Beyond STM: high resolution imaging of a prototypical molecule-insulator system with NC-AFM

The recent attention given to the idea of molecular electronics has sparked much interest in the study of organic molecules on surfaces. The addition of non-contact atomic force microscopy (NC-AFM) to the surface science toolbox has opened up the possibility of studying insulator surfaces, previously inaccessible to many other surface science techniques including scanning tunneling microscopy (STM). In this study, C_60 on the alkali-halide KBr is used as a prototypical molecule-insulator system in order to better understand the interactions between molecules and insulator substrates. The KBr(100) surface was studied with NC-AFM, and atomically resolved at room temperature. Sub-monolayer coverages of C_60 molecules were deposited on KBr at room temperature by thermal evaporation (T_D≃ 330^rcC) and imaged with molecular resolution. Multi-layer, hexagonally shaped islands of close-packed C_60 molecules were observed predominantly over step edges showing dislocations along the underlying step. Also, a statistical analysis of the C_60 islands revealed a minimum average size of ∼20 nm, and a narrow size distribution (standard deviation of ∼ 20%), possibly indicating heterogeneous nucleation of the clusters.