Atomistic simulations of semiconductor and metallic nanoparticles

Abstract

Semiconductor and metallic nanoparticles have recently become an attractive area of intensive research due to their unique and diverse properties, that differ significantly from bulk materials. With a wide range of applications and potential uses in nanoelectronics, catalysis, medicine, chemistry or physics an important amount of experimental and theoretical investigations aim to facilitate deeper understating in their physical and chemical behaviour. Within this context, this thesis is focused on the theoretical investigation of silicon, gold and platinum nanoclusters and nanoalloys, in order to provide support for experimental data obtained from collaborating researchers and scientists. Modelled structures of the above nanoparticles were constructed and studied by using a variety of computational tools such as, classical force field MD (DL POLY [1]), tightbinding DFT (DFTB+ [2]), conventional DFT (CASTEP [3]) and linear-scaling DFT (ONETEP [4]). A brief introduction regarding some basic principles of quantum mechanics (QM) and of solid state physics is presented in the first chapter; followed by a general chapter about the classical molecular dynamics (MD) method and its utilisation within the DL POLY code [1]. The last part of the second chapter is devoted to the introduction, validation and implementation of a non-default force field in the source code of DL POLY. The third chapter contains a brief description of some important theorems and terms used in density functional theory (DFT), with some basic information about linear-scaling DFT, as developed in the ONETEP code [4], and tight-binding DFT, reported in the last sections. Chapter 4, includes the results of a series of DFT calculations performed on silicon nanorods, with diameters varying from 0.8 nm to 1.3 nm and about 5.0 nm long. While up to now, similar computational works were conducted on periodic nanowires, in our case, the calculations were performed on the entire nanorods without imposing any symmetry. The fifth chapter proposes a new methodology for calculating extended x-ray absorption fine structure (EXAFS) spectra from modelled geometries of gold nanoparticles by exploiting some of the capabilities of the FEFF code [5]. From several snap-shots of a classical MD simulation, a probability distribution function is calculated for sampling the photoabsorbing and the scattering atoms of the simulated system. The results are then compared with experimental EXAFS data showing a good agreement between the predicted and the measured structures. Finally, in the last two chapters, classical MD simulations on gold and platinum nanoparticles and nanoalloys are reported, which have been performed to support the structural characterisation and analysis of synthesised gold and platinum nanoparticles. Within this framework, DFT calculations have also been attempted on ultrasmall gold nanoparticles and on gold nanosurfaces with one or two thiols attached to them, as a preliminary stage towards the application of linear-scaling DFT in simulating the properties of large metallic systems, currently being studied with semi-empirical quantum approaches or empirical force fields

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ORCID for Chris-Kriton Skylaris: orcid.org/0000-0003-0258-3433

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