Synthesis of highly luminescent wurtzite CdSe/CdS giant-shell nanocrystals using a fast continuous injection route

S Christodoulou,A Casu,G Vicidomini,A Genovese,F De Donato,A Diaspro,S Brovelli,V Pinchetti,J Q Grim,L Manna,G Vaccaro,I Moreels

doi:10.1039/c4tc00280f

Abstract

We synthesized CdSe/CdS giant-shell nanocrystals, with a CdSe core diameter between 2.8 nm and 5.5 nm, and a CdS shell thickness of up to 7–8 nm (equivalent to about 20 monolayers of CdS). Both the core and shell have a wurtzite crystal structure, yielding epitaxial growth of the shell and nearly defect-free crystals. As a result, the photoluminescence (PL) quantum efficiency (QE) is as high as 90%. Quantitative PL measurements at various excitation wavelengths allow us to separate the nonradiative decay into contributions from interface and surface trapping, giving us pathways for future optimization of the structure. In addition, the NCs do not blink, and the giant shell and concurring strong electron delocalization efficiently suppress Auger recombination, yielding a biexciton lifetime of about 15 ns. The corresponding biexciton PL QE equals 11% in 5.5/18.1 nm CdSe/CdS. Variable-temperature time-resolved PL and PL under magnetic fields further reveal that the emission at cryogenic temperature originates from a negative trion-state, in agreement with other CdSe/CdS giant-shell systems reported in the literature.

Highlights

Colloidal nanocrystals (NCs), in particular CdSe/CdS core/shell heterostructures, have proven to be a versatile class of nanomaterials with potential applications in light-emitting diodes,[1,2] lasers,[3,4,5,6] bio-labels[7,8] and single photon sources.[9,10] Control over the synthesis parameters currently permits embedment of nanocrystalline CdSe cores into a variety of shell structures
Transmission electron microscopy (TEM) images of aliquots taken during synthesis yield a direct view on the NC growth, here starting from 5.5 nm CdSe seeds and aiming for a CdS shell of 20 ML (Fig. 1)
The reduced value compared to 5.5/18.1 nm NCs is in line with the increased PL intermittence observed for this sample, yet more importantly, the biexciton PL quantum efficiency (QE) of our CdSe/CdS NCs is comparable to other CdSe/CdS giant-shell systems[43] and even CdSe/CdS NCs with an alloyed interface,[44] again con rming the high quality of our material

Summary

Introduction

Colloidal nanocrystals (NCs), in particular CdSe/CdS core/shell heterostructures, have proven to be a versatile class of nanomaterials with potential applications in light-emitting diodes,[1,2] lasers,[3,4,5,6] bio-labels[7,8] and single photon sources.[9,10] Control over the synthesis parameters currently permits embedment of nanocrystalline CdSe cores into a variety of shell structures. According to progressive delocalization of electrons in thicker-shell CdSe/CdS NCs, the exciton lifetimes are in the range of 400–700 ns, depending on the core diameter.

Results

Conclusion