Direct observation of local chemical surface properties by scanning tunneling microscopy

Vapor deposited Pt films with a thickness locally varying between one and two atomic layers on Ru(0001) are studied as model surfaces with a well-defined lateral variation of the chemical properties. These are probed by reversible CO adsorption at 300 K and with CO pressures in the range $pCO = 10−10...10−5$ mbar under real-time STM observation. Upon exposure to 10⁻⁵ mbar CO, no densely packed adlayer is formed on the areas with a local thickness of one atomic layer, whereas a c(4×2) adlayer containing 0.5 CO molecules per surface atom is reversibly formed on areas with locally two atomic layers of Pt. Based on experimental and theoretical data points and parameters from the literature we calculate adsorption isotherms for Pt(111) and the two Ru(0001) supported Pt thin films at 300 K. For all three surfaces, STM observations on CO adsorption are found to be fully consistent with the respective isotherms.