The mechanism of nanostructuring upon nanosecond laser irradiation of a STM tip

The mechanism of nanostructuring by illumination of scanning probe tips is examined. For that purpose the tip of a scanning tunneling microscope is illuminated with nanosecond and femtosecond laser pulses. The observation of a transient increase of the tunneling current on the timescale of ms is indicative of thermal expansion, which amounts to several nm for the energy density necessary for the purpose of nanostructuring. Furthermore, quantized electrical resistance can be observed upon illumination, which shows the formation of mechanical contact between tip and surface. Thus it is concluded that the appearance of nanostructures (craters or mounds) is dominated by the cohesion properties of the tip=sample combination. Finally it is shown that even a huge increase of the involved electromagnetic field, reached by the reduction of the pulse length from 10 ns to 100 fs, does not change this scenario