Graphene’s Amazing Properties Continue To Expand
While increasingly more research is focused on manufacturing and applications of graphene, its incredible capabilities continue to grow
Creating Large-Area Graphene With Magnetic Properties
Back in 2013, researchers in Spain were the first to impart magnetic properties into graphene. Now researchers at the U.S. Naval Research Laboratory (NRL) have taken this a step further by creating a large-area magnetic graphene that can be easily patterned. The researchers believe that if the process proves stable it could lead to a million-fold increase in storage capacity over current hard drives.
Where previous method of magnetizing graphene have involved imparting defects into the graphene, in research published in the journal Advanced Materials, the NRL researchers developed a simple method using hydrogen that magnetizes the graphene.
The method developed by the NRL team starts with placing the graphene on a silicon wafer. From there, the graphene-silicon wafer is placed in cryogenic ammonia containing a small amount of lithium. This is all it takes to impart hydrogen onto the wafer and make it ferromagnetic.
Not only did the method make the wafer magnetic but it also spread the magnetism uniformly 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 NRL researchers further discovered that it was possible to remove hydrogen atoms from the material using an electron beam and thereby alter its magnetism. The electrons break the bond between the hydrogen and the graphene, removing the magnetic properties of the graphene at those points where the electron beam is focused. In this way, large patterns can be etched into the wafer.
"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.
The key to determining whether this research can achieve its lofty goal is determining how fine the patterns can be made and how long the ferromagnetism remains stable. If both of these issues can be satisfactorily addressed, it could lead to a storage medium in which a single hydrogenated-carbon pair could storing a single magnetic bit of data—thereby leading to a million-fold improvement on today’s hard drives.
Bringing Both the Electric and Magnetic Together in Graphene
There is increasing research interest in materials that display both magnetic and electric properties at the same time. The main focus in these types of materials—dubbed multiferroics—has been bismuth ferrite, which displays this capability naturally.
If the hope for these materials is realized, then they could lead to low-power, instant-on computing. That would translate into a smartphone that would switch on instantly and consume a fraction of the energy, enabling it to last much longer on a charge.
Graphene has been looked at for creating these materials that combine both the electric and magnetic. But it involves extensive doping of it that ends up introducing so many impurities that all the properties that make graphene attractive in the first place are lost, such as its high conductivity.
Now, however, researchers at the University of California Riverside have managed to bring this combination of electrical and magnetic properties into graphene without sacrificing any of its attractive capabilities.
The research, which was published in the journal Physical Review Letters, was able to achieve this combination by bringing a sheet very close to an electrical insulator with magnetic properties, which in this case was an atomically flat yttrium iron garnet (YIG) ferromagnetic thin film that the researchers had grown.
Simply by being in such close proximity the YIG film, the graphene just adopted its magnetic properties. But because the YIG film is also an electrical insulator it didn’t disrupt the graphene’s electrical conductivity.
“This is the first time that graphene has been made magnetic this way,” said Jing Shi, a professor of physics and astronomy, whose lab led the research, in a press release. “The magnetic graphene acquires new electronic properties so that new quantum phenomena can arise. These properties can lead to new electronic devices that are more robust and multi-functional.”
Graphene Withstands Both Heat and Humidity
Graphene has been looked at for photovoltaic applications largely as a replacement for the relatively expensive indium-tin oxide (ITO), which is used in the electrodes of solar cells.
The Achilles Heel of graphene in this application is that solar cells have to be outdoors where they are exposed to rain and high temperatures, both of which can ruin graphene.
Now researchers at the University of Exeter in the UK have discovered that a graphene-based hybrid material they developed three years ago dubbed GraphExeter is resistant to both high temperatures and humidity.
"By demonstrating its stability to being exposed to both high temperatures and humidity, we have shown that it is a practical and realistic alternative to indium tin oxide (ITO),” said Monica Craciun, University of Exeter engineer and lead researcher, in a press release. “This is particularly exciting for the solar panel industry, where the ability to withstand all weathers is crucial."
The researchers demonstrated in experiments, which were published in the journal Scientific Reports, that GraphExeter could withstand temperatures of 150 degrees Celsius at atmosphere and 620 degrees Celsius in a vacuum and survive at 100 percent humidity for 25 days.
"Having a metallic conductor stable at temperatures above 600°C, that is also optically transparent and flexible, can truly enable novel technologies for space applications and harsh environments such as nuclear power centrals," said Saverio Russo, a researcher at Exeter, in the press release.
Graphene Can Be Crumpled Then Flattened and Still Be Effective
As we have detailed previously in this newsletter, one of the key factors in improving supercapacitors is increasing the surface area of electrodes so more ions can stored on its surface and therefore the greater its storage capacity.
Unfortunately, graphene can’t really beat out the most advanced versions of activated carbon in surface area. But because of its high conductivity and ability to be shaped into attractive structures for supercapacitor applications, graphene remains a target of interest for supercapacitors.
Last October, researchers at MIT showed that you could crumple graphene up and flatten it out again up to 1000 times without losing any performance, translating to an interesting possibility of flexible supercapacitors—a feat that activated carbon was unlikely to achieve.
Building on this research, a team out of the University of Illinois at Urbana-Champaign showed that if you kept graphene at its crumpled state it could be used in new applications in electronics and biomateirals.
In addition to higher surface area electrodes for battery and supercapacitor applications, the researchers believe the enhanced surface area will allow even more sensitive and intimate interactions with biological systems, leading to high sensitivity devices.
“As a coating layer, 3D textured/crumpled nano-topographies could allow omniphobic/anti-bacterial surfaces for advanced coating applications,” said SungWoo Nam, an assistant professor of mechanical science and engineering at Illinois, in a press release.
The key feature of this latest crumpling of graphene, which is described in the journal Nano Letters, is that the method they used makes it possible to selectively pattern the crumples.
“In this study, we developed a novel method for controlled crumpling of graphene and graphite via heat-induced contractile deformation of the underlying substrate,” said Michael Cai Wang, a graduate student and first author of the paper, in the release. ”While graphene intrinsically exhibits tiny ripples in ambient conditions, we created large and tunable crumpled textures in a tailored and scalable fashion.”
Ballistic Transport Offers a New Capability in Graphene
Last year, we reported on work that showed that the ballistic transport of graphene (the speeds at which electrons flow through a material at room temperature) were ten times faster than previous theoretical limits.
At the time, this research caught hold of the imagination because of comments from Walt de Heer, a Regent's professor in the School of Physics at the Georgia Institute of Technology.
"This work shows that we can control graphene electrons in very different ways because the properties are really exceptional," said de Heer at the time. "This could result in a new class of coherent electronic devices based on room temperature ballistic transport in graphene. Such devices would be very different from what we make today in silicon."
Another group at the University of Basel in Switzerland have been so focused on this ability of electrons to travel through graphene at speeds that made them behave almost like photons that they spent years experimenting in the area.
Now all that research has paid off with their discovery that it’s possible to direct the electrons in graphene across a predefined path.
In research published in the journal Nature Communications, the Swiss-based researchers discovered that when they stretched, or otherwise manipulated, the honeycomb structure of the graphene and applied both an electrical and magnetic field to it, they could direct the flow of electrons. This marks the first time that anyone has successfully switched the guidance of electrons on and off and guided them without any loss.
In a bit of irony, it is graphene’s lack of an inherent band gap, which has remained such a thorny obstacle for its prospects in digital logic applications, that makes it possible for the electrons to be directed in this way.
“A nano-switch of this type in graphene can be incorporated into a wide variety of devices and operated simply by altering the magnetic field or the electrical field,” said Christian Schönenberger, one of the researchers, in a press release.
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