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‘Hopelessly attached’: Scientists discover new 2D material that sticks the landing

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Discovery confirms decade-old prediction and sheds light on underexplored class of 2D materials

Atomic-resolution scanning tunneling microscopy image (left) overlapped with the copper boride (Cu8B14) structural model.

More than ten years ago, researchers at Rice University led by materials scientist Boris Yakobson predicted that boron atoms would cling too tightly to copper to form borophene, a flexible, metallic two-dimensional material with potential across electronics, energy and catalysis. Now, new research shows the prediction holds up, but in an unexpected way.

Unlike systems such as graphene on copper, where atoms may diffuse into the substrate without forming a distinct alloy, the boron atoms in this case formed a defined 2D copper boride  — a new compound with a distinct atomic structure. The finding, published in Science Advances by researchers from Rice and Northwestern University, sets the stage for further exploration of a relatively untapped class of 2D materials.

– See more at Rice News

Rice scientists pull off quantum coup

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Study validates method for guided discovery of 3D flat-band materials

Rice University scientists have discovered a first-of-its-kind material, a 3D crystalline metal in which quantum correlations and the geometry of the crystal structure combine to frustrate the movement of electrons and lock them in place.

The find is detailed in a study published in Nature Physics. The paper also describes the theoretical design principle and experimental methodology that guided the research team to the material. One part copper, two parts vanadium and four parts sulfur, the alloy features a 3D pyrochlore lattice consisting of corner-sharing tetrahedra.

Yakobson Research Group performed first-principle calculations that quantified the flat-band effects produced by geometric frustration.

Quantum materials are a likely place to look, especially if they host strong electron interactions that give rise to quantum entanglement. Entanglement leads to strange electronic behaviors, including frustrating the movement of electrons to the point where they become locked in place.

–  See more at Rice News

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Growing pure nanotubes is a stretch, but possible

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Rice theorists show how tight “diet” could produce single-chirality carbon nanotubes

Like a giraffe stretching for leaves on a tall tree, making carbon nanotubes reach for food as they grow may lead to a long-sought breakthrough.

Materials theorists Boris Yakobson and Ksenia Bets at Rice University’s George R. Brown School of Engineering show how putting constraints on growing nanotubes could facilitate a “holy grail” of growing batches with a single desired chirality.

Their paper in Science Advances describes a strategy by which constraining the carbon feedstock in a furnace would help control the “kite” growth of nanotubes. In this method, the nanotube begins to form at the metal catalyst on a substrate, but lifts the catalyst as it grows, resembling a kite on a string.

Carbon nanotube walls are basically graphene, its hexagonal lattice of atoms rolled into a tube. Chirality refers to how the hexagons are angled within the lattice, between 0 and 30 degrees. That determines whether the nanotubes are metallic or semiconductors. The ability to grow long nanotubes in a single chirality could, for instance, enable the manufacture of highly conductive nanotube fibers or semiconductor channels of transistors.

– See more at Rice News

Nanotube fibers stand strong – but for how long?

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Rice scientists calculate how carbon nanotubes and their fibers experience fatigue

Up here in the macro world, we all feel fatigue now and then. It’s the same for bundles of carbon nanotubes, no matter how perfect their individual components are.

A Rice University study calculates how strains and stresses affect both “perfect” nanotubes and those assembled into fibers and found that while fibers under cyclic loads can fail over time, the tubes themselves may remain perfect. How long the tubes or their fibers sustain their mechanical environment can determine their practicality for applications.

That made the study, which appears in Science Advances, important to Rice materials theorist Boris Yakobson,graduate student Nitant Gupta and assistant research professor Evgeni Penev of Rice’s George R. Brown School of Engineering. They quantified the effects of cyclic stress on nanotubes using state-of-the-art simulation techniques like a kinetic Monte Carlo method. They hope to give researchers and industry a way to predict how long nanotube fibers or other assemblies can be expected to last under given conditions.

– See more at Rice News

Weak bonds a strength in making borophene

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Rice theory shows potential to synthesize material on an insulator

Borophene may be done tantalizing materials scientists and start serving their ambitions, if a new approach by Rice University researchers can be turned into practice.

Materials theorist Boris Yakobson of Rice’s George R. Brown School of Engineering and his group suggest a method to synthesize borophene, the 2D version of boron, in a way that could make it easier to free up or manipulate.

According to the group’s paper in the American Chemical Society journal ACS Nano, that would involve growing the exotic material on hexagonal boron nitride (hBN), an insulator, rather than the more traditional metallic surfaces typically used in molecular beam epitaxy (MBE).

The Yakobson team, including lead author and graduate student Qiyuan Ruan and co-authors Luqing Wang, a Rice alumnus, and research scientist Ksenia Bets, calculated the atom-level energies of borophene and hBN. They found the step-and-plateau hBN substrate encouraged boron atoms floating in the MBE chamber to alight, nucleating growth.

– See more at Rice News

Bilayer borophene is a first

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Rice theories, Northwestern experiments combine to produce exotic material

If one layer of borophene is good, will two be better? Scientists at Rice University and Northwestern University hope so, because they’ve now made the elusive material.

Borophene is a one-atom-thick material made of boron atoms, which mostly fall together in neat triangles when grown in a furnace on a proper substrate. Its high strength and excellent conductivity make it a good candidate for use in quantum electronics, energy storage and sensors.

Unlike graphene, which can be exfoliated from bulk graphite, borophene can only be synthesized. And until now, it was only possible to make it in a single layer.

But the theory group at Rice led by Boris Yakobson and experimentalists at Northwestern led by Mark Hersam have given borophene a second deck. Their success at making bilayer borophene is detailed in Nature Materials.

“This is a significant step up, because it should enhance the coveted properties of 2D borophene as well as bring about new ones,” said Yakobson, a materials physicist at Rice’s Brown School of Engineering whose lab designed and performed simulations to guide the experiments.

– See more at Rice News

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2D-materials prediction is on the Cover of ACS Nano

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ACS Nano features our review on theoretical prediction of two-dimensional materials, behavior, and properties on its April front cover

Predictive modeling of 2D materials is at the crossroad of two current rapidly growing interests: 2D materials per se, massively sought after and explored in experimental laboratories, and materials theoretical-computational models in general, flourishing on a fertile mix of condensed-matter physics and chemistry with advancing computational technology. In the April issue of ACS Nano we briefly overview the general methods and specific techniques of modeling, along with a somewhat philosophical assessment of what “prediction” is, followed by selected practical examples for 2D materials, from structures and properties, to device functionalities and synthetic routes for their making. We conclude with a brief sketch–outlook of future developments.

The cover image represents the persistence of the scientific quest for two-dimensional materials that motivates their prediction. In the extraterrestrial landscape, inspired by an iconic Salvador Dalí painting, one should wonder what magnitude of gravity g would shape the graphene flake this way? Theory can predict, from first principles.

‘Defective’ carbon simplifies hydrogen peroxide production

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Plasma processing modifies carbon black powder to catalyze valuable chemical

Rice University researchers have created a “defective” catalyst that simplifies the generation of hydrogen peroxide from oxygen.

Rice scientists treated metal-free carbon black, the inexpensive, powdered product of petroleum production, with oxygen plasma. The process introduces defects and oxygen-containing groups into the structure of the carbon particles, exposing more surface area for interactions.

When used as a catalyst, the defective particles known as CB-Plasma reduce oxygen to hydrogen peroxide with 100% Faradaic efficiency, a measure of charge transfer in electrochemical reactions. The process shows promise to replace the complex anthraquinone-based production method that requires expensive catalysts and generates toxic organic byproducts and large amounts of wastewater, according to the researchers.

The research by Rice chemist James Tour and materials theorist Boris Yakobson appears in the American Chemical Society journal ACS Catalysis.

– See more at Rice News

A little friction goes a long way toward stronger nanotube fibers

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Rice model may lead to better materials for aerospace, automotive, medical applications

Carbon nanotube fibers are not nearly as strong as the nanotubes they contain, but Rice University researchers are working to close the gap.

A computational model by materials theorist Boris Yakobson and his team at Rice’s Brown School of Engineering establishes a universal scaling relationship between nanotube length and friction between them in a bundle, parameters that can be used to fine-tune fiber properties for strength.

The model is a tool for scientists and engineers who develop conductive fibers for aerospace, automotive, medical and textile applications like smart clothing. Carbon nanotube fibers have been considered as a possible basis for a space elevator, a project Yakobson has studied.

The research is detailed in the American Chemical Society journal ACS Nano.

– See more at Rice News

CNT-graphene-borophene specialty cake

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A sweet way to say “Happy Holidays”

Not everyone can make cakes, but a great cake can come from anyone… or so it seems. Combining many years of experience in modeling the synthesis of carbon nanotubes (CNTs) and 2D materials, and obviously some serious culinary skills Ksenia Bets has scaled-up their production to say “Happy Holidays” on behalf of Yakobson’s group and the MSNE department.

The cake blends a candy nanotube (CNT), graphene and borophene from sugar (or possibly other solid sources) on a stepped glazed surface. The MSNE logo can also be seen on the top terrace.

For recipes visit this page.

Excitons form superfluid in certain 2D combos

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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

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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

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‘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

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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

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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