DARPA: New Nano-Material Could Change How We Work and Play

newmaterialworkplayx250Serendipity has as much a place in sci­ence as in love.

That’s what North­eastern Univ. 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 in 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.”


An artistic ren¬dering of novel mag¬netism in 2D-BNCO sheets, the new mate¬rial Swastik Kar and Srinivas Sridhar cre¬ated. Image: Northeastern Univ.

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 U.S., 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.”

Source: Northeastern Univ.

Laser Blast Makes Pure (Green) Quantum Dots

Laser PW-2014-12-08-Dume-quantumQuantum dots made of pure selenium can be made by simply firing a laser beam at selenium powder mixed into a glass of water. The easy and inexpensive process was developed by researchers at the University of Texas at San Antonio and Northeastern University in the US, and unlike other techniques, does not involve potentially toxic chemicals. The high-quality nanostructures could be used in two very different applications: as antibacterial agents and as light harvesters in solar cells.

Quantum dots are tiny pieces of semiconductor – such as selenium – that are typically tens of nanometres across. The size of a quantum dot dictates how their charge-carrying electrons and holes interact with light. As a result, they are of great interest to researchers trying to develop photonic technologies and especially solar cells. However, growing quantum dots that are pure and all the same size can be a challenge.A usingsolaren

Green and easy

The researchers, led by Gregory Guisbiers in San Antonio, created their pure selenium quantum dots using a technique called pulsed laser ablation in liquids (PLAL), which involves simply firing a pulsed laser beam at a target – in this case selenium powder in water. “Our method is ‘green’ because it does not involve any dangerous solvents, only water, and there are no toxic adducts or by-products, like those often encountered in many wet chemistry processes,” explains Guisbiers. “It is also cheap and easy because we do not need a vacuum chamber or clean room – everything is done in a beaker of water.” The pure nanoparticles produced are also easy to collect and store because they are directly synthesized in solution, he adds.

This is the first time that selenium quantum dots have been synthesized using PLAL at ultraviolet and visible wavelengths, he says. These wavelengths are particularly interesting because they are better at reducing the size of particles compared with light at near-infrared wavelengths. Guisbiers and colleagues also showed that the crystallinity of the nanoparticles created by this technique depends on their size – that is, the smallest particles are crystalline while the largest ones are amorphous.

Antibacterial and anti-cancer

Selenium nanoparticles have antibacterial and anti-cancer properties, and could be used in medicine because the material is biocompatible and already exists in our bodies. However, nanoparticles need to be free of surface contaminants if they are to be employed in a biomedical setting – something that has proved difficult to achieve in the past.

The team, which has already tested its nanoparticles on E. coli, is now looking to see if they are efficient at killing other types of bacteria. “We are particularly interested in other bacteria involved in nosocomial diseases, like the methicillin-resistant Staphylococcus aureus,” Guisbiers says. “I’m told that [hospital-acquired infections] cause roughly 100,000 deaths every year in the US alone because bacteria are becoming more and more resistant to existing antibiotics. What’s more, these so-called super-germs are spreading worldwide, making this a major international health concern.”

The researchers will report their work in an upcoming issue of Laser Physics Letters. The team is also planning to incorporate the pure selenium quantum dots that they made into third-generation solar cells. “Indeed, since the element itself is a p-type semiconductor, when combined with an n-type semiconductor, we can build p–n junctions (the building blocks of all modern-day electronics) at the nanoscale,” adds Guisbiers.