The isolation of various two-dimensional (2D) materials, and the possibility to combine them in vertical stacks, has created a new paradigm in materials science: heterostructures based on 2D crystals. Such a concept has already proven fruitful for a number of electronic applications in the area of ultrathin and flexible devices. Here, we expand the range of such structures to photoactive ones by using semiconducting transition metal dichalcogenides (TMDCs)/graphene stacks. Van Hove singularities in the electronic density of states of TMDC guarantees enhanced light-matter interactions, leading to enhanced photon absorption and electron-hole creation (which are collected in transparent graphene electrodes). This allows development of extremely efficient flexible photovoltaic devices with photoresponsivity above 0.1 ampere per watt (corresponding to an external quantum efficiency of above 30%).

It has been well established that single layer MX 2 (M=Mo,W and X=S,Se) are direct gap semiconductors with band edges coinciding at the K point in contrast to their indirect gap multilayer counterparts. In few-layer MX 2 , there are two valleys along the Γ-K line with similar energy. There is little understanding on which of the two valleys forms the conduction band minimum (CBM) in this thickness regime. We investigate the conduction band valley structure in

We have studied the optical conductivity of two-dimensional (2D) semiconducting transition metal dichalcogenides (STMDC) using ab initio density functional theory (DFT). We find that this class of materials presents large optical response due to the phenomenon of band nesting. The tendency towards band nesting is enhanced by the presence of van Hove singularities in the bandstructure of these materials. Given that 2D crystals are atomically thin and naturally transparent, our results show that it is possible to have strong photon-electron interactions even in 2D. PACS numbers: 71.20.Mq,78.40.Fy, Semiconductor transition metal dichalcogenides (STMDC) are a family of crystals with a chemical formula MX 2 where M = W, Mo, Ti, Zr, Hf, Pd, Pt, and others, and X = S, Se, Te, 1-3 which can exist in a two-dimensional (2D) structure consisting of one layer of transition metal atoms sandwiched by two layers of chalcogens, all in hexagonal sublattices. They have two known structural polytypes, trigonal prismatic (T) and octahedral (O), which can be distinguished by the relative stacking of the chalcogenide layers. Most 2D STMDC have band gaps in the visible range, between 1 eV and 3 eV, and have been the subject of study in the last few years 4,5 since the emergence of the field of 2D crystals. 6 Because of these band gaps, in a technologically interesting range, these materials are being considered for a new generation of 2D transistor, sensor, and photovoltaic applications.It was discovered recently 7 that these materials have strong optical properties even when they are only three atoms thin. This is rather surprising because atomically thin films like these, only tens of Ångströms in thickness, are naturally transparent and we would not expect a strong photon-electron coupling a priori. In this article, we show that this extraordinary optical response is due to the phenomenon of "band nesting", namely, the fact that in the bandstructure of these materials there are regions where conduction and valence bands are parallel to each other in energy. Band nesting implies that when the material absorbs a photon, the produced electrons and holes propagate with exactly the same, but opposite, velocities. We find that band nesting is present in the bandstructure of all these materials. Furthermore, the existence of strong van Hove singularities (VHS) facilitates the phenomenon of band nesting. In two-dimensional materials, the band-nesting results in a divergence of the joint density of states, leading to very high optical conductivity. We present calculations of the optical response of the 2D STMDC with X =S,Se, illustrating how it is enhanced by the phenomenon of band nesting. A. Band nestingIn semiconductors, the band gap plays an important role in what concerns optical absorption. It defines the threshold after which there is absorption of electromagnetic radiation, by the promotion of an electron from the valence band to the conduction band. But the largest absorption is usually not at the band gap edge; it is often consider...