Direct and indirect light emissions from layered ReS_2-x Se _x (0 ≤ x ≤ 2)

ReS₂ and ReSe₂ have recently been enthusiastically studied owing to the specific in-plane electrical, optical and structural anisotropy caused by their distorted one-layer trigonal (1 T) phase, whereas other traditional transition-metal dichalcogenides (TMDCs, e.g. MoS₂ and WSe₂) have a hexagonal structure. Because of this special property, more and versatile nano-electronics and nano-optoelectronics devices can be developed. In this work, 2D materials in the series ReS_2-x Se _x (0 ≤ x ≤ 2) have been successfully grown by the method of chemical vapor transport. The direct and indirect resonant emissions of the complete series of layers can be simultaneously detected by polarized micro-photoluminescence (μPL) spectroscopy when the thickness of the ReS_2-x Se _x is greater than ∼70 nm. When it is less than 70 nm, only three direct excitonic emissions-E ₁^ex, E ₂^ex and E _S^ex-are detected. For the thick (bulk) ReS_2-x Se _x , more stacking of the ReX₂ monolayers even flattens and shifts the valence-band maximum from Γ to the other K- or M-related points, thus leading to the coexistence of direct and indirect resonant light emissions from the c-plane ReX₂. The transmittance absorption edge of each bulk ReX₂ (a few microns thick) usually has a lower energy than those of the direct E ₁^ex and E ₂^ex excitonic emissions to form indirect absorption. The coexistence of direct and indirect emissions in ReX₂ is a unique characteristic of a 2D layered semiconductor possessing triclinic low symmetry.