A reliable method has been developed for making through-bond electrical contacts to molecules. Current-voltage curves are quantized as integer multiples of one fundamental curve, an observation used to identify single-molecule contacts. The resistance of a single octanedithiol molecule was 900 +/- 50 megohms, based on measurements on more than 1000 single molecules. In contrast, nonbonded contacts to octanethiol monolayers were at least four orders of magnitude more resistive, less reproducible, and had a different voltage dependence, demonstrating that the measurement of intrinsic molecular properties requires chemically bonded contacts.
This is the report of a DOE-sponsored workshop organized to discuss the status of our understanding of
charge-transfer processes on the nanoscale and to identify research and other needs for progress in nanoscience
and nanotechnology. The current status of basic electron-transfer research, both theoretical and experimental,
is addressed, with emphasis on the distance-dependent measurements, and we have attempted to integrate
terminology and notation of solution electron-transfer kinetics with that of conductance analysis. The interface
between molecules or nanoparticles and bulk metals is examined, and new research tools that advance
description and understanding of the interface are presented. The present state-of-the-art in molecular electronics
efforts is summarized along with future research needs. Finally, novel strategies that exploit nanoscale
architectures are presented for enhancing the efficiences of energy conversion based on photochemistry,
catalysis, and electrocatalysis principles.
Electrical contacts between a metal probe and molecular
monolayers have been characterized using conducting atomic force
microscopy in an inert environment and in a voltage range that yields
reversible current-voltage data. The current through alkanethiol
monolayers depends on the contact force in a way that is accounted for by
the change of chain-to-chain tunnelling with film thickness. The
electronic decay constant, βN, was obtained from
measurements as a function of chain length at constant force and
bias, yielding βN = 0.8±0.2 per methylene over a
±3 V range. Current-voltage curves are difficult to reconcile
with this almost constant value. Very different results are obtained
when a gold tip contacts a 1,8-octanedithiol film. Notably, the
current-voltage curves are often independent of contact
force. Thus the contact may play a critical role both in the nature
of charge transport and the shape of the current-voltage curve.
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