Crystal momentum K is not a good quantum number to describe band structures in quantum confined nanocrystalline systems, thereby blurring the distinction between direct and indirect semiconductors as conventionally defined in the bulk. A transition to indirect behavior in the bulk In1-xGaxP alloy phase with higher gallium content is well documented. However, the effect of quantum confinement on the direct to indirect transition in this system has yet to be established because high quality colloidal nanocrystal samples have remained inaccessible. Herein we report a molten salt In3+ to Ga3+ cation exchange to prepare In1-xGaxP nanocrystals and subsequent ZnS shelling protocol to achieve high radiative efficiency. The synthesis is rigorously oxygen free, allowing us to obtain bright In1-xGaxP/ZnS core-shell particles of decent emission color purity with emission quantum yields exceeding 80%. Steady-state and time-resolved photoluminescence studies indicate that radiative decay lifetime in these highly emissive samples increases monotonically, while quantum yield falls with higher gallium content, indicating an increasingly larger contribution from Gamma to X valley excitonic transitions. Transient absorption studies on these core-shell nanocrystals allow us to establish direct or indirect-like behavior in these samples in terms of the presence or absence of excitonic bleach features. These insights establish cation exchange in molten inorganic salts as a viable route towards synthesizing nontoxic In1-xGaxP/ZnS QD emitters with good optoelectronic properties. We utilize this optimized synthesis to further introduce composition as a tunable parameter for obtaining direct to indirect behaving quantum confined nanocrystalline systems.

Direct to indirect transition in core-shell In1-xGaxP/ZnS colloidal quantum dots

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