Probing surfaces insight into atomic-scale chemistry and physics /

by Han, Patrick.

We use a low temperature scanning tunneling microscope to investigate the chemistry and physics of small molecules adsorbed on Au{111}23 × ?3 surface at 4 K. The focus of our experiments is placed on surface systems involving weak adsorbate-surface interactions, in order to observe subtle chemical and physical effects resulting from the competition between weak forces. This is demonstrated with a first set of experiments comparing the physical behaviors of C2N2 and CS2—two linear quadrupolar molecules. We show that although the physical properties of the two molecules are comparable, their overlayer structures on Au{111} differ greatly due to the surface reconstruction. Based on our measurements, we confirm that molecular aggregation depends on the delicate balance between intermolecular interactions and the adsorbate-substrate interactions. We also investigate the phenomenon of substrate-mediated interactions that involve weak electron-scattering adsorbate species. Firstly, we induce adsorbate-substrate interaction between the two-dimensional gas phase CS2 with the Au{111} substrate by manipulating the tunneling conditions. In so doing, we show that the surface states involved in the adsorbate-substrate interactions are close to the surface Fermi energy level. Secondly, we use a combination of time-lapse imaging with digital tracking to calculate the magnitude of substrate-mediated interactions for a physisorbed benzene overlayer at a coverage of 0.9 ML on Au{111}. Focusing on the two-dimensional desorption of the benzene molecules, we find an exponential relationship between the number of nearest neighbors iv before desorption and the desorption occurrence. Based on these measurements, we use an Arrhenius-like approach to calculate the magnitude of substrate-mediated interactions. The subtle effects of these close-range substrate-mediated interactions are apparent over the “pinwheel” structured benzene overlayer, as time-lapse images reveal benzene cascade motions. We demonstrate that the cascade motions are concerted due to substrate-mediated interactions, not the result of random individual molecular motions. Finally, we push the limit of the time-resolution of scanning tunneling microscopy by designing and testing a novel set of control electronics capable of video-rate imaging. We show that by utilizing the error signal as a means of tip-surface distance measurement, we can circumvent the problems associated with the bandwidth of both the feedback electronics and that of the signal amplifier. We demonstrate that interactive video-rate imaging is now possible. v