Synthesis and Evaluation of Ideal Core/Shell Quantum Dots with Precisely Controlled Shell Growth: Nonblinking, Single Photoluminescence Decay Channel, and Suppressed FRET

Abstract

Due to the unique optical properties, colloidal quantum dots (QDs) are excellent candidates for developing next-generation display and solid-state lighting technologies. However, some factors including photoluminescence blinking and Förster resonance energy transfer (FRET) still affect their practical applications. Herein, a series of ZnCdSe-based core/shell QDs with low optical polydispersity have been successfully synthesized by a “low-temperature injection and high-temperature growth” precisely controlled method. The alloyed ZnCdSe core with a certain ratio of Cd and Zn was presynthesized first. Followed by accurate ZnS shell growth, the as-synthesized core/shell QDs are nonblinking with the nonblinking threshold volume of ∼137 nm3. The PL decay dynamics are all single-exponential for both QDs in solutions and close-packed solid films when ZnS shell thickness varying from 2 to 20 monolayers. FRET can be effectively suppressed after growing 10 monolayers of ZnS shell. All of these superb characteristics including nonblinking, single-exponential PL decay dynamics and suppressed FRET can be beneficial to high-quality QD-based light-emitting devices (QLEDs). By applying the ZnCdSe-based core/shell QDs with 10 monolayers ZnS shell, the highest external quantum efficiency of ∼17% was reached, which could compare favorably with the highest efficiency of green QLEDs with traditional multilayered structures.

High‐Efficiency Green InP Quantum Dot‐Based Electroluminescent Device Comprising Thick‐Shell Quantum Dots

“…For the blue InP‐based QLED, there have been no record on the efficiency being reported so far. The low efficiency of InP‐based QLED is mainly attributed to the low quantum yield (QY) of the emitting QDs film, which results in low radiative recombination efficiency of the electrons and holes injected into the InP QDs emitting layer, therefore causing a poor performance of QLED . Although there have been many reports of InP core/shell QDs possessing excellent QYs in solution state (>70%), their QYs in the form of closely packed thin film become severely reduced as a result of the efficient nonradiative Förster resonant energy transfer (FRET) due to small particle size (<6 nm) and poor uniformity, typically retaining <20%.…”

Quantum‐Dot Light‐Emitting Diodes for Outdoor Displays with High Stability at High Brightness

“…To obtain QDs with a graded interlayer (gi) structure, we have modified the synthesis scheme of thick shell ZnCdSe/ZnS QDs, as reported earlier by our group, which are also synthesized as reference samples in this study. Novel QDs with a ZnCdSe core, thick ZnSe/ZnSeS gi, and ultrathin outmost ZnS shell are synthesized by adopting a “low temperature nucleation and high temperature shell growth” method . Figure a presents the evolution of the UV‐vis absorption and PL emission spectra for ZnCdSe/gi/ZnS QDs upon shell growth.…”

“…A near‐unity (≈97%) PL QY can be obtained for ZnCdSe/7ZnSe/2ZnSeS QDs, and no significant peak shifting occurs with the further growth of the ZnS outermost shell (as shown in Figure d). This result suggests that the ZnSe/ZnSeS gi releases the strain caused by lattice mismatch between the core and shell and that two monolayers of the ZnSeS shell can provide sufficient confinement for charge carriers . The introduction of the outermost ZnS shell is intended to improve charge balance by partially inhibiting electron injection in devices and pinning the alloy layer in a more inner part to protect the QDs from degradation, and thus maintaining the high PL QY of the QDs.…”

“…As a reference, ZnCdSe/ZnS QDs with a similar PL peak (≈527 nm) were also synthesized following previous reports . For ZnCdSe/ZnS QDs, Se can be detected in just the core region, and S has a wider distribution throughout the whole of the QD (Figure S3, Supporting Information).…”

“…In this study, we report on a “low‐temperature injection and high‐temperature growth” method to synthesize a new type of ZnCdSe/ZnSe/ZnSeS/ZnS core/shell QD, contributing to the best‐performing solution‐processed green QLEDs demonstrated up to now. The specially engineered core/shell QDs feature a compositionally graded interlayer and a final barrier layer, which result in QDs with high stability and ideal emissive properties, including a high PL QY of >95% and a single PL decay channel (single exponential decay with the best value for a goodness‐of‐fit (χ 2 ) of 1.04).…”