(a) Schematic geometry of a CdSbulk−CdSe−CdS−ZnS quantum shell. The graph shows calculated radial probability distributions of electron and hole wave functions in each region. (b) High-angle annular dark-field (HAADF)-STEM image of 7.2-nm-core quantum shells, illustrating the presence of the CdSe shell layer. (c) TEM image of 9.0-nm-core CdSbulk−CdSe−CdS−ZnS QSs. Inset: High-resolution TEM image of a 9.0-nm-core QS.
Our lab aims to explore the lasing properties and general optical characteristics of quantum shells (QSs), focusing on their unique excitonic behavior and potential applications in optoelectronics. Quantum shells are engineered nanostructures that enable efficient light emission with well-defined exciton and biexciton transitions. By leveraging their distinct energy separation, we can generate correlated photon pairs, making them promising candidates for quantum light sources. Amplified spontaneous emission (ASE) techniques allow us to characterize the spectral properties of these states, revealing that, unlike epitaxial quantum dots (QDs), QSs exhibit biexciton transition energies higher than excitons, leading to clear spectral distinction between emission bands. (ACS Nano 2024, 18, 44, 30863–30870)
A critical aspect of our studies is the suppression of Auger recombination, which typically limits the efficiency of semiconductor nanostructures. By designing CdS−CdSe−CdS−ZnS core−shell−shell−shell QSs with optimized morphology, we have achieved significantly prolonged Auger lifetimes (~22–110 ns), among the longest reported for colloidal nanocrystals. These structures demonstrate remarkably high photoluminescence (PL) quantum yields of up to 90% and biexciton emission quantum yields nearing 79%. Furthermore, the low-threshold ASE observed in QS thin films (biexciton ASE onset fluence of 5.3 μJ/cm²) highlights their strong optical gain properties, positioning them as ideal materials for lasing applications. (J. Am. Chem. Soc. 2023, 145, 24, 13326–13334)
(a) Absorption and emission spectra of 7.2-nm-core CdS bulk−CdSe−CdS−ZnS QSs (RCdS = 3.0 nm, HSe ≈ 1.8 nm, HCdS ≈ 3.8 nm, and HZnS < 1.5 nm). (b) Absorption and emission spectra of 8.7-nm-core CdS bulk−CdSe−CdS−ZnS QSs (RCdS = 4.3 nm, HSe ≈ 1.8 nm, HCdS ≈ 4.2 nm, and HZnS < 1.5 nm). (c, d) Lagtime dependence of the cross-correlation function, g(2), that was used to determine the ratio of biexciton to exciton QY (QYXX/QYX) for (c) 7.2-nm core QSs and (d) 8.7-nm core QSs. The displayed cross-correlation function represents the integrated intensity of the correlation peak as detailed in ref 45.
Additionally, we investigate the integration of QSs into photonic crystal cavities to enhance lasing performance. These specially designed cavities facilitate ultra-wide spectral ASE, low-threshold excitation, and high modal gain. In particular, red-emissive quantum shells embedded in photonic crystal nanopillar cavities exhibit efficient low-threshold lasing with high Q-factors. By optimizing exciton-exciton interactions and stabilizing biexciton states, our research advances the potential of QSs for next-generation laser technologies, quantum optics, and other optoelectronic applications.