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Rice University scientists use tiny gaps between
gold electrodes to simultaneously perform electronic and optical
measurements of the same molecule. These scanning electron images
show electrodes and gaps on a silicon chip. The color insets show
optical signals due to the chip (top) and a gap (bottom).
Image by D. Natelson/Rice University
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"We can mass-produce these in known locations,
and they have single-molecule sensitivity at room temperature in open
air," said study co-author Douglas Natelson, associate professor of
physics and astronomy and co-director of Rice's Quantum Magnetism
Laboratory (QML). "In principle, we think the design may allow us to
observe chemical reactions at the single-molecule level."
While scientists have used electronic and optical instruments to
measure single molecules before, Rice's system is the first that
allows both simultaneously - a process known as "multimodal" sensing -
on a single small molecule.
The research sprang from a collaboration between Natelson's group -
where the electrodes were developed - and Rice's Laboratory for
Nanophotonics (LANP), where the simultaneous electronic and optical
testing was performed. In research published last year, the two groups
explained how the electrodes focus near-infrared light into the
molecule-sized gap, increasing light intensity in the gap by as much
as a million times. The increased intensity allows the team to collect
unique optical signatures for molecules trapped there via a technique
called surface enhanced Raman spectroscopy (SERS).
"Our latest results confirm that we have the sensitivity required to
measure single molecules," said LANP Director Naomi Halas, the Stanley
C. Moore Professor of Electrical and Computer Engineering and
professor of chemistry. "That sensitivity, and the multimodal
capabilities of this system, gives us a great tool for fundamental
science at the nanoscale."
Daniel Ward, a student in Natelson's research group, built the
electrodes from tiny gold wires on silicon wafers and performed the
critical measurements. The group specializes in studying the
electronic and magnetic properties of nanoscale objects -- particles
and devices that are built with atomic precision. The devices are so
small they can only be seen with certain types of microscopes, and
even those provide unclear pictures at best. Natelson said the new
multimodal device gives researchers a much clearer idea of what is
going on by combining two different kinds of measurements, electronic
and optical.
"Conduction across our electrodes is known to depend on a quantum
effect called 'tunneling,'" Natelson said. "The gaps are so small that
only one or two molecules contribute to the conduction. So when we get
conduction, and we see the optical fingerprint associated with a
particular molecule, and they track each other, then we know we're
measuring a single molecule and we know what kind of molecule it is.
We can even tell when it rotates and changes position.
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