Scientists Unleash “Designer Electrons”; Synthesize Graphene Atom-By Atom

Stanford University researchers have come up with ‘designer electrons’  – electrons whose properties can be controlled and fine-tuned. Leading the research is Hari Manoharan, associate professor at the Stanford Physics department. He reports that his group has created graphene-like electrons and then tweaked them!

The honeycomb potential

Why is this important? Let Prof. Manoharan answer that question:

The behavior of electrons in materials is at the heart of essentially all of today’s technologies. We’re now able to tune the fundamental properties of electrons so they behave in ways rarely seen in ordinary materials.

Working with individual molecules

So, this is what they did. They placed carbon monoxide molecules on an atomically smooth copper substrate using a Scanning Tunneling Microscope (STM), which then created the potential mimicking that of the carbon atoms in graphene, forcing the electrons to behave like those of graphene. The peculiarity of graphene electrons is that, within graphene, they act as if they have no mass. They thus move at the maximum speed possible within the substrate. Manoharan calls this ‘molecular graphene’, built from scratch atom-by-atom.

Making electrons go crazy – at will

Manoharan’s group could then tweak the potential properties by changing the positions of the Carbon Monoxide molecules and then the electrons suddenly gained mass! More than that, they could make the electrons behave as if they were in an electric or magnetic field. Says Manoharan:

One of the wildest things we did was to make the electrons think they are in a huge magnetic field when, in fact, no real field had been applied

The team is now looking to synthesize more semiconductor substances and also replicate certain properties of graphene. The team promises that more ‘Dirac materials’ are on the way! We live in exciting times.

The work is featured in Nature and it appeared yesterday. Here is the link: http://www.nature.com/nature/journal/v483/n7389/full/nature10941.html

More info from Stanford news: http://news.stanford.edu/news/2012/march/molecules-designer-electrons-031412.html

Also, do watch the video below, where the group explains their work. You do NOT want to miss this.

Novel New Material Graphyne Can Be A Serious Competitor To Graphene

The carbon nanostructure revolution refuses to cease. First, it was carbon nanotubes, followed by graphene. After these two “hot” materials, it may now be the dawning of another wonder material called ‘Graphyne’. Graphyne may surpass even graphene in its electrical properties. While graphyne has been researched for the last 30 years, it has suddenly become a hot material for condensed matter physics.

An artist's impression of graphene

Graphene is known for its extremely high conductivity owing to a peculiar property of graphene electrons. In graphene, the so-called valence and conduction bands touch. Near the points where the two bands touch, called the Dirac point, the energy-momentum relation of the electrons is linear (graph, right), instead of quadratic as seen for other particles. This leads to the mass of the charge carriers (electrons or holes) inside the material being effectively zero. This allows them to travel at extremely high speeds, giving rise to very high mobilities and superior conduction properties. It was precisely this that led to the 2010 Nobel Prize being awarded to Andre Geim and his student Konstantin Novoselov.

Graphene and Graphyne

Graphyne doesn’t really exist; it has to be synthesized using special techniques. However, computer simulations show that its conduction properties can be better than graphene. Graphyne is a 2D lattice, just like graphene, but with double and triple bonds, rather than just single bonds as it is with graphene. The graphene lattice is strictly hexagonal, while graphyne lattice can take up an arbitrary shape due to the presence of the double and triple bonds. In particular, it can take up a rectangular lattice shape.

The key to good conduction is not only high mobility of the electrons, but also directionality. The electrons should be free to travel in a straight line. For graphene, the lattice has no preferred direction, but for graphyne, the lattice being rectangular, prefers conduction in one direction over the other. This means that it has gating properties depending on the direction of passage of current.

6,6,12- Graphyne

Simulations and predictions

A recent paper in Physical Review Letters, by Andreas Gorling and colleagues (link) presents simulations of electronic properties of graphyne. They discuss the so-called 6,6,12-graphyne (pic above) and simulate its properties. Density functional simulations predict the presence of Dirac cones in graphyne, which were thought to be unique to graphene. Moreover, the conduction turns out to be superior to graphene.

We should stress the fact that graphyne has not been made in the laboratory in significant quantities as yet; only trace amounts have been fabricated. Only proper experiments on real samples can verify the simulation results, but Mikhail Katsnelson, a big name in the field of graphene physics, expresses confidence in the density functional methods. The next step would be to prepare proper graphyne samples for study. Only then can all the fancy experimental tests be applied.

New material on the block and a lot of new physics to be known – it’s a mouth-watering prospect for physicists.

More about the mechanical properties of graphyne: http://web.mit.edu/mbuehler/www/papers/Carbon_2011.pdf

Fabricated: The World’s First Two-Dimensional Glass

Researchers from the Cornell University and the University of Ulm have created the world’s thinnest pane of glass – and all this due to a failed attempt at making graphene. The glass is a mere 3 atoms thick! It’s the world’s first pseudo two-dimensional glass sheet.

The image as seen by the electron microscope. Inset: What the theorist had predicted about glass's structure in 1932

Making Graphene… err Glass

Graphene is a novel material, which is just one atom thick. It is basically one atomic layer of graphite, having honeycomb shaped lattices, with carbon atoms at each of the lattice points. The researchers were trying to synthesize graphene, using a technique called Chemical Vapour Deposition (CVD). They were trying to make a graphene sheet on a copper-covered quartz layer, which is a standard technique. What they found instead was a glass layer that had formed alongwith the graphene layer.

The scientists believe that an air leak allowed the copper to react with the quartz in the presence of oxygen and under high temperature. This led to the creation of a very thin layer of glass just 3 atoms in thickness.

A pleasant surprise

The greater surprise – and a pleasant one – emerged from the fact the glass structure looks like what a theorist had actually envisioned way back in 1932. Look at the inset figure. It clearly shows a few honey-comb structures alongwith many irregular ones. While oxygen appears white, silicon is marked in black. The use of ultra-thin glass sheets is innumerable. It can be used as miniature dielectric layers. Furthermore, this ‘accident’ reveals how such thin glass sheets can be produced.

The scientists have published this finding in Nano Letters.

Ecole Makes World’s First Molybdenite Chip

After Silicon and Graphene, it might be the turn of Molybdenite to steal the limelight. Scientists at  Ã‰cole Polytechnique Fédérale de Lausanne,  or EPFL, have synthesized the world’s first Molybdenite chip. It has a number of advantages over the conventional silicon chip, including greater energy efficiency and integration of transistors on a larger scale.

The Molybdenite Chip (Courtesy: EPFL, 2011)

Molybdenite vs Silicon vs Graphene!

Molybdenum disulphide, or MoS2, is found much more widely than Silicon. It has high flexibility and also good semi-conducting properties. The greatest advantage of Molybdenite is that it allows for drastic miniaturization. Silicon cannot be made less than 2 nm thick, since, on further thinning, it oxidizes and the surface properties are lost. Molybdenite can be made as thin as 3 atomic layers.

Molybdenite can even rival graphene. The main problem with Graphene is the lack of any natural bandgap. Silicon is extremely convenient in this respect with a 0.7 V bandgap at room temperature. Molybdenite has no such bandgap problems. Though electronic mobility for molybdenite is much less than that of Graphene, for normal circuitry (i.e. not RF circuits) Molybdenite can be easily implemented. The switching speed is much higher than silicon transistors, but less than Graphene. However, the on-off voltage ratio is much higher for molybdenite than for Graphene, making it a better switch, if the switching operations need not be very fast.

All of this coupled with the obvious amplification properties, make Molybdenite a good option for future electronics.

The flexibility of Molybdenite inspires flexible sheets of chips that can be strapped onto a human hand! How’s that for a futuristic vision?

Transparent Loudspeakers Made From Graphene Using Inkjet Printing Technology

Researchers at Seoul University have come up with the utmost innovation in sound technology transparent loud speakers, made using Graphene. The team used a special kind of plastic material called Poly Vinylidene Flouride (PVDF), on which a layer of Graphene Oxide was printed’ in order to achieve this.

Arrangement of Graphitic layers. Graphene is simply one graphite layer.

Graphene is a single layer of carbon (picture above), manufactured by industrial methods like Carbon Vapour Deposition or by simply stripping away at Graphite using Scotch Tape. It is the material, hot and happening, in today’s material science research.

The Ink

Graphene Oxide was used as the ink. Prepared using known and tested methods, the synthesized Graphene oxide was filled in an empty inkjet printer cartridge. This would be the ink’ for printing on the PVDF.

The PVDF

The PVDF was treated using low oxygen plasma treatment, so that the surface is amiable for printing’.

The technology used for printing was the regular inkjet printer technology. The moment two layers of Graphene oxide were printed’ uniformly on the two sides of the PVDF, the entire material behaved like an electrolyte and the Graphene layer acted as the electrodes. Dipping the printed sheet of PVDF into hydrazine and ammonia solution completed the printing process.

A sheet of graphene

The rest of the process is straight forward. Regular digital pulses of electricity excite the PVDF sheet and due to the piezoelectric effect (or its inverse, if you prefer), the PVDF sheet bends in specific ways so as to produce sound waves.

Where might this be useful?

The applications are immense. Soon, one might have screens with a thin PVDF-Graphene layer, doubling up as the primary speakers on his/her laptop. Giant screens would be their own audio sources. Even the car windshields or windows might double as the entertainment devices. There is also the huge possibility of inducing anti-noise vibrations, making these PVDF speakers perfect for noise reduction.

The good news is that these are extremely inexpensive and quite durable. The bad news is that the sound quality needs a lot to be desired, especially at low scales.

Wonder material Graphene does it again, but there are still chinks to iron out. Graphene is hot, really hot. Is that loud and clear?

Revolution in Communication: Berkeley Scientists Create World’s Smallest, Fastest Optical Modulator Using Graphene

The wonder material Graphene continues to amaze. A research team, headed by Xiang Zhang, a UC Berkeley engineering professor, has built an ultra-small optical device that can control the switching on and off of light pulses, using Graphene. This extraordinary device is guaranteed to revolutionise communication, both in terms of speed and how we do it, in the very near future.

Graphene is a one-atom thick sheet of carbon atoms arranged in a hexagonal pattern the so called sp2 hybridized structure. (Read more about Graphene here). Graphene can be switched on and off extremely fast and this is the property exploited here. According to an externally applied voltage, Graphene can modulate pulses of light by letting some go through and restricting some others.

An artist's impression of Graphene

But why Graphene?

The modulator will not only be fast, it will be very compact. It only takes a 25 micron (that’s roughly 400 times thinner than a human hair) a side square of Graphene to make this modulator. Graphene also has the added advantage of supporting a huge bandwidth of optical frequencies. Any frequency of light ranging across the optical range up to ultra-violet and down to even infra-red can be effectively modulated. Graphene is also quite cheap to produce, especially with improved techniques like Chemical Vapor Deposition (CVD). It can be easily integrated with other materials, like Silicon, without bothering much about contamination. Graphene has the tremendous advantage of being highly conducting in pure form. It needs no doping, unlike Silicon. Further, its conductivity remains constant down to very low temperatures, like a few Kelvin.

A disk of Graphene
A disk of Graphene (coated with an oxide)

How the modulation happens: Interplay between electrons and light

UC Berkeley researchers were attracted by the behavior of electrons and light within Graphene, especially their interaction. Here’s the watered down version of the technicalities: The electrons that matter are called valence electrons. They lie near the top of a so-called valence band, having energy equal to that of the Fermi level (the level till which the electrons are filled here the top of the valence band). Electrons can jump from just below the Fermi level to above the level by absorbing light. The Fermi level can, however, be varied by an external voltage. Given enough negative voltage, electrons can be drawn out of the bands completely or packed tight (Technically, external voltage alters the Fermi level). In both these cases, light cannot be absorbed by the electrons (either because they are absent or because they have no place to jump to). The material is then transparent to light. Researchers hit upon the Goldilocks voltage range’ in which Graphene is opaque and used it to turn the modulator off. Due to the high mobility of electrons in Graphene, a square voltage pulse with Gigahertz frequency easily dumps Graphene in and out of the opaque state, effectively modulating the transmission of light pulses. This is the first time light has been modified and guided at such small scales. Generally, light requires bulky mirrors or photonic crystals.

The Schematic for Graphene modulatoes
Schematic for Graphene Modulators. The honeycomb layer is the Graphene which sits on top of the Silicon waveguide. Pulses of light are sent through the waveguide and according to the voltage applied (the square pulses on the left), light is either transmitted or absorbed. (Courtesy: UC Berkeley)

The team put a Graphene layer on top of a Silicon waveguide (images above and below). They were able to achieve 1GHz modulation speed. Theoretically, 500 GHz is possible, so the 1 GHz figure will definitely be revised.

An image showing the actual fabrication (Courtesy: UC Berkeley)

The Future … is Here

In the near future, Zhang says, Instead of broadband, we will have ‘extremeband’, because the bandwidth offered will be huge, up to 10 nanometers (above 1000 Terahertz).

So there is another glimpse of the future, courtesy Graphene: It’s super-fast, super-small, cheap and offers huge bandwidth. Hope you download a lot of HD movies in 3D on your phone in future it’ll, after all, take just a few seconds.

IBM Creates World’s Fastest Transistor Using Graphene

IBM on Tuesday, 12th April, announced that they have made the world’s fastest transistor using graphene and also hinted that they might go into commercial production very soon. This is major news, as graphene might revolutionize the current semi-conductor industry scenario. Graphene may even be good enough to replace silicon, the standard material used in all of today’s semi-conductor devices, in the near future.

What is Graphene? How is it produced?

Graphene is a mono-layer of carbon, with the atoms in hexagonal configuration. Each of the carbon atoms has bonds with three neighboring carbon atoms, maintaining, what is called, a sp2 hybridization. Basically, it is one layer of graphite.

Graphite Structure
The Structure of Graphite

It is produced in the most mundane way you can think of. A pure crystal of graphite is repeatedly stripped off using Scotch Tape, until, about 50 or more repetitions later, graphene is found buried amongst poly- or bi-layered graphite (Pic below). This has to be verified, and can be done so optically, after the extracted graphene is mounted on a Silicon Dioxide (SiO2) substrate of correct thickness. Often, Raman spectroscopy is used for verification.

Graphene under light
How Graphene looks under light and the right thickness of Si layer. (Courtesy: Graphene Industries)

Other methods, which allow graphene to be grown for commercial purposes, are also known. Primary among these is Chemical Vapour Deposition (CVD), in which carbon vapour (obtained from carbon rich substances like acetylene) is deposited on a Ni or Cu substrate.

 

How did IBM do it?

Graphene grown directly on a SiO2 substrate suffers from the problem of scattering of electrons, resulting in the deterioration of the transport properties and also producing non-uniformity across the SiO2 wafer. The IBM team used a novel substrate called Diamond-like Carbon’ (DLC) on top of the SiO2 layer so as to reduce the scattering. DLC is loosely amorphous (i.e. powdered) diamond. It has all atoms in sp3 configuration (i.e. each atom bonds to four neighbors) and tetrahedral arrangement, but, being a powder, lacks any fracture planes. Thus, having the necessary properties of diamond, it is also flexible and can easily be used as a coating over a substrate.

Graphene couples weakly to the DLC layer and this greatly reduces the scattering, as also the temperature dependence of the material. In fact, the transport properties of DLC-grown graphene remains almost (maybe, exactly) temperature independent right up to 4.3K (which is minus 269 Centigrade).

The IBM team took graphene, made using CVD on a Cu layer, and, after protecting with polymethylmethacrylate (PMMA), dissolved the Cu layer using FeCl3. Then, the PMMA-graphene was transferred to a DLC layer on the SiO2 substrate and the PMMA was got rid of. Raman spectroscopy was used to verify the quality of the graphene layer.

Graphene lends itself readily to fabrication of Field Effect Transistors (FETs). By constructing the gate, drain and source contacts using pure metal and properly calibrating their device, the IBM team achieved input-output characteristics similar to a Si FET. Further, they achieved very high switching speeds up to 26 GHz for a 550 nm long device.

 

Arrangement of a FET Graphene transistor
Schematic representation of a graphene FET (Courtesy: Nature)

What a graphene transistor actually looks like

How will it score over Si transistors?

Graphene transistors will be ideal in radio frequency (RF) range signals, due to their high switching speeds. Unlike Si transistors, their properties don’t degrade at low temperatures. This means that there will be no unnatural change in transport properties as the temperature is varied.

The problem graphene transistors have is that they have low on-off voltage ratio. However, that is not a very strict condition for RF communication. A more serious problem is that of high contact resistance, which cannot be minimized, unlike MOSFET‘s in case of Si.

 

Conclusion:

IBM reports that transistors with cut-off frequencies (the frequency at which the current gain becomes unity) as high as 155 GHz have been achieved on a 40 nm device using short gate lengths.

A figure of merit is the product of cut-off frequency and gate length. IBM reports the figure of 13 GHz mm for the 550 nm device, which beats the highest value of 9 GHz mm for Si MOSFETS by a long margin.

The future may be here already.