In this study, we have demonstrated a facile, simple one-pot and low cost method for the synthesis of 3-mercaptopropionic acid (MPA)-capped, water-soluble CdS quantum dots (QDs) with highly tunable optical properties. Initially Cd2+ coordinates with MPA at about pH 5, and the CdS QDs were then formed at a higher pH (7–12) under refluxing conditions through the disruption of coordination interaction with the release of sulfur. Here MPA played a dual role, as both, a source of sulfur and as a stabilizer. The particle size and the optical properties of the as-prepared CdS QDs were found to be dependent on the refluxing time for a given concentration ratio of the reactants and pH of the initial mixture. The broadness and large Stokes shift of emission of MPA–CdS QDs are due to the surface-trap state photoluminescence (PL). The PL peak around 510 nm–650 nm is due to the recombination of shallow trapped electrons in sulfur vacancy defect states with holes in the valence band, and a ∼665 nm peak (shoulder) arises from deep-trap states. The origin of the longer lifetime is presumed to be due to the involvement of surface-trap states and their environment. Use of MPA as a capping agent eventually enhances the water solubility as well as the stability of CdS QDs, which makes them useful for the ultra-sensitive detection of Co2+ and Ni2+. The selective coordination interaction of Co2+ and Ni2+ with MPA–CdS QDs through the carboxyl group of MPA provides a turn-off photoluminescence-based assay for sensitive detection of these metal ions without any interference of other commonly coexisting metal ions. The limit of detection (LOD) is 10 nM for Co2+ ions and 50 nM for Ni2+ ions. Co2+-induced color (from colorless to yellow) and UV-vis spectral change of MPA–CdS QDs is the simple way to distinguish Co2+ from Ni2+ in a higher concentration range (more than 5 µM). On the other hand the lower stability of the Co(II)–MPA complex than the Ni(II)–MPA complex provides a disodium salt of ethylenediaminetetraacetic acid (EDTA)-induced, time dependent turn-on photoluminescence-based technique to distinguish Co2+ from Ni2+ in the entire range of concentrations. EDTA-induced time dependent PL recovery of MPA–CdS QDs occurs via rapid dissociation of Co2+ ions from the surface of QDs than that of Ni2+. Thus our synthesized MPA–CdS QDs offer a very simple, rapid, cost-effective, turn-off–on photoluminescence-based technique for ultra-sensitive and selective detection of either Co2+ or Ni2+ in aqueous solution without interference of other common metal ions.

A single source-precursor route for the one-pot synthesis of highly luminescent CdS quantum dots as ultra-sensitive and selective photoluminescence sensor for Co2+ and Ni2+ ions

Photoluminescence quenching in ligand free CdS nanocrystals due to silver doping along with two high energy surface states emission

Controllable synthesis and characterization of CdS quantum dots by a microemulsion-mediated hydrothermal method

Biomolecule assisted morphology-controllable synthesis of Zinc Sulphide nanomaterials for efficient photocatalytic activity under solar irradiation

ZnS quantum dots doped with magnetic transition metal (Zn1−x
 TM
 x
 S; where x = 0.04, 0.08 and transition metal = Ni, Mn, Fe, Co and Cr) were synthesized using a chemical co-precipitation method. To prevent agglomeration, samples were capped with polyvinylpyrrolidone. X-ray diffraction peaks confirmed pure cubic phases of all samples. The crystallite dimensions of the samples are within the scale of 2.0–2.6 nm, which was calculated using Scherrer formula. A band gap varying from 4.1 eV to 4.24 eV was estimated from their ultraviolet–visible absorption spectroscopy. The synthesized samples show a strong blue shift in their emission spectroscopy along with emissions from inherent Zn and S point defects (interstitial and vacancy). Superconducting quantum interference device studies at 300 K reveal that all samples show room temperature canted ferromagnetism at low magnetic fields which does not saturate even up to a fields of 5 T. We study the defects as seen through emission spectroscopy and correlate with the magnetic properties of the doped semiconducting quantum dots.

Super Exchange-Induced Canted Ferromagnetism in Transition Metal-Doped ZnS Quantum Dots

With the features of convenience and eco-friendly, the low-temperature solid-state
reaction synthesis was successfully developed as a new approach to prepare quantum-sized
ZnS nanocrystals. One major achievement is that the size and shape of ZnS nanocrystals
can be tuned by adjusting the surfactant and its feed. The UV-Vis absorption spectra
of quasispherical and one-dimensional quantum-sized ZnS nanocrystals all showed
a blue-shift from the bulk counterpart, indicating large quantum confinement effects
of ZnS nanocrystals. These ZnS nanocrystals all showed well-defined excitonic emission
features. Contrastive studies on photoluminescence performances
indicated that the bandedge emission experienced only the size-dependent quantum
confinement effect, while the trap-state emission experienced the size- and shape-dependences.
So we can design a purposeful synthesis route to ZnS nanocrystals with target luminescence
emission performances.

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Meng Cao

Papers

A New Simple Route to ZnS Quantized Particles with Tunable Size and Shape, and Size/Shape-Dependent Optical Properties