Characterizing mechanical behavior of atomically thin films: A review
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Atomically thin films, such as graphene, graphene oxide, hexagonal-boron nitride (h-BN), and molybdenum disulfide (MoS2), have attracted intensive studies to explore their properties and potential applications as next generation materials due to their outstanding mechanical, electrical, thermal, and optical properties. The study of the mechanical behavior of this class of materials is in particular interesting as it not only physically determines the potential application fields where these materials can be utilized but also has revealed unique mechanical size effects and phenomena. Researchers have been studying the mechanical properties such as elastic modulus, strength, friction, and fracture behavior of atomically thin films for over a decade now. Here, we review recent results of the mechanical characterization and understanding of this class of materials. I. INTRODUCTION
A. Mechanical exfoliation
Single atomic layer two-dimensional films with thicknesses of typically less than 1 nm are commonly known as atomically thin films. Such a class of materials includes (but is not limited to) graphene, graphene oxide, hexagonalboron nitride (h-BN), molybdenum disulfide (MoS2), and niobium diselenide (NbSe2). Ever since Geim and Novoselov1 won the Nobel prize in physics for their identification and characterization of monolayer graphene (;0.333 nm thick), studies on this class of materials have boomed. Their potential industrial applications to composite materials,2–4 lubricants,5,6 electronics,7–9 batteries,10–12 optics,13,14 as well as many more emerging fields have attracted much attention in the last decade. In addition to the well-documented electronic benefits of graphene, its mechanical behavior has become of particular interest recently with respect to the above-mentioned applications. For example, graphene has been demonstrated to be one of the stiffest and strongest materials found in nature,15 which makes it a great candidate for reinforcement in composite materials. Here, we review recent experimental results on the mechanical behavior (e.g., elastic modulus, strength, friction, and fracture) of the most studied atomically thin films which include graphene, graphene oxide, and h-BN.
Mechanically exfoliating graphite using scotch tape to produce graphene films was the first approach demonstrated to obtain pristine graphene [Fig. 1(a)].16 Empirical parameters such as scotch tape adhesion characteristics and exfoliation time have been demonstrated to be important for producing monolayer or few-layers graphene. The advantages of this method are that the graphene films achieved are monocrystalline, and no complicated experimental setup is needed. However, the number of layers and the size of crystal domains are not easily controllable; in addition, the application of this method is limited due to its difficulty of application in mass production. B. Chemical modification of graphite
II. MATERIAL SYNTHESIS
Oxidizing graphite to form graphite oxide or starting with expandable graphite or graphite intercalation compounds to gen
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