Estados eletrônicos e alinhamento de bandas em heterojunções por espectroscopia de elétrons

Orientadores: Fernando Alvarez, Florestano EvangelisteTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Apresentamos um novo método experimental para determinar o alinhamento de banda em heterojunções e aplicamos este para estudar a heterojunção c-Si/a-SixC1-x:H. Esse método utiliza uma versão moderna de uma espectroscopía muito utilizada: a espectroscopía de yield fotoelétrico excitada com fótons na região do ultra-violeta próximo. No espectro de yield podemos determinar as bordas da banda de valência do filme e do substrato devido ao grande intervalo dinâmico desta técnica. Também apresentamos um novo método para carbonizar a superfície de silício. Aplicando este método fomos capazes de crescer heterojunções c-SiC/c-Si a temperaturas inferiores das que são reportadas na literatura. A estrutura do filme e a morfologia foram estudadas in situ por RHEED e ex-situ por AFM, respectivamente. Encontramos que o crescimento ocorre em ilhas cristalinas orientadas preferencialmente na direção (001). As propriedades eletrônicas do filme foram investigadas in situ por XPS, UPS e LEYS-CFS. A análise das estruturas associadas aos níveis de caroço e a associadas aos plamons mostrou que temos um interface abrupta entre o filme estequiométrico e o substrato. O uso do LEYS-CFS permitiu a determinação da descontinuidade de banda de valência na interface embora o filme de SiC tenha nucleação em ilhas e não recobra completamente o substrato de Si. O valor encontrado para DEv foi 0.75± 0.08 eV. Esse resultado é discutido dentro da teoria existente na literaturaAbstract: We present a new experimenta1 method for determining band lineups at the semiconductor heterojunctions and apply it to the c-Si(100)/a-SixC1-x:H heterostructure. This method uses a modern version of an old spectroscopy technique: the photoeletric yield spectroscopy excited with photons in the near ultra-violet UV range. It is shown that both substrate and overlayer valence- and tops can be identified in the yield spectrum due to the high escape depth and the high dynamica1 range of the technique, thus a1lowing a direct and precise determination of the band lineup. We a1so present a new method to carbonize silicon surface. Applying this method we were able to grow c-SiC/c-Si heterojunctions at lower temperatures than the one reported in the literature. The c-SiC film structure and morphology were investigated in situ by Reflection High-Energy Electron Diffraction (RHEED) and ex-situ by Atomic Force Microscopy (AFM), respectively. It was found that the growth proceeds through the nucleation of cubic and relaxed crysta1line islands preferentia1ly oriented in (001) direction. The average island size increases with carbonization time. The electronic properties were investigated in situ by X-ray and UV photoemission spectroscopies and by photoelectric yield spectroscopy excited with low energy photons (visible and near -UV range). The ana1ysis of high resolution core level and plasma excitation spectra unambiguously shows that only stoichiometric SiC is present at the interface and demonstrates that the interface itself is abrupt. The use of the photoelectric yield technique operated in the constant final state mode a1lowed us to determine the va1ence band discontinuity a1though the SiC overlayer was cluster-like and did not cover uniform1y the Si substrate. The found va1ue is 0.75± 0.08 eV. This result is discussed in the framework of the current lineup theoriesDoutoradoFísicaDoutor em Ciência

A bibliometric and classification study of Project-based Learning in engineering education

Engineering education has been the subject of studies in search of approaches that provide better results in terms of learning. The Project Based Learning approach (PBL) is the subject of this study from the point of view of its application in Engineering. The objective is to present a classification and bibliometric analysis of PBL in Engineering. Publications on the subject were identified through queries at the journal databases at ISI Web of Science and Scopus SCImago between 2000 and 2016. The results highlight the benefits from the use of the PBL approach to learning in Engineering showing increased absorption of technical content by students and the development of soft and multi-disciplinary skills. The bibliometric analysis revealed the most relevant journals in the subject, authors and the most cited papers and keywords. New horizons to advance the use of PBL in engineering education are discussed

Synthesis and Characterization of Highly Crystalline Vertically Aligned WSe2 Nanosheets

Here, we report on the synthesis of tungsten diselenide (WSe2) nanosheets using an atmospheric pressure chemical vapor deposition technique via the rapid selenization of thin tungsten films. The morphology and the structure, as well as the optical properties, of the so-produced material have been studied using electron microscopies, X-ray photoelectron spectroscopy, photoluminescence, UV–visible and Raman spectroscopies, and X-ray diffraction. These studies confirmed the high crystallinity, quality, purity, and orientation of the WSe2 nanosheets, in addition to the unexpected presence of mixed phases, instead of only the most thermodynamically stable 2H phase. The synthesized material might be useful for applications such as gas sensing or for hydrogen evolution reaction catalysis

The Role of Cerium Valence in the Conversion Temperature of H2_2Ti3_3O7_7 Nanoribbons to TiO2_2-B and Anatase Nanoribbons, and Further to Rutile

CeO$_2$-TiO$_2$ is an important mixed oxide due to its catalytic properties, particularly in heterogeneous photocatalysis. This study presents a straightforward method to obtain 1D TiO$_2$ nanostructures decorated with CeO$_2$ nanoparticles at the surface. As the precursor, we used H$_2$Ti$_3$O$_7$ nanoribbons prepared from sodium titanate nanoribbons by ion exchange. Two cerium sources with an oxidation state of +3 and +4 were used to obtain mixed oxides. HAADF–STEM mapping of the Ce$^{4+}$-modified nanoribbons revealed a thin continuous layer at the surface of the H$_2$Ti$_3$O$_7$ nanoribbons, while Ce$^{3+}$ cerium ions intercalated partially between the titanate layers. The phase composition and morphology changes were monitored during calcination between 620 °C and 960 °C. Thermal treatment led to the formation of CeO$_2$ nanoparticles on the surface of the TiO$_2$ nanoribbons, whose size increased with the calcination temperature. The use of Ce$^{4+}$ raised the temperature required for converting H$_2$Ti$_3$O$_7$ to TiO$_2$-B by approximately 200 °C, and the temperature for the formation of anatase. For the Ce$^{3+}$ batch, the presence of cerium inhibited the conversion to rutile. Analysis of cerium oxidation states revealed the existence of both +4 and +3 in all calcined samples, regardless of the initial cerium oxidation state

A Project-based Learning curricular approach in a Production Engineering Program

The Industrial Engineering undergraduate program offered at the University of Brasília was structured on Project Based Learning (PBL) methodology. This innovative educational proposal allows the students to deploy their technical competencies through real problem solving situations. The methodology also stimulates the development of the students’ soft skills, by exposing them to a challenging environment. At the end of each project, the results are presented to an external agent, and are subsequently implemented. The objective of this paper is to present the evolution of the PSP courses, which adopt as guidelines both the PMBOK project management framework, as well as content from specific technical courses related to the project’s subject. One other relevant aspect of the PSP courses is their involvement of both undergraduate and graduate students from different Engineering areas, such as Industrial, Mechanical and Civil Engineering, which serves to foster a more holistic and integrated problem-solving skillset

Modelo Multicritério para Avaliação de Ciclo de Vida de Ativos de TI/ A Multi-Criteria Decision Support Framework for IT Assets Life Cycle

Este artigo apresentou um modelo de decisão multicritério para avaliação de ciclo de vida de equipamentos de TI de uma instituição financeira pública nacional, com foco na melhoria do processo de gestão de ativos de TI e na eficiência das contratações públicas. Para tanto, um framework de decisão multicritério (MCDA) foi utilizado para incorporar critérios relevantes que permeiam todo o ciclo de vida dos equipamentos, incluindo a fase inicial, que compreende a decisão pela aquisição (compra, projeto e fabricação), a entrega e instalação; a fase de operação e manutenção; a fase de atualização tecnológica e a última fase que é o defeito irrecuperável e o fim de vida útil do equipamento. Esses critérios fornecem uma combinação dos fatores internos e externos à organização responsáveis pela inservibilidade ou obsolescência dos equipamentos. Os resultados obtidos servirão de subsídio para definir a melhor decisão a ser tomada em relação à estratégia de manutenção ou substituiçao do parque de equipamentos instalados da organização