Author Archives: Evgeni Penev

Wrinkles in atomically thin materials unlock ultraefficient electronics

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Representation of electron spins in a bent 2D material exhibiting the persistent spin helix structure. (Image courtesy of Sunny Gupta)

Wrinkles can be an asset — especially for next-generation electronics. Rice University scientists have discovered that tiny creases in two-dimensional materials can control electrons’ spin with record precision, opening the path to ultracompact, energy-efficient electronic devices.

If most devices today use the charge of electrons flowing through silicon to process and encode information, future computing may instead harness spin — a quantum property of electrons that takes on either an “up” or “down” value. Computing with spin could overcome the limitations of current silicon-based technology, reducing the energy footprint of devices and data centers at a moment when computing-driven energy use is soaring globally.

However, spintronics has to contend with a major challenge: Information encoded in spin is quick to decay and can be lost when the electrons in a material scatter and collide with atoms.

In a new study published in the journal Matter, Rice materials scientists have reported that bending atomically thin layers of materials like molybdenum ditelluride, MoTe2, gives rise to a unique spin texture called persistent spin helix, or PSH, which can preserve spin state even in scattering collisions.

– See more at Rice News

‘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

Nicholas Tjahjono wins the Outstanding Teaching Assistant Award

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Nicholas Tjahjono, a fourth-year doctoral student in our group and a NASA Space Technology Graduate Research Opportunities Fellow has been awarded the Outstanding Teaching Assistant Award of the Rice’s MSNE Department.

Tjahjono’s research aims to aid in NASA’s Artemis mission to the moon and beyond. His objective is to develop mechanically robust and self-healing materials capable of operating under, and protecting astronauts from, extreme space environments such as extreme heat and cold, bombardment from micrometeoroids and orbital debris, and galactic cosmic radiation.

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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Bending 2D nanomaterial could ‘switch on’ future technologies

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Rice University scientists find new workable feature of a class of 2D materials

Rice University materials scientist Boris Yakobson and collaborators uncovered a property of ferroelectric 2D materials that could be exploited as a feature in future devices.

Because they bend in response to an electrical stimulus, single-layer ferroelectric materials can be controlled to act as a nanoscale switch or even a motor, according to the study published in ACS Nano.

Polarization drives the larger atoms to one side of the 2D-material layer and the smaller atoms to the other side. This asymmetrical distribution of the atoms or ions causes the material surface to bend in ferroelectric state.

The study looked at 2D indium phosphide (InP) as a representative of the class of ferroelectrics for which it predicts this property.

“This new property or flexing behavior has to be tested in a laboratory for specific substances,” Yakobson said. “Its most likely use will be as a type of switch. This behavior is very fast, very sensitive, which means that with a very tiny local signal you can maybe switch on a turbine or electrical engine, or control adaptive-optics telescopes’ mirrors. That’s basically the essence of these actuators.”

–  See more at Rice News

A crystal shape conundrum is finally solved

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Rice theorists’ method can predict shapes of crystals that lack symmetry

A crystal’s shape is determined by its inherent chemistry, a characteristic that ultimately determines its final form from the most basic of details. But sometimes the lack of symmetry in a crystal makes the surface energies of its facets unknowable, confounding any theoretical prediction of its shape.

Theorists at Rice University say they’ve found a way around this conundrum by assigning arbitrary latent energies to its surfaces or, in the case of two-dimensional materials, its edges.

Yes, it seems like cheating, but in the same way a magician finds a select card in a deck by narrowing the possibilities, a little algebraic sleight-of-hand goes a long way to solve the problem of predicting a crystal’s shape.

The method described in Nature Computational Science shows using what they call auxiliary edge energies can bring predictions back in line with the Wulff construction, a geometrical recipe in use for more than a century to determine how crystals arrive at their final equilibrium shapes.

–  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

Bumps could smooth quantum investigations

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Rice University models show unique properties of 2D materials stressed by A new theory by Rice University researchers suggests that 2D materials like hexagonal boron nitride, at top, could be placed atop a contoured surface, center, and thus be manipulated to form 1D bands that take on electronic or magnetic properties. Courtesy of the Yakobson Research Groupcontoured substrates

Atoms do weird things when forced out of their comfort zones. Rice University engineers have thought up a new way to give them a nudge.

Materials theorist Boris Yakobson and his team at Rice’s George R. Brown School of Engineering have a theory that changing the contour of a layer of 2D material, thus changing the relationships between its atoms, might be simpler to do than previously thought.

While others twist 2D bilayers — two layers stacked together — of graphene and the like to change their topology, the Rice researchers suggest through computational models that growing or stamping single-layer 2D materials on a carefully designed undulating surface would achieve “an unprecedented level of control” over their magnetic and electronic properties.

They say the discovery opens a path to explore many-body effects, the interactions between multiple microscopic particles, including quantum systems.

The paper by Yakobson and two alumni, co-lead author Sunny Gupta and Henry Yu, of his lab appears in Nature Communications.

– See more at Rice News

Don’t underestimate undulating graphene

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Rice theorists show unique electronics made possible by wavy patterns that channel electrons

A theory by Rice researchers suggests growing graphene on a surface that undulates like an egg crate would stress it enough to create a minute electromagnetic field. The phenomenon could be useful for creating 2D electron optics or valleytronics devices. Illustration by Henry Yu

Lay some graphene down on a wavy surface, and you’ll get a guide to one possible future of two-dimensional electronics.

Rice University scientists put forth the idea that growing a tom-thick graphene on a gently textured surface creates peaks and valleys in the sheets that turn them into “pseudo-electromagnetic” devices.

The channels create their own minute but detectable magnetic fields. According to a study by materials theorist Boris Yakobson, alumnus Henry Yu and research scientist Alex Kutana at Rice’s George R. Brown School of Engineering, these could facilitate nanoscale optical devices like converging lenses or collimators.

Their study appears in the American Chemical Society’s Nano Letters.

– See more at Rice News

Rusting iron can be its own worst enemy

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Rice team’s simulations show iron catalyzes corrosion in ‘inert’ carbon dioxide

 Iron that rusts in water theoretically shouldn’t corrode in contact with an “inert” supercritical fluid of carbon dioxide. But it does.

Materials theorist Boris Yakobson and his colleagues at Rice’s George R. Brown School of Engineering found through atom-level simulations that iron itself plays a role in its own corrosion when exposed to supercritical CO2 (sCO2) and trace amounts of water by promoting the formation of reactive species in the fluid that come back to attack it.

In their research, published in the Cell Press journal Matter, they conclude that thin hydrophobic layers of 2D materials like graphene or hexagonal boron nitride could be employed as a barrier between iron atoms and the reactive elements of sCO2.

“Eliminating corrosion is a constant challenge, and it’s on a lot of people’s minds right now as the government prepares to invest heavily in infrastructure,” said Yakobson, the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry. “Iron is a pillar of infrastructure from ancient times, but only now are we able to get an atomistic understanding of how it corrodes.”

– See more at Rice News

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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

Nickel’s need for speed makes unusual nanoribbons

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Rice lab studies mechanism behind bilayer molybdenum disulfide ribbons

It’s now possible to quickly make ultrathin nanoribbons of molybdenum disulfide, with a speedy nickel nanoparticle leading the way.Materials theorist Boris Yakobson and his team at Rice University’s George R. Brown School of Engineering collaborated with the Honda Research Institute and others to make tightly controlled bilayer nanoribbons of the material commonly known as MoS2, a step forward with potential applications in quantum computing.

Honda, with scientists at Rice, Columbia University and Oak Ridge National Laboratory, found that nanoparticles of nickel exposed to molybdenum oxide and sodium bromide powders and sulfur gas in a chemical vapor deposition furnace wrangle the resulting nanoribbons into shape, constraining their width to several micrometers. At the same time, the nickel catalyzes a thinner second layer of less than 30 nanometers, roughly equivalent to the width of the nanoparticle itself.

The study appears in Science Advances.

– 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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Carbon nanotube research in its 30th year

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Our work featured in ACS Nano virtual issue celebrating CNT research

Since its inception in August 2007, ACS Nano has published over 1000 papers on carbon nanotubes (CNTs). To celebrate the 30th year of CNT research, the journal Editors have collected ~60 papers on CNTs published in ACS Nano, forming a virtual issue. The papers are listed by topic and reflect current trends in the field: preparation, macroassemblies and composites, electronics, energy and sustainability, bioapplications and bioeffects, and optics.

Our recent work on Janus segregation at the CNT–catalyst Interface appears in the Synthesis section of the virtual issue, and Macroassemblies & Composites features our recent comprehensive analysis of the universal strength scaling in CNT bundles with frictional load transfer.

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