Quantum Dots (QDs) are semiconductor nanocrystals with a wide range of potential applications including displays, photovoltaics, and biological labeling and tracking. Understanding the charge carrier dynamics of emerging quantum dot materials and correlating them to their structure, composition, and surface chemistry allows for QDs to be developed for different applications. One such system is InP QDs, which are being developed as a cadmium-free alternative to traditional CdSe QDs. InP offers size-tunable emission across the visible and near infrared spectral range. Shelling an InP core with a wide band gap material such as ZnSe increases the quantum yield from 2-3% up to ~50%, improves the QD’s photostability, and suppresses blinking. Many additional properties of InP QDs are yet to be fully studied and understood. Ultrafast fluorescence upconversion spectroscopy is a powerful technique used to characterize QDs by providing valuable information about their ensemble charge carrier and trapping dynamics. Femtosecond decay constants and their relative amplitudes provide insight into how these QDs’ trapping dynamics differ from those of traditional cadmium-based core-shell QDs. Traditionally, shelling a QD core leads to confinement of the electron and hole leading to improved photoluminescent properties by reducing the number of defects observed on the QD’s surface. Initial ultrafast measurements on a thick-shell InP-ZnSe QD sample show a fast initial 6.2 ± 1.7 picoseconds decay, traditionally associated with hole trapping. A second longer lived component represents radiative recombination.
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