Ultrahigh-efficiency aqueous flat nanocrystals of CdSe/CdS@Cd_1−xZn_xS colloidal core/crown@alloyed-shell quantum wells

Abstract

Colloidal semiconductor nanoplatelets (NPLs) are highly promising luminescent materials owing to their exceptionally narrow emission spectra. While high-efficiency NPLs in non-polar organic media can be obtained readily, NPLs in aqueous media suffer from extremely low quantum yields (QYs), which completely undermines their potential, especially in biological applications. Here, we show high-efficiency watersoluble CdSe/CdS@Cd 1−x Zn x S core/crown@shell NPLs formed by layer-by-layer grown and compositiontuned gradient Cd 1−x Zn x S shells on CdSe/CdS core/crown seeds. Such control of shell composition with monolayer precision and effective peripheral crown passivation, together with the compact capping density of short 3-mercaptopropionic acid ligands, allow for QYs reaching 90% in water, accompanied by a significantly increased photoluminescence lifetime (∼35 ns), indicating the suppression of nonradiative channels in these NPLs. We also demonstrate the controlled attachment of these NPLs without stacking at the nanoscale by taking advantage of their 2D geometry and hydrophilicity. This is a significant step in achieving controlled assemblies and overcoming the stacking process, which otherwise undermines their film formation and performance in optoelectronic applications. Moreover, we show that the parallel orientation of such NPLs achieved by the controlled attachment enables directed emission perpendicular to the surface of the NPL films, which is highly advantageous for light extraction in light-emitting platforms. † Electronic supplementary information (ESI) available: Details of synthesis procedures, experimental set-up, theoretical modelling and additional figures. See

Coreless Fiber‐Based Whispering‐Gallery‐Mode Assisted Lasing from Colloidal Quantum Well Solids

“…In this study, we synthesized CdSe/CdS@Cd 1– x Zn x S C/C@GAS via a colloidal atomic layer deposition technique where Cd 1– x Zn x S shells were grown on the seed 4‐monolayer (4‐ML) CdSe/CdS core/crown CQWs . Thanks to this specially engineered heterostructure of our CQWs, the electron wavefunction feels soft confinement potential in the vertical direction of the heterostructure owing to the small offset between the conduction bands of the core and gradually alloyed shell, while the hole wavefunction is mostly localized in the core region due to the large valence band offset .…”

“…Among these nanocrystals, the colloidal quantum wells (CQWs), also known as nanoplatelets deserve a unique place considering their superior optical and electronic characteristics including giant oscillator strength, exceptionally large linear and nonlinear absorption cross‐sections, purely homogeneous broadening, and ultralarge modal optical gain coefficients . In addition, the excitonic features of these atomically flat NCs can be tuned by using their heterostructures (i.e., core/crown and core/shell) which result in highly efficient optical gain performances . Previously, various heterostructures of CQWs have been extensively used as gain media in optical gain and colloidal laser applications, in the close‐packed solid film phase, integrated into different optical resonator structures.…”

Record High External Quantum Efficiency of 19.2% Achieved in Light‐Emitting Diodes of Colloidal Quantum Wells Enabled by Hot‐Injection Shell Growth

“…Optimized low AR cores were used to grow HIS for these experiments and CdSe/Cd 0.25 Zn 0.75 S core/HIS was found to be the best CQW emitter for the enhancement of balanced charge injection. With the increasing of Cd in the shell, the bandgap of CdSe/Cd 1− x Zn x S is decreasing since the bandgap of CdS is narrower than that of ZnS (e.g., the bandgap of CdSe/Cd 0.25 Zn 0.75 S is 1.87 eV), as shown in Figure S8 (Supporting Information). According to the ultraviolet photoelectron spectroscopic (UPS) measurement (Figure S9 and Table S3, Supporting Information), the energy level of CdSe/Cd 0.25 Zn 0.75 S can be more effectively matched with the device architecture thanks to the reduced charge injection barrier.…”

“…On the other hand, the stacking in CQW films is detrimental to the efficiency through fast nonradiative exciton transfer giving defected sub‐populations, which act as exciton sinks, undermining the efficiency of LEDs . As HAADF TEM images clearly show in Figure S6 (Supporting Information), negligible stacking is observed in low AR CQWs while stacking exists in high AR CQWs, suppressing the efficiency reduction in low AR devices . Nevertheless, despite low AR‐based CQW‐LEDs show superior performance among all devices possessing different ARs, further investigations are needed to better understand their role in the high performance.…”

Colloidal semiconductor nanocrystals for light-emitting devices : from materials to device perspectives

“…CdSe/CdS@Cd 1-x Zn x S Core/Crown@Shell Nanoplatelets and Their Aqueous DispersionThe contents of this chapter are partially based on the publication: "Ultrahigh-Nanoscale, 2019, 11, 301-310. Reproduced (adapted) from ref [89]. with permission from the Royal Society of Chemistry.…”