Compositional Grading for Efficient and Narrowband Emission in CdSe-Based Core/Shell Nanoplatelets

Abstract

Colloidal semiconductor nanoplatelets exhibit exceptionally narrow photoluminescence spectra. This occurs because samples can be synthesized in which all nanoplatelets share the same atomic-scale thickness. As this dimension sets the emission wavelength, inhomogeneous linewidth broadening due to size variation, which is always present in samples of quasi-spherical nanocrystals (quantum dots), is essentially eliminated. Nanoplatelets thus offer improved, spectrally pure emitters for various applications. Unfortunately, due to their non-equilibrium shape, nanoplatelets also suffer from low photo-, chemical, and thermal stability, which limits their use. Moreover, their poor stability hampers the development of efficient synthesis protocols for adding high-quality protective inorganic shells, a well-known strategy to improve the performance of quantum dots. Herein, we report a general synthesis approach to highly emissive and stable core/shell nanoplatelets with various shell compositions, including CdSe/ZnS, CdSe/CdS/ZnS, CdSe/CdxZn1-xS, and CdSe/ZnSe. Motivated by previous work on quantum dots, we find that slow, high-temperature growth of shells containing a compositional gradient reduces strain-induced crystal defects and minimizes the emission linewidth while maintaining good surface passivation and nanocrystal uniformity. Indeed, our best core/shell nanoplatelets (CdSe/CdxZn1-xS) show photoluminescence quantum yields of 90% with linewidths as low as 56 meV (19.5 nm at 655 nm). To confirm the high quality of our different core/shell nanoplatelets for a specific application, we demonstrate their use as gain media in low-threshold ring lasers. More generally, the ability of our synthesis protocol to engineer high-quality shells can help further improve nanoplatelets for optoelectronic devices.

CdS crown growth on CdSe nanoplatelets: core shape matters

“…44 It needs to be noted, that under the applied reaction conditions the formation of a complete CdS layer on the top and bottom facets of the core NPLs is generally unlikely, as it requires higher reaction temperatures and/or an excess of the chalcogenide precursor. 16,23,28,52,53 In addition to this rst population of NPLs, a second NPL population with an even thicker CdS crown and a bathochromically shied emission seems to be present. The excitation spectra recorded at this emission peak [ Fig.…”

“…To date, a manifold of ways to modify the NPL structure has already been developed, including the growth of shells, crowns and domains around the core NPLs. 19 Depending on the desired application, the modication material can either be a semiconductor (wide or narrow bandgap) [20][21][22][23][24] or a metal. 25,26 If a shell is grown around a core NPL, the shell material is deposited on all surfaces of the NPL, which results in a lateral extension as well as in an increase in the NPL thickness.…”

“…This, in turn, leads to a good surface passivation, thus enhanced photoluminescence quantum yields (PLQYs) and chemical stabilities but also to bathochromically shied emission features independent of the actual band structures due to the weakening of the quantum connement. 23,[27][28][29][30] So-called core/crown structures, in contrast, are obtained by the lateral extension of the core NPLs with the crown material while retaining the core thickness. The optical properties of these heterostructures, however, strongly depend on the band positions of the applied crown material, ranging from nearly unaltered emission wavelengths for CdSe/CdS core/crown NPLs to strong bathochromic emission shis and even the appearance of dual wavelength emission for CdSe/CdTe and CdSe/CdSe 1Àx CdTe x core/crown NPLs, respectively.…”

“…33,34 In order to further enhance the PLQY of core/crown NPLs or core/shell nanocrystals (NCs) in general, the interface between core and shell needs to be optimised. Several approaches to tackle this issue have already been reported for core/shell NPLs, quantum dots (QDs) or nano rods, including high-temperature shell growth, 16,23,28,30 the introduction of post-synthetic annealing steps 35 and the utilisation of low-reactivity shell precursors to slow down the growth rate. 36 To the synthesis of highly luminescent core/crown NPLs the latter way is particularly applicable, as core-only NPLs are comparatively temperaturesensitive and as higher temperatures could moreover promote the three-dimensional shell growth.…”

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

“…However, to reduce the lattice mismatch and strain build-up in core/shell CQWs, there is a definite need for the fine control of the core/shell interface. [39,41] To address the above-mentioned challenges, here we have introduced HIS approach with a precise control of CQW aspect ratio (AR) and shell alloying which lead to core/HIS grown materials, namely CdSe/Cd 1−x Zn x S CQWs. The alloying mechanism allows for the fine tuning of the bandgap, which is demonstrated to be of critical importance for achieving highperformance LEDs.…”