Low-Dimensional Semiconductor Nanomaterials And Hetero-structures:The First Principle Calculations

Nanomaterials have received steadily growing interests because of their peculiar and fascinating properties, and applications superior to their bulk counterparts. The ability to generate nanostructures is essential to much of modern science and technology, especially energy industry. As a clean renewable source of energy, solar energy is always a subject that people are quite keen on. Considerable efforts have been devoted to improve the solar cell energy conversion efficiencies for over decades. Besides, solar hydrogen production by electrolysis of water is also of prime interest. A number of theoretical and experimental works have been undertaken in this direction. For example, hydrogen production by electrolysis of water is a key point in the national program on key basic research project whose goal is to improve the reaction rate and conversion efficiencies of Catalytic extraction of stored solar energy.The first principle calculations on the basis of density functional theory (DFT) have been proved to be an efficient theoretical method in revealing the structures and properties of nanomaterials. The geometric configurations, electronic structures, optoelectronic absorption and excitation etc., can be predicted using this theoretical scheme. Combined with experiments, first principle calculations have also been successfully employed to deal with the problem of defect devising and restoring, doping and functionalization, gas sensor and storage, and nanoelectronic circuit device, which increase the efficiency and shorten the development period.In this paper, we performed first principle calculations to study the structures and electronic properties of different nanostructures and heterostructures for several semiconductors, SiC, GaN and ZnO. The tubable electronic properties of these nano-and hetero-structures are crucial for their applications in nanoscaled devices. The main conclusions are summarized as follows.(1) SiC low-dimensional nanomaterials, such as clusters, nanotubes, and nanowires, have attracted considerable attention because of their superior properties resulting from large surface-to-volume ratios, quantum size-confinement effects, and accordingly, potential applications in various fields. Our DFT calculations of faceted single-crystalline SiC NWs and NTs show that the surface relaxation plays an important role in the energetics and electronic properties of these nanostructures. The Eform of NWs decreases with increasing wire radius whereas that of NTs decreases with the increase of wall thickness, irrespective of the tube radius. The faceted SiC NWs and NTs are energetically more favorable than the cylindrical single-walled SiC NTs built analogously to the BN nanotubes. Both the bared faceted SiC NWs and NTs are indirect-band-gap semiconductors with the band gaps narrower than that of2H-SiC crystals, due to the surface states. Surface hydrogenation passivates the surface states of NWs and NTs, making them display the characters of direct-band-gap semiconductors. The band gaps of H-NWs and H-NTs are wider than that of2H-SiC crystal and decrease with the increase of wire radius or tube wall thickness.(2) SiC material is particularly attractive due to not only its excellent mechanical properties, high chemical resistivity, and its widespread applications in high-temperature, high-voltage electronics and short-wavelength optics, but also its abundant polytypes. Our first-principles study of SiCNWs growing along different directions with zinc blende and wurtzite structures shows that the SiCNWs growing along [111] direction in zinc blended form are energetically most favorable, in good agreement with the experimental results. In contrast to the indirect band gaps of bulk crystals, all the NWs have direct band gaps at Γ points, except the ones orientating along [112] directions which have indirect band gaps. The direct-band-gap features of [111]-orientated SiCNWs arising from the quantum-confinement effects are preserved for the NWs with diameters up to2.8nm. The band gap values decrease with the increase of wire diameters.(3) Recently, radially-modulated (core/shell) heterostructured nanowires (NWs) have been explored extensively as photovoltaic (PV) materials to produce high-efficiency, robust, integrated PV power sources at low cost. Our first-principles calculations show that the <0001>orientated ZnO-core/GaN-shell heterostructured NWs have natural charge spatial separation with electrons and holes being confined within core and shell regions, respectively. The built-in electric field pointing toward shell region formed across the interface facilities the electron-hole separation. The electrons excited into the conduction bands near the Fermi level exhibit near-free electron features with effective mass of only a half of free electron mass. The band offset and band gap of these core-shell heterostructured NWs can be modulated by the quantum confinement effects, resulting in broad absorption band ranging from2.21to3.71eV. These results suggest that the ZnO-core/GaN-shell NWs are promising candidates for new generation solar cells.(4) The DFT calculations within different exchange-correlation (XC) functionals indicate that GGA (or GGA+U) functionals underestimate the band gaps, electron effective masses of w-ZnO and w-GaN crystals, as well as the valence band offset at ZnO/GaN interface, whereas those obtained from the mod-HSE06functional are comparable to the experimental data. The (Ga1-xZnx)(N1-xOx) solid solution with x=0.125has a direct band gap of2.608eV at the Γ point, which is narrower than those of w-ZnO and w-GaN crystals, due to the strong N2p-Zn3d interactions. Both nonpolar ZnO/(Ga0.875Zn0.125)(N0.875O0.125) and (Ga0.875Zn0.125)(N0.875O0.125)/GaN interfaces exhibit type-Ⅱ band alignment features. Based on these results, we propose that the formation of (Ga1-xZnx)(N1-xOx) solid solution in the region between GaN and ZnO components of GaN/ZnO core-shell heterostructured nanowires can improve the visible light adsorption ability and carrier collection efficiency, which are crucial for the design of next-generation solar cells.