Optical Signatures of 2D Materials

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. (Source: Yakobson Research Group, Rice Univ.)

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 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 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,” Shirodkar said. “In the latter case, hetero­structured 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!” (Source: Rice U.)

Reference: S. Gupta et al.: In Pursuit of 2D Materials for Maximum Optical Response, ACS Nano, online 18 September 2018; DOI: 10.1021/acsnano.8b03754

Link: Dept. of Materials Science and NanoEngineering, Rice University, Houston, USA

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