Viewpoint: Graphene Is Skinny, however Not Infinitely So

John Proctor, College of Computing, Science & Engineering, College of Salford, Salford, United Kingdom

September 25, 2019• Physics 12, 104

Atomically skinny graphene is taken into account a prototypical 2D materials, however high-pressure experiments now reveal the 3D nature of its mechanical properties.

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Determine 1: Graphene is just one carbon atom thick and appears 2D. However it has digital orbitals that stretch perpendicularly to the atomic airplane, such because the 2pz orbitals (mild pink). These orbitals resist compression within the route perpendicular to graphene’s lattice, a attribute of 3D supplies.Graphene is just one carbon atom thick and appears 2D. However it has digital orbitals that stretch perpendicularly to the atomic airplane, such because the 2pz orbitals (mild pink). These orbitals resist compression within the route perpendicular to graphene’s… Present extra

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Determine 1: Graphene is just one carbon atom thick and appears 2D. However it has digital orbitals that stretch perpendicularly to the atomic airplane, such because the 2pz orbitals (mild pink). These orbitals resist compression within the route perpendicular to graphene’s lattice, a attribute of 3D supplies.×

We dwell in a 3D world, but 2D supplies are one of many hottest subjects in physics proper now. Graphene (Fig. 1) is probably essentially the most well-known instance. Fairly merely, the fabric “seems to be” 2D as a result of it is just one atom thick. However does it act 2D? By way of digital properties, the reply is sure, as graphene has an digital band construction that’s completely different from its 3D counterpart, graphite. Equally, graphene’s thermal growth reveals telltale 2D traits. However what about graphene’s mechanical properties? Yiwei Solar of Queen Mary College of London and colleagues have addressed this query by subjecting this prototypical 2D materials to a conventional 3D experiment [1]. The workforce compressed flakes of graphene to a stress of 12 GPa (120,000 instances higher than atmospheric stress) and confirmed that the ensuing vitality shift of graphene’s vibrations (phonons) is in step with a 3D materials slightly than a 2D one. The discovering could have implications for purposes of graphene and different 2D supplies as mechanical sensors and structural reinforcements.

The very first thing to make clear is that no utterly flat, freestanding materials would ever be steady. The reason being that phonons perpendicular to the 2D airplane will, colloquially talking, all the time shake the fabric aside at any temperature above absolute zero [2–4]. The foundation of this impact is how phonons behave in a 2D materials: the phonon density of states is linear with wave vector okay, leading to quite a lot of phonons at low vitality. As compared, the density of phonon states in 3D supplies falls off a lot quicker at low vitality as a result of it varies as k2. Theorists knew the disruptive impact of phonons lengthy earlier than 2D supplies got here into vogue [2], so discovering a genuinely steady 2D materials would have raised some critical theoretical issues.

Researchers have been in a position to stabilize graphene and associated households of supplies by supporting them on a substrate—although the supplies nonetheless have ripples of their form. Nevertheless, using a substrate implies that measurements of graphene’s properties must account for the impact of the fabric assist. A few decade in the past, my colleagues and I turned this downside into a possibility through the use of measurements on a graphene pattern supported by a substrate and at excessive stress to deduce properties of graphene underneath recognized pressure [5]. In our experiment, the graphene adhered strongly to the substrate, so any compression of the substrate translated right into a recognized pressure on the graphene. However due to the substrate, such an experiment can’t instantly decide the stress on graphene, which is the parameter utilized in fashions of mechanical properties.

Of their work, Solar et al. go one higher than the earlier examine. They cast off the supporting substrate by suspending flakes of monolayer graphene in a viscous liquid [1]. The liquid prevents the flakes from crumpling and/or bonding collectively to type graphite for lengthy sufficient to carry out an experiment. With this method, an utilized stress on the liquid will translate instantly right into a recognized stress on the graphene.

Solar et al. squeezed the graphene-containing liquid between two anvil-shaped diamonds—a setup often known as a diamond anvil cell (DAC). Utilizing optical Raman spectroscopy, they made an in depth and cautious examine of the vibrations within the graphene sheet at excessive stress. Their evaluation demonstrates that—so far as mechanical properties are involved—we can not deal with graphene as a totally 2D materials. Particularly, they discover that the vibrations shift to larger values with stress. The identical qualitative habits is seen in each 2D and 3D supplies, however to precisely mannequin and clarify the noticed shifts in graphene, Solar et al. needed to deal with it as a 3D materials with an out-of-plane elastic fixed—which means graphene can turn into thinner underneath compression. Graphene, it appears, is extra like cardboard than a sheet of paper.

Excessive-pressure experiments like these are simple to explain, however they’re notoriously troublesome to carry out. Fiddly handiwork is required to align the DAC and pattern with micrometer-precision. Due to these calls for, such experiments even have a excessive failure charge. Solar and colleagues’ capability to check graphene underneath a recognized excessive stress—a primary—is due to this fact a significant achievement.

The workforce’s outcomes will even have concrete implications. If graphene had been infinitely skinny, one would solely have to know its in-plane elastic fixed to work out how it will behave underneath any type of stress. The truth that graphene is 3D means the out-of-plane route issues as nicely. From a microscopic perspective, this conclusion is smart: though graphene is one atom thick, every atom has digital orbitals (the 2pz orbitals) that stretch a ways above and beneath the graphene sheet and resist compression (Fig. 1). Subsequently, it’s significant to explain elastic properties in relation to this axis in addition to the in-plane axes.

We are able to count on to see this analysis used within the growth of pressure sensors primarily based on graphene. It could additionally have an effect on how Raman spectroscopy is used as a diagnostic device for brand new kinds of graphene composites that serve to bolster different supplies. Right here, the spectroscopy helps decide the extent to which stress or pressure is transferred from the host materials to the graphene reinforcement. Realizing graphene’s 3D traits will assist researchers optimize this reinforcing habits.

This analysis is revealed in Bodily Evaluation Letters.


Y. W. Solar, W. Liu, I. Hernandez, J. Gonzalez, F. Rodriguez, D. J. Dunstan, and C. J. Humphreys, “3D pressure in 2D supplies: To what extent is monolayer graphene graphite?,” Phys. Rev. Lett. 123, 135501 (2019).A. Ok. Geim and Ok. S. Novoselov, “The rise of graphene,” Nat. Mater. 6, 183 (2007).A. H. Castro Neto, F. Guinea, N. M. R. Peres, Ok. S. Novoselov, and A. Ok. Geim, “The digital properties of graphene,” Rev. Mod. Phys. 81, 109 (2009).J. E. Proctor, D. A. Melendrez Armada, and A. Vijayaraghavan, An Introduction to Graphene and Carbon Nanotubes (CRC Press, Boca Raton, 2017)[Amazon][WorldCat].J. E. Proctor, E. Gregoryanz, Ok. S. Novoselov, M. Lotya, J. N. Coleman, and M. P. Halsall, “Excessive-pressure Raman spectroscopy of graphene,” Phys. Rev. B 80, 073408 (2009).

Concerning the Writer

Image of John Proctor

John Proctor obtained his Ph.D. from the College of Manchester in 2008. Since then, his analysis has been dedicated to the sector of high-pressure physics. Primarily utilizing the diamond anvil high-pressure cell, he has targeted on nanotubes and graphene, superhard supplies, hydrides, and, in recent times, supercritical fluids. He’s now a senior lecturer in Physics and head of the Supplies and Physics analysis group on the College of Salford. He not too long ago revealed his first textbook, An Introduction to Graphene and Carbon Nanotubes, and he has written for Physics World.

Topic Areas

GrapheneCondensed Matter Physics

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