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This is an abstract for a poster to be presented at the Fifth Foresight Conference on Molecular Nanotechnology.
The Laboratory for Molecular Robotics at the University of Southern California is a highly interdisciplinary research group focused on understanding and developing enabling technologies for the computer-controlled, proximal probe based, direct manipulation of nanoscale three-dimensional (3D) features ranging in size from a few tens of nm to less than a nm. In this paper we describe the development of novel non-contact atomic force microscope (NC-AFM) based techniques for nanomanipulation and illustrate the potential applicability of these techniques via the demonstration of the NC-AFM probe-assisted, directed assembly of nanoscale gold particles (in the size range of 5-30 nm) on a mica surface in air at room temperature. We address in parallel, the two important and interconnected issues of: (a) the scientific basis of NC-AFM imaging and manipulation of nanoscale 3D objects, and (b) the translation of the knowledge base accumulated through (a) to the development of reliable protocols for NC-AFM based nanomanipulation.
While the NC-AFM has the significant advantage of being a non-destructive imaging tool, its underlying imaging mechanism allows us to exploit the microscope for nanomanipulation by computer-controlled operation of the latter in a regime where tip-sample contact is selectively induced. The need to switch to a different imaging technique such as contact AFM to achieve the manipulation is therefore avoided in our protocols. One of the remarkable features of NC-AFM that allows us to achieve the above is the observation of complete reversal of NC-AFM imaging contrast of nanoscale 3D objects, from positive to negative as a function of the NC-AFM imaging conditions. Our results suggest the universality of this contrast reversal for it is observed in air and ultra-high vacuum for a variety of materials systems. Using a force-gradient model of NC-AFM imaging we show that such contrast reversals are accompanied by the excursions of the tip from a regime of tip-sample attractive forces to potentially, a regime of repulsive forces. Using the contrast reversal as a qualitative reference point for delineating the tip-sample interaction force regime, we exploit the tip excursions towards the sample concomitant with the change to negative contrast, through computer-controlled changes in the NC-AFM imaging setpoint while scanning over the 3D feature of interest. The consequence is that the tip-particle repulsive interactions cause the nanoparticles to move on the surface in a pre-specified direction. The choice of the setpoints for manipulation would be relatively easy to establish due to the universality of the contrast reversal and this allows our technique to be tested on a wide variety of materials systems. In addition, we also report on the successful demonstration of another NC-AFM based protocol involving the selective disabling of the feedback while scanning over the particle of interest. Also in this case, we show via systematic studies that repulsive interactions cause the particle to be "pushed" in the desired direction. We demonstrate the reproducibility and reliability of these protocols by assembling 5-30 nm wide gold particles on a mica surface in a pre-determined pattern.
T. R. Ramachandran, Department of Materials Science, VHE 512, 3651 Watt Way, University of Southern California, Los Angeles, CA 90089-0241. ph: (213) 740-4324, fax: (213) 740-4333. E-Mail: email@example.com
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