Recently, I discussed some possible advantages of using ultrasound in Atomic Force Microscopy (AFM)-based nanofabrication procedures [1], both in top-down approaches in which the tip of a cantilever is expected to act as a plow or as an engraving tool, and in bottom-up methods such as the assembly of nanostructures from individual nano-objects. Since ultrasonic vibration may reduce friction (sonolubrication effect), surface acoustic vibrations could assist in the assembly of nano / micro-objects. In AFM manipulation, the mechanical actuation with the tip of an AFM cantilever upon an individual nano-object provides the opportunity to measure the forces required for its controlled displacement, and the dissipated energy [2]. In this contribution, I will discuss recent data from our lab on ultrasonic-assisted AFM manipulation of nanoparticles. We have performed experiments at Sb nanoparticles deposited on HOPG and MoS2 substrates [3], and magnetic nanoparticles adsorbed on Si surfaces from a ferrofluid solution, covered by poly-l-lysine. The excitation of ultrasound allows us to modify the dynamic response of the nanoparticles upon tip actuation. In the presence of ultrasonic vibration, large Sb nanoparticles on HOPG and MoS2 substrates were laterally displaced using the tip of a compliant AFM cantilever by increasing the ultrasonic amplitude [4]. In contrast, magnetic nanoparticles (of about 10-20 nm in diameter) swept by the tip when scanning in contact mode in the absence of ultrasound, were kept undisturbed with surface ultrasonic vibration. Ultrasonic excitation may modify both tip-particle and particle-surface frictional properties. The exposure of nanostructures to ultrasound renders the opportunity to explore novel methods to control their assembly.
[1] M. T. Cuberes, J. of Physics: Conf. Ser., Proceedings of ICN+T06 (May, 2007).
[2] R. Luthi et al. Science 266 (1994) 1979.
[3] The samples with Sb nanoparticles were provided by C. Ritter and U. Schwarz, Humbold University Berlin; see M. T. Cuberes et al. Ultramicroscopy (in press) for a description of their UFM contrast.
[4] M. T. Cuberes, Proceedings of the 30th Annual Meeting of the Journal Adhesion Society in Tampa, Florida (February 18-21, 2007).
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