Chemical identification of individual organic molecules by noncontact atomic force microscopy

H. Onishi, A. Sasahara and H. Uetsuka

Kanagawa Academy of Science and Technology

Noncontact atomic force microscopy (NC-AFM) employs weak attractive force to regulate tip-surface distance instead of tunneling current in scanning tunneling microscopy (STM). Topographic image of materials is constructed regardless of conductivity. This provides a great opportunity in characterizing complex organic molecules adsorbed on solid surfaces which will play a primary role in the forthcoming age of nano-technology. In the present study, mixed monolayers of a series of organic molecules RCOO (R=CH₃, CHF₂, and CF₃) were prepared on a TiO₂(110) surface. By imaging the mixed monolayers, molecule-dependent topography could be evaluated in constant frequency-shift topography while minimizing tip-dependent artifacts. CF₃COO was represented to be a protrusion shorter than CH₃COO. The downward permanent dipole of the CF₃ group was proposed to compensate the upward dipole of the COO group of the molecule and to reduce electrostatic force between the tip and molecule.