A TUNABLE SEMICONDUCTOR MATERIALS SCIENCE: Bilayer graphene has controllable bandgap
T
HE ELECTRONIC properties
of graphene—a single layer of carbon atoms configured like chicken wire—are compelling enough. But now, researchers say, by connecting two layers of A bilayer of graphene. graphene, they have achieved what could be an extraordinary breakthrough in electronics: a device with a tunable bandgap. The defining property of any semiconductor or insulating material is the size of its bandgap—the amount of energy between the material’s valence band and conduction band. This intrinsic, fixed characteristic determines the material’s ability to transport electrons or absorb photons and thus what role it can play in devices such as transistors and photodiodes. University of California, Berkeley, physics professor Feng Wang and colleagues report that by placing two sheets of graphene on top of each other and putting
MICROTUBES FOLLOW DIRECTIONS
COURTESY OF LEROY CRONIN
A microtube’s direction (top) and diameter (bottom) can be controlled in real time, which could enable the growth of sophisticated microfluidic devices.
NANOTECH: Researchers control the growth, direction, and size of spontaneously assembling microtubes
F
ABRICATING microfluidic devices is generally a
painstaking process that requires a unique mold or mask for each device configuration. Geoffrey J. T. Cooper and Leroy Cronin of the University of Glasgow, in Scotland, have now taken a step toward a more flexible approach to device fabrication by developing a way to control, in real time, the growth, direction, and diameter of self-fabricating polyoxometalate (POM) microtubes. POMs are oxo-anion clusters of early transition metals. In a previous study, Cooper, Cronin, and colleagues observed spontaneous growth of micrometer-scale tubes from a tungstate POM crystal upon immersion in an aqueous solution of a polyaromatic organic cation (Nat. Chem. 2009, 1, 47). The interaction of POM anions with organic cations causes a semipermeable membrane to form around the crystal, and osmotic pressure within the membrane drives miWWW.CEN-ONLINE.ORG
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the layers between two electrical gates, they are able to adjust the bandgap by changing the applied voltage (Nature 2009, 459, 820). Researchers had predicted the potential for a tunable bandgap in bilayer graphene and have been intensely interested in the implications of this type of material for flexible nanoelectronic and nanophotonic devices. But efforts to fabricate such a device hadn’t been successful, and some researchers had been skeptical about whether such devices could be created at all. Dirk van der Marel, materials science professor at the University of Geneva, says the report “looks like a very beautiful paper,” adding that tunable bandgaps offer innovative ways of manipulating electrical transport properties in devices. Since its discovery in 2004, graphene has grabbed much attention (C&EN, March 2, page 14). The material’s single, incredibly strong sheets appear to conduct electrons almost effortlessly, and researchers are expending considerable effort to learn how to synthesize it more easily. On the horizon are graphene-based transistors, frequency multipliers, and light-emitting diodes. The Wang group’s work on bilayer graphene may lay the foundation for a new direction in graphene research—giving scientists a chance to double their fun.—ELIZABETH WILSON
crotube growth. The microtubes are uniform in diameter and sufficiently robust to allow the flow of liquid, thereby raising the possibility of their use as channels in microfluidic devices. In their latest paper, Cooper and Cronin developed a method to precisely control, in real time, the direction of microtube assembly with the help of an applied electric field (J. Am. Chem. Soc., DOI: 10.1021/ja902684b). To prepare microtube assemblies, the researchers introduce POM crystals to the center of a reaction vessel containing an organic cation solution. The vessel has four electrodes that are perpendicular to each other, and by varying the direction and duration of the applied field, the researchers produce complex patterns of microtubes, such as zigzags and 90° and 180° bends. They also control the diameters of microtubes by changing the concentration of the cation solution. Because POMs have semiconducting, catalytic, and optical properties, “this method could provide the base material for numerous microreactor systems,” says J. Tanner Nevill, applications engineer at Fluxion Biosciences, a manufacturer of microfluidic devices. “Also, the ability to change the tubing diameter during growth offers interesting potential for tapered, axially symmetric microfluMORE ONLINE idic channels, which are extremely difficult to achieve with conventional microfabrication techniques. The real challenge will be interfacing these tubes in a practical manner.”—LAURA CASSIDAY
JUNE 15, 2009
FENG WANG
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