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.