Scanning Auger microscopy study of W tips for scanning tunneling microscopy

Tungsten tips used in scanning tunneling microscopy (STM) (prepared via electrochemical etching with a 2 N KOH or NaOH solution) have been studied with state of the art scanning Auger microscopy (SAM) with chemical lateral resolution of 10 nm. The experiments were focused on the investigation of the W tips' apex shape and surface composition, for tips as etched, or after various postetching treatments performed for cleaning, sharpening, and surface oxide removal purposes. Ultrasonic cleaning likely bend the tip apex. Hydrofluoride etching successfully removes the native WO3 oxide layer, but this happens at the expense of the tip sharpness. Ion sputtering in ultrahigh vacuum is not always effective in sharpening and cleaning the tungsten tip apex, and we sometimes observed the formation of needle like nanotips, mostly composed of WO3. Direct resistive annealing of the tip (operated in the STM at 10 V, 50 nA set-point sample bias voltage and current, respectively) to remove the oxide layer, produces a coiling of the tip apex. In this case, atom transfer from the sample to the tip is directly demonstrated with Auger spectra taken at the tip apex. (C) 2003 American Institute of Physics.