Evidence for atomic-scale resolution in atomic-force microscopy of layer silicates

We have recorded atomic force microscopy (AFM) images of the surfaces of ka olinite and lizardite. Typically, the AFM images of the rings of basal-O at oms of these structures have two-fold rotational symmetry, and there is an apparent difference in height between adjacent tetrahedra. These are anomal ous features not found in the known structures, and comparison with calcula ted images of graphite suggest that both features are due to the interactio n of a double-atom AFM tip with the atoms of the surface of the sample. How ever, in some cases we have recorded the first images of the O-atoms in the basal-O plane of sheet silicates at atomic-scale resolution. Currently, th ere are two views of such AFM images: (1) they show arrays of individual at oms; (2) they show arrays of averaged atom positions. There are two interpr etations for the second view. The first relates to the use of two-dimension al fast-Fourier transforms (2DFFT) to enhance detail in the image, the argu ment being that the use of 2DFFT introduces spurious features and averages the atomic positions. The other interpretation is that the periodic repeat- units of the structure being scanned set up harmonic vibrations in the cant ilever and produce an image of an average structure. We have recorded a sin gle image that shows two different atomic repeat-units in different parts o f the raw image. Enhancement using 2DFFT significantly increases the resolu tion of each part of the image but maintains the two discrete images of the different structural elements; it does not average the separate parts of t he image. In another image, we have recorded atoms that are displaced from the expected atomic position, a feature that would not be recorded if avera ging took place. We conclude that these images represents individual atoms on the surface.