Hydrothermal synthesis of high-quality type-II CdTe/CdSe quantum dots with near-infrared fluorescence

Abstract

A simple hydrothermal method is developed for the synthesis of high-quality, water-soluble, and near-infrared (NIR)-emitting type-II core/shell CdTe/CdSe quantum dots (QDs) by employing thiol-capped CdTe QDs as core templates and CdCl₂ and Na₂SeO₃ as shell precursors. Compared with the original CdTe core QDs, the core/shell CdTe/CdSe QDs exhibit an obvious red-shifted emission, whose color can be tuned between visible and NIR regions (620–740 nm) by controlling the thickness of the CdSe shell. The photoluminescence quantum yield (PL QY) of CdTe/CdSe QDs with an optimized thickness of the CdSe shell can reach up to 44.2% without any post-preparative treatment. Through a thorough study of the core/shell structure by high-resolution transmission electron microscopy (HRTEM), ultraviolet–visible (UV–vis) absorption spectra, fluorescence spectra, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the as-prepared CdTe/CdSe QDs demonstrate good monodispersity, hardened lattice structure and excellent photostability, offering a great potential for biological application.

Introduction

Semiconductor nanocrystals (or quantum dots, QDs), as a new class of very promising materials, have been widely used in biological fields owing to their unique optical properties during the past decade [1], [2], [3]. In this area, the near-infrared (NIR) QDs with emission wavelengths between 650 and 900 nm are of particular interest to researchers because the tissue auto-fluorescence and absorption are much more reduced compared to those in other ranges [4], [5].

Until now, there have been two strategies to fabricate NIR-emitting QDs. One is to select suitable semiconductor materials with bulk band gap energy lower than the NIR energy to prepare mononuclear or alloyed semiconductor nanocrystals by controlling their growth sizes or structure compositions, respectively, such as PbSe, InAs, HgTe, CdHgTe, CdTeS and CdSeTe [6], [7], [8], [9], [10], [11]. The other is to fabricate core/shell QDs. Most recently, the synthesis of core/shell (type-I and type-II) NIR-emitting QDs has become an active field; not only do core/shell heterostructure QDs effectively reduce surface defects and nonradioactive decay, but the emission wavelengths of QDs could also be tuned by their sizes and compositions [12], [13], [14], [15]. Compared with type-I core/shell QDs, type-II nanocrystals, with the valence band edge or the conduction band edge of the shell material located in the band gap of the core, have many novel properties, such as the red-shift of the emission and a longer decay lifetime due to the formation of an indirect excitation [12], [13], [14], [15]. Thus far, there have been reported many type-II systems, including CdTe/CdSe, ZnTe/CdTe (CdSe, CdS), CdSe/ZnTe and ZnSe/CdS [13], [14], [15], [16], [17], [18], and most of these QDs are prepared through organometallic routes. However, for biological applications, additional surface modification is required to disperse the QDs in aqueous phase, for example, wrapping an amphiphilic polymer around the QDs or exchanging original hydrophobic ligands with thiols. Moreover, these further processes often lead to the increment of QD size and the as-prepared QDs usually have a significant decrease of photoluminescence quantum yield (PL QY). As a result, direct aqueous synthesis of type-II core/shell NIR-emitting QDs has become an attractive and challenging task in QD research. Recently, Zhu et al. have obtained NIR-emitting CdTe/CdSe QDs in aqueous solution, and then the as-prepared QDs were applied for ultrasensitive Cu²⁺ sensing [19]. Subsequently, Yan and his co-workers successfully synthesized water-soluble l-cysteine-capped CdTe/CdSe QDs with NIR fluorescence by successive ion-layer adsorption and reaction (SILAR) for bioimaging applications [20]. However, their methods are considerably complicated, because they are time-consuming and the PL QYs of the QDs are much lower than those synthesized in organic phase. Therefore, it is desirable to develop a facile method for the preparation of high-quality type-II core/shell NIR-emitting QDs in aqueous medium.

Here we present a layer-by-layer colloidal epitaxial growth of 3-mercaptopropionic acid (MPA)-capped CdTe/CdSe type-II core/shell QDs in aqueous solution via hydrothermal technique by using preformed MPA-capped CdTe QDs as core templates and CdCl₂ and Na₂SeO₃ as shell precursors. A CdSe shell is formed on the CdTe core through the reduction of Na₂SeO₃ by NaBH₄. With the increase of the thickness of the CdSe shell on the CdTe core QDs, the fluorescence emissions of the prepared MPA-capped CdTe/CdSe QDs are tunable between 620 and 740 nm. The as-prepared core/shell QDs show a PL QY as high as 44.2% with three layers of CdSe shells, which is much better than previous reports [19], [20]. Optical and physical properties have been studied by high-resolution transmission electron microscopy (HRTEM), ultraviolet–visible (UV–vis) absorption spectra, fluorescence spectra, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The resulting NIR-emitting QDs are expected to have a great potential for biological application.