiO2 (x = 3, 5 e 7 and y = 3, 5 e 7) CSN samples demonstrated an inclination change of the cumulative mass gain line with the Ti(OnPr)2(hex)2 and Ce(hex)3 precursor alternation. From the estimated Eg* for the PVG/3TiO2@xCeO2 (x = 1, 2 e 3) samples compared with PVG@xCeO2 samples, it was possible to relate the energies with the CSN formation. The PVG/3TiO2@3CeO2, 0.75 mol L CSN present a particles size of 6.9 nm, constituting a TiO2 core around 4.7 nm (Raman/PCM, TEM and DRS/EMAM) and a CeO2 shell insert from 3 ICD from cerium precursor (PVG/3CeO2 presents 4.12 nm by PCM). The data suggest that the CeO2 shell nucleated around the PVG/3TiO2 core, corroborating with the inclination change of the cumulative mass gain line. It was observed that the PVG/5CeO2@3TiO2 system presents an absorption edge around 3.23 eV. This shows qualitative evidence about the PVG/5CeO2@3TiO2 CSN formation. Therefore, if the PVG/5CeO2 core is not covered, it will show an absorption edge around 3.17 eV. The PVG/xCeO2@yTiO2 (x = 3, 5 e 7 e y = 3, 5 e 7) system was studied by Raman spectroscopy. The results showed systematic shifts in TiO2 A Eg band dependent for the TiO2 shell thickness and the CeO2 T2g band stabilization related to the CeO2 covered in the core. Raman spectra on the PVG/xTiO2@yCeO2 (x = 3, 5 e 7 e y = 3, 5 e 7) system showed a big band shift when CeO2 was in the shell / Mestrado / Quimica Inorganica / Mestre em Química

The growth of atomically flat silver films on Si(111) usually requires a two-step growth, including deposition at low temperature (≈100 K) followed by slowly annealing to room temperature. In addition, flat silver films are usually only obtained on Si(111) for film thicknesses larger than the critical thickness of 6 monolayer. In this work, Ag thin film formation at ambient temperature on Sn/Si(111)-√3×√3 has been investigated experimentally using a combination of scanning tunneling microscopy, scanning tunneling spectroscopy and low-energy electron diffraction. The first buffer layer, probably consisting of both Ag and Sn, formed a partially ordered structure consisting of atomic rows which mainly followed the high-symmetry directions of the underlying Si(111) lattice. From 3 ML coverage, an atomically flat Ag film was formed. Low-energy electron diffraction confirmed that the films grew in the [111]-direction. This shows that atomically flat Ag films as thin as 3 ML can be grown on Sn/Si(111)-√3×√3 by conventional deposition at room temperature. The electronic structures of the films were studied for a range of different coverages by scanning tunneling spectroscopy. The normalized tunneling conductance showed quantum well states in the occupied electronic states, which moved towards the Fermi energy with increasing film thicknesses.