News

Study explains strength gap between graphene, carbon fiber

posted Oct 27, 2016, 9:09 AM by Evgeni Penev   [ updated Oct 27, 2016, 9:13 AM ]

Rice researchers simulate defects in popular fiber, suggest ways to improve it


Carbon fiber, a pillar of strength in materials manufacturing for decades, isn’t as good as it could be, but there are ways to improve it, according to Rice University scientists.

They found the polymer chains that make up a common carbon fiber are prone to misalign during manufacture, a defect the researchers compared with a faulty zipper that weakens the product.

The Rice lab of theoretical physicist Boris Yakobson set out to analyze these overlooked defects and suggest how they might be curtailed. The lab’s work appears this month in Advanced Materials.

– See more at Rice News

Can Two-Dimensional Boron Superconduct?

posted Oct 13, 2016, 8:26 AM by Evgeni Penev

Nano Letters features our work on its April 2016 issue

Two-dimensional boron is expected to exhibit various structural polymorphs, all being metallic. Additionally, its small atomic mass suggests strong electron–phonon coupling, which in turn can enable superconducting behavior.  In a work that appears this month in the American Chemical Society journal Nano Letters,  we report first-principles analysis of electronic structure, phonon spectra, and electron–phonon coupling of selected 2D boron polymorphs and show that the most stable structures predicted to feasibly form on a metal substrate should also exhibit intrinsic phonon-mediated superconductivity, with estimated critical temperature in the range of T≈ 10–20 K.

The Carbon NanoSolenoid on the Cover of Nano Letters

posted Oct 13, 2016, 8:24 AM by Evgeni Penev

Nano Letters features my image on the Cover of its January 2016 issue

Graphene forms helicoids, akin to the mathematical Riemann surfacefor log(z), naturally occurring as screw dislocations in graphite or anthracite. In the Nano Letters paper, my colleagues demonstrate that the miniscule pitch of such winding carbon ribbons endows them with largest magnetic inductance per volume, which surpasses any current technologies. If voltage is applied, electrical current must flow helically, producing near the center strong magnetic field orders of magnitude greater than that of planet Earth.

The image was produced from the actual atomic geometry, provided by Henry Yu, using VMD, our own Edgecount tool, MeshLabPython scripting, and the mighty Gimp.

Nano Lett. about Nano Letters

posted Oct 13, 2016, 8:22 AM by Evgeni Penev

…or the indentation response of individual CNT junctions

Never mind the ABCs. Rice University scientists interested in nanotubes are studying their XYΩs.

Carbon nanotubes grown in a furnace aren’t always straight. Sometimes they curve and kink, and sometimes they branch off in several directions. The Rice researchers realized they now had the tools available to examine just how tough those branches are.

They used experiments and simulations to study the stiffness of joined nanotubes and found significant differences that are defined by their forms. It turned out that some types are tougher than others, and that all may have their uses if and when nanotubes are used to build macroscale structures.

The team led by Rice materials scientist Pulickel Ajayan and theoretical physicist Boris Yakobson named their nanotubes for their shapes: I for straight nanotubes, Y for branched, X for covalently joined tubes that cross, the lambda symbol (an upside-down “V”) for nanotubes that join at any angle and the omega symbol (Ω) for noncovalent tubes that bind through van der Waals and other forces.

The study was published by the American Chemical Society’s Nano Letters.

– See more at Rice News

Editor’s Highlights for Carbon selects our work on fibers

posted Oct 13, 2016, 8:19 AM by Evgeni Penev   [ updated Oct 13, 2016, 8:20 AM ]

recent work from the group on atomistic modeling of carbon fibers appears in the quarterly Editor’s Highlights for Carbon. These articles are handpicked by the Editors for the reader community and are made freely available for a limited time.

Carbon fiber structure is excessively complex and modeling attempts necessarily rely on various approximations. We have designed structural faults with atomistic details, pertaining to polyacrylonitrile (PAN) derived fibers, and probed them using large-scale molecular dynamics simulations to uncover trends and gain insight into the effect of local structure on the strength of the basic structural units (BSUs) and the role of interfaces between regions with different degrees of graphitization. Besides capturing the expected strength degrading with increasing misalignment, the designed basic structural units reveal atomistic details of local structural failure upon tensile loading.

The image shows an atomistic representation of a BSU (~ 40,000 atoms); for clarity part of the geometry is not rendered. A misoriented block  is highlighted. Load is applied along the fiber axis, as indicated by the thick arrow, by displacing thin slabs (“handles”) at the top and bottom of the system, schematically represented as plates.

Sparking Industrial Breakthroughs

posted Oct 13, 2016, 8:12 AM by Evgeni Penev

2015 CASC Brochure features my artwork

Images illustrating two works I have co-authored are featured in the 2015 Brochure published by the Coalition for Academic Scientific Computation - an alliance of 79 of America’s most forward thinking research universities, national labs and computing centers, including Rice’s Ken Kennedy Institute for Information TechnologyOn page 2, the  highlight box “Something new under the sun” shows a collage based on a recent Nanoscale paper, and our extensive sampling of the CNT end-caps energy landscape, published in ACS Nano, is featured in the top box on page 3.

2D Phosphorus on the Cover of Nano Letters

posted Oct 13, 2016, 8:08 AM by Evgeni Penev   [ updated Oct 13, 2016, 8:10 AM ]

Nano Letters features our work on the cover of its December 2014 issue

In a recent Nano Lett. article, we demonstrate that a 2D mono-elemental semiconductor is a promising candidate. This is exemplified by first-principles study of 2D phosphorus (P), a recently fabricated high-mobility semiconductor. Most of the defects, including intrinsic point defects and grain boundaries, are electronically inactive, thanks to the homoelemental bonding, which is not preferred in heteroelemental system such as MX2. Unlike MX2, the edges of which create deep gap states and cannot be ubeliminated by passivation, the edge states of 2D P can be removed from the band gap by hydrogen termination. We further find that both the type and the concentration of charge carriers in 2D P can be tuned by doping with foreign atoms.

The cover image I have designed represents a “phosphorescent” rendering of some structural and electronic signatures of 2D phosphorus arranged in a collage inspired by the digital rain from “The Matrix” movie.

See more at: Rice News: Phosphorus ‘rain’

Why do nanotubes grow chiral?

posted Oct 11, 2016, 3:49 PM by Evgeni Penev

A question no more...

Carbon nanotubes hold enormous technological promise. It can only be harnessed if one controls their chirality, the feature of the tubular ​carbon topology that governs all the properties of nanotubes—electronic, optical, mechanical. Experiments in catalytic growth over the last decade have repeatedly revealed a puzzling strong preference towards minimally chiral (near-armchair) tubes, challenging any existing hypotheses and making chirality control ever more tantalizing, yet leaving its understanding elusive.

In a just-published article in Nature Communications, we combine the nanotube/catalyst interface thermodynamics with the kinetic growth theory to show that the unusual near-armchair peaks 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 image illustrates a very specific feature of the minimally chiral tubes: they can tilt off the vertical! And when it comes to tilting, or perhaps leaning, there is nothing more iconic than the Leaning Tower of Pisa!

In The News

Imperfect 2D Phosphorus: Yet A Perfect Semiconductor

posted Oct 11, 2016, 12:16 PM by Evgeni Penev

The deep gap states created by defects in semiconductors typically deteriorate the performance of (opto)electronic devices. This has limited the applications of two-dimensional (2D) metal dichalcogenides (MX2) and underscored the need for a new 2D semiconductor without defect-induced deep gap states.

In a recent Nano Lett. article, we demonstrate that a 2D mono-elemental semiconductor is a promising candidate. This is exemplified by first-principles study of 2D phosphorus (P), a recently fabricated high-mobility semiconductor. Most of the defects, including intrinsic point defects and grain boundaries, are electronically inactive, thanks to the homoelemental bonding, which is not preferred in heteroelemental system such as MX2. Unlike MX2, the edges of which create deep gap states and cannot be eliminated by passivation, the edge states of 2D P can be removed from the band gap by hydrogen termination. We further find that both the type and the concentration of charge carriers in 2D P can be tuned by doping with foreign atoms.

The image represents a "phosphorescent" rendering of a major defect in 2D phosphorus: a puckered pair of a heptagon and a pentagon. Although it alters the ideal hexagonal bonding pattern, it is innocuous to the ultimate electronic properties that make this material truly attractive for lossless electronics.

In The News

Caps story

posted Oct 11, 2016, 12:11 PM by Evgeni Penev

Caps not the culprit in nanotube chirality

In the formation of a carbon nanotube (CNT) nucleus, a hemispherical fullerene end-cap, a specific pattern of six pentagons encodes what unique (n,m) chirality a nascent CNT would inherit, with many possible pentagon patterns corresponding to a single chirality. This configurational variety and its potential role in the initial stages of CNT catalytic growth remain essentially unexplored. Here we present large-scale calculations designed to evaluate the intrinsic energies of all possible CNT caps for selected chiralities corresponding to tube diameters d  1 nm. Our quantitative analysis reveals that for all chiral angles χ the energy scale variability associated with the CNT caps is small, compared to that of the CNT/catalyst interface. Such a flat energy landscape cannot therefore be a dominant factor for chiral distribution and lends further credibility to interface-controlled scenarios for selective growth of single-walled CNT of desired chirality.

In The News

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