Determination of the atomic structure of scanning probe microscopy tungsten tips by field ion microscopy

Detailed knowledge of the tip apex structure is necessary for quantitative comparison between theory-based simulations and experimental observations of tip-substrate interactions in scanning probe microscopy (SPM). Here, we discuss field ion microscopy (FIM) techniques to characterize and atomically define SPM tungsten tips. The tip radius can be estimated from field emission data, while FIM imaging allows the full atomic characterization of the tip apex. We find that when FIM is applied to tips with a radius of a few nanometers (as is desirable for high-resolution atomic force microscopy imaging), limitations not apparent with less sharp tips arise; successful resolution of these limitations will extend the utility of FIM. Field evaporation can be used to atomically engineer the apex into a desired atomic configuration. Starting from a W(111) wire, a tip terminating in three atoms can reproducibly be fabricated; due to its geometry and stability, this apex configuration is well suited for application as an atomically defined electrical contact in SPM experiments aimed at understanding contact mechanics at the atomic scale.