外电场下STM钨针尖电子结构的理论研究

THEORETICAL STUDY ON THE ELECTRONIC STRUCTURE OF STM TUNGSTEN TIP UNDER EXTERNAL ELECTRIC FIELDS

采用离散变分局域密度泛函方法，研究了外电场对钨（１１１）面针尖电子结构的影响．详细计算和分析了在不同偏压和距离条件下，钨针尖的隧道激活轨道和电荷分布．研究结果表明：隧道激活轨道中针尖原子的成分对外偏压的极性、大小以及针尖与样品之间的距离都较敏感．与过去理论计算结果不同，钨针尖原子的５ｄｚ２轨道对隧道激活轨道有一定贡献，但并不是最主要的．在加正偏压时对隧道激活轨道贡献最大的为５ｄｘｚ和５ｄｙｚ轨道，而在加负偏压时则为包括６ｓ轨道在内的多个轨道的共同贡献．这一结果可以用来解释同一针尖在不同极性偏压下，人们获得不同分辨率ＳＴＭ图象的实验事实，并可预言在ＳＴＭ实验中如用（１１１）面的钨探针，能观测到图象反转现象．

Abstract The electronic structure of tungsten tip of (111) surface under external electric fields is studies by using discrete variational local density functional method. The tunneling orbitals(TOs) and the charge distributions of the tip are calculated and analysed in detail under different conditions. The results show that the components of TOs are sensitive to the polarity, amplitude of the external electric field and the distances between tip and sample. It is found that although 5d z 2 has some contribution to TOs, it is not dominant, which differs from the previous theoretical calculations. The dominating contributions are from 5d xz and 5d yz orbitals under positive bias voltages. However, under negative bias voltages, TOs are made of many orbitals including 6s orbital. Our results can explain the phenomenon in STM experiments that the resolution of STM images will be changed using the same tip when the polarity of the electric field is reversed, and can also predict that if one uses tungsten tip of (111) surface, the inverted STM images should be observed.