Size dependent electronic properties of silicon quantum dots - an analysis with hybrid, screened hybrid and local density functional theory

We use an efficient projection scheme for the Fock operator to analyze the size dependence of silicon quantum dots (QDs) electronic properties. We compare the behavior of hybrid, screened hybrid and local density functionals as a function of the dot size up to $\sim$800 silicon atoms and volume of up to $\sim$20nm$^3$. This allows comparing the calculations of hybrid and screened hybrid functionals to experimental results over a wide range of QD sizes. We demonstrate the size dependent behavior of the band gap, density of states, ionization potential and HOMO level shift after ionization. Those results are compared to experiment and to other theoretical approaches, such as tight-binding, empirical pseudopotentials, TDDFT and GW

Dielectric dependent hybrid functionals for heterogeneous materials

We derive a dielectric-dependent hybrid functional which accurately describes the electronic properties of heterogeneous interfaces and surfaces, as well as those of three- and two-dimensional bulk solids. The functional, which does not contain any adjustable parameter, is a generalization of self-consistent hybrid functionals introduced for homogeneous solids, where the screened Coulomb interaction is defined using a spatially varying, local dielectric function. The latter is determined self-consistently using density functional calculations in finite electric fields. We present results for the band gaps and dielectric constants of 3D and 2D bulk materials, and band offsets for interfaces, showing an accuracy comparable to that of GW calculations.Comment: 27 pages, 8 figure

GeVn complexes for silicon-based room-temperature single-atom nanoelectronics

We characterize germanium-vacancy GeVn complexes in silicon using first-principles Density Functional Theory calculations with screening-dependent hybrid functionals. We report on the local geometry and electronic excited states of these defects, including charge transition levels corresponding to the addition of one or more electrons to the defect. Our main theoretical result concerns the GeV complex, which we show to give rise to two excited states deep in the gap, at -0.51 and -0.35 eV from the conduction band, consistently with the available spectroscopic data. The adopted theoretical scheme, suitable to compute a reliable estimate of the wavefunction decay, leads us to predict that such states are associated to an electron localization over a length of about 0.45 nm. By combining the electronic properties of the bare silicon vacancy, carrying deep states in the band gap, with the spatial controllability arising from single Ge ion implantation techniques, the GeVn complex emerges as a suitable ingredient for silicon-based room-temperature single-atom devices

Modelling and Simulation of materials for Photovoltaic applications

The global need for energy is predicted to double by 2050 and triple by the end of the 21st century. Today, fossil fuels are the primary source for energy supply in the world. However, the excessive consumption of fossil fuels has led to global warming and has resulted in severe environmental impacts. Growing population demands a lot of energy in the future, and there will be limited fossil fuels resources available. Thus, alternative clean energy resources will be the hour of need. Solar energy is probably the most promising source of clean and abundant energy that we have now. An enormous technological and political effort has been undertaken to harness the solar energy more directly. However, the challenge is that solar energy technologies should become cheaper, flexible, energy effective and harmless to the environment. This research focuses on materials for new generation solar cell technologies that fulfil these demands. Third generation solar cells such as intermediate band solar cells and non-silicon solar cells are a newer type of solar cells. They have attained considerable attention in the last two decades, as a potentially cost-effective alternative to conventional costly silicon solar cells. Intermediate band solar cells and non-silicon solar cells are complex devices, which is relied on the interplay of several key components. The unique architecture of intermediate band solar cells provides balancelimiting efficiencies of 63.2%. As a result, an extensive and increasing amount of research effort has been devoted to design and synthesize novel materials. However, most of such efforts have been expensive and time-consuming synthesis procedure. To overcome this drawback, modelling and simulation of new materials is a better method to study and verify the properties of the materials for photovoltaic applications. This thesis has focussed on a theoretical calculation of properties like structural prediction, electronic structure, optical properties, structural stability and mechanical stability behaviour of photovoltaic materials. The aim of the study is fivefold: The first is to study and gain knowledge on the fundamental properties of the matter governed by the electronic structure of a variety of bulk materials. The second is to study novel materials and determine the adaptability and the applicability of theoretical calculation as an accompaniment to experiments for the material scientist in his/her search for novel photovoltaic materials. The third is to investigate materials numerically with intermediate bandgaps that could pave the way for higher cell efficiencies than the theoretically limited efficiency of 32%. Fourth is to carry out an in-depth analysis of low-cost, direct band gap, non-silicon materials for PV applications. Fifth is to implement efficient approximations, methods and algorithms to derive accurate numerical results for electronic and optical properties of a variety of novel materials for PV applications. We expect these findings of novel materials in this thesis will lead to immediate concern and interest to an extensive audience in the scientific society

Optical excitations in organic molecules, clusters and defects studied by first-principles Green's function methods

Spectroscopic and optical properties of nanosystems and point defects are discussed within the framework of Green's function methods. We use an approach based on evaluating the self-energy in the so-called GW approximation and solving the Bethe-Salpeter equation in the space of single-particle transitions. Plasmon-pole models or numerical energy integration, which have been used in most of the previous GW calculations, are not used. Fourier transforms of the dielectric function are also avoided. This approach is applied to benzene, naphthalene, passivated silicon clusters (containing more than one hundred atoms), and the F center in LiCl. In the latter, excitonic effects and the $1s \to 2p$ defect line are identified in the energy-resolved dielectric function. We also compare optical spectra obtained by solving the Bethe-Salpeter equation and by using time-dependent density functional theory in the local, adiabatic approximation. From this comparison, we conclude that both methods give similar predictions for optical excitations in benzene and naphthalene, but they differ in the spectra of small silicon clusters. As cluster size increases, both methods predict very low cross section for photoabsorption in the optical and near ultra-violet ranges. For the larger clusters, the computed cross section shows a slow increase as function of photon frequency. Ionization potentials and electron affinities of molecules and clusters are also calculated.Comment: 9 figures, 5 tables, to appear in Phys. Rev. B, 200