Researchers at the University of Illinois at Chicago have discovered a route to alter boron nitride, a layered 2D material, so that it can bind to other materials, like those found in electronics, biosensors and airplanes, for example. Being able to better-incorporate boron nitride into these components could help dramatically improve their performance.

The scientific community has long been interested in boron nitride because of its unique properties –it is strong, ultrathin, transparent, insulating, lightweight and thermally conductive — which, in theory, makes it a perfect material for use by engineers in a wide variety of applications. However, boron nitride’s natural resistance to chemicals and lack of surface-level molecular binding sites have made it difficult for the material to interface with other materials used in these applications.

UIC’s Vikas Berry and his colleagues are the first to report that treatment with a superacid causes boron nitride layers to separate into atomically thick sheets, while creating binding sites on the surface of these sheets that provide opportunities to interface with nanoparticles, molecules and other 2D nanomaterials, like graphene. This includes nanotechnologies that use boron nitride to insulate nano-circuits.

Their findings are published in ACS Nano, a journal of the American Chemical Society.

“Boron nitride is like a stack of highly sticky papers in a ream, and by treating this ream with chlorosulfonic acid, we introduced positive charges on the boron nitride layers that caused the sheets to repel each other and separate,” said Berry, associate professor and head of chemical engineering at the UIC College of Engineering and corresponding author on the paper.

Berry said that “like magnets of the same polarity,” these positively charged boron nitride sheets repel one another.

“We showed that the positive charges on the surfaces of the separated boron nitride sheets make it more chemically active,” Berry said. “The protonation — the addition of positive charges to atoms — of internal and edge nitrogen atoms creates a scaffold to which other materials can bind.”

Berry said that the opportunities for boron nitride to improve composite materials in next-generation applications are vast.

“Boron and nitrogen are on the left and the right of carbon on the periodic table and therefore, boron-nitride is isostructural and isoelectronic to carbon-based graphene, which is considered a ‘wonder material,'” Berry said. This means these two materials are similar in their atomic crystal structure (isostructural) and their overall electron density (isoelectric), he said.

“We can potentially use this material in all kinds of electronics, like optoelectronic and piezoelectric devices, and in many other applications, from solar-cell passivation layers, which function as filters to absorb only certain types of light, to medical diagnostic devices,” Berry said.

The ability of metallic or semiconducting materials to absorb, reflect and act upon light is of primary importance to scientists developing optoelectronics – electronic devices that interact with light to perform tasks. Rice University scientists have now produced a method to determine the properties of atom-thin materials that promise to refine the modulation and manipulation of light.

Two-dimensional materials have been a hot research topic since graphene, a flat lattice of carbon atoms, was identified in 2001. Since then, scientists have raced to develop, either in theory or in the lab, novel 2D materials with a range of optical, electronic and physical properties.

Until now, they have lacked a comprehensive guide to the optical properties those materials offer as ultrathin reflectors, transmitters or absorbers.

The Rice lab of materials theorist Boris Yakobson took up the challenge. Yakobson and his co-authors, graduate student and lead author Sunny Gupta, postdoctoral researcher Sharmila Shirodkar and research scientist Alex Kutana, used state-of-the-art theoretical methods to compute the maximum optical properties of 55 2D materials.

“The important thing now that we understand the protocol is that we can use it to analyze any 2D material,” Gupta said. “This is a big computational effort, but now it’s possible to evaluate any material at a deeper quantitative level.”

Their work, which appears this month in the American Chemical Society journal ACS Nano, details the monolayers’ transmittance, absorbance and reflectance, properties they collectively dubbed TAR. At the nanoscale, light can interact with materials in unique ways, prompting electron-photon interactions or triggering plasmons that absorb light at one frequency and emit it in another.

Manipulating 2D materials lets researchers design ever smaller devices like sensors or light-driven circuits. But first it helps to know how sensitive a material is to a particular wavelength of light, from infrared to visible colors to ultraviolet.

“Generally, the common wisdom is that 2D materials are so thin that they should appear to be essentially transparent, with negligible reflection and absorption,” Yakobson said. “Surprisingly, we found that each material has an expressive optical signature, with a large portion of light of a particular color (wavelength) being absorbed or reflected.”

The co-authors anticipate photodetecting and modulating devices and polarizing filters are possible applications for 2D materials that have directionally dependent optical properties. “Multilayer coatings could provide good protection from radiation or light, like from lasers,” Shirodkar said. “In the latter case, heterostructured (multilayered) films — coatings of complementary materials — may be needed. Greater intensities of light could produce nonlinear effects, and accounting for those will certainly require further research.”

The researchers modeled 2D stacks as well as single layers. “Stacks can broaden the spectral range or bring about new functionality, like polarizers,” Kutana said. “We can think about using stacked heterostructure patterns to store information or even for cryptography.”

Among their results, the researchers verified that stacks of graphene and borophene are highly reflective of mid-infrared light. Their most striking discovery was that a material made of more than 100 single-atom layers of boron — which would still be only about 40 nanometers thick — would reflect more than 99 percent of light from the infrared to ultraviolet, outperforming doped graphene and bulk silver.

There’s a side benefit that fits with Yakobson’s artistic sensibility as well. “Now that we know the optical properties of all these materials – the colors they reflect and transmit when hit with light – we can think about making Tiffany-style stained-glass windows on the nanoscale,” he said. “That would be fantastic!”

Should you care that scientists can control a baffling current? Their research results could someday affect your daily living.

Physicists have managed to send and control a spin current across longer distances than ever before – and in a material that was previously considered unsuitable for the task.

We’ll return to what that strange sentence really means. But a spin current is a current that is kept going without relying on a simultaneous current of electrical charges.

“We’ve transferred spins more than 80 microns in an antiferromagnet,” says Arne Brataas, a professor at the Norwegian University of Science and Technology’s (NTNU) Department of Physics, and head of the university’s recently launched Center for Quantum Spintronics (QuSpin).

Eighty microns – a mere 8/100 000th of a metre – is that so impressive?

“We’re not exactly sending signals to the other side of the city. But this is far in the world of nanoelectronics,” says Brataas.

Nanoelectronics forms the basis for all the smart technology we surround ourselves with.

Right about now you can start doing your happy dance. That’s because 80 microns is getting to be a great enough distance to matter to people besides the scientists who are interested in knowledge for its own sake.

QuSpin has been collaborating with international physicists, including several in Germany and the Netherlands. The results are so intriguing that they are being published in the latest issue of the journal Nature.

So what is spintronics?

The technology of the future may depend on spintronics. If you don’t know what it is, you might as well learn about it. But you can also jump ahead to the next section if you just want to learn about its practical uses.

Atoms have several parts. Electrons are the negatively charged particles, as many of us learned in science class.

But electrons don’t only have a charge, they also have spin, an apparent internal rotation.

The spin has a direction, which is the basis of magnetism. A ferromagnetic material has a preference to align the spins in one particular direction. These materials are the magnets that you put on the refrigerator door.

Antiferromagnetic materials are also magnetic, but you don’t notice their magnetic quality. The atoms in the material alternate between spins in opposite directions. These alterations effectively zero out the total spin so that the material itself does not have a magnetic moment. These materials don’t work on your refrigerator door.

How spins are organized can therefore have very clear consequences for how a material behaves. The spin can be exploited.

Magnetism can transfer signals

Brataas and his colleagues aren’t currently focusing so much on practical uses, so that’s up to us to do.

Today’s technology transmits signals in a computer’s microchips by means of an electric charge. In an electrical current, electrons and spin both flow through the material.

But in the future a spin current will be able do parts of the same job, using magnetism to transmit the signals instead, without electrons passing through with the current.

What are the benefits of spin currents?

Well, for one, a spin current can sometimes flow more readily than an electrical charge current, since only the spin moves and not the electrons. This results in less energy loss in transmitting the signal.

A spin current does not generate a lot of heat. As the transistors on microchips have become ever smaller, overheating has become a growing problem, causing microchips to melt. Using spin current means smaller transistors can be used -another practical feature as new electronic gizmos pop up everywhere.

Spin current can also be controlled much more quickly. So all the new gizmos can be a lot faster.

Controlling spin current

These results would not be nearly as exciting if physicists couldn’t control the spin current at the same time. But they can.

Physicists can start the spin current by applying an electric field at one end of the material. The signal flows through the material without the electrons moving from one end to the other.

Physicists can also do the opposite at the other end, and transfer the spin current to an electrical current.

They have managed to do this at temperatures approaching room temperature. Granted, 200 degrees Kelvin – or -73.15 degrees Celsius – is a bit chilly for a room, but it falls within the range of naturally occurring temperatures on Earth. The researchers expect that they will be able do this experiment at a more comfortable room temperature pretty soon as well.

The research group used the antiferromagnet hematite, an iron oxide (Fe2O3), in their experiments.

The results this time are clearly just a step along the way. The research team will continue to test other materials and look at how these materials respond to different types of influences.

High risk, but important

NTNU’s QuSpin was awarded Norwegian Centre of Excellence (SFF) status last year, a highly regarded recognition. The centre was created to combine theory with experimental physics in the spintronics field and can already show world-leading results.

“The centre focuses on high risk projects of major importance in many different directions,” says Brataas.

The status that SFF confers provides more stable research funding, since QuSpin is guaranteed support for ten years. The funding facilitates high-risk research that can fail too. And they do, all the time.

Many of their experiments do not match the theories or vice versa, and that is important in its own way. But some experiments are spot on, and they can have particularly great significance.