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Graphene Properties
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With New Properties Come New Potential Applications for 2D Materials


 

Better thermal management in electronics and improved magnetic sensors all come down to a better understanding of the properties of 2D materials

 

While there may be a fair amount of frustrated hand wringing among some in the investment community that graphene hasn’t provided a huge revenue windfall as of yet, many in the scientific community are just excited at the prospect of all the new properties they are discovering and will continue to discover in the world of two-dimensional (2D) materials. 

 

This past quarter hasn’t revealed much in the way of 2D materials demonstrating any greater capacity in enabling digital logic. However, in the ancillary areas of thermal management and memory, 2D materials, like graphene and black phosphorus, have shown some new properties that should excite the electronics community.

 

Graphene and Black Phosphorous Compete for Thermal Management Bragging Rights

 

 

Illustration: Berkeley Lab

 

Researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have taken advantage of black phosphorus’ in-plane anisotropy, which means its properties are dependent on the direction of the crystal, to reveal the material’s thermal management capabilities. Because of its in-plane anisotropy, the Berkeley Lab researchers were able to demonstrate for the first time outside of a computer model that heat flows in black phosphorus more easily in a direction in which electricity flows with more difficulty. 

 

“Our study shows that in a similar manner heat flow in the black phosphorous nanoribbons can be very different along different directions in the plane,” said Junqiao Wu, one of the Berkeley Lab researchers, in a press release. “This thermal conductivity anisotropy has been predicted recently for 2-D black phosphorous crystals by theorists but never before observed.”

 

In simple terms, if black phosphorus is used in an electronic device, as soon as the device starts to heat up that heat will dissipate far more efficiently than today.

 

“This anisotropy can be especially advantageous if heat generation and dissipation play a role in the device operation,” said Wu in the press release. “For example, these orientation-dependent thermal conductivities give us opportunities to design microelectronic devices with different lattice orientations for cooling and operating microchips. We could use efficient thermal management to reduce chip temperature and enhance chip performance.”

 

When it comes to thermal management in electronics, graphene was not going to be outdone by black phosphorous. 

 

An international team of researchers, organized by a team at the University of Michigan, has found that layered graphene can provide a key mechanism for dispersing heat in electronics. 

 

In research published in the journal Nature Communications, the scientists demonstrated that the electrostatic interactions between electrically charged particles—known as Coulomb interactions—between different layers of multi-layered graphene offers a key mechanism for dispersing heat. This occurs despite the fact that all electronic states are strongly confined within individual 2D layers.

 

“We believe that this cooling mechanism is not limited to multilayer graphene samples but is likely to be important in many other new, layered nanomaterials under active development by the scientific community,” said Theodore Norris, who led the research, in a press release.

 

The researchers were a bit nonplussed to discover this cooling mechanism. The prevailing wisdom suggested that the heat building up in the electrons of the graphene would not travel well between the layers because the interaction between the layers was so weak. 

 

In a way, the prevailing wisdom was not wrong. The electrons in the different layers don’t come in contact with each other mechanically. However, they do manage to interact with each through their respective electrical charges.

 

The mechanism works likes this: When the negative charges begin to repel each other, the electrons take on effective size that extends between the layers. Once in contact, the hotter electrons transfer their heat to the colder ones. This process continues until the heat works its way towards the outer layer near the substrate and then is transferred to the substrate.

 

The researcher’s detailed mechanism of this phenomenon could become an important tool for thermal management in electronics for years to come.

 

A Ubiquitous Sensor Chip Used in Home Appliances May Have Just Gone From Silicon to Graphene

 

 

Photo: National University of Singapore

 

There is a type of sensor known as the magnetoresistance (MR) sensor that is ubiquitous in our everyday lives. These are the sensors that turn our refrigerators and washing machines into smart devices.

 

These MR sensors operate on the mechanism of detecting a change in electrical resistance brought on by the presence of a magnetic field and have to date been fabricated using silicon.

 

Now researchers at the National University of Singapore (NUS) have produced these MR sensor chips out of graphene and boron nitride. Their version is 200 times as sensitive to electrical resistance as its silicon counterpart. 

 

In research published in the journal Nature Communications, the NUS researchers demonstrated that a chip made out of boron nitride as a substrate with graphene sheets layered on top formed an interface that allows electrons to pass through the material very quickly.

 

This design also afforded the chip high sensitivity to both high and low intensity magnetic fields, and neither its tunability nor its resistance was substantially impacted by changes in temperature.

 

This stands in contrast to silicon chips in which their properties begin to change as their temperatures approach 127 degrees Celsius—the maximum temperature at which most electronics operate—causing their sensitivity to diminish. Instead, the graphene-based chip actually improves as the temperature rises. When these chips operate in temperatures at 127 °C, their ability to sense resistance improves by as much as eight times greater than it is at room temperature.

 

This would eliminate the need in today’s MR sensors for expensive and complicated temperature correction mechanisms. This would be huge win for the automotive industry where MR sensors are exposed to extreme temperatures in the working of an automobile.

 

Graphene Enable Disappearing Chips for Use in Biomedical Applications

 

 

Illustration: Fedorov Laboratory, Georgia Tech

 

In most chip design research, materials are investigated to enable things like better thermal management, better electron conductivity or any property that will allow the chips to become smaller and run faster. What we don’t typically see is an interest in a material property that will make the chip invisible. 

 

Now this may seem like a property that would only be attractive for spy thrillers, but, in fact, it’s a feature that has very real benefits for biomedical applications. To this end, researchers from the Georgia Institute of Technology have created circuits than change over time. 

 

In research published in the journal Nanoscale, the Georgia Tech researchers deposited carbon atoms onto graphene using a focused electron beam process to create patterns that evolve over time on the graphene.

 

“We will now be able to draw electronic circuits that evolve over time,” said Andrei Fedorov, a professor at Georgia Tech, in a press release. “You could design a circuit that operates one way now, but after waiting a day for the carbon to diffuse over the graphene surface, you would no longer have an electronic device. Today the device would do one thing; tomorrow it would do something entirely different.”

 

Their discovery was serendipitous since they really only had set out to find a way to remove hydrocarbon contaminants from graphene. Instead what they found was that when they deposited the carbon atoms on the graphene, they could produce patterns and these patterns served to create negatively charged areas in the graphene.

 

This was an interesting observation, but what happened next was what surprised them: the carbon atoms moved around the surface over time. The carbon atoms would continue to move around the surface until they spread uniformly across the entire surface. This process took around tens hours and at the end of which positively charged (p-doped) surface regions had been converted into surfaces with a uniformly negative charge (n-doped). And, perhaps most importantly, an intermediate p-n junction domain forms during this transformation.

 

“The electronic structures continuously change over time,” Fedorov explained. “That gives you a reconfigurable device, especially since our carbon deposition is done not using bulk films, but rather an electron beam that is used to draw where you want a negatively-doped domain to exist.”

 

Graphene Could Usher in Lower-Dose X-Rays in the Future

 

X-rays remain a mainstay of medicine. However, the technology that makes it possible is both nearly a century old and somewhat dangerous because of the high dose of radioactive energy involved.

 

Now researchers at the Massachusetts Institute of Technology (MIT) and from the Singapore Institute of Manufacturing Technology believe that graphene could offer an alternative based on computer simulations. 

 

What the international team of researchers discovered was that graphene generates something called plasmons when it is struck by photons from a laser beam. 

 

Plasmons generated by graphene are similar to surface plasmons, which are the oscillations that occur on the structure of a metal surface when photons strike its surface exciting its electrons. Recently, research has revealed that graphene and other two-dimensional materials produce plasmons as well. These graphene plasmons as they’ve come to be know have an advantage over the surface plasmons that metal surfaces produce because they can be tuned and controlled by a voltage gate.

 

What the MIT researchers did was discover that the graphene plasmons could be used to trigger a sharp pulse of radiation that could be tuned to wavelengths in the range from infrared light to X-rays.

 

The research, which was published in the journal Nature Photonics, further determined that radiation that was produced by the graphene was uniform and tightly aligned like that that would come out of a laser beam. This would translate into lower-dose X-ray systems that would be significantly safer.

 

The traditional way that X-rays work is to produce high-energy electrons using an accelerator and then wiggle them in a way that their oscillations create X-rays. It is hard and, most importantly, expensive to create these high-energy electrons.

 

“The reason this is unique is that we’re substantially bypassing the problem of accelerating the electrons,” said Ido Kaminer, one of the researchers, in a press release. “Every other approach involves accelerating the electrons. This is unique in producing X-rays from low-energy electrons.”

 

Now that they have demonstrated this property in simulations, the researchers are attempting to produce a physical device based on their simulations.

 

“We hope to have solid confirmation of the principles within a year, and X-rays, if that goes well, optimistically within three years,” said MIT professors Marin Soljacic in a press release.