AsianScientist (Jun. 22, 2018) – In a study published in Advanced Materials, scientists in South Korea have devised a method to measure the tensile strength of centimeter-scale, atom-thick graphene.
Graphene is one of the stiffest and thinnest materials in the world. Recently, new and impressive characteristics of ultrathin films of graphene have been unveiled, with promising applications in microelectromechanics, optoelectronics and biology.
However, the mechanical properties of monolayer graphene pieces with lengths or widths bigger than a few micrometers have never been characterized, simply because moving larger pieces of such an ultrathin film to a standard testing apparatus has not been possible.
In this study, researchers at the Center for Multidimensional Carbon Materials of the Institute for Basic Science (IBS) have managed to measure the tensile strength of centimeter-scale monolayer graphene films, using camphor—a chemical that is highly volatile at room temperature—as a temporary support layer.
Measuring the tensile strength of materials involves pulling samples until they break. However, testing two-dimensional (2D) materials is a complex task. Traditionally, a layer of another material, known as the substrate, is used as a ‘tray’ to help with the transfer and to give these ultrathin films some support.
There are pros and cons to the use of such substrates—there is the risk of damaging the film when the substrate is peeled away, and if the substrate layer is too thick, distinguishing the properties of the substrate from those of the 2D material of interest becomes impossible.
Hence, the researchers used camphor as a transient support. Because camphor sublimes in air at room temperature, ultrathin films of graphene with an area larger than 1 cm x 1 cm were successfully transferred without damage. The researchers were thus able to make tensile measurements of centimeter-scale, 300 nm-thick graphene oxide film specimens—almost ten times thinner than previously reported. They also managed to work with a graphene oxide film that was only 35 nm thick and suspend it over a 1 cm x 1 cm hole.
Moreover, the IBS researchers tested the camphor 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 could not be obtained because the thick PC film dominates the response. However, IBS scientists managed to distinguish monolayer graphene properties using the camphor method and a 100 nm-thick PC substrate.
The tensile tests demonstrated that these large-scale samples do have high stiffness. The team measured graphene mechanical properties in terms of the Young’s modulus, which characterizes the intrinsic stiffness of a material. They reported that centimeter-scale polycrystal monolayer graphene had a Young’s modulus of 637-793 GPa, but that single-crystal graphene had very high values at or near 908 GPa. For comparison, a block of high-strength steel has a Young’s modulus of about 200 GPa.
“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,” said Professor Rodney S. Ruoff, director of the IBS Center for Multidimensional Carbon Materials.
The article can be found at: Wang et al. (2018) Camphor-Enabled Transfer and Mechanical Testing of Centimeter-Scale Ultrathin Films.
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Source: Institute for Basic Science; Photo: Shutterstock.
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