Electrons in QDs seen absorbing, emitting light

The special energy states of electrons confined in quantum dots have been observed for the first time, a feat that could help exploit the unique properties of the nanoscale semiconductor materials for technological applications.

Because they are easy to synthesize and their behavior is akin to that of single atoms, quantum dots are generally considered to hold great potential for technological applications. But to take advantage of these properties, scientists must first understand how the electrons trapped inside quantum dots absorb energy and emit it again as light.

There are typically one or two electrons inside the minuscule pyramid-like structure of quantum dots. The constricted movement in the dots allows electrons to occupy only specific energy levels; these depend on the composition of the semiconductor material and the size of the nanopyramid.

The two free-electron lasers at HZDR. The Dresden researchers are the first to use IR light to scan transitions between energy levels in single quantum dots.

Using scanning near-field microscopy, scientists from Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Leibniz Institute for Solid State and Materials Research Dresden (IFW) and TU Dresden observed the special energy states of electrons confined in the dots.

“These sharply defined energy levels are exploited, for example, in highly energy-efficient lasers based on quantum dots,” said Dr. Stephan Winnerl of HZDR. “The light is produced when an electron drops from a higher energy level into a lower one. The energy difference between the two levels determines the color of the light.”

The Dresden researchers are the first to successfully scan transitions between energy levels in single quantum dots using infrared light. Since electrons in different-sized nanopyramids respond to different IR energies, it is possible to obtain only blurred signals by using IR light. For this reason, it’s important to view the electrons confined to a single quantum dot.

The new technique involves shining laser light onto a metallic tip less than 100 nm thick, which strongly collimates the light to 100 times smaller than the wavelength of light – the spatial resolution limit for conventional optics. By focusing this collimated light precisely onto one pyramid, energy is donated to the electrons, exciting them to a higher energy level. The energy transfer can be measured by watching the IR light scattered from the tip in this process. The technique is sensitive enough to generate a distinct nanoscale image of the electrons inside a quantum dot.

Shining laser light onto a metallic tip less than 100 nm thick strongly collimates the light; focusing this collimated light precisely onto one pyramid allows energy to be donated to the electrons, exciting them to a higher energy level. The energy transfer can be measured by watching the IR light scattered from the tip in this process. Here, near-field microscopy using the free-electron laser at HZDR: An adjusting laser aligns the measuring tip of the microscope as it comes from above. Below, the movable sample stage is seen.

“Next, we intend to reveal the behavior of electrons inside quantum dots at lower temperatures,” Winnerl said. “From these experiments, we hope to gain even more precise insights into the confined behavior of these electrons. In particular, we want to gain a much better understanding of how the electrons interact with one another as well as with the vibrations of the crystal lattice.”

The work appeared in Nano Letters (doi: 10.1021/nl302078w).