Building Devices Remains the Name of the Game in 2D Material Research
Graphene and 2D materials only matter if you can make something useful from them
While the properties of graphene and other 2D materials and the methods for producing them continue to expand, the real aim is make devices that perform better than those we have today. They can be devices for electronics, energy storage or generation or medical devices. The number of potential applications is enormous, but fabricating a device from these new emerging materials is a challenge.
This challenge is even more acute when you recognize that they need to not only compete with today’s current technologies and materials that have been here for generations, but they need to outperform them on everything from capabilities to cost.
The range of devices better enabled by graphene and other 2D materials over the last quarter is quite diverse. Everything from better enabling next-generation vehicles to better microphones are represented, and, of course, there is the ever present proposed improvements to electronics.
Graphene and Nanotubes Join Forces to Tackle the Digital Switch
Separately, graphene and nanotubes have struggled to have an impact on electronics. Their respective issues are fairly different from one another: graphene lacks an inherent band gap and a single batch of nanotubes can have a mix of the metallic and semiconductor variety making the necessary uniformity a challenge. So, maybe together they can fill in for the weaknesses of the other.
At least that’s what researchers at the Michigan Technological University (MTU) believed and their belief led them create digital switches by making a sandwich of nanotubes and graphene.
“When we put these two aliens together, we create something better,” said Yoke Khin Yap, a professor at MTU, in a press release. “When we put them together, you form a band gap mismatch—that creates a so-called ‘potential barrier’ that stops electrons.”
The trick for the researchers was how to bring these two nanomaterials together. In research published in the journal Scientific Reports, were able to achieve this pairing by exfoliating the graphene in such a way that its surface was covered in small holes. It was inside these holes on the surface of the graphene that the researchers grew the nanotubes.
What this combination achieves is to bring the extraordinary conductivity of graphene with the ability to slow down the electrons with the boron nitride nanotubes. So what happens is that the electrons pass along the surface of the graphene until they come up to the hair-like sprouts of the nanotubes. The huge band gap of the boron nitride nanotubes makes the semiconductor act almost like an insulator.
It is at these points of where the nanotubes sprout up out of the graphene—points known as heterojunctions—that the digital switches become possible.
“Imagine the electrons are like cars driving across a smooth track,” Yap said in the release. “They circle around and around, but then they come to a staircase and are forced to stop.”
One of the aims of a good electronic switch is that it can turn on and off very quickly. The MTU researchers claim that this design is several orders of magnitude faster than current graphene switches.
Ultrasonic Microphones Made From Graphene
Inspired by the sonar-locating hearing capabilities of bats, researchers at the University of California, Berkeley have developed tiny ultrasonic microphones made from graphene that in combination with an ultrasonic radio could be used for wireless communication.
Graphene is extremely sensitive to a wide-range of frequencies. As a result, the graphene-enabled microphone can pick up frequencies from across the human hearing range—from subsonic (below 20 hertz) to ultrasonic (above 20 kilohertz)—and as high as 500 kHz. (A bat hears in the 9 kHz to 200 kHz range.)
While graphene struggles to meet the demands of electronic applications, this particular use may be fairly easy to achieve on the commercial level, according to the researchers.
“There’s a lot of talk about using graphene in electronics and small nanoscale devices, but they’re all a ways away,” said UC Berkeley physicist, Alex Zettl, in the press release. “The microphone and loudspeaker are some of the closest devices to commercial viability, because we’ve worked out how to make the graphene and mount it, and it’s easy to scale up.”
Is Graphene the Answer for Thermoelectric Materials for Next-Generation Vehicles?
Something called the thermoelectric effect has been tantalizing researchers for a while. Essentially the thermoelectric effect occurs when you have a material that on one side is quite hot and on the other side quite cool. This difference in temperature generates an electrical current. So, for instance, the difference in temperature on your laptop’s surfaces could generate a current that could keep it charged up.
That’s the idea at least. The problem has been that the available materials for exploiting this effect had poor thermoelectric conversion efficiency or were prohibitively expensive for commercial use.
A joint industry and academia research project between the University of Manchester in the U.K. and the company European Thermodynamics Ltd. have suggested that graphene may be the material to finally make the thermoelectric effect commercially viable.
The joint academic-industrial team reported in the journal Applied Materials and Interfaces a method in which they added a small amount of graphene to strontium titanium dioxide (STO), a thermoelectric material that, by itself, generates a current only at extremely high temperatures. The researchers discovered that the graphene made it possible for the STO to operate at room temperature.
“Current oxide thermoelectric materials are limited by their operating temperatures which can be around 700 degrees Celsius,” said Robert Freer, one of the lead University of Manchester researchers, in a press release. “This has been a problem which has hampered efforts to improve efficiency by utilizing heat energy waste for some time.”
The researchers have also shown that the graphene-enabled thermoelectric material has significantly better conversion efficiencies. The new material can convert 3 to 5 percent of the heat into electricity, a big bump from the 1 percent of other materials.
Based on this increased conversion efficiency, the researchers estimate that since a vehicle loses 70 percent of the energy in fuel via waste heat and friction, applying this material for improved thermal energy recovery will lead to a substantial boost in energy efficiency.
Graphene-Based Magnetic Sensors Outperform Silicon Variety
This past June, the European Commission’s $1-billion investment in graphene research, The Graphene Flagship, held a conference entitled Graphene Week 2015 where quite a stir was created by the presentation of a research paper that reported on a graphene-based magnetic sensor that is 100 times more sensitive than the silicon variety.
In joint research between industry and academia, a research team from the German company Bosch and the Max-Planck Institute for Solid State Research haved graphene sensors are based on the Hall effect, in which a magnetic field focused on a conductor causes a Lorentz force that deflects charge carriers in a current and leads to a measurable voltage.
Since Bosch was interested in pursuing this research for the purpose of creating a commercial product they needed to come up with a cost-effective way of producing the graphene. So, top-down techniques such as mechanical and chemical exfoliation were immediately taken off the table. Instead, Bosch looked at bottom-up techniques such as the thermal decomposition of silicon carbide, and chemical vapor deposition onto metal surfaces.
The research team is not really expecting that these sensors are going to be available in the next 5-10 years. Nonetheless they expect them to perform well with performance parameters at least being equal to today’s silicon-based sensors. In their more optimistic outlook, they believe that the graphene-based sensors will be two orders of magnitude more sensitive than its silicon-based counterparts.
Wrapping Up Wires With Graphene Boosts Chip Speeds
While the coatings for copper wires in silicon chips may not rise to the level of a “device”, in terms of their impact on how these devices operate they can have an enormous influence.
As such, researchers at Stanford University have demonstrated that by wrapping graphene around the copper wires of computer chips instead of tantalum nitride boosts speeds in the chips by 30 percent.
“Researchers have made tremendous advances on all of the other components in chips, but recently there hasn’t been much progress on improving the performance of the wires,” said H.-S Wong, who led the research, in a press release.
The researchers concede that if you replaced the tantalum nitride wrappings with graphene ones on the copper wires in today’s computer chips, the improvements would be modest—in the range of four to 17 percent faster wire speeds. However, as transistors and chip dimensions inevitably continue to shrink, the researchers believe this new graphene sheathing could increase wire speeds by 30 percent within the next two generations of computer chips.
Wong added: “Graphene has been promised to benefit the electronics industry for a long time, and using it as a copper barrier is perhaps the first realization of this promise.”
Graphene Enables World’s Thinnest Light Bulb
In our last quarter’s newsletter we covered the announcement that graphene was going to be hitting store shelves in the form of coatings for LED light bulbs.
While we still await whether graphene-enabled LED bulbs will make a big commercial splash, researchers at the Columbia University in cooperation with a team at Korea Research Institute of Standards and Science (KRISS) have created the first on-chip incandescent visible light source using graphene as the filament.
“We've created what is essentially the world's thinnest light bulb,” said James Hone, a professor at Columbia Engineering, in a press release. “This new type of 'broadband' light emitter can be integrated into chips and will pave the way towards the realization of atomically thin, flexible, and transparent displays, and graphene-based on-chip optical communications.”
The international research team reported in the journal Nature Nanotechnology, suspending graphene above a silicon substrate by attaching it to two metal electrodes and then passing current through the graphene-based filament, causing it to heat up.
This research should be significant for so-called integrated photonic circuits in which photons are used in place of electrons for processing information. For integrated photonic circuits to operate it sometimes is necessary to shine light onto the chip itself. This research marks the first time that anyone has been able to incandescent light to serve this purpose.
The reason incandescent light has never been used previously is because it produces so much heat. Attempts to remove that level of heat from the chips had always resulted in damaging them.
Graphene was able to address this issue because of its odd property of becoming an increasingly worse conductor of heat the hotter it gets. As a result, the lack of heat conduction confined the heat to a very small hot spot in the middle of the graphene filament.