RESEARCH NEWS

OCTOBER 2008 | VOLUME 11 | NUMBER 10 13

Graphine is a one atom thick crystal layer, a chemically

stable and electrically conducting membrane

exhibiting a variety of unique properties due to its

novel molecular structure One of the big question still

remaining unanswered was; can such membranes be

impermeable to atoms, molecules and ions?

Researchers at Cornell University in the US have

addressed this question for gases: They successfully

used micron-scale graphene sheets to create the

world’s smallest balloons.

The team isolated graphene sheets by mechanical

exfoliation, placing them across wells that had been

created in silica substrates. Van der Waals forces

held the sheets in place around their circumference,

forming sealed microchambers nearly five microns on

a side.

Both positive and negative pressure differentials were

created across the atom-thick membranes by placing

the microchambers under pressure or in vacuum

and then allowing the pressure in the chambers to

equilibrate over hours or days. The sheets were then

imaged by atomic force microscopy, showing that they

bulged inward or outward significantly (see image).

The approach was attempted with nitrogen, air, and

helium as the high pressure gases, and with graphene

thicknesses from just one to 75 layers. The team found

that the timescale of the equilibration to ambient

pressure was not dependent on the thickness of the

graphene. Thus, any leakage was either through the

glass or the interface.

The result is important, says lead author Scott Bunch,

because it shows that “a single sheet of graphene is

impermeable to helium gas atoms and therefore free

of any significant vacancy over micron size areas.”

Measuring the size of the bulges above or below the

substrate for given pressure differentials allowed

estimates of the sheets’ elasticity, which the

researchers found to be more or less equal to that

of graphite. That solves a longstanding question

about the use of bulk elastic constants for nanoscale

materials [Bunch, et. al., Nano Lett. (2008), DOI:

10.1021/nl801457b].

The work suggests graphene sheets are applicable

as incredibly sensitive pressure sensors, and

selectively patterning the sheets would make them

ideal for ultrafiltration, the authors say. Graphene

drumheads can also offer the opportunity to probe

the permeability of gases through atomic vacancies in

single layers of atoms.

And what next for the team? Of course, Bunch says,

there’s an inevitable, irresistible desire: to pop the

balloons. “By popping graphene balloons we will

determine how various gases diffuse through the

atomic size openings created by our ‘pops’.”

Jason Palmer

Graphene puffs up under pressureCARBON

Researchers from the US and Germany have

developed a new polymeric material that allows

organic thin-film transistors (OTFTs) to operate

stably in water [Roberts et al., Proc. Natl. Acad. Sci. (2008) doi: 10.1073/pnas.0802105105]. The

advance could be a boon for low-cost, disposable

chemical and biological sensors.

OTFTs are attractive for sensing applications

because they can be fabricated on large-area,

flexible substrates and have active layers that can

be tuned to detect a variety of different analytes.

Exposing OTFTs to a variety of solvents in the

vapor phase produces a change in the device

current – which is straightforward to detect. But

the high operating voltages, degradation, and

delamination of OTFTs under humid or aqueous

conditions have limited their use as sensors in

real applications.

Zhenan Bao’s team at Stanford University,

together with colleagues from the Max Planck

Institute for Polymer Research, have created

OTFTs that operate at low voltage and is stable

under water.

The device relies on a new cross-linked

polymer gate dielectric and a stable organic

semiconductor. “We successfully cross-linked

poly(4-vinylphenol) or PVP with commercially

available dianhydride molecules at relatively low

temperatures, yielding well-insulated films with

high capacitance,” explains Bao.

To test the sensing capabilities of the OTFT,

the researchers then constructed an elastomeric

flow cell directly on the surface. The OTFT is

sensitive to trinitrobenzene down to 300 ppb,

glucose down to 10 ppm, and cystine down to

100 ppb.

“OTFTs can be used to detect low

concentrations of chemicals in a complex

environment without encapsulation,” says Bao.

The researchers are now working on a variety of

other interesting analytes including the explosive

trinitrotoluene, a chemical warfare nerve agent,

and DNA.

It is a big plus for OTFTs to be able to interact

chemically with many different analytes, says

Ananth Dodabalapur of the University of Texas

at Austin. “[The results] are very interesting in

that low voltages are used to operate the organic

transistor, which is very helpful in avoiding

ionic currents,” he adds. “This is an important

advance.”

Cordelia Sealy

New material puts organic transistors under waterSENSORS/POLYMERS

Pressure differentials across the atom thick

membranes, imaged by atomic force microscopy.

Image credits Zoom in Schematic - Victor Yu-Juei

Tzen.

A water droplet with trace amount of

trinitrobenzene on the surface of an organic

transistor. The presence of the analytes in the

semiconductor channel results in a disturbance to

the charge transport causing a change in output

current. Plastic materials form the basis of new

electronic sensors for chemical detection in air or

water. (Courtesy of Stefan C. B. Mannsfeld, Mark

Roberts and Zhenan Bao, Stanford University.)

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