Theoretical and experimental investigation of force imaging at the atomic scale on alkali halide crystals

Assuming a model tip (${\mathrm{Si}}_4$ ${\mathrm{O}}_{10}$ ${\mathrm{H}}_{10}$) as a reasonable representation of the surface of a ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$ cantilever stylus having a hydrogen-terminated asperity and a broader load-bearing base, we investigate the interaction of an atomic force microscope (AFM) with an alkali halide crystal by quantum chemical methods. Structural relaxation of the sample during engagement is allowed, and defect formation is investigated. Force curves above cation and anion positions are calculated, determining maximum sustainable loads and indicating a basis for atomic contrast. Experiments using a ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$ cantilever for AFM imaging of 12 alkali halide and alkaline earth fluoride crystals in air and desiccated helium are reported, in the widest AFM survey of such materials to date. Adsorbed water is shown to significantly enhance the observation of atomic periodicity on ionic halide samples, and rapid surface diffusion on alkali halide crystals is illustrated as it affects prospects for defect investigations. Observations of step edges and point-defect candidates at atomic 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 to examine a possible basis for atomic resolution in the presence of long-range attractive forces.