Spectroscopic Signatures of Charged States in CdSe Nanoplatelets: Direct Observation of Oscillator Strength and Spin Center Identification

Abstract The low-temperature photoluminescence of CdSe nanoplatelets exhibits a characteristic two-peak structure that has been widely attributed to negative trion emission. Despite this consensus, direct spectroscopic evidence for the trion oscillator strength and chemical identification of the resident charge have remained elusive. Here, a combination of differential absorption spectroscopy, photoluminescence excitation spectroscopy, and electron paramagnetic resonance was used to provide evidence for a trion state in 4 monolayer CdSe nanoplatelets. Under illumination at cryogenic temperatures, we observe the growth of a new absorption feature red-shifted by ∼30 meV from the heavy-hole exciton, consistent with trion formation. The magnitude of this photoinduced absorption correlates quantitatively with the fractional increase in red-shifted emission. PLE measurements at 80 K reveal that the red emission feature possesses a distinct excitation spectrum with measurable oscillator strength below the exciton energy. Photo-EPR measurements generate a spin signal attributed to an electron localized on selenium atoms (i.e., VCd–). These findings establish a direct connection between the spectroscopic signatures of trions and specific point defects in CdSe nanoplatelets. The data also suggest a reconciliation with the known sensitivity of the red emission to sample preparation and aggregation state through homo-FRET-mediated energy transfer and dielectric enhancement effects in aggregated systems.