水分子在Bi(111)表面上的吸附和自组装

Adsorption and self-assembly of water molecules on Bi(111)

利用低温扫描隧道显微镜(STM)研究了水分子在半金属Bi(111)表面的吸附和自组装.当沉积速率比较低时,水分子主要以单体和二聚体的形式吸附在衬底表面.高分辨STM图表明,水单体平躺在衬底表面且占据on-top位.水二聚体表现为平行于衬底晶轴方向的两个亮点.当沉积速率比较高时,水分子自组装为单分散的六聚体阵列.每个六聚体表现为由3个亮点组成的三角形,其3个边分别平行于衬底晶轴方向.进一步分析表明,六聚体的3个亮点来自处于高位的3个水分子,一个H原子指向衬底上方,另一个H原子用于分子间氢键.处于低位的另外3个水分子,两个H原子参与分子间的氢键作用,因而在STM图中不易被观察到.我们认为,在半金属Bi(111)表面上水分子的自组装是一个动力学控制的生长过程.

We have investigated the adsorption and self-assembly of water molecules on the self-metallic Bi（111） surface. When the deposition rate is very low, individual water monomers and dimers have been observed on Bi（111） surface. High-resolution STM images indicate that the monomers are flat-lying and occupy the on-top site of Bi substrate. The dimers exhibit two protrusions which are aligned at the principal axes of Bi（111） lattices. When the deposition rate is high, large scale of monodispersed water hexamer have been found on the substrate. Each hexamer manifests as a triangle composing three bright protrusions aligned at the directions of Bi（111） principal axes. By further analysis, it is found that the three bright spots of hexamers arise from the three higher water molecules. One H atom is pointing to the vacuum while the other one takes part in the hydrogen-bond. The other three water molecules are absent due to their lower height. We attribute the self-assembly of water on Bi（111） to a kineticslimited growth process.