The great research progresses in materials sciences in the past decades have revealed that reduced dimension can have significant influences on the properties of materials. Since the successful fabrication of graphene in 2004 by Novoselov and Geim, two-dimensional (2D) crystals have attracted great attention because of their novel properties and great application potentials. 2D materials are single and fewatom- thick layers of crystalline materials. Within a single layer of 2D materials, atoms are connected through strong covalent bonds, whereas neighboring layers are typically only bonded via weak van der Waals interactions. With the rapid developments in both top-down and bottom-up synthesis methods, a lot of new 2D materials beyond graphene have been uncovered, such as BN sheet, silicene, graphdiyne, transition metal chalcogenides and black phosphorus. In 2D materials, electrons are mainly confined into planes, leading to many novel electronic properties such as massless carriers, strong excitonic effects and valley polarization that have potential application in the next generation of electronic devices. In addition, due to the 2D character, 2D materials have maximum mechanical flexibility and optical transparency which are feasible for highly flexible and transparent electronic/optoelectronic devices. Moreover, van der Waals heterostructures can be formed through stacking of different 2D materials. Such heterostructures have atomic sharp interface without dangling bond, and there is no lattice-match requirement. In terms of material choice and layer thickness control, there can be enormous variability in designing van der Waals heterostructures, which is promising to fulfill the demands of future nanoelectronic industry on multifunctionality. Although the industrialization of 2D materials is still at a relatively early stage, based on the research achievements made, we fully believe that 2D materials could be utilized for daily life in the future.

Here we organized a specific topic on 2D materials and devices, and we are grateful to the 12 groups of researchers who are working in 2D materials and were invited to either overview recent important progress in literature or contribute their research progress in 2D materials. We have 5 reviews focusing on the synthesis of 2D BN, the Raman properties of transition metal dichalcogenides, and van der Waals heterostructures, optoelectronics based on 2D transition metal dichalcogenides (TMDs) and heterostructures, respectively. In addition, we have 7 research papers covering the synthesis and imaging, photoresponse, doping properties, band structure modulation, and quantum transport in various 2D materials. The topics presented in this special issue are all the main and/or future research directions in 2D materials. We sincerely hope that the published papers in this special issue can provide useful information to the readers working in this hot area.