Thermoreflectance characterization of the band-edge excitons observed in multilayered CuInP2S6

Abstract Exciton is a bound state that consisted of one electron and one hole attracted each other by electrostatic Columbic force in materials. Especially in a two-dimensional (2D) semiconductor, an exciton is generated by absorption of a photon with energy larger than band gap and it may survive stably with higher binding energy due to the reduced 2D dimension for weakening dielectric screening effect. Here we report Rydberg-series band-edge excitons of multilayered CuInP2S6 observed by micro-thermal-modulated reflectance (μTR) measurements from 10 to 300 K. Bulk CuInP2S6 is one of the renowned ferroelectric 2D materials with wider piezoelectric hysteresis loop for electrical memory use. The μTR result demonstrated that multi-layered CuInP2S6 of ferroelectric (FE) phase displays prominent band-edge excitonic series that consists of E1 = 2.958 eV (n = 1), E2 = 2.993 eV (n = 2) and E∞ = 3.004 eV (direct gap) at 10 K. The transition amplitude of the E1 feature is approximately 3 to 4 times larger than that of the E2 transition for verification of the intrinsic behavior of Rydberg series starting from n = 1. At 300 K, E2 is ionized and the observed transitions by μTR are E1 = 2.748 eV and E∞ = 2.794 eV, respectively. The spontaneous band-edge excitons observed in CuInP2S6 render it available for application in optoelectronics devices.