Synthesis and Characterization of Poly(acrylic acid) Stabilized Cadmium Sulfide Quantum Dots

Abstract

Cadmium sulfide (CdS) nanoparticles (NPs) capped with poly(acrylic acid) (PAA) were prepared in aqueous solutions from Cd(NO3)2 and Na2S. Influence of the COOH/Cd ratio (0.8-12.5), reaction pH (5.5 and 7.5), and PAA molecular weight (2100 and 5100 g/mol) on the particle size, colloidal stability, and photoluminescence were investigated. A Cd/S ratio of <1 causes ineffective passivization of the surface with the carboxylate and therefore results in a red shift of the absorption band and a significant drop in photoluminescence. Therefore, the Cd/S ratio was fixed at 1.1 for all experiments studying the mentioned variables. PAA coating provided excellent colloidal stability at a COOH/Cd ratio above 1. Absorption edges of PAA-coated CdS NPs are in the range of 460-508 nm. The size of the NPs increases slightly with increasing PAA molecular weight and COOH/Cd ratio at pH 7.5. It is demonstrated that there is a critical COOH/Cd ratio (1.5-2) that maximizes the photoluminescence intensity and quantum yield (QY, 17%). Above this critical ratio, which corresponds to smaller crystal sizes (3.7-4.1 nm) for each reaction set, the quantum yield decreases and the crystal size increases. Moreover, CdS NPs prepared at pH 7.5 have significantly higher QY and absorb at lower wavelengths in comparison with those prepared at pH 5.5. Luminescence quenching has not been observed over 8 months.

Photocurrent application of Zn-doped CdS nanostructures grown by thermal evaporation method

“…Furthermore, the yellow emission of the Zn-doped CdS has a tail in the red region around 700 nm (red band). A red band related to sulfur vacancies, as a consequence of the doping defect [17][18]. This indicates that the undoped CdS nanostructures have a better relative crystalline quality.…”

Stability of core/shell quantum dots—role of pH and small organic ligands

“…A gradual decrease of the l max was obtained during the first 30 min after what the value kept constant and nearly the same as the one obtained after 24 h. This indicates the importance of the exposure time after a QD surface perturbation. At higher pH values the NPs have a much better surface passivation because the polymer is mostly ionized (Celebi et al 2007), which results in a much more stable fluorescence intensity; in fact ViveNano Inc. provides the QD stock at pH 9.0. Despite the better surface passivation given by the polymer at the higher pH values the stability of the QD will also depend not only on the environmental pH but also on the pK a of the stabilizer.…”

One-Pot Synthesis, Encapsulation, and Solubilization of Size-Tuned Quantum Dots with Amphiphilic Multidentate Ligands

“…However, the resulting nanocrystals are often hydrophobic and must be encapsulated and solubilized for many important applications. Aqueous synthetic procedures have been used as alternative approaches to prepare water soluble QDs, using small thiol-containing molecules or polymers with carboxylic acid functional groups as stabilizing agents 8–10. But these methods do not yield QDs with the fluorescence brightness or size monodispersity that are often achieved with the high-temperature organic procedures.…”