Thick-shell (>5 nm) InP-ZnSe colloidal quantum dots (QDs) grown by a continuous-injection shell growth process are reported. The growth of a thick crystalline shell is attributed to the high temperature of the growth process and the relatively low lattice mismatch between the InP core and ZnSe shell. In addition to a narrow ensemble photoluminescence (PL) line-width (∼40 nm), ensemble and single-particle emission dynamics measurements indicate that blinking and Auger recombination are reduced in these heterostructures. More specifically, high single-dot ON-times (>95%) were obtained for the core-shell QDs, and measured ensemble biexciton lifetimes, τ ∼ 540 ps, represent a 7-fold increase compared to InP-ZnS QDs. Further, high-resolution energy dispersive X-ray (EDX) chemical maps directly show for the first time significant incorporation of indium into the shell of the InP-ZnSe QDs. Examination of the atomic structure of the thick-shell QDs by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals structural defects in subpopulations of particles that may mitigate PL efficiencies (∼40% in ensemble), providing insight toward further synthetic refinement. These InP-ZnSe heterostructures represent progress toward fully cadmium-free QDs with superior photophysical properties important in biological labeling and other emission-based technologies.

Copper sulfide particles across a wide range of stoichiometries are obtained depending on the ratio of cosolvents from which they are grown. Copper sulfides are abundant, low-cost materials with phase-dependent properties relevant to solar energy conversion and (opto)electronic devices. For this reaction, the reducing ability of dodecanethiol versus oleic acid affects the speciation of the precursors, as determined using UV−vis absorption spectroscopy. The ratio of dodecanethiol to oleic acid in the synthetic medium affects the solid-state structure and stoichiometry, as determined by powder X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and detailed investigation of the band edge positions and plasmon behavior using visible−NIR optical absorption spectroscopy and cyclic voltammetry. A range of phases was obtained, including monoclinic chalcocite, tetragonal chalcocite, digenite, and covellite. The thermodynamic relationships between these phases were elucidated using equilibration experiments, revealing methods for postsynthetic property alteration. Particle size and morphology were also affected by solvent ratio, as shown using scanning or transmission electron microscopy. Oleic acid accelerated particle growth and resulted in particles with an unusual faceted shape.

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Thick-shell InP/ZnSe III–V/II–VI quantum dots (QDs) were synthesized with two distinct interfaces between the InP core and ZnSe shell: alloy and core/shell. Despite sharing similar optical properties in the spectral domain, these two QD systems have differing amounts of indium incorporation in the shell as determined by high-resolution energy-dispersive x-ray spectroscopy scanning transmission electron microscopy. Ultrafast fluorescence upconversion spectroscopy was used to probe the charge carrier dynamics of these two systems and shows substantial charge carrier trapping in both systems that prevents radiative recombination and reduces the photoluminescence quantum yield. The alloy and core/shell QDs show slight differences in the extent of charge carrier localization with more extensive trapping observed in the alloy nanocrystals. Despite the ability to grow a thick shell, structural defects caused by III–V/II–VI charge carrier imbalances still need to be mitigated to further improve InP QDs.