Following adsorption, the next key steps in surface catalytic reactions are diffusion and collision of adsorbed reactants and intermediates. Experimentally measuring the energetics of surface diffusion in isolation is challenging in its own right; under reaction conditions, the complexity of the possible molecular interactions makes the problem seemingly intractable. Advances in molecular manipulation using scanning probe microscopy methods together with the ability to quantitatively measure the forces imparted by the scanning probe tip using non-contact atomic force microscopy (NC-AFM) methods offers promise to measure how surface diffusion barriers are affected by the surrounding environment. This talk will focus on the diffusion of benzene and phenyl radicals on Cu(100). For benzene, the results indicate that the molecules were either pushed, pulled, jumped to the tip or did not move, depending on both the chemical surroundings on the surface as well as the chemical identity of the tip. Comparison with theory suggests that the tip lowers the energy barrier the benzene must overcome to translate on the surface, which evolves with the chemical identity of the tip. In comparison, the missing H of the phenyl radical opens up additional motions including rotation and flipping of the molecule around the C-Cu bond. Different jump distances and barriers were seen that may be associated with these motions. Together, the results indicate that a reliable methodology where surface reactions can be induced by the tip of a scanning probe microscope and the reaction pathway is chosen at will could be in reach, which would ultimately enable an individualized, site-specific understanding of single-molecule surface chemistry.

Measuring energetics of molecular motions of surface species with non-contact atomic force microscopy

ACS Spring 2022

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