Synthesis of Air-Stable CdSe/ZnS Core–Shell Nanoplatelets with Tunable Emission Wavelength

Abstract

In the past few years, several protocols have been reported on the synthesis of CdSe nanoplatelets with narrow photoluminescence (PL) spectrum, high PL quantum efficiency, and short exciton lifetime. The corresponding core/shell nanoplatelets are however still mostly based on CdSe/CdS, which possess an extended lifetime and a strong red shift of the band-edge absorption and emission, in accordance with a quasi-type-II band alignment. Here we report on a robust synthesis procedure to grow a ZnS shell around CdSe nanoplatelets at moderate temperatures of 100–150 °C, to improve the optical properties of CdSe nanoplatelets via a type-I core/shell heterostructure. The shell growth is performed under ambient atmosphere, in either toluene or 1,2-dichlorobenzene. The variation of the shell thickness induces a continuous red shift of the PL peak, eventually reaching 611 nm. The PL quantum efficiency is increased compared to the original CdSe cores, with values up to 60% depending on the shell thickness. High-resolution transmission electron microscopy reveals a bending of the nanoplatelets caused by strain due to 12% lattice mismatch between CdSe and ZnS. The present procedure can easily be translated to other core/shell nanocrystals, such as CdSe/CdS and CdSe/CdZnS nanoplatelets.

Excitons in core-only, core-shell and core-crown CdSe nanoplatelets: Interplay between in-plane electron-hole correlation, spatial confinement, and dielectric confinement

“…In the type-I limit (cbo = 1 eV), the exciton emission energy experiences a net blueshift of 15 meV, which is very close to recent theoretical estimates for CdSe/ZnS NPLs [19]. Such a blueshift, although with larger magnitude, was already reported in core-only CdSe NPLs [16,19], and takes place because, in anisotropic objects like NPLs, the self-energy repulsion terms exceed the binding energy enhancement. The blueshift is less severe in type-I core-shell NPLs than in core-only ones because the shell separates excitonic carriers from their image charges in the dielectric environment.…”

“…Two important physical factors affect the exciton properties in that case. First, the thickness of the shell tends to reduce the influence of the dielectric environment [19]. Second, different core-shell band alignments can be used to concentrate or separate electrons from holes.…”

“…Self-interaction of carriers with their image charges is also important. In NPLs it has been found to exceed the exciton binding energy enhancement, thus leading to a net blueshift of the emission energy [16,18,19].…”

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

“…[54] However, it is deserved to point out that such PLQY is consistent with that of reported core/shell CQWs utilizing c-ALD, with levels in the range of 0.5-11%. [55,56] The XRD and XPS characterizations of the CdSe/Cd 0.25 Zn 0.75 S core/c-ALD and core/HIS are shown in Figures 3c,d, respectively. Both XRD and XPS studies reveal that Zn composition is very low in the c-ALD approach, since it cannot penetrate into the structure due to the low temperature shell growth route followed in c-ALD approach.…”

Wet-Chemical Synthesis and Applications of Semiconductor Nanomaterial-Based Epitaxial Heterostructures

“…8a-c). The synthetic approach is very general and can be extended to other types of shell materials, as long as the reactivities of the core surface and the precursor are high [147][148][149][150]. Although the layer-by-layer method is very useful to finely control the shell thickness and chemical composition, this protocol is very time-consuming, and the NCs need to be carefully washed between each deposition process to avoid the secondary nucleation.…”