Srinivas Sridhar, left, the Arts and Sciences Distinguished Professor of Physics and director of Northeastern’s Electronic Materials Research Institute, and Swastik Kar, an assistant professor of physics in the College of Science. File photo
Srinivas Sridhar, left, the Arts and Sciences Distinguished Professor of Physics and director of Northeastern’s Electronic Materials Research Institute, and Swastik Kar, an assistant professor of physics in the College of Science. File photo

by Thea Singer

Serendipity has as much a place in sci­ence as in love.

That’s what North­eastern physi­cists Swastik Kar and Srinivas Sridhar found during their four-​​year project to modify graphene, a stronger-​​than-​​steel infin­i­tes­i­mally thin lat­tice of tightly packed carbon atoms. Pri­marily funded by the Army Research Lab­o­ra­tory and Defense Advanced Research Projects Agency, or DARPA, the researchers were charged with imbuing the decade-​​old mate­rial with thermal sen­si­tivity for use in infrared imaging devices such as night-​​vision gog­gles for the military.

What they unearthed, pub­lished Friday in the journal Sci­ence Advances, was so much more: an entirely new mate­rial spun out of boron, nitrogen, carbon, and oxygen that shows evi­dence of mag­netic, optical, and elec­trical prop­er­ties as well as DARPA’s sought-​​after thermal ones. Its poten­tial appli­ca­tions run the gamut: from 20-​​megapixel arrays for cell­phone cam­eras to photo detec­tors to atom­i­cally thin tran­sis­tors that when mul­ti­plied by the bil­lions could fuel computers.

We had to start from scratch and build every­thing,” says Kar, an assis­tant pro­fessor of physics in the Col­lege of Sci­ence. “We were on a journey, cre­ating a new path, a new direc­tion of research.”

The pair was familiar with “alloys,” con­trolled com­bi­na­tions of ele­ments that resulted in mate­rials with prop­er­ties that sur­passed graphene’s—for example, the addi­tion of boron and nitrogen to graphene’s carbon to con­note the con­duc­tivity nec­es­sary to pro­duce an elec­trical insu­lator. But no one had ever thought of choosing oxygen to add to the mix.

What led the North­eastern researchers to do so?

Well, we didn’t choose oxygen,” says Kar, smiling broadly. “Oxygen chose us.”

Oxygen, of course, is every­where. Indeed, Kar and Sridhar spent a lot of time trying to get rid of the oxygen seeping into their brew, wor­ried that it would con­t­a­m­i­nate the “pure” mate­rial they were seeking to develop.

That’s where the Aha! moment hap­pened for us,” says Kar. “We real­ized we could not ignore the role that oxygen plays in the way these ele­ments mix together.”

So instead of trying to remove oxygen, we thought: Let’s con­trol its intro­duc­tion,” adds Sridhar, the Arts and Sci­ences Dis­tin­guished Pro­fessor of Physics and director of Northeastern’s Elec­tronic Mate­rials Research Institute.

Oxygen, it turned out, was behaving in the reac­tion chamber in a way the sci­en­tists had never antic­i­pated: It was deter­mining how the other elements—the boron, carbon, and nitrogen—combined in a solid, crystal form, while also inserting itself into the lat­tice. The trace amounts of oxygen were, metaphor­i­cally, “etching away” some of the patches of carbon, explains Kar, making room for the boron and nitrogen to fill the gaps.

It was as if the oxygen was con­trol­ling the geo­metric struc­ture,” says Sridhar.

They named the new mate­rial, sen­sibly, 2D-​​BNCO, rep­re­senting the four ele­ments in the mix and the two-​​dimensionality of the super-​​thin light­weight mate­rial, and set about char­ac­ter­izing and man­u­fac­turing it, to ensure it was both repro­ducible and scal­able. That meant inves­ti­gating the myriad per­mu­ta­tions of the four ingre­di­ents, holding three con­stant while varying the mea­sure­ment of the remaining one, and vice versa, mul­tiple times over.

After each trial, they ana­lyzed the struc­ture and the func­tional prop­er­ties of the product— elec­trical, optical—using elec­tron micro­scopes and spec­tro­scopic tools, and col­lab­o­rated with com­pu­ta­tional physi­cists, who cre­ated models of the struc­tures to see if the con­fig­u­ra­tions would be fea­sible in the real world.

Next they will examine the new material’s mechan­ical prop­er­ties and begin to exper­i­men­tally val­i­date the mag­netic ones con­ferred, sur­pris­ingly, by the inter­min­gling of these four non­mag­netic ele­ments. “You begin to see very quickly how com­pli­cated that process is,” says Kar.

Helping with that com­plexity were col­lab­o­ra­tors from around the globe. In addi­tion to   North­eastern asso­ciate research sci­en­tists, post­doc­toral fel­lows, and grad­uate stu­dents, con­trib­u­tors included researchers in gov­ern­ment, industry, and acad­emia from the United States, Mexico, and India.

There is still a long way to go but there are clear indi­ca­tions that we can tune the elec­trical prop­er­ties of these mate­rials,” says Sridhar. “And if we find the right com­bi­na­tion, we will very likely get to that point where we reach the thermal sen­si­tivity that DARPA was ini­tially looking for as well as many as-​​yet unfore­seen applications.”

Originally published in news@Northeastern on July 31, 2015.