Read this one carefully. Unless I am way off nearly perfect energy
conversion devices can exist. This is
another raw beginning from a completely unexpected behavior. We have already
been putting graphene through cartwheels and now it is turning into pure magic.
Whatever we can imagine, it wants
to produce.
As I have posted before, graphene
is a materials revolution way out in front of all other elements.
Electrons heat up in graphene
Oct 17, 2011
Graphene has once again amazed researchers with its bizarre properties
– this time in the way it reacts to light. A team in the US
Graphene is a layer of carbon just one atom thick that has a range of
unique electronic, mechanical and optical properties that could have great
technological promise. Indeed, since its discovery in 2004 the "wonder
material" has been used to create transistors and other prototype
components.
Hot carriers at all temperatures
Researchers are also keen to create optical devices using graphene and
this latest discovery by Pablo Jarillo-Herrero and colleagues at the
Massachusetts Institute of Technology and Harvard University
When a conventional semiconductor is excited with light, high-energy
electron–hole pairs are produced. These charge carriers subsequently generate a
photocurrent, which is usually driven by an electrostatic potential difference.
Such processes form the basis of modern optoelectronics devices.
Graphene is different
Until now, researchers believed that graphene was no different in its
reaction to light – although some suspected that thermoelectric processes could
be at play in the material. The new research by the MIT–Harvard team has
unambiguously confirmed for the first time that these processes are indeed
responsible for photocurrent generation in graphene.
The researchers obtained their results by making a host of
optoelectronic measurements on complex graphene p–n-junction nanodevices that
they had fabricated themselves in the laboratory. In particular, they performed
precise spatially resolved optical-excitation microscopy and electron-transport
measurements by shining laser light with a wavelength of 850 nm onto the
graphene p–n interfaces. They then measured the photocurrent produced in the
devices as the laser spot was scanned over the samples.
The team observed that a strong photocurrent was produced at the p–n
contact that increased as the power of the laser beam was increased. The
maximum photocurrent recorded was 5 mA/W at low temperatures, a value that
is six times higher than that seen in previous graphene optoelectronic devices.
Running hot and cold
According to the researchers, such high values are a result of the
photothermoelectric effect. "It turns out that when you shine a light on
graphene, the electrons in the material heat up, and remain hot, while the
underlying carbon lattice remains cool," explains Jarillo-Herrero.
"It is these hot electrons that then produce a current." The electrons
in the excited graphene cannot cool down easily because they couple poorly to
the carbon lattice and so cannot transfer their heat to it, he adds.
"Our study is of a very fundamental nature," says
Jarillo-Herrero, "and forces us to ask myriad questions." For
example, how efficient are the photogenerated charge carriers and can the
dimensions of the devices we made be optimized to maximize the current
produced? What happens if the number of graphene layers is changed and what
happens if the devices are coupled to optical cavities?
"All of these questions will be relevant when making new types of
ultrafast and highly efficient photodetectors and energy-harvesting devices,
the basic operating principles of which could be quite different from those of
standard semiconductor devices because they rely on hot-carrier
generation," he says.
"Graphene with its new exciting properties allows for
unprecedented engineering of novel thermo-optoelectronic structures,"
Gerasimos Konstantatos of the Institut de Ciències Fotòniques in Barcelona , Spain
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