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A semiconductor quantum well (shown in the illustration in purple) is electrically excited by current from a battery (marked as V). The released energy is non-radiatively transferred to the nanocrystal quantum dots - shown as red spheres at the top of the system. The nanocrystal quantum dots (red spheres) are covered by the stabilizing layer of organic molecules shown in the image as white "cones." The transferred energy is released as optical radiation, indicated by the red glow. The frequency of the light is tunable by virtue of the quantum dot size. Graphic courtesy of Chemistry (C) Division/Mollie Boorman

Pumping energy to nanocrystals from a quantum well

Scientists working at the Laboratory with a colleague from Sandia National Laboratories have developed a new method for exciting light emission from nanocrystal quantum dots. The discovery provides a way to supply energy to quantum dots without wires, and paves the way for a potentially wider use of tunable nanocrystalline materials in a variety of novel light-emitting technologies, ranging from electronic displays to solid-state lighting and electrically pumped nanoscale lasers.

In a paper published in today's issue of the scientific journal Nature, scientist Victor Klimov of Physical Chemistry and Applied Spectroscopy (C-PCS) and his colleagues describe their method for using non-contact, non-radiative energy transfer from a quantum well to produce light from an adjacent layer of nanocrystals. A quantum well is a semiconductor structure in which an electron is sandwiched between two barriers so that its motion is confined to two dimensions. In a real-life device, the quantum well would be pumped electrically in the same way a common quantum-well light-emitting diode is pumped.

According to Klimov, "The transfer of energy is fast enough to compete with exciton recombination in the quantum well, and that allows us to 'move' more than 50 percent of the excitons to adjacent quantum dots. The recombination of these transferred excitons leads to emission of light with color that can be controlled by quantum dot size. The high efficiency of energy transfer in combination with the exceptional luminescent properties of nanocrystal quantum dots make hybrid quantum-well/nanocrystal devices feasible as efficient sources of any color light -- or even white light."

In addition to Klimov, project scientists include Marc Achermann and Melissa Petruska both of C-PCS, Simon Kos of the Center for Nonlinear Studies (T-CNLS) and Darryl Smith of Condensed Matter and Statistical Physics (T-11), along with Daniel Koleske from Sandia National Laboratories.

Quantum dot research at Los Alamos has led to a number of innovations over the past several years, including news ways to observe and manipulate nanodots and methods for making semiconductor nanocrystals respond to photons by producing multiple electrons as a result of impact ionization (http://www.lanl.gov/orgs/pa/newsbulletin/2004/05/03/text02.shtml). That innovation has potential applications in a new generation of solar cells that would produce as much as 35 percent more electrical output than current solar cells.

The nanocrystal quantum dot research is funded by DOE's Office of Basics Energy Sciences, the Laboratory's Center for Integrated Nanotechnologies (CINT) and by the Los Alamos Laboratory-Directed Research and Development (LDRD) program. LDRD funds basic and applied research and development focusing on employee-initiated creative proposals selected at the discretion of the Laboratory director.

CINT is a Department of Energy/Office of Science Nanoscale Science Research Center operating as a national user facility devoted to establishing the scientific principles that govern the design, performance and integration of nanoscale materials.

Additional information on Los Alamos quantum dot research is available at http://quantumdot.lanl.gov/ online. More information about CINT is available at http://cint.lanl.gov/ online.

-- Todd Hanson

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