S掺杂调节SnTiO3带隙的第一性原理研究

First Principles Study on the Bandgap Modulation of SnTiO3 by S Doping

获得窄的带隙和大的极化强度是提高铁电光伏材料性能的关键。本文采用第一性原理方法,研究了S掺杂对SnTiO3带隙和极化强度的影响。通过计算两种最可能的替换掺杂位置,确定了稳定的SnTiO3-xSx掺杂结构。研究发现,S的掺杂会引起SnTiO3沿c轴拉伸,使c/a值变大,带隙宽度变小,极化强度变大。SnTiO3-xSx在x=0.33和x=0.5时的带隙相同,此时S掺杂量的改变并没有引起带隙的变化;但S掺杂量增加到x=1时(即SnTiO2S)具有金属性。进一步的态密度分析表明,S适量掺入后与Ti和Sn成键,降低了Ti的非占据态能量,导带的Ti3d轨道向低能级移动,从而使掺杂SnTiO2S的带隙由SnTiO3的1.87eV降低到1.01eV。通过Bader电荷分析SnTiO2S带隙降低的原因,结果显示,S的引入导致SnTiO2S中离子电荷的总数变小,体系整体电离度变弱,从而引起带隙变小。使用Berry相方法计算表明,随着掺杂量的增加,SnTiO3-xSx的极化强度也会增加。本工作从理论上预测S掺杂能够降低无Pb材料SnTiO3的带隙并使其保持大的极化强度,为改善SnTiO3的铁电光伏特性提供了一种有效手段。

The narrow bandgap and the large polarization play important roles to improve the ferroelectric photovoltaic properties of ferroelectric materials.In this work,the effects of S doping to the bandgap and polarization of the Pb-free SnTiO3 are investigated.The stable structure of SnTiO3-xSx is obtained by the calculation of the two possible S doping sites.It is found that the c axils would be stretched to induce the increase of c/a,and to obtain the narrow bandgap and the large polarization.The bandgaps of SnTiO3-xSx is nearly same when x is 0.33 and 0.5.However,the SnTiO2S is not semiconductor but metal when x=1.Furthermore,the density of states analysis demonstrates that the energy of unoccuping state of SnTiO2S reduced comparing with SnTiO3 when S atom bond with Ti and Sn atom after S doping.The bandgap of SnTiO2S decrease as the Ti3d ortitals moves to the lower energy.Bader charge analysis shows that the total ionizability of SnTiO2S is lower than SnTiO3,inducing the bandgap decrease of SnTiO2S.The polarization of SnTiO3-xSx increases with the increase of S doping by the Berry phase method.In this work,it is predicted that the narrow bandgap and the large polarization of Pb-free SnTiO3 can be obtained by the S doping.It provides an effective way to improve the ferroelectric photovoltaic properties of SnTiO3.