ADSORBATE-SUBSTRATE AND ADSORBATE-ADSORBATE INTERACTIONS OF NA AND K ADLAYERS ON AL(111)
ADSORBATE-SUBSTRATE AND ADSORBATE-ADSORBATE INTERACTIONS OF NA AND K ADLAYERS ON AL(111)
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DOI：
10.1103/physrevb.46.16067
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发表时间：
1992-12-15
影响因子：
3.7
通讯作者：
SCHEFFLER, M
中科院分区：
文献类型：
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作者：
NEUGEBAUER, J;SCHEFFLER, M
我们提出了总能量，力，和电子结构的计算Na和K吸附在Al（111）表面上的各种几何形状。计算应用密度泛函理论与低密度近似和从头赝势形式主义。两个吸附物网格，即（平方根3 X平方根3）R30度和（2 X 2），被认为是和他们每个的吸附层相对于基板的几何形状是在很宽的范围内变化的可能性。通过总能量最小化，我们确定稳定和亚稳的几何形状。对于钠，我们发现对于两个吸附物网格，每个吸附原子的计算结合能的顺序是这样的取代几何形状，其中每个钠原子取代表面铝原子，是最有利的和最不利的顶部位置。（平方根3 X平方根3）R30度结构具有比（2 X 2）结构低的能量。这被证明是一个基板的效果，而不是吸附物-吸附物相互作用的效果。与Na的结果相反，我们发现对于（平方根3 X平方根3）R30度K吸附，顶部、三重中空和取代位点的计算吸附能在我们的计算精度内相等，为+/-0.03 eV。表面上的吸附位点的能量的相似性被解释为较大尺寸的K的结果，这意味着吸附原子经历一个相当小的衬底电子密度ε。因此，对于钾，在非弛豫Al（111）衬底的顶部和中空位置的能量已经接近。由于在顶部网站的弛豫能大于三重空心网站的两个网站实际上收到相同的吸附能。意外的可能性的表面取代的网站被解释为结果的离子性质的键合，在较高的覆盖率，可以开发最强的吸附原子时，可以潜入基板尽可能深。有趣的研究系统的结果是，“正常”和替代几何之间的键强度的差异是足够大的踢出表面铝原子。
We present total-energy, force, and electronic-structure calculations for Na and K adsorbed in various geometries on an Al(111) surface. The calculations apply density-functional theory together with the low-density approximation and the ab initio pseudopotential formalism. Two adsorbate meshes, namely (square-root 3 X square-root 3)R30-degrees and (2 X 2), are considered and for each of them the geometry of the adlayer relative to the substrate is varied over a wide range of possibilities. By total-energy minimization we determine stable and metastable geometries. For Na we find for both adsorbate meshes that the ordering of the calculated binding energies per adatom is such that the substitutional geometry, where each Na atom replaces a surface Al atom, is most favorable and the on-top position is most unfavorable. The (square-root 3 X square-root 3)R30-degrees structure has a lower energy than the (2 X 2) structure. This is shown to be a substrate effect and not an effect of the adsorbate-adsorbate interaction. In contrast to the results for Na, we find for the (square-root 3 X square-root 3)R30-degrees K adsorption that the calculated adsorption energies for the on-top, threefold hollow, and substitutional sites are equal within the accuracy of our calculation, which is +/-0.03 eV. The similarity of the energies of the on-surface adsorption sites is explained as a consequence of the bigger size of K which implies that the adatom experiences a rather small substrate electron-density corrugation. Therefore for potassium the on-top and hollow sites are close in energy already for the unrelaxed Al(111) substrate. Because the relaxation energy of the on-top site is larger than that of the threefold hollow site both sites receive practically the same adsorption energy. The unexpected possibility of surface-substitutional sites is explained as a consequence of the ionic nature of the bonding which, at higher coverages, can develop strongest when the adatom can dive into the substrate as deep as possible. The interesting result of the studied systems is that the difference in bond strengths between the ''normal'' and substitutional geometries is sufficiently large to kick out a surface Al atom.