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.

Treatment with a superacid causes boron nitride layers to separate and become positively charged, allowing for it to interface with other nanoparticles, like gold. Credit: Berry, et al

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.

Rice University researchers modeled two-dimensional materials to quantify how they react to light. They calculated how the atom-thick materials in single or stacked layers would transmit, absorb and reflect light. The graphs above measure the maximum absorbance of several of the 55 materials tested. Credit: Yakobson Research Group/Rice University

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!”