Near-Unity Biexciton Quantum Yields and Broadband, Low Threshold Lasing from Colloidal Quantum Shells

Spherically shaped semiconductor nanoplatelets, called quantum shells (QSs) have recently attracted considerable attention for various optoelectronic applications. We developed a range of CdSe quantum-confined spherical shell sandwiched between wider-bandgap CdS core and CdS outer layers in which separation of charge carriers is defined not only by the mutual attraction or repulsion of the exciton pairs, but also by the core size. At the same time, it allows for a smaller electron-hole overlap, promoting longer radiative times. We demonstrate that such QSs with a large-size core component exhibit excellent multi-exciton emission characteristics with strongly suppressed Auger recombination. By using a combination of photoluminescence (PL) blinking and single-photon statistics measurements, we show that biexciton quantum yield increases from QY^BX ~45 % for small-core CdS/CdSe/CdS QSs (D_core = 4.5 nm) to an average QY^BX of ~ 82 % for large-core QSs (D_core = 8.2 nm) with individual particles exhibiting QY^BX of up to 100%. Efficient emission and ultralong BX lifetimes (&gt; 15 ns) allowed us to observe low-threshold amplified spontaneous emission (ASE) with modal gain<i> g </i>~ 1000 cm^-1 in large-core QS films. To utilize broad gain bandwidth, we demonstrated narrow line lasing action using QSs in tunable optical cavity configurations. We design distributed feedback (DFB) cavities using lithographically defined SiO₂ nanopillar arrays etched on Si substrates and filled with close-packed QS films. Using only one QSs size (i.e., confinement), we demonstrate emission coupling and low threshold, narrowband lasing across wide spectral range, from single exciton (l_X~640 nm) to biexciton (l_BX~625 nm) to multiple exciton (l_MX~615-565 nm) transitions. The ensemble-averaged gain threshold of &lt;<i>N</i>&gt;~1.4 electron-hole pairs per QS particle and lowest pump fluence of ~ 4 mJ/cm² result from almost completely impeded Auger recombination and low optical losses in the nanopillar cavity. Lasing emission tuning shows excellent agreement with calculations as an array period is continuously varied, while maintaining the mode confinement and quality (<i>Q</i>) factors. These results represent a significant advance towards the development of future electrically pumped, colloidal nanocrystal lasers.