From e4engineering.com, 23
June 2004
Nanocrystals light the way
By Neal Singer
A wireless nanodevice that functions like a fluorescent light - but
potentially far more efficiently - has been developed in a joint project
between the US 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
work.
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 (quantum dots) can be varied by
controlling the size of the dot rather than by the standard 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. They could even be combined to generate white light.
The quantum well, about three nanometres 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.
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