Light-emitting diodes (LEDs) at infrared wavelengths are the magic behind
such things as night vision and optical communications, including the streaming
data that comes through Netflix. Cornell
have advanced the process of making such LEDs cheaper and easier to fabricate,
which could lead to ultrathin LEDs painted onto silicon to replace computer
wiring with light waves.
The research group led by Frank Wise, professor of applied and engineering
physics, reported online in Nature
Nanotechnology that they have used solution chemistry to make infrared LEDs
out of nanocrystals, commonly known as quantum dots, out of lead sulfide.
Their process, which involves tuning emitted wavelengths based on
controlling the size of the nanocrystals, could rival the effective, but
expensive, practice of growing semiconductor materials using the atom-by-atom
process known as epitaxy. The Cornell nanocrystal LEDs are about as bright as
epitaxially grown LEDs, but they were made using low-temperature, solution-based
processing that is much cheaper.
Infrared LEDs are usually made of crystals of such materials as indium
gallium arsenide, and they cannot be grown on silicon due to their different
crystal structures, Wise explained. Thus far there has been no natural way to
make light-emitting materials on silicon.
Getting electrons to flow through nanocrystals is a major challenge, Wise
said. The Cornell team did it with some clever chemistry: They changed the
distance between the nanocrystals by changing the molecules on their surfaces.
Longer carbon chains produced bigger spacing, which dramatically affected the
efficiency of light emission. Changing the distance between nanocrystals by
half a nanometer made the devices 100 times more efficient, Wise said. The
researchers found the optimum distances between nanocrystals to make the LEDs
emit the brightest light. They measured those distances using X-ray scattering
technology provided by the Cornell High Energy Synchrotron Source (CHESS).
Because the Cornell-developed LEDs were made through solution processing,
they can be more easily integrated with other materials. They could lead to
such breakthroughs as the ability to “paint” the LEDs onto silicon,
for example. Such an application would hold sway in optical interconnects,
replacing electrical wires that are now a bottleneck for speed of the modern
computer chip. Communication between chips with a light wave, rather than a
wire, is expected to revolutionize information processing.
The nanocrystals the researchers used have struck interest among people
making photovoltaic cells, too. A solar cell absorbs light and emits electrons
as electric current, which can supply power. Lead sulfide and lead selenide
nanocrystals are leading candidates for replacing cadmium telluride and other
materials found in commercial solar cells today.