Bidentate structures of acetic acid on Ge(100): The role of carboxyl oxygen

The adsorption structures of acetic acid on the Ge(100) surface at room temperature have been investigated by using both scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. The conversion of acetic acid to acetate on the Ge(100) surface occurs through dissociation of the O-H bond and the formation of a Ge-O bond. The abstracted hydrogen atom saturates the dangling bond of the other Ge dimer atom or diffuses to a neighboring dimer. Our STM results enable the identification of several adsorption configurations, which arise from bonding involving either one oxygen atom (the monodentate structure) or two oxygen atoms (the bidentate structures). We observe two types of bidentate structures: the end-bridged and on-top structures. The calculations show that the on-top structure is the most stable among many possible adsorption products on the Ge(100) surface considered. Surfaces exposed to acetic acid are annealed to investigate the annealing effect. The end-bridged bidentate and monodentate structures are observed before annealing, while the on-top bidentate structure appears only after annealing. These behaviors are explained in terms of reaction pathways obtained from theoretical calculations.