Blanco-Rey M, Tremblay J C
Departamento de Física de Materiales, Facultad de Químicas UPV/EHU, Apartado 1072, 20018 Donostia-San Sebastián, Spain.
Institute for Chemistry and Biochemistry, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany.
J Chem Phys. 2015 Apr 21;142(15):154704. doi: 10.1063/1.4917537.
Past scanning tunneling microscopy (STM) experiments of H manipulation on Pd(111), at low temperature, have shown that it is possible to induce diffusion of surface species as well as of those deeply buried under the surface. Several questions remain open regarding the role of subsurface site occupancies. In the present work, the interaction potential of H atoms with Pd(111) under various H coverage conditions is determined by means of density functional theory calculations in order to provide an answer to two of these questions: (i) whether subsurface sites are the final locations for the H impurities that attempt to emerge from bulk regions, and (ii) whether penetration of the surface is a competing route of on-surface diffusion during depletion of surface H on densely covered Pd(111). We find that a high H coverage has the effect of blocking resurfacing of H atoms travelling from below, which would otherwise reach the surface fcc sites, but it hardly alters deeper diffusion energy barriers. Penetration is unlikely and restricted to high occupancies of hcp hollows. In agreement with experiments, the Pd lattice expands vertically as a consequence of H atoms being blocked at subsurface sites, and surface H enhances this expansion. STM tip effects are included in the calculations self-consistently as an external static electric field. The main contribution to the induced surface electric dipoles originates from the Pd substrate polarisability. We find that the electric field has a non-negligible effect on the H-Pd potential in the vicinity of the topmost Pd atomic layer, yet typical STM intensities of 1-2 VÅ(-1) are insufficient to invert the stabilities of the surface and subsurface equilibrium sites.
过去在低温下对钯（111）表面进行氢操纵的扫描隧道显微镜（STM）实验表明，有可能诱导表面物种以及深埋在表面以下的物种发生扩散。关于次表面位点占据的作用，仍有几个问题悬而未决。在本工作中，通过密度泛函理论计算确定了在各种氢覆盖条件下氢原子与钯（111）的相互作用势，以便回答其中两个问题：（i）次表面位点是否是试图从体相区域出现的氢杂质的最终位置，以及（ii）在密集覆盖的钯（111）表面氢耗尽期间，表面穿透是否是表面扩散的竞争途径。我们发现，高氢覆盖率会阻止从下方移动的氢原子重新出现在表面，否则这些氢原子会到达表面面心立方位点，但它几乎不会改变更深层的扩散能垒。穿透不太可能发生，并且仅限于六方密堆积空位的高占据情况。与实验一致，由于氢原子被阻挡在次表面位点，钯晶格会垂直膨胀，并且表面氢会增强这种膨胀。STM针尖效应在计算中作为外部静电场自洽地包含在内。对感应表面电偶极矩的主要贡献源自钯衬底的极化率。我们发现，电场对最顶层钯原子层附近的氢 - 钯势有不可忽略的影响，但典型的1 - 2 VÅ⁻¹的STM强度不足以反转表面和次表面平衡位点的稳定性。
