We present a systematic investigation of the electronic properties of bulk and few layer ReS2 van der Waals crystals using low temperature optical spectroscopy. Weak photoluminescence emission is observed from two non-degenerate band edge excitonic transitions separated by ∼ 20 meV. The comparable emission intensity of both excitonic transitions is incompatible with a fully thermalized (Boltzmann) distribution of excitons, indicating the hot nature of the emission. While DFT calculations predict bilayer ReS2 to have a direct fundamental band gap, our optical data suggests that the fundamental gap is indirect in all cases.Emerging transition metal dichalcogenides (TMDs) such as MoS 2 , MoSe 2 , MoTe 2 , WS 2 and WSe 2 are attracting great attention due to their remarkable electronic properties. In particular, the energy and the character of the band gap can be easily tuned by varying the number of atomic layers in the crystal. 1-5 The two dimensional confinement and reduced dielectric screening in the single and few layer limit result in significantly enhanced exciton and trion binding energies, 6-9 while the lack of inversion symmetry in the monolayer leads to valley-selective optical selection rules. [10][11][12][13] Recently, layered semiconductors with in-plane anisotropy such as black phosphorus, 14,15 and rhenium dichalcogenides (ReX 2 , where X stands for Se or S atoms) 16 have joined the family of intensively investigated van der Waals crystals. The sizable in-plane crystal asymmetry results in anisotropic optical, 3,5,[17][18][19][20][21][22][23][24][25] and electrical 17,23,[26][27][28] properties, which can be employed in field effect transistors, 29-32 polarization sensitive photodetectors, 33 and new plasmonic devices. 34 The major advantage of Re dichalcogenides over black phosphorus is their stability under ambient conditions, 35 making them potentially interesting for applications.Unlike the more extensively studied Mo and W based dichalcogenides, Re based TMDs crystallize in the distorted 1T' structure (schematically shown in Fig. 1(a)) of lower triclinic symmetry. 4,[36][37][38] In rhenium dichalcogenides, two non-degenerate direct excitons couple to light, as observed in the reflectivity contrast and photoluminescence (PL) spectra. 3,5,39 The strong linear polarization of the excitonic transitions provides a new degree of freedom to control the optical response 40,41 of this material. Despite the flurry of recent investigations of the ReS 2 and ReSe 2 , 16 knowledge about their fundamental electronic properties is extremely limited. For example, the nature of the fundamental band gap remains controversial.Existing band structure calculations provide no consensus concerning the nature of the fundamental band gap. 4,5,38,[42][43][44] Studies regarding the absorption edge indicate that both materials have an indirect band gap in the bulk form. [18][19][20][21]45 This assignment is supported by recent angle-resolved photoemission spectroscopy (ARPES) 44 and photoemission electron microscopy (PEEM) 4...

Monolayers (MLs) of group‐6 transition‐metal dichalcogenides (TMDs) are semiconducting 2D materials with direct bandgap, showing promising applications in various fields of science and technology, such as nanoelectronics and optoelectronics. These MLs can undergo strong elastic deformations, up to about 10%, without any bond breaking. Moreover, the electronic structure and transport properties, which define the performance of these TMD MLs in nanoelectronic devices, can be strongly affected by the presence of point defects, which are often present in the synthetic samples. Thus, it is important to understand both effects on the electronic properties of such MLs. Herein, the electronic structure and energetic properties of defective MoS2 MLs are investigated as subject to various strains, using density functional theory simulations. The results indicate that strain leads to strong modifications of the defect levels inside the bandgap and their orbital characteristics. Strain also splits the degenerate defect levels up to an amount of 450 meV, proposing novel applications.