CASTEP Study on Electronic and Optical Properties of Zinc Oxide

Zinc oxide (ZnO) is a promising direct wide band gap semiconductor, which has many advantages such as good optical, electronic properties and low cost [1,2]. ZnO has received considerable attention as a potential solar material due to its photostability and excellent charge transport [3]. Currently, a significant amount of theoretical research is being devoted to ZnO nanoparticles (Figure 1). In particular, zero-dimensional (0D) ZnO nanostructures such as nanoparticles have attracted considerable attention because they would have improved applications for their large surface-to-volume ratio, controllable shapes and quantum confinement effect [4]. The 0D ZnO nanostructures have been synthesized by various methods and characterized to have some unique properties. In contrast to the large amount of experimental investigations, only few theoretical studies on the nano structural ZnO have been done so far (Figure 2). In this regard, ab-initio calculations have contributed to explore the electronic structures of ZnO nanocluster and bulk for their promising application in building nano devices [5-9]. Theoretically, their structure, electronic, and optical properties have also been investigated.


Introduction
Zinc oxide (ZnO) is a promising direct wide band gap semiconductor, which has many advantages such as good optical, electronic properties and low cost [1,2].ZnO has received considerable attention as a potential solar material due to its photostability and excellent charge transport [3].Currently, a significant amount of theoretical research is being devoted to ZnO nanoparticles (Figure 1).In particular, zero-dimensional (0D) ZnO nanostructures such as nanoparticles have attracted considerable attention because they would have improved applications for their large surface-to-volume ratio, controllable shapes and quantum confinement effect [4].The 0D ZnO nanostructures have been synthesized by various methods and characterized to have some unique properties.In contrast to the large amount of experimental investigations, only few theoretical studies on the nano structural ZnO have been done so far (Figure 2).In this regard, ab-initio calculations have contributed to explore the electronic structures of ZnO nanocluster and bulk for their promising application in building nano devices [5][6][7][8][9].Theoretically, their structure, electronic, and optical properties have also been investigated.In this paper, we first calculated the band structure, density of states and optical properties of ZnO nanocluster and bulk by using CASTEP code based on density functional theory (DFT) (Figure 3).The electronic and optical properties were calculated with the optimized structures.The photo catalysts are often semiconductors that contain transitionmetal or post-transition-metal ions with d0 or d10 electronic configuration as cations and along with VA or VIA group ions as counter anions [4,10,11].As a semiconductor of this type, ZnO is a low cost and environmentally friendly material with predominant chemical and physical properties, thus it has been widely used as photo catalyst (Figure 4).So, we also discussed the photo catalytic activity of ZnO nancluster following the part of optical properties.In the present work, CASTEP module of Accelrys material studio based on density function theory and plane wave pseudopotential method was used to perform simulations [12].Zinc oxide structure was imported and simulations were performed for band structure, density of states and optical properties.The (Figure 5) shows the imported structure of ZnO, build nanocluster and electron density around nanocluster.The ground state geometries for ZnO were relaxed using CASTEP (Figure 6).With CASTEP, the wave functions of valence electrons were expanded in a plane wave basis set with k vectors within a specified energy cutoff [13].The core region and valence electrons of the atoms in the super cell of intrinsic ZnO are described within the generalized gradient approximation (GGA), employing the Perdew-Burke-Ernzerhof (PBE) functional form [14] (Figure 7).

Conclusion
The CASTEP calculations based on the density function theory (DFT) have been carried out in studying the electronic and optical properties ZnO bulk and nanocluster.The results show reduction in band gap from 0.822 eV to 0.013 eV which means low energy required for transition between valence bands to conduction band.The ZnO nanocluster show low absorption as compared to bulk due to small band gap.The high dielectric function shows good response in electric field.The results show practical applications in nano fluids, photo voltaic sand solar cell.

Figure 2 :
Figure 2: Band structure and density of states of ZnO bulk.

Figure 3 :
Figure 3: Band structure and density of States of ZnO nanocluster.

Figure 6 :
Figure 6: Real and imaginary dielectric Function of ZnO bulk.

Figure 2
Figure 2 and 3shows the band structure and density of states of ZnO bulk and nanocluster respectively.The results show reduction in band gap from 0.822 eV to 0.013 eV.The band structure calculations of ZnO nanocluster show splitting of energy levels and shift in Fermi level as shown (Figure 8 & 9).

Figure 9 :
Figure 9: Comparison of dielectric Function of ZnO bulk and nanocluster.