The band offsets and heterostructures of monolayer and few-layer transition-metal dichalcogenides MX 2 (M ¼ Mo, W; X ¼ S, Se, Te) are investigated from first principles calculations. The band alignments between different MX 2 monolayers are calculated using the vacuum level as reference, and a simple model is proposed to explain the observed chemical trends. Some of the monolayers and their heterostructures show band alignments suitable for potential applications in spontaneous water splitting, photovoltaics, and optoelectronics. The strong dependence of the band offset on the number of layers also implicates a possible way of patterning quantum structures with thickness engineering. V

Semiconducting transition metal dichalcogenides consist of monolayers held together by weak forces where the layers are electronically and vibrationally coupled. Isolated monolayers show changes in electronic structure and lattice vibration energies, including a transition from indirect to direct bandgap. Here we present a new member of the family, rhenium disulphide (ReS 2 ), where such variation is absent and bulk behaves as electronically and vibrationally decoupled monolayers stacked together. From bulk to monolayers, ReS 2 remains direct bandgap and its Raman spectrum shows no dependence on the number of layers. Interlayer decoupling is further demonstrated by the insensitivity of the optical absorption and Raman spectrum to interlayer distance modulated by hydrostatic pressure. Theoretical calculations attribute the decoupling to Peierls distortion of the 1T structure of ReS 2 , which prevents ordered stacking and minimizes the interlayer overlap of wavefunctions. Such vanishing interlayer coupling enables probing of two-dimensional-like systems without the need for monolayers.

Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandgap in the bulk limit over to direct bandgap in the quantum (2D) limit. Such a crossover can be achieved by peeling off a multilayer sample to a single layer. For exploration of physical behavior and device applications, it is much desired to reversibly modulate such crossover in a multilayer sample. Here we demonstrate that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion. Such a situation is realized in few-layer MoSe 2 , which shows stark contrast from the well-explored MoS 2 where the indirect and direct bandgaps are far from degenerate. Photoluminescence of few-layer MoSe 2 is much enhanced with the temperature rise, much like the way that the photoluminescence is enhanced due to the bandgap crossover going from the bulk to the quantum limit, offering potential applications involving external modulation of optical properties in 2D semiconductors. The direct bandgap of MoSe 2 , identified at 1.55 eV, may also promise applications in energy conversion involving solar spectrum, as it is close to the optimal bandgap value of single-junction solar cells and photoelechemical devices. KEYWORDS: 2D-Semiconductors, MoSe 2 , MoS 2 , photoluminescence, bandgap, temperature dependence T wo-dimensional (2D) materials have attracted much interest mainly owing to their exotic physical properties that are strikingly different from their three-dimensional (bulk) counterparts. Even though graphene, the most famous member of the 2D material family, possesses extraordinary properties 1 and is readily integrated in various applications, 2−4 the lack of a native bandgap in graphene has led to a broad search for other 2D semiconducting materials. More recently, the transitionmetal dichalcogenide (TMD) semiconductor MoS 2 has been focused on and has shown great potential in the field; singlelayer MoS 2 has been used as an integral part of transistors, 5−8 sensors, 9 and magnetic materials. 10 However, beyond MoS 2 , other layered TMDs offer a large variety of 2D materials with distinct properties.In this work we studied, for the first time, single-layer MoSe 2 mechanically exfoliated onto SiO 2 /Si. 11 Single-layer MoSe 2 displays good thermal stability with a 1.55 eV direct bandgap as determined from photoluminescence (PL) measurements. The PL peak intensity is enhanced dramatically from few-layer to single-layer as a result of the crossover from indirect bandgap in the bulk limit to direct bandgap in the quantum (2D) limit, similar to the behavior of MoS 2 . 12−14 More interestingly, we find that few-layer MoSe 2 flakes posssess a nearly degenerate indirect and direct bandgap, and an increase in temperature can effectively push the system toward the quasi-2D limit by thermally reducing the coupling between the layers. This response in f...