The Sharpest Ever Optical Image of Molecular Vibrations

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 /png/2003/182.htm ). 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 more information. 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. With 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)