The sharpest ever optical image of molecular vibrations, revealing details
as small as 20 nanometers, has been produced by a Rochester-Harvard-Portland
State collaboration (Lukas Novotny, 585-275-5767 ,
The image shows individual carbon
nanotubes with single-atom-thick walls (see figure at www.aip.org/mgr/png ).
Looking beyond this result, the researchers are striving for even higher
sensitivity, which could supply very useful images of proteins, only 5-20
nanometers in size. Other, non-optical imaging techniques, such as scanning
tunneling microscopy, can show smaller details, but this is the highest
resolution image that uses light, a probe that can potentially extract lots
The researchers employed a sophisticated version of
"near-field optical microscopy," in which a small probe (in this case, a
gold wire with an extremely narrow tip) is placed very close to the surface.
With the wire only a few nanometers away from the surface, researchers
circumvented the usual roadblock to resolution, known as the "diffraction
limit," in which optical details are ordinarily limited to half the
wavelength of the light being used. In their technique, called "near-field
Raman spectroscopy," the researchers shine laser light at the gold wire. The
light strikes the wire's electrons, which then generate electric fields.
These fields interact with vibrating atoms in the sample, which then release
light of specific colors (frequencies).
The spectrum of frequencies provides
information on the chemical composition and molecular structure of the
sample. From this information, an image can be created.
In designing their
probe, the researchers made use of the "surface-enhanced Raman scattering
effect," in which the interaction with atomic vibrations is greatly
increased by the use of nanometer-sized metal particles (in this case, the
tip itself). In the future, researchers hope to use their technique to
determine presently unknown structural details of carbon nanotubes, such as
the different ways the nanotubes can interconnect with one another.
better resolution, the researchers hope to take detailed pictures of
proteins in cell membranes. Such data can potentially shed new insights on
how proteins act in a cell membrane and offer clues for designing better
drugs. (Hartschuh et al., Physical Review Letters, 7 March 2003)