Synthesis and photoluminescence properties of Ru-doped ZnS quantum dots

Synthesis and photoluminescence properties of Ru-doped ZnS quantum dots

Journal of Luminescence 187 (2017) 421–427 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/loca...

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Journal of Luminescence 187 (2017) 421–427

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Synthesis and photoluminescence properties of Ru-doped ZnS quantum dots Reza Sahraei a,n, Farnaz Mohammadi a, Ehsan Soheyli a,b, Mahmoud Roushani a a b

Department of Chemistry, Faculty of Science, University of Ilam, 65315-516, Ilam, Iran Department of Physics, Faculty of Science, University of Arak, Arak 3815688394, Iran

art ic l e i nf o

a b s t r a c t

Article history: Received 19 November 2016 Received in revised form 11 March 2017 Accepted 11 March 2017 Available online 21 March 2017

In the present work, the feasibility study of growth doping strategy for direct preparation of Ru-doped ZnS quantum dots (QDs) in aqueous solution is investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), UV–vis absorption, and photoluminescence (PL) emission spectroscopies. Structural investigations indicated that the Ru-doped ZnS QDs were about 3 nm in size, showing size-dependent optical properties of the prepared QDs. Specifically, three different peaks are observed in photoluminescence spectra of Ru-doped ZnS specimens; a very weak excitonic emission at about 350 nm, the blue trap-related emission around 420 nm, and a strong cyan dopant-related emission at about 450 nm. Various parameters such as solution pH, refluxing temperature and precursors molar ratios were observed to have significant effects on emission properties of the as-prepared QDs. By optimization of each parameter, the best result for preparation of the Ru-doped QDs was determined to be; solution pH ¼ 11, refluxing temperature of 100 °C and the Zn:ME:Ru:S molar ratio of 1:50:0.01:0.5. Considering the simplicity, fastness, and their notable chemical and optical stabilities, the present QDs can be a candidate for variety of applications in biotechnology and optoelectronic devices. & 2017 Elsevier B.V. All rights reserved.

Keywords: Semiconductors Quantum dots Ru-doped ZnS Photoluminescence properties

1. Introduction Semiconductor nanocrystals, also known as quantum dots (QDs), with different size-dependent properties from their bulk counterparts have attracted more attention of researchers in recent years [1]. The small sizes of such structures have been proven to cause deep impacts on their physical and chemical properties, which candidate them for variety of applications in electrochemical reduction reactions, sensors, optoelectronic devices, biomedical imaging, and labeling [2–5]. The high quality binary, alloy and core/shell structures of II-VI group are several examples of semiconductor QDs which have been highly studied at the recent decades [6–10]. However, the necessity of finding the more thermal, chemical and photochemical stable QDs with desired characteristics and zero self-quenching reabsorption, has motivated the researchers to work on doped QDs; the one which contains an intentionally introduced amount of impurity. Transition metal ions are hence good choices for this goal. Mn, Cu, Ni, Fe and Co ions are some of the most used dopant ions [11–16] which can represent their own emission centers, enhance the magnetic character or improve the electronic properties. These elements n

Corresponding author. E-mail addresses: [email protected], [email protected] (R. Sahraei).

http://dx.doi.org/10.1016/j.jlumin.2017.03.020 0022-2313/& 2017 Elsevier B.V. All rights reserved.

have been highly used during two-past decades and their emission characteristics have been also studied. Hence, it can be suitable to find, prepare and generally introduce a new element for concept of the doping. Ru3 þ ion is another dopant element which can makes its own energy level, depending on the band edge position of host matrix, at forbidden band gap region as efficient center for recombination of excited carriers [17]. From this point of view and regarding very few works on Ru-doped semiconductor nanostructures, we are going to emphasize on a chemical route to prepare the Ru-doped colloidal QDs and to investigate the optimized conditions for their emission properties. A fundamental work to find a suitable chemical route for successful preparation of doping structures has been carried out by Pradhan et al. [18]. They state that, this kind of structures can be achieved by nucleation and growth doping strategies through separation of doping and growth steps. Colloidal semiconductor QDs have born out of the pioneering efforts of researchers, especially in organic solvents [19]. Although, the results are highly interesting and the emission quantum yield is suitable for further applications, however, the organic-based methods are handled with more toxic-expensive precursors, full of complicated processes, and at high temperature conditions, compared to aqueous-based routes [20]. Hence, despite all the remarkable attentions in recent years, the further studies are still required to explore the possibility of introduction of other ions to

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enhance the physiochemical characteristics of the host matrix, especially in the aqueous solutions. In addition to be an excellent inorganic passivation shell, ZnS structure is also the most promising semiconductor to impart dopant ions due to wide-direct band gap of about 3.6 eV, high stability and low toxicity [21]. To the best of our knowledge, there are no any reports on Ru-doped ZnS QDs and several works have just studied the effect of Ru3 þ dopant ions on physiochemical properties of metal oxide thin films or nanoparticles, like ZnO, CuO, TiO2 and SnO2 [22–25]. Ru-doped nanostructures have the potential applications as good candidates as electrodes for supercapacitor [23], in photoelectron-catalytic processes [24] and gas sensors [25]. Herein, we report, for a first time, direct aqueous synthesis of Ru-doped ZnS QDs using 2-mercaptoethanol (ME) as capping agent. The growth doping strategy is used and the effect of solution pH, refluxing temperature and precursors’ molar ratio on optical properties of Ru:ZnS QDs is investigated to obtain the optimum conditions for the best emission character.

2. Experimental All chemicals including ZnCl2, 2-mercaptoethanol (ME), RuCl3.3H2O, Na2S.9H2O, 2-propanol, and NaOH were of analytical reagent grade and purchased from Merck Company. Deionized water was also used during all experiments. The Ru:ZnS QDs were synthesized as follows; 20 ml of 0.01 M (mol L  1) ZnCl2 solution was added to a mixture of ME and 65 mL deionized water in a three-necked flask. Next, appropriate amounts of 0.02 M RuCl3 solution were added to change the Ru3 þ : Zn2 þ molar ratio from 1:200 to 5:100. The pH of the stirring solution was adjusted to desire value (from 8 to 11) by dropwise addition of 1 M NaOH solution and the resulting mixture was deaerated for 15 min by N2 gas. Subsequently, appropriate amounts of 0.01 M Na2S solution were slowly injected into the mixture under stirring. The reaction mixture was then heated up to specific temperature and further refluxed for 1 h. After cooling to room temperature, each specimen absorption and emission spectra were recorded. The Zn2 þ concentration was 2  10  3 M in a total volume of 100 mL. The molar ratios of Zn2 þ :ME used in our experiment are 1:25, 1:30, 1:40, 1:50, 1:62.5, and 1:75 in sequence. The molar ratios of S2-:Zn2 þ used in our experiment are also, 1:100, 1:25, 1:10, 3:10, 5:10, 6:10, and 1:1, in sequence. For further analysis, a mixture of 2-propanol and as-prepared Ru:ZnS QDs was centrifuged at 6000 rpm for 15 min and the precipitations were washed by water and ethanol for several times to remove the unreacted species. The obtained powders were finally dried at 40 °C overnight. PL measurements were obtained by a Cary Eclipse fluorescence spectrophotometer (Agilent technology). The UV-visible spectra were also recorded through Cary 300 Bio UV–vis spectrophotometer (VARIAN) in the wavelength range of 250–800 nm. X-ray diffraction (XRD) pattern, transmission electron microscopy (TEM) image and energy-dispersive X-ray analysis (EDX) were performed using automated Philips X'Pert X-ray diffractometer with Cu Kα radiation (40 kV and 30 mA), Philips CM30 electron microscope at an operating voltage of 300 kV and Oxford INCA II energy solid state detector, respectively. Inductively coupled plasma atomic emission spectroscopy (ICPAES; Varian Vista-Pro) was employed to determine the elemental content of the Ru:ZnS QDs. All of the measurements were carried out at room temperature and under ambient conditions. 3. Results and discussion The crystallographic property of the prepared Ru-doped ZnS

QDs and the possible effect of Ru ions in their solid state structure were characterized by XRD measurements. The standard XRD pattern for ZnS (Joint Committee for Powder Diffraction Standards, JCPDS card No. 05–0566) has been given at the bottom of Fig. 1a. The three broad peaks observed in the diffractogram at around 28.66°, 47.74°, and 56.75° can be assigned to the planes (111), (220), and (311), respectively, of the cubic zinc blend structure [26]. Interestingly, incorporation of Ru3 þ ions into ZnS host QDs, do not make any substantial change in the number of peaks or creation of extra Ru2S3-related peaks. It means that the Ru3 þ ions are just incorporated into ZnS matrix as dopant. Low intensity and broadness of all peaks can be related to nano-scale size of structure. The size of specific nanocrystal was estimated using wellknown Debye–Scherrer formula as follows [27]:

D=

0.94λ β cos θhkl

(1)

where λ is a wavelength of the X-ray radiation used (1.5406 Å), θhkl is the Bragg angle correspond to the diffraction maximum, and β is full width at half maximum (FWHM) of the XRD peak appearing at the diffraction angle θhkl. The calculation showed an average diameter size of 2.7 nm. TEM image and size distribution of the specific Ru:ZnS QDs (Fig. 1b and c) demonstrate that the specimen is a combination of well-dispersed dots with relatively narrow size uniformity of 37 0.8 nm. The shape of all QDs is quasi-sphere and their average diameter is slightly higher than the XRD result. This observation is because of the disregarding the strain and instrument-related broadening of the XRD peaks [6] in Eq. 1. Chemical composition of the Ru:ZnS QDs was analyzed by EDX and ICP-AES measurements. Fig. 2 shows the EDX spectrum of the Ru:ZnS QDs prepared at the Zn:ME:Ru:S molar ratio of 1:50:0.01:0.5, pH of 11 and at 100 °C. As can be seen in Fig. 2, the very strong peaks for Zn and S are observed in the spectrum. A detectable small amount of Ru, also indicates that the Ru impurity was incorporated into the lattice of ZnS host matrix. As the percentage of ruthenium in some of QDs was less than the detection limit of EDX, however, the Ru:Zn atomic ratios in Ru:ZnS QDs were determined by ICP-AES measurements. The Ru:Zn atomic ratios were 0.35:100, 0.85:100, 1.8:100, 2.7:100, and 4.8:100 for the purified QDs prepared at different Ru:Zn molar ratios of 0.5:100, 1:100, 2:100, 3:100, and 5:100, respectively. The absorption and emission spectra of the undoped ZnS and Ru-doped ZnS QDs are shown in Fig. 3. The absorption spectra of the both samples are approximately the same which demonstrate that the absorption process is occurred through host structure of the semiconductor and it would be independent on the dopant ions. As can be seen in Fig. 3, the absorption spectra of the undoped ZnS and Ru-doped:ZnS QDs prepared at pH ¼ 11 show a small blue shift, regarding that of bulk ZnS (325 nm against 337 nm). Due to the very low excitonic Bohr radius of ZnS structure (2.5 nm) [28,29], the quantum confinement related-phenomena are hardly observed at the present QD size. To demonstrate that small blue shift in band edge position has been observed in all conditions, we will also present the absorption spectrum (correspond to best emission result) together with the PL spectra in all investigated conditions. The emission spectrum of the undoped ZnS QDs (Fig. 3) shows two peaks at around 350 nm and 420 nm. The first one with small Stokes shift of about 25 nm is attributed to excitonic emission due to recombination of electron in excitonic states with holes in valence band. The second one can be related to trap states [30]. As discussed by Denzler et al. [31], the bulk defects such as vacancies (Schottky defects) and interstitials (Frenkel defects) were the main source of trap states in the aqueous-based ZnS QDs. With cubic

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Fig. 1. (a) XRD pattern of the Ru:ZnS QDs prepared at the Zn:ME:Ru:S molar ratio of 1:50:0.01:0.5, pH of 11 and at 100 °C. The vertical bars indicate the position of the peaks in cubic ZnS bulk structure. (b) TEM image of the same specimen and (c) Its size distribution.

Fig. 2. EDX spectrum of the Ru-doped ZnS QDs prepared at the Zn:ME:Ru:S molar ratio of 1:50:0.01:0.5, pH of 11 and at 100 °C.

zinc blende structure, ZnS usually has Schottky defects predominant over Frenkel defects [32]. Therefore, the observed photoluminescence of the aqueous-based ZnS QDs can be attributed to a recombination of electrons at the sulfur vacancy donor level or in the conduction band with holes trapped at the zinc vacancy acceptor level or in the valence band [33]. There are many possible recombination paths and thereby many trap-state emissions with different energies, causing the relatively wide emission peak. However, the PL spectrum of the Ru-doped ZnS QDs contains three

different peaks at around 350, 420 and 450 nm. As can be seen, the excitonic emission of the undoped ZnS QDs experiences a complete disappearance while the dopant emission is appeared. Indeed, the last peak at around 450 nm can be attributed to the Ru3 þ dopant ions. As a transition metal ion, Ru3 þ ions can make their individual states within the forbidden band gap of ZnS and the carriers can recombine thorough these levels. Scheme 1 displays the suggested different pass ways for recombination of excited electrons and holes in the present Ru:ZnS QDs.

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Fig. 3. Absorption and emission spectra of the undoped ZnS and Ru-doped ZnS QDs.

Fig. 4. pH-dependent variation of PL spectra for the Ru-doped ZnS QDs. The absorption spectrum of the QDs prepared at pH ¼ 11 is also shown.

Fig. 5. Temperature-dependence variation of PL spectra of Ru-doped ZnS QDs. The absorption spectrum of the QDs prepared at 100 °C has been also shown. Scheme 1. Schematic illustration of the observed peaks in PL spectra. Each symbol has been referenced at the bottom of the scheme.

Because of the high surface to volume ratio in QDs, various parameters would have the direct effect on optical properties of them. To ensure this and to survey the effect of each one on emission spectra, influence of solution pH, reaction temperature and molar ratios of precursors is investigated where we fixed all of the other experimental variables when one is changed. The pH of reaction solution has significant effect on emission intensity of the as-prepared QDs (Fig. 4). As can be seen, increasing of the solution pH results in two alterations in PL spectra; First, small red-shift in peak positions which can be related to easier growth of QDs in strong alkaline solution. It is well known that due to the reduction of electrostatic repulsion forces between monomers and other species at higher pH values, the growth of the QDs is facilitated [34]. On the other hand, owing to the size dependency of the optical properties in nano-scale semiconductors, the growth of Ru:ZnS QDs at higher pH values leads to convergence of band edge positions toward each other [35]. The rational result of such a behavior is, decreasing the energy of the emitted photon or correspondingly increasing of their wavelength. Secondly, the obvious enhancement of the peaks’ intensity is observed in Fig. 4, which can be related to easier deprotonation of thiol group in ME molecules. As is seen,

increasing of solution pH to 11, results in a 10-fold enhancement compared with Ru-related peak intensity at pH¼8 which can be illustrated as follows; The higher pH values leads to more ejection of thiolate (the pKa value for thiol head is about 9.72 [36]) and easier formation of Zn-thiolate complexes can lead to better passivation of the surface of the Ru:ZnS QDs. This happening may lead to better internally doping structure, and subsequent enhancement of dopant-related peak intensity. Other researchers believe that the alkaline solution can effect on Zn2 þ and Ru3 þ ions to form ZnS or Ru2S3 as the mutually diffused cores [37]. Indeed, the presence of ̄ higher amount of OH ions has dual effect; better formation of smallsized Ru2S3 cores and presenting the ZnS species on the surface of the mutually diffused core which both are necessary for enhancement of emission properties. In addition, the meta-stability of inorganic core-ligand complex in aqueous solution can be solved at high enough alkaline condition. This factor can discourage the ligand protonation and so do the aggregation of QDs which is undesirable for biological applications. The thermodynamic of the growth process in colloidal routs is mainly controlled by the temperature of reaction solution. It has been proved that this factor has direct influence on doping phenomena [38]. As is seen in Fig. 5, increasing of temperature from 50 °C to 100 °C, leads to enhancement of the dopant-related

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Table 1 Band gap energy and size values of the Ru:ZnS QDs prepared at different Zn:ME molar ratios. Zn:ME molar ratio

Band gap energy (eV)

QDs size (nm)

1:25 1:30 1:40 1:50 1:62.5 1:75

3.85 3.88 3.94 4.10 4.08 4.04

4.05 3.92 3.69 3.24 3.29 3.39

absorption spectra and these two equations, one can optically estimate the size of the nanoparticles. Here, regarding the physical constants for the ZnS zinc blend structure [36] and the obtained band gap energy (Eg) for the Ru-doped ZnS QDs, the diameter (D) of the spherical nanocrystals prepared at different Zn:ME molar ratios has been calculated by following equation [44]: Fig. 6. Absorption and emission spectra of Ru:ZnS QDs prepared at different Zn:ME molar ratios.

emission peak. This may has two main reasons; higher temperatures are extremely suitable for better migration of dopant ions to the host matrix [39] and the higher possibility of internal doping can result in higher emission intensity. In addition, increasing of temperature may lead to faster deprotonation of sulfhydryl group (R–SH) and subsequent better surface passivation over the surface of the Ru-doped ZnS QDs, which can also justify the enhanced dopant ions’ emission. The peaks position also experience a small red shift with refluxing temperature which can be attributed to faster growth rate of the QDs and therefore formation of larger QDs. The molar ratio of precursors is another factor which has been proved to have key role in the emission properties of QDs. As is clear in Fig. 6, the ratio of the Zn-to-ME is observed to have an effective role not only on the nucleation and growth process of the QDs, but also on their colloidal stabilities and PL intensity [40]. As an important factor, the variation of the absorption spectrum is also displayed and surveyed, here. As shown in Fig. 6, with decreasing the Zn:ME molar ratio from 1:25 to 1:50, the absorption edge shifted toward shorter wavelength and subsequently the growth of QDs is reduced. The ME (HO–CH2CH2–SH) ligand has two terminal groups of sulfhydryl (–SH) and hydroxyl (–OH). During the refluxing processes in alkaline solution, the –SH groups can be deprotonated and due to the higher reactivity of thiolate ions toward Zn2 þ ions till hydroxyl groups, Zn-thiolate complexes are formed at the surface of the growing QDs, while the –OH groups have a very key role in chemical stability of the Ru:ZnS QDs and their correlation with solution via the formation of hydrogen bonding with water molecules. The hydration of the Ru:ZnS QDs through hydroxyl groups can therefore effectively hinder the growth of the QDs. In addition to this factor, the steric prohibition is enhanced with increasing of the ligand concentration results in formation of smaller QDs. However, with a further decreasing of the Zn:ME molar ratio from 1:50 to 1:75, the absorption edge position slightly shifted toward longer wavelengths and simultaneously the intensity of absorption spectra is increased. It should be noted that in Zn:ME molar ratio of 1:75, larger amount of NaOH was required to justify the solution pH at 11. The presence of notable amount of NaOH in the Zn:ME molar ratio of 1:75 probably leads to decreasing of repulsive forces between monomers and QDs surface, resulting in easier growth of the Ru:ZnS QDs [41]. To prove our assertion for variation of QDs size at different Znto-ME molar ratios, we do use two well-known formulas called as Tauc and Brus equations [42,43]. This so-called quantum size-related effect allows one to tune the emission and excitation wavelengths of a nanocrystal by tuning the crystal diameter. Using

D=

0.32 − 2.9 Eg − 3.49 (3.5 − Eg )

(2)

The obtained diameters (D) for the QDs are listed in Table 1. The optical estimations on size of the QDs are in good agreement with XRD and TEM results. It should be noted that, as the Zn:ME molar ratio decreases from 1:25 to 1:50, the size of the Ru:ZnS QDs firstly decreases and then increases to 3.39 nm at 1:75 M ratio. The emission characteristic of the present Ru:ZnS QDs is also impressed by amount of the capping agent. Decreasing of the Zn:ME molar ratio to 1:50 makes more effective surface passivation and favors the formation of ME-rich QDs surface which can provide better condition for internally doped structure and therefore the Ru-related emission intensity goes up. Nevertheless, as shown in Fig. 6, the PL emission intensity of Ru:ZnS QDs prepared at the Zn: ME molar ratio of 1:75 is slightly decreased. A possible reason is that a large amount of ME ligands decreases the reactivity of Ru3 þ ions through formation of Ru3 þ –ME complexes [45]. As a result, the Ru3 þ ions can not effectively incorporate into the ZnS host matrix and hence the intensity of PL emission is reduced. Furthermore, at very high amounts of capping agent, the formation possibility of ZnS or Ru2S3 cores may be decreased and subsequently the intensity of PL emission is quenched [46]. The Ru3 þ ions concentration is another factor which is necessary to be optimized. Just as Fig. 7 shows, the PL intensity changed with Ru-to-Zn molar ratio and reached a maximum at 1:100 M

Fig. 7. PL spectra of Ru:ZnS QDs prepared at different Ru:Zn molar ratios. The absorption spectrum has been also shown for 1:100 M ratio of Ru doping.

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Fig. 8. PL spectra of Ru:ZnS QDs prepared at different S:Zn molar ratios. The absorption spectrum has been also shown for 1:2 M ratio of S-to-Zn precursors.

ratio. Increasing of the dopant content from 0.5:100 to 1:100 (relative to Zn2 þ ) would rationally increase the number of dopantrelated states in the forbidden band gap and subsequent enhancement of the recombination process through these states. This is maybe a reason for why we see the initial enhancement in PL spectrum (Fig. 7). Nonetheless, addition of Ru3 þ dopant ions, more than the 1:100 M ratio, gives rise to appearance of concentration quenching effect due to non-radiative transitions between the neighboring dopant ions. Many researchers report this effect in transition metal-doped II-VI QDs [47–49]. The possible reason is that, all the Ru3 þ ions can’t enter in the host matrix in a replacement manner at a higher concentration of Ru, but may probably locates at the surface or interstitial positions and introduce non-radiative centers for carriers’ recombination. Fig. 8 shows the influence of different molar ratios of S:Zn on PL intensity of the Ru:ZnS QDs. It can be seen that, in a low precursor ratio of 1:100 for S:Zn, no fluorescence emission is detected. While, as the S:Zn molar ratio is increased from 1:100 to 5:10, the

PL intensity of the Ru:ZnS QDs is significantly enhanced and then gradually decreased by further increasing of the sulfur precursor to 1:1 M ratio. The reason of such non-uniform experimental behavior can be explained as follows; there is a competition between Zn2 þ and Ru3 þ ions to react with anionic sites but Zn2 þ ions can react more easily with S2- ions rather than Ru3 þ ones. At very low concentration of S2- ions (even at high concentration of Zn2 þ and Ru3 þ ions), there aren’t enough anionic sites to simultaneously form stable ZnS or Ru2S3 nanocrystals. Hence, the PL emission can’t be observed. With an increase in the sodium sulfide amount, actually, the possibility of formation of mentioned structures goes up. Therefore, ZnS core structure can be initially formed and the Ru3 þ ions can be then incorporated as dopant, results in higher dopant emission intensity. As is seen from Fig. 8, further increasing of Na2S content [50] gives rise to reduction of PL intensity which can be attributed to increase of defect sites. These defect states may act as non-radiative centers for carriers, resulting in quenching of emission intensity. The PL stability of the samples was analyzed from chemio- and photo- stabilities point of views. The chemical stability was investigated using H2O2 as oxidizing agent. The process was carried out according to previous report [37]. As has been shown in Fig. 9, after more than 30 min of etching time by H2O2, the emission intensity of the Ru-doped ZnS QDs was found to be 40% of the initial emission intensity. It can demonstrate that the emission centers (especially the dopant states) in the present specimens are probably located at the inner host structure of the QDs rather than their surface. Additionally, in order to investigate the photo-stability of the QDs, they were continuously illuminated by a 100 W xenon lamp with radiation wavelength of 365 nm. As can be observed in Fig. 10, the UV illumination doesn’t create a meaningful variation in PL spectrum of the Ru-doped QDs even after 5 hour of irradiation. However, the PL intensity enhances slightly with illumination time and reaches a maximum for 95 min of irradiation. When the illumination time is further increased, the PL intensity decreases. The initial small increase in PL emission would be probably due to passivation of non-radiative surface states at the surface of the Ru-doped ZnS QDs by slight formation of ZnO or Zn (OH)2 species [51]. Increasing of the surface defect sates may be an

Fig. 9. The chemical stability of the Ru:ZnS QDs at the presence of the H2O2. Time-dependent variation of a) PL emission spectra and b) intensity of the Ru-related peak.

R. Sahraei et al. / Journal of Luminescence 187 (2017) 421–427 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] Fig. 10. The photo-stability of the Ru:ZnS QDs under UV irradiation for different time intervals. Inset shows the variation of the Ru-related peak's intensity at different times of UV irradiation.

ensuing happening which results in small reduction of the PL emission intensity.

[19] [20] [21] [22] [23] [24]

4. Conclusions Monodisperse, fluorescent, and water-soluble Ru-doped ZnS QDs were prepared by growth-doping route in an aqueous solution using 2-mercaptoethanol as a capping agent. The XRD and TEM results indicated that the Ru:ZnS QDs are in nano-scale size with zinc blend cubic structure. The chemical composition analysis, showed the presence of small amount of Ru as dopant in the host matrix of ZnS. As a very important factor on growth and chemical stability of the QDs, the absorption spectra of the Ru:ZnS QDs prepared at different Zn:ME molar ratios were studied and the size variation of the QDs was estimated using Brus equation. The results of the emission spectra showed that, in spite of the very weak excitonic and trap emissions, the Ru-related emission at about 450 nm is appeared. To gain further insight toward the emission properties of the prepared QDs, the effect of the several experimental factors was surveyed and the optimum conditions was found to be; the solution pH of 11, the refluxing temperature of 100 °C, and the Zn:ME:Ru:S molar ratio of 1:50:0.01:0.5. The remarkable resistance of the prepared QDs against chemical and optical corrosions was also concluded using H2O2 etching and UVradiation, respectively. Water-solubility, durability of the fluorescence intensity, and permanence of the sub-10 nm QDs size are all the essential features that suggest the present aqueous-based and Cd-free Ru:ZnS QDs for potential biological applications.

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