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Updated by Angela Laguipo on 23 July 2019
The outstanding thermal, electronic, and mechanical properties of graphene made it possible for it to become one of the most popular materials to be used in almost every industry. These include biological engineering, composite materials, photovoltaic cells, and energy source, among others.1
While these properties are impressive, the functionality of graphene is extremely sensitive to any cracks or defects present within its sheets.
As the interest in utilizing graphene and other similar two-dimensional (2D) materials, such as silicone, boron nitride, and molybdenum disulfide continues to rise, a further understanding into how these defects occur and affect the various properties of this material follow.
The unique properties of graphene also paved the way for it to be used as a potential artificial robotic skin. To understand further, it’s important to know more about graphene’s properties.
Graphene’s Mechanical Behavior and Design
By obtaining the ability to predict the ultimate behavior of graphene and relating materials in the presence of defects, researchers and interested industries can have a full understanding of the mechanical behavior and design of these materials. These represent an advantage in predicting the behavior of graphene when manipulated into nano-based systems.
The ability of graphene to self-heal any defects and topological cracks have been recorded when metal doping has been applied. Metal doping, a process in which impurities are intentionally introduced into an intrinsic semiconductor, allows for the current electrical properties of the semiconductor to be modulated and often enhanced.2
Graphene’s Self-Healing Properties
Until recently, the self-healing properties of graphene without the aid of any external stimulus have not been recorded. A group of researchers located in Hyderabad, India have studied these self-healing properties of graphene, and have confirmed the phenomena of this material to auto-correct ‘in-situ’ cracks present within a single layer of the material, without the assistance from any other processes.3
A team of researchers led by Amit Acharyya simulated defects onto the graphene through a tensile test. The tensile testing of the graphene sheet, which measured 5 x 5 nm, was performed in a controlled setting where a strain loading was put onto the graphene in increments of 0.001/picosecond (ps) until the ultimate tensile strength of the test material was achieved.3
To ensure that the self-healing phenomena of graphene was true, each of the forces being put onto the material were removed following the tensile test, which was followed by a relaxation period of 150 ps. Once the graphene sheet completed this stage of relaxation, the cracks present within the material were found to heal themselves, if they fell within the opening displacement range of 0.3-0.5 nm3.
The effect of the various types of applied imperfects and defects applied to the graphene sheets were analyzed by molecular dynamic (MD) simulations, which were performed using large-scale atomic/molecular massively parallel simulator (LAAMPS). Running on a single processor, LAAMPS is a type of MD code that models a variety of particles within a liquid, gaseous or solid-state. MD simulation confirmed the self-healing properties of graphene through a mechanism that relies heavily on the deletion of atoms during the development of the defects.5
While atoms that are initially deleted cannot heal because of the lack of any extra atoms to fill in the defect, crack healing remained an evident process of treatment in response to these cracks, if the critical gap between the two separated graphene segments is less than or equal to 0.5 nm. It was also observed that as the length of the defect, such as single to multiple cracks or vacancies, increases, the time required for the crack within the material to heal increased as well.
Potential of Graphene as an Artificial Skin
The work performed in this study showed the potential of graphene sheets to be utilized for artificial skin that could be applied in future robots. Skin, which is the largest organ in the body, exhibits exceptional self-healing properties that have, until now, been almost impossible to replicate.4 To date, the artificial skin that has been applied to robots are extremely susceptible to accidental scratches and unprecedented stretching or bending.
In their proposal, Acharyya’s team hopes that a sub-nano sensor composed of graphene could potentially sense a crack as soon as it begins nucleation, which could allow researchers and developers to address a crack or rupture immediately before it spreads.
In a recent study titled, “Triboelectric electronic-skin based on graphene quantum dots for application in self-powered, smart, artificial fingers”, the researchers formulated e-skins or electronic skin that can respond sensitively to various stimuli. The artificial skin made use of graphene quantum dot-coated nano-wires to boost the sensitivity.
- "Graphene Applications & Uses." Graphenea. Web. https://www.graphenea.com/pages/graphene-uses-applications#.WNmRMVeTTww.
- A. H. Wilson (1965). The Theory of Metals (2nd ed.). Cambridge University Press.
- K. VijayaSekhar, Swati Ghosh Acharyya, Sanghamitra Debroy, V. Pavan Kumar Miriyala, Amit Acharyya. Self-healing phenomena of graphene: potential and applications. Open Physics, 2016; 14.
- "Self-healing Graphene Holds Promise for Artificial Skin in Future Robots." ScienceDaily. ScienceDaily, 21 Mar. 2017. Web. https://www.sciencedaily.com/releases/2017/03/170321125011.htm.
- "LAAMPS." Sandia National Laboratories.
This article was updated on the 23rd July 2019.