Writing with nanoparticles.

David Nesbitt and his group invented a novel method using lasers for make silver nanoclusters. The clusters measure 5 × 30 nm and resemble brightly luminescent pancakes. The group is currently using these clusters to develop an exquisitely sensitive technique, known as surface-enhanced Raman spectroscopy, for detecting and identifying as few as 1–2 unknown molecules. Raman spectra are unique for each molecule, and the silver nanoclusters can theoretically amplify the molecular signature of a particular molecule by a factor of a trillion. Thus far, the researchers have shown that the collective oscillation of the electrons in the clusters, or plasmon frequency, enhances local electric fields, which makes it easier to detect unknown molecules. The plasmon frequency, in turn, depends on the shape of the silver nanoclusters and is highest in the junctions between two silver particles.

In other work, the Nesbitt group uses scanning laser microscopy to study quantum dots, or artificial atoms, made from CdSe and silver nanocrystals. (Quantum dots are objects that confine electrons in all three dimensions.) By carefully separating the fluorescent light emitted by quantum dots from the incident laser light, the researchers can count and graph the number of photons emitted every millisecond in the field of view. This process allows them to "see" individual quantum dots and study their kinetics.

The Nesbitt group has discovered that quantum dots do not fluoresce uniformly; rather, they blink on and off in a pattern that most closely resembles fractal kinetics (chaotic motion). The researchers are now working to explain this phenomenon, which had not been observed prior to their experiments. They propose that a photon of light creates an electron-hole pair in the quantum dot. However, the quantum dot retains electrical neutrality. The wave functions of both the positively charged hole and the negatively charged electron are somehow squeezed into the dimensionless quantum dot because they can't go anywhere else. Even so, they don't exactly fit where they are. As Nesbitt says, "Clearly, there is new physics to be learned at the nanoscale."