Mn2+-doped ZnS semiconductor quantum dots reveal remarkably intense photoluminescence with the 4T1(4G) f6A1(6S) transition. In this study, following growth doping technique, Mn2+-doped ZnS quantum dots (ZnS:Mn2+ QDs) with high-quality optical properties and narrow size distribution were synthesized successfully. The dopant emission has been optimized with various reaction parameters, and it has been found that the percentage of introduced dopant, reaction temperature, and time as well as the pH of a reaction mixture are key factors for controlling the intensity. Photoluminescence emission (PL) measurements of ZnS:Mn2+ QDs show Mn2+ d-d orange luminescence along with band-edge blue luminescence. Moreover, the electron transfer from singlet states of hypocrellin A (HA) to colloidal ZnS:Mn2+ QDs has been examined by absorption spectra and fluorescence quenching. The absorption spectrum gave an evidence of the increases in the extinction coefficient and the red-shift of the absorption maxima in the absorption spectra of HA in the presence of ZnS:Mn2+ QDs, demonstrating the occurrence of surface interactions between the sensitizer and the particle surface. Fluorescence quenching by ZnS:Mn2+ QDs also suggested that there were a complex association between HA and ZnS:Mn2+ QDs, which was necessary for observing the heterogeneous electron-transfer process at the interface of sensitizer-semiconductor.
Light emitting semiconductor nanocrystals, otherwise known as quantum dots (QDs), have been widely investigated during the last two decades in view of their size-tunable optical properties, wide range of excitations, emission color purity, high quantum efficiency, and applications as a light emitting source in various optoelectrical devices, imaging, solar cells, environment, remediation, and therapeutics and in biological applications [
ZnS is the most studied II–VI compound with band gap energy of 3.6 eV and has sufficiently intrigued an enormous amount of researchers to devote themselves to researching in this hot field for its excellent properties [
The photosensitization of a stable, large-band gap semiconductor ZnS by organic dyes is an interesting and useful phenomenon that was used to extend its absorptive range [
Chemical structures of hypocrellin A.
ZnSO4, L-cysteine, sodium sulfide (55–58%), manganese acetate tetrahydrate (99%), and NaOH were analytical grade reagents. All chemicals were purchased from Aldrich. In the reaction solution, the Mn concentration was varied (by varying the Mn(CH3COO)2·4H2O amount in the organometallic precursor) to values of 0.0%, 1.0%, 2.0%, 3.0%, 4.0%, and 5.0%.
The UV-visible spectra (Perkin-Elmer Lambda 900 USA) and photoluminescence (PL) emission spectra were recorded in a Hitachi F-7000 fluorescence photometer. All measurements were conducted at room temperature.
For the synthesis of ZnS QDs, we have employed a simple aqueous method using zinc sulfate, sodium sulfide, and manganese acetate tetrahydrate as the starting materials. The L-cysteine was used as the complexing agent and Milli-Q grade water as the solvent. First, 50 mL 0.03 M L-cysteine and 5 mL 0.1 M ZnSO4 solutions were placed in three-necked flask equipped with magnetic stirring bars. Second, a certain amount of NaOH solutions was slowly dropped into flask to make pH of the reaction mixture to 11.5 through a syringe controlled by a step motor. Then, N2 was introduced to rule out oxygen in solution for 30 min while thoroughly stirring. Finally, 5 mL 0.1 M Na2S was rapidly dropped into the mixtures through a syringe. After the addition of Na2S, the final solutions were stirred for another 4.5 h under totally enclosed condition at ambient temperature. The ZnS QDs were collected by centrifuging at 4,000 rpm for 20 min and washed 2-3 times with Milli-Q water.
A main objective of our research in this area of ZnS:Mn2+ quantum dots is to investigate the optimal conditions for their synthesis in the aqueous medium from readily available precursors. The dopant emission has been optimized with varying reaction parameters, and it has been found that the pH of an aqueous solution, the reaction temperature, and reaction time as well as the percentage of introduced dopant in the reaction mixture are key factors for controlling the intensity. A second objective of our program is to investigate photochemical events during the photosensitization of hypocrellin A on highly luminescent Mn2+-doped ZnS nanocrystals.
The ZnS:Mn2+ QDs were synthesized by the following general procedure. 50 milliliters of an aqueous solution of L-cysteine (0.03 M) was added to 5 mL of ZnSO4 (0.1 M) under vigorous stirring. Then a certain amount of Mn(CH3COO)2 and 4H2O (0.2 M) was added to this solution, and a certain amount of NaOH solutions was slowly dropped to adjust to the pH value of the mixtures. Interestingly, L-cysteine can resolve to −SCH2CH(NH2)COO− complexation with Zn2+ and Mn2+ easily in alkaline condition, which is apt to prepare ZnS:Mn2+ QDs. N2 was introduced to rule out oxygen into solution for 30 min while thoroughly stirring. Next, 5 mL of Na2S (0.5 M) was rapidly dropped into the mixtures through a syringe. The reaction temperature was maintained at various certain temperatures (30°C, 40°C, 50°C, 60°C, and 70°C) for a certain period of time (3 h, 3.5 h, 4 h, 4.5 h, 5 h, and 5.5 h) under constant stirring in totally enclosed condition. Finally, the ZnS:Mn2+ QDs were isolated by centrifugation, washed three times with Milli-Q water and then with cold ethanol (95% v/v), dried under vacuum for 24 h, and ground in an agate mortar.
HA was extracted and purified according to the method described by Brockman and Spitzner [
Doping introduces new levels within the band gap, modifying the linear photophysical properties of ZnS QDs, particularly the photoluminescence of ZnS QDs. Room-temperature photoluminescence emission (PL) spectra in Mn2+-doped ZnS QDs (Figure
Fluorescence emission spectra of ZnS and ZnS:Mn2+ QDs. The excitation wavelength for all the samples was fixed at 300 nm.
The measured UV-visible absorption spectrum shows resolvable one-photon absorption induced optical transitions from valence subbands to conduction subbands of ZnS and ZnS:Mn2+ QDs, depicted in Figure
UV-visible spectra of ZnS QDs (black) and ZnS:Mn2+ QDs (red).
The introduction of impurity ions into the host lattice can significantly depend on the reaction parameters, so the Mn precursor concentration, the pH of an aqueous solution, the reaction temperatures, and reaction times were investigated in this literature.
Band-edge luminescence and Mn2+-related luminescence are competing processes. The absorption of a photon leads to the formation of an exciton. The Mn precursor concentration in the reaction solutions was varied from 1.0% to 5.0%. Nevertheless, an increase in the Mn2+ concentration in the precursor solution above 5.0% caused significant deterioration in the luminescence, perhaps because of multiphase formation in the quantum dots (leading to defect states, which quench the luminescence). In this report, we mainly discuss ZnS:Mn2+ QDs formed with amounts of Mn in the solution less than or equal to 5.0%. As shown in Figure
Fluorescence emission spectra of ZnS:Mn2+ QDs under the different doping level of Mn (from 1.0% to 5.0%). The excitation wavelength for all the samples was fixed at 300 nm.
The synthesis of ZnS:Mn2+ QDs is dependent on the pH of an aqueous solution; L-cysteine can resolve to −SCH2CH(NH2)COO− complexation with Zn2+ and Mn2+ easily in alkaline condition, which is apt to prepare ZnS:Mn2+ QDs. As shown in Figure
Fluorescence emission spectra of ZnS:Mn2+ QDs under the different pH value of an aqueous solution (from 10.5 to 12.5), obtained using an excitation wavelength of 300 nm.
The reaction temperature is one of key factors to influence the size and shape of ZnS:Mn2+ QDs and affect high quantum efficiency consequently. When the ZnS:Mn2+ QDs were prepared using Mn(CH3COO)2·4H2O as the source of Mn and different reaction temperatures, a significant change in fluorescence intensity was observed (Figure
Fluorescence emission spectra of ZnS:Mn2+ QDs under the different reaction temperature (from 30°C to 70°C), obtained using an excitation wavelength of 300 nm.
PL spectra were recorded at the reaction times ranging from 3 h to 5.5 h (Figure
Fluorescence emission spectra of ZnS:Mn2+ QDs under different reaction times (from 3 h to 5.5 h), obtained using an excitation wavelength of 300 nm.
In conclusion, the optimal reaction conditions of ZnS:Mn2+ QDs were obtained as follows: the doping level of Mn2+ is 3.0%, the pH of solution is 11.5, the reaction temperature is 50°C, and reaction time is 4.5 h. ZnS:Mn2+ QDs were prepared under such optimal reaction conditions having high quality of photophysical properties and narrow size distribution.
It has been shown that HA is not only an effective phototherapeutic agent but also a good photosensitizer [
Absorption spectra of HA (A), ZnS:Mn2+ QDs (B), and HA-ZnS:Mn2+ QDs system (C) are showed in Figure
(a) Absorption spectra of HA (A), ZnS:Mn2+ QDs (B), and HA-ZnS:Mn2+ QDs system (C); (b) an image of absorption spectra of HA.
The present study employed HA as sensitizer to investigate the adsorption details on the surface of the ZnS:Mn2+ QDs and the mechanism of the interfacial electron-transfer process from singlet state of HA to conduction band of ZnS:Mn2+ QDs in aqueous solution. In an aqueous solution, the colloidal ZnS:Mn2+ QDs can be assumed to have predominantly Zn-OH groups at the surface of ZnS:Mn2+ QDs, through which polar species in solution can strongly interact with the particles; HA can associate with ZnS:Mn2+ QDs through its polar groups such as OH and/or C=O, and this association will lead to a change of the p-electron distribution in HA molecule. As a result, as shown in Figure
Absorption spectra of HA-ZnS:Mn2+ QDs system. The HA concentration was fixed at 1.8315 × 10−4 M and the concentration of ZnS:Mn2+ QDs (8.0143 × 10−3 M) increased from top to bottom.
The fluorescence emission of HA was quenched upon successive addition of ZnS:Mn2+ QDs to a solution of 1.8315 × 10−4 M HA (Figure
Fluorescence emission spectra of HA-ZnS:Mn2+ QDs system. The HA concentration was fixed at 1.8315 × 10−4 M and the concentration of ZnS:Mn2+ QDs (8.0143 × 10−3 M) increased from 0
Following growth doping technique, highly luminescent Mn-doped ZnS nanocrystals were synthesized and revealed remarkably intense photoluminescence with the 4T1(4G) f6A1(6S) transition in this literature. The dopant emission has been optimized with varying reaction parameters and found the optimal doping level of Mn. The method is simple, is hassle-free, and can be prepared to the high quality of Mn-doped ZnS nanocrystals. Photoluminescence (PL) emission measurements of ZnS:Mn2+ QDs show Mn2+-related orange luminescence. Moreover, the electron transfer from singlet states of hypocrellin A (HA) to colloidal ZnS:Mn2+ QDs has been examined by absorption and fluorescence quenching. The absorption spectrum gave an evidence of the strong association between HA and ZnS:Mn2+ QDs; the electron could be injected from singlet excited states of HA to conduction band of colloidal ZnS:Mn2+ QDs. PL measurements also suggested that there was a complex association between HA and ZnS:Mn2+ QDs.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge the financial support from the National Science Foundation of China (61361002 and 21362010) and Yunnan Provincial Department of Education General Project (2013Y067).