Spectroscopy of Nanomaterials

To study the transient dynamics, we apply femtosecond transient absorption (TA) experiments to probe the evolution of the pump-induced band-edge (1S) absorption bleaching (Δα1S) and pump-intensity-dependent transient PL measurements with pump photons of various energy. The transient dynamics measurements has been used to reveal high efficient spin-exchange process. Recently, QD-Mn interactions were exploited to realize highly efficient spin-exchange carrier multiplication (SE-CM) in Mn-doped PbSe/CdSe QDs, in which a lower-bandgap PbSe core was enclosed in a higher-bandgap CdSe shell.(1)  Due to extremely short SE time scales, SE steps occurred without considerable interference from phonon emission, resulting in high SE-CM efficiency. More recently, to enable the practical utilization of carriers generated by SE-CM, Mn-doped CdSe/HgSe QDs with an inverted band structure were developed.(2) In these QDs, the lower bandgap material is located in the shell region, making electrons and holes generated by SE-CM easily electrically accessible. 

1. Nat. Mater. 22, 1013-1021 (2023).

2. Nat. Commun. 16, 2952 (2025).

To clearly reveal the spectral details obscured in the ensemble optical spectra of 2D QDs, we apply the single-particle optical measurements of individual 2D QDs. The effects of the lateral quantum confinement on the optical transition in single-layer WSe2 are unambiguously identified.(1) A much broader PL is observed than the exciton PL of single-layer WSe2 sheets, which suggests possible defect emissions and/or heterogeneous broadening, and the lack of identifiable peaks in the absorption spectra hampers the characterization of the nature of the absorption and emission. We resolve these issues by employing single-particle spectroscopy, providing the clearer spectral features without complications from the ensemble heterogeneity. Further examination of the similarities and differences between WSe2 SQDs and single-layer WSe2 sheets is performed via single-particle PL polarization anisotropy measurements. Because the shape of WSe2 SQDs is highly symmetric, the transition dipole of the absorption that gives rise to the PL in WSe2 SQDs is in-plane and 2D isotropic. 

1. Nano Lett. 17, 7471-7477 (2017).