Near-infrared emitting CdTe0.5Se0.5/Cd0.5Zn0.5S quantum dots: synthesis and bright luminescence

Ping Yang,Norio Murase,Shiquan Wang

doi:10.1186/1556-276x-7-615

Abstract

We present how CdTe0.5Se0.5 cores can be coated with Cd0.5Zn0.5S shells at relatively low temperature (around 200°C) via facile synthesis using organic ammine ligands. The cores were firstly fabricated via a less toxic procedure using CdO, trioctylphosphine (TOP), Se, Te, and trioctylamine. The cores with small sizes (3.2-3.5 nm) revealed green and yellow photoluminescence (PL) and spherical morphologies. Hydrophobic core/shell CdTe0.5Se0.5/Cd0.5Zn0.5S quantum dots (QDs) with tunable PL between green and near-infrared (a maximum PL peak wavelength of 735 nm) were then created through a facile shell coating procedure using trioctylphosphine selenium with cadmium and zinc acetate. The QDs exhibited high PL efficiencies up to 50% because of the formation of a protective Cd0.5Zn0.5S shell on the CdTe0.5Se0.5 core, even though the PL efficiency of the cores is low (≤1%). Namely, the slow growth process of the shell plays an important role for getting high PL efficiencies. The properties of the QDs are largely determined by the properties of CdTe0.5Se0.5 cores and shells preparation conditions such as reaction temperature and time. The core/shell QDs exhibited a small size diameter. For example, the average diameter of the QDs with a PL peak wavelength of 735 nm is 6.1 nm. Small size and tunable bright PL makes the QDs utilizable as bioprobes because the size of QD-based bioprobes is considered as the major limitation for their broad applications in biological imaging.

Highlights

Semiconductor quantum dots (QDs) have attracted considerable interest in the past decade because of their excellent properties such as narrow, symmetric, and tunable emission spectra, broad absorption spectra, superior photostability, high photoluminescence (PL) efficiency, and the capacity of simultaneous excitation of multiple fluorescence colors compared with the organic dyes and fluorescent proteins [1,2,3,4]
CdTe and CdTe/CdSe QDs prepared by an organic synthesis are normally unstable, which results in low PL efficiency and fast photobleaching
The structure of these QDs differs from that of core/shell QDs, where a thin layer of a wider band gap semiconductor is grown on the surface of a core semiconductor

Summary

Introduction

Semiconductor quantum dots (QDs) have attracted considerable interest in the past decade because of their excellent properties such as narrow, symmetric, and tunable emission spectra, broad absorption spectra, superior photostability, high photoluminescence (PL) efficiency, and the capacity of simultaneous excitation of multiple fluorescence colors compared with the organic dyes and fluorescent proteins [1,2,3,4]. CdSe (Eg = 1.76 eV, aB = 9.6 nm) QDs can be tuned through quantum confinement to emit visible fluorescent spectrum only reaching typically 650–700 nm. It is difficult to grow CdSe-based QDs with NIR emission because of their growth kinetics. Compared with CdSe, the band gap of bulk CdTe is narrow (1.56 eV, corresponding to approximately 795 nm wavelength). Composited CdTe and CdSe QDs or CdTe-based QDs with protective inorganic shells are expected. These QDs with NIR emission and small size are necessary. There have been few reports of a higher band gap shell growth around CdTe or CdTeSe QDs. Rogach and co-workers described the aqueous synthesis of 8–10 nm CdTebased QDs emitting at approximately 800 nm [7]

Methods

Results

Conclusion