A química computacional apresenta a grande vantagem de prover informações fundamentais para espécies moleculares propostas, antes mesmo de sua síntese em laboratório. A Teoria do Funcional da Densidade é bastante utilizada nesta área, produzindo resultados satisfatórios para um grande número de propriedades e sistemas, mas com uma menor demanda por recursos computacionais que métodos mais avançados. Entretanto, o desenvolvimento de funcionais que incluem efeitos relativísticos ainda se encontra num estágio inicial. Em geral, tais efeitos são importantes em compostos de átomos pesados, embora devam ser considerados também em sistemas com átomos mais leves se a propriedade em estudo for particularmente sensível, como é o caso do gradiente de campo elétrico na posição de núcleos em moléculas. Assim, na primeira etapa desta dissertação foi avaliado o desempenho de funcionais comuns de troca-correlação não relativísticos, quando utilizados em conjunto com o formalismo de quatro componentes (tratamento relativístico), no estudo dos gradientes de campo elétrico em núcleos de átomos (índio, antimônio, iodo, lutécio e háfnio) constituindo moléculas diatômicas. Foram investigados funcionais baseados nas aproximações da densidade local e do gradiente generalizado, funcionais híbridos e que incluem correções em termos da atenuação com a distância. Nossos resultados, que estão em acordo com observações da literatura, ressaltam o melhor desempenho de funcionais híbridos e com correções de atenuação para esta propriedade e demonstram a importância do uso do método indireto. Posteriormente, foi feita uma nova parametrização de alguns dos melhores funcionais não relativísticos selecionados na etapa anterior (B3LYP, PBE0 e CAM-B3LYP), dentro do formalismo de quatro componentes, para uso no cálculo destes mesmos gradientes num grupo teste de átomos (cobre, iodo, lantânio e ouro) em moléculas lineares. Nestes casos, os funcionais modificados propostos tiveram um bom desempenho geral e foram particularmente bem sucedidos para cobre e ouro. Finalmente, é possível destacar o funcional híbrido PBE0 e sua modificação, proposta neste estudo, por conta de seu desempenho excelente, tanto para os metais como para os demais elementos que tiveram seus EFGs investigados aqui. / The computational chemistry has the great advantage of providing fundamental information for proposed molecular species even before their synthesis in laboratory. The Density Functional Theory is widely used in this area, producing satisfactory results for a large number of properties and systems, but with a lower demand for computational resources than that of more advanced methods. However, the development of functionals that include relativistic effects is still at an early stage. In general, these effects are important in compounds containing heavy elements, but they must also be considered in systems of lighter atoms if the studied property was particularly sensitive, as occurs for the electric field gradient at the position of nuclei in molecules. Thus, the first step of this dissertation was to evaluate the performance of common non-relativistic exchange-correlation functionals when used in conjunction with the four component formalism (relativistic treatment) in the study of electric field gradients at the nuclei of atoms (indium, antimony, iodine, lutetium and hafnium) forming diatomic molecules. Functionals based on the local density approximation and generalized gradient approximation, hybrid functionals and the ones that include attenuation corrections were investigated. Our results, which are in agreement with observations in the literature, highlight the best performance of hybrid functionals and attenuation corrections for this property and demonstrate the importance of using the indirect approach. Subsequently, there was a new parameterization of some of the best non-relativistic functionals selected in the previous step (B3LYP, PBE0 and CAM - B3LYP) within the four component formalism for calculations of these same gradients in a trial group of atoms (copper, iodine, lanthanum and gold) into linear molecules. In these cases, the modified functionals proposed had a satisfactory overall performance and were particularly successful for copper and gold. Finally, it is possible to mention the excellent performance of the hybrid functional PBE0 and its modification proposed in this study for both metals and the other elements that had their EFGs investigated here.

Neste trabalho de mestrado apresentamos um estudo sobre a manifestação do efeito Rashba em isolantes topológicos na ausência de simetria de inversão estrutural. Os cálculos das propriedades atomísticas, energéticas e as estruturas eletrônicas são abordados através de métodos de primeiros princípios baseados na teoria do funcional da densidade. E seus resultados foram utilizados para o desenvolvimento de hamiltoniana efetiva baseado no modelo de Zhang. Realizamos o estudo de dois sistemas: 1) Bi$_2$Se$_3$ com átomos de Sn depositados na superfície: Este sistema pode ser entendido através da manifestação do efeito Rashba sobre um isolante topológico dada a quebra de simetria de inversão estrutural. Para um sítio de deposição específico, os átomos de Sn causam uma reconstrução da superfície e um terceiro cone de Dirac é observado na estrutura eletrônica. Este terceiro cone é não localizado na superfície e pode ser entendido como a manifestação do efeito Rashba. 2) PbBiI: Reportado aqui como um novo isolante topológico 2D com efeito Rashba. Descobrimos este sistema por um estudo sistemático sobre uma família de materiais formados por átomos tipo IV, V, e VII, cuja estrutura cristalina é hexagonal e não centrossimétrica. Mostramos que o PbBiI possui: i) Estabilidade mecânica, ii) Spin-splitting Rashba de 60 meV, iii) um gap de energia não trivial de 0.14 eV, iv) retroespalhamento proibido entre os estados de borda e v) retroespalhamento proibido entre os estados do bulk no entorno do nível de Fermi. Estas propriedades fazem do PbBiI um candidato para construção de dispositivos de spintrônica que atenua a perda de energia. / In this work, were studied the Rashba effect in topological insulators without structural inversion symmetry. We performed a first principles study based on density functional theory to calculate the atomistic properties, formation energy and electronic structure. These results were used to development a effective Hamiltonian based on Zhang model. They were studied two systems: 1) Bi$_2$Se$_3$ with Sn atoms deposited on the surface: This system can be seen as the Rashba effect manifestation on a topological insulator due to the structural inversion symmetry breaking. For a specific deposition site, the Sn atoms cause a reconstruction of the surface and display a third Dirac cone in the electronic structure. This third cone is not located on the surface and can be understood as the giant Rashba effect manifestation. 2) We propose a new non-centrosymmetric honeycomb-lattice QSH insulator family formed by the IV, V, and VII elements. The system formed by Bi, Pb and I atoms is reported here as a new 2D topological insulator with Rashba effect. We show that this system has: i) Mechanical stability, ii) spin-splitting Rashba of 60 meV, iii) nontrivial energy gap of 0.14 eV, iv) backscattering forbidden for both edge and bulk conductivity channels in the nanoribbon band structure. These properties make PbBiI a good candidate to construct spintronic devices with less energy loss.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-06-15T11:52:02Z No. of bitstreams: 1 carlavieirasoares.pdf: 4663634 bytes, checksum: 2b83afaaaca5d4ad67efb3c7313d7bb2 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-07-13T14:09:33Z (GMT) No. of bitstreams: 1 carlavieirasoares.pdf: 4663634 bytes, checksum: 2b83afaaaca5d4ad67efb3c7313d7bb2 (MD5) / Made available in DSpace on 2016-07-13T14:09:33Z (GMT). No. of bitstreams: 1 carlavieirasoares.pdf: 4663634 bytes, checksum: 2b83afaaaca5d4ad67efb3c7313d7bb2 (MD5) Previous issue date: 2016-02-29 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Cálculos de primeiros princípios baseados na Teoria do Funcional da Densidade (DFT) foram utilizados para investigar a adsorção de moléculas (CO2, CO, H2, CH4, N2, H2S, C4H4S, C2H4O2, C2H4O e H2O) na cavidade porosa da MOF formada por Zircônio e Tereftalato. A estrutura alvo deste trabalho denominada MIL-140A foi primeiramente validada, mostrando boa concordância com os dados espectroscópicos (Infravermelho e RMN), difratograma de raios X e parâmetros estruturais entre o modelo simulado e a estrutura experimental. Com os cálculos de estrutura eletrônica foi possível prever os sítios preferenciais para a adsorção, avaliar a força das interações adsorvente/adsorbato e determinar a ordem de seletividade para as moléculas na MIL-140A. O uso do funcional PBE aliado à correção DFT-D2 foi uma ferramenta valiosa para descrever a interação nesses sistemas. Devido ao fato das interações serem relativamente fracas, a contribuição da energia de ponto zero (EZPE) e da energia térmica (Evib(T)) são pequenas se comparadas com a contribuição das interações de dispersão. Assim, as energias vibracionais foram negligenciadas e H pôde ser aproximada para ΔEDFT. O bom acordo encontrado entre ΔEDFT e a entalpia de adsorção experimental para as moléculas (CO2, CO, CH4 e N2) comprova essa hipótese. A estabilidade da MIL-140A foi explorada mediante o cálculo da barreira de energia associada à reação de dissociação da molécula de água. O valor muito alto encontrado para a dissociação da molécula de água confirma que essa MOF se mantém estável. Os resultados demonstram que a MIL-140A pode ser um adsorvente ideal para a remoção de CO2 a partir de uma série de gases, por exemplo, gás natural, gás de síntese, biogás e gás de combustão. Além disso, a MIL-140A também pode ser utilizada na remoção do sulfeto de hidrogênio e tiofeno dos combustíveis, atuar como uma plataforma para reações envolvendo o ácido acético e reciclar o acetaldeído ejetado das indústrias. / First principle calculations based on Density Functional Theory (DFT) were used to investigate the adsorption of molecules (CO2, CO, H2, CH4, N2, H2S, C4H4S, C2H4O2, C2H4O and H2O) in a porous cavity of Zirconium terephthalate MOF. The MIL-140A was first validated by a very good agreement between the simulation and the experimental spectroscopic (Infra-red and NMR), X-ray diffraction and structural parameters of the selected MOF. The calculations of electronic structure further predicted the preferential adsorption sites, the strength of the host/guest interactions and determine the selectivity for molecules in MIL-140A. The use of the PBE XC functional integrating the DFT-D2 correction was valuable to accurately describe the interaction of those systems. Due relative weak interactions it is expected that the contribution of the zero point energies (ΔEZPE) and thermal energy contributions (ΔETE) are very small in comparison to the dispersion interactions contributions. Thus, the vibrational energies can be neglected and thus ΔH can be approximated to ΔEDFT. A very good agreement between ΔEDFT and the experimental adsorption enthalpy determined for the molecules (CO2, CO, CH4 e N2) confirms this assumption. The water stability of the MIL-140A was explored through the activation barrier associated to the dissociative chemisorption of water. Its high value confirms the very good water stability of this MOF. These results demonstrate that the MIL-140A could act as an ideal adsorvent for the removal of CO2 from a series of gases (e.g. natural gas, syngas, biogas and flue gas). Furthermore the MIL-140A can be able to remove hydrogen sulfide and thiophene from the fuels, act as a platform for reactions with acid acetic and recycle the acetaldehyde ejected from the factories.

Orientador: Edison Zacarias da Silva / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-22T13:09:46Z (GMT). No. of bitstreams: 1 Nascimento_AnaPaulaFavaro_D.pdf: 24653202 bytes, checksum: cb09f1eecbbf100b37ad50fd3a26f857 (MD5) Previous issue date: 2013 / Resumo: A fabricação e o estudo de nanofios de ouro despertam grande interesse na comunidade científica, na tentativa de maior entendimento de efeitos quânticos de sistemas em escala reduzida, assim como na possibilidade de seu uso em aplicações tecnológicas. Uma vez que os nanofios de ouro apresentam propriedades surpreendentes quando dopados por impurezas atômicas fomos motivados a estudar como estas se formam. Devido ao fato de em escala nanoscópica o ouro apresentar atividade catalítica, consideramos que a presença de impurezas se deve a reação de pequenas moléculas em nanofios de Au. O estudo foi realizado por meio de cálculos abinitio via Teoria do Funcional da Densidade, usando o código computacional SIESTA. A metodologia para o estudo da estrutura eletrônica desses sistemas foi a de otimização de geometria e de dinâmica molecular ab initio. Nosso foco de estudo foi encontrar caminhos reacionais para a formação de impurezas atômicas de carbono ou de oxigênio nas cadeias atômicas lineares de nanofios monoatômicos. A análise se baseou na interação entre duas moléculas catalisada pelo nanofio, as moléculas consideradas em nosso estudo foram CO e O2. Um estudo extensivo e detalhado das possíveis reações foi feito. Dentre os vários caminhos estudados, uma reação sequencial onde uma molécula de O2 é adsorvida por apenas um dos seus oxigênios, inicia um processo, que seguido pela adsorção de uma molécula de CO, leva a formação de um complexo O2-CO ligado a cadeia atômica do nanofio. Nós mostramos situações onde este complexo fica ativado e reage formando uma molécula de CO2 que vai para a fase gasosa deixando o nanofio dopado com um átomo de oxigênio. Portanto este trabalho apresenta um caminho reacional para a formação de uma impureza atômica na cadeia atômica de um nanofio de ouro, uma questão que esperava uma solução a quase uma década. / Abstract: The manufacture and the study of gold nanowires weakened great interest of the scientific community in the quest for better understanding of the quantum effects in systems with reduced scales and also due to the possibility of their use in technological applications. Since gold nanowires present novel and surprising properties when doped with atomic impurities, this led us into the search to understand how these impurities can be produced. Due to the fact that gold in nanoscale presents catalytic activity, we considered the possibility of chemical reactions with small molecules in the presence of gold nanowires. The present study was performed with ab initio calculations based in the density functional theory as implemented by the SIESTA code. The methodology for the electronic structure studies was the geometry optimization using conjugated gradient method and abinitio molecular dynamics. Our focus was to find reaction paths to produce atomic impurities of carbon and oxygen in linear atomic chains of gold nanowires. The analysis was based in the reaction of two molecules catalized by the nanowire, the molecules considered in this study were CO and O2. An extensive and detailed study of possible pathways was undertaken. Among the various paths, a sequential reaction where only one O of a adsorbed O2 molecule attached to the nanowire, started the process, followed by the adsorption of a CO molecule nearby that formed an O2-CO complex attached to the atomic chain of the nanowire. We presented circumstances in which this complex becomes activated and evolves to form a CO2 molecule that goes into the gas phase leaving an atomic oxygen impurity attached to the linear chain. Therefore, this work presented a reactional path to the formation of an atomic impurity in the atomic chain of a gold nanowire, a question that waited an answer for almost ten years. / Doutorado / Física / Doutora em Ciências

Abstract In this thesis, the magnetic resonance and magneto-optical rotation parameters are studied in single-layer carbon systems of two different dimensionalities. Based on electronic structure calculations, the spectral parameters are predicted for both extended (2D) and finite, molecular (0D) systems consisting of pure sp²-hybridised pristine graphene (G), as well as hydrogenated and fluorinated, sp³-hybridised graphene derivatives, graphane (HG) and fluorographene (FG), respectively. Nuclear magnetic resonance (NMR) parameters are calculated for G, HG and FG systems at their large-system limit. For their 0D counterparts, graphene flakes, qualitative spectral trends are predicted as functions of their size and perimeter type. The last group of studied carbon systems consists of 2D graphenes containing spin-1/2 paramagnetic defects. Electron spin resonance (ESR) parameters and paramagnetic NMR shieldings are predicted for four different paramagnetic systems, including the vacancy-defected graphane and fluorographene, as well as graphene with hydrogen and fluorine adatoms. The magneto-optic properties of G and HG flakes are studied in terms of Faraday optical rotation and nuclear spin optical rotation parameters, to investigate the effects of their finite size and also the different level of hydrogenation. All the different investigated parameters displayed characteristic sensitivity to the electronic and atomic structure of the studied graphenes. The parameters obtained provide an insight into the physics of these 0D and 2D carbon materials, and encourage experimental verification.

15N-based nuclear magnetic resonance techniques are considered very powerful to study the molecular properties of platinum-containing anticancer agents, these properties being responsible for the efficacy of the compounds, but also for the understanding of resistance mechanisms and toxicity. Therefore, the first part of the present work aimed to develop a new method for synthesizing 15N-labeled, chiral platinum compounds. A theoretical discussion on the nucleophilic ring-opening of aziridines has also been envisaged, rationalizing an interesting regiochemistry question. Indeed, a surprising inversion of regiochemistry arose during the development of the above-mentioned synthetic pathway, and density functional theory calculation brought a rational framework to the experimental findings.<p><p>Infrared spectroscopy probes the global chemical composition of a sample and has been used to produce a snapshot of cancer cells contents after treatment with platinum coordinates. Indeed, in vitro studies focused here on the use of modern spectroscopic methods to fingerprint the cellular impact of platinum complexes. These drug signatures help to classify and select promising compounds. It makes no doubt that such systemic approaches for compound discovery are helpful technologies. Also, we made the use of the COMPARE algorithm from the NCI, which analyzes similarity between any active compounds previously tested by the NCI large scale in vitro screening program of anticancer agents. <p><p>The last chapter aimed to study the interactions between a series of platinum coordinates and DNA. Binding mode to telomeric-like sequences and binding kinetics to genomic-like sequences were assessed to investigate any differences between the compounds and to gain insight into structure-activity relationships. <p> / Doctorat en Sciences biomédicales et pharmaceutiques / info:eu-repo/semantics/nonPublished

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work properties were studied the structural, electronic and magnetic semiconductors PbSe and PbTe doped with europium (Eu) in the crystalline phase. The investigation is performed within the density functional theory (DFT). We observe that due a strong spin-orbit coupling, relativistic corrections are necessary to describe the PbSe and PbTe. To obtain a good description of crystals where Eu is present (EuSe and EuTe) the standard DFT fails. This fail is due to the fact that the Eu atom presents f electrons in the valence band, which are strongly localized. To describe the localized f electrons an additional procedure from the many body theory is necessary. We observe that using an additional term (the U term) from the Hubbard model, the DFT is able to describe the localized f electrons, such the theory now is called DFT+U. The formation energy results show that the Eu atom is more stable in Pb sites for both PbSe and PbTe. Eu in a Pb site in PbSe introduces electronic levels inside the band gap while the new electronic levels from Eu in a Pb site in PbTe are resonant with the top of the valence band. The analysis from the character of these electronic levels reveled that they come from the 4f electrons from the Eu atom. These results allow us to conclude that Eu is a good dopant to improve the thermoelectric properties of PbTe while the same is not observed for PbSe. Finally, we investigate the magnetic properties for Eu doping PbSe and PbTe, we observe that in both semiconductors there is a magnetic moment of the 1.0 μB localized in the Eu atom. / Neste trabalho foi estudado as propriedades estruturais, eletrônicas e magnéticas dos semicondutores de PbSe e PbTe dopados com európio (Eu) na fase cristalina. Utilizamos cálculos de primeiros princípios dentro do formalismo da teoria do funcional da densidade (DFT). Observamos que para uma boa descrição do PbSe e PbTe dentro da DFT é necessário a inclusão das correções relativísticas através da interação spin-órbita. Para a descrição de cristais onde o európio é um dos constituintes (EuSe e EuTe) se observou a necessidade de correções à DFT para que se possa ter uma boa descrição dos elétrons f provenientes do Eu e que são fortemente localizados. A teoria DFT+U, onde U é proveniente da teoria de muitos corpos dentro do modelo de Hubbard se mostrou eficiente. Com a melhor metodologia estabelecida iniciamos os cálculos das energias de formação. Os resultados mostraram que o Eu é mais estávelr em sítios de Pb, tanto no cristal de PbSe como no cristal de PbTe. Nesses sítios a análise da parte eletrônica diz que no PbSe existem níveis no gap provenientes dos elétrons 4f do Eu, enquanto que no PbTe esses níveis estão ressonantes com o topo da banda de valência. Isso permite concluir que o Eu é um bom dopante para melhorar as propriedades termoelétricas do PbTe, o mesmo não pode ser dito com relação a dopagem com Eu em PbSe. Com relação a parte magnética observa-se um momento magnético de 1,0 μB tanto no cristal de PbSe como no cristal de PbTe. Esse momento magnético é localizado no átomo de európio para ambos semicondutores.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / The observation of the quantum spin Hall effect in the HgTe/CdTe heterostructure triggered the study of materials exhibiting a spin polarized electronic current at their surfaces/ interfaces. These states are topologically protected against perturbations preserving time reversal symmetry and presenting a linear dispersion, forming a Dirac cone. However, non-magnetic perturbations (that preserve time reversal symmetry) will certainly affect these surface/interface states. In this work we user the density functional theory to characterize the topologically protected states of the (001) HgTe/CdTe heterostructure. We observed that for a correct description of the HgTe band structure we use a GGA+U method. The topological states showed a Rashba-like in-plane spin texture. We analyzed the effects of external pressures and electric fields in the HgTe/CdTe heterostructures. We show that these perturbations modify the energetics and dispersion of the protected states, although not destroying the topological phase. Also, we study defects like antisite, vacancy and a Fe magnetic impurity at the interface of the (001) HgTe/CdTe heterostructure. We show that the antisite and the vacancy do not affect the spin polarization nor the energy dispersion of the protected states. On the other hand, the magnetic impurity significantly affects the topological states, degrading the spin polarization for the states close to the magnetic impurity and inducing out-of-plane spin components. Further, we study the (001) HgTe surface for different thicknesses of the HgTe sample, and with different terminations (Hg and Te). To the (001) HgTe samples with a thickness of 38 Å , the spin polarized states do not show a linear dispersion, however, when the thickness is increased we observe the formation of spin-polarized surface states with linear dispersion, characterizing the formation of a Dirac cone. Also, we show that biaxial pressures modify the energy dispersion of the spin polarized states. Finally, we study materials that turn topological insulators under external pressures as the anti-perovskite structures Sr3BiN and Ca3BiN, using the self-consistent GW method. We show that these materials present an inversion of the Bi-pz and Bi-s band edge states when subjected to biaxial tensile stress. We conclude that these materials can be characterized Topological Insulators under pressure. / A observação do efeito Spin Hall Quântico na heteroestrutura HgTe/CdTe motivou o estudo de materiais que exibem uma corrente eletrônica spin-polarizada nas suas interfaces/ superfícies. Estes estados são topologicamente protegidos frente a perturbações que preservam a simetria de reversão temporal e apresentam uma dispersão linear formando um Cone de Dirac. Entretanto, perturbações não-magnéticas (que preservam a reversão temporal) irão certamente afetar estes estados de interface/superfície. Neste trabalho, usamos a Teoria do Funcional da Densidade (DFT), para caracterizar os estados topologicamente protegidos da heteroestrutura HgTe/CdTe (001), que é um Isolante Topológico (IT) 2D. Para uma descrição mais correta das posições dos níveis na estrutura de bandas do HgTe, nós usamos o método GGA+U. Na heteroestrutura, a caracterização dos estados topologicamente protegidos mostrou uma textura de spin no plano da interface, do tipo Rashba. Analisamos os efeitos de perturbações externas na heteroestrutura HgTe/CdTe (001), como pressões e campo elétrico. Mostramos que ambas perturbações modificam a energia do ponto de cruzamento e a dispersão dos estados protegidos, mas não destroem a fase topológica. Estudamos também a presença de defeitos na interface HgTe/CdTe (001), como um anti-sítio, uma vacância e uma impureza magnética de Fe. A presença de um anti-sítio e de uma vacância não afetam a polarização de spin dos estados protegidos e nem sua dispersão. Por outro lado, a presença de uma impureza magnética afeta significantemente estes estados, degradando a polarização de spin para os estados próximos a impureza magnética e fazendo que o sistema apresente componentes de spin fora do plano da interface/superfície. Além disso, estudamos a superfície de HgTe com diferentes espessuras (38, 64, e 129 Å ) e terminações (Hg e Te). Para as estruturas com uma espessura de 38 Å , os estados com polarização de spin não apresentam uma dispersão linear, entretanto, quando aumentamos a espessura do material, observamos a formação dos estados de superfície com uma dispersão linear e polarização de spin, caracterizando a formação do cone de Dirac. Mostramos também, que pressões biaxiais modificam a dispersão dos estados com polarização de spin. Realizamos um estudo de materiais que são Isolantes Topológicos quando submetidos a pressões externas. Neste caso estudamos as estruturas antiperovsquitas Sr3BiN e Ca3BiN, usando método GW auto-consistente. Mostramos que esses materiais apresentam uma inversão dos níveis de energia Bi-pz e Bi-s quando sujeitos a pressão externa biaxial distensiva. Concluímos que estes materiais podem ser caracterizados como Isolantes Topológicos sob pressão.

While group VII 4d Tc and 5d Re have hexagonally close-packed (hcp) ground states, 3d Mn adopts the complex chi-phase which exhibits non-collinear magnetism. Density functional theory (DFT) calculations have shown that without magnetism the chi-phase remains the ground state of Mn implying that magnetism is not the critical factor, as is commonly believed, in driving the anomalous stability of the chi-phase over hcp. Using a tight-binding (TB) model it is found that while harder potentials stabilise close-packed hcp, a softer potential stabilises the more open chi-phase. By analogy with the structural trend from open to close-packed phases down the group IV elements, the anomalous stability of the chi-phase in Mn is shown to be due to 3d valent Mn lacking d states in the core which leads to an effectively softer atomic repulsion between the atoms than in 4d Tc and 5d Re. Subsequently an analytic Bond-Order Potential (BOP) is developed to investigate the structural and magnetic properties of elemental Mn at 0 K. It is derived within BOP theory directly from a new short-ranged orthogonal d-valent TB model of Mn, the parameters of which are fitted to reproduce the DFT binding energy curves of the five experimentally observed phases of Mn, alpha, beta, gamma, delta, and epsilon-Mn. Not only does the BOP reproduce qualitatively DFT binding energy curves of the five different structure types, it also predicts the complex collinear antiferromagnetic (AFM) ordering in alpha-Mn, the ferrimagnetic (FiM) ordering in beta-Mn and the AFM ordering in the other phases that are found by DFT. A BOP expansion including 14 moments is sufficiently converged to reproduce most of the properties of the TB model with the exception of the elastic shear constants which require further moments. Magnetic analytic BOPs are also developed for Fe and Fe-Mn. The Fe model correctly reproduces trends in the structural stabilities of the common metallic structures except that AFM hcp is overstabilised. Reproduction of the elastic constants with a 9-moment BOP is reasonable although as is found for the Mn BOP the elastic shear constants require more moments to converge. Vacancy formation energies are close to those determined by experiment and DFT and the relative stabilities of self-interstitial atom (SIA) defects in ferromagnetic bcc Fe are correctly reproduced. The SIA formation energies are found to be better than those calculated with existing BOP models. The Fe-Mn TB and BOP models were challenging to fit and nonmagnetic face-centred cubic (fcc) structures are overstabilised. Furthermore within BOP an incorrect magnetic solution is predicted for one fcc structure resulting in poor reproduction of the DFT stacking fault energies. Refitting the bond integrals might help to better reproduce the nonmagnetic hcp-fcc energy differences while an environment-dependent Stoner parameter could help provide the flexibility needed to correctly capture the magnetic energy differences.

Graphene – a two-dimensional honeycomb lattice of sp2-bonded carbon atoms – has been attracting a great deal of research interest since its first experimental realization in 2004, due to its various novel properties and its potential for applications in futuristic nanodevices. Being the fundamental building block for carbon allotropes of other dimensionality, it can be stacked to form 3d graphite or rolled into 1d nanotube. Graphene is the thinnest known material in the universe, and one of the strongest materials ever measured in terms of its in-plane Young modulus and elastic stiffness. The charge carriers in graphene exhibit giant mobility as high as 20 m2/Vs, have almost zero effective mass, and can travel for micrometers without scattering even at ambient conditions. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and renders easy accessibility to optical probes. Electron transport in graphene is described by a Dirac-type equation, which allows the investigation of “relativistic” quantum phenomena in a benchtop experiment. This results in the observation of a number of very peculiar electronic properties from an anomalous quantum Hall effect to Kien paradox and the absence of localization. All these enticing features make this material an excellent candidate for application in various electronic, photonic and optoelectronic devices. For instance, its ballistic ambipolar transport and high carrier mobility are the most useful traits for making ultrafast and low-power electronic devices. Its high surface area shouldmake it handy in manufacturing tough composite materials. The extreme thinness of graphene could also lead to more efficient field emitters that release electrons in the presence of strong electric fields. Its robustness and light weight are useful for micromechnical resonators. The tunability of its properties could make it possible to build so-called spin-valve transistors, as well as ultra-sensitive chemical detectors. Many of such applications of graphene require tuning of its properties, which can be achieved by varying the number of layers or/and by doping. There are several ways to dope graphene: (i)electrochemically gated doping, (ii)molecular charge-transfer doping, and (iii) substitutional doping by atoms like Boron or Nitrogen.Moreover, for graphene, a zero band gap semiconductor in its pristine form, to become a versatile electronic device material it is mandatory to find means to open up a band gap and tune the size of the band gap. Several strategies have been adopted to engineer such a band gap in graphene in a controlled way. Some of these are based on the ability to control the geometry of graphene layers, some use graphene-substrate interactions, while others are based on chemical reactions of atoms or molecules with the graphene layer. Motivated by these considerations, in this thesis we present a systematic and thorough study of the structural, electronic and vibrational properties of graphene and their dependence on the number of layers, and on doping achieved electrochemically, molecularly and substitutionally, using first principles density functional theory (DFT). In Chapter 1, we give an introduction to the hitherto beguiling world of graphene. Here, we briefly discuss the structure, novel properties and potential applications of graphene, and the motivation for this thesis. In Chapter 2, an overview of the DFT formalism adopted here is given. We clearly state the theorems of the formalism and the approximations used when performing calculations. We succinctly explain how the various quantities like total energies, forces, stresses etcetera are calculated within this formalism. We also discuss how phonon frequencies, eigenvectors, electron-phonon couplings are obtained by using density functional perturbation theory (DFPT), which calculates the full dynamical matrices through the linear response of electrons to static perturbations induced by ionic displacements. Calculations are done first using a fully ab-initio approach within the standard Born-Oppenheimer approximation, and then time-dependent perturbation theory is used to explore the effects of dynamic response. In Chapter 3, using such first-principles density-functional theory calculations, we determine the vibrational properties of ultra-thin n(1,2,...,7)-layer graphene films and present a detailed analysis of their zone-center phonons. We present the results (including structural relaxations, phonons, mode symmetries, optical activities) for bulk Graphite, single-layer graphene and ultrathin n-layer graphene films. and discuss the underlying physics of our main results together with a pictorial representation of the phonon modes. We demonstrate that a low-frequency (∼ 112 cm−1 ) optical phonon with out-of-plane displacements exhibits a particularly large sensitivity to the number of layers, although no discernible change in the interlayer spacing is found as n varies. Frequency shifts of the optical phonons in bilayer graphene are also calculated as a function of its interlayer separation and interpreted in terms of the inter-planar interaction. The surface vibrational properties of n-layer graphene films are presented in Chapter 4, which renders a detailed and thorough analysis of all the surface phonon modes by determining, classifying and identifying them accurately. The response of surface modes to the presence of adsorbed hydrogen molecules is determined. As an illustrative adsorbate, hydrogen is chosen here mainly because of its huge importance in fuel cell technology and as a molecular sensor. We demonstrate that a doubly degenerate surface phonon mode with low-frequency (~ 35cm−1)exhibits a particularly large sensitivity to the adsorption of hydrogen molecules, as compared to other surface modes. Futhermore, we show that a low-frequency (108.8 cm−1)bulk-like phonon with out-of-plane displacements is also very sensitive and gets upshifted by as much as 21 cm−1 due to this adsorption. In Chapter 5, we determine the adiabatic frequency shift of the and phonons in a monolayer graphene as a function of both electron and hole doping. The doping is simulated here to correspond to electrochemically gated graphene. Compared to the results for the E2g -Γ phonon (Raman G band), the results for the phonon are dramatically different, while those for the phonon are not so different. Furthermore, we calculate the frequency shifts, as a function of the charge doping, of the (K + ΔK) phonons responsible for the Raman 2D band –a key finger print of graphene, where [ΔK] is determined by the double resonance Raman process. Doping graphene with electron donating or accepting molecules is an interesting approach to introduce carriers into it, analogous to electrochemical doping accomplished in graphene when used in a field-effect transistor. In Chapter 6, we use first-principles density-functional theory to determine changes in the electronic structure and vibrational properties of graphene that arise from the adsorption of aromatic molecules such as aniline and nitrobenzene. Identifying the roles of various mechanisms of chemical interaction between graphene and the adsorbed molecules, we bring out the contrast between electrochemical and molecular doping of graphene. Our estimates of various contributions to shifts in the Raman active modes of graphene with molecular doping are fundamental to the possible use of Raman spectroscopy in (a)characterization of the nature and concentration of carriers in graphene arising from molecular doping, and (b) graphene-based chemical sensors. Graphene doped electrochemically or through charge-transfer with electron-donor and acceptor molecules, shows marked changes in electronic structure, with characteristic signatures in the Raman spectra. Substitutional doping, universally used in tuning properties of semiconductors, could also be a powerful tool to control the electronic properties of graphene. In Chapter 7, we present the structure and properties of boron and nitrogen doped graphenes, again using first-principles density functional theory. We demonstrate systematic changes in the carrier-concentration and electronic structure of graphenes with B/N-doping, accompanied by a stiffening of the G-band and change of the defect related D-band in the Raman spectra. Such n/p -type graphenes obtained without external fields or chemical agents should find device applications.