Nanostructure formation on amorphous WO3 thin films in air by scanning tunneling microscopy

Scanning tunnelling microscope (STM) modifications on alpha -WO3-x thin fil ms were carried out in air. Holes and bumps were produced. Two mechanisms w ere clarified with respect to the water layer attached mon or less in modif ications. By applying voltage pulses, the high humidity (>60%) raised the p robability of hole formations. Connection lines were formed between the hol es, confirming a dominant electrochemical nature. The low humidity (<20%) r aised the probability of bump formations. Similar heights in both STM and a tomic force microscopy (AFM) images imply a physical transformation in the process. Depressed sides near the bumps strongly manifested the diffusion o f surface molecules: to the tip induced by the electric field gradient. In the intermediate humidity (20-60%), there was a higher voltage threshold fo r bump formations than for. hole formations, as well as for positive voltag e pulses than for negative voltage pulses. The discrepancies were reasonabl y attributed to the different tip-sample distances and the consequently ind uced electric fields.