The David Nesbitt group has figured out the central role of plasmon resonances in light-induced emission of electrons from gold or silver nanoparticles. Plasmons are rapid-fire electron oscillations of freely moving (conduction) electrons in metals. In metal nanoparticles, plasmons are caused by light of just the right frequency—a frequency that exquisitely depends on the shape of the particle as well as its size and material.
Master glass blowers figured out how to take advantage of plasmon resonances during the Middle Ages, even though they didn’t understand the chemical physics of the process. Glass blowers added tiny particles of gold and silver during glass making to produce the vibrant reds, blues, yellows, and purples of the stained glass windows in the great cathedrals of Europe. The tiny metal particle were not only responsible for the gorgeous colors, but have also prevented the hues from degrading over time, in some cases for more than a thousand years.
Today, chemical physicists are working to understand the intricacies of plasmon resonances and their relationship to the photoelectric effect. In the photoelectric effect, a photon of light of the right frequency will eject an electron from a metal surface. And, if the frequency of a single photon is not high enough to dislodge an electron, intense light can also cause electron ejection if the metal surface simultaneously absorbs several photons whose collective energy is high enough. In the multiphoton photoelectric effect, it usually takes three or more photons to eject just one electron.
Then, if the frequency of light hitting the metal surface happens to resonate with the surface’s plasmon oscillations, billions more electrons will be ejected than would normally occur! This plasmon-induced photoelectron emission is the focus of the Nesbitt group, which recently completed a study of the role of the intense electric field that accompanies plasmon resonances in photoelectron emission.
The group showed how plasmon resonances significantly increase the coherent multiphoton photoelectron yield from gold nanorods. By adjusting laser alignment and choosing a laser frequency that vibrated in sync with the nanorod’s plasmon resonances, the group was able to increase the light-induced emission rate of electrons from a gold nanorod surface by a factor of 10 billion!
The group is also investigating the responses to laser light of tiny gold-coated nanospheres and thin sheets of gold peppered with tiny nanoscale holes. The metal surrounding the holes creates plasmonic properties that can be observed and analyzed. This work also encompasses graphene sheets and nanopyramids.
Future investigations of plasmon resonances could yield such applications as ultrashort-pulsed electron sources, more efficient solar cells, light-activated cancer agents, high-density storage drives, and ultrasensitive chemical detectors.
Wandering Quantum Dots
When the Kapteyn/Murnane group studied quantum dots, it got some surprising results. The quantum dots responded differently depending on their size. Electrons from the smallest quantum dots traveled the farthest away and stayed outside the longest—roughly 10% of the time. This fraction was the important factor in explaining the results.
If an electron is already part way outside a quantum dot, it's much easier to completely dislodge it. One reason is that electrons inside of quantum dots don't absorb photons from laser light as easily as electrons outside do. And, even if an electron absorbs a photon while it's inside a quantum dot, there's a fair chance that it won't make it all the way out of the quantum dot. If the electron comes from a larger dot, its time outside is quite short, making it less likely the electron will be kicked all the way out.
In other words, the farther away an electron wanders from its quantum dot, the easier it is for a second light pulse to knock it al the way out of the ball park.
The next step for the researchers is to seek a better understanding of how electrons move from quantum dots to other nano materials. They plan to attach quantum dots to quantum rods, molecules, and other nano structures to investigate more about the physics of electron transfer.
Blinking Quantum Dots
Quantum dots are made of semiconductor materials such as cadmium selenide (CdSe). They are so small they constrain their constituents in 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 can’t get out of the dot. When these pairs recombine inside the dot, they release energy as light.
When quantum dots are continuously illuminated by a laser, they blink on and off seemingly at random. The on-and-off periods can last from a microsecond to several minutes. The David Nesbitt group has figured out what turns the blinking off and then back on again!
The key to solving the mystery was a paradox: The amount of time it takes an “on” quantum dot to blink off appeared to depend on when it was observed and for how long. The longer a dot had been on, the more likely it was to turn off. And, the use of higher-power lasers greatly increased the rate at which the dots turned off.
The group figured out that when a laser illuminated a quantum dot, there was a very small chance that two photons would interact with the dot, creating two electron/hole pairs, known as biexcitons. The chance of making a biexciton was only about one in a million in a microsecond. But, if a laser shined on a quantum dot for an entire second, the chances were much better of forming a biexciton. And, biexcitons stopped the quantum dots from blinking!
It worked like this: First, one of the electron/hole pairs recombined. Second, the energy released in this merger kicked the second electron out of the quantum dot onto the dot’s surface where it became trapped. The trapped electron created a huge electric field across the dot. This field prevented the dot from emitting a photon until the trapped electron finally tunneled back into the dot and recombined with its hole.