Chemical vapor deposition growth and characterization of ReSe₂

Two-dimensional (2D) ReSe₂ flakes were grown by chemical vapor deposition and investigated at room temperature using Raman, photoluminescence, and absorption spectroscopies. The Raman spectra revealed eighteen phonon modes in the range of 100-300 cm^-1, which were found to be in good agreement with the density functional theory (DFT) calculations. The thickness profiles of the ReSe₂ flakes are in the range of 5-50 nm. The ReSe₂ crystal structure and morphology were investigated using XRD, atomic force microscopy and scanning electron microscopy. Energy dispersion spectroscopy confirmed the 1 : 2 elemental composition. The absorption spectra of ReSe₂ flakes exhibited excitonic peaks in the spectral region of 885-942 nm (1.401-1.316 eV). These peaks are used to define the band gap of the material. The DFT calculations predicted an indirect bandgap of 0.88 eV for the bulk structure, while a direct bandgap of 1.26 eV was predicted for the monolayer.