Excitons form superfluid in certain 2D combos

Rice researchers find ‘paradox’ in ground-state bilayers

Mixing and matching computational models of 2D materials led scientists at Rice University to the realization that excitons — quasiparticles that exist when electrons and holes briefly bind — can be manipulated in new and useful ways.

The researchers identified a small set of 2D compounds with similar atomic lattice dimensions that, when placed together, would allow excitons to form spontaneously. Generally, excitons happen when energy from light or electricity boosts electrons and holes into a higher state.

But in a few of the combinations predicted by Rice materials theorist Boris Yakobson and his team, excitons were observed stabilizing at the materials’ ground state. According to their determination, these excitons at their lowest energy state could condense into a superfluid-like phase. The discovery shows promise for electronic, spintronic and quantum computing applications.

The open-access study by Yakobson, graduate student Sunny Gupta and research scientist Alex Kutana, all of Rice’s Brown School of Engineering, appears in Nature Communications.

– See more at Rice News

Double-walled nanotubes have electro-optical advantages

Rice University calculations show they could be highly useful for solar panels

One nanotube could be great for electronics applications, but there’s new evidence that two could be tops.

Rice University engineers already knew that size matters when using single-walled carbon nanotubes for their electrical properties. But until now, nobody had studied how electrons act when confronted with the Russian doll-like structure of multiwalled tubes.

The Rice lab of materials theorist Boris Yakobson has now calculated the impact of curvature of semiconducting double-wall carbon nanotubes on their flexoelectric voltage, a measure of electrical imbalance between the nanotube’s inner and outer walls.

This affects how suitable nested nanotube pairs may be for nanoelectronics applications, especially photovoltaics.

The theoretical research by Yakobson’s Brown School of Engineering group appears in the American Chemical Society journal Nano Letters.

– See more at Rice News

Rice lab turns trash into valuable graphene in a flash

‘Green’ process promises pristine graphene in bulk using waste food, plastic and other materials

A new process introduced by the Rice University lab of chemist James Tour can turn bulk quantities of just about any carbon source into valuable graphene flakes. The process is quick and cheap; Tour said the “flash graphene” technique can convert a ton of coal, food waste or plastic into graphene for a fraction of the cost used by other bulk graphene-producing methods.

As reported in Nature, flash graphene is made in 10 milliseconds by heating carbon-containing materials to 3,000 Kelvin (about 5,000 degrees Fahrenheit). The source material can be nearly anything with carbon content. Food waste, plastic waste, petroleum coke, coal, wood clippings and biochar are prime candidates, Tour said. “With the present commercial price of graphene being $67,000 to $200,000 per ton, the prospects for this process look superb,” he said.

Atom-level simulations by Rice researcher and co-author Ksenia Bets confirmed that temperature is key to the material’s rapid formation.“It is amazing how state-of-the-art computer simulations, notoriously slow for observing such kinetics, reveal the details of high temperature-modulated atomic movements and transformation,” Bets said.

– See more at Rice News

Borophene on silver grows freely into an atomic ‘skin’

Rice scientists lead effort to improve manufacture of valuable 2D material

Borophene has a nearly perfect partner in a form of silver that could help the trendy two-dimensional material grow to unheard-of lengths.

A well-ordered lattice of silver atoms makes it possible to speed the growth of pristine borophene, the atom-thick allotrope of boron that so far can only form via synthesis by molecular-beam epitaxy (MBE).

By using a silver substrate and through careful manipulation of temperature and deposition rate, scientists have discovered they can grow elongated hexagon-shaped flakes of borophene. They suggested the use of a proper metal substrate could facilitate the growth of ultrathin, narrow borophene ribbons.

New work published in Science Advances by researchers at Rice and Northwestern universities, Nanjing University of Aeronautics and Astronautics and Argonne National Laboratory will help streamline the manufacture of the conductive material, which shows potential for use in wearable and transparent electronics, plasmonic sensors and energy storage.

– See more at Rice News

Oddball edge wins nanotube faceoff

Rice theory shows peculiar ‘Janus’ interface a common mechanism in carbon nanotube growth

When is a circle less stable than a jagged loop? Apparently when you’re talking about carbon nanotubes.

Rice University theoretical researchers have discovered that nanotubes with segregated sections of “zigzag” and “armchair” facets growing from a solid catalyst are far more energetically stable than a circular arrangement would be.

Under the right circumstances, they reported, the interface between a growing nanotube and its catalyst can reach its lowest-known energy state via the two-faced “Janus” configuration, with a half-circle of zigzags opposite six armchairs.

The terms refer to the shape of the nanotube’s edge: A zigzag nanotube’s end looks like a saw tooth, while an armchair is like a row of seats with armrests. They are the basic edge configurations of the two-dimensional honeycomb of carbon atoms known as graphene (as well as other 2D materials) and determine many of the materials’ properties, especially electrical conductivity.

The Brown School of Engineering team of materials theorist Boris Yakobson, researcher and lead author Ksenia Bets and assistant research professor Evgeni Penev reported their results in the American Chemical Society journal ACS Nano.

– See more at Rice News

Borophene shines alone as 2-D plasmonic material

Rice University scientists calculate flat boron capable of visible plasmon emissions

Illustration by Sharmila Shirodkar.

An atom-thick film of boron could be the first pure two-dimensional material able to emit visible and near-infrared light by activating its plasmons, according to Rice University scientists. That would make the material known as borophene a candidate for plasmonic and photonic devices like biomolecule sensors, waveguides, nanoscale light harvesters and nanoantennas. Plasmons are collective excitations of electrons that flow across the surface of metals when triggered by an input of energy, like laser light. Significantly, delivering light to a plasmonic material in one color (determined by the light’s frequency) can prompt the emission of light in another color.

Models by Rice theoretical physicist Boris Yakobson and his colleagues predict that borophene would be the first known 2-D material to do so naturally, without modification. The lab’s simulations are detailed in a paper by Yakobson with lead authors Yuefei Huang, a graduate student, and Sharmila Shirodkar, a postdoctoral researcher, in the Journal of the American Chemical Society.

– See more at Rice News

Boron atoms stretch out, gain new powers

Rice simulations demonstrate 1-D material’s stiffness, electrical versatility

Hold on, there, graphene. You might think you’re the most interesting new nanomaterial of the century, but boron might already have you beat, according to scientists at Rice University.

A Rice team that simulated one-dimensional forms of boron — both two-atom-wide ribbons and single-atom chains — found they possess unique properties. The new findings appear this week in the Journal of the American Chemical Society.

For example, if metallic ribbons of boron are stretched, they morph into antiferromagnetic semiconducting chains, and when released they fold back into ribbons.

In The News

Nano-chimneys can cool circuits

chim_v2Rice scientists calculate tweaks to graphene would form phonon-friendly cones

A few nanoscale adjustments may be all that is required to make graphene-nanotube junctions excel at transferring heat, according to Rice University scientists.

The Rice lab of theoretical physicist Boris Yakobson found that putting a cone-like “chimney” between the graphene and nanotube all but eliminates a barrier that blocks heat from escaping.

The research appears in the American Chemical Society’s Journal of Physical Chemistry C.

Heat is transferred through phonons, quasiparticle waves that also transmit sound. The Rice theory offers a strategy to channel damaging heat away from next-generation nano-electronics.

– See more at Rice News

 

Can Two-Dimensional Boron Superconduct?

Rice University scientists predict 2-D material – no longer theoretical – has unique propertiesimg4news_small

Rice University scientists have determined that two-dimensional boron is a natural low-temperature superconductor. In fact, it may be the only 2-D material with such potential.

Rice theoretical physicist Boris Yakobson and his co-workers published their calculations that show atomically flat boron is metallic and will transmit electrons with no resistance. The work appears this month in the American Chemical Society journal Nano Letters.

The hitch, as with most superconducting materials, is that it loses its resistivity only when very cold, in this case between 10 and 20 kelvins (roughly, minus-430 degrees Fahrenheit). But for making very small superconducting circuits, it might be the only game in town.

– See more at Rice News

The latest fashion: Graphene edges can be tailor-made

Rice University theory shows it should be possible to tune material’s properties

Graphene nanoribbons can be enticed to form favorable "reconstructed" edges by pulling them apart with the right force and at the right temperature, according to researchers at Rice University. The illustration shows the crack at the edge that begins the formation of five- and seven-atom pair under the right conditions. Illustration by ZiAng Zhang

Theoretical physicists at Rice University are living on the edge as they study the astounding properties of graphene. In a new study, they figure out how researchers can fracture graphene nanoribbons to get the edges they need for applications.

New research by Rice physicist Boris Yakobson and his colleagues shows it should be possible to control the edge properties of graphene nanoribbons by controlling the conditions under which the nanoribbons are pulled apart.

In the work, which appeared this month in the Royal Society of Chemistry journal Nanoscale, the Rice team used sophisticated computer modeling to show it’s possible to rip nanoribbons and get graphene with either pristine zigzag edges or what are called reconstructed zigzags.

– See more at: Rice News

Why do nanotubes grow chiral?

Rice University theorists determine factors that give tubes their chiral angles

Many a great idea springs from talks over a cup of coffee. But it’s rare and wonderful when a revelation comes from the cup itself.

Rice University theoretical physicist Boris Yakobson, acting upon sudden inspiration at a meeting last year in Arlington, Va., obtained a couple of spare coffee cups from a server and a pair of scissors and proceeded to lay out – science fair-style – an idea that could have far-reaching implications for the nanotechnology industry.

As reflected in a new paper in Nature Communications, Yakobson and his Rice colleagues, postdoctoral researcher Vasilii Artyukhov and research scientist Evgeni Penev, had come up with the seed (or perhaps, bean) of a simple formula that describes why nanotubes have chirality. Chirality is the property that describes the angle of the carbon atom hexagons that make up a nanotube’s walls.

– See more at: Rice News

Phosphorus a promising semiconductor

Rice University physicists find 2-D form pays no heed to defects

Defects damage the ideal properties of many two-dimensional materials, like carbon-based graphene. Phosphorus just shrugs.

That makes it a promising candidate for nano-electronic applications that require stable properties, according to new research by Rice University theoretical physicist Boris Yakobson and his colleagues.

In a paper in the American Chemical Society journal Nano Letters, the Rice team analyzed the properties of elemental bonds between semiconducting phosphorus atoms in 2-D sheets. Two-dimensional phosphorus is not theoretical; it was recently created through exfoliation from black phosphorus.

– See more at: Rice News

Carbyne morphs when stretched

Rice University calculations show carbon-atom chain would go metal to semiconductor

Applying just the right amount of tension to a chain of carbon atoms can turn it from a metallic conductor to an insulator, according to Rice University scientists.

Stretching the material known as carbyne — a hard-to-make, one-dimensional chain of carbon atoms — by just 3 percent can begin to change its properties in ways that engineers might find useful for mechanically activated nanoscale electronics and optics.

The finding by Rice theoretical physicist Boris Yakobson and his colleagues appears in the American Chemical Society journal Nano Letters.

– See more at: Rice News

Caps not the culprit in nanotube chirality

Rice study narrows the possibilities for gaining control of nanotube type

A single-walled carbon nanotube grows from the round cap down, so it’s logical to think the cap’s formation determines what follows. But according to researchers at Rice University, that’s not entirely so.

Theoretical physicist Boris Yakobson and his Rice colleagues found through exhaustive analysis that those who wish to control the chirality of nanotubes – the characteristic that determines their electrical properties – would be wise to look at other aspects of their growth.

In the study by Yakobson, research scientist Evgeni Penev and postdoctoral researcher Vasilli Artyukhov that was published recently by the American Chemical Society journal ACS Nano, the Rice researchers found that the elastic energy landscapes involved in cap formation are not strong enough to dictate the nanotube’s chirality….more

Flat boron by the numbers

Rice University researchers calculate what it would take to make new two-dimensional material

It would be a terrible thing if laboratories striving to grow graphene from carbon atoms kept winding up with big pesky diamonds.

“That would be trouble, cleaning out the diamonds so you could do some real work,” said Rice University theoretical physicist Boris Yakobson, chuckling at the absurd image.

Yet something like that keeps happening to experimentalists working to grow two-dimensional boron. Boron atoms have a strong preference to clump into three-dimensional shapes rather than assemble into pristine single-atom sheets, like carbon does when it becomes graphene. And boron clumps aren’t nearly as sparkly…more