Raman and photoluminescence characterization of two-dimensional Janus atomic layers synthesized from single-crystalline gallium selenide by thermal annealing in sulfur atmosphere

Abstract We report a sulfur-induced Janusization of GaSe (Se–Ga–Ga–Se) to form two-dimensional Janus S–Ga–Ga–Se atomic layers. GaSe single crystals were synthesized by vertical Bridgman method, and their exfoliated flakes were annealed in a sulfur atmosphere. Distinct Raman spectra were obtained above 390 °C with new peaks observed around 250 and 300 cm −1 . This indicates the onset of Janus formation in consistent with first-principles calculations. Photoluminescence measurements yielded an emission peak at 2.15 eV, which lies between those of GaS and GaSe. Scanning electron microscope observations and energy-dispersive X-ray spectroscopy mappings had further confirmed the selective sulfur incorporation into the GaSe flakes. This study provides the first experimental evidence for the Janusization of GaSe. It establishes a basis for further structural analyses and a possible way for the Janusization of other layered group-13 monochalcogenides.