Test checks how atom-thin materials interact with light

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Researchers have created a method to determine the light-related properties of atom-thin 2D materials.

The ability of metallic or semiconducting materials to absorb, reflect, and act upon light is important to scientists developing optoelectronics—electronic devices that interact with light to perform tasks.

Two-dimensional materials have been a hot research topic since researchers identified graphene, a flat lattice of carbon atoms, in 2001. Since then, scientists have raced to develop, either in theory or in the lab, new 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.

Searching for TAR

Boris Yakobson, a materials theorist in the materials science and nanoengineering department at Rice University, took up the challenge. Yakobson and his colleagues 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,” says graduate student and lead author Sunny Gupta. “This is a big computational effort, but now it’s possible to evaluate any material at a deeper quantitative level.”

Their work, which appears in the 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 says. “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 researchers 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,” says coauthor Sharmila Shirodkar, a postdoctoral researcher. “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.”

Stacks and singles

The researchers modeled 2D stacks as well as single layers. “Stacks can broaden the spectral range or bring about new functionality, like polarizers,” says coauthor Alex Kutana. “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 says. “That would be fantastic!”

The US Army Research Office and the Robert Welch Foundation supported the work. The National Science Foundation-supported DAVinCI cluster at Rice, which the Center for Research Computing administers and the Ken Kennedy Institute for Information Technology, the Department of Defense High Performance Computing Modernization Program, and the Department of Energy Office of Science’s National Energy Research Scientific Computing Center helped procure provided computing resources.

Source: Rice University