α-Al_2O_3(0001)表面原子缺陷对ZnO吸附影响

Effects of atomic defects of α-Al_2O_3(0001) on ZnO adsorption 

采用基于密度泛函理论的分子动力学方法 ,对α Al2 O3(0 0 0 1)表面Al,O原子空位缺陷及其对ZnO吸附进行了理论计算 .电子局域函数显示了表面空位处的电子密度变化 ,表面Al原子空缺处有非常明显的缺电子区域 ,悬挂键临近O的电子密度增大 ,有利于对Zn的吸附 ;O原子空缺处的Al原子处存在孤立电子 ,其ELF值为 0 0 5— 0 3,将有利于同电负性较大的O或O2 - 结合 .通过吸附动力学模拟与体系能量的计算发现 ,表面缺陷显著增强了表面的化学吸附 ,空缺原子处都被吸附原子填补 ,吸附结合能远大于单晶表面的情况 .在Al空缺的表面 ,由于ZnO的O与表面O形成双键 ,破坏了α Al2 O3(0 0 0 1)表面O六角对称结构 ,减小了O的表面扩散 ,从而不利于规则的ZnO薄膜生长 .相反 ,O的空缺表面 ,弥补了α Al2 O3(0 0 0 1)表面O空位缺陷 ,不影响基片表面O六角对称结构 .

The Al and O atomic vacancies on α-Al2O3(0001) surface and the adsorption of ZnO are calculated theoretically by using a plane wave ultrasoft pseudo-potential method based on density functional theory. The electron localization function (ELF) shows the electronic density changes at the atomic vacancies. There is an obvious absence of electronic areas where the vacancies of Al have the dangling bonds of oxygen, which are favourable for the adsorption of Zn atoms. In contrast, at the vacancies of O, there are more electron distribution(ELF:0.05-0.3), forming lone electron pairs which are profitably available to combine O atoms or O2- ions. Through the dynamics calculation, we find that surface defects can significantly strengthen particle adsorption, and the vacancies are filled by adatoms (Al or O), with the adsorption energy much higher than the case of the adsorption on a perfect surface. The surface Al-vacancies where the ZnO adsorption clearly breaks the hexagonal symmetry of oxygen on the α-Al2O3(0001) surface because of the formation of the O double bonds, which apparently weakens the reaction diffusivity of O, and has a further negative impact on the regular ZnO films formation. Accordingly, more defects and dislocations in films growth might be produced. Contrarily, the O-vacancies on the α-Al2O3(0001) surface, the chemisorption and dissociation of ZnO do not have an effect on the surface hexagonal symmetry.