THEORETICAL AND EXPERIMENTAL INVESTIGATION OF FORCE IMAGING AT THE ATOMIC-SCALE ON ALKALI-HALIDE CRYSTALS

Assuming a model tip (Si4O10H10) as a reasonable representation of the surface of a Si3N4 cantilever stylus having a hydrogen-terminated asp erity and a broader load-bearing base, we investigate the interaction of an atomic force microscope (AFM) with an alkali halide crystal by q uantum chemical methods. Structural relaxation of the sample during en gagement is allowed, and defect formation is investigated. Force curve s above cation and anion positions are calculated, determining maximum sustainable loads and indicating a basis for atomic contrast. Experim ents using a Si3N4 cantilever for AFM imaging of 12 alkali halide and alkaline earth fluoride crystals in air and desiccated helium are repo rted, in the widest AFM survey of such materials to date. Adsorbed wat er is shown to significantly enhance the observation of atomic periodi city on ionic halide samples, and rapid surface diffusion on alkali ha lide crystals is illustrated as it affects prospects for defect invest igations. Observations of step edges and point-defect candidates at at omic scale are reported. The theoretical and experimental results are discussed together in the effort to provide a quantum-mechanical model for observations of alkali halide samples at atomic resolution, and t o examine a possible basis for atomic resolution in the presence of lo ng-range attractive forces.