First Principles Study Of Electronic Properties Of Nanostructure Diamond And Related Materials

Group-IV semiconductors have been the focused central materials in semiconductor fields. Following the advent of nanoscience and nanotechnology at the end of last century, the nanoscale counterpart of the group-IV semiconductors have been paid much attention, particularly for the zero-dimensional quantum dots, which have shown great potential for fabricating nanoelectronic, optoelectronic devices and the candidate of building blocks for nanodevices. Quantum dots (QD) can exhibit quantum confinement effects, which can substantially affect the electronic, optical properties of nanostructure systems. In this dissertationthesis, based on first principles density functional calculations, we investigated the influence of surface conditions and vacancy defects on the quantum confinement of group-IV quantum dots, including the electronic property, electron affinities and so on.In 2003, Dahl et al. reported their successful isolation of diamondoids, a new class of nanosized hydrocarbons with high thermodynamic stability in, which have three-dimensional diamond-cage structures. This experimental finding makes the study of electronic structure of sp3 hybridizational system reach molecular level. At nanoscale, quantum dots exhibit quantum confinement effect, generally, with decreasing the size, their valence band edge (conduction band edge) will move downwards (upwards) resulting in the increasing of the band gap. In 2005 and 2006, Willey et al. reported their experimentally studies of quantum confinement effect on the band edge states of hydrogenated diamond quantum dots. Their resutls showed that the valence band edge have blueshift with decreasing dot size, while the conduction band edge exhibits non-shift feature, which is mainly composed of the C-H bonds at surface. Their results confirmed that the gap of larger diamond QD may be smaller than that of the bulk diamond. For Si and Ge QDs, both the valence band edge and the conduction band edge exhibit quantum confinement effect. Moreover, density functional calculations showed that the gap of Si and Ge QDs are always larger than that of the corresponding bulks. Experiments displayed that hydrogenated diamond QDs have negative electron affinities, which has potential applications for diamond based electron-emitter devices. In contrast, hydrogenated Si and Ge QDs have positive electron affinities, and their values increase with the dot size. For SiC QDs composed of Si and C elements, experimental observations of quantum confinement have been reported, but there is rare related theoretical studies.Investigations on group-IV QDs also concern the magnetic property, such as the room temperature ferromagnetism observed in nanosized diamond QDs by C-/N- ion implantation. In 2007, Liou et al. reported the room temperature ferromagnetism in Ge nanostructures, which is also affected by quantum confinement effect.It is known that nanoscale systems have relatively larger surface-to-volume ratio, thus the surface has substantial influence on the properties of such systems. In this dissertation, we investigated the effect of surface enviroment and reconstruction on the electronic structure by means of density functional theory (DFT) calculations, and the quantum confinement effect on the spin polarization of vacancy defects in group-IV QDs, and such effect on the HOMO/LUMO levels of SiC QDs are also examined and explored.In chapter 1, we present a brief introduction to the study background of group-IV QDs and the origin of quantum confinement effect. We outlined the questions to be resolved and clarified the significance of the dissertation.In chapter 2, we briefly introduce the basic concept of density functional theory and reviewed its recent progress. Developing good approximation for exchange-correlation functional is one of the main targets of DFT research. Following the development of exchange-correlation functionals, DFT method can give more and more accurate description from the initial LDA, GGA to hybridization functional. At the end, we briefly introduce the ADF code.In chapter 3, we investigated the influence of the surface reconstructions on the geometries, stability based on DFT calculations. Our results show that the changes of the geometries can modulate the energy gaps. The spatial variation of the LUMOs depends rather on the C-H bond length than on the respective surface sites and the causes are analyzed. For the hydrogenated surface, the values of the negative electron affinity show lowering trend with the hydrogen coverage decreasing due to the increase of the surface C-H dipoles. These results are heplfull for the design of nanoscale diamond-based optoelectronic and electron-emitter devices.In chapter 4, we investigate the effect of different surface terminations on the geometry, electronic structure, electron affinity and the stability of hydrogenated diamond QDs. Our results indicated that with the existence of CH3 species, the QDs with size larger than 1 nm have larger gaps than that without CH3 species, and even larger than that of bulk diamond. The studies of electroinc structure indicate that the compositions of HOMO and LUMO are responsible for the individual behavior associated with the quantum confinement, which agrees with the experimentally observed spectral feature in the x-ray absorption measurement. In addition, our results show that the negative electron affinity is strongly dependent on the C/H ratio for the hydrogenated diamond nanoparticles.In chapter 5, we examine the vacancy-induced spin polarization in diamond, silicon and germanium nanoparticles and the magnetic coupling between the vacancy-induced defect states in those nanoparticles. Our results show that the vacancy-induced defect states are spin-polarized in diamond nanoparticles regardless of their size, but this happens in silicon and germanium nanoparticles only when their size is small, which is in reasonable agreement with the experimentally observed magnetic behaviors. It is found that the vacancy-induced defect states on adjacent vacancies prefer to couple ferromagnetically in C nanoparticles, but antiferromagnetcally in Si and Ge nanoparticles.In chapter 6, by density functional calculations and chemical bonding analysis, we examine the quantum confinement effect and electron affinities of hydrogen-passivated group-IV quantum dots (C, Si, Ge, SiC and GeC). The results show that the HOMOs of SiC and GeC quantum dots show quantum confinement effect, but their LUMOs do not. SiC and GeC quantum dots have negative and positive electron affinities when their surfaces are terminated by C-H and A-H (A = Si, Ge) bonds, respectively. The chemical bonding analysis reveals that the differences in the quantum confinement effect and electron affinities of the group-IV quantum dots originated from the nearest-neighbor and next-nearest-neighbor interactions between the valence atomic p-orbitals in their HOMO/LUMO levels.In chapter 7, we summarized the contents of the dissertation and draw a preview for future works.