Preparation of green-emitting InP-based quantum dots with controlled shell thickness and their photoluminescence quantum yield upon silica encapsulation

Silica encapsulation of colloidal quantum dots (QDs) is an effective method for preserving their distinctive photoluminescence properties. However, applying this encapsulation method, initially developed for CdSe-based QDs, to InP-based QDs results in a significant decrease in photoluminescence quantum yield (PLQY). To understand this discrepancy, we prepared three types of QDs (InP/(ZnSe)_n/ZnS, with n = 4, 6, 8 monolayers) that emit in the green region and encapsulated them into silica particles (~ 30 nm in size, typically containing ~ 10 QDs per particle). Increasing the thickness of the intermediate ZnSe layer from 1.3 (4 monolayers) to 2.7 nm (8 monolayers) using the same core size (1.6 nm) effectively suppressed the decrease in PLQY after encapsulation. Quantum mechanical calculation revealed that compared to CdSe-based QDs, the excited electron in InP-based QDs tends to spread significantly due to the lighter effective electron mass and lower barrier height from the InP core to the ZnSe and ZnS shells. As the ZnSe layer thickness increases, the amount of spread electron reduces, thereby better maintaining the PLQY after encapsulation. The calculations further suggest that larger cores (> 2.2 nm) and thicker shells (> 2.5 nm) are preferable for achieving high PLQY after silica encapsulation. This knowledge serves as a guideline for developing ideal QDs with bright, robust, and non-toxic features as user-friendly phosphors.