Molecularly resolved observation of anisotropic intermolecular force in a formate-ion monolayer on a TiO2(110) surface by scanning tunneling microscopy

The diffusion kinetics of adsorbed formate ions have been observed on a TiO2 (110) surface by scanning tunneling microscopy (STM) operated under ultrahigh vacuum. Formate ions in a desired area are removed without damage to the substrate, by rastering the monolayer with a high-biased STM tip. A patch of uncovered substrate is thus created in a (2 x 1)-formate monolayer. Time-resolved STM observation reveals the transport kinetics of individual formate ions filling the void. The anisotropic rate of migration suggests that a row of Ti ions exposed on the substrate presents a one-dimensional channel for formate transport. A significant anisotropy in formate-formate interaction is further postulated from the kinetics; a repulsive force between formate ions neighboring on a Ti row drives them to migrate, whereas an attractive force between formates on adjacent Ti-rows plays a secondary role in regulating overlayer structure. The present study shows how STM images document the anisotropy in intermolecular force regulating an adsorbate overlayer on a solid surface, by taking advantage of the relaxation of a nanometer-sized structure fabricated in the overlayer.