High-Temperature Synthesis of CdSe-Based Core/Shell, Core/Shell/Shell, and Core/Graded-Shell Nanoplatelets for Stable and Efficient Narrowband Emitters

Abstract

<div>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, which are well known to improve the performance of quantum dots. <br></div><div>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/Cd<sub>x</sub>Zn<sub>1–x</sub>S, 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/Cd<sub>x</sub>Zn<sub>1–x</sub>S) 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.</div>

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS–PIETA NMR Spectroscopy

“…A popular approach to enhance the optical properties of CdE (E = S, Se, Te) NPLs, foremost to maximize their PL quantum yield (QY) and to impart suited environmental stability, invokes epitaxial overgrowth of the NPLs with another, wider-band gap semiconductor shell. Core/shell CdSe/CdS NPLs ,,, and CdSe/ZnS NPLs − are thus by far the most commonly studied colloidal NPL heterostructures. The most recent additions to the class of colloidal NPLs are CdSe/CdS/CdSe, CdSe/CdS/ZnS, and CdSe/CdSe/CdZnS core/shell1/shell2 NPLs. , With the epitaxial extension of the crystallites, the band gap and other physical parameters are substantially altered. ,,,− ,,− For instance, the PL QY raises from about 30% for core-only CdSe NPLs (green-emissive, 4.5 monolayers thick) to 85–98% upon the addition of a wider-band gap shell (red-emissive) .…”

“…Core/shell CdSe/CdS NPLs ,,, and CdSe/ZnS NPLs − are thus by far the most commonly studied colloidal NPL heterostructures. The most recent additions to the class of colloidal NPLs are CdSe/CdS/CdSe, CdSe/CdS/ZnS, and CdSe/CdSe/CdZnS core/shell1/shell2 NPLs. , With the epitaxial extension of the crystallites, the band gap and other physical parameters are substantially altered. ,,,− ,,− For instance, the PL QY raises from about 30% for core-only CdSe NPLs (green-emissive, 4.5 monolayers thick) to 85–98% upon the addition of a wider-band gap shell (red-emissive) . With the goal of putting the exceptional PL properties to work, numerous applications of CdSe-based NPLs are being pursued, e.g., light-emitting devices, − solar cells, , photodetectors, , and lasers. ,,,,,− …”

“…The versatility in the growth of heterostructures has increased in the past few years such that the thickness is controllable down to a monolayer for both the core ,,, and the shell. ,,,,, Furthermore, lateral extension of the platelet with a chemically distinct material is possible without altering the NPL thickness, forming what are known as core/crown NPLs. ,− , A complete enclosure of the NPL core is attainable as well, thereby harnessing the benefits of both the core/shell and core/crown heterostructures. , The PL tuning range obtainable with these different NPL compositions encompasses a wide range from 395 nm (core-only CdSe NPLs) , to ∼720 nm (6-monolayer-CdSe/CdS/ZnS core/shell1/shell2 NPLs) . While the thickness of the NPLs directly influences their band gap energy in discrete steps, a more continuous (fine) tuning of the NPLs emission wavelength can be obtained compositionally by the formation of solid solutions of the pristine NPLs (e.g., CdSe 1– x S x NPLs, , CdS x Se 1– x NPLs, CdSe 1– x Te x NPL), or likewise adjusting shell and crown compositions, ,,,,− , or by doping the CdSe NPLs with Ag to introduce lower-energy transitions via midgap trap states . Next to the formation of heterostructures, diverse postsynthetic chemical modifications are pursued to impart desired optical and electronic characteristics of NPLs.…”

“…Core/shell CdSe/CdS NPLs 16,26,38,39 and CdSe/ZnS NPLs 39−42 are thus by far the most commonly studied colloidal NPL heterostructures. The most recent additions to the class of colloidal NPLs are CdSe/CdS/CdSe, 27 CdSe/CdS/ZnS, 42 and CdSe/CdSe/CdZnS core/shell1/shell2 NPLs. 42,43 With the epitaxial extension of the crystallites, the band gap and other physical parameters are substantially altered.…”

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

“…The wide‐bandgap shell (e.g., ZnS) provides high photostability and reduced Auger recombination for exciton harvesting applications. 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 . 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.…”