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 μs 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. PACS: 42.62.-b; 61.16.Ch; 65.70.+y Recent experiments have shown the possibility of surface nanostructuring by illumination of a scanning microscope tip with ns laser pulses [1–5]. This seems to be a promising method since a high degree of reproducibility has been reported [3–5]. Furthermore this method is applicable to different materials (for example Au, Au/Pd, Ti, PMMA, polycarbonate, organic media) as well as with different kinds of microscopes (AFM and STM) [3–5]. However, the responsible mechanism is controversial. On one hand field enhancement in the vicinity of the tip and subsequent field evaporation is proposed, while thermal effects are ruled out [1–5]. On the other hand mechanical contact as a result of thermal expansion is discussed as well [6–8]. Such thermal effects have also been observed in other combinations of pulsed laser light with scanning microscope tips [9–11]. We performed transient measurements of the tunneling current during such experiments, which should enable us to distinguish between these two models due to the different time scales involved. Whereas a field evaporation mechanism should take place on the time scale of the laser pulse (ns), a thermal effect occurs with a typical cooling time in the order of μs (from τ = d2/2D one can estimate a relaxation time of 73 μs for a tungsten tip with a thermal diffusivity D of 0.68 cm2/s and a focus diameter d = 100 μm). Following the thermal model it might also be possible to observe indications of the mechanical contact, for example quantized conductance of the junction formed upon illumination. Therefore we present measurements of the gap resistance of a STM upon illumination with a ns laser pulse. Finally a comparison of current transients after illumination with ns and fs laser pulses, respectively, is shown.