Sublattice Identification in Scanning Force Microscopy on Alkali Halide Surfaces

Abstract

We propose and apply to the KBr(001) surface a new procedure for species recognition in scanning force microscopy (SFM) of ionic crystal surfaces which show a high symmetry of the charge arrangement. The method is based on a comparison between atomistic simulations and site-specific frequency versus distance measurements. First, by taking the difference of force-distance curves extracted at a few judiciously chosen surface sites we eliminate site-independent long-range forces. The obtained short-range force differences are then compared with calculated ones assuming plausible tip apex models. This procedure allows for the first time identification of the tip apex polarity and of the positive and negative sublattices in SFM images of the (001) cleavage surface of an ionic crystal with the rock salt structure.

Chemical Resolution at Ionic Crystal Surfaces Using Dynamic Atomic Force Microscopy with Metallic Tips

“…Previous studies demonstrate that on ionic surfaces atomic resolution with a pure Si tip is expected for separations below 0.4 nm with the largest attraction above anions [16]. Using force spectroscopy above different ionic sites to in situ characterize the tip apex atom in combination with atomistic modeling of different types of tips requires distances of about 0.3 nm [17,18].…”

Site-specific force-distance characteristics on NaCl(001): Measurements versus atomistic simulations

“…The latter may even change during the course of an experiment by picking up material from the surface. In the abovementioned study of KBr͑001͒, 7 a KBr cluster was used to model the tip apex with good results, further supporting the idea that the tip is often contaminated with material from the surface. Such studies represent significant progress in understanding tip-surface interactions in SFM, but the validation of the assumed model tips requires extensive computations.…”

“…From the calculated force differences and a thorough comparison to carefully measured experimental data, identification of the positively and negatively charged sublattices is possible following the method described in Ref. 7.…”

“…7 For this reason the desired comparison can only be made for the force differences between the minimum ͑2͒ and the maximum ͑1͒ in the image ͓Fig. 6͑a͔͒.…”

“…5 By comparing drift-free measurements above different sites, the short-range component of the force could be determined and related to the formation of a single chemical bond. Since then, site-specific surface forces have been studied in detail on a few other samples, including KBr͑001͒, 6,7 NiO͑001͒, 8,9 CaF 2 ͑111͒, 10 a carbon nanotube, 11 and graphite. 12 Such measurements are useful for understanding SFM resolution and contrast mechanisms and are more generally of interest for gauging interactions at surfaces.…”