Wireless nanocrystals efficiently radiate visible light

ALBUQUERQUE, N.M. - A wireless nanodevice that functions like a fluorescent light - but potentially far more efficiently - has been developed in a joint project between the National Nuclear Security Administration's Los Alamos and Sandia national laboratories. The experimental success, reported in the June 10 issue of Nature, efficiently causes nanocrystals to emit light when placed on top of a nearby energy source, eliminating the need to put wires directly on the nanocrystals.

The energy source is a so-called quantum well that emits energy at wavelengths most easily absorbable by the nanocrystals.

The efficiency of the energy transfer from the quantum well to the nanocrystals was approximately 55 percent - although in theory nearly 100 percent transfer of the energy is possible and might be achieved with further tweaking.

The work is another step in creating more efficient white-light-emitting diodes - semiconductor-based structures more efficient and hardier than the common tungsten light bulb.

Reduction of lighting costs is of wide interest because on a world scale, lighting uses more electrical energy per year than any other human invention.

Nanocrystals pumped by quantum wells generate light in a process similar to the light generation in a fluorescent light bulb.

There, a captive gas permeated by electricity emits ultraviolet light that strikes the phosphor-coated surface of the bulb, causing the coat to emit its familiar, overly white fluorescent light.

The current work shows that the nanocrystals can be pumped very efficiently by a peculiar kind of energy transfer that does not require radiation in the usual sense. The process is so efficient, reports Los Alamos National Laboratory (LANL) researcher Marc Achermann, because unlike the fluorescent bulb, which must radiate its ultraviolet energy to the phosphor, the quantum well delivers its ultraviolet energy to the nanocrystal very rapidly before radiation occurs.

Because the emissions of nanocrystals (a.k.a. quantum dots) can be varied merely by controlling the size of the dot rather than by the standard, cumbersome process of varying the mix of materials, no known theoretical or practical barriers exist to pumping different-sized quantum dots that could individually emit blue, green, or red light, or be combined to generate white light.

The quantum well, about three nanometers thick, is composed of a dozen atomic layers. It coats a wafer two inches in diameter and is composed of indium gallium nitride. The film is not fabricated but rather grown as crystal, with an energy gap between its different layers that emits energy in the ultraviolet range at approximately 400 nm.

In this proof-of-principle work, the energy in the quantum well was delivered with a laser. Although the difficulties of inserting energy into the quantum well using an electrical connection rather than laser light are significant, it is considered to be feasible.

The thin-film quantum well crystal film was grown at Sandia by chemist Daniel Koleske.

Sandia researchers are reputed to be among the finest epitaxial crystal-growers in the world.

LANL researchers Achermann, Melissa Petruska, Simon Kos, Darryl Smith, and Victor Klimov attached the semiconductor nanocrystals, made the measurements, and created the theory.

### Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.

...and I am more certain than ever that the two biggest advances in civilization were electric lights, and refrigeration.

Amazingly this article on refrigeration just came out today.

Tiny iron supplement has chilling effect

NIST materials scientist Robert Shull loads an alloy sample into a chamber surrounded by a superconducting-magnet as he prepares to make measurements of the magnetocaloric effect--the property that causes certain materials to heat up when exposed to a magnetic field and to cool down when the field is removed. Photo by Kathie Koenig

A pinch of iron dramatically boosts the cooling performance of a material considered key to the development of magnetic refrigerators. The accomplishment, reported by National Institute of Standards and Technology (NIST) researchers in the June 24 issue of Nature, might move the promising technology closer to market and open the way to substantial energy and cost savings for homes and businesses.

By adding a small amount of iron (about 1 percent by volume), the NIST team enhanced the effective cooling capacity of the so-called "giant magnetocaloric effect" material by 15 to 30 percent. The result, writes materials scientist Virgil Provenzano and his NIST colleagues, "is a much-improved magnetic refrigerant for near-room-temperature applications."

The original material---a gadolinium-germanium-silicon alloy---already is considered an attractive candidate for a room-temperature magnetic refrigerant. However, its cooling potential is undercut by significant energy costs exacted during the on-and-off cycling of an applied magnetic field, the process that drives the refrigeration device. These costs---called hysteresis losses---translate into commensurate losses of energy available for cooling.

Hysteresis, which results in magnetized remnants that persist after an applied magnetic field is relaxed, can be technologically useful. For example, it enables bits of data to be stored on magnetic disks and tapes. In the case of the gadolinium-germanium-silicon alloy, however, it penalizes cooling performance.

The iron supplement overcomes this disadvantage. It nearly eliminates hysteresis and the associated energy cost, permitting the material to perform near the peak of its potential. Independently suggested by two scientists in the 1920s, earning one the Nobel Prize in 1949, magnetic refrigerators offer sizable prospective advantages over today's vapor-compression cooling systems, a century-old technology. Potential pluses include substantial gains in energy efficiency, lower cost of operation, elimination of environmentally damaging coolants, and nearly noise- and vibration-free operation.

With recent progress in materials science and engineering, magnetic refrigeration technology is edging into contention for specialized uses, such as cooling sensors in spacecraft and liquefying gases. Further advances, like the one reported by the NIST team, are necessary if the technology is to replace household refrigerators, freezers, dehumidifiers, and air conditioners, which account for about 25 percent of residential energy usage.

When exposed to a magnetic field, the gadolinium alloy and other magnetocaloric-effect materials heat up as their spinning electrons align with the field, thereby magnetizing the materials and raising their temperature. When the external field is removed, the materials demagnetize -- the electrons revert to a disordered magnetic-spin state -- and their temperature drops. The two-stage process forms the magnetic refrigeration cycle.

In the case of the iron-free gadolinium alloy, however, the electrons become disorganized at field strengths different from those required to align them. Consequently, energy is required to cycle the field, diminishing the material's effective capacity for cooling.

Adding the iron supplement largely suppresses a rearrangement of atoms that occurs as the applied magnetic field increases (or temperature decreases) in the original gadolinium alloy. In turn, stifling the shift in atomic structure all but eliminates the hysteresis loss, the NIST team found. This substantially increased the efficiency of the refrigerant by reducing the energy cost corresponding to the application and, then, removal of a magnetic field.

As important, the iron broadened the range of temperatures over which the material achieves acceptable cooling performance, peaking at 32 degrees Celsius (89 degrees Fahrenheit), compared with 0 degrees Celsius (32 degrees Fahrenheit) for the iron-free alloy.

NIST magnetics researcher Robert Shull, one of the NIST inventors of the new nanocomposite material, notes that the modest addition of iron results in the formation of nanometer-sized magnetic clusters in the gadolinium alloy. In earlier research, Shull and his colleagues demonstrated that dispersing what are essentially tiny magnets throughout a material can greatly enhance the size of the temperature increase resulting from the application of an external magnetic field.

"How such a nanomagnetic structure developed is unknown," Shull explains. "It's existence is certain, but its cause is very subtle."

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As a non-regulatory agency of the U.S. Department of Commerce's Technology Administration, NIST develops and promotes measurement, standards and technology to enhance productivity, facilitate trade and improve the quality of life.

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