From Cameras to Computers, New Material Could Change How We Work and Play

From Cameras to Computers, New Material Could Change How We Work and Play

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.

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.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.

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.”


Bend Me, Shape Me, Any Way You Want Me: Scientists Curve Nanoparticle Sheets into Complex Forms

Bend Me, Shape Me, Any Way You Want Me: Scientists Curve Nanoparticle Sheets into Complex Forms

Scientists have been making nanoparticles for more than two decades in two-dimensional sheets, three-dimensional crystals and random clusters. But they have never been able to get a sheet of nanoparticles to curve or fold into a complex three-dimensional structure. Now researchers from the University of Chicago, the University of Missouri and the U.S. Department of Energy’s Argonne National Laboratory have found a simple way to do exactly that.

The findings open the way for scientists to design membranes with tunable electrical, magnetic and mechanical properties that could be used in electronics and may even have implications for understanding biological systems.

Bend me, shape me, any way you want me: Scientists curve nanoparticle sheets into complex forms
Argonne researchers are able to fold gold nanoparticle membranes in a specific direction using an electron beam because two sides of the membrane are different. Image credit: Xiao-Min Lin et. al, taken at Argonne’s Electron Microscopy Center. Credit: Argonne National Laboratory.

Working at the Center for Nanoscale Materials (CNM) and the Advanced Photon Source (APS), two DOE Office of Science User Facilities located at Argonne, the team got membranes of gold nanoparticles coated with organic molecules to curl into tubes when hit with an electron beam. Equally importantly, they have discovered how and why it happens.

The scientists coat gold nanoparticles of a few thousand atoms each with an oil-like organic molecule that holds the gold particles together. When floated on water the particles form a sheet; when the water evaporates, it leaves the sheet suspended over a hole. “It’s almost like a drumhead,” says Xiao-Min Lin, the staff scientist at the Center for Nanoscale Materials who led the project. “But it’s a very thin membrane made of a single layer of nanoparticles.”

To their surprise, when the scientists put the membrane into the beam of a scanning electron microscope, it folded. It folded every time, and always in the same direction.

“That got our curiosity up,” said Lin. “Why is it bending in one direction?”

The answer lay in the organic surface molecules. They are hydrophobic: when floated on water they try to avoid contact with it, so they end up distributing themselves in a non-uniform way across the top and bottom layers of the nanoparticle sheet. When the electron beam hits the molecules on the surface it causes them to form an additional bond with their neighbors, creating an asymmetrical stress that makes the membranes fold.

Zhang Jiang and Jin Wang, X-ray staff at the APS, came up with an ingenious way to measure the molecular asymmetry, which at only six angstroms, or about six atoms thick, is so tiny it would not normally be measurable.

Subramanian Sankaranarayanan and Sanket Deshmukh at CNM used the high-performance computing resources at DOE’s National Energy Research Scientific Computing Center and the Argonne Leadership Computing Facility (ALCF), both DOE Office of Science User Facilities, to analyze the surface of the nanoparticles. They discovered that the amount of surface covered by the organic molecules and the molecules’ mobility on the surface both have an important influence on the degree of asymmetry in the membrane.

“These are fascinating results,” said Fernando Bresme, professor of chemical physics at the Imperial College in London and a leading theorist on soft matter physics. “They advance significantly our ability to make new nano-structures with controlled shapes.”

In principle, scientists could use this method to induce folding in any nanoparticle membrane that has an asymmetrical distribution of surface molecules. Said Lin, “You use one type of molecule that hates water and rely on the water surfaces to drive the molecules to distribute non-uniformly, or you could use two different kinds of molecules. The key is that the molecules have to distribute non-uniformly.”

The next step for Lin and his colleagues is to explore how they can control the molecular distribution on the surface and therefore the folding behavior. They envision zapping only a small part of the structure with the electron beam, designing the stresses to achieve particular bending patterns.

“You can maybe fold these things into origami structures and all sorts of interesting geometries,” Lin said. “It opens the possibilities.”

Z. Jiang, J. He, S. A. Deshmukh,  P. Kanjanaboos, G. Kamath, Y. Wang, S K. R. S. Sankaranarayanan, J Wang, H. M. Jaeger and X. Lin. “Subnanometre ligand-shell asymmetry leads to Janus-like nanoparticle membranes.”Nature Materials.