Teoretické simulace transportu náboje v nanostrukturách

Při hlubším pochopení fyzikálních a chemických vlastností nanostruktur hrají ab initio výpočty významnou roli umožňující získat detailní informace o jejich atomární a elektronové struktuře. Cílem doktorské práce je studium elektronové struktury nanosystémů a jejich vliv na fyzikální a chemické vlastnosti studovaného systému, jakož i studium přenosu náboje. Součástí by měl být další rozvoj výpočetní ab initio metody Fireball (www.fireball-dft.org/) umožňující provádět efektivní ab initio simulace komplexních nanostruktur obsahujících řádově 100-1000 atomů. Powered by TCPDF (www.tcpdf.org)Ab initio simulations play an important role for deeper understanding of physical and chemical properties of nanostructures and allow to obtain basic information about their atomic and electronic structure. The goal of the doctoral thesis is an examination of electronic structure of nanosystems and its impact on physical and chemical properties of the systems under study, as well as charge transfer. The work should include further development of ab initio method (www.fireball-dft.org/), which enables effective ab initio simulations of complex nanostructures comprising 100-1000 atoms. Powered by TCPDF (www.tcpdf.org)Matematicko-fyzikální fakultaFaculty of Mathematics and Physic

Theoretical analysis of electronic band structure of 2-to-3-nm Si nanocrystals

We introduce a general method which allows reconstruction of electronic band structure of nanocrystals from ordinary real-space electronic structure calculations. A comprehensive study of band structure of a realistic nanocrystal is given including full geometric and electronic relaxation with the surface passivating groups. In particular, we combine this method with large scale density functional theory calculations to obtain insight into the luminescence properties of silicon nanocrystals of up to 3 nm in size depending on the surface passivation and geometric distortion. We conclude that the band structure concept is applicable to silicon nanocrystals with diameter larger than $\approx$ 2 nm with certain limitations. We also show how perturbations due to polarized surface groups or geometric distortion can lead to considerable moderation of momentum space selection rules

The origin of high-resolution IETS-STM images of organic molecules with functionalized tips

Recently, the family of high-resolution scanning probe imaging techniques using decorated tips has been complimented by a method based on inelastic electron tunneling spectroscopy (IETS). The new technique resolves the inner structure of organic molecules by mapping the vibrational energy of a single carbonmonoxide (CO) molecule positioned at the apex of a scanning tunnelling microscope (STM) tip. Here, we explain high-resolution IETS imaging by extending the model developed earlier for STM and atomic force microscopy (AFM) imaging with decorated tips. In particular, we show that the tip decorated with CO acts as a nanoscale sensor that changes the energy of the CO frustrated translation in response to the change of the local curvature of the surface potential. In addition, we show that high resolution AFM, STM and IETS-STM images can deliver information about intramolecular charge transfer for molecules deposited on a~surface. To demonstrate this, we extended our numerical model by taking into the account the electrostatic force acting between the decorated tip and surface Hartree potential.Comment: 5 pages, 4 figure

The mechanism of high-resolution STM/AFM imaging with functionalized tips

High resolution Atomic Force Microscopy (AFM) and Scanning Tunnelling Microscopy (STM) imaging with functionalized tips is well established, but a detailed understanding of the imaging mechanism is still missing. We present a numerical STM/AFM model, which takes into account the relaxation of the probe due to the tip-sample interaction. We demonstrate that the model is able to reproduce very well not only the experimental intra- and intermolecular contrasts, but also their evolution upon tip approach. At close distances, the simulations unveil a significant probe particle relaxation towards local minima of the interaction potential. This effect is responsible for the sharp sub-molecular resolution observed in AFM/STM experiments. In addition, we demonstrate that sharp apparent intermolecular bonds should not be interpreted as true hydrogen bonds, in the sense of representing areas of increased electron density. Instead they represent the ridge between two minima of the potential energy landscape due to neighbouring atoms

Calculation Of Non-Adiabatic Coupling Vectors In A Local-Orbital Basis Set

The following article appeared in Journal of Chemical Physics 138.15 (2013): 154106 and may be found at http://scitation.aip.org/content/aip/journal/jcp/138/15/10.1063/1.4801511Most of today's molecular-dynamics simulations of materials are based on the Born-Oppenheimer approximation. There are many cases, however, in which the coupling of the electrons and nuclei is important and it is necessary to go beyond the Born-Oppenheimer approximation. In these methods, the non-adiabatic coupling vectors are fundamental since they represent the link between the classical atomic motion of the nuclei and the time evolution of the quantum electronic state. In this paper we analyze the calculation of non-adiabatic coupling vectors in a basis set of local orbitals and derive an expression to calculate them in a practical and computationally efficient way. Some examples of the application of this expression using a local-orbital density functional theory approach are presented for a few simple molecules: H3, formaldimine, and azobenzene. These results show that the approach presented here, using the Slater transition-state density, is a very promising way for the practical calculation of non-adiabatic coupling vectors for large systems.This work was partially supported by Spanish Ministerio de Economía y Competitividad (Contract No.FIS2010-16046), the Comunidad de Madrid (Contract No.S2009/MAT-1467), the Office of Science, Basic Energy Sciences in the US Department of Energy (Grant No. DEFG02-10ER16164), the Czech Science Foundation (GAČR)(Project No. 204/10/0952), the Grant of the MŠMT of the Czech Republic (Grant No. ME 09048), and COST-CMTS Action CM1002 (CODECS). J.O. gratefully acknowledges support from the Spanish Ministerio de Ciencia e Innovación (PR2008-0027). E.A. gratefully acknowledges financial support by the Consejería de Educación de la Comunidad de Madrid and Fondo Social Europeo