Graphene Advances

Mass produced graphene Transistors just got a little closer with this laboratory advance in graphene film fabrication.

"Before we can fully utilize the superior electronic properties of graphene in devices, we must first develop a method of forming uniform single-layer graphene films on nonconducting substrates on a large scale," says Yuegang Zhang, a materials scientist with the Lawrence Berkeley National Laboratory (Berkeley Lab). Current fabrication methods based on mechanical cleavage or ultrahigh vacuum annealing, he says, are ill-suited for commercial-scale production. Graphene films made via solution-based deposition and chemical reduction have suffered from poor or uneven quality.

Zhang and colleagues at Berkeley Lab's Molecular Foundry, a U.S. Department of Energy (DOE) center for nanoscience, have taken a significant step at clearing this major hurdle. They have successfully used direct chemical vapor deposition (CVD) to synthesize single-layer films of graphene on a dielectric substrate. Zhang and his colleagues made their graphene films by catalytically decomposing hydrocarbon precursors over thin films of copper that had been pre-deposited on the dielectric substrate. The copper films subsequently dewetted (separated into puddles or droplets) and were evaporated. The final product was a single-layer graphene film on a bare dielectric.

"This is exciting news for electronic applications because chemical vapor deposition is a technique already widely used in the semiconductor industry," Zhang says.

It is always nice to have a process that just needs to be adjusted. Even nicer is that it is a production process. There are probably 1,000 more steps like that required before your next microprocessor is made of charcoal (with a few enhancements).

Graphene has some exceptional properties.

In a semiconductor there is a quadratic relationship between the energy and momentum of the electrons. But in graphene that relationship is linear. Papers #2 (Geim’s group) and #3 (Philip Kim’s group, Columbia University, New York), published side by side in Nature, report on an important consequence of the linear relationship. They independently discovered that electrons move through the films as if they have no mass. That’s because the energy-momentum relationship means that electron transport is governed by the relativistic Dirac equation.

In semiconductors, electron transport is ruled by the non-relativistic Schrödinger equation. So electrons in graphene behave like relativistic particles and travel at about 106 m s-1. Although that speed is about 300 times slower than the velocity of light, it is much faster than the speed of electrons in conductors. The electrons travel sub-micron distances without scattering, something unheard of in semiconductors. Suddenly, ballistic transistors, in which electrons barrel through the device like a bullet, begin to look feasible.

I think massless electrons could come in quite handy. Provided you could produce them on demand and control them. You know. Power and Control.

Here is a fairly recent book on the subject that may help you get up to speed:

Graphene

H/T DavidWillard at Talk Polywell

Cross Posted at Classical Values

That Is Fast

Carbon (graphene) transistors are getting really fast.

IBM Research has demonstrated a 100GHz transistor. Fabricated on new 2-inch graphene wafers and operating at room temperature, the RF graphene transistors are said to beat the speeds of all but the fastest GaAs transistors, paving the way to commercialization of high-speed, carbon-based electronics.

"There are all kinds of extraordinary claims being made every day for graphene semiconductors, but this is the first demonstration of a RF graphene transistor that was made under technologically relevant conditions and scale," said IBM Fellow Phaedon Avouris, who oversees carbon-based materials efforts at IBM Research.

The graphene RF transistors were created for the Defense Advanced Research Project Agency under its Carbon Electronics for RF Applications (CERA) program. Almost four times faster than previous demonstrations, the graphene transistors were fabricated at the wafer scale using epitaxially grown graphene processing techniques that are compatible with those used to fabricate silicon transistors.

Big transistors are are not too hard. You essentially lay down a sheet of graphene, dope it (or dope it while it is part of the SiC substrate), and then put a gate pattern over it. You get a power transistor. At 100 GHZ that probaly is indicative of the ability to work at 50 GHz.

But they have set their sights on bigger game.

There are several relatively easy steps to further widen the gap between graphene and silicon. For instance, graphene suspended over an air gap and supercooled has achieve carrier mobilities of up to 200,000cm²/Vs compared to silicon's 1400cm²/Vs.

IBM's demonstration of room-temperature graphene on an insulating substrate only achieved 1500cm²/Vs.

The gate length of IBM's graphene transistor was 240nm, nearly 10x larger than the smallest gate lengths achievable with current lithographic techniques (under 35nm). By optimizing its process to increase mobility and shortening the gate length, IBM will next aim to increase the speed of its graphene transistor up to 1THz, which is the goal for the CERA program.

Computers are a little different. Take the top speed and divide it by 4. Then allow for 6 levels of logic (AND, OR, and NOT) plus wiring delays and you can divide that number again by 10. So 1 THz/40 = 25 GHz. About 10X faster than today's computers. If they can cut the heat load by a factor of 5 to 10 they will have one screaming machine. Who will be the early adopters? Gamers, server farms, and of course the guys who funded it. The military through the CERA Program.

Long Lines

Technology Review reports on progress in Making Carbon Nanotubes into Long Fibers

A new method for assembling carbon nanotubes has been used to create fibers hundreds of meters long. Individual carbon nanotubes are strong, lightweight, and electrically conductive, and could be valuable as, among other things, electrical transmission wires. But aligning masses of the nanotubes into well-ordered materials such as fibers has proven challenging at a scale suitable for manufacturing. By processing carbon nanotubes in a solution called a superacid, researchers at Rice University have made long fibers that might be used as lightweight, efficient wires for the electrical grid or as the basis of structural materials and conductive textiles.

Yep. It could be a very good replacement for copper or aluminum wires. And the base material is rather abundant. Coal mines are full of it. On the other hand petroleum or natural gas might be easier to process.

But we are not quite there yet.

So far, the group has made fibers that are highly conductive but not as strong as other carbon materials. Pasquali says the strength of the fibers could probably be improved tenfold by using longer carbon nanotubes. "We're now working on a project for making electrical transmission lines," says Pasquali. "Metallic nanotubes conduct electricity better than copper, they're lighter, and they fail less often."

One important hurdle for large-scale manufacturing of carbon nanotubes remains: Today, there aren't any good methods for making the nanotubes themselves in large, pure batches. In order to make nanotube transmission lines, for example, the Rice group would need to start with a large batch of nanotubes containing all metallic nanotubes and no semiconducting ones. Last month, chemists at the Honda Research Institute published a paper in Science describing a method for making large amounts of metallic nanotubes that Pasquali says is promising. "For transmission lines you need to make tons, and there are no methods now to do that," he says. "We are one miracle away."

And that miracle may have already happened.

What remains to be done after the breakthrough: making enough Carbon Nanotube (CNT) wire to build a test section into the grid. Developing methods for joining the wire to other non CNT segments of the grid. Developing methods for joining CNT segments. Testing it against weather and lightning strikes. And at least 10,000 other details (hand tools among them) will need to be worked out including crew training. Miracles take time to unfold.

And power lines might not be the prime candidate. Lowering the weight of wiring harnesses in automobiles might be a more favorable initial application since weight reduction is worth real money.

H/T GPecchia at Talk Polywell

Nanotubes In Semiconductors

Carbon Nanotubes may be coming soon to a semiconductor near you.

Surrey NanoSystems, a University of Surrey spin-out working on a low-temperature growth process for carbon nanotubes, has secured second round funding of £2.5m

Surrey NanoSystems was established in 2006 as a spin-out from the University of Surrey’s Advanced Technology Institute (ATI) to develop intellectual property which supports the fabrication of carbon nanotubes at low temperature.

The company developed a platform called NanoGrowth which creates conditions for the growth of precision carbon nanotubes at both the temperatures and densities needed for CMOS process technology.

The company is now optimising its technology for the mass-volume manufacturing environment, by scaling the hardware and refining and scaling the materials processing technology.

The new funding will allow Surrey NanoSystems to scale the materials growth technology from its current 100mm wafer size capability to the 300mm sizes used in commercial wafer fabrication plants.

"The semiconductor industry urgently needs a new interconnection technology. If you can solve the problem of growing precision carbon nanotubes at silicon-friendly temperatures - and we have - it opens up a massive potential market," said Ben Jensen, CTO of Surrey NanoSystems.

Yes it does. Right now with large integrated circuits more energy is lost in the connections than in the transistors. Carbon nanotubes are more conductive than the commonly used copper. This should make integrated circuits both faster and cooler. Lower resistance works like that.

It is also possible that things will get even better when carbon nanotube transistor fabrication becomes a manufacturable technique. Faster transistors with lower losses that can operate at higher temperatures. Just what is needed to advance power control technology.

Carbon Chips

Carbon is going to be the next big thing in computer chips.

Carbon—the basis of all organic compounds—seems destined to displace silicon as the material of choice for future semiconductors. According to researchers, various structures based on the element that sits just above silicon on the Periodic Table can surpass silicon's abilities in thermal performance, frequency range and perhaps even superconductivity.

"Of the carbon technologies, diamond is probably the closest [to commercialization] at this time, as work in diamond has been taking place for 15 years or longer," said Dean Freeman, senior analyst at Gartner Inc. "Most of the others still have a ways to go."

Three-dimensional carbon—diamond—offers 10x the heat dissipation of silicon, according to suppliers currently hawking 40nm to 15µm diamond films on silicon wafers. Two-dimensional carbon—3-angstrom-thick monolayers called graphene—could dismantle silicon's roadblock to terahertz performance by attaining 10x the electron mobility of silicon.

Likewise, one-dimensional carbon—1nm-diameter nanotubes—could solve digital silicon's speed woes. Nanotubes will appear first as printable "inks" that are 10 times faster than competing organic transistors.

Meanwhile, zero-dimensional carbon—60-atom, hollow spheres of carbon called fullerenes—could answer silicon's inability to attain high-temperature superconductivity. Tightly packed fullerenes intercalcated with alkali-metal atoms superconduct at 38K.

Over the next few years, carbon process technologies will become available to replace nearly every circuit material in use today: conductors, for interconnecting devices; semiconductors; and insulators, for isolating devices. But how quickly the industry embraces the carbon-based materials, especially during uncertain economic times, remains to be seen.

So how about some details on how the work is progressing? I have them. Lets look at some Buckyball research first.

Add a drop of oil to buckyballs, and they join together to form wires like strings of pearls.Finally! Something useful from buckyballs.

Junfeng Geng at the University of Cambridge, in the U.K., and buddies have found a way to polymerize these microballs so that they line up into buckywires.

The trick that Geng and co have found is a way to connect two buckyballs together using a molecule of 1,2,4-trimethylbenzene--a colorless aromatic hydrocarbon. Repeat that and you've got a way to connect any number of buckyballs. And to prove it, the researchers have created and studied these buckywires in their lab, saying that the wires are highly stable.

Buckywires ought to be handy for all kinds of biological, electrical, optical, and magnetic applications. The gist of the paper is that anything that traditional carbon nanotubes can do, buckywires can do better. Or at least more cheaply.

The exciting thing about this breakthrough is the potential to grow buckywires on an industrial scale from buckyballs dissolved in a vat of bubbling oil. Since the buckywires are insoluble, they precipitate out, forming crystals. (Here it ought to be said that various other groups are said to have made buckywires of one kind or another, but none seem to have nailed it from an industrial perspective.)

There is more. Go have a look.

Next there is a most interesting substance called graphene, which is a very thin layer of carbon. Its properties can be tuned by applying an electric field to the material.

Semiconductors, for example, can be turned off because of a finite bandgap between the valence and conduction electron bands.

While a single layer of graphene has a zero bandgap, two layers of graphene together theoretically should have a variable bandgap controlled by an electrical field, Wang said. Previous experiments on bilayer graphene, however, have failed to demonstrate the predicted bandgap structure, possibly because of impurities. Researchers obtain graphene with a very low-tech method: They take graphite, like that in pencil lead, smear it over a surface, cover with Scotch tape and rip it off. The tape shears the graphite, which is just billions of layers of graphene, to produce single- as well as multi-layered graphene.

Wang, Zhang, Tang and their colleagues decided to construct bilayer graphene with two voltage gates instead of one. When the gate electrodes were attached to the top and bottom of the bilayer and electrical connections (a source and drain) made at the edges of the bilayer sheets, the researchers were able to open up and tune a bandgap merely by varying the gating voltages.

The team also showed that it can change another critical property of graphene, its Fermi energy, that is, the maximum energy of occupied electron states, which controls the electron density in the material.

"With top and bottom gates on bilayer graphene, you can independently control the two most important parameters in a semiconductor: You can change the electronic structure to vary the bandgap continuously, and independently control electron doping by varying the Fermi level," Wang said.

For those of you not conversant with transistor design and terms like band gap and Fermi energy you might find this book helpful. It is a history of physics and includes the work by Bell Labs on the transistor.

Crystals, Electrons, Transistors



Cross Posted at Classical Values

Graphene Is Strange

NIST and Georgia Tech have been looking at graphene (a single layer of carbon atoms in a hexagonal grid pattern) and they are seeing some strange behavior.

Graphene’s exotic behaviors present intriguing prospects for future technologies, including high-speed, graphene-based electronics that might replace today’s silicon-based integrated circuits and other devices. Even at room temperature, electrons in graphene are more than 100 times more mobile than in silicon.

Graphene apparently owes this enhanced mobility to the curious fact that its electrons and other carriers of electric charges behave as though they do not have mass. In conventional materials, the speed of electrons is related to their energy, but not in graphene. Although they do not approach the speed of light, the unbound electrons in graphene behave much like photons, massless particles of light that also move at a speed independent of their energy.

This weird massless behavior is associated with other strangeness. When ordinary conductors are put in a strong magnetic field, charge carriers such as electrons begin moving in circular orbits that are constrained to discrete, equally spaced energy levels. In graphene these levels are known to be unevenly spaced because of the “massless” electrons.

When something so unusual comes up in a lab it means a lot of potential applications that can't even be imagined now. One of the reasons for the lack of imagination is that such a material was not even suspected so why would you even spend any time thinking about what you could do with such a material? However, with the current research mapping out such properties the floodgates are open.

The Georgia Tech/NIST team tracked these massless electrons in action, using a specialized NIST instrument to zoom in on the graphene layer at a billion times magnification, tracking the electronic states while at the same time applying high magnetic fields. The custom-built, ultra-low-temperature and ultra-high-vacuum scanning tunneling microscope allowed them to sweep an adjustable magnetic field across graphene samples prepared at Georgia Tech, observing and mapping the peculiar non-uniform spacing among discrete energy levels that form when the material is exposed to magnetic fields.

The team developed a high-resolution map of the distribution of energy levels in graphene. In contrast to metals and other conducting materials, where the distance from one energy peak to the next is uniformly equal, this spacing is uneven in graphene.

The researchers also probed and spatially mapped graphene’s hallmark “zero energy state,” a curious phenomenon where the material has no electrical carriers until a magnetic field is applied.

Expect to hear more about this material and its close relative, carbon nanotubes, in the not too distant future. I can't wait to find out what the applications might be.

Carbon Nanotube Production

Increased production of multi-wall carbon nanotubes is causing a drop in prices.

Multiwall carbon nanotubes are rapidly emerging as an interesting alternate to carbon black for electrostatic discharge applications.

"There will be three important applications for multiwall carbon nanotubes," says Page McAndrew, senior research scientist at Arkema, one of the leading developers of the new reinforcement technology. "One is imparting electrical conductivity, primarily for ESD applications. Two is for improving mechanical performance, and three is for flame retardancy."

ESD applications means using a conductive material to prevent static electricity buildup. ESD stands for ElectroStatic Discharge. Important in things like fuel lines.

And what is this fabulous new rate of world production that is driving down prices?

Production for multiwall carbon nanotubes is still in the pilot stage and global capacity today is estimated by trade sources at less than 500 metric tons. Capacity could grow tenfold or more based on announced increases in capacity.

Obviously prices will come down further and make more applications attractive as production increases.

A New Kind Of Transistor

There is some very promising research that promises the development of a new kind of transistor.

A team of Duke University chemists has modified a method for growing long, straight, numerous and well-aligned carbon cylinders only a few atoms thick that paves the way for manufacturing reliable electronic nanocircuits.

The team had already described a method last April for growing the crystals, but the modification is targeted at making a process specifically for producing semiconducting versions of the single-walled carbon nanotubes, sometimes called "buckytubes" because their ends, when closed, take the form of soccer ball-shaped carbon-60 molecules known as buckminsterfullerines, or "buckyballs".

The effort is being led by Jie Liu, Duke's Jerry G. and Patricia Crawford Hubbard professor of chemistry.

"I think it's the holy grail for the field," Liu said. "Every piece is now there, including the control of location, orientation and electronic properties all together. We are positioned to make large numbers of electronic devices such as high-current field-effect transistors and sensors."

A report on their achievement, co-authored by Liu and a team of collaborators from his Duke laboratory and Peking University in China, has just been published in the research journal Nano Letters.

What does this portend? Well quite a few things actually. Carbon Nano Tubes (CNTs) are five times as conductive as copper, electron mobility is about 70 times that of silicon and it should be able to withstand much higher temperatures than silicon without losing its semiconducting properties. Not only that, the material is abundant. So once the manufacturing process is worked out it will mean high power, low loss, extremely high speed transistors.

How soon you ask? First off not all the bugs have been worked out in the laboratory models.

That earlier JACS report described how the researchers coaxed nanotubes to form in long, parallel paths that will not cross each other to impede potential electronic performance. Their method grows the nanotubes on a template made of a continuous and unbroken kind of single quartz crystal used in electronic applications. Copper is also used as a growth promoter.

But that method left one unresolved issue blocking the use of such nanotubes as electronic components. Only some of the resulting nanotubes acted electronically as semiconductors. Others were the electronic equivalent of metals. To work in transistors, the nanotubes must all be semiconducting, Liu said.

The researchers now say they have achieved virtually all-semiconductor growth conditions by making one modification.

In their earlier work they had used the alcohol ethanol in the feeder gas to provide carbon atoms as building blocks for the growing nanotubes. In the new work, they describe how they tried various ratios of two alcohols—ethanol and methanol—combined with two other gases they also used previously—argon and hydrogen.

"We found that by using the right combination of the two alcohols with the argon and hydrogen we could grow exclusively semiconducting nanotubes," Liu said. "It was like operating a tuning knob." The inert argon gas was used to provide a steady feed of the ethanol and methanol, with hydrogen to keep the copper catalyst from oxidizing.

After making the nanotubes by chemical vapor deposition in a small furnace set to a temperature of 900°C, the researchers assembled some of them into field-effect transistors to test their electronic properties.

"We have estimated from these measurements that the samples consisted of 95 to 98 percent semiconducting nanotubes," the researchers reported.

Now that is probably good enough for first generation transistors in some applications if those kind of numbers can be achieved in production. However what you want for general use is 99.9% or 99.99% semiconducting CNTs. The more nines the better. So how soon? I'd say pilot production in five years, and full scale production (10s of millions of devices) in about eight years. Fortunately it builds on the base of silicon semiconductor production so the equipment needed is likely to be very similar to what is already in use.

The best power conversion equipment we have using silicon has efficiencies topping out at around 95% with the more typical units running at 85% to 90% efficiency. With these new devices we could reach 99% or better. They could also mean 20 times faster computers that use 1/10th as much power as current devices. Considering that we already have chips on the market that deliver 25,000 MIPS for 360 milliwatts that would be something. It would be roughly equivalent to 1,000 Cray 1s in your pocket that could be powered from an AA cell for a month. Cell phones could run for weeks on a charge. Laptops that could run for many days. Faster please.

Cross Posted at Classical Values

Room Temperature Superconductors?

We normally think of carbon as a high resistance material. The first practical electric light bulbs produced by Edison had carbon filaments. However, there is a new kid on the block based on carbon and it is not a superconductor, but it is close. Some recent research in nanotube properties shows very high current carrying capacities.

Relatively early in the research of nanotubes, Thess et al. calculated the resistivity of ropes of metallic SWNTs to be in the order of 1E-4 ohm-cm at 300 K. They did this by measuring the resistivity directly with a four-point technique. One of their values they measured was 0.34E-4 ohm-cm, which they noted would indicate that the ropes were the most highly conductive carbon fibers known, even factoring in their error in measurement. In the same study his measurements of the conductivity, Frank et al. was able to have reach a current density in the tube greater than 1E7 A/sq cm. Later, Phaedon Avouris suggested that stable current densities of nanotubes could be pushed as high as 1E13 A/cm2.

A SWNT is a Single Walled Nano Tubes.

So how does that compare to copper? For household wiring typical current density is 500A/sq cm and ultimate current density is maybe 10X that with the wires near the melting point or beyond. In round numbers 1E4 A/sq cm vs 1E7 A/sq cm for carbon nanotubes. In other words 1,000 times the current density. At a weight per unit volume of about 1/4 that of copper. Copper resistivity at room temperature is about 1.7E-4 ohm-cm. So carbon nanotubes can carry about 5X as much current as an equivalent volume of copper for the same losses.

If we can get this stuff into mass production - which is likely to take twenty or thirty years - we can rewire the grid we have for 5X times as much power as it handles now or the same power with 1/5th the losses. Not room temperature superconductors, but a definite improvement.

H/T IntLibber at Talk Polywell

Cross Posted at Classical Values