David Kirkpatrick

July 2, 2010

Graphene 2.0

Yep, I’m going to be lazy just cop part of the title of this release, well really more of an article than an out-and-out press release. Sounds like a pretty cool graphene transistor with potential real world applications.

The release:

Graphene 2.0: a new approach to making a unique material

June 30, 2010

Since its discovery, graphene—an unusual and versatile substance composed of a single-layer crystal lattice of carbon atoms—has caused much excitement in the scientific community. Now, Nongjian (NJ) Tao, a researcher at the Biodesign Institute at Arizona State University has hit on a new way of making graphene, maximizing the material’s enormous potential, particularly for use in high-speed electronic devices.

Along with collaborators from Germany’s Max Planck Institute, the Department of Materials Science and Engineering, University of Utah, and Tsinghua University, Beijing, Tao created a graphene transistor composed of 13 benzene rings.

The molecule, known as a coronene, shows an improved electronic band gap, a property which may help to overcome one of the central obstacles to applying graphene technology for electronics. Tao is the director of the Biodesign Institute’s Center for Bioelectronics and Biosensors and electrical engineering professor in the Ira A. Fulton Schools of Engineering. The group’s work appears in the June 29 advanced online issue of Nature Communications.

Eventually, graphene components may find their way into a broad array of products, from lasers to ultra-fast computer chips; ultracapacitors with unprecedented storage capabilities; tools for microbial detection and diagnosis; photovoltaic cells; quantum computing applications and many others.

As the name suggests, graphene is closely related to graphite. Each time a pencil is drawn across a page, tiny fragments of graphene are shed. When properly magnified, the substance resembles an atomic-scale chicken wire. Sheets of the material possess exceptional electronic and optical properties, making it highly attractive for varied applications.

“Graphene is an amazing material, made of carbon atoms connected in a honeycomb structure,” Tao says, pointing to graphene’s huge electrical mobility—the ease with which electrons can flow through the material. Such high mobility is a critical parameter in determining the speed of components like transistors.

Producing usable amounts of graphene however, can be tricky. Until now, two methods have been favored, one in which single layer graphene is peeled from a multilayer sheet of graphite, using adhesive tape and the other, in which crystals of graphene are grown on a substrate, such as silicon carbide.

In each case, an intrinsic property of graphene must be overcome for the material to be suitable for a transistor. As Tao explains, “a transistor is basically a switch—you turn it on or off. A graphene transistor is very fast but the on/off ratio is very tiny. ” This is due to the fact that the space between the valence and conduction bands of the material—or band gap as it is known—is zero for graphene.

In order to enlarge the band gap and improve the on/off ratio of the material, larger sheets of graphene may be cut down to nanoscale sizes. This has the effect of opening the gap between valence and conductance bands and improving the on/off ratio, though such size reduction comes at a cost. The process is laborious and tends to introduce irregularities in shape and impurities in chemical composition, which somewhat degrade the electrical properties of the graphene.  “This may not really be a viable solution for mass production,” Tao observes.

Rather than a top down approach in which sheets of graphene are reduced to a suitable size to act as transistors, Tao’s approach is bottom up—building up the graphene, molecular piece by piece. To do this, Tao relies on the chemical synthesis of benzene rings, hexagonal structures, each formed from 6 carbon atoms. “Benzene is usually an insulating material, ” Tao says. But as more such rings are joined together, the material’s behavior becomes more like a semiconductor.

Using this process, the group was able to synthesize a coronene molecule, consisting of 13 benzene rings arranged in a well defined shape. The molecule was then fitted on either side with linker groups—chemical binders that allow the molecule to be attached to electrodes, forming a nanoscale circuit. An electrical potential was then passed through the molecule and the behavior, observed. The new structure displayed transistor properties, showing reversible on and off switches.

Tao points out that the process of chemical synthesis permits the fine-tuning of structures in terms of ideal size, shape and geometric structure, making it advantageous for commercial mass production. Graphene can also be made free of defects and impurities, thereby reducing electrical scattering and providing material with maximum mobility and carrier velocity, ideal for high-speed electronics.

In conventional devices, resistance is proportional to temperature, but in the graphene transistors by Tao et al., electron mobility is due to quantum tunneling, and remains temperature independent—a signature of coherent process.

The group believes they will be able to enlarge the graphene structures through chemical synthesis to perhaps hundreds of rings, while still maintaining a sufficient band gap to enable switching behavior. The research opens many possibilities for the future commercialization of this uncommon material, and its use in a new generation of ultra high-speed electronics.

Written by Richard Harth
Biodesign Institute Science Writer

December 24, 2009

Introducing the one-molecule transistor

I’ve already introduced the one-atom transistor earlier this month. Now here’s new research offering a transistor created from a single molecule.

The release:

Scientists create world’s first molecular transistor

IMAGE: Engineers adjusted the voltage applied via gold contacts to a benzene molecule, allowing them to raise and lower the molecule’s energy states and demonstrate that it could be used exactly…

Click here for more information.

This release is available in Chinese.

New Haven, Conn.—A group of scientists has succeeded in creating the first transistor made from a single molecule. The team, which includes researchers from Yale University and the Gwangju Institute of Science and Technology in South Korea, published their findings in the December 24 issue of the journal Nature.

The team, including Mark Reed, the Harold Hodgkinson Professor of Engineering & Applied Science at Yale, showed that a benzene molecule attached to gold contacts could behave just like a silicon transistor.

The researchers were able to manipulate the molecule’s different energy states depending on the voltage they applied to it through the contacts. By manipulating the energy states, they were able to control the current passing through the molecule.

“It’s like rolling a ball up and over a hill, where the ball represents electrical current and the height of the hill represents the molecule’s different energy states,” Reed said. “We were able to adjust the height of the hill, allowing current to get through when it was low, and stopping the current when it was high.” In this way, the team was able to use the molecule in much the same way as regular transistors are used.

The work builds on previous research Reed did in the 1990s, which demonstrated that individual molecules could be trapped between electrical contacts. Since then, he and Takhee Lee, a former Yale postdoctoral associate and now a professor at the Gwangju Institute of Science and Technology, developed additional techniques over the years that allowed them to “see” what was happening at the molecular level.

Being able to fabricate the electrical contacts on such small scales, identifying the ideal molecules to use, and figuring out where to place them and how to connect them to the contacts were also key components of the discovery. “There were a lot of technological advances and understanding we built up over many years to make this happen,” Reed said.

There is a lot of interest in using molecules in computer circuits because traditional transistors are not feasible at such small scales. But Reed stressed that this is strictly a scientific breakthrough and that practical applications such as smaller and faster “molecular computers”—if possible at all—are many decades away.

“We’re not about to create the next generation of integrated circuits,” he said. “But after many years of work gearing up to this, we have fulfilled a decade-long quest and shown that molecules can act as transistors.”


Other authors of the paper include Hyunwook Song and Yun Hee Jang (Gwangju Institute of Science and Technology); and Youngsang Kim and Heejun Jeong (Hanyang University).

Citation: 10.1038/nature08639

December 7, 2009

Introducing the one-atom transistor

Via KurzweilAI.net — This really is just amazing. A transistor made of one single atom of phosphorus.

Single-atom transistor created
KurzweilAI.net, Dec. 7, 2009

A working transistor whose active region comprises only of a single phosphorus atom in silicon has been built byresearchers from Helsinki University of Technology, University of New South Wales, and University of Melbourne.

The device uses sequential tunneling of single electrons between the phosphorus atom and the source and drain leads of the transistor. The tunneling can be suppressed or allowed by controlling the voltage on a nearby metal electrode with a width of a few tens of nanometers.

The researchers plan to use the spin degree of freedom of an electron of the phosphorus donor as a quantum bit (qubit). They were able to observe spin up and down states for a single phosphorus donor in a magnetic field for the first time–a crucial step towards the control of these states in realizing a qubit.

(a) Scanning electron microscope image of single-atomtransistor
(b) Differential conductance through the transistor with 4 Tesla magnetic field

More informationHelsinki University of Technology news

November 13, 2008

First plasma transistor

Researchers at the University of Illinois at Urbana-Champaign have created a micro-sized plasma transistor. Big news for a number of applications, but particularly for high resolution displays in smaller devices.

From the link:

“As you might imagine, this first plasma transistor has not yet been engineered to the degree necessary for a commercial product,” Eden told PhysOrg.com. “Nevertheless, it should be mentioned that a microplasma transistor is advantageous in those situations requiring the transistor to handle high voltages and power. Unlike conventional transistors that can be damaged by a voltage transient, for example, the microplasma transistor is expected to be quite rugged because a gas (and plasma) cannot be ‘burnt.’”

In the plasma transistor, the electron emitter injects electrons in a controlled manner into the sheath of a partially ionized neon gas (the plasma). The scientists discovered that even a voltage as low as 5 volts can change the properties of the microplasma, including quadrupling the current and increasing the visible light emission.

By controllably altering the microplasma’s properties, the electron emitter effectively transforms the plasma microcavity device into a three-terminal transistor. Like a regular transistor, the microplasma transistor has the ability to control the current traveling through the terminals, and act as a switch or amplifier.