Estudo das propriedades estruturais e magnéticas de nanopartículas de SnO2 dopadas com Gd

Dissertação (mestrado)—Universidade de Brasília, Instituto de Física, Programa de Pós-graduação em Física, 2013.Esse trabalho envolve o estudo de propriedades estruturais e magnéticas de nanopartículas de SnO2 dopadas em até 10,0% de gadolínio, preparadas pelo método dos precursores poliméricos (método de Pechini). Resultados obtidos pela técnica de EDS evidenciam a presença de Gd aproximadamente na mesma proporção designada nominalmente na síntese das nanopartículas dopadas com Gd. Ajustes pelo método de Rietveld (utilizando o programa GSAS) dos difratogramas de raios X indicam a formação de fase cristalina tetragonal do tipo rutilo em todas as amostras estudadas. Além disso, foram estimados os diâmetros médios dos cristalitos utilizando a equação de Scherrer em todos os compostos. Nesse contexto, foi observado um decréscimo no tamanho das nanopartículas em função do aumento da concentração de Gd na matriz semicondutora. O diâmetro médio obtido através da análise de imagens de TEM corrobora com o diâmetro obtido por difração de raios X. Medidas de espectroscopia Raman confirmaram a formação da fase rutilo em todas as amostras estudadas a partir da presença de modos vibracionais típicos correspondentes a essa estrutura (Eg, A1g, B2g). Foi observado um deslocamento no modo principal A1g para menores frequências, o qual foi associado ao efeito da diminuição do tamanho. Além disso, observaram-se bandas adicionais S1, S2 e S3 cujas áreas espectrais crescem ao aumentar-se a concentração de dopante na matriz semicondutora. Essas bandas foram associadas a modos ativados pela desordem na superfície das partículas, cujo sinal é forte devido ao tamanho reduzido destas partículas. Foram realizadas medidas magnéticas em uma ampla faixa de temperatura (5 - 300 K) aplicando campos magnéticos de até ±80 kOe. A partir das curvas de M vs. H a 5 K foi observado um aumento acentuado nos valores da magnetização de saturação em função do aumento da concentração de Gd, o que confirma a entrada de Gd na estrutura. Ajustes usando a lei de Curie-Weiss das curvas de susceptibilidade vs.temperatura, mostram que o momento magnético efetivo por átomo de Gd está próximo do valor de 7,94 µB para o gadolínio no estado Gd3+. A análise das medidas de magnetização em função do campo magnético aplicado sugere que, para amostras com baixa concentração de Gd (< 2,5%), acontece a coexistência de ferromagnetismo e paramagnetismo; entretanto, para as amostras com alta concentração de Gd, foi determinado somente a presença de paramagnetismo. A natureza dessa contribuição ferromagnética foi atribuída à presença de pólarons magnéticos ligados (BMP - Bound Magnetic Polarons), que são formados pela geração de vacâncias de oxigênio para compensar a carga do sistema. _______________________________________________________________________________________ ABSTRACTIn this work the structural and magnetic properties of SnO2 nanoparticles doped up to 10,0% of gadolinium is studied. These nanoparticles were prepared by the polymer precursor method (Pechini’s method). Results obtained from the EDS technique demonstrated that the Gd doping level is consistent with the nominal amount need to synthesize the Gd-doped SnO2 nanoparticles. Rietveld refinement (using the GSAS program) of the X ray diffractograms indicated the formation of the tetragonal rutile type crystalline phase for the whole set of samples. Besides, the mean crystallite size was estimated using the Scherrer equation. It was observed that the crystallite size decreases as the Gd concentration increases in the semiconducting matrix. The mean particle size determined from TEM images corroborate to the crystallite size determined from XRD data. Based on presence of canonical vibrational modes of the rutile structure (Eg, A1g, B2g), Raman spectroscopy measurements confirmed the formation of the cassiterite phase for all samples. The main mode A1g shows a shift to lower wave numbers which was associated to the particle size effect. Besides that, it was observed the presence of additional bands S1, S2 and S3 whose spectral areas increase with the increase of the dopant concentration. Those bands have been assigned to disorder-activated modes at the particle surface, which signal is strong due to the small particle size. Magnetic measurements were carried out in a broad range of temperatures (5 – 300 K) and applying magnetic fields up to ±80 kOe. From the M vs. H curves obtained at 5 K, it was confirmed an increase in the saturation magnetization values as a function of the Gd concentration, which confirmed the entrance of Gd atoms inside the crystalline structure. Fits using the Curie-Weiss law of the susceptibility vs. temperature curves demonstrated that the effective magnetic moment, per Gd atom, is near to the value 7,94 µB expected for Gd ion in the valence state Gd3+. The analyses of magnetic measurements as a function of the applied magnetic field suggest the coexistence of ferromagnetic and paramagnetic phases in samples with low Gd concentration (< 2,5%); however, only a paramagnetic phase was determined for samples with higher Gd concentration. The nature of the ferromagnetic contribution was assigned to the presence of bound magnetic polarons (BMP), which are formed due to generation of oxygen vacancies in order to compensate the charge of the system

Effect of thermal annealing on the optical and structural properties of (311)B and (001) GaAsBi/GaAs single quantum wells grown by MBE

The effect of Furnace Annealing (FA) and Rapid Thermal annealing (RTA) on the structural and optical properties of GaAs1 − xBix/GaAs single quantum wells grown on (001) and (311)B substrates by molecular beam epitaxy was investigated. The structural properties were investigated by high-resolution x-ray diffraction (HR-XRD) and Transmission Electron Microscopy. The Bi concentration profiles were determined by simulating the HR-XRD 2θ−ω scans using dynamical scattering theory to estimate the Bi content, lattice coherence, and quality of the interfaces. The Bi composition was found to be similar for both samples grown on (001) and (311)B GaAs substrates. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi quantum well (QW) layer but also extends out of the GaAsBi QW toward the GaAs barrier. Photoluminescence (PL) measurements were performed as a function of temperature and laser power for samples with a nominal Bi composition of 3%. PL spectra showed that (001) and (311)B samples have different peak energies at 1.23 eV and 1.26 eV, respectively, at 10 K. After RTA at 300 °C for 2 min, the PL intensity of (311)B and (001) samples was enhanced by factors of ∼2.5 and 1.75, respectively. However, for the (001) and (311)B FA samples, an enhancement of the PL intensity by a factor of only 1.5 times could be achieved. The enhancement of PL intensity in annealed samples was interpreted in terms of PL activation energies, with a reduction in the alloy disorder and an increase in the Bi cluster

Revealing the true impact of interstitial and substitutional nitrogen doping in TiO2 on photoelectrochemical applications

Application of photocatalysts that strongly absorb within the visible range is a common strategy to improve the efficiency of photoelectrochemical (PEC) systems; this may translate to high photocurrents, but it is not always the case. Here, we show that nitrogen doping enhances visible light absorption of TiO2; however, it does not necessarily result in improved PEC performance. Depending on the applied external potential, N-doping can improve, or degrade, PEC performance either under water oxidation conditions or via hole scavenging (Na2S/Na2SO3). In this work, we developed a holistic approach to evaluate the true impact of N doping in TiO2 on PEC performance. Interstitial and substitutional N doping are experimentally explored for the first time through a simple and novel PEC approach which complemented X-ray photoelectron analyses. Using this approach, we show that interstitial N doping of anatase TiO2 dominates up to 400 °C and substitutional doping up to ca. 600 °C, without rutile formation. This reveals that the bottleneck for doping higher N-concentrations in TiO2 is the direct transformation to thermodynamically favorable N-rich phases, such as TiN/Ti2N at 700 °C, inhibiting the formation of rutile phase. Transmission electron microscopy revealed that N doping proceeds mainly from the inner to the outer tube walls via nitridation and follows a preferential pathway from interstitial to substitutional doping. Direct PEC experimental evidence on visible light activation of N doped TiO2, and the location of interband states, showed acceptor levels of 1.0 eV for substitutional and 0.7 eV for interstitial doping above the TiO2 valence band maximum. In addition, due to O vacancies and Ti3+ species, donor levels below the conduction band minimum were also created. These levels act as trapping/recombination centers for charge carriers and, therefore, the gain in the visible range due to N doping does not translate to an improved PEC performance by these structural defects. Ultimately, we show that whilst there is a benefit of visible light absorption through N doping in TiO2, the PEC performance of the samples only surpasses pristine TiO2 at relatively high biasing (>0.3 V vs. Ag/AgCl)

Síntese por feixe de íons de GaN-layer sobre GaAs

O Nitreto de Gálio (GaN) é um semicondutor de gap direto, motivo de numerosas pesquisas científicas, principalmente devido a sua importância na fabricação de dispositivos de alta potência e optoeletrônicas. Ligas de GaN como InGaN e AlGaN, por exemplo, possibilitam a fabricação de LEDs e LASERs azuis. Neste nosso estudo selecionamos o Arseneto de Gálio (GaAs) como um substrato viável para síntese de GaN mediante a permuta de Arsênio (As) por Nitrogênio (N) fundamentada em três passos: a) incorporação de N por implantação iônica em GaAs (à 350, 450 ou 550 ºC) em elevadas fluências (1, 2, 3 ou 4 × 1017 N/cm2); b) maior estabilidade das ligações Ga-N frente às de Ga-As; e c) expurgo de As da região contendo N implantado mediante recozimentos (à 550, 650, 750, 850 ou 1000 ºC) sob fluxo de N2. Uma capa de ~ 125 nm de Nitreto de Silício (Si3N4) foi depositada por sputtering sobre o GaAs previamente a implantação à quente: camada de sacrifício que pode ser removida após a síntese. Análises por Microscopia Eletrônica de Transmissão (TEM) e Espectroscopia de Raios- X por Dispersão em Energia (EDS) demonstraram que, no estado como-implantado da fluência de 3 × 1017 N/cm2, formam-se bolhas de N para ambos os lados da interface Si3N4/GaAs e a região implantada do GaAs amorfiza. Após um recozimento à 850 ºC/5 min, observou-se uma elevada degradação da camada de Si3N4, fragilizada pela formação das bolhas de N. Formou-se uma camada contínua de GaN (GaN-layer) de ~ 70 nm na sua fase hexagonal, sustentada por “pilares” no substrato GaAs, entre os quais existem extensos vazios. Medidas TEM em alta resolução (HRTEM) e por Difração de Elétrons de Área Selecionada (SAED) revelaram que a GaN-layer apresenta forte tendência à epitaxia com o substrato GaAs (relações de epitaxia são aqui apresentadas), e regiões estruturalmente espelhadas (i.e., twins). SAED sobre os pilares evidenciaram uma fase transicional cúbica, com um parâmetro de rede substancialmente menor (0,42 ± 0,01) nm que o reportado na literatura (0,45 nm). Estudos por Espectrometria de Retroespalhamento de Rutherford e Canalização (RBS/C) mostraram que a GaN-layer é rica em N (Ga1,00N1,90, para 3 × 1017 N/cm2) e apresenta canalização (para implantações de 2 e 3 × 1017 N/cm2), confirmando o caráter monocristalino identificado por TEM. Medidas de Fotoluminescência (PL) confirmam emissão na região do gap de banda do α-GaN (~ 3,4 eV), bem como bandas associadas a defeitos estruturais do material. Também foi investigado o efeito de campos de tensão provenientes de bolhas de Hélio (He) mediante a realização da síntese a partir de substrato GaAs pré-implantado com He. Neste caso, as bolhas, que se formam no GaAs durante a implantação de N à quente e extinguem-se após recozimentos, limitam a difusão de N para o interior do substrato, conduzindo a formação de uma GaN-layer mais espessa (~ 120 nm) e com bem mais N (Ga1,00N2,80). Como consequência, a GaNlayer apresentou um caráter mais policristalino.The Gallium Nitride (GaN) is a direct gap semiconductor, is issue of numerous scientific research, mainly due to its importance in the manufacture of high power devices and optoelectronic devices. GaN alloys, as InGaN and AlGaN, for example, enable the production of LEDs and blue LASERs. In this study, we have selected Gallium Arsenide (GaAs) as a suitable substrate for GaN synthesis through Arsenic (As) replacement by Nitrogen (N), based on three steps: a) incorporation of N by ion implantation into GaAs (at 350, 450 or 550 ºC) at high fluences (1, 2, 3 or 4 × 1017 N/cm2); b) higher stability of the Ga-N bonds compared to the Ga-As ones; and c) purge of As from the region containing implanted N by annealing (at 550, 650, 750, 850 or 1000 ºC) under N2 flow. A 125-nm cap-layer of Silicon Nitride (Si3N4) was deposited by sputtering on GaAs prior to the hot implantation: it is a sacrifice layer which can be removed after the synthesis. Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDS) analyzes demonstrated that, on the as-implanted state of the fluence of 3 × 1017 N/cm2, N bubbles are formed on both sides of the Si3N4/GaAs interface and the implanted region of GaAs amorphizes. After annealing at 850 °C/5min, a high degradation of the Si3N4 layer was observed, weakened by the formation of N bubbles. A continuous layer of GaN (GaN-layer) of ~ 70 nm was formed in its hexagonal phase, supported by “pillars” on the GaAs substrate, with extensive voids in between them. High-Resolution TEM (HRTEM) and Selected Area Electron Diffraction (SAED) measurements revealed that the GaN-layer exhibits a strong tendency to epitaxy with the GaAs substrate (epitaxial relationships are here presented), and structurally mirrored regions (i.e., twins). SAED on the pillars showed a transitional cubic phase, with a lattice parameter substantially smaller (0.42 ± 0.01) nm than the one reported in the literature (0.45 nm). Rutherford Backscattering Spectrometry studies and Channeling (RBS/C) showed that the GaN-layer is rich in N (Ga1.00N1.90, for 3 × 1017 N/cm2) and presents channeling (for implantations of 2 and 3 × 1017 N/cm2), corroborating the monocrystalline nature identified by TEM. Photoluminescence (PL) measurements confirm emission in the band gap region of a- GaN (~ 3.4 eV), as well as bands associated to structural defects in the material. It was also investigated the effect of strain fields from Helium (He) bubbles through synthesis starting up from He pre-implanted GaAs substrate. In this case, the bubbles, which are formed in the GaAs during the hot N-implantation and are annihilated after annealing, limit the N diffusion into the substrate, leading to the formation of a thicker GaN-layer (~ 120 nm) and with much more N (Ga1.00N2.80). As a result, the GaN-layer presented an aspect more polycrystalline

Homogeneous V incorporation via single-step anodization : structural doping or heterostructure formation?

Two distinct-phenomena were reported for anodic elemental-incorporation; structural-doping/heterostructure formation. Herein, by adding V for the first time in TiO2-nanotubes via anodization; the foreign elemental incorporation is peered. The bandgap (3.2 eV), flat band (0.07 VRHE) and valence band (VB) positions of V incorporated anatase-TiO2 nanotubes remained invariant which ruled out structural doping of TiO2. The first step in anodization is Ti-oxidation that occurs momentarily; thereby, the oxidation/dissolution dominancy of anodization in later stages impeded V-structural doping even for varying times (1–24 h) and voltages (6–60 V) and thus a small fraction of V2O5 was observed in surface and microscopic studies. Density functional theory realistic simulations revealed TiO2/V2O5 forms commensurate-heterostructure at common lattice constants with charge transfer from TiO2 to V2O5 at the heterostructure interface; exhibiting type-II band-alignment having CB from V2O5 and VB from TiO2 with decreasing bandgap. In photoelectrochemical-setup, the photogenerated electron transfer from V2O5 to the heterostructure interface was suppressed due to barrier-height which decreased photocurrent for TiO2/V2O5 (52 µA/cm2 ) as compared to pure-TiO2 (140 µA/cm2 ) at ~1.0 VRHE. Contrarily, in photolysis, improved light absorption and facile carrier migration from TiO2/V2O5 resulted in better photodegradation activity of methylene-blue. We unlocked anodization is ineffective for structural-doping, indeed it results in uniform low-content elemental incorporation forming oxides heterostructure

Representações sociais do medicamento genérico por consumidores residentes em Natal, Rio Grande do Norte, Brasil Social representations of generic drugs by consumers from Natal, Rio Grande do Norte, Brazil

O trabalho objetivou determinar os núcleos central e periférico das representações sociais do medicamento genérico por consumidores, estabelecendo mecanismos que poderão ser utilizados no aprimoramento da política desse tipo de medicamento no Brasil. A pesquisa foi realizada no período de abril de 2002 a fevereiro de 2003, na Cidade do Natal, Rio Grande do Norte, com quatrocentos consumidores. O teste utilizado foi o de associação de palavras, e o estímulo indutor, as palavras medicamento genérico. Foi solicitada a evocação de três palavras, de acordo com a estratégia de acesso ao Núcleo Central de Vergès. A análise dos dados foi realizada com o auxílio do programa EVOC 2000 e da análise de conteúdo preconizada por Bardin. Os resultados demonstraram que o núcleo central era composto pelas categorias preço, qualidade e equivalência farmacêutica, e o sistema periférico, representado pelas categorias opção, eficácia, governo, benefício social e acessibilidade.<br>This paper aimed to determine the central and peripheral roles of consumers' social representations concerning generic drugs, establishing mechanisms that could be used to improve policies for this type of medication in Brazil. The research was done from April 2002 to February 2003 in the city of Natal, Rio Grande do Norte, with 400 consumers. The study employed the word association test with the words "generic drug" as the inductive stimulus. Evocation of three words was requested, according to the access strategy to Vergès' Central Nucleus. Data analysis used the EVOC 2000 software and the content analysis proposed by Bardin. The results demonstrated that the central nucleus consisted of the categories price, quality, and pharmaceutical equivalence, while the peripheral system was represented by the categories option, effectiveness, government, social benefit, and accessibility