Identification of individual 4-methylpyridine molecules physisorbed and chemisorbed on TiO2(110)-(1 x 1) surface by STM

Abstract

We succeeded in identifying geometries of 4-methylpyridine (4-MP) molecules adsorbed on a TiO2(110)-(1 x 1) surface based on sequential STM imaging. Characteristics in mobility, topographic height, and location allowed us to distinguish three adsorption states at room temperature: a chemisorbed state with the upright geometry (A1), a flat-lying state localized at specific sites (A2), and a flat-lying physisorbed state mobile over the surface (B). The concentration of A1 and A2 species was restricted at an order of 0.01 ML. The A2 state was related to the adsorption at oxygen vacancies resident on the vacuum-annealed surface. The present study demonstrates the promising ability of STM to identify the adsorption geometry of small probe molecules, 4-methylpyridine in the present case, and to provide atom-level information on the origin of acidic property of oxide surfaces.