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Colloidal nanoplatelets (NPLs) are promising materials for lasingapplications. The properties are usually discussed in the framework of 2Dmaterials, where strong excitonic effects dominate the optical properties nearthe band edge. At the same time, NPLs have finite lateral dimensions such thatNPLs are not true extended 2D structures. Here we study the photophysics andgain properties of CdSe/CdS/ZnS core-shell-shell NPLs upon electrochemical ndoping and optical excitation. Steady-state absorption and PL spectroscopy showthat excitonic effects are weaker in core-shell-shell nanoplatelets due to thereduced exciton binding energy. Transient absorption studies reveal a gainthreshold of only one excitation per nanoplatelet. Using electrochemical ndoping we observe the complete bleaching of the band edge exciton transitions.Combining electrochemical doping with transient absorption spectroscopy wedemonstrate that the gain threshold is fully removed over a broad spectralrange and gain coefficients of several thousand cm-1 are obtained. These dopedNPLs are the best performing colloidal nanomaterial gain medium reported todate. The low exciton binding energy due to the CdS and ZnS shells, incombination with the relatively small lateral size of the NPLs, result inexcited states that are effectively delocalised over the entire platelet.Core-shell NPLs are thus on the border between strong confinement in QDs anddominant Coulombic effects in 2D materials. We demonstrate that this limit isin effect ideal for optical gain, and that it results in an optimal lateralsize of the platelets where the gain threshold per nm2 is minimal.

Zero-threshold optical gain in electrochemically doped nanoplatelets and the physics behind it