Direct and Controlled Manipulation
of Nanometer-Sized Particles
Using Novel Non-Contact AFM Based Techniques
Laboratory
for Molecular Robotics,
University of Southern California, Los Angeles, CA 90089
This is an abstract
for a poster to be presented at the
Fifth
Foresight Conference on Molecular Nanotechnology.
The full paper is available here.
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.
*Corresponding Address:
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 protected]
WWW: http://alicudi.usc.edu:80/~lmr/
|