Graphene `light bulb' is world's thinnest
Researchers create bright,
visible light emission from one-atom-thick carbon.
The breakthrough may lead to a light source for optical computers in the future
The breakthrough may lead to a light source for optical computers in the future
Led by Young Duck Kim, a
postdoctoral research scientist at Columbia Engineering, a team of scientists
from Co lumbia, Seoul National University (SNU), and Korea Research Institute
of Standards and Science (KRISS) have demonstrated for the first time an
on-chip visible light source using graphene, an atomically thin and perfectly
crystalline form of carbon, as a filament.
They attached small strips of
graphene to metal electrodes, suspended the strips above the substrate, and
passed a current through the filaments to cause them to heat up. The study
appears in the Advance Online Publication on Nature Nanotechnology's Web site.
“We've created what is essentially
the world's thinnest light bulb,“ says Hone, co-author of the study.
“This new type of `broadband' light
emitter can be integrated into chips and will pave the way towards the
realisation of atomically thin, flexible, and transparent displays, and
graphene-based on-chip optical communications.“
Creating light in small structures
on the surface of a chip is crucial for developing fully integrated “photonic“
circuits that do with light what is now done with electric currents in
semiconductor integrated circuits.
Researchers have developed many
approaches to do this, but have not yet been able to put the oldest and
simplest artificial light source the incandescent light bulb onto a chip.
This is primarily because light bulb filaments must be extremely hot
thousands of degrees Celsius in order to glow in the visible range and
micro-scale metal wires cannot withstand such temperatures.
In addition, heat transfer from the
hot filament to its surroundings is extremely efficient at the microscale,
making such structures impractical and leading to damage of the surrounding
chip.
By measuring the spectrum of the
light emitted from the graphene, the team was able to show that the graphene
was reaching temperatures of above 2500 degrees Celsius, hot enough to glow
brightly.
“The visible light from atomically
thin graphene is so intense that it is visible even to the naked eye, without
any additional magnification,“ explains Young Duck Kim, first and co-lead
author on the paper, who works in Hone's group at Columbia Engineering.
Interestingly, the spectrum of the
emitted light showed peaks at specific wavelengths, which the team discovered
was due to interference be tween the light emitted directly from the graphene
and light reflecting off the silicon substrate and passing back through the
graphene.
Kim notes, “This is only possible
because graphene is transparent, unlike any conventional filament, and allows
us to tune the emission spectrum by changing the distance to the substrate.“
The ability of graphene to achieve
such high temperatures without melting the substrate or the metal electrodes is
due to another interesting property: as it heats up, graphene becomes a much
poorer conductor of heat. This means that the high temperatures stay confined
to a small “hot spot“ in the center.
“At the highest temperatures, the
electron temperature is much higher than that of acoustic vibrational modes of
the graphene lattice, so that less energy is needed to attain temperatures
needed for visible light emission,“ Myung-Ho Bae, co-lead author, observes.
“These unique thermal properties
allow us to heat the suspended graphene up to half of temperature of the sun,
and improve efficiency 1000 times, as compared to graphene on a solid
substrate.“
Yun Daniel Park, professor in the
department of physics and astronomy at Seoul National University and co-lead
author, notes that they are working with the same material that Thomas Edison
used when he invented the incandescent light bulb: “Edison originally used
carbon as a filament for his light bulb and here we are going back to the same
element, but using it in its pure form graphene and at its ultimate size
limit one atom thick.“
The group is currently working to
further characterise the performance of these devices for example, how fast
they can be turned on and off to create “bits“ for optical communications and
to develop techniques for integrating them into flexible substrates.
Hone adds, “We are just starting to
dream about other uses for these structures for example, as microhotplates
that can be heated to thousands of degrees in a fraction of a second to study high-temperature
chemical reactions or catalysis.“
MM16JUN15
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