The band-edge excitons observed in few-layer NiPS3

Abstract

Band-edge excitons of few-layer nickel phosphorous trisulfide (NiPS₃) are characterized via micro-thermal-modulated reflectance (μTR) measurements from 10 to 300 K. Prominent μTR features of the A exciton series and B are simultaneously detected near the band edge of NiPS₃. The A exciton series contains two sharp A₁ and A₂ levels and one threshold-energy-related transition (direct gap, E_∞), which are simultaneously detected at the lower energy side of NiPS₃. In addition, one broadened B feature is present at the higher energy side of few-layer NiPS₃. The A series excitons may correlate with majorly d-to-d transition in the Rydberg series with threshold energy of E_∞ ≅ 1.511 eV at 10 K. The binding energy of A₁ is about 36 meV, and the transition energy is A₁ ≅ 1.366 eV at 300 K. The transition energy of B measured by μTR is about 1.894 eV at 10 K. The excitonic series A may directly transit from the top of valence band to the conduction band of NiPS₃, while the B feature might originate from the spin-split-off valence band to the conduction band edge. The direct optical gap of NiPS₃ is ~1.402 eV at 300 K, which is confirmed by μTR and transmittance experiments.