Halide Ligands To Release Strain in Cadmium Chalcogenide Nanoplatelets and Achieve High Brightness

Abstract

Zinc blende II-VI semiconductor nanoplatelets (NPLs) are defined at the atomic scale along the thickness of the nanoparticle and are initially capped with carboxylates on the top and bottom [001] facets. These ligands are exchanged on CdSe NPLs with halides that act as X-L-type ligands. These CdSe NPLs are costabilized by amines to provide colloidal stability in nonpolar solvents. The hydrogen from the amine can participate in a hydrogen bond with the lone pair electrons of surface halides. After ligand exchange, the optical features are redshifted. Thus, ligand tuning is another way, in addition to confinement, to tune the optical features of NPLs. The improved surface passivation leads to an increase in the fluorescence quantum efficiency of up to 70% in the case of bromide. However, for chloride and iodide, the surface coverage is incomplete, and thus, the fluorescence quantum efficiency is lower. This ligand exchange is associated with a decrease in stress that leads to unfolding of the NPLs, which is particularly noticeable for iodide-capped NPLs.

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

“…CQW‐LEDs are highly promising for display and lighting applications, owing to their high color purity with ultranarrow full‐width at half‐maximum (FWHM), easy solution‐processed fabrication procedures, and good compatibility with flexible electronics . In the past few years, some efforts have been made to develop CQW‐LEDs .…”

“…Most importantly, the CQWs should have an excellent photoluminescence quantum yield (PLQY), which is essential to warrant the high efficiency . However, the PLQY of CQWs used in LEDs is not sufficiently high . In the case of core CQWs, the surface nonradiative recombination sites will easily deteriorate the efficiency, leading to the inefficient core‐only‐based CQW‐LEDs .…”

“…In addition, the effect of the physical shape of CQWs on the device performance remains unknown, which might affect the film morphology of spin‐coated CQWs in LEDs. Furthermore, the surface of previously reported CQW films is not smooth enough, which may lead to detrimental current leakage and other losses . Moreover, the employment of hybrid organic‐inorganic device architectures is considered to be the most effective scheme to ensure high‐efficiency CQD‐LEDs .…”

“…The core CQWs, in their synthesized form, suffer from low stability and are prone to degradation due to their interaction with the surrounding medium, giving rise to suppressed PL by surface trap sites. This has tremendously affected the in‐film QY of core CQWs only reaching 5%, despite the in‐solution QY reached 70% by using halide ligands . Hence, the overcoating shell material is usually needed for the c‐ALD approach in CQWs.…”

“…Consequently, CQW‐LEDs using CdSe/Cd 0.25 Zn 0.75 S core/c‐ALD shell show poor performance. However, it is deserved to note that no CQW film used in LEDs has been previously demonstrated to exhibit an RMS roughness of ≤3 nm, indicating that the HIS technique (e.g., an RMS roughness of 2.23 nm) provides a new pathway to effectively smoothen the film surface.…”