Selective Raman modes and strong photoluminescence of gallium selenide flakes on sp² carbon

Two-dimensional materials awakened a strong interest in the scientific and technological communities due to their exceptional properties that can be tuned by the material thickness and chemistry. In order to correlate optical properties with crystallographic structure and morphology, in this work, the authors aim at studying GaSe nanoflakes deposited on highly ordered pyrolytic graphite by means of atomic force microscopy, Raman, and photoluminescence (PL) spectroscopies. The authors found that the basal plane of the flakes can be attributed to the ε-phase expected for bulk samples grown by the Bridgman method. However, a strong difference in the Raman spectra was systematically found at the edge of our GaSe flakes. Forbidden Raman modes located around 250 cm^-1 were selectively observed at specific locations. These modes could not be directly attributed to the ε-phase observed in the basal plane or in the bulk. The atomic force microscopy investigations show that high topographical features characterize the regions with the highest photoluminescence and the forbidden Raman modes. This change in crystal orientation at selective locations of the flake could activate the E′(LO) Raman mode at 255 cm^-1. The enhancement of the PL emission around 620 nm is also linked to the change in crystal orientation due to an increase in the probability of electronic transitions. These results show the rich physics of GaSe flakes in contrast to what is known from its bulk counterpart with possible applications in photovoltaics and photodetectors for nanoflakes with engineered geometries.