THE MECHANISM OF NANOSTRUCTURING UPON NANOSECOND LASER IRRADIATION OFA STM TIP

The mechanism of nanostructuring by illumination of scanning probe tip s is examined. For that purpose the tip of a scanning tunneling micros cope is illuminated with nanosecond and femtosecond laser pulses. The observation of a transient increase of the tunneling current on the ti mescale of mu s is indicative of thermal expansion, which amounts to s everal nm for the energy density necessary for the purpose of nanostru cturing. Furthermore, quantized electrical resistance can be observed upon illumination, which shows the formation of mechanical contact bet ween tip and surface. Thus it is concluded that the appearance of nano structures (craters or mounds) is dominated by the cohesion properties of the tip/sample combination. Finally it is shown that even a huge i ncrease of the involved electromagnetic field, reached by the reductio n of the pulse length from 10 ns to 100 fs, does not change this scena rio.