One of the landmark features of sTMDs is that they undergo a crossover from indirect bandgap in the bulk to direct bandgap in monolayers 4, and as a result, monolayer sTMDs become direct-gap semiconductors that absorb and emit light efficiently. Monolayer sTMDs are promising functional materials for next-generation flexible optoelectronics and photovoltaics applications owing to their mechanical flexibility, chemically and environmentally stability, optical properties and low operating voltages in various device configurations 1, 2, 3. Such vanishing interlayer coupling enables probing of two-dimensional-like systems without the need for monolayers.Ītomically thin monolayer transition metal dichalcogenides (sTMDs) are a new class of two-dimensional (2D) materials with the chemical formula MX 2, where M is a transition metal (Mo, W and Re) and X is a chalcogen (S, Se and Te) element. 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.
Interlayer decoupling is further demonstrated by the insensitivity of the optical absorption and Raman spectrum to interlayer distance modulated by hydrostatic pressure. From bulk to monolayers, ReS 2 remains direct bandgap and its Raman spectrum shows no dependence on the number of layers. 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. Isolated monolayers show changes in electronic structure and lattice vibration energies, including a transition from indirect to direct bandgap. Semiconducting transition metal dichalcogenides consist of monolayers held together by weak forces where the layers are electronically and vibrationally coupled.
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