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New technique opens a gap in graphene



Researchers in Germany and Switzerland have developed a new way to make extremely narrow graphene ribbons with specific widths and electronic bandgaps. The ribbons also have smooth edges, something that is crucial for making electronic devices out of graphene.
Graphene is a flat sheet of carbon just one atom thick – with the carbon atoms arranged in a honeycomb lattice. Since the material was first created in 2004, its unique electronic and mechanical properties have amazed researchers who say that it could be used in a host of device applications. Indeed, graphene might even replace silicon as the electronic material of choice in the future.
However, unlike the semiconductor silicon, graphene has no gap between its valence and conduction bands. Such a bandgap is essential for electronics applications because it allows a material to switch the flow of electrons on and off. One way of introducing a bandgap into graphene is to make extremely narrow ribbons of the material.

Cutting or unzipping
Until now, these graphene nanoribbons were made using top-down approaches, such as "cutting" the ribbons from larger graphene sheets or "unzipping" carbon nanotubes. Such methods produce ribbons that are relatively wide (more than 10 nm across) with rough edges. For high-efficiency electronics devices, the ribbons need to be much smaller than 10 nm wide and, importantly, their edges need to be smooth because even minute deviations from the ideal edge shapes, "armchair" and "zigzag", seriously degrade graphene's electronic properties.
The new technique, developed by a team led by Roman Fasel of the Swiss Federal Laboratories for Materials Science and Technology (Empa) and Klaus Müllen from the Max Planck Institute for Polymer Research in Germany, is a simple, surface-based bottom-up chemical process. It involves first spreading specially designed halogen-substituted bianthryl monomers onto gold and silver surfaces under a high vacuum. Next, the monomers are made to link up to form polyphenylene chains.

'Important first step'
Fasel and colleagues then remove hydrogen atoms from the polymers by heating up the ensemble. This leads to the polymer chains interconnecting to form planar, aromatic graphene ribbons that are just one atom thick, 1 nm wide and up to 50 nm long. The ribbons are narrow enough to have an electronic bandgap and thus switching properties. "Such switching is an important first step for the shift from silicon microelectronics to graphene nanoelectronics," say the researchers.
And that is not all: the edges of the graphene ribbons are smooth and armchair-shaped, and the ribbons themselves are either straight or zigzagged, depending on the monomers used to make them. The smooth edges will be important for studying fundamental experimental physics too, says the team – for example, observing how magnetic properties of the ribbons change with different edge structures. Until now, previous methods to make graphene nanoribbons always produced rough edges that were difficult to study
The new technique could also be used to dope the graphene ribbons by using monomers containing nitrogen or boron atoms. And monomers with additional functionalities should allow the researchers to create positively and negatively doped ribbons – for making p–n junctions in transistors, for instance.

Solar cells
Going further still, a combination of various monomers might even allow heterojunctions (interfaces between different types of graphene nanoribbon, such as those with large or small bandgaps) to be created. Such structures could be used in applications like solar cells or high-frequency devices. Fasel and colleagues have already shown that this technique is viable by connecting three separate graphene ribbons together using two suitable monomers.
The team, which includes scientists from ETH Zürich and the Universities of Zürich and Bern, is now working on creating the nanoribbons on semiconductor surfaces, rather than on just metallic substrates as in this work. This will be critical for making real-world electronic devices.

YOSEPH BUITRAGO C.I. 18257871 EES SECCION2

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power portable electronics Nanofibres New transistor breaks speed

A new kind of miniature energy harvesting device that generates electricity using nanometre-sized fibres has been unveiled in the US. The nanogenerator could harvest energy from human or other motion to power wireless sensors, personal electronics and even medical implants, claim its inventors at Stevens Institute of Technology and Princeton University.

"We are particularly excited about using the nanofibre-based generators in bio-compatible situations, like embedding the devices in shoes and clothing to harvest energy from the motion of the human body to charge personal electronics such as iPod batteries and cell phones," says team leader Yong Shi, who is a mechanical engineer at Stevens.

The new high power output devices are based on lead zirconate titanate (PZT) nanofibres. PZT has a high piezoelectric voltage and dielectric constants – ideal properties for converting mechanical energy into electrical energy. Unlike bulk thin films or microfibres, PZT nanofibres prepared by electrospinning processes are also highly bendable and mechanically strong.

Embedded in a soft polymer

Shi's team made the nanogenerator by depositing electrospun PZT nanofibres on preformed arrays of electrodes on a silicon substrate. The nanofibres are around 60 nm in diameter and they were embedded in a soft polymer (polydimethylsiloxane, PDMS) matrix. The finished device can be released from the silicon substrate or prepared on flexible substrates, depending on the application desired.

When mechanical pressure is applied on the top surface of the ensemble, it is transferred to the nanofibres via the PDMS matrix. This results in electrical charge being generated thanks to the combined tensile and bending stresses in the nanofibres as they move. This results in a voltage between two adjacent electrodes.

The researchers say that, for a given volume of nanogenerator, the nanofibre device generates much higher voltages and power than devices made from semiconductor piezoelectric nanowires for the same energy input. In theory, the maximum output power from a piezoelectric nanogenerator depends on the properties of the active materials, so the higher the piezoelectric voltage constant of the material between two electrodes, the higher the output voltage and power. What is more, varying the length of the active materials between the two electrodes will also vary the voltage output and current at the same time, explains Shi.

The devices could be used to power wireless sensors, personal electronics and, in the future, biosensors and bioactuators that are directly injected into the human body.

Powered by blood flow

Arthur Ritter – who is director of biomedical engineering at Stevens and was not involved in the research – said, "One of the major limitations of current active implantable biomedical devices is that they are battery powered. This means that they either have to be recharged or replaced periodically. Shi's group has demonstrated a technology that will allow implantable devices to recover some of the mechanical energy in flowing blood or peristaltic fluid movement in the gastro-intestinal tract to power smart implantable biomedical devices."

And, because the technology is based on nanostructures, it could provide power to nanorobots in the blood stream for extended periods of time, he adds. Such robots could transmit diagnostic data, take biopsy samples and/or send wireless images directly to an external database for analysis.

The team now plans to optimize the structure of its nanodevice and simplify the fabrication process. "We are also working hard on implantable bio-applications," revealed Shi.

YOSEPH BUITRAGO  C.I. 18257871   EES   SECCION2

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A pair of physicists in the US has built the fastest ever transistor: one that can operate at a frequency of over 600 gigahertz. Developed by Walid Hafez and Milton Feng at the University of Illinois at Urbana-Champaign, the device is made from the semiconductors indium phosphide and indium gallium arsenide (Appl. Phys. Lett. 86 152101). The work demonstrates the feasibility of making transistors that can operate at frequencies of several terahertz, which could be used in ultrafast communications, high-speed computing, medical imaging and sensors.
The new device is a so-called bipolar transistor, which is very different from the more well-known field-effect transistor. In it, electrons are injected from the "emitter" terminal, travel towards the "base" and are then received by the "collector", an arrangement that allows the device to work faster than a field-effect transistor.
Hafez and Feng have previously built a high-frequency bipolar transistor, but this earlier work focused on reducing the time it takes electrons to pass through the device by minimizing the device's vertical thickness. Their new research further increases electron speeds through the device by slightly varying, or "grading", the composition of the semiconductor layers. This, say the researchers, lowers the band gap in selected areas of the transistor and makes it easier for electrons to travel across the device.
The two physicists have shown their transistor can operate at a frequency of 604 gigahertz, a new record. However, according to Hafez, what is more important is that they have developed a technology that could be used to build transistors operating in the terahertz range. "Projections from our earlier high-frequency devices indicated that in order to create a transistor with a cutoff frequency of 1 terahertz, the devices would have to operate above 10,000 degrees C," he says. "By introducing the grading into the layer structure of the device, we have been able to lower the potential operating temperature for a terahertz transistor to within an acceptable range."
Devices operating at terahertz frequencies (the far infrared) could be used in communications applications or as sensors to detect toxic gases. They could also be used for medical imaging, since the radiation is long enough to Penetrate skin and image What lies underneath.
The Researchers' next step is to show That Can Be Their devices assembled Into circuits.


Yoseph 18257871 ESS SECTION 2 CI BUITRAGO