Predicting Surface Clustering at Ambient Conditions from Thermodynamic Data

Scanning tunneling microscopy (STM) has proved to be a prime tool to characterize the atomic structure of crystal surfaces under UHV conditions. With the development of high-pressure scanning tunneling microscopy (HP-STM), the scope of this technique has been largely extended, as new structures were found to occur under gas phase chemical potentials achieved under ambient conditions. Particularly interesting is the substantial restructuring of initially flat and stable surfaces into new orientations by formation of nanoclusters. Here we discuss the possible generality of this phenomenon by analyzing cases where atomically flat surfaces of certain transition metals undergo such changes in the presence of CO at room temperature (RT) while some remain unchanged. From our analysis we argue that such changes can be predicted from thermodynamic data published in the literature, particularly from the difference in adsorption energy on low- and high-coordination sites, like terrace and step sites, which can be obtained from thermal desorption spectroscopy (TDS) measurements, and possibly also from theoretical calculations. For the vicinal surfaces with high Miller indices, changes in the repulsive elastic interactions between the ordered steps due to adsorbates may also play an important role.