7/29/2019 483S38a
1/2S 3 8 | N A T U R E | V O L 4 8 3 | 1 5 M A R C H 2 0 1 2
BYUNGH
EE
HONG
Picture caption
Graphenes first major appearances out-side laboratories may be
at the inter-section of electronics and optics, in the
form of transparent electrodes. Such electrodes which carry
current but allow light to passthrough let touch screens operate,
collectcurrent from solar cells without blocking thelight needed to
produce that current, and inject
current to power organic light-emitting diodes(OLEDs) while
allowing most of the photonsto escape.
Much graphene research has focused on thematerials remarkable
electronic abilities andthe possibility of using it to build
transistors.However, it competes there with a well-estab-lished
technology silicon. As James Tour, achemist at Rice University in
Houston, Texas,puts it: Silicon has had a 50-year head
start,trillions of dollars and millions of person years.So some
researchers are starting to explore pos-sibilities that combine
graphenes electronicproperties with its light-handling
characteris-
tics, as well as other attractive properties suchas strength and
flexibility. Transparent elec-trodes today generally rely on indium
tin oxide(ITO). But indium is rare, and its price is risingas
demand for smartphones and tablets soars.And because ITO is
brittle, it must be depos-ited on an unbending surface (usually
glass),thus placing constraints on device design. Witha thin film
of graphene as a conductive mate-rial, manufacturers could build
flexible organicsolar cells; OLED panels could be wrappedaround
furniture, automobile frames or a wrist.Graphene electrodes could
make possible light-weight smartphones that could be folded
orrolled up and whose screen wont crack whendropped. At the
Consumer Electronics Showin early 2011, Korean electronics firm
Samsungdisplayed a flexible active-matrix OLED phonescreen based on
graphene, but the company hasnot announced any products
incorporating thistechnology.
Tour concedes that graphenes performancecharacteristics are not
as good as ITOs, at leastnot yet. A thin film of ITO lets about 90%
ofthe light pass through it, and has electricalresistance of 50
ohms. A single-layer sheet ofpristine graphene allows more light
through 97.7% but has far higher resistance, between2,000 and 5,000
ohms. Adding more layers of
graphene improves resistance but worsenstransparency; four
layers provide transparencyroughly comparable to ITOs but, at 350
ohms,it still has seven times the resistance.
But graphene may not need to be as goodelectrically as ITO to
get into the marketplace being cheaper may be enough. And in
themeantime, researchers are working to enhancethe carbon
monolayers performance. Tour isbuilding a touchscreen that contains
a thin meshof aluminium nanowires between the substrateand the
graphene. Only 5 micrometres in diam-eter, the wires are too
slender to block light butthey make the device dramatically more
con-
ductive. A screen with 90% transparency now
O P T O E L E C T R O N I C S
Come intothe lightTransparency across the spectrum combinedwith
electronic prowess makes graphene an ideal
photonic material.
B Y N E I L S A V A G E
Touch screens with a graphene electrode could bring flexibility
to mobile phones and other devices.
GRAPHENEOUTLOOK
2012 Macmillan Publishers Limited. All rights reserved
7/29/2019 483S38a
2/21 5 M A R C H 2 0 1 2 | V O L 4 8 3 | N A T U R E | S 3 9
has a resistance of only 20 ohms. Researchers atSamsung and at
Sungkyunkwan University inSeoul have collaborated to obtain similar
results 90% transparency with 30 ohms resistance by mixing in
nitric acid. And these South Koreangroups have shown they can
produce sheets ofthe improved graphene that measure 30
inchesdiagonally, using a roll-to-roll process that would
be convenient for manufacturing.Finding efficient production
processes is partof the challenge of introducing graphene
trans-parent electrodes. Tour has devised a methodto create thin
ribbons of graphene by treatingcarbon nanotubes with sulphuric
acid, causingthem to unzip into ribbons. He then attaches tothe
ribbons edges molecular groups that allowthe material to be
suspended in a solution andsprayed onto a surface. This approach
could allowtransparent electrodes to be painted onto large-area
solar cells or OLEDs, potentially providingan inexpensive
alternative to silicon solar cells.
Andrea Ferrari, a nanotechnologist at the
University of Cambridge in the United King-dom, has made
graphene-based inks that aninkjet printer can use to make
transparent elec-trodes and circuits; graphene ink and
polymerscould be used to create thin-film solar cells, hesuggests.
These wouldnt convert as much lightto electricity as a silicon
solar cell, but becausetheyre so lightweight and transparent they
couldbe installed on the facades of buildings, captur-ing sunlight
that would otherwise go to wastewithout inducing any structural
strain or ruin-ing the aesthetics. A thin-film solar cell could
betransparent enough to cover a window pane, yetstill produce
enough energy to make a dent in anelectricity bill. Filling niches
where silicon isntappropriate could be a small but still
valuablerole for graphene, Ferrari says. Even though itworks much
worse than the state of the art, it hasthe advantage of being
flexible and cheap.
GAPS IN THE SPECTRUM
Graphenes most attractive optical property,according to Ferrari,
is that it works at absolutelyany wavelength whatsoever. That is,
the mate-rial absorbs the same fraction of light roughly2.3% all
across the spectrum, from the ultra-violet into the far infrared.
This property arisesfrom the electronic structure of graphene: it
lacksa bandgap a range of energy states where elec-
trons are prohibited from existing. Graphenesability to absorb
across the spectrum is impor-tant because the way light interacts
with differentmaterials whether its absorbed or scattered orpasses
though varies depending on its wave-length. One wavelength may be
ideal for cuttinga certain material, another for detecting
particu-lar organic molecules. But at some wavelengths,theres no
device to perform those tasks, at leastone that isnt prohibitively
expensive. Much ofthe electromagnetic spectrum, Ferrari notes,
hasnever been exploited, because its hard to makesemiconductors
that work at those wavelengths.
One such untapped portion of the spectrum
is between the far infrared and microwaves, or
frequencies between about 100 and 10,000 giga-hertz (0.110
terahertz). Because such terahertzradiation penetrates a wide
variety of materialsbut is non-ionizing, it has potential
applicationsin security scans, environmental sensing, andmedical
imaging. Currently, however, sourcesand detectors that operate at
those frequenciesare few and complex; some terahertz detectors
require cryogenic cooling, whereas a graphenedetector could work
at room temperature. Somepeople refer to it as the last gap in the
electromag-netic wavelengths, says physicist Feng Wang,head of the
Ultrafast Nano-Optics Group at theUniversity of California,
Berkeley.
Wang built a terahertz detector based onnanoribbons of graphene
ranging in widthbetween 1 and 4 micrometres. The detector isbased
on plasmon resonance: when light strikesa material whose electrons
resonate with a fre-quency that matches the lights wavelength,
theresult is oscillations in electron density calledplasmons, which
can enhance the strength of
a signal. (The colours in stained glass windowsare so vibrant
because of plasmon resonancebetween visible light and metal
nanoparticlesembedded in the glass.) In graphene, this plas-mon
resonance happens at terahertz frequen-cies. Wang designs devices
to detect particularfrequencies by choosing the width of the
ribbons,and then further tunes the device by applying anelectrical
field. Creating such a detector is the
beginning of developing a set of tools for theterahertz range,
he says.
Graphene may be valuable not only as adetector of terahertz
radiation, but also as asource, Wang says. There are already
quantumcascade lasers that emit at frequencies of 10 to100 THz. The
most useful frequencies, however,lie between about 500 GHz and 4
THz, and theonly existing sources in that range are limited tovery
low output power. Wang points out that asresearchers push to create
faster transistors thatoperate at these speeds, they could in
effect becreating an emission source in that range as well just as
conventional megahertz-speed tran-sistors produce radio waves.
Pushing emitters tohigher and higher frequencies opens up the
pos-
sibility of higher bandwidth communications.
Although the terahertz range is the area thatsmost sorely
lacking emitters and detectors, gra-phene could fill in other gaps
in the spectrum aswell, because you have a sort of universal
detec-tor, says Phaedon Avouris, head of the nanoscalescience and
technology group at IBMs WatsonResearch Center in Yorktown Heights,
New York.Current technology does not offer fast photo-
detectors at mid- to far-infrared wavelengths;Avouriss team
built an experimental graphene-based detector for this region of
the spectrumthat operates at a speed of 40 GHz. And,
Avouriscontends, because of imperfect measurementtools, its
possible that the graphene device couldactually operate at 500 GHz
or higher.
As for emitters, Ferrari says graphene mightsoon be used in
ultrafast lasers. Such lasers pro-duce short, high-energy bursts of
light, lastingonly picoseconds (trillionths of a second)
orfemtoseconds (thousandths of a picosecond).Delivering a lot of
energy in a short burst letsthe lasers cut very precisely without
damag-
ing nearby material or tissue; theyre used inapplications from
eye surgery to printed-cir-cuit-board processing. Most ultrafast
lasersuse a technique called mode-locking, in whicha device known
as a saturable absorber soaksup continuous laser emission and
releases itin a series of pulses. The absorbers are
usuallysemiconductor mirrors. But such systems arecomplex to build,
and they generally emit lightat only a narrow range of wavelengths.
Usinggraphene as the saturable absorber would notonly allow a
simpler design but also permitbroader spectrum operation.
SMALL AND FAST
In addition to emitting and detecting light, pho-tonics systems
often require an optical modu-lator, which encodes signals onto
light beams.Optical telecommunications use a modulatorbuilt of
lithium niobate, and thats unlikely tobe displaced for the majority
of applications,says Wang. His group demonstrated a
graphenemodulator about 25 square micrometres in size 4 million
times smaller than a conventionalmodulator made from lithium
niobate. Such asmall device could be integrated on a computerchip
for optical interconnections, replacing cop-per wires with light
beams and clearing a bot-tleneck for processing speed. The
modulators
should also work up to 10 times faster than exist-ing devices
and, unlike lithium niobate, could beused in the mid-infrared,
giving system design-ers more flexibility.
It will take some time to figure out the impacton photonics of
these emerging graphene tech-nologies, Ferrari says. Graphene
optoelectron-ics is only three years old, he says. We need togive
it another two or three years to see wherewe are going. By that
time, this toddler technol-ogy might be starting to light the way
forward inways not yet conceived.
Neil Savage is a science and technology
journalist based in Lowell, Massachusetts.
NATURENANOTECHNOLOGY
A graphene-based touch screen in operation.
OUTLOOKGRAPHENE
2012 Macmillan Publishers Limited. All rights reserved
