. (a) The model unitcell view of ReS₂^[33]; (b) Top view of the crystalline structure of distorted-1T phase of monolayer ReX₂(Black balls represent Re atoms and yellow balls represent S or Se atoms); (c, d) Schematic images of 1T lattice symmetries and energy levels of d-orbital electrons induced by the crystal field^[39,40]; (e) First-principles scalar relativistic projector augmented wave calculations of electronic band structures for bulk (top) and single-layer (down) ReSe₂^[50]; (f) Band structure of monolayer, trilayer and five-layer ReS₂ by ab initio-calculations^[51]

. PL spectra of ReS₂ flakes with different number of layers; (b) Integrated PL intensity as a function of number of layers (normalized to that of monolayer) in ReS₂, MoS₂, MoSe₂, WS₂ and WSe₂^[50]; Raman spectra recorded on (c) N-layer ReS₂ and (d) N-layer ReSe₂ in the parallel polarization configuration^[58]; (e) Schematic for the process of oriented self assembly of ReS₂ nanoscrolls^[59]; (f) Schematic for the TIB of a single ReS₂ nanowall^[60]

. (a) SEM image of ReS₂ powders and TEM image of as-exfoliated ReS₂ nanosheets with inset showing photograph of a typical dark-brown exfoliated ReS₂ suspension in water; (b) High-resolution STEM image of as-exfoliated ReS₂ nanosheets^[70]; (c) Schematics for different density gradient ultracentrifugation ReS₂ nanosheets through iDGU; (d) Atomic force microscopy image of solution-processed ReS₂ following deposition on a Si wafer; (e) Raman spectrum of ReS₂ nanosheets^[63]

. (a) Schematic diagram of synthesized ReS₂ film by PVD; (b) Raman spectrum of ReS₂ film ; (c) Optical photograph of grown ReS₂ film on the SiO₂/Si substrate with inset showing the AFM and TEM images^[88]; (d) A picture of bare and as-grown ReS₂ bilayer film on sapphire wafer by CVD; (e) Optical microscope image of the ReS₂ hexagons^[74]

. (a) Schematic for the tellurium-assisted CVD growth approach; (b) Optical image of ReS₂ after transferred onto SiO₂/Si (300 nm) substrate with inset showing AFM image of ReS₂ on mica substrate^[77]; (c) Schematic of the CVD growth of ReSe₂ in the confined reaction space and the surface reaction during the epitaxial growth of the ReSe₂ atomic layer on mica; (d) Optical image of ReSe₂ in A and B face^[89]

. (a) A schematic illustrating the pump-probe experiment of few-layer ReS₂ with inset showing optical image of few-layer ReS₂; (b) Polarization-dependent absorption spectra of few-layer ReS₂; (c) Corresponding spectral weights of Lorentzian contributions of X₁ (blue dots) and X₂(red dots). Yellow line represents the b-axis^[122]; (d) Raman spectrum for bulk ReS₂^[128] and Low-frequency Raman spectroscopy of few layer ReS₂^[131]; (e) Unpolarized Raman spectra as a function of sample orientation angle; (f) High-magnification ADF-STEM image and corresponded polarization-and orientation-resolved Raman spectra^[130]

. (a) Optical microscope image of ReS₂ four probe transistor; (b) The magnified ADF images taken from the sample in (a); (c) The direction-dependent I-V characteristics with inset showing nonlinear I-V behavior indicate the Schottky Au/ReS₂ contacts; (d) The direction-dependent transfer characteristics^[137]; (e) Transfer curves of anisotropic ReS₂ FETs along two sides with top inset showing optical image of the devices (Scale bar, 10 μm) and low inset showing the 4-probe resistance of the same devices. (f) Normalized field-effect mobility of a six-layer device with inset showing the optical image of the device^[51]; (g) Angle-dependent transfer curves of ReS_1.23Se_0.77 alloy device with inset showing optical image of ReS₂ device^[76]

. (a) The photocurrent of ReS₂ change as a function of drain bias under different polarization light illuminations; (b) The change of the photocurrent under different drain biases plotted as a function of polarization angle^[112]; (c) Photocurrent response of ReS_1.06Se_0.94 alloy device under light on and off irradiation, and under light with different polarization direction; (d) Polar plots for the photocurrent with respect to the polarization angle of the incident light^[76]; (e) Schematic structure of ReSe₂ photodetectors; (f) The SEM image and polarization-dependent photocurrent mapping of the device^[15]

. (a) Optical microscopy image of an exfoliated ReS₂ ?ake; (b) Through-plane TDTR data at two modulation frequencies; (c) In-plane TDTR data at f = 1.1 MHz and time delay of -50 ps. The dashed lines are the intensity profile of the laser beam; (d) 2D beam-offset scan of the TDTR signal; (e) In-plane thermal conductivity of exfoliated ReS₂ ?akes as a function of thickness^[151]

Table 1. Original unit-cell lattice parameters of ReS₂ and ReSe₂^[36,37]

Table 2. The 18 Raman active frequencies in bulk and monolayer ReS₂ under 633 nm solid state laser excitation^[35]