Physicists use commercial graphene for T-wave detection

Graphene-based transistor with a metal grating. Courtesy of the researchers. (Image: MIPT)

https://www.nanowerk.com/nanotechnology-news2/newsid=53081.php?utm_source=feedblitz&utm_medium=FeedBlitzRss&utm_campaign=nanowerkemergingtechnologiesnews

(Nanowerk News) Russian researchers from the Moscow Institute of Physics and Technology (MIPT) and Valiev Institute of Physics and Technology have demonstrated resonant absorption of terahertz radiation in commercially available graphene (check out this graphene infographic). This is an important step toward designing efficient terahertz detectors, which would enable faster internet and a safe replacement for X-ray body scans.

The research findings were published in Physical Review Applied ("Tight-Binding Terahertz Plasmons in Chemical-Vapor-Deposited Graphene").

Graphene optoelectronics

Since Andre Geim and Kostya Novoselov received the 2010 Nobel Prize in physics for studying the unique electronic properties of graphene, the interest toward this material has never waned. Graphene is truly two-dimensional: It consists of a one-atom-thick layer of carbon, which is one of the reasons why its properties are so amazing. It is thin but mechanically strong, impermeable even to helium atoms, and conducts electricity and heat extremely well. The high mobility of electrons in graphene makes it a promising material for ultrafast photodetectors, including those operating in the terahertz range.

THz radiation, also known as T-waves, is equally difficult to generate and to detect. This gave rise to the notion of a “terahertz gap,” which refers to the roughly 0.1-10 THz frequency band in the electromagnetic spectrum. There are no efficient devices for generating and detecting radiation in this range. Nevertheless, T-waves are very important for humanity: They do not harm the body and so could replace X-rays in medical scans. Also, T-waves could make Wi-Fi much faster and unlock a poorly studied band of cosmic radiation for astronomical research.

Despite the great potential of graphene for photodetection, its monolayer by itself absorbs only about 2.3% of external radiation, which is not enough for reliable detection. A way around this is to strongly localize the field near graphene, forcing an electromagnetic wave to couple with graphene electrons and excite resonant oscillations. The resulting collective wave of the electromagnetic field and conduction electrons is known as a surface plasmon. The corresponding phenomenon of plasmon resonance is the enhanced light absorption due to the excitation of surface plasmon waves.

Unfortunately, this phenomenon is not observed in a continuous sheet of a conductor illuminated with plane waves. The plasmon wavelength is too short compared with that of the photon, that’s why these two waves can hardly be synchronous. To address this disparity, a metal grating is placed above the graphene film. It resembles a tiny comb with teeth less than a micrometer apart.

Graphene: Expectations vs. reality

Dozens of techniques are available for producing graphene. They differ in terms of end product quality and labor intensity. Researchers praising the high electron mobility in graphene have often played down how difficult this material is to manufacture.

The highest-quality graphene is produced by mechanical exfoliation. This involves placing a piece of graphite between two sticky tapes, which then rip off progressively thinner layers in multiple iterations. At some point, fragments of graphene — that is, monolayer graphite — emerge. Such “handmade” graphene has the best characteristics for applied devices, such as the resonant T-wave detector based on encapsulated graphene created by researchers from MIPT, Moscow State Pedagogical University, and the University of Manchester (Nature Communications, "Resonant terahertz detection using graphene plasmons").

Unfortunately, graphene flakes manufactured by mechanical exfoliation are only micrometers across, take several months to produce, and end up too expensive for serial device design.

There is an easier and scalable alternative technique for graphene synthesis called chemical vapor deposition (CVD). It involves decomposing gases — normally, a mix of methane, hydrogen, and argon — in a special furnace. The process leads to a graphene film forming on a copper or nickel substrate. The resulting graphene has poorer characteristics and more defects than the mechanically exfoliated one. But CVD is currently the technology best-suited for scaling up device production.

The Russian physicists set out to test whether such commercial-grade graphene is good enough for THz plasmon resonance excitation, which would make it a valid material for T-wave detectors.

“Actually, a CVD-produced graphene film is not homogeneous. Like a polycrystal, it consists of numerous merged grains. Each one is an ordered region with a completely symmetrical atomic pattern. Grain boundaries, along with defects, make working with such graphene far from easy,” study co-author and MIPT graduate student Elena Titova said.

It took the team over a year to master working with CVD graphene at the Institute’s Center of Shared Research Facilities. Meanwhile, the colleagues from the lab’s theoretical department were convinced that no plasmon resonance would be observed. The reason is that resonance visibility is determined by the so-called quality factor — that is, how many periods the field passes before the electron encounters a lattice defect. Theoretical estimates predicted a very low Q factor limited by frequent electron-defect collisions in CVD graphene. That said, the high electron mobility in graphene emerges not due to infrequent electron collisions, but due to a low mass of electrons, which enables their fast acceleration to a high velocity.

Theory and experiment

Despite the pessimistic theoretical predictions, the authors of the paper decided to still do the experiment. Their resolve was rewarded: The absorption spectra exhibited the peaks indicative of plasmon resonance in CVD-synthesized graphene.

“The thing is that not all defects are the same, and electrons collide with different defects in direct current measurements and THz absorption measurements,” comments the research supervisor, Dmitry Svintsov, who heads the MIPT Laboratory of 2D Materials for Optoelectronics. “In a DC experiment, an electron will inevitably encounter grain boundaries on the way from one electrical contact to the other.But when exposed to T-waves, it will mostly fluctuate within a single grain, away from its boundaries. This means that defects impairing DC conductivity are actually ‘safe’ for T-wave detection.”

A further mystery had to do with the frequency of resonant plasmon excitation, which disagreed with the previously existing theories. It turned out to be related to the geometry of the metal grating in an unexpected way. The team found that when positioned close to graphene, the grating (depicted in orange in figure 1) modified the plasmon field distribution. This led to plasmon localization under the “comb teeth,” whose edges acted as mirrors for plasmons. The researchers formulated a very simple theory describing the phenomenon based on an analogy with the tight-binding model from solid-state physics. The theory reproduces the experimental data well without resorting to fitting parameters and can be used to optimize future T-wave detectors.

Source: Moscow Institute of Physics and Technology

The dispute about the origins of terahertz photoresponse in graphene results in a draw

CAPTION Photoresponse in graphene.

 CREDIT Lion_on_helium, MIPT press office

MOSCOW INSTITUTE OF PHYSICS AND TECHNOLOGY

https://www.eurekalert.org/pub_releases/2018-04/miop-tda042418.php

Physicists at MIPT and their British and Russian colleagues revealed the mechanisms leading to photocurrent in graphene under terahertz radiation. The paper published in Applied Physics Letters not only puts a period to a long-lasting debate about the origins of direct current in graphene illuminated by high-frequency radiation but also sets the stage for the development of high-sensitivity terahertz detectors. Such detectors are highly demanded in medical diagnostics, wireless communications and security systems.

In 2005 MIPT alumni Andre Geim and Konstantin Novoselov experimentally studied the behavior of electrons in graphene, a flat honeycomb lattice of carbon atoms. They found that electrons in graphene respond to electromagnetic radiation with an energy of quantum, whereas the common semiconductors have an energy threshold below which the material does not respond to light at all. However, the direction of electron motion in graphene exposed to radiation has long remained a point of controversy, as there is a plenty of factors pulling it in different directions. The controversy was especially stark in the case of the photocurrent caused by terahertz radiation.

What sets terahertz radiation apart is its unique set of properties. As an example, it easily passes through many dielectrics without ionizing them: this is of particular value to medical diagnostic or security systems. A terahertz camera can "see" the weapons concealed under a person's clothes, and a medical scanner can detect skin diseases at early stages by the spectral lines ("fingerprints") of characteristic biomolecules in the terahertz range. Finally, raising the carrier frequency of Wi-Fi devices from several to hundreds of gigahertz (into the sub-terahertz range) will proportionally increase the bandwidth. But all these applications need a sensitive and low-noise terahertz detector which is simple in fabrication.

A terahertz detector designed by researchers at MIPT, MSPU and the University of Manchester (the place where graphene was first discovered) is a graphene sheet (colored green in figures 1 and 2) sandwiched between dielectric layers of boron nitride and electrically coupled to a terahertz antenna--a metal spiral about a millimeter in size. As radiation impinges on the antenna, it rocks electrons on one side of the graphene sheet, while the resulting direct current is measured on the other side. It is the "packing" of graphene into boron nitride that enables record-high electric characteristics, giving the detector a sensitivity that is a cut above the earlier designs. However, the main result of the research is not a better-performing instrument; it is the insight into the physical phenomena responsible for the photocurrent.

There are three main effects leading to the electric current flowing in graphene exposed to terahertz radiation. The first one, the photothermoelectric effect, is due to the temperature difference between the antenna terminal and the sensing terminal. This sends electrons from the hot terminal to the cold one, like air rising up from a warm radiator up to cold ceiling. The second effect is the rectification of current at the terminals: it turns out that the edges of graphene let through only the high-frequency signal of a certain polarity. The third and most interesting effect is called plasma wave rectification. We can think of the antenna terminal as stirring up "waves in the electronic sea" of the graphene strip, while the sensing terminal registers the average current associated with these waves.

"Earlier attempts to explain the photocurrent in such detectors used only one of these mechanisms and excluded all the others," says Dmitry Svintsov, head of the Laboratory of 2d Materials' Optoelectronics at MIPT. "In reality, all three of them are at play, and our study found which effect dominates at which conditions. Thermoelectric effects dominate at low temperatures, while plasmonic rectification prevails at high temperatures and in longer-channel instruments. And the main thing is that we figured out how to make a detector in which the different photoresponse mechanisms will not cancel each other, but rather reinforce each other"

These experiments will help choose the best design for terahertz detectors and bring us closer to remote detection of dangerous substances, safe medical diagnostics, and high-speed wireless communications.

###

The work was supported by the Russian Science Foundation, the Ministry of Education and Science of the Russian Federation, the Leverhulme Trust (Great Britain) and the Russian Foundation for Basic Research

Graphene brings tuneable properties to terahertz devices

15th January 2016 12:08 pm

http://www.theengineer.co.uk/graphene-brings-tuneable-properties-to-terahertz-devices/

Researchers at Manchester University have found that graphene can be used to control the frequency of terahertz lasers, a development that may lead to safer alternatives to X-rays.

The benefit of using terahertz lasers instead of X-rays is that they can accurately scan materials without causing damage. Furthermore, by utilising the electrical properties of graphene, highly sensitive scanners for sensing minute defects in manufactured drugs, or detecting concealed weapons at airports could become more accurate and much safer for frequent use using the new technology.

The researchers, led by Dr Subhasish Chakraborty and Sir Kostya Novoselov, have reported their findings – Tuning THz lasers via graphene plasmons – in Science.

“Terahertz technologies have been somewhat ignored by industry, partly due to the complexity of creating high-power tuneable devices, as lots of different materials were necessary,” said Dr Chakraborty. “Graphene can now fill in the gaps of this technology by creating a platform to electronically control devices and flexibly engineer device output.”

According to Manchester University, the introduction of tuneable properties thanks to graphene could also potentially increase internet bandwidth capabilities up to and beyond one terabyte per second.

Sir Kostya said: “Current terahertz devices do not allow for tuneable properties, a new device would have to be made each time requirements changed, making them unattractive on an industrial scale.

“Graphene however, can allow for terahertz devices to be switched on and off, as well as altering their state.”

Graphene – a one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice – was discovered at Manchester University in 2004.

Pioneering work with graphene at Manchester University led to Sir Kostya and colleague Prof Andre Geim being awarded the Nobel Prize for Physics in 2010.

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Graphene Global Research and Technology Hub to be established in the UK

http://www.manchester.ac.uk/aboutus/news/display/?id=7504

The world's thinnest, strongest and most conductive material, discovered in 2004 at the University of Manchester by Professor Andre Geim and Professor Kostya Novoselov, has the potential to revolutionize material science. Demonstrating the remarkable properties of graphene won the two scientists the Nobel Prize for Physics last year and Chancellor of the Exchequer George Osborne has just announced plans for a £50m graphene research hub to be set up.Now, writing in the journal Nature Physics, the University of Manchester team have for the first time demonstrated how graphene inside electronic circuits will probably look like in the future.­By sandwiching two sheets of graphene with another two-dimensional material, boron nitrate, the team created the graphene 'Big Mac' - a four-layered structure which could be the key to replacing the silicon chip in computers.Because there are two layers of graphene completed surrounded by the boron nitrate, this has allowed the researchers for the first time to observe how graphene behaves when unaffected by the environment.Dr Leonid Ponomarenko, the leading author on the paper, said: "Creating the multilayer structure has allowed us to isolate graphene from negative influence of the environment and control graphene's electronic properties in a way it was impossible before."So far people have never seen graphene as an insulator unless it has been purposefully damaged, but here high-quality graphene becomes an insulator for the first time."The two layers of boron nitrate are used not only to separate two graphene layers but also to see how graphene reacts when it is completely encapsulated by another material.Professor Geim said: "We are constantly looking at new ways of demonstrating and improving the remarkable properties of graphene.""Leaving the new physics we report aside, technologically important is our demonstration that graphene encapsulated within boron nitride offers the best and most advanced platform for future graphene electronics. It solves several nasty issues about graphene's stability and quality that were hanging for long time as dark clouds over the future road for graphene electronics.We did this on a small scale but the experience shows that everything with graphene can be scaled up.""It could be only a matter of several months before we have encapsulated graphene transistors with characteristics better than previously demonstrated."Graphene is a novel two-dimensional material which can be seen as a monolayer of carbon atoms arranged in a hexagonal lattice.Its remarkable properties could lead to bendy, touch screen phones and computers, lighter aircraft, wallpaper-thin HD TV sets and superfast internet connections, to name but a few.The £50m Graphene Global Research and Technology Hub will be set up by the Government to commercialise graphene. Institutions will be able to bid for the money via the Engineering and Physical Sciences Research Council (EPSRC) - who funded work leading to the award of the Nobel prize long before the applications were realised.­