First-principles Calculations Of Zinc Oxide Low-dimensional Nanostructure

As an n-type semiconductor material with direct and wide band gap, zinc oxide was widely used in transparent conductive glass, solar cell, flat panel display, gas sensors cell and other fields. Due to its excellent optical and electrical properties, zinc oxide with low-dimensional structure has broad potential application in optoelectronic devices, ultraviolet laser system and many other fields, which attracts wide attention of the researchers. In this paper, SIESTA software based on Density Functional Theory is employed calculating Zinc Oxide low-dimensional nanostructure. The principal elements are listed as follows:1. Electric structures are investigated with the method of LCAO combined with normconversingpseudopotential for the [0001] wurtzite zinc oxide nanowires of different crosssectionaldiameters. Theoretical calculations shows that [0001] wurtzite zinc oxide nanowires have direct and wide band gap while bottom of conduction band and top of valence band, which are mainly composed of Zn-4s&:Zn-4p orbits and Zn-3d&O-2p orbits respectively and are both relatively smooth, are located inΓpoint of Brillouin zone. Effective mass of electron in conduction band and hole in valence band are both large, thus their mobility are both low. It indicates that pure [0001] wurtzite zinc oxide nanowires without any defects have relatively poor conductive capability. The width of band gap decreases with the cross-sectional diameter increased, which shows that it happens obvious quantum confinement effects in a nanometer. Formation energy, which is less than zero, decreases with the cross-sectional diameter increased, which shows that [0001] wurtzite zinc oxide nanowires could be fabricated by bulk zinc and oxygen under some condition and the larger the cross-sectional diameter the easier its fabrication. The width of band gap is gradually increasing with tension increased, which could be ascribed to quantum confinement effects.2. Electric structures are investigated with the method of LCAO combined with normconversingpseudopotential for zigzag (n,0) and armchair (n,n) zinc oxide single-walled nan-otubes, respectively. Theoretical calculations shows that these two kinds of nanotubes both have direct and wide band gap while bottom of conduction band and top of valence band, which are mainly composed of Zn-4s&Zn-4p orbits and Zn-3d&O-2p orbits respectively, are located inΓpoint of Brillouin zone. To zigzag (n,0) zinc oxide single-walled nanotubes, conduction band is steep but valence band is relatively smooth. Effective mass of electron in conduction band is small but that of hole is relatively large, thus mobility of electron is high but that of hole is relatively low. It indicates that zigzag (n,0) zinc oxide single-walled nanotubes could have a certain conductive capability. To armchair (n,n) zinc oxide single-walled nanotubes, conduction band and valence band are both relatively steep. Effective mass of electron in conduction band and hole in valence band are both relatively small, whose mobility are both relatively high. It indicates that armchair (n,n) zinc oxide single-walled nanotubes could have a good conductive capability. As tube's diameter increases, the surfaces of these two kinds of nanotubes are smoothing but the width of their band gap does almost not change compared with that of [0001] wurtzite zinc oxide nanowires, which shows that there is almost not obviousquantum confinement effects in a nanometer. Formation energy, which is less than zero, decreases with the cross-sectional diameter increased, which shows that these two kinds of zinc oxide nanowires could be fabricated by bulk zinc and oxygen under some condition and the larger the cross-sectional diameter the easier their fabrications, but compared with that of [0001] wurtzite zinc oxide nanowires, their formation energy decrease much slower, which shows that their fabrications is much harder. The width of band gap is gradually decreasing with tension increased, which is associated with the variation of Zn-O bonds' lengths and angles.3. Energy level distribution are investigated with the method of LCAO combined with norm-conversing pseudopotential for zinc oxide nanoparticle. Theoretical calculation shows that zinc oxide nanoparticle can be considered as a two-level system at the temperature of 0k. Provided these two energy level meet transition conditions, the generalized optical Bloch equations(GBE) are employed calculating photon emission from zinc oxide nanoparticle under single pulse laser field such like photon count by time, Mandel Q and probability of n photon emission. Specially, the maximum of probability of single photon emission is nearly 1 under single pulse laser field.