Quantum Dots

The David Nesbitt group uses scanning laser microscopy to study artificial atoms, known as quantum dots, made from CdSe and silver nanocrystals. With diameters of 1–5 nm, quantum dots are small enough to confine their constituents in all three dimensions. This constraint means that when a photon of light knocks an electron into the conduction band and creates an electron/hole pair, the pair (called an exciton) can’t get out of the dot. In terms of quantum mechanics, this confinement means that the wavelengths of the wave functions of both the electron and the hole are forced to be significantly smaller and more on top of each other inside a quantum dots than they would be in ordinary semiconductor material.

When electron/hole pairs recombine, they can release energy as light. 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. What they see is that quantum dots do not fluoresce uniformly when continuously illuminated by a laser; rather, they randomly blink on and off like Christmas tree lights.

Recently, Nesbitt and his colleague Jeff Peterson (State University of New York at Geneseo) figured out why this happens. They observed that the amount of time it takes an "on" quantum dot to blink off depends on when it was observed and for how long. The longer a dot had been on, the more likely it was to turn off. The reason for this mysterious behavior is simple. When a laser illuminates a dot, there is a very small chance that two photons will interact with the dot, creating two electron/hole pairs, known as biexcitons. The chance of making a biexciton is only about one in a million in a microsecond. However, if a laser shines on a dot for a million times longer (i.e., 1 second), the chances of forming a biexciton are good. And, biexcitons turn off emission: First, one of the electron/hole pairs recombines. The energy released from this merger kicks the second electron out of the quantum dot onto the dot’s surface where it remains trapped, suppressing blinking. The electron eventually tunnels back into the dot, recombines with its hole, and the dot blinks back on.

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."