We hence perform the present study to implement such strategy for deposition of a coherent thick shell on strongly confined spherical QDs in order to achieve heavy-metal-free core/shell QDs with superior optical properties. This prior investigation of the shell growth mechanism on nanorods revealed the advantageous pathway to avoid interfacial trap formation while using the slow thermodynamic growth conditions. The morphology of the islands-shell case also enables avoidance of interfacial traps, much better than the case of the flat-shell growth, but the helical-shell morphology offers the optimal effective interfacial passivation. In the case of the islands-shell growth, growth of a wetting layer with a critical thickness of ∼3.3 MLs was identified before the islands appeared on the surface of the nanorods. Lowering the Zn/oleic acid molar ratio to 1/6.3 (1/4) transformed the shell morphology of ZnS to islands-shell (flat-shell), as the growth conditions were changed gradually from thermodynamic into kinetic control. (28) The helical shell morphology allows one to maintain high quality band gap emission from the rods even upon growth of thicker shells, consistent with avoidance of creation of defects at the core/shell interface, which leads to carrier trapping and quenching band gap emission. We have recently shown that the use of zinc oleate with low reactivity (molar ratio of Zn/oleic acid shell precursor of 1/10) led to unique growth of a helical shell of ZnS on ZnSe nanorods under thermodynamic growth conditions. (24−27) This emphasizes the need for additional high-quality shell growth strategies. For instance, in order to obtain highly fluorescent ZnSe QDs, ZnS is the typical choice as the shell material to form type-I band alignment ( Figure 1a). However, considering heavy-metal-free Zn-chalcogenides, the choice of suitable high bandgap semiconductor shell materials becomes much more limited. (18−23) These are particularly well developed for the Cd-chalcogenide semiconductor QDs. Among these are the growth of alloyed shells, and shells with graded composition designated to relax the strain upon increasing shell thickness. Various approaches were introduced to achieve high quality shell growth. (12,13) Large lattice mismatch actually limits the ability to achieve epitaxial growth of a thick shell and often other architectures may form. (12) Typically, the lattice mismatch between the core and the shell semiconductors needs to be small, so that the epitaxial shell growth can take place to passivate the surface of the core effectively without inducing interfacial defects. (10,11) A type-I band alignment, in which the semiconductor shell material with a larger band gap straddles the core semiconductor band gap, is the typical architecture that confines both electron and hole in the core and alleviates nonradiative decay via trap sites at the QD surface. (3−9) For over two decades, the efficient strategy to achieve these stringent demands has been through the formation of core/shell structures. (1,2) These properties, combined with the solution-processability of QDs, are the basis for their widespread implementation as building blocks in optical and optoelectronic applications including in commercial displays, lasing, light-emitting diodes (LEDs), and bioimaging that require high photoluminescence (PL) quantum yields (QY) and photostability. Our study points to a general strategy to obtain high-quality core/shell QDs with enhanced optical properties through controlled reactivity yielding shell growth in the thermodynamic limit.Ĭolloidal quantum dots (QDs) are exceptional fluorescence emitters manifesting continuous color-tunability and narrow emission line widths. This high performance is ascribed to the effective avoidance of traps at the interface between the core and the shell, which are detrimental to the emission properties. In the thermodynamic growth regime, enhanced fluorescence quantum yields and reduced on–off blinking are achieved. Tuning the precursor reactivity modifies the growth mode of ZnS shells on ZnSe cores transforming from kinetic (fast) to thermodynamic (slow) growth regimes. Herein we investigate the effect of shell growth rate on the structure and optical properties in blue-emitting ZnSe/ZnS QDs with narrow emission line width. Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the key strategy for achieving high fluorescence quantum efficiency and essential stability for optoelectronic applications and biotagging with emissive QDs.