Functional group identification in scanning tunneling microscopy of molecular adsorbates

Monolayer films of several primary substituted hydrocarbons CH3(CH2)(n)CH(2)X (X = CH3, OH, NH2, SH, Cl, Br, I; n = 16-30) have been imaged on graphite at the solution-substrate interface using a scanning tunneling microscope(STM). The straight chain hydrocarbons form well-ordered 2-D films on graphite and physisorb with their molecular axes parallel to the surface. The NH2, SH, Br, and I end groups are observed as bright spots in the STM image corresponding to an enhancement in the tunnel current in the vicinity of the functional group, relative to the remainder of the carbon chain. On the other hand, the OH and Cl substituents were not distinguishable from the alkyl chain in the STM images. Comparison of the relative ''brightness'' of the functional groups with respect to the carbon chain reveals an empirical relationship between increasing relative brightness and increasing molecular polarizability. A model is proposed to describe the STM imaging mechanism for these insulating, physisorbed films in which the role of the adsorbate's polarizability, electronic structure, and orientation with respect to the surface are considered.