New method to determine mechanical stiffness and strength of centimeter-scale atom thick Graphene

A new camphor-method.

Distinguished Professor Rodney S. Ruoff from the Center for Multidimensional Carbon Materials (CMCM), within the Institute for Basic Science (IBS) at UNIST.
Distinguished Professor Rodney S. Ruoff from the Center for Multidimensional Carbon Materials (CMCM), within the Institute for Basic Science (IBS) at UNIST.

An international team of scientists led by UNIST has successfully developed a method to measure the tensile strength of centimeter-scale monolayer graphene for the first time, using camphor as a naturally-volatilizing support.

Until now, it was difficult to determine the mechanical properties of monolayer graphene pieces, as they are bigger than a few micrometers. Therefore, moving such an ultrathin film to a standard testing apparatus has not been possible.

Now, scientists under the guidance of Professor Rodney S. Ruoff from Center for Multidimensional Carbon Materials (CMCM), within the Institute for Basic Science (IBS) at UNIST, have unveiled new and impressive characteristics of ultrathin films of graphene. Those specifications involve: the combination of graphene’s transparency, impermeability, conductivity, and elasticity could be used for flexible electronics, transparent protective coatings, and barrier films.

Estimating the tensile strength of materials includes pulling samples until the point when they break. In any case, testing 2D materials is a complex task. Generally, a layer of another material (substrate) is utilized as a “tray”, to help with the exchange and to give these ultrathin films some help.

Such substrates have advantages and disadvantages: there is the danger of damaging the film when the substrate is peeled away, or if the substrate layer is too thick, recognizing the properties of the substrate from the ones of the 2D material of interest becomes impossible.

Scientists used camphor as a transient help. Camphor, which is established in the bark and wood of the camphor laurel tree, is a waxy, combustible, white or transparent solid with a strong, aromatic fragrance. It is ineffectively dissolvable in water, however exceedingly dissolvable in organic solvents. Thus, it is usually utilized as a preservative or an embalming fluid and also for medical purposes.

What differentiates this from conventional methods is that it is sublimed away in air at room temperature naturally, or at higher temperatures for faster processing.

Scientists tested their method with one-atom-thick graphene films. In this case, stretching a freestanding monolayer 2D film is not yet possible, and a polycarbonate (PC) film is used as a support layer. Without camphor, the thinnest PC/monolayer graphene assembly tested was around 1 μm thick, and graphene’s mechanical properties couldn’t be gotten as the thick PC film overwhelms the response. Although, IBS researchers figured out how to recognize monolayer graphene properties utilizing the camphor technique and a 100 nm thick PC substrate.

The tensile tests showed that these substantial scales samples do have high firmness. The group estimated graphene mechanical properties in terms of Young’s modulus, which describes the natural firmness of a material. The investigation reports that centimeter-scale polycrystal monolayer graphene has Young’s modulus of 637-793 GPa, however, that single-crystal graphene had top of the line esteems at or near 908 GPa. As a matter of examination, a block of high strength steel has Young’s modulus of around 200 GPa, yet representing the higher density of steel, that is, revised for weight, the intrinsic stiffness of large-scale graphene is around 20 times higher.

Professor Ruoff said, “I think this method might become a standard around the world for testing single layer 2D materials, including graphene at length scales meaningful for applications.”

Bin Wang, the first author of the study said, “Strong carbon-carbon bonds confer graphene with an impressive ideal strength, which is however lowered if there are defects in its structure. The ideal strength of graphene is about 130 GPa, second only to the ideal strength of diamond along with a particular crystallographic direction (240 GPa). But these are calculated strengths, and are never obtained for “large” specimens, like a centimeter or larger—because such specimens always have defects, also called flaws, like small cracks.”

“Now that we have this new method for measuring large graphene specimens, we can begin to understand why the strength for our graphene samples, even the single crystal graphene samples, is typically around 8 GPa, and not higher. Understanding this will provide us a “road map” to make graphene that is much stronger—that breaks at a much higher value of percent elongation,” continues Professor Ruoff. “And when we do that—that will change the world.”

Scientists reported their findings in the journal Advanced Materials.