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Preface to the Special Issue on 2D-Materials-Related Physical Properties and Optoelectronic Devices

Recent advances in two-dimensional (2D) materials following the successful fabrication of graphene in 2004 by Novoselov and Geim is expected to grow into the new silicon, offering a lifeline for Moore’s law. With the rapid development of the synthesis methods, more and more 2D materials, such as transition metal dichalcogenides (TMDs, MX 2 ), black phosphorus (BP) and InSe with a finite gap are reported to be more promising for achieving this dream since they often offer alternative solutions to compensate for the gapless graphene’s weaknesses. 2D materials are single or few-layer thick, in which the atoms in a single layer connect with the strong covalent bonds while the adjacent layers are bonded via weak van der Waals (vdW) interactions. The ultrathin thickness and high mobility of 2D materials have opened new pathways for materials with just one-atom thickness to be used in the post-silicon electronic era. Furthermore, the electrons in 2D materials are almost confined in a 2D plane, leading to massless carriers, strong excitonic effects and valley polarization. In addition, some 2D materials, such as BP and ReS(Se) 2 , exhibit strong in-plane anisotropy, offering a new degree of freedom to deliver various physical properties compared with the isotropic 2D materials. Any two or more 2D materials with similar properties can be alloyed into a new layered material, namely, 2D alloy. More interestingly, the combination of these emergent 2D materials with rich properties paves way for fabricating vdW heterostructures. The electronic and optoelectronic