Appl Phys A (2010) 99: 1–8 DOI 10.1007/s00339-010-5601-8
R A P I D C O M M U N I C AT I O N
Heating of an Atomic Force Microscope tip by femtosecond laser pulses Alexander A. Milner · Kaiyin Zhang · Valery Garmider · Yehiam Prior
Received: 25 February 2010 / Accepted: 26 February 2010 / Published online: 9 March 2010 © Springer-Verlag 2010
Abstract For various applications of nanoscale surface modification by an Atomic Force Microscope, one would like to maintain the AFM tip near the surface and at an accurately controlled elevated temperature. We study the laser heating of an ordinary AFM silicon tip under ambient conditions, and show that a tightly focused laser beam can heat the tip apex to the desired temperature, while affecting the cantilever quite moderately. We demonstrate that the observation of the shift of the silicon Raman line scattered from the tip is an efficient and accurate way to determine the tip temperature, and we substantiate our observations by theoretically modeling the dynamics of heat accumulation in the tip-cantilever system. For situations where Raman measurements are not feasible, we introduce a new method for estimating the tip temperature by monitoring the mechanical resonance frequency shift of the probe.
1 Introduction Surface modification on the nanometric scale by the tip of an Atomic Force Microscopes (AFM) is an active field of research. Whereas top down optical lithography and direct laser writing are diffraction limited by the wavelength of
These authors had equal contribution. A.A. Milner () · K. Zhang · V. Garmider · Y. Prior Department of Chemical Physics, Weizmann Institute of Science, Rehovot, 76100, Israel e-mail:
[email protected] Fax: +972-8-9344123 Present address: K. Zhang Department of Physics, Fuyang Normal College, Fuyang, Anhui 236032, China
the light used in the experiment, tip-induced interactions are limited by the sharpness (radius of curvature) of the tip which can be as small as a few nanometers. Various types of surface modifications were demonstrated. These include, among others, adding minute amounts of material to the surface as in Dip Pen Nanolithography (DPN) and its many variants [1], where the AFM tip is ‘soaked’ in a liquid (ink) which is then deposited on the surface; mechanical ‘scratching’ of the surface either directly [2] or via a mask followed by deposition of another material; electrochemical tip-induced surface interactions [3]; hot-tip mechanical surface modification, where the tip is heated either electrically [4] or by a laser [5]; or laser-induced tip-enhanced ablation, where the laser action on the surface is manifested either in terms of mechanical action of a tip elongated by its heating or by the enhancement of the electromagnetic field due to plasmonic effects in the tip [6, 7]. Several demonstrations of the use of an AFM tip as a sharp, nanoscale knife were reported [8, 9], including the use of the tip as a hot-knife, but these pose a major challenge due to lack of accurate control (or knowledge) of the Tip-Sample Gap (TSG) in the standard noncontact operation of an AFM, and the absence of direct information of the tip temperature when in close proximity to the surface. To address these last two critical issues, we have recently introduced [10]. Floating Tip Nanolithography (FTN), where by means of a new, nonresonant feedback loop we control the TSG accurately in the range of 1–3 nm, and are able to maintain this well controlled gap while scanning the sample and with the tip hovering over the sample and not touching it for very long durations. We have further demonstrated that by tightly focusing a femtosecond laser on or near the tip apex, the tip is significantly heated, and due to the very small (∼1 nm) distance between the tip and the surface, heat is effectively transferred to the surface [11] and lo-
2
calized surface heating gives rise to hot-tip-induced surface modification. Laser illumination under a sharp tip leads to enhancement of electromagnetic fields, and indeed, we observed both hot-tip nanowriting on soft polymer surfaces as well as modification of gold surfaces (which are not affected by a tip which is maintained at temperatures which are hundreds of degrees below the gold melting temperature). In both cases we have demonstrated nanowriting with resolution of
