Computational Study of the Imaging of a Molecular Crystal with the Atomic Force Microscope

Atomic force microscopy (AFM) has the capacity to probe the surface topography of a variety of materials with atomic scale resolution.Unlike scanning tunneling microscopy AFM does not depend on the sample being an electrical conductor and therefore provides an ideal way to image the surfaces of nonconducting molecular crystals. Many studies have focused on characterizing the surfaces of metals or semiconductor crystals. An alternative and potentially important application of AFM is to the characterization of the surfaces of biochemical crystals, with a view to mapping out active sites in proteins. While AFM has been able to resolve the surface structure of crystals with atomic scale resolution, an important issue is the unique identification of individual surface atoms; for example, to use AFM to determine which cleavage plane of a molecular crystal is exposed. This letter reports a numerical simulation of a recent AFM study of the {100} surface of a DL-leucine crystal performed by Hansma and co-workers using a shattered diamond tip.