Nanotechnology Now

Our NanoNews Digest Sponsors





Heifer International

Wikipedia Affiliate Button


android tablet pc

Home > Press > Breaking Kasha’s Rule: Berkeley Lab Scientists Find Unique Luminescence in Tetrapod Nanocrystals

Berkeley Lab researchers have developed unqiue semiconductor tetrapods that under illumination break Kasha’s rule for photoluminescence by emitting two colors of light.
Berkeley Lab researchers have developed unqiue semiconductor tetrapods that under illumination break Kasha’s rule for photoluminescence by emitting two colors of light.

Abstract:
Observation of a scientific rule being broken can sometimes lead to new knowledge and important applications. Such would seem to be the case when scientists with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) created artificial molecules of semiconductor nanocrystals and watched them break a fundamental principle of photoluminescence known as "Kasha's rule."

Breaking Kasha’s Rule: Berkeley Lab Scientists Find Unique Luminescence in Tetrapod Nanocrystals

Berkeley, CA | Posted on July 2nd, 2011

Named for chemist Michael Kasha, who proposed it in 1950, Kasha's rule holds that when light is shined on a molecule, the molecule will only emit light (fluorescence or phosphorescence) from its lowest energy excited state. This is why photoluminescent molecules emit light at a lower energy than the excitation light. While there have been examples of organic molecules, such as azulene, that break Kasha's rule, these examples are rare. Highly luminescent molecular systems crafted from quantum dots that break Kasha's rule have not been reported - until now.

"We have demonstrated a semiconductor nanocrystal molecule, in the form of a tetrapod consisting of a cadmium-selenide quantum dot core and four cadmium sulfide arms, that breaks Kasha's rule by emitting light from multiple excited states," says Paul Alivisatos, director of Berkeley Lab and the Larry and Diane Bock Professor of Nanotechnology at the University of California (UC) Berkeley. "Because this nanocrystal system has much higher quantum yield and is relatively more photostable than organic molecules, it holds promising potential for optical sensing and light emission-based applications, such as LEDs and imaging labels."

Alivisatos, an internationally recognized authority on nanochemistry, is one of two corresponding authors, along with Sanjeevi Sivasankar of DOE's Ames Laboratory and Iowa State University, on a paper describing this work in the journal Nano Letters. The paper is titled "Spatially Indirect Emission in a Luminescent Nanocrystal Molecule." Co-authoring the paper were Charina Choi, Prashant Jain and Andrew Olson, all members of Alivisatos' research group, plus Hui Li, a member of Sivasankar's research group.

Semiconductor tetrapods make exceptionally good subjects for the study of electronically coupled nanocrystals as Charina Choi, lead author of the Nano Letters paper, explains.

"For the study of nanocrystal molecules, it is important to be able to grow complex nanocrystals in which simple nanocrystal building blocks are connected together in well-defined ways," Choi says. "Although there are many versions of electronically coupled nanocrystal molecules, semiconductor tetrapods feature a beautiful symmetry that is analogous to the methane molecule, one of the fundamental units of organic chemistry."

In this study, Choi, Alivisatos and their co-authors designed a cadmium-selenide (CdSe)and cadmium-sulfide (CdS) core/shell tetrapod whose quasi-type-I band alignment results in high luminescence quantum yields of 30- to 60-percent. The highest occupied molecular orbital (HOMO) of this tetrapod involves an electron "hole" within the cadmium-sulfide core, while the lowest unoccupied molecular orbital (LUMO) is centered within the core but is also likely to be present in the four arms as well. The next lowest unoccupied molecular orbital (LUMO+1) is located primarily within the four CdS arms.

Through single particle photoluminescence spectroscopy carried out at Ames, it was determined that when a CdSe/CdS core/shell tetrapod is excited, not only is a photon emitted at the HOMO-LUMO energy gap as expected, but there is also a second photon emitted at a higher energy that corresponds to a transition to the HOMO from the LUMO+1.

"The discovery that these CdSe/CdS core/shell tetrapods emit two colors was a surprise," Choi says. "If we can learn to control the frequency and intensity of the emitted colors then these tetrapods may be useful for multi-color emission technologies."

For example, says co-author Prashant Jain, "In the field of optical sensing with light emitters, it is impractical to rely simply on changes in emission intensity as emission intensity can fluctuate significantly due to background signal. However, if a molecule emits light from multiple excited states, then one can design a ratiometric sensor, which would provide more accurate readouts than intensity magnitude, and would be more robust against fluctuations and background signals."

Another promising possibility for CdSe/CdS core/shell tetrapods is their potential application as nanoscale sensors for measuring forces. Previous work by Alivisatos and Choi showed that the emission wavelengths of these tetrapods will shift in response to local stress on their four arms.

"When a stress bends the arms of a tetrapod it perturbs the electronic coupling within the tetrapod's heterostructure, which in turn changes the color of the emitted light, and also likely alters the ratio of emission intensity from the two excited states," Choi says. "We are currently trying to use this dependence to measure biological forces, for example, the stresses exerted by a beating heart cell."

By adjusting the length of a CdSe/CdS core/shell tetrapod's arms, it is possible to tune band alignment and electronic coupling within the heterostructure. The result would be tunable emissions from multiple excited states, an important advantage for nano-optic applications.

"We've demonstrated that the oscillator strength of LUMO+1 to HOMO light emissions can be tuned by changing the arm length of the tetrapod," Choi says. "We predict that the lifetime and energy of the emissions can also be controlled through appropriate structural modifications, including arm thickness, number of arms, chemical composition and particle strain."

This research was primarily supported by DOE's Office of Science.

####

About Berkeley Lab
Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 12 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

For more information, please click here

Contacts:
Lynn Yarris
(510) 486-5375

Copyright © Berkeley Lab

If you have a comment, please Contact us.

Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

Bookmark:
Delicious Digg Newsvine Google Yahoo Reddit Magnoliacom Furl Facebook

Related News Press

News and information

Materials for the next generation of electronics and photovoltaics: MacArthur Fellow develops new uses for carbon nanotubes October 21st, 2014

Special UO microscope captures defects in nanotubes: University of Oregon chemists provide a detailed view of traps that disrupt energy flow, possibly pointing toward improved charge-carrying devices October 21st, 2014

Super stable garnet ceramics may be ideal for high-energy lithium batteries October 21st, 2014

Could I squeeze by you? Ames Laboratory scientists model molecular movement within narrow channels of mesoporous nanoparticles October 21st, 2014

Imaging

Special UO microscope captures defects in nanotubes: University of Oregon chemists provide a detailed view of traps that disrupt energy flow, possibly pointing toward improved charge-carrying devices October 21st, 2014

Super stable garnet ceramics may be ideal for high-energy lithium batteries October 21st, 2014

Laboratories

Super stable garnet ceramics may be ideal for high-energy lithium batteries October 21st, 2014

Could I squeeze by you? Ames Laboratory scientists model molecular movement within narrow channels of mesoporous nanoparticles October 21st, 2014

ORNL research reveals unique capabilities of 3-D printing October 15th, 2014

Scientists Map Key Moment in Assembly of DNA-Splitting Molecular Machine: Crucial steps and surprising structures revealed in the genesis of the enzyme that divides the DNA double helix during cell replication October 15th, 2014

Display technology/LEDs/SS Lighting/OLEDs

QD Vision Wins Prestigious Presidential Green Chemistry Challenge Award from the U.S. Environmental Protection Agency October 16th, 2014

Beyond LEDs: Brighter, new energy-saving flat panel lights based on carbon nanotubes - Planar light source using a phosphor screen with highly crystalline single-walled carbon nanotubes (SWCNTs) as field emitters demonstrates its potential for energy-efficient lighting device October 14th, 2014

Govt.-Legislation/Regulation/Funding/Policy

Materials for the next generation of electronics and photovoltaics: MacArthur Fellow develops new uses for carbon nanotubes October 21st, 2014

Special UO microscope captures defects in nanotubes: University of Oregon chemists provide a detailed view of traps that disrupt energy flow, possibly pointing toward improved charge-carrying devices October 21st, 2014

Super stable garnet ceramics may be ideal for high-energy lithium batteries October 21st, 2014

Could I squeeze by you? Ames Laboratory scientists model molecular movement within narrow channels of mesoporous nanoparticles October 21st, 2014

Sensors

Imaging electric charge propagating along microbial nanowires October 20th, 2014

Graphenea opens US branch October 16th, 2014

IRLYNX and CEA-Leti to Streamline New CMOS-based Infrared Sensing Modules Dedicated to Human-activities Characterization October 15th, 2014

Nanodevices for clinical diagnostic with potential for the international market: The development is based on optical principles and provides precision and allows saving vital time for the patient October 15th, 2014

Discoveries

Special UO microscope captures defects in nanotubes: University of Oregon chemists provide a detailed view of traps that disrupt energy flow, possibly pointing toward improved charge-carrying devices October 21st, 2014

Super stable garnet ceramics may be ideal for high-energy lithium batteries October 21st, 2014

Could I squeeze by you? Ames Laboratory scientists model molecular movement within narrow channels of mesoporous nanoparticles October 21st, 2014

Detecting Cancer Earlier is Goal of Rutgers-Developed Medical Imaging Technology: Rare earth nanocrystals and infrared light can reveal small cancerous tumors and cardiovascular lesions October 21st, 2014

Announcements

Special UO microscope captures defects in nanotubes: University of Oregon chemists provide a detailed view of traps that disrupt energy flow, possibly pointing toward improved charge-carrying devices October 21st, 2014

Super stable garnet ceramics may be ideal for high-energy lithium batteries October 21st, 2014

Could I squeeze by you? Ames Laboratory scientists model molecular movement within narrow channels of mesoporous nanoparticles October 21st, 2014

Detecting Cancer Earlier is Goal of Rutgers-Developed Medical Imaging Technology: Rare earth nanocrystals and infrared light can reveal small cancerous tumors and cardiovascular lesions October 21st, 2014

NanoNews-Digest
The latest news from around the world, FREE





  Premium Products
NanoNews-Custom
Only the news you want to read!
 Learn More
NanoTech-Transfer
University Technology Transfer & Patents
 Learn More
NanoStrategies
Full-service, expert consulting
 Learn More














ASP
Nanotechnology Now Featured Books




NNN

The Hunger Project







© Copyright 1999-2014 7th Wave, Inc. All Rights Reserved PRIVACY POLICY :: CONTACT US :: STATS :: SITE MAP :: ADVERTISE