In recent year, the quantum device has been rapid developed. The quantum conductor has been of great interest for most authors, and one of that is gold nanowire. As the diameter of the gold nanowire is smaller than 2nm, the structure arrangement is affected by surface tensor, and therefore the FCC structure will self assemble to a helical structure. However, the nanowire would be used in quantum devices, therefore, the material property must be understood and investigated. The properties of nanowire would be a significant on development of quantum device in the future. In this study, the molecular dynamics is employed to investigate the mechanical properties of the helical multi-shall gold nanowires and nanowries of the bulk FCC. The stress and strain relationship is obtained form the tensile and compressed tests. In addition, the yielding stress, maximum stress, Young¡¦s modulus, and breaking force can be determined from the tensile test and compressed test. Moreover, the different length/diameter ratio, temperature, and strain rate effects on mechanical properties and deformation behaviors are also investigated. The structure transform from crystalline to non-crystalline is also observed by the variation of radial distribution function (RDF) and angular correlation function (ACF). In this study, the tight-binding many body potential is employed to model the interaction between gold atoms.

It has been shown recently that high-quality epitaxial graphene (EPG) can be grown on the SiC substrate that exhibits interesting physical properties and has great advantages for varies device applications. In particular, the multilayer graphene films grown on the C-face show rotational disorder. It is expected that the twisted layers exhibit unique new physics that is distinct from that of either single layer graphene or graphite. In this work, by combining density functional and tight-binding model calculations, we investigate the electric field and doping effects on twisted bilayer graphene (TBG), multiple layer effects on twisted triple-layer graphene, and wave packet propagation properties of TBG. Though these studies, we obtain a comprehensive description of the interesting interlayer interaction in this twisted multilayer graphene system.

The growth of the Si(001) surface from gas sources such as disilane is technologically important, as well as scientifically interesting. The aspects of growth covered are: the clean surface, its defects and steps; the action of bismuth, a surfactant; the diffusion behaviour of hydrogen in different environments; and the entire pathway for formation of a new layer of silicon from adsorption of fragments of disilane to nucleation of dimer strings. The theoretical methods used, density functional theory and tight binding, are described. Four linear scaling tight binding methods are compared. The construction of the tight binding parameterisations used is also explained. The structure of the most common defect on the Si(001) surface is identified by comparison of the electronic structure with scanning tunneling microscopy (STM) images. The energy and structure of steps is calculated, and their kinking behaviour is modelled, achieving good agreement with experimental results. Two unusual features which form when bismuth is placed on the surface and annealed are investigated. The first has possible applications as a quantum wire, and its structure and growth are described. The second relates to a controversial area in the field; a structure is proposed which fits all available experimental evidence. The behaviour of hydrogen is vital to understanding growth, as large amounts are deposited during disilane growth. After validating the tight binding parameterisation against DFT and experiment for the system of a single hydrogen diffusing on the clean Si(001) surface, the barriers for diffusion on the saturated surface, down a step and away from a defect are found, and prove to be in good agreement with available experimental data. The pathway for the formation of a new layer of silicon from disilane is described step by step, giving barriers and structures for all events. The interaction with experiment is highlighted, and demonstrates that great benefit accrues from such close work, and that the atomistic modelling techniques used in the thesis produce results in close agreement with reality.

Transition metal oxides constitute a large class of materials with variety of very interesting properties and important technological utility. A subset with perovskite structure has been the subject matter of the current theoretical investigation with an emphasis on their electronic and structural behavior. An analytical and a computational method are used to calculate physical entities like lattice parameters, bulk moduli, band structures, density of electronic states and charge density distributions for various topologies. Results are discussed and compared with the available experimental findings.

In this thesis we describe the development of a real-space implementation of the Bethe-Salpeter equation (BSE) and use it in conjunction with a semi-empirical tight-binding model to investigate the optoelectronic properties of colloidal quantum- confined nanostructures. This novel implementation exploits the limited radial extent and small size of the atomic orbital basis to treat finite systems containing up to ∼4000 atoms in a fully many-body framework. In the first part of this thesis our tight-binding model is initially benchmarked on zincblende CdSe nanocrystals, before subsequently being used to investigate the electronic states of zincblende CdSe nanoplatelets as a function of thickness. The band-edge electronic states are found to show minimal variation for a range of thicknesses and the results of our tight-binding model show good agreement with those predicted using a 14-band k·p model for a nanoplatelet of 4 monolayers (ML) in thickness. Optical absorption spectra were also computed in the independent-particle approximation. While the results of the tight-binding model show good agreement with those of the 14-band k·p model in the low-energy region of the spectrum, agreement with experiment was poor. This reflects the need for a many-body treatment of optical absorption in nanoplatelet systems. In the second part of this thesis we apply our tight-binding plus BSE model to study the excitonic properties of CdSe nanocrystals and nanoplatelets. Simulations performed on CdSe nanocrystals examined an approximation of the BSE equivalent to configuration interaction singles (CIS), and found that both the optical gap and the low-energy spectral features were unaffected by the approximation. A comparison of exciton binding energies with those predicted by CIS demonstrates the sensitivity of results to the exact treatment of dielectric screening and the decision of whether or not to screen exchange. Our model predicts optical gaps that are in strong agreement with average experimental data for all but the smallest diameters, but was not able to reproduce low-energy spectral features that were fully consistent with experiment. This was attributed to the absence of the spin-orbit interaction in the model. Simulations performed on CdSe nanoplatelets investigate the optical gaps and exciton binding energies as a function of thickness. Exciton binding energies were found to reach ∼200 meV for the thinnest system, however, optical gaps were slightly overestimated in comparison to experiment. This is attributed to the reduced lateral dimensions used in our simulations and our bulk treatment of dielectric screening. A two-dimensional treatment of dielectric screening is expected to further increase binding energies. Calculations of the excitonic absorption spectrum reproduce the characteristic spectral features observed in experiment, and show strong agreement with the spectra of nanoplatelets, with thicknesses ranging from 3 ML to 5 ML.

No description available.

Multiscale computational simulations are performed to investigate how electronic structure and optical absorption characteristics of recently reported nanostructured III-nitride core-shell MQW solar cells are governed by an intricate coupling of size-quantization, atomicity, and built-in structural and polarization fields. The core computational framework, as available in our in-house QuADS 3-D simulator, is divided into four coupled phases: 1) Geometry construction for the wurtzite lattice having hexagonal crystal symmetry and non-conventional crystal orientations; 2) Structural relaxation and calculation of atomistic strain distributions using the VFF Keating molecular-mechanics model, which employs a conjugate gradient energy minimization scheme; 3) Obtaining the induced polarization and internal potential distributions using a 3-D atomistic Poisson solver; 4) Computing the single-particle electronic structure and optical transition rates using a 10- band sp3 s*-spin tight-binding framework; and 5) Using a TCAD toolkit, study the carrier transport and obtain the device terminal characteristics. Special care was taken in incorporating the nonpolar m-plane crystallographic orientation within the simulator via appropriate lattice vectors, rotational matrices, neighboring atom co-ordinates and sp3-hybridized passivation scheme. Numerical calculations of electronic structure properties are generally based on non-primitive rectangular unit cell. The rectangular geometry approximation is still valid and can be considered even in the presence of strain in nanostructures such as quantum wells, nanowires, and even in self-assembled quantum dots with varying composition. With this approximation, atoms are grouped into traditional unit cells resulting in simpler analysis and better storage scheme, which results in more dynamic and easily debugged algorithms. Note that the contribution of the second-order piezoelectric polarization is small in the nonpolar m-plane structure (as compared to the polar c-plane counterpart) and was neglected in this study. Besides, the spontaneous polarization is non-existent in m-plane structure. The polarization fields are incorporated in the Hamiltonian as an external potential within a non-self-consistent approximation. From the simulations, it is found that, even without the inclusion of any internal fields, the crystal symmetry is lowered compared to ideal geometries, which is due mainly to the fundamental atomicity and interface discontinuities. However, with the inclusion of internal polarization fields, although the symmetry is lowered further, the m-plane structure exhibits a stronger overlap and localization of the wavefunctions, as compared to the c-plane counterpart. Importantly, strain, in the m-plane structure, causes a larger splitting of the topmost valence band and the interband transition probability involving the 4th valence band was found to be highest. Overall, the m-plane structure offers higher spontaneous emission rate and internal quantum efficiency (IQE) as well as an improved fill-factor.

abstract: In this work, transport in nanowire materials and nanowire field effect transistors is studied using a full band Monte Carlo simulator within the tight binding basis. Chapter 1 is dedicated to the importance of nanowires and nanoscale devices in present day electronics and the necessity to use a computationally efficient tool to simulate transport in these devices. Chapter 2 discusses the calculation of the full band structure of nanowires based on an atomistic tight binding approach, particularly noting the use of the exact same tight binding parameters for bulk band structures as well as the nanowire band structures. Chapter 3 contains the scattering rate formula for deformation potential, polar optical phonon, ionized impurity and impact ionization scattering in nanowires using Fermi’s golden rule and the tight binding basis to describe the wave functions. A method to calculate the dielectric screening in 1D systems within the tight binding basis is also described. Importantly, the scattering rates of nanowires tends to the bulk scattering rates at high energies, enabling the use of the same parameter set that were fitted to bulk experimental data to be used in the simulation of nanowire transport. A robust and efficient method to model interband tunneling is discussed in chapter 4 and its importance in nanowire transport is highlighted. In chapter 5, energy relaxation of excited electrons is studied for free standing nanowires and cladded nanowires. Finally, in chapter 6, a full band Monte Carlo particle based solver is created which treats confinement in a full quantum way and the current voltage characteristics as well as the subthreshold swing and percentage of ballistic transport is analyzed for an In0.7Ga0.3As junctionless nanowire field effect transistor. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2016

Submitted by Allan Victor Ribeiro null (allan_vr@fc.unesp.br) on 2017-07-12T19:01:29Z No. of bitstreams: 1 Tese - Allan Victor Ribeiro - Posmat.pdf: 17273743 bytes, checksum: 654df5020a2a453977468f1145d42794 (MD5) / Approved for entry into archive by Monique Sasaki (sayumi_sasaki@hotmail.com) on 2017-07-14T17:31:10Z (GMT) No. of bitstreams: 1 ribeiro_av_dr_bauru.pdf: 17273743 bytes, checksum: 654df5020a2a453977468f1145d42794 (MD5) / Made available in DSpace on 2017-07-14T17:31:10Z (GMT). No. of bitstreams: 1 ribeiro_av_dr_bauru.pdf: 17273743 bytes, checksum: 654df5020a2a453977468f1145d42794 (MD5) Previous issue date: 2017-04-28 / Gregory H. Wannier, em 1937, introduziu uma representação dos orbitais eletrônicos cristalinos em termos de funções ortogonais localizadas relacionadas com os orbitais atômicos. Posteriormente, tais funções foram denominadas de funções de Wannier. Nos últimos 30 anos, estudos têm apontado um crescente interesse da comunidade científica por estas funções, as quais se apresentam como uma poderosa ferramenta para a investigação de propriedades eletrônicas dos materiais. No presente trabalho, calculamos as funções de Wannier de sistemas nanométricos uni e bidimensionais. Inicialmente abordamos o cumuleno, que consiste em uma cadeia de átomos de carbono equidistantes. As funções de Bloch são obtidas por meio de uma aproximação tight binding e as funções de Wannier, usuais e generalizadas, são calculadas a partir delas. São discutidas as relações entre as funções de Wannier generalizadas obtidas por meio da aproximação tight binding e os orbitais híbridos sp. Isto é explicado mediante um cálculo alternativo das funções de Wannier, com a resolução de um problema de autovalores generalizado. As funções de Wannier das bandas pz do grafeno também são calculadas a partir das funções de Bloch obtidas por meio de uma aproximação tight binding. Elas assemelham-se a um par ligante-antiligante de orbitais moleculares, e suas propriedades de simetria e localização são discutidas. Finalmente, por meio de uma combinação dos pacotes PWscf (baseado em ondas planas e na teoria do funcional da densidade) e wannier90, são calculadas as funções de Bloch e as funções de Wannier de máxima localização para arranjos atômicos com periodicidade em uma (cumuleno) e duas (grafeno) dimensões. Há boa concordância qualitativa entre os resultados da aproximação tight binding e da teoria do funcional da densidade. Deve-se ressaltar que a primeira abordagem não usa réplicas dos sistemas nanométricos e permite aprofundar o entendimento das propriedades e do significado físico das funções de Wannier. / Gregory H. Wannier, in 1937, introduced a representation of crystalline electronic orbitals in terms of localized orthogonal functions related to the atomic orbitals. Subsequently, these functions were called as Wannier functions. Over the past 30 years, studies have shown a growing interest of the scientific community on these functions, which are presented as a powerful tool to investigate the electronic properties of materials. In this work, we calculate the Wannier functions of one and two-dimensional nanometric systems. Initially, we deal with cumulene, which consists of a chain of equidistant carbon atoms. The Bloch functions are obtained by means of a tight binding approximation, and the standard and the generalized Wannier functions are derived from them. The relations between the generalized Wannier functions and the sp hybrid orbitals is discussed. This is explained through an alternative calculation of the Wannier functions, solving a generalized eigenvalue problem. The pz Wannier functions of graphene are also calculated from the Bloch functions obtained by means of a tight binding approximation. They resemble a bonding-antibonding pair of molecular orbitals, and their symmetry and localization properties are discussed. Finally, by combining the computational codes PWscf (based on plane waves and the Density-functional Theory) and wannier90, the Bloch functions and the maximally localized Wannier functions are calculated for atomic arrangements which are periodic in one (cumulene) and two (graphene) dimensions. There is a good qualitative agreement between the results of the tight binding and density-functional approaches. It should be noted that the former does not involve replicas of the nanometric systems and allows a deeper understanding of the properties and the physical meaning of the Wannier functions.

Utilizando a Dinâmica Molecular Tight Binding (TBMD), parametrizada para sistemas de carbono e hidrogênio, simulamos com condições periódicas de contorno e modelos de fatia, superfícies de diamante (100) puras e hidrogenadas em modelos de reconstruções ideais usualmente presentes na literatura, analisando o seu comportamento geométrico e eletrônico. Em seguida abordamos o comportamento morfológico e eletrônico, em simulações com temperaturas que variam entre 100K e 2000K de dois modelos de superfícies monohidrogenadas, que apresentam dois domínios em torno de uma estrutura de depressão local, característica de filmes de alta rugosidade. Em oposição à grande estabilidade térmica exibida pelo modelo monohidrogenado ideal e pelas colunas contínuas de dímeros, os modelos com depressão apresentaram significativa migração de átomos de hidrogênio para regiões subsuperficiais. Em nossas simulações os átomos de hidrogênio ficaram confinados nas regiões subsuperficiais, introduzindo uma desordem morfológica na superfície e nas regiões internas à fatia, induzindo estados eletrônicos nesta região, que levam ao fechamento do gap, passando a caracterizar uma fase quase-metálica. / By using the Tight Binding Molecular Dynamics (TBMD), parametrized to describe carbon and hydrogen atoms composed of systems, we apply periodic boundary conditions, slab models in order to simulate (100) clean and hydrogenated diamond surfaces. We study first the standard models used in the literature, analyzing their geometrical and eletronic behavior. We then focus on the morphological and electronic properties, in simulations under finite temperature dynamics ranging from 100K up to 2000K, of two distinct models of monohydride surfaces; Each model exhibits two distincts domains in the surface pattern characterized by a local depression, characteristic of rough surfaces. In opposition to the high thermal stability observed for ideal monohydrogenated surfaces and the extended dimer rows, these models showed an expressive hydrogen migration to the subsurface regions. In our simulations the hydrogen atoms remain in the subsurface regions, but introduce morphological disorder at the surface and in the slab internal regions. These hydrogen atoms induce electronic states mostly localized in the subsurface region, which are responsible for closing the gap, and leading the system to exhibit a quasi-metallic phase.