电场对GaN/g-C_(3)N_(4)异质结电子结构和光学性质影响的第一性原理研究

First-principles study of influence of electric field on electronic structure and optical properties of GaN/g-C_(3)N_(4) heterojunction

采用基于密度泛函理论的第一性原理平面波超软赝势方法研究了GaN/g-C_(3)N_(4)异质结的稳定性、电子结构、光学性质及功函数,同时考虑了电场效应.结果表明:GaN/g-C_(3)N_(4)范德瓦耳斯异质结的晶格失配率(0.9%)和晶格失配能极低(-1.230 meV/Å^(2),1Å=0.1 nm),说明该异质结稳定性很好,且该异质结在很大程度上保留了GaN和g-C_(3)N_(4)的基本电子性质,可作为直接带隙半导体材料.同时,GaN/g-C_(3)N_(4)异质结在界面处形成了从GaN指向g-C_(3)N_(4)的内建电场,使得光生电子-空穴对可以有效分离,这有利于提高体系的光催化能力.进一步分析可知,外加电场使GaN/g-C_(3)N_(4)异质结的禁带宽度有着不同程度的减小,使得电子从价带跃迁至导带更加容易,有利于提高体系的光催化活性;此外,当外加电场高于0.3 V/Å以及低于-0.4 V/Å时,异质结的能带排列由Ⅰ型向Ⅱ型过渡,更好地实现光生电子-空穴对的分离,进一步提高了体系的光催化活性.因此,本文提出的构建异质结及施加外电场是提高体系光催化活性的有效手段.

In this paper,the stability,electronic structure,optical properties,and work function of GaN/g-C_(3)N_(4) heterojunction are studied by using the first-principles plane wave ultra-soft pseudopotential method based on density functional theory.The electric field effect is also considered.The results show that the total energy for each of the three stacking modes changes little for using the two different dispersion correction methods,i.e.Tkatchenko-Scheffler and Grimme,and the total energy of modeⅡis the lowest,indicating that the structure of modeⅡis the most stable.The lattice mismatch ratio and lattice mismatch energy of GaN/g-C_(3)N_(4) van der Waals heterojunction are very low,indicating that the heterojunction has good stability.The heterojunction retains the basic electronic properties of GaN and g-C_(3)N_(4) to a great extent and can be used as a direct bandgap semiconductor material.It can be known from the work function and differential charge diagram that the charge on the heterojunction interface is transferred from GaN to g-C_(3)N_(4),and a built-in electric field orientating g-C_(3)N_(4) from GaN is formed at the interface.The built-in electric field of the heterojunction can effectively separate the photogenerated electron-hole pairs,which is conducive to improving the photocatalytic capability of the system.Further analysis shows that the applied electric field reduces the bandgap of GaN/g-C_(3)N_(4) heterostructure to varying degrees.It makes it easier for electrons to transit from valence band to conduction band,which is conducive to improving the photocatalytic activity of the system.In addition,when the applied electric field is-0.6 V/Å and 0.5 V/Å separately,the semiconductor metal phase transition occurs in the heterojunction.When the applied electric field is higher than 0.3 V/Å and lower than-0.4 V/Å,in the energy band arrangement of the heterojunction there occurs the transition from typeⅠto typeⅡ.This can better realize the separation of photogenerated electron-hole pairs and further improve the photocatalytic capactivity of the system.Therefore,the construction of heterojunction and application of external electric field proposed in this work constitute an effective means to improve the photocatalytic activity of the system.