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

Abstract

Abstract. 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. 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][2][3][4][5]. This seems to be a promising method since a high degree of reproducibility has been reported [3][4][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][4][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][2][3][4][5]. On the other hand mechanical contact as a result of thermal expansion is discussed as well [6][7][8]. Such thermal effects have also been observed in other combinations of pulsed laser light with scanning microscope tips [9][10][11]. PACS: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 = d 2 /2D one can estimate a relaxation time of 73 µs for a tungsten tip with a thermal diffusivity D of 0.68 cm 2 /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.

Field‐enhanced scanning near‐field optical microscopy

“…He then estimated that a modelocked excitation would provide sufficient peak power for a good signal-to-noise, while avoiding unwanted photothermal effects (Boneberg et al, 1998;Kawata et al, 1999;Novotny et al, 1997). Additionally, the use of ultrafast laser sources would allow triggering of nonlinear processes.…”

A Review of Heat Transfer Physics

“…Although laser-assisted STM-based nanoprocessing schemes have yielded resolutions on the order of $30 nm [83,84], restrictions on the electrical conductivity of both the material to be processed and the scanning probe tip as well as the ambient working conditions make these methods unsuitable for a wide range of electrically nonconductive and biological materials. Laser nanoprocessing schemes coupled with AFM operated either in contact or noncontact mode provide effective means for overcoming aforementioned drawbacks and enable the development of nanoscopic features of lateral dimensions less than 20 nm.…”

Optical near-field effects in surface nanostructuring and laser cleaning

“…Thus the setup seemed to be promising both for the study of field enhancement effects at sharp tips, a question of great interest in optics, and for surface structuring applications. Further investigations showed that the structuring observed resulted from a thermal expansion of the tip and its subsequent mechanical contact with the surface rather than from field induced effects [4][5][6]. With regard to its application for structuring, the main disadvantage of the illumination of an STM tip is that the process is serial.…”