Impact of a CdS Crown on Optical Gain in CdSe/CdS Core/Crown Nanoplatelets

Type‐I core/crown CdSe/CdS nanoplatelets, with fast emission lifetime, narrow emission spectrum, and high photoluminescence quantum efficiency, are interesting candidates for lasing applications. Using femtosecond transient absorption spectroscopy, the influence of the CdS crown area on optical gain parameters, such as the exciton density and excitation fluence at gain threshold, the gain lifetime, and the gain bandwidth, is assessed. Growing a small CdS crown lowered the biexciton gain threshold by a factor of three compared to core‐only nanoplatelets, down to an exciton density of 0.005 nm −2 , corresponding to, on average, a single exciton per nanoplatelet. It also augmented the gain lifetime to about 600 ps, close to the estimated radiative biexciton recombination rate of 875 ps. Increasing the lateral size of the crown, on the other hand, allowed to widen the gain band width toward shorter wavelengths, by exploiting the increased absorption cross section of the CdS crown and inducing a sizeable and relatively long‐lived free‐carrier gain, with maximal lifetime of about 110 ps, compared to 25–40 ps for nanoplatelets with a small crown. The results demonstrate that type‐I core/crown nanoplatelets with carefully engineered crown dimensions can offer an interesting platform for both low‐threshold as well as broadband lasing applications.