Graphene and 2D Materials Are Not Just for Digital Logic Applications
The range of applications for graphene in 2D materials in electronics extends far beyond just digital logic
While much research effort is devoted to seeing if graphene can replace silicon as the basis for the next generation of computer chips, this is not the only potential application of graphene in the broader field of electronics.
In addition to its use in transistors for digital logic, graphene is also being investigated for use in flexible electronics where graphene can enable conductive inks and can serve as a replacement for indium tin oxide (ITO) as the transparent conductors in these devices. There are also the more workhorse roles as an alternative material for interconnects and heat sinks.
Graphene Makes Possible the First Textile Electrode
Along the lines of flexible electronics, a collaborative research team of scientists from the University of Exeter in the U.K. and the Institute for Systems Engineering and Computers, Microsystems and Nanotechnology (INESC-MN) in Lisbon, the Universities of Lisbon and Aveiro in Portugal and the Belgian Textile Research Centre (CenTexBel) used graphene to develop what’s being dubbed the first textile electrode.
In a research published in the journal Scientific Reports, the international team of researchers used chemical vapor deposition (CVD) to fabricate monolayer graphene.
Typically, CVD methods for manufacturing graphene are compromised when the graphene that has been grown on top of a copper substrate in an oven has to be peeled off. This research developed a way to peel the graphene off of the copper sheet and transfer it to a yarn without compromising the electronic properties of the graphene.
“The methodology that we have developed to prepare transparent and conductive textile fibers by coating them with graphene will now open the way to the integration of electronic devices on these textile fibers,” said Ana Neves, currently a researcher at Exeter and previously a researcher at INESC, in a press release.
The researchers envision this process enabling a range of potential applications, including textile GPS systems, biomedical monitoring, personal security or even communication tools for those who are sensory impaired.
Graphene Coating Improves Copper Nanowires for Use in Flexible Electronics
In research that sort of bridges flexible displays with interconnects, researchers at Purdue University have coated copper nanowires with graphene to lower the resistance and susceptibility to heating of the copper wires. This research could allow copper wires to be used in a range of electronics, including the flexible variety.
“Highly conductive copper nanowires are essential for efficient data transfer and heat conduction in many applications like high-performance semiconductor chips and transparent displays,” said Purdue doctoral student Ruchit Mehta in a press release.
In research published in the journal Nano Letters, the Purdue team developed a method for encapsulating the wires with graphene. The results were striking. The encapsulated wires can transmit data 15 percent faster while reducing the peak temperature by 27 percent compared to uncoated wires.
“This is compelling evidence for improved speed and thermal management by adapting the copper-graphene hybrid technology in future silicon chips and flexible electronic applications,” Mehta added in the release.
This is not the first time that the potential for coating nanowires with graphene has been investigated. But in previous inquiries, the process was too difficult because the CVD process, which operates at 1000 degrees Celsius, can ruin both copper thin films and small-dimension wires.
The key development in this research was to use a plasma-enhanced CVD process that can be run at a lower temperature of 650 degrees Celsius, which keeps the small wires intact.
New Tool Should Aid the Use of Graphene in Thermal Management
As the circuit densities and clock speeds of chips are steadily rising, thermal management issues have becoming increasingly paramount. Because of graphene’s high thermal conductivity it has been investigated as a potential solution to some of these thermal management issues.
Now researchers at EPFL (École Polytechnique Fédérale de Lausanne) have brought that promise of graphene one step closer with the discovery that heat propagates in the form of a wave, just like sound in air.
“We can show that the thermal transport is described by waves, not only in graphene but also in other materials that have not been studied yet,” explained Andrea Cepellotti, the first author of the report, in a press release. “This is extremely valuable information for engineers, who could adapt the design of future electronic components using some of these novel two-dimensional materials’ properties.”
While this work, which was published in the journal Nature Communications, is still just computer models, it should help engineers better understand the mechanisms of thermal conductivity in graphene and other two-dimensional materials, and become a valuable tool for those who are looking to use graphene for thermal management solutions.
Molybdenum Disulfide Enables Next Step in Memristor Design
Graphene’s two-dimensional cousin molybdenum disulfide (MoS2) has offered a new way to pursue the potential for two-terminal non-volatile memory devices based on resistance switching, known in some circles as the memristor.
The memristor, or memory resistor, was theorized back in the 1970s to be the fourth fundamental electronic component, joining the resistor, the capacitor, and the inductor. According to the theory, a memristor would provide a similar relationship between magnetic flux and charge that a resistor gives between voltage and current.
In real world terms that would translate into a device that behaved like a resistor and whose value could vary according to the current passing through it and which would remember that value even after the current disappeared. It could serve as a kind of non-volatile memory device.
The potential of memristor devices looked very attractive to researchers at Northwestern University who saw that it could serve as a fundamental component in future computers that could mimic the neurons in the human brain.
To squeeze that potential out of two-terminal non-volatile memory devices, which can only control one voltage channel, the Northwestern researchers created a device with a third terminal.
In research published in the journal Nature Nanotechnology, the Northwestern team discovered that the grain boundaries of MoS2 provided a better way to modulate resistance than can be achieved with memristors consisting of metal–insulator–metal structures with insulating oxides.
Grains are essentially the direction that atoms are arranged in a material; the grain boundaries are the interface where these grains come together and meet.
"Because the atoms are not in the same orientation, there are unsatisfied chemical bonds at that interface," Hersam explained in the release. "These grain boundaries influence the flow of current, so they can serve as a means of tuning resistance."
In actual practice, when a voltage is applied to the MoS2-based memristor the grain boundaries physically move, which is the mechanism that changes the device’s resistance. The result is a three-terminal memristive device that can be tuned by a gate electrode.
"With a memristor that can be tuned with a third electrode, we have the possibility to realize a function you could not previously achieve,” Hersam said.
Magnetized Graphene Promises a Million-fold Increase in Data Storage
While the overriding preoccupation with graphene in electronics may be to see it used in digital logic applications, it may provide enormous improvements to today’s digital memory solutions.
In research out of the U.S. Naval Research Laboratory (NRL), graphene has been imbued with magnetic properties that could lead to a million-fold increase in capacity over today’s hard drives.
While other research teams have managed to magnetize graphene, the NRL team was able to achieve it with a relatively simple and scalable process.
The researchers first put a layer of graphene over a silicon wafer. The graphene-covered silicon wafer is then placed into cryogenic ammonia that contains a small amount of lithium. This process adds hydrogen to the wafer, which makes it ferromagnetic.
While that is a pretty neat feat, what surprised the researchers was just how evenly the magnetism was spread across the wafer.
"I was surprised that the partially hydrogenated graphene prepared by our method was so uniform in its magnetism and apparently didn't have any magnetic grain boundaries," said Dr. Paul Sheehan of NRL's Chemistry Division in a press release.
The key discovery was that the researchers were able to remove the hydrogen atoms from the material using an electron beam. This maintained magnetic areas in some parts of the graphene and removed the magnetism from others. This means that large areas of the graphene can be patterned with the electron beam to precisely tune the magnetism.
"Since massive patterning with commercial electron beam lithography system is possible, we believe that our technique can be readily applicable for current microelectronics fabrication," said Dr. Woo-Kyung Lee, materials research scientist in the Chemistry Division at NRL and project lead in the press release.
While the researchers concede that this work doesn’t promise that graphene will be the memory storage medium of the future, if they can work out some of the issues with the material they believe that it could lead to a storage medium in which a single hydrogenated-carbon pair could store a single magnetic bit of data—a million-fold increase over today’s hard drives.
Graphene Looks to Play a Role in Spintronics
Spintronics, in which the spin of electrons is used to encode information rather than charge, has long promised to be the next step in the evolution of computing. We have already seen evidence of that migration with today’s disk drives that are capable of much greater storage capacity than the previous generation due to the material phenomenon known as giant magneto resistance.
However, it didn’t appear as though graphene was going to have much of an impact on the future of spintronics since if you laid it out flat it didn’t seem to have any affect on the spin of electrons. This changed when it was discovered that if you put a small bend in graphene it could influence the spin of electrons.
The latest piece of work that has continued to pursue this line of research comes out of Chalmers University in Sweden in which an electron spin has been preserved for an extended distance using large-area graphene.
"We believe that these results will attract a lot of attention in the research community and put graphene on the map for applications in spintronic components," said Saroj Dash, one of the Chalmers researchers, in a press release.
The Chalmers researchers reported in the journal Nature Communications, that they were able to achieve precise pure spin transport over lengths of 16 micrometers with a spin lifetime of 1.2 nanoseconds. According to the research paper, these spin parameters are “six times higher than previous reports and highest at room temperature for any form of pristine graphene on industrial standard [silicon/silicon-dioxide] substrates.”
"In future spin-based components, it is expected that the electrons must be able to travel several tens of micrometers with their spins kept aligned. Metals, such as aluminum or copper, do not have the capacity to handle this,” said Saroj Dash. “Graphene appears to be the only possible material at the moment.”