The mechanical properties of materials are not only indispensable key factors in their application fields, but are also fundamentally important in terms of materials science. Since the successful isolation of graphene with an atomic thickness, two-dimensional (2D) materials have attracted enormous attention over the past decade due to their unique properties. In particular, 2D materials are of interest owing to their outstanding mechanical properties, such as high Young’s modulus and strength, despite their ultrathinness and low weight in comparison to their bulk counterparts. However, studies on the mechanical properties of various 2D materials have been limited, with the exception of graphene, leaving many open questions and challenges. In this article, recent empirical and theoretical advances in studies of the mechanical properties of 2D materials and their applications are reviewed. First, mechanical characterization methods, which are widely used for ultrathin membranes, are summarized. The effects of defects on the mechanical properties of 2D materials are reviewed, including naturally (or intentionally) generated defects and chemically functionalized 2D materials. Finally, we discuss recent advances and the possibility of using 2D materials in diverse mechanical applications. The summary of the unique mechanical properties of 2D materials and their derivatives in this article would be beneficial for the study of 2D materials and their applications in lightweight, flexible, and transparent systems.

The advent of 3D printing has had a disruptive impact in manufacturing and can potentially revolutionize industrial fields. Thermoplastic materials printable into complex structures are widely employed for 3D printing. Polylactic acid (PLA) is among the most promising polymers used for 3D printing, owing to its low cost, biodegradability, and nontoxicity. However, PLA is electrically insulating and mechanically weak; this limits its use in a variety of 3D printing applications. This study demonstrates a straightforward and environment-friendly method to fabricate conductive and mechanically reinforced PLA composites by incorporating graphene nanoplatelets (GNPs). To fully utilize the superior electrical and mechanical properties of graphene, liquid-exfoliated GNPs are dispersed in isopropyl alcohol without the addition of any surfactant and combined with PLA dissolved in chloroform. The GNP–PLA composites exhibit improved mechanical properties (improvement in tensile strength by 44% and maximum strain by 57%) even at a low GNP threshold concentration of 2 wt %. The GNP–PLA composites also exhibit an electrical conductivity of over 1 mS/cm at >1.2 wt %. The GNP–PLA composites can be 3D-printed into various features with electrical conductivity and mechanical flexibility. This work presents a new direction toward advanced 3D printing technology by providing higher flexibility in designing multifunctional 3D printed features.

Monolayer transition metal dichalcogenides (TMDs) are promising for optoelectronics because of their high optical quantum yield and strong light-matter interaction. In particular, the van der Waals (vdW) heterostructures consisting of monolayer TMDs sandwiched by large gap hexagonal boron nitride have shown great potential for novel optoelectronic devices. However, a complicated stacking process limits scalability and practical applications. Furthermore, even though lots of efforts, such as fabrication of vdW heterointerfaces, modification of the surface, and structural phase transition, have been devoted to preserve or modulate the properties of TMDs, high environmental sensitivity and damage-prone characteristics of TMDs make it difficult to achieve a controllable technique for surface/interface engineering. Here, we demonstrate a novel way to fabricate multiple two-dimensional (2D) vdW heterostructures consisting of alternately stacked MoS2 and MoO x with enhanced photoluminescence (PL). We directly oxidized multilayer MoS2 to a MoO x /1 L-MoS2 heterostructure with atomic layer precision through a customized oxygen plasma system. The monolayer MoS2 covered by MoO x showed an enhanced PL intensity 3.2 and 6.5 times higher in average than the as-exfoliated 1 L- and 2 L-MoS2 because of preserved crystallinity and compensated dedoping by MoO x . By using layer-by-layer oxidation and transfer processes, we fabricated the heterostructures of MoO x /MoS2/MoO x /MoS2, where the MoS2 monolayers are separated by MoO x . The heterostructures showed the multiplied PL intensity as the number of embedded MoS2 layers increases because of suppression of the nonradiative trion formation and interlayer decoupling between stacked MoS2 layers. Our work shows a novel way toward the fabrication of 2D material-based multiple vdW heterostructures and our layer-by-layer oxidation process is beneficial for the fabrication of high performance 2D optoelectronic devices.

Covalent functionalization of the surface is more crucial in 2D materials than in conventional bulk materials because of their atomic thinness, large surface-to-volume ratio, and uniform surface chemical potential. Because...

No abstract