A Layer-by-Layer Growth Strategy for Large-Size InP/ZnSe/ZnS Core–Shell Quantum Dots Enabling High-Efficiency Light-Emitting Diodes

Abstract

Shell is of great significance to the enhancement in the photoluminescence quantum yield (PLQY) and stability of core–shell-type quantum dots (QDs). InP/ZnS core–shell QDs without intrinsic toxicity have shown huge potential as a replacement for the widely used cadmium-containing QDs; however, it is still challenging to control the growth of InP-based core–shell QDs due to the lattice mismatch between the InP core and ZnS shell. Here, we report on the synthesis of ∼15-nm-size InP/ZnSe/ZnS QDs with a thick ZnS outer shell by a layer-by-layer shell growth strategy. The ZnS shell was prepared by a circularly gradient temperature rise and long reaction procedure in each step, which not only ensures relatively low precursor concentration preventing the anisotropic growth of QDs but also allows the low-reactivity source to be decomposed sufficiently to achieve layer-by-layer growth of a thick ZnS shell. The resulting QDs show the highest PLQY of 73%, narrow emission line width of up to 40 nm, wide spectrum tunability, and excellent stability. Furthermore, the thick ZnS shell also effectively suppresses nonradiative Förster resonant energy transfer and Auger recombination within QDs. As a result, these enable our quantum dot light-emitting diodes (QLEDs) to achieve a record external quantum efficiency of 6.6% in heavy-metal-free red QLEDs.

Preparation of Highly Stable and Photoluminescent Cadmium‐Free InP/GaP/ZnS Core/Shell Quantum Dots and Application to Quantitative Immunoassay

“…The PL QY can maintain up 72%, which is equivalent to the optimal values of InP core/shell QDs with thin shell reported before . This result shows that the GaP middle shell layer to reduces the internal defects of QDs and the thick ZnS outer shell results in better surface passivation effect of InP/GaP/ZnS core/shell QDs. The larger size of InP/GaP/ZnS core/shell QDs were obtained through the multiple shell precursor injections and extension of shelling period of time.…”

“…Multiple injections of shell precursor, the relatively low concentration of shell precursor could restrain the anisotropic growth of InP/GaP/ZnS QDs at each time. Meanwhile, the relatively low reaction temperature prevents the InP/GaP core/shell proceed Ostwald ripening at high temperature and the prolonged reaction time enables OT to be fully decomposed to realize the growth of ZnS thick shell. Finally, we obtained the thick shell InP/GaP/ZnS core/shell QDs under greatly prolong shelling periods of time up to 22 h along with multiple injections of shell precursor by using OT as S source at the reaction temperature of 260 °C (Figure a).…”

“…It can be seen from TEM image that the corresponding average size of InP/GaP/ZnS core/shell QDs was increased to 5.2 ± 0.9 nm (Figure b), and the thickness of ZnS shell was estimated to be ≈1.8 nm. The surface passivation effect of ZnS outer shell could reduce the PL quenching centers (dangling bond), thus resulting in the InP/GaP/ZnS core/shell QDs improved PL QY of 83% with an emission FWHM of 61 nm. Here, we define this InP/GaP/ZnS core/shell QDs with thin shell as conventional QDs, due to it has the same characteristics as previous prepared high quality InP core/shell QDs, i.e., high QY with thin shell.…”

Synthesis of Blue-Emissive InP/GaP/ZnS Quantum Dots via Controlling the Reaction Kinetics of Shell Growth and Length of Capping Ligands

“…The synthesis of InP QDs is, however, relatively challenging compared with that of Cd QDs because of their covalent character, vulnerability to oxidizing environments, and lattice mismatch between the InP core and ZnS shell. The optical properties of InP QDs have been improved by the introduction of inner shells such as ZnSe [14][15][16], ZnSeS [17][18][19][20], and GaP [21][22][23] to alleviate abrupt interfacial strain and the removal of the oxide surface of the InP core through in situ etching [15,16]. However, the development of blue-emissive InP QDs still lags behind that of red and green QDs because of the difficulty in controlling the reactivity of the small InP cores.…”

All-Inorganic Red-Light Emitting Diodes Based on Silicon Quantum Dots

“…However, we expect a dramatically improved device performance for a current-driven LED with Si QDs because external quantum efficiency (EQE) should be dominated by recombination rate between electrons and holes injected from electrodes. In fact, the best value of EQE for Si-QLEDs is as high as 8.6%, which is still a record value in cadmium-free QLEDs [4,9,10]. The next important step is the development of an advanced Si-QLED, in addition to achieving an optimized optical performance in terms of low turn-on voltage, high EQE, strong brightness, Gaussian-shaped electroluminescence (EL) spectra, narrow emission band tunable over a wide range from visible to near-infrared (NIR) wavelengths, a long device lifetime, resistance characteristics against humidity and oxidation, no parasitic emission, and spectral stability over a broad range of operation voltages.…”