Tip sharpening by normal and reverse electrochemical etching

This article deals with the characteristics and formation by electrochemical etching under ac voltage of very sharp metal tips usable in several applications and in particular in scanning probe microscopy scanning tunneling microscopy (STM) and atomic force microscopy. An exhaustive survey of other existing mechanical and physicochemical procedures for producing sharp tips is also included for background comparison. Because tip sharpness is central to the atomic or near-atomic resolution attainable by STM, yet appears to be so far incompletely studied or documented in the literature, it is argued that high-resolution evidence is required for proper tip characterization as a prerequisite toward adequate performance in the nanometer range. Although atomic-resolution imaging of two-dimensional (flat) surfaces by STM has been possible with tips of ill defined or large apex radii, comparable performance on three-dimensional (rough) surfaces requires the use of tips with sleek shanks and apex radii smaller than or at least commensurate with the desired resolution. The central role played in electrochemical etching under ac voltage by bubble dynamics in shaping the tip apex is analyzed. Experimental results embodied in high-magnification micrographs obtained by high-resolution transmission electron microscopy are presented to illustrate both the intrinsic limitation imposed by large apex radii usually obtained by electrochemical etching in normal configuration (tip oriented downward) and the greatly enhanced sharpening action of the reverse configuration (tip oriented upward) that produces ultrasharp tips of nanometer and subnanometer apex dimensions (nanotips).