Atomically sharp tips are a requirement for scanning-probe microscopy, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Compared with STM, AFM imaging is more sensitive to the sharpness of tip apices because long-range forces act as a background signal on the high-resolution AFM images originating from short-range forces. Here we report the investigation of in situ reproducible sharp tips for AFM. We make an Ag2S crystal, a mixed ionic and electronic conductor, on a conventional Si cantilever, and controllably grow and shrink the Ag nanoprotrusion by changing the polarity of the bias voltage between the tip and the sample. We are able to reduce the contribution of long-range forces by growing a Ag nanoprotrusion on the Ag2S tip, and obtain atomic-resolution AFM images. We also confirm that the Ag2S tip with a Ag nanoprotrusion, the end of which presumably terminates in Si atoms, is capable of simultaneous AFM and STM measurements.
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