Water-Soluble CdTe/CdS Core/Shell Semiconductor Nanocrystals: How Their Optical Properties Depend on the Synthesis Methods

Abstract

Abstract:We conducted a comparative synthesis of water-soluble CdTe/CdS colloidal nanocrystalline semiconductors of the core/shell type. We prepared the CdS shell using two different methods: a one-pot approach and successive ionic layer adsorption and reaction (SILAR); in both cases, we used 3-mercaptopropionic acid (MPA) as the surface ligand. In the one-pot approach, thiourea was added over the freshly formed CdTe dispersion, and served as the sulfur source. We achieved thicker CdS layers by altering the Cd:S stoichiometric ratio (1:1, 1:2, 1:4, and 1:8). The Cd:S ratios 1:1 and 1:2 furnished the best optical properties; these ratios also made the formation of surface defects less likely. For CdTe/CdS obtained using SILAR, we coated the surface of three differently sized CdTe cores (2.17, 3.10, and 3.45 nm) with one to five CdS layers using successive injections of the Cd 2+ and S 2-ions. The results showed that the core size influenced the optical properties of the materials. The deposition of three to five layers over the surface of smaller CdTe colloidal nanocrystals generated strain effects on the core/shell structure.

Ligand induced switching of the band alignment in aqueous synthesized CdTe/CdS core/shell nanocrystals

“…The synthesis of CdTe QDs could be successfully realized with all proposed ligands. Powder XRD analyses of the final products confirm that the crystal structure of the obtained QDs can be assigned to the cubic zinc-blende phase (JCPDS 93942) 26,27 . Figure 1 shows the diffractograms of CdTe QDs capped with different SL.

Figure 1Powder X-ray diffractograms of CdTe QDs capped with different surface ligands.

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“…To better understand the relationship between the CdS shell thickness and the PL lifetime data, we point out that in the case of a thin shell a type I band alignment is expected, keeping the electron-hole pair confined within the CdTe core, which leads to a direct transition 27,40,44 . With increasing CdS shell thickness, a transition to a type II heterostructure occurs, with the CB and VB of the core located above those of the shell, respectively, confining the electron within the shell and the hole within the core, and leading to the a change of the transition to indirect 39,40,44 .…”

“…These characteristics result in longer PL lifetimes and smaller oscillator strength for the transitions within the type II and quasi-type II structures when compared to the type I and inverted type I. Several studies indicate that core-shell systems with smaller cores are more sensitive to the compression effects compared to the larger ones 27,45 . Thus, because of the short lifetime (~20 ns), large QDs size and intermediate Cd/S ratio of CdTe/MPA and CdTe/TGA QDs, we conclude that the CdTe/CdS core-shell structures formed are of type I.…”

Size-Dependent Photobleaching Mechanism and Kinetics Induced by Nanosecond Laser Pulses in Colloidal Semiconductor Quantum Dots

“…According to refs , , , in the case of a thin CdS shell deposited on a small CdTe core (relative to the exciton Bohr radius), a standard type I band alignment is expected in the CdTe/CdS CSQDs since the valence and conduction band-edges of the core material are located within the energy bandgap of the shell. In this configuration, the bandgap of the shell semiconductor (CdS) is, in fact, wider than that of the core (CdTe), and so the exciton tends to localize predominantly in the core, where stronger quantum confinement of the charge carriers is established.…”

“…In fact, more dramatic changes are expected in the transition from type I to type II, such as dramatically elongated PL lifetimes 64 and significantly larger emission and absorption red shifts when CdTe cores as small as 2 nm are covered by thicker CdS shells (three to five monolayers), which have already been successfully monitored using the SILAR (successive ionic layers and adsorption reaction) technique. 66 Furthermore, in quasi-type II CSQDs, the hole is strongly confined to the core, whereas the electron is only weakly confined, being largely delocalized across the entire QD volume, with the electron probability density spreading into the core/shell interface. This partial spatial separation between charge carriers decreases the electron−hole overlap integral and hence the band-edge oscillator strength, thus contributing to the occurrence of attenuated optical absorption features, such as the already mentioned reduction in the absorption and emission intensities for the irradiated QD sample CdTe-555 (Figure 1e).…”