Science News Everyday 08.06.15

CHEMISTRY: Small-Cluster Coexistence

Phil Szuromi

When metal clusters are adsorbed on metal oxide surfaces, their properties can change, especially if they interact with defect sites and undergo charge transfer. Simic-Milosevicet al.explored the extent of such charge transfer in a model system, in which magnesium oxide films [three monolayers (MLs) of MgO(001)] were grown on a Ag(001) surface; Au was then deposited at low coverages (0.03 ML) at very low temperatures (5 K), and its structure was studied with a scanning tunneling microscope (STM). Although most of the Au was present as isolated atoms, two types of dimer could be observed that either lay flat on the surface (shown at left) or stood upright. Manipulations with the STM tip could form. the flat dimers from the atoms and then convert them to the upright form. Density functional theory calculations indicated that the upright form. is more stable and adsorbs as a neutral species onto surface oxygen sites, whereas the flat form. is negatively charged and places the Au atoms over Mg. These results show that the spread in energy between different electronic states may be low enough to allow substantial coexistence on a surface.

化学：微小金属簇的共存现象

如果金属簇吸附在金属氧化物的表面，其性质就会发生变化，尤其是当其与金属氧化物的缺陷位相互作用，发生电荷转移时。Simic-Milosevic 等人提出一种模型系统，以研究这种电荷转移的程度。这一模型系统是在氧化镁（MgO）薄膜（实为三个MgO单分子层）上培植一个金Au的表层，然后让Au在极低温（5K）下以低覆盖度（0.03ML）沉淀，然后用隧穿扫描显微镜（STM）研究其结构。绝大部分的金原子表现为孤立原子，但可以观察到有两种二聚体，一种水平排列在表面，一种与Au表面垂直。利用STM探针可以识别出水平构型的二聚体，然后将之转化为垂直构型。根据密度泛函理论计算可得，垂直构型更稳定，为中性构型，与表面的的氧基吸附；而水平构型带负电，使Au原子趋向于Mg。这一研究结果表明，这两种不同电子态之间的能量分散度不大，使得实际上二者在表面共存。