Here, we designed and synthesized alloyed heterostructures of CdSexS1–x nanoplatelets (NPLs) using CdS coating in the lateral and vertical directions for the achievement of highly tunable optical gain performance. By using homogeneously alloyed CdSexS1–x core NPLs as a seed, we prepared CdSexS1–x/CdS core/crown NPLs, where CdS crown region is extended only in the lateral direction. With the sidewall passivation around inner CdSexS1–x 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 CdSexS1–x alloyed core NPLs. In addition, we further extended the spectral tunability of these solution-processed NPLs with the synthesis of CdSexS1–x/CdS core/shell NPLs. Depending on the sulfur composition of the CdSexS1–x core and thickness of the CdS shell, CdSexS1–x/CdS core/shell NPLs possessed highly tunable emission characteristics within the spectral range of 560–650 nm. F...