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

Step right up for bigger 2D sheets

Rice theory shows how monocrystals of hexagonal boron nitride come together

Very small steps make a big difference to researchers who want to create large wafers of two-dimensional material.

Atom-sized steps in a substrate provide the means for 2D crystals growing in a chemical vapor furnace to come together in perfect rank. Scientists have recently observed this phenomenon, and now a Rice University group has an idea why it works.

Rice materials theorist Boris Yakobson and researcher Ksenia Bets led the construction of simulations that show atom-sized steps on a growth surface, or substrate, have the remarkable ability to keep monolayer crystal islands in alignment as they grow.

If the conditions are right, the islands join into a larger crystal without the grain boundaries so characteristic of 2D materials like graphene grown via chemical vapor deposition (CVD). That preserves their electronic perfection and characteristics, which differ depending on the material.

The Rice theory appears in the American Chemical Society journal Nano Letters.

– See more at Rice News

Imperfections make photons perfect for quantum computing

Rice scientists show how atom-flat materials could produce polarized photons on demand

If you can make a single photon, tell it how to spin and tell it where to go, you have a basic element for next-generation computers that work with light instead of wires.

That appears to be possible with atom-thick materials, as demonstrated by several labs. Now, Rice University scientists have developed an understanding of the mechanism by which two-dimensional materials can be manipulated to produce the desired photons.

The Rice lab of materials theorist Boris Yakobson reported this month that by adding pre-arranged imperfections to atom-thick materials like molybdenum disulfide, they become perfectly capable of emitting single photons in either left or right polarization on demand.

The discovery through first-principles simulations is detailed in the American Chemical Society journal Nano Letters.

– See more at Rice News

Two-faced edge makes nanotubes obey

Rice theorists find mechanism behind nearly pure nanotubes from the unusual catalyst

Growing a batch of carbon nanotubes that are all the same may not be as simple as researchers had hoped, according to Rice University scientists.

Rice materials theorist Boris Yakobson and his team bucked a theory that when growing nanotubes in a furnace, a catalyst with a specific atomic arrangement and symmetry would reliably make carbon nanotubes of like chirality, the angle of its carbon-atom lattice.

Instead, they found the catalyst in question starts nanotubes with a variety of chiral angles but redirects almost all of them toward a fast-growing variant known as (12,6). The cause appears to be a Janus-like interface that is composed of armchair and zigzag segments – and ultimately changes how nanotubes grow.

The Rice theoretical study detailed in the American Chemical Society journal Nano Letters could be a step toward catalysts that produce homogeneous batches of nanotubes, Yakobson said.

– 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

2D Boron on the Cover of Chem Soc Rev

Chemical Society Reviews features our review on two-dimensional boron on its front cover

In a recent article, we review the current theoretical and experimental progress in realizing boron atomic layers. Starting by describing a decade-long effort towards understanding the size-dependent structures of boron clusters, we present how theory plays a role in extrapolating boron clusters into 2D form, from a freestanding state to that on substrates, as well as in exploring practical routes for their synthesis that recently culminated in experimental realization. While 2D boron has been revealed to have unusual mechanical, electronic and chemical properties, materializing its potential in practical applications remains largely impeded by lack of routes towards transfer from substrates and controlled synthesis of quality samples.

The review is on the list of referee-recommended articles, HOT Chem Soc Rev articles for October, and is free to access until 13th December 2017.

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

Ultra-flat circuits will have unique properties

Rice University lab studies 2-D hybrids to see how they differ from common electronics

The old rules don’t necessarily apply when building electronic components out of two-dimensional materials, according to scientists at Rice University.2d-junct

The Rice lab of theoretical physicist Boris Yakobson analyzed hybrids that put 2-D materials like graphene and boron nitride side by side to see what happens at the border. They found that the electronic characteristics of such “co-planar” hybrids differ from bulkier components.

Their results appear this month in the American Chemical Society journal Nano Letters.

Shrinking electronics means shrinking their components. Academic labs and industries are studying how materials like graphene may enable the ultimate in thin devices by building all the necessary circuits into an atom-thick layer.

– See more at Rice News

Polyphony in B flat

At last, experiments offer two-dimensional boron… on a silver platterb_cards

In an extensive “News & Views” in Nature Chemistry, we provide a critical view of the latest breakthrough in materials flatland: the synthesis of two-dimensional boron. It also reflects on a decade-long effort in Yakobson’s group towards understanding and eventually predicting the structure of low-dimensional boron: from the B80 fullerene, to the polymorphism of 2D boron, to practical routes for its synthesis.

New Materials for Better Electronics

2016 CASC Brochure features work from the group

casc2016An image illustrating recent work from the group is featured in the 2016 Brochure
published by the Coalition for Academic Scientific Computation – an alliance of 85 of America’s most forward thinking research universities, national labs and computing centers, including Rice’s Ken Kennedy Institute for Information Technology.
On page 13, the highlight box “New Materials for Better Electronics” features our recent work on two-dimensional black phosphorus.

2D Boron among the Angew. Chem. covers

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Two-dimensional boron would take different forms, depending on the substrate used in chemical vapor deposition growth. Image by Zhuzha Zhang

Our most recent work on 2D boron will be featured on a cover of the upcoming issue of Angewandte Chemie International Edition. The study builds on two of our previous works on two-dimensional boron  and provides further clues as to how this elusive material can be synthesized and what the product may look like.

Calculation of the atom-by-atom energies involved in creating a sheet of boron revealed that the metal substrate – the surface upon which two-dimensional materials are grown in a chemical vapor deposition (CVD) furnace – would make all the difference.

The new calculations show it may be possible to guide the formation of 2D boron by tailoring boron-metal interactions.Theoretical physicist Boris Yakobson and his Rice colleagues discovered that copper, a common substrate in graphene growth, might be best to obtain flat boron, while other metals would guide the resulting material in their unique ways.

– See more at: Rice News

“Why nanotubes grow chiral” earns a spot in C&EN Nanotube hiStory

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JUNE 8, 2015 ISSUE, VOL. 93 | ISS. 23 Twists And Shouts: A Nanotube Story Nanotechnology’s chiral superstars were overshadowed by hype, but researchers believe they still have potential

The June 8 issue of the American Chemical Society‘s C&EN magazine quotes Boris Yakobson in its Cover Story “Twists And Shouts: A Nanotube Story“.

The timeline of major events in the history of carbon nanotubes features the Artyukhov–Penev–Yakobson (APY) theory of nanotube chirality in the most recent “Nanotubes Today” chapter. The APY theory combines the nanotube/catalyst interface thermodynamics with the kinetic growth theory to show that the unusual near-armchair peaks, repeatedly revealed in catalytic growth experiments over the last decade, emerge from the two antagonistic trends at the interface: energetic preference towards achiral versus the faster growth kinetics of chiral nanotubes. This narrow distribution is inherently related to the peaked behaviour of a simple function, xe−x.

The ultimate diamond slab…

among the most cited articles in Diamond and Related Materials published since 2010
fig6-diam_reatl_mater_2010
The ultimate diamond slab: GraphAne versus graphEne” is one of the early works from the group presenting a comprehensive characterization of three carbon nanomaterials of technological interest: graphEne, graphAne, and fluorinated graphene. By means of first principles and tight-binding calculations in combination with analytical methods, we carried out detailed comparative studies of their structural, mechanical, thermal, and electronic properties. The calculated elastic properties of these materials confirmed their high mechanical stability and stiffness, which in association with their low dimensionality, translates into a large ballistic thermal conductance. Furthermore, we showed that while graphene is a zero-gap semi-metal, graphane and fluorinated graphene are wide gap semiconductors. Here we also discussed designed interfaces between these systems, and showed that their physical properties have potential applications in nanoelectronic devices.