Alloyed Heterostructures of CdSe_<i>x</i>S_1–<i>x</i> Nanoplatelets with Highly Tunable Optical Gain Performance

Here, we designed and synthesized alloyed heterostructures of CdSe_<i>x</i>S_1–<i>x</i> nanoplatelets (NPLs) using CdS coating in the lateral and vertical directions for the achievement of highly tunable optical gain performance. By using homogeneously alloyed CdSe_<i>x</i>S_1–<i>x</i> core NPLs as a seed, we prepared CdSe_<i>x</i>S_1–<i>x</i>/CdS core/crown NPLs, where CdS crown region is extended only in the lateral direction. With the sidewall passivation around inner CdSe_<i>x</i>S_1–<i>x</i> cores, we achieved enhanced photoluminescence quantum yield (PL-QY) (reaching 60%), together with increased absorption cross-section and improved stability without changing the emission spectrum of CdSe_<i>x</i>S_1–<i>x</i> alloyed core NPLs. In addition, we further extended the spectral tunability of these solution-processed NPLs with the synthesis of CdSe_<i>x</i>S_1–<i>x</i>/CdS core/shell NPLs. Depending on the sulfur composition of the CdSe_<i>x</i>S_1–<i>x</i> core and thickness of the CdS shell, CdSe_<i>x</i>S_1–<i>x</i>/CdS core/shell NPLs possessed highly tunable emission characteristics within the spectral range of 560–650 nm. Finally, we studied the optical gain performances of different heterostructures of CdSe_<i>x</i>S_1–<i>x</i> alloyed NPLs offering great advantages, including reduced reabsorption and spectrally tunable optical gain range. Despite their decreased PL-QY and reduced absorption cross-section upon increasing the sulfur composition, CdSe_<i>x</i>S_1–<i>x</i> based NPLs exhibit highly tunable amplified spontaneous emission performance together with low gain thresholds down to ∼53 μJ/cm²

CdSe/CdSe_1–<i>x</i>Te_<i>x</i> Core/Crown Heteronanoplatelets: Tuning the Excitonic Properties without Changing the Thickness

Here we designed and synthesized CdSe/CdSe_1–<i>x</i>Te_<i>x</i> core/crown nanoplatelets (NPLs) with controlled crown compositions by using the core-seeded-growth approach. We confirmed the uniform growth of the crown regions with well-defined shape and compositions by employing transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. By precisely tuning the composition of the CdSe_1–<i>x</i>Te_<i>x</i> crown region from pure CdTe (<i>x</i> = 1.00) to almost pure CdSe doped with several Te atoms (<i>x</i> = 0.02), we achieved tunable excitonic properties without changing the thickness of the NPLs and demonstrated the evolution of type-II electronic structure. Upon increasing the Te concentration in the crown region, we obtained continuously tunable photoluminescence peaks within the range of ∼570 nm (for CdSe_1–<i>x</i>Te_<i>x</i> crown with <i>x</i> = 0.02) and ∼660 nm (for CdSe_1–<i>x</i>Te_<i>x</i> crown with <i>x</i> = 1.00). Furthermore, with the formation of the CdSe_1–<i>x</i>Te_<i>x</i> crown region, we observed substantially improved photoluminescence quantum yields (up to ∼95%) owing to the suppression of nonradiative hole trap sites. Also, we found significantly increased fluorescence lifetimes from ∼49 up to ∼326 ns with increasing Te content in the crown, suggesting the transition from quasi-type-II to type-II electronic structure. With their tunable excitonic properties, this novel material presented here will find ubiquitous use in various efficient light-emitting and -harvesting applications