Synthesis and Characterizations of ZnS:Cu/ZnS Assisted by 3-Mercaptopropionic Acid
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3-Mercaptopropionic acid capped core/shell ZnS:Cu(3%)/ZnS doped quantum dots (QDs) where synthesized at 95 °C in basic aqueous solution using the nucleation doping approach methods. The structural and optical properties of the QDs were characterized by X-ray diffraction (XRD) which confirms the cubic phase blende, whereas transmission electron microscopy (TEM) which shows the nanoparticles are spherical. UV–vis spectroscopy and photoluminescence (PL) spectroscopy. The obtained nearly monodisperse QDs have an average diameter of ca. 3.31 nm and a zinc blende crystal structure. The PL emission wavelength was limited between 500 and 510 nm but PL quantum efficiency of ZnS:Cu/ZnS core/shell nanocrystals increased up to 5%. The experimental results demonstrate that this method is effective for the preparation of ZnS:Cu/ZnS quantum dots.
KeywordsQuantum dots Semiconductor Core/shell Fluorescence
Semiconductor nanocrystals, or quantum dots (QDs), have attracted much attention over the past years as a novel class of material with unique electronic and optical properties [1, 2]. QDs display high extinction coefficients, PL quantum yields, and photostability, and their narrow emission spectra can be tuned by size and composition.
These semiconductor nanocrystals (quantum dots, QDs) have specific structural, optical and electronic properties [3, 4]. When the QDs’ properties are reduced to a few nanometers, the number of atoms located at the surface becomes predominant compared to the atoms in the core and a new phenomenon appears: the quantum confinement effect [5, 6].
Semiconductor cubic ZnS, with a bulk band gap of 3.7 eV at room temperature, is a common and attractive option as a doping host semiconductor for the production of dots for low cost and with a high stability [7, 8]. Research work on ZnS:Cu nanoparticles, including material preparation [9, 10] and property characterization [11–13], has been approved out. However, as far as we know, studies on the effect of Cu2+ concentration on the luminescence of ZnS nanoparticles are very limited. In the case of nanosized ZnS:Cu, the luminescence properties are still controversial. Two emission bands (blue and green) have often been observed together in the same sample, such as the 420 nm and 520 nm bands described by Lee et al.  and the 460 nm and 507 nm ones described by Xu et al. . The photoluminescence observed after Cu2+ doping is more complicated because Cu2+ is reduced by S2− into Cu2+ during the QDs synthesis. The green emission observed with Cu-doped ZnS QDs probably originates from the excited electrons’ transition from defect states of the host material to holes in the d-orbitals of the copper ions [14, 15]. The shell, generally composed of a wide band gap material such as ZnS, prevents degradation and preserves the optical properties [16, 17].
Herein, we demonstrate that the tunable photoluminescence of ZnS:Cu/ZnS QDs prepared by nucleation-doping strategy shifts from ca. 495 to 506 nm simply by changing the cation associated to the MPA ligand. The overall PL increase of the ZnS:Cu nanocrystals with the ZnS shell growth indicates that the Cu2+ dopant is non-fluorescent near the surface sites of the core ZnS QDs but becomes fluorescent when confined to the core of the nanocrystals there for improving the emission line profile due to a reduction of surface defects.
2 Experimental Section
Zinc sulfate heptahydrate (ZnSO4. 7H2O, 99.99%), copper chloride (CuCl2), Na2S. 9H2O (98 + %), 3-mercaptopropionic acid (MPA, 99%), cesium chloride (> 99%), and i-PrOH were used as received without additional purification. All solutions were prepared using Milli-Q water (18.2 MΩ cm, Millipore) as the solvent.
2.2 Synthesis of ZnS:Cu/ZnS QDs
The synthesis of ZnS:Cu(3%) core was described in the paper Labiadh et al. 2014 . The solution described above, was used to prepare ZnS:Cu(3%)/ZnS nanoparticles. In order to achieve shell growth, 0.7 ml of ZnSO4.7H2O (0.15 M) and 0.7 ml of Na2S.9H2O (0.1 M)/0.04 M (MPA) (pH set to 11 using CsOH) were injected drop by drop into solution ZnS:Cu under reflux. After heating for 1 h under reflux, another similar injection was performed. Each injection of Zn and S precursors is considered as the addition of a new monolayer. After injecting five layers, a colorless core/shell solution is obtained which is then purified through successive precipitation/centrifugation processes where iso-propanol (i-PrOH) is used as counter-solvent. After centrifugation the powder (precipitated QDs) is recovered. The obtained nanocrystals may be put in water in order to form a stable and light colloidal solution.
2.3 Characterizations of QDs
Transmission electron microscopy (TEM) images were taken by placing a drop of aqueous suspension of particles onto a carbon film-supported copper grid. Samples were studied using a Philips CM20 instrument operating at 200 kV equipped with Energy Dispersive X-ray Spectrometer (EDS). The X-ray powder diffraction data were collected from an X’Pert MPD diffractometer (Panalytical AXS) with a goniometer radius 240 mm, fixed divergence slit module (1/2°divergence slit, 0.04 rd Soller slits) and an X’Celerator as a detector. The powder samples were placed on zero background quartz sample holders and the XRD patterns were recorded at room temperature using Cu Kα radiation (λ = 0.15418 nm).
Absorption spectra were recorded on a PerkineElmer (Lambda 2) UV–Visible spectrophotometer. Steady state fluorescence spectra were recorded on a fluorolog-3 spectrofluorimeter F222 (Jobin–Yvon) using a 450 W xenon lamp.
3 Results and Discussion
3.2 Structural Characterization: XRD and TEM of QDs
Measured and calculated lattice parameters
3.3 Optical Properties of ZnS:Cu(3%)/ZnS QDs
The absorption shoulder is considerably blue shifted due to a quantum confinement effect compared to the absorption onset of bulk ZnS (337 nm). The excitonic absorption shoulder is shifted gradually to 278 nm from about 306 nm, suggesting that the quantum dots with a smaller size are produced at lower laser fluence. The result explained by laser-matter interaction mechanics .
In order to demonstrate the PL origin of the obtained colloidal particles, the PL characteristic of the bulk ZnS:Cu (3% Cu by atoms) crystal is compared as shown in Fig. 4. The blue-Cu (peaked at about 450 nm) and green-Cu (peaked at about 506 nm) emissions, are attributed to transitions from the conduction band of ZnS to the ‘e’ and ‘t’ levels of the excited Cu2+(d9) state in the ZnS band gap [23, 24], respectively.
The blue-Cu emission has been seldom detected in ZnS:Cu nanostructures and little is known about its PL decay dynamics. Even for green-Cu emission in ZnS:Cu nanostructures, the experimental results are also quite different. Al though there has been no uniform model for explaining the PL decay characteristic of Cu doped ZnS quantum dots by now, such a short luminescence decay time in quantum dots can be attributed  to the large overlap of electron–hole wave function in quantum dots due to the quantum confinement effect.
An increase in the fluorescence (PL) intensity, which reaches a maximum after the addition of 5 monolayers, was also observed . Similar results were obtained for the ZnS:Cu/ZnS nanocrystals prepared with NaOH however, the corresponding quantum yield was slightly lower than in the case of those prepared with CsOH. The increase in the quantum yield after the shell growth indicates that the surface defects in the ZnS:Cu(3%) core are significantly reduced and that the ZnS:Cu(3%)/ZnS emission intensities clearly increased from 2% without a shell to 5% with 5 ZnS monolayers. The increase of PL QY after shell growth indicates that the surface defects in the core ZnS:Cu@MPA QDs could be greatly reduced and that the emission intensity ZnS:Cu/ZnS core/shell QDs could be markedly enhanced for nanocrystals prepared with CsOH.
In summary, Cu2+ doped ZnS QDs stabilized by MPA were synthesized via a facile aqueous synthesis based on nucleationdoping strategy. X-ray diffraction and electron microscopy show the particles to be nanometric in size with the cubic phase. The result of ZnS shell growth around Cu doped ZnS core QDs was a slight blue shift of the PL and an amplitude increase of PL, attributed to the Cu dopant incorporation into the core ZnS nanocrystal lattice aided by the extra ZnS shell acting as a passivating layer. The concentration quenching of the luminescence may be caused by the formation of CuS compound. The result of this study is significant in better understanding the fundamental optical properties of doped semiconductor nanocrystals and the dependence of these properties on dopant location and surface charge density, which can be used in different opto-electronic applications. Those characteristics of the as-prepared quantum dots make them very attractive for biomedical labels .
The authors are grateful to the team at the Laboratory of Synthesis and Structure of Nanomaterials and laboratoire Réactions et Génie des Procédés (LRGP), UMR 7274, CNRS, 1 rue Grandville, BP 20451, 54001 Nancy Cedex, France.
LH and LS drafted and revised the manuscript. LH carried out the synthetic experiments and characterizations. LH and LS participated in the scientific flow. All authors read and approved the final manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no competing interests.
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