The Source of Image Contrast in STM Images of
Functionalized Alkanes on Graphite:
A Systematic Functional Group Approach
California Institute of Technology
This is an abstract
for a talk to be given at the
Fifth
Foresight Conference on Molecular Nanotechnology.
There will be a link from here to the full article when it is
available on the web.
A series of functionalized alkanes and/or alkyl alcohols have
been prepared and imaged by scanning tunneling microscopy (STM)
methods on graphite surfaces. The stability of these ordered
overlayers has facilitated reproducible collection of STM images
at room temperature with sub-molecular resolution, in most cases
allowing identification of individual hydrogen atoms in the
alkane chains, but in all cases allowing identification of
molecular length features and other aspects of the image that can
be unequivocally related to the presence of functional groups in
the various molecules of concern. Functional groups imaged in
this study include halides (X=F, Cl, Br, I), amines, alcohols,
nitriles, alkenes, alkynes, ethers, thioethers, and disulfides.
Except for -Cl and -OH, all of the other functional groups could
be distinguished from each other and from -Cl or -OH through an
analysis of their STM metrics and image contrast behavior. The
dominance of molecular topography in producing the STM images of
alkanes and alkanols was established experimentally and also was
consistent with quantum chemistry calculations. Unlike the
contrast of the methylene regions of the alkyl chains, the STM
contrast produced by the various functional groups was not
dominated by topographic effects, indicating that variations in
local electronic coupling were important in producing the
observed STM images of these regions of the molecules. For
molecules in which electronic effects overwhelmed topographic
effects in determining the image contrast, a simple model is
presented to explain the variation in the electronic coupling
component that produces the contrast between the various
functional groups observed in the STM images.
*Corresponding Address:
Christopher Claypool, California
Institute of Technology, Dept. of
Chemistry,127-72, Pasadena, CA 91125 ph: (818) 395-3964
e-mail: [email protected]
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