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First-principles study of the electronic and optical properties of ZnO nanowires



1. Introduction

Hexagonal zinc oxide (ZnO), a typical wide bandgap (Eg = 3:37 eV) II-VI semiconductor with a large exciton binding energy (60 meV), has a stable wurtzite structure with lattice spacing a = 0:325 nm and c = 0:521 nm. It has recently attracted the most intensive research for many properties and potential applications in building optical and optoelectronic nanodevices.[1-6] ZnO nanostructures such as nanowires, nanotubes, nanobelts, nanosheets and nanorods are promising building blocks for optical and electronic devices because of the poten-tial applications in the fields of blue-light emitting, short wavelength laser diodes with low thresholds in the UV light-emitting diodes, solar cells, transparent and conducting electrodes in solar cells, surface acoustic wave devices, and chemical and biological sensors.[7-12] Many experimental and theoretical investigations have been reported as regards ZnO NWs of various electronic and optical devices. Wang et al showed an approach to gaining the piezoelectric nanogenerators.[13] Huang et al measured the photoluminescence spectra of ZnO NWs and advanced the ZnO NWs nanolasers,[14] Luo et al studied the charac-terization of ZnO NWs-based ultraviolet photodiodes. But the electronic and optical properties of ZnO NWs are debated in the literature.[15-18] To our knowledge, very few systematical studies on the dimension and size-dependent properties of ZnO NWs have been made theoretically until now. In particular, the effects of surface electronic states due to the appearance of the sidewall in ZnO NWs on their electronic and optical properties are unclear. It is necessary for us to use the first principle method to study these issues for developing future applications of ZnO NWs.

2. Theoretical approaches

Our NWs are generated from a ZnO wutzite crystal structure, cut along the axis with period c as shown in Fig.1. The NWs have a diameter from 3A to 20 A (1

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A=0.1 nm), including 12, 20, 26, 32, 38, 48 and 108 atoms per unit cell.

In our study, all the structural optimizations, band structures, optical properties


and energy calculations are performed by using the first principles calculations with the CASTEP software package based on density functional theory (DFT). To minimize the surface-surface interaction effect due to the finite supercell, we placed the ZnO NWs within a square supercell with a vacuum 10.0 A thick. The Perdew and Wang 91-parametrization is taken as the exchange-correlation potential in the generalized gradient approximation (GGA). The cutoff energy of the plane-wave is set to be 420 eV. The maximum root-mean-square convergent tolerance is less than2105 eV /atom, that is, the force imposed on each atom is not greater than 0.1 eV and 0.1 GPa for stress. The Brillouin zone integration is approximated by using a special K-points sampling scheme of Monkhorst-Pack and 1124 K-point grids are used. All cal-culations are performed in the reciprocal space.

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4. Conclusions

In summary, we have studied the electronic and optical properties of ZnO NWs by using the first-principles calculation of the plane wave ultra-soft pseudo-potential technology based on the density functional theory (DFT). We find that the band gaps of ZnO NWs have a direct band gap, which is larger than that of ZnO bulk due to the quantum confinement effects, and the surface states of ZnO NWs greatly affect the electronic structure and optical properties. The calculated results also indicate that the absorption edges shift towards the blue-light region with the decrease of the ZnO NWs size, and the imaginary part of the dielectric function move stowards the higher energy, the optical band gap is broadened. These results promote the application of ZnO NWs as nanodevices, nanoactuators and ultraviolet optoelectronic devices

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