First-principles Study Of Doping And Surface Adsorption Of ZnO

ZnO has some potential applications in many fields as a new multi-functional material,such as used for optoelectronic devices, gas sensors, solar cells, photocatalyst and so on. Inrecent years, with the development of the theoretical calculation software, materialstheoretical calculation and design have become an important method for modern materialsresearchs. In this paper, we will research the related properties of wurtzite ZnO through thefirst principles method based on density functional theory. We both consider the bulk andsurface properties of ZnO. The concrete contents are as follows:1. Calculated the electronic structure and optical properties of S atom doped bulk ZnO.For the models design, we consider the influence of S doping concentration (x=0%,2.78%,5.56%), doping sites and the introduced intrinsic defects (VZn, VO) to the properties of ZnO.From the models calculation, we find that S doping reduces the band gap of ZnO and theoptical absorption edge has a slight red shift. With the increase of S doping concentrationand the changed of doping sites, we find the further influence of them to the electronicstructure and optical properties of ZnO is a little. Absorption coefficients show that theintrinsic defects of VOand VZnintroduced by S doping have significant effects on the bulkZnO. VOintrinsic defect increases the UV light absorption intensity of S doped ZnO. VZnintrinsic defects increases the visible light absorption range of S doped ZnO.2. Calculated the electronic structure and optical properties of the four main low millerindex surfaces of ZnO: nonpolar (1010) and (1120) surfaces as well as polar (0001)-Znand (0001)-O surfaces. For (1010) and (1120) surfaces, the surface Zn and O atoms havedifferent convergence behavior, which lead the Zn-O dimers tilting to the surface and havean obvious contraction. For (0001)-Zn and (0001)-O surfaces, the surface Zn or O atomstend to relax inward, but the largest relaxation belongs to the (0001)-O surface. Thecalculated band gaps are0.56,0.89,0.21and0.71eV for (1010),(1120),(0001)-Zn and(0001)-O surfaces. In these four surface, the absorption edge of (0001)-Zn surface has themost obvious red shift phenomenon and has the highest utilization rate to the sunlight. Thecalculated results are in accordance with the experimental observations that preferentialexposure of (0001)-Zn surface is an advisable method to improve the photocatalyticefficiency of ZnO. 3. Calculated the adsorption behavior of O2and H2O molecules on ZnO (1010)surfaces. In all our considered adsorption models, the calculated formation energy indicatethat the formation energy of O2molecule adsorbed on ZnO (1010) surface is positive andthe value is negative when the H2O molecule adsorbed on ZnO (1010) surface. For O2molecule adsorbed on ZnO (1010) surface, the most favorable adsorption model is that theadsorbed Oads1and Oads2atoms are located at the above right side of the middle of Znslab1and Znslab2atoms. The adsorbed O2molecules formed Znslab1-Oads1-Oads2-Znslab2bond withsurface Zn atoms. For H2O molecule adsorbed on ZnO (1010) surface, the most favorableadsorption model is that Oadsatom located at the above of Znslab1and Oslab3atoms.Simultaneously, one H atom of H2O molecule forms a hydrogen bond with Oslab3atom.4. Calculated the electronic structure and optical properties of Si atom adsorbed at Top,T4and H3sites of ZnO (0001) surface. In the model design, we consider the influence ofdifferent adsorption sites of Si atom to the properties of ZnO (0001) surface. We alsoconsider different monolayer (ML) coverage at the same adsorption site. In all consideredSi/ZnO (0001) system, Si atom will preferentially adsorb at the T4site when the coverageis1/4ML. When the coverage is2/4ML, structural optimization shows that the most stableconfiguration is two Si atoms adsorbed on T4and H3sites respectively. When continue toincrease the coverage to3/4and4/4ML, the adsorbed Si atoms will form triangular orquadrilateral Si structures on the surface, and they just belong to Si3and Si4clusters.Absorbed Si atoms decrease the band gaps of the Si/ZnO (0001) system. The width andposition of the band gaps are influenced by the adsorption models. Similarly, absorptioncoefficient is also influenced by the adsorption models.5. Calculated the formation energies, electronic structures and optical properties of â…¢A (B, Al, Ga, In) atoms adsorbed at Top, T4and H3sites of ZnO (0001) surface. Thecalculation results show that the formation energies of B, Al, Ga and In atoms adsorbed atTop sites are the highest, and then are T4and H3sites. The same atoms adsorbed atdifferent adsorption sites will have different formation energies, but the difference valuewill decrease with the atom periodic increase of B, Al, Ga and In atoms. The electronicstructure of ZnO (0001) surface has a little change when B, Al, Ga and In atoms adsorbed atTop sites. When these atoms adsorbed at T4and H3sites, impurity states will appear above the valence band of the corresponding models, which lead the band gap of modelsnarrowing. Absorption coefficient shows that B, Al, Ga and In atoms adsorbed at Top sitescan increase the utilization rate of sunlight in the low energy region, but Top site isinsensitive to the variety of the adsorbed atoms.