First-principles Study On The Pristen And Doped ZnO Monolayer

Two-dimensional material has recently attracted much attention due to its potential application in optoelectronic devices, sensor and catalytic. ZnO is an important multifunctional material with wide band gap, high exaction binding energy and excellent photoelectric properties. Therefore, two-dimensional ZnO has become the focus of research. In order to design two-dimensional ZnO-based optoelectronic devices, one important issue is the realization of tuning band gap and physical properties of two-dimensional ZnO. Thus, in this thesis, the geometric configurations and band structures of the ZnO monolayer have been investigatd using the first principles calculation method. Moreover, the effect of doping of Mg, Cd, Ag and rare earth elements on electronic structure, optical properties and magnetic property of ZnO monolayer and its mechanism have been revealed.The calculation results show that with LDA+U method, we can precisely predict the band gap of bulk and monolayer ZnO by introducing the on-site Coulomb interaction into both the Zn 3d and O 2p orbits. The band-gap problem in conventional first-principles density-functional theory calculation has been overcomed. Moreover, ZnO monolayer is non-magnetic direct band gap semiconductor. The top of valence band and bottom of conduction band locate in the same G point of brillouin zone. In particular, the band gap of monolayer ZnO is larger than that of bulk count part due to quantum size effect. In addition,whenever applying compressing or stretching strain, the monolayer ZnO always keeps the characteristic of direct band gap.The results of formation energies indicate that the structure of ZnO monolayer doped with Mg, Cd and Ag could be stability. The band gap of Mgdoped ZnO monolayer increases with increasing Mg concentration. In contrast,the band gap of Cd-doped ZnO monolayer decreases with the increase of Cdconcentration. With Ag doping, the impurity energy level is introduce into the ZnO monolayer. These trendency become more obvious at high doping level. The changes of band gap by doping would affect the optical properties. The absorption spectrum of monolayer ZnO exhibits the blue shift with Mg doping and the red shift with Cd doping. The visible region would have new absorption peak with doped Ag in monolayer ZnO. Cd doping increases the absorption in visible region, implying that Cd-doped ZnO monolayer could display potential applicationin photocatalysis field.The optimization of geometric configuration shows that ZnO monolayer doped rare earth keep the planar hexagonal structure like graphene. With the doping of rare earth, the magnetic moment is introduced in ZnO monolayer. This is due to 4f electron of rare earth by analysis density of electronic states. The ZnO monolayer exhibits high magnetic moment by Eu and Gd doping.Furthermore, rare earth-doped ZnO monolayer could be half-metallic or metallic,broaden the band gap. The phenomenon of blue shift is also observed in ZnO monolayer after doped rare earth.