Paper SS1-MoA2
Vibrational Study of Individual Water Dimers on Pt(111) using Scanning Tunneling Microscope

Monday, October 15, 2007, 2:20 pm, Room 608

The adsorption and diffusion of water molecules on metal surfaces play an important role in various fields of surface science, such as electrochemistry, heterogeneous catalysis, corrosion, and so on. Although the adsorption structure of water on a variety of metal surfaces at low coverage has been extensively investigated with various techniques, still unclear is the microscopic feature of adsorbed isolated water molecules. On the other hand, Vibrational spectroscopy methods have been used as powerful tools to get information of bond specific interactions, but in spite of that, the spatial resolution and sensitivity of conventional methods are not sufficient to detect vibrational signals from individual water monomers and dimers. To solve these problems, we investigated and report microscopic and vibrational study of adsorption behaviors of individual water molecules, especially water dimers, on Pt (111) by use of the scanning tunneling microscopy (STM) at 4.7 K. The Pt (111) single crystal surface was dosed with a small amount of water molecules (< 0.01 ML) at a temperature lower than 20 K. This temperature is low enough to prevent water molecules from thermal processes, such as diffusion, desorption and chemical reaction, on the surface, so that one can easily observe isolated water monomers and without forming large clusters. A water dimer appears as a flower like protrusion in the STM images, while a water monomer as a single protrusion as shown in Figure 1. It can be explained as one of the water molecules in a dimer rotating around the other. Indeed, we have succeeded in forming a dimer by manipulating two monomers with an STM tip, and also breaking a dimer into two monomers by inducing pulse bias. We obtained vibrational information by analyzing diffusion behavior of individual water dimers using STM, which method is so called "Action spectroscopy". The result of this measurement provides us not only the information of bond specific interaction but also the detailed features of adsorption structure. We found a kind of hydrogen bonding between one water molecule and Pt surface atoms in adsorption structure of a dimer.