Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Discovery of nonblinking semiconductor nanocrystals advances their applications

18.05.2009
Substantial advances for applications of nanocrystals in the fields requiring a continuous output of photons and high quantum efficiency may soon be realized due to discovery of non-blinking semiconductor nanocrystals.

This discovery recently announced by scientists at the Naval Research Laboratory (NRL), University of Rochester, Cornell University and Eastman Kodak Company is an important step to the use of the nanocrystals in various practical devices ranging from low-threshold lasers to the solar cells and biological imaging and tracking. The complete findings of the study are published on line in the May 10, 2009, issue of the journal Nature.

Colloidal nanocrystals are a new class of optical materials that essentially constitute a new form of matter that can be considered as "artificial atoms." Like atoms, they have discrete optical energy spectra that are tunable over a wide range of wavelengths by varying the nanocrystals' size. The widely tunable absorption band edge is controlled mainly by the nanocrystal size, resulting in widely tunable emission spectra. This tunability combined with the optical stability of nanocrystals and the great chemical flexibility in the nanocrystal growth have resulted in the widespread nanocrystal applications in use today.

Nanocrystals show quite high photoluminescence quantum efficiency of up to 70% at room temperature. The missing 30% efficiency turns out to be an intrinsic property of nanocrystals. Studies of single colloidal nanocrystals show that they randomly turn their photoluminescence on and off even under continuous light illumination. Dr. Alexander Efros, from NRL's Center for Computational Material Science, describes the blinking problem in this way, "Imagine the irritation and frustration you would feel if the bulb in your reading lamp started to blink. These same emotions are experienced by engineers and scientists who study single colloidal nanocrystals and try to use their fluorescent properties for biological imaging or lasing" (Nature Materials, vol. 7, 612 (2008)).

The blinking in nanocrystals was first reported 13 years ago, and it came as a surprise to researchers. Today, researchers agree that the blinking happens because when illuminated, nanocrystals can be charged (or ionized) and then neutralized. Under normal conditions when nanocrystal is neutral, a photon excites an electron-hole pair, which then recombines, emitting another photon and leading to photoluminescence. This process is called radiative recombination. If however, the nanocrystal is charged, the extra carrier triggers a process called non-radiative Auger recombination, where exciton energy is transferred to an extra electron or hole. Auger recombination occurs orders of magnitude faster than the radiative recombination. So photoluminescence is almost entirely suppressed in charged nanocrystals. Scientists still do not fully understand the origin of the charging and neutralization process. One of the photoexcited carriers (the electron or the hole) must be ejected from the nanocrystal. At some later time, the ejected charge returns to the nanocrystal (restoring charge neutrality and therefore radiative recombination). The details of these processes occur still are not understood.

Scientists are attempting to eliminate the problem of blinking nanocrystals. One common solution is to suppress nanocrystal ionization. This could be done, for example, by growing a very thick semiconductor shell around the nanocrystal core. However, blinking was reduced, not eliminated, because the fundament processes responsible for blinking - the non-radiative Auger recombination- were still present.

The team of researchers at University of Rochester, Eastman Kodak Company, Cornell University and NRL have taken a significant step toward solving this problem by synthesizing gradually-graded alloy CdZnSe core nanocrystals capped with a ZnSe semiconductor shell that never blinks. The highly unusual multi-peaked photoluminescence spectra clearly indicates also that these nanocrystals always have an extra charge. The observation of photoluminescence from charged (ionized) nanocrystals is direct proof that the nonradiative Auger recombination has been weakened by three orders of magnitude.

The Auger rate suppression is connected with softening the abrupt confined potential of typical core/shell nanocrystals in the structures with a radially graded alloy of CdZnSe into ZnSe. Future efforts will be focused on optimization of these nanocrystal structures with a goal to eliminate the nonradiative Auger processes completely. By completely suppressing blinking associated with Auger processes and "keeping the nanocrystal light bulb turned on," as Dr. Efros explains, researchers look to future breakthroughs for photonics, laser, and other optical applications of nanocrystals.

The research was conducted by Dr. Xiaoyong Wang, Dr. Megan Hahn and Prof. Todd Krauss from the Department of Chemistry at the University of Rochester; Dr. Xiaofan Ren, Dr. Keith Kahen and Dr. Manju Rajeswaran from Eastman Kodak Company; Mrs. Sara Maccagnano-Zacher and Prof. John Silcox, from School of Applied and Engineering Physics at Cornell University; and Dr. George Cragg and Dr. Alexander Efros from NRL's Material Science and Technology Division.

Donna McKinney | EurekAlert!
Further information:
http://www.nrl.navy.mil

More articles from Physics and Astronomy:

nachricht Hope to discover sure signs of life on Mars? New research says look for the element vanadium
22.09.2017 | University of Kansas

nachricht Calculating quietness
22.09.2017 | Forschungszentrum MATHEON ECMath

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: The pyrenoid is a carbon-fixing liquid droplet

Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.

A warming planet

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Rainbow colors reveal cell history: Uncovering β-cell heterogeneity

22.09.2017 | Life Sciences

Penn first in world to treat patient with new radiation technology

22.09.2017 | Medical Engineering

Calculating quietness

22.09.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>