Featured Research

from universities, journals, and other organizations

Electrons confined inside nano-pyramids

Date:
September 28, 2012
Source:
Helmholtz Association of German Research Centres
Summary:
Quantum dots are nanostructures of semiconducting materials that behave a lot like single atoms and are very easy to produce. Given their special properties, researchers see huge potential for quantum dots in technological applications. Before this can happen, however, we need a better understanding of how the electrons "trapped" inside them behave. Physicists have recently observed how electrons in individual quantum dots absorb energy and emit it again as light.

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 to be seen.
Credit: Image courtesy of Helmholtz Association of German Research Centres

Quantum dots are nanostructures of semiconducting materials that behave a lot like single atoms and are very easy to produce. Given their special properties, researchers see huge potential for quantum dots in technological applications. Before this can happen, however, we need a better understanding of how the electrons "trapped" inside them behave. Dresden physicists have recently observed how electrons in individual quantum dots absorb energy and emit it again as light.

Their results were recently published in the journal Nano Letters.

Quantum dots look like miniscule pyramids. Inside each of these nano-pyramids are always only one or two electrons that essentially "feel" the constricting walls around them and are therefore tightly constrained in their mobility. Scientists from Helmholtz-Zentrum Dresden-Rossendorf (HZDR), TU Dresden. TU Dresden and the Leibniz Institute for solid State and Materials Research Dresden (IFW) have now studied the special energy states of the electrons trapped inside individual quantum dots.

Sharp energy levels

The behaviour of electrons in a material essentially determines its properties. Being spatially constrained in all three spatial dimensions, electrons inside a nano-pyramid can only occupy very specific energy levels -- which is why quantum dots are also called "artificial atoms." Where these energy levels lie depends on the chemical composition of the semiconductor material as well as the size of the nano-pyramid. "These sharply defined energy levels are exploited, for example, in highly energy-efficient lasers based on quantum dots. 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 colour of the light," Dr. Stephan Winnerl of HZDR explains.

Seeing electrons inside individual quantum dots

The researchers in Dresden working with Dr. Winnerl were recently the first to succeed in scanning transitions between energy levels in single quantum dots using infrared light. Although, they could only do this after overcoming a certain hurdle: While the pyramids of indium arsenide or indium gallium arsenide form spontaneously during a specific mode of crystal growth, their size varies within a certain range. Studying them with infrared light, for example, one obtains blurred signals because electrons in different sized pyramids respond to different infrared energies. This is why it is so important to obtain a detailed view of the electrons trapped inside a single quantum dot.

The scientists approached this task with the special method of scanning near-field microscopy. Laser light is shone onto a metallic tip less than 100 nanometers thick, which strongly collimates the light to a hundred times smaller than the wavelength of light, which is the spatial resolution limit for "conventional" optics using lenses and mirrors. By focusing this collimated light precisely onto one pyramid, energy is donated to the electrons, thereby exciting them to a higher energy level. This energy transfer can be measured by watching the infrared light scattered from the tip in this process. While near-field microscopy involves major signal losses, the light beam is still strong enough to excite the electrons inside a nano-pyramid. The method is also so sensitive that it can create a nanoscale image in which the one or two electrons inside a quantum dot stand out in clear contrast. In this fashion, Stephan Winnerl and his colleagues from HZDR, plus physicists from TU and IFW Dresden, studied the behaviour of electrons inside a quantum dot in great detail, thereby contributing towards our understanding of them.

Infrared light from the free electron laser

The infrared light used in the experiments came from the free electron laser at HZDR. This special laser is an ideal infrared radiation source for such experiments because the energy of its light can be adjusted to precisely match the energy level inside the quantum dots. The laser also delivers such intense radiation that it more than makes up for the unavoidable losses inherent to the method.

"Next, we intend to reveal the behaviour of electrons inside quantum dots at lower temperatures," Dr. Winnerl says. "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." Thanks to its intense laser flashes in a broad, freely selectable spectral range, the free electron laser offers ideal conditions for the method of near-field microscopy in Dresden, which benefits particularly from the close collaboration with Prof. Lukas Eng of TU Dresden in the scope of DRESDEN-concept.


Story Source:

The above story is based on materials provided by Helmholtz Association of German Research Centres. Note: Materials may be edited for content and length.


Journal Reference:

  1. Rainer Jacob, Stephan Winnerl, Markus Fehrenbacher, Jayeeta Bhattacharyya, Harald Schneider, Marc Tobias Wenzel, Hans-Georg von Ribbeck, Lukas M. Eng, Paola Atkinson, Oliver G. Schmidt, Manfred Helm. Intersublevel Spectroscopy on Single InAs-Quantum Dots by Terahertz Near-Field Microscopy. Nano Letters, 2012; 12 (8): 4336 DOI: 10.1021/nl302078w

Cite This Page:

Helmholtz Association of German Research Centres. "Electrons confined inside nano-pyramids." ScienceDaily. ScienceDaily, 28 September 2012. <www.sciencedaily.com/releases/2012/09/120928103756.htm>.
Helmholtz Association of German Research Centres. (2012, September 28). Electrons confined inside nano-pyramids. ScienceDaily. Retrieved September 18, 2014 from www.sciencedaily.com/releases/2012/09/120928103756.htm
Helmholtz Association of German Research Centres. "Electrons confined inside nano-pyramids." ScienceDaily. www.sciencedaily.com/releases/2012/09/120928103756.htm (accessed September 18, 2014).

Share This



More Matter & Energy News

Thursday, September 18, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Stocks Hit All-Time High as Fed Holds Steady

Stocks Hit All-Time High as Fed Holds Steady

AP (Sep. 17, 2014) The Federal Reserve signaled Wednesday that it plans to keep a key interest rate at a record low because a broad range of U.S. economic measures remain subpar. Stocks hit an all-time high on the news. (Sept. 17) Video provided by AP
Powered by NewsLook.com
Space Race Pits Bezos Vs Musk

Space Race Pits Bezos Vs Musk

Reuters - Business Video Online (Sep. 16, 2014) Amazon CEO Jeff Bezos' startup will team up with Boeing and Lockheed to develop rocket engines as Elon Musk races to have his rockets certified. Fred Katayama reports. Video provided by Reuters
Powered by NewsLook.com
MIT's Robot Cheetah Unleashed — Can Now Run, Jump Freely

MIT's Robot Cheetah Unleashed — Can Now Run, Jump Freely

Newsy (Sep. 16, 2014) MIT developed a robot modeled after a cheetah. It can run up to speeds of 10 mph, though researchers estimate it will eventually reach 30 mph. Video provided by Newsy
Powered by NewsLook.com
Manufacturer Prints 3-D Car In Record Time

Manufacturer Prints 3-D Car In Record Time

Newsy (Sep. 15, 2014) Automobile manufacturer Local Motors created a drivable electric car using a 3-D printer. Printing the body only took 44 hours. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins