Search
Menu
Rocky Mountain Instruments - Custom Assemblies LB

Electrons in QDs Observed Absorbing, Emitting Light

Facebook X LinkedIn Email
The special energy states of electrons confined in quantum dots were 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 of 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.

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.


Near-field microscopy using the free-electron laser at HZDR: An adjusting laser is employed to align the measuring tip of the microscope that comes from above. Below, the movable sample stage is seen. Courtesy of HZDR.

“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.”

Ohara Corp. - Optical Glass, Polish substrates 10-23

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 two free-electron lasers at HZDR. Courtesy of Sven Claus.

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 using mirrors and lenses. 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.

“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

For more information, visit: www.hzdr.de

Published: October 2012
Glossary
collimation
1. The process of aligning the optical axes of optical systems to the reference mechanical axes or surfaces of an instrument. 2. The adjustment of two or more optical axes with respect to each other. 3. The process by which a divergent beam of radiation or particles is converted into a parallel beam.
nano
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
quantum dots
A quantum dot is a nanoscale semiconductor structure, typically composed of materials like cadmium selenide or indium arsenide, that exhibits unique quantum mechanical properties. These properties arise from the confinement of electrons within the dot, leading to discrete energy levels, or "quantization" of energy, similar to the behavior of individual atoms or molecules. Quantum dots have a size on the order of a few nanometers and can emit or absorb photons (light) with precise wavelengths,...
collimationelectron trappingelectronsEuropefree electron laserHelmholtz-Zentrum Dresden-RossendorfHZDRIFWImagingLeibniz Institute for Solid State and Materials Research DresdenlensesMicroscopymirrorsnanonanopyramidsOpticsphotonicsquantum dotsResearch & TechnologyScanning near-field microscopysemiconductor materialssingle quantum dotStephan WinnerlTU DresdenLasers

We use cookies to improve user experience and analyze our website traffic as stated in our Privacy Policy. By using this website, you agree to the use of cookies unless you have disabled them.