Graphene and transition metal dichalcogenides (TMDCs) are the two major types of layered materials under intensive investigation. However, the zero-bandgap nature of graphene and the relatively low mobility in TMDCs limit their applications. Here we reintroduce black phosphorus (BP), the most stable allotrope of phosphorus with strong intrinsic in-plane anisotropy, to the layered-material family. For 15-nm-thick BP, we measure a Hall mobility of 1,000 and 600 cm 2 V À 1 s À 1 for holes along the light (x) and heavy (y) effective mass directions at 120 K. BP thin films also exhibit large and anisotropic in-plane optical conductivity from 2 to 5 mm. Field-effect transistors using 5 nm BP along x direction exhibit an on-off current ratio exceeding 10 5 , a field-effect mobility of 205 cm 2 V À 1 s À 1 , and good current saturation characteristics all at room temperature. BP shows great potential for thin-film electronics, infrared optoelectronics and novel devices in which anisotropic properties are desirable.

Abstract. The emerging two-dimensional (2D) materials exhibit a wide range of electronic properties, ranging from insulating hexagonal boron nitride (hBN), semiconducting transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS 2 ) and tungsten diselenide (WSe 2 ), to semi-metallic graphene. The plethora of 2D materials together with their heterostructures, which are free of the traditional "lattice mismatch" issue, brings new opportunities for exploring novel optical phenomena. In this review, we first discuss the optical properties and applications of a variety of 2D materials, followed by two different approaches to enhance their interactions with light: through their integration with external photonic structures and through their intrinsic polaritonic resonances. Finally, we cover a narrow bandgap layered material, black phosphorus, which serendipitously bridges the zero gap graphene and the relatively large-bandgap TMDCs such as MoS 2 and WSe 2 . The combination of these materials and the approaches for enhancing light-matter interaction offers the promise of scientific discoveries and nanophotonics technologies across a wide range of electromagnetic spectrum.

operation, and good internal quantum efficiency. ╪These authors contributed equally to this work *

Moreover, owing to the unique band structure and exceptional electronic properties of graphene, high speed photodetectors with an ultra-wide operational wavelength range at least from 300 nm to 6 µm 10, 11 can be realized using this fascinating material.

The isolation of graphene in 2004 from graphite was a defining moment for the "birth" of a field: two-dimensional (2D) materials. In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement. Here, we review significant recent advances and important new developments in 2D materials "beyond graphene". We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies. Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (i.e., silicene, phosphorene, etc.) and transition metal carbide- and carbon nitride-based MXenes. We then discuss the doping and functionalization of 2D materials beyond graphene that enable device applications, followed by advances in electronic, optoelectronic, and magnetic devices and theory. Finally, we provide perspectives on the future of 2D materials beyond graphene.