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