Oxygen‐induced sharpening process of W(111) tips for scanning tunneling microscope use

Atomic resolution in scanning tunneling microscopy is usually obtained by a trial and error procedure for formation of the tunneling tip. The success of such attempts is particularly small when rough and contaminated surfaces have to be measured, where the macroscopic tip radius must be as small as possible in order to obtain a reasonable image of the sample. Here we report on a method by heating the tip in an oxygen atmosphere without an electric field, which routinely allows the generation of W(111) tips with an average tip radius of <2 nm. The sharpening of W tips by oxidation and annealing has already been reported by Müller [Z. Physik 108, 668 (1938)]. We have studied the mechanism of tip sharpening by scanning electron microscopy and in situ by field emission and field ion microscopy. The main effect is a preferential removal of (110)‐like planes, which gives rise to strong facetting of the tip apex and the formation of sharp W microtips. The arrangement of microtips is essentially determined by the initial shape of the tip apex. In case of a very blunt tip the oxygen treatment produces several microtips. Normally, only the front microtip will contribute to the field ion microscopical images. By field desorption the outermost atomic layers were consecutively removed and the atomic structure of the microtip elucidated.