Nanotechnology
ACCEPTED MANUSCRIPT
Facile, one-pot and scalable synthesis of highly emissive aqueousbased Ag,Ni:ZnCdS/ZnS core/shell quantum dots with high chemical and optical stability To cite this article before publication: Reza Sahraei et al 2017 Nanotechnology in press https://doi.org/10.1088/1361-6528/aa92b2
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Facile, one-pot and scalable synthesis of highly emissive aqueousbased Ag,Ni:ZnCdS/ZnS core/shell quantum dots with high
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chemical and optical stability
Reza Sahraeia*, Ehsan Soheylia,b, Zahra Farajia, Mohammad Soleiman-Beigia a
Department of Chemistry, Faculty of Science, University of Ilam, 65315-516, Ilam, Iran b
Department of Physics, Faculty of Science, University of Ilam, 65315-516, Ilam, Iran *Corresponding author. Tel/ fax: +98 841 222 7022
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E-mail address: [email protected] & [email protected]
Abstract
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We report here a one pot, mild and low cost aqueous-based synthetic route for preparation of colloidally stable and highly luminescent dual-doped Ag,Ni:ZnCdS/ZnS core/shell quantum dots (QDs). The pure dopant emission of the Ni-doped core/shell quantum dots was found to
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be highly effected at the presence of second dopant ion (Ag+). Results showed that the PL emission intensity increases while its peak position experiences an obvious blue shift with increasing the content of Ag+ ions. Regarding the optical observations, we simply provide a scheme for absorption-recombination processes of the carriers through impurity centers. To
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obtain an optimum conditions with better emission characteristic, we also study the effect of different reaction parameters such as: refluxing temperature, core and shell solutions pH, molar ratio of the dopant ions (Ni:(Zn+Cd) and Ag:(Zn+Cd)), and concentration of the core and shell precursors. Nonetheless, the most effective parameter is the presence of the ZnS
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shell with suitable amount to eliminate the surface trap states and enhance their emission intensity. It can also, improve the bio-compatibility of the prepared QDs by restricting the Cd2+ toxic ions inside the core of the QDs. The present suggested route was also yielded to
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remarkable optical and chemical stability of the colloidal QDs which introduce them as a decent kind of nano-scale structures for light emitting applications, especially in the 1
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biological technologies. The suggested process has also this interesting potential to be scaledup while remaining the emission characteristics and structural quality which is inevitable for
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industrial applications in optoelectronic devices.
Keywords: Aqueous-based synthesis; ZnCdS quantum dots; dual doping; optical properties; chemical stability; up-scaling
Introduction
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1.
The advent of quantum mechanics at the beginning of the 20th century and finding the
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possible ways through chemical preparation of the extremely low size/dimension structures at the recent three decades has opened a very interesting window to achieve more applicable materials with peculiar behaviors. One of these wonderful structures, is quantum dots (QDs);
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a zero dimensional systems with semiconducting electronic nature [1]. The fabulous sizedependent optical properties can be observed in such materials as their size reaches to their corresponding excitonic Bohr radius [2] which leads to remarkable applications of colloidal QDs in optoelectronic devices, biological systems, photo-catalysis, chemical probes and etc
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[3-9]. The diverse nanostructures of binary, ternary, alloy and core/shell QDs have been synthesized using different elements, ligands and in various media [10-13]. Nonetheless, creation of desired characteristics, improvement of the intrinsic properties (such as longer excited state lifetimes, and enhanced thermal and chemical stability) and specifically
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eradication of the re-absorption in emitting-required applications have been led to tendency of the researchers to doped-QDs (d-dots) [14, 15]. Pioneer researchers have been concentrated on the preparation of high-quality d-dots
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in organic solvents [16, 17]. But, direct synthesis of the d-dots in aqueous media using low
temperatures and non-toxic precursors can be a good alternative to overcome the difficulties 2
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of the organometallic methods [18]. As presented by Peng’s group [19], internally doped
be employed in direct aqueous-based methods.
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nanostructures can be achieved by separation of doping and growth processes which can also
As suggested by Zhang and co-workers as well as Jana and co-workers, ZnCdS alloys
with zinc blend structure can be a suitable host structure for doping different types of
transition metal ions [20, 21] ]. Nickel is a transition metal element which has been rarely employed as dopant element in direct preparation of high quality d-dots in aqueous media. A
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simultaneous enhancement in optical, electrical and magnetic properties of the core structure
(by insertion of Ni2+ ions) is an indication over multifunctional utilization of such d-dots
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which has to be studied as they deserve [22]. However, the presence of the second dopant ion may have different effects on emission properties of the d-dots. As showed by Cui’s group, introduction of Ag+ dopant ion leads to quenching of dopant-related emission in Cu:ZnSe
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QDs while it improves that of Mn:ZnSe QDs [23]. Pradhan’s group also studied the variation of the emission color for Cu, Mn-doped CdZnS NCs, as the band edge positions of the host matrix was changing [24]. They realized from their hot injection-based method that the allowed transition would be occurred in selective way which involves just one dopant-related
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state. In a systematic investigation, Huang et. Al [25], also reported the nontoxic Ag- and Mn-doped ZnInS/ZnS dual-emission QDs with simultaneous tunable emission wavelengths. However, they also use a high temperature route of about 230 °C in organic solvents. They reported that the ratio-metric of Ag and Mn dual emissions can be tuned by controlling Ag–
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Mn and Mn–Mn dopant coupling.
In order to achieve high quantum yield emission, passivation of the surface states is
necessary which can be obtained by over coating of the core nuclease with an inorganic shell
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of the wider band gap semiconductor. The Schneider’s group prepared high quality ZnSe:Mn/ZnO core/shell QDs by aqueous route [26]. The results were very interesting but, 3
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they had to use more than 16 h refluxing temperature to obtain ZnSe:Mn core QDs. Recently, Levchuk et. Al [27], developed a one-pot, two-step synthetic route for highly luminescent
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Mn-doped ZnxCd1-xS/ZnS core/shell NCs in organic medium and at 230 °C. They also scaled up their industrially compatible method and used the obtained NCs for fabrication of downshifting layers in monocrystalline silicon-based solar cells.
Herein we present, for a first time, a growth doping strategy for cost-effective and
one-pot synthesis of water soluble dual-doped Ag,Ni:ZnCdS/ZnS core/shell QDs using N-
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acetyl-L-cysteine as a green stabilizer. Since the dopant emission is not an accidental phenomenon and strongly depends on the kind of the host matrix, its quality and dopant
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itself, beside of the successful preparation of QDs, our attention will be paid to possible correlation effect between the two different dopant ions and finding the best conditions to achieve better emission characters. Regarding the necessity of the resistance of the specimens
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against optical and chemical corrosions, their stabilities is examined and in order to prove the feasibility of the present method for industrial usage, the large-scale preparation of the high quality core/shell QDs is also introduced. Due to the fastness and simplicity of the present method, very cheapness and biocompatibility of the precursors, specially capping agent and
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high emission intensity [28], the present core/shell QDs have this potential to be used in biolabeling, solar cells and light emitting devices.
2.
Experimental
2.1.
Chemical
Merck’s
chemicals
of
zinc
sulfate
heptahydrate
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Materials.
(ZnSO4.7H2O, 99-103%), cadmium acetate dehydrate (Cd(OAc)2.2H2O, +98%), N-acetyl-Lcysteine (NAC, 99%), silver sulfate (Ag2SO4, +99.5%), nickel acetate tetrahydrate (Ni(OAc)2.4H2O, 99%), Thiourea (TU, +99%), H2O2 (30%), NaOH (+99%), 2-propanol
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(+99.8%) and ethanol (+99.9%) were used in the present work. Extra pure sodium sulfide
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hydrate (Na2S.H2O, 60-63%) was purchased from Acros Organics. Deionized water was also utilized, as solvent, in all experiments. All of the precursor materials were used without any
2.2.
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purification.
Synthesis of Ag,Ni:ZnCdS/ZnS core/shell QDs. Water soluble ZnCdS QDs with
small amount of two dopant elements and additional inorganic shell of ZnS have been prepared using growth doping strategy. Namely, for preparation of Ag,Ni:ZnCdS core QDs,
equal amounts of Cd and Zn precursors were added to the mixture of NAC and 87 mL
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deionized water. Then, desired molar ratio of Ag+ and Ni2+ precursors were added to solution and it was adjusted to specific pH by using 1 molL-1 (M) NaOH. After a while, the solution
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was transferred to three-necked flask and appropriate amount of a 0.1 M Na2S stock solution was injected to solution as fast as possible we can, under vigorous stirring. The total volume of the solution was close to 100 mL. Next, the flask was heated up to desired temperature and
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refluxed for 90 min. The reagents molar ratio of Zn:Cd:NAC:S was taken as 1:1:5:3 for different amounts of dopant ions. Actually, the amount of Ni2+ ions was firstly justified in Nidoped ZnCdS/ZnS core/shell QDs and then at the optimized amount of Ni2+, the second dopant ion (Ag+) was added at different amounts. In order to investigate the effect of core
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precursors concentration, the amounts of (Zn2+ + Cd2+) ions has been adjusted at 0.06, 0.12 and 0.24 mmol, where the amounts of other precursors varied based on their molar ratios. Over coating of the core QDs with ZnS shell was also carried out as follows; firstly, Zn2+ and TU precursors were respectively added to a 20 mL of NAC-containing deionized water at
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molar ratios of 1:1.5:1 (Zn:NAC:TU) and the solution was brought to specific pH values by 1 M NaOH. Then, desired amounts of the ZnS shell precursor solution (0.07, 0.14, 0.28 and 0.42 mmol) was injected in two steps into the stirring Ag,Ni:ZnCdS core QDs at its refluxing
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temperature. Following extra 90 min, for efficient shelling, the heater and stirrer were turned off and flask was cooled down to room temperature. All of the optical measurements were 5
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carried out immediately after this step. To obtain a suitable powder of the specimens, the asprepared QDs were precipitated by initial combination with acetone (1:1 volume ratio) and
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ensuing centrifugation at 4500 rpm for 10 min. After that, the precipitates were washed by mixture of ethanol and deionized water for several times and then dried overnight at 40 °C.
As one can see in Scheme S1 (See Supporting Information), the suggested strategy is really facile and one-pot/two steps method which all of its procedures are carried out at one three-
necked flask. The measurement details and instruments used have been mentioned in the
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Supporting Information. Results and Discussion
3.1.
Structural Characterizations. The XRD patterns of the Ag,Ni:ZnCdS QDs before
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3.
and after covering by ZnS shell are shown in Figure 1 which are broadened because of the nano-scale size of the structures. As can be seen, the peaks position of the core QDs has been
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located between those for ZnS and CdS ones with cubic zinc blend phase. It means that the prepared specimens have been crystallized in alloy form. However, by over coating of the core with ZnS shell, the peaks positions are shifted toward larger angles correspond to ZnS cubic structure. Interestingly, there are no any traces of the Ag2S and NiS-related peaks
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which shows that the small amount of silver and nickel ions are just introduced as dopant ions. The slight increasing of the first peak intensity after shell overcoating also can be
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attributed to increasing of the average size of the QDs.
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Figure 1: XRD patterns of the dual doped core and core/shell QDs.
Figure 2 shows TEM images of the prepared QDs. Both core and core/shell structures are nearly mono-disperse QDs with average sizes of 2.9±0.2 and 4.9±0.4 nm, respectively.
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Since, the excitonic Bohr radius of the ZnCdS structure with similar amount of Cd+2 and Zn+2 ions is about 2.6 nm [29], the strong quantum confinement effect has to be presented at the present dual-doped QDs. Also, based on the dynamic light scattering measurements, the size and size distribution of the QDs was investigated (See Figure S1). The average size of the
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core and core/shell QDs were around 3.1±0.1 and 4.8±0.3 nm, respectively.
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Figure 2: TEM images of, (a) Ag,Ni:ZnCdS QDs and (b) Ag,Ni:ZnCdS/ZnS core/shell QDs. 7
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Figure S2 shows the FT-IR spectra of pure NAC and NAC-capped Ag,Ni:ZnCdS core QDs before and after covering by ZnS shell. As is seen, the infrared spectra of all samples are
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quite similar to that of NAC, except in one case. The characteristic IR absorption bond of thiol group (2570 cm-1) in pure NAC powder has been completely disappeared in both types of prepared core and core/shell QDs. This observation shows that the deprotonated thiol
terminals of the NAC molecules form the metal-thiol complexes with unreacted cations at the surface of the QDs and effectively passivate them.
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The elemental analysis for dual-doped ZnCdS QDs before and after covering by ZnS shell has been showed in Figure S3. The presence of the Zn, Cd and S elements are clear. It
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can be also observed that the amount of Zn increases after shell injection. There are no any traces regarding Ag or Ni species. It means that the used-amount of these ions are as low as which cannot be detected by EDX analysis. To ensure the dopants collaboration into lattice
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structure of the QDs, ICP-AES measurements were carried out on Ag,Ni:ZnCdS QDs at different Ag+ concentrations. Based on these measurements, the nominal Ni:(Cd+Zn) and Ag:(Cd+Zn) molar ratios used in the synthesis and the real Ni:(Cd+Zn) and Ag:(Cd+Zn) molar ratios incorporated into the lattice structure of the QDs were determined and given in
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Table 1.
Table 1: Nominal Ni:(Cd+Zn) and Ag:(Cd+Zn) molar ratios used in the synthesis and the real Ni:(Cd+Zn) and Ag:(Cd+Zn) molar ratios in Ag,Ni:ZnCdS core QDs samples determined by ICP-AES measurements.
nominal Ni:(Cd+Zn) molar ratio
ICP results of Ag:(Cd+Zn) molar ratio
ICP results of Ni:(Cd+Zn) molar ratio
0.16:100
0.66:100
0.15:100
0.42:100
0.33:100
0.66:100
0.30:100
0.43:100
0.50:100
0.66:100
0.48:100
0.45:100
0.66:100
0.66:100
0.65:100
0.44:100
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nominal Ag:(Cd+Zn) molar ratio
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It can be found that the real Ag:(Cd+Zn) molar ratios in actual samples are quite similar to the nominal values, while the real Ni:(Cd+Zn) molar ratio is lower than the
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nominal values. This can be attributed to enormous difference in the solubility products of Ag2S (Ksp=3×10-50) and NiS (Ksp=1×10-21). It also explains the difficulty of the coincorporation of Ag+ and Ni2+ ions into the host matrix. 3.2.
Optical properties. As described in Scheme S1, the Ag,Ni:ZnCdS/ZnS core/shell
QDs have been prepared in two steps. Firstly, the NAC-capped ZnCdS cores QDs with
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suitable amount of Ag and Ni ions have been synthesized and then, they were over-coated
with additional ZnS shell. The presence of the strong quantum confinement effect in as-
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prepared QDs can be concluded based on a considerable blue-shift of the absorption edge, compared to band edge position of the corresponding bulk structure (from 540 to 405 nm). However, there are several factors which can be highly effective on emission characters of
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the prepared QDs. Refluxing temperature, the solution pH of core and shell precursors, concentration of core and shell precursors, effect of dopant ions and their concentrations on optical properties of QDs are all of the factors which we are going to report and discuss about their influence.
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3.2.1. Influence of the dopant ions. UV-Vis and normalized PL spectra of undoped, Nidoped, Ag-doped and Ag,Ni-dual-doped ZnCdS/ZnS core/shell QDs are shown in Figure 3(a). The ZnCdS/ZnS QDs were prepared at core and shell precursors concentrations of 0.12 (amount of (Zn2++Cd2+) ions) and 0.28 (amount of Zn2+ ions) mmol, respectively. The molar
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ratios of 1:1:5:3, for Cd:Zn:NAC:Na2S in the core reaction mixture and 1:1.5:1 for Zn:NAC:TU in shell precursor were used. The pH of core solution was 11.4 and that of injected ZnS shell was about 11.3. The refluxing time and temperature of both core and shell
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formation were also set at 90 min and 110 °C, respectively. As it has been shown in Figure 3(a), the undoped ZnCdS/ZnS QDs have an absorption edge around 405 nm and with 9
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relatively large stokes shift, a prominent PL emission peak at about 562 nm. This remarkable energy difference between two spectra indicates that the emission should not be happened
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through excitonic states of the ZnCdS. Therefore, this broad peak can be attributed to trap
states emission (a very weak peak at 420 nm is also observed due to excitonic recombination). Despites the core over-coating with higher band gap ZnS shell, the intense trap-related peak is still observed that has been previously attributed to intrinsic trap sites of
such structures for hole carriers [30]. However, by introduction of small amount of Ni2+
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dopant ions (Ni/(Zn+Cd) molar ratio of 0.66%), the emission peak depicts a red-shift to 576 nm while its absorption edge doesn’t change, remarkably. This red-shift is probably because Ni2+-related levels in band gap region are located at higher energies rather than trap states
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which results in emission at longer wavelengths. As Figure 3 shows, when we used Ag+ ions (Ag/(Zn+Cd) molar ratio of 0.5%) instead of Ni2+ ions, in spite of the unaltered absorption
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spectrum, the dopant-related emission peak experiences a clear blue shift with higher intensity relative to both of undoped and Ni-doped QDs. However, co-existence of the both dopant ions has interesting results. It causes an meaningful red-shift in emission spectrum regarding that of Ag+-doped QDs which has to be due to the lower energy of Ag+ states relative to Ni2+ ones, at forbidden band gap region. This kind of behavior has been reported
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by Jana et. al [21], in high temperature prepared dual-doped ZnCdS NCs. These observations in absorption and emission spectra can be illustrated as follows, respectively; the band edges position of the present Ni-doped QDs are highly dependent on size and composition of the
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samples and in the case of fixed reaction conditions (solution pH, refluxing time and temperature, precursor’s molar ratio and concentration), band gap of the specimens remains unchanged even when different kinds of impurities are used, which is confirmed by approximately similar absorption spectra for all of as-prepared core/shell QDs in Figure 3(a).
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On the other hand, the emission peak position changes considerably, due to a chemical
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competition between Ag+ ions (as a soft acid) and Ni2+ ions (as a borderline acid) to react with S2- ions (as a soft base). This intrinsic characteristic, leads to significantly higher
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reactivity of the silver ions thorough S2- ions, rather than nickel ions (solubility product of
Ag2S; Ksp=3×10-50 compared to NiS; Ksp=1×10-21). Hence, as previously determined by ICPAES measurements (Table 1), in a fixed amount of S2- ions, the Ag2S specimens are formed more easily which yields to observation of the Ag+-related emission and due to the lower
energy of Ag+ states, the blue shift in emission spectrum is comes to pass. Based on optical
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observations, we suggest a scheme for possible absorption-recombination processes in the
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present Ag,Ni:ZnCdS/ZnS core/shell QDs (Scheme 1).
Figure 3: (a) UV-Vis absorption and normalized PL spectra of undoped, Ni-doped, Ag-doped and dualdoped (Ni and Ag) ZnCdS/ZnS core/shell QDs. Weak excitonic peak has been indicated by dot-line. (b)
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Evolution of intensity (circles) and position (squares) of the emission peaks in different samples.
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Scheme 1: Schematic diagram for the position of Ag+, trap and Ni2+ states and their possible
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recombination process in the Ag,Ni:ZnCdS/ZnS core/shell QDs.
In order to study the effect of the second dopant ions (Ag+) concentration on PL
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emission properties of Ag,Ni:ZnCdS/ZnS core/shell QDs, a series of samples with different Ag/(Cd+Zn) molar ratios of 0.00%, 0.16%, 0.33%, 0.50%, and 0.66% were prepared while keeping the other experimental variables fixed. As shown in Figure 4, with increasing of Ag/(Zn+Cd) molar ratio, the dopant emission intensity increases until it reaches to its maximum at Ag/(Zn+Cd) molar ratio of 0.50%, whilst the position of PL emission peaks of
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the Ag,Ni:ZnCdS/ZnS core/shell QDs is shifted to shorter wavelengths (from 585 nm to 545 nm). However, with further increasing in Ag/(Zn+Cd) molar ratio (0.66%), the PL intensity decreased, where quenching phenomenon is appeared. It can be attributed to energy transfer between Ag dopant states in a non-radiative manner. Compared to this PL behavior, the UV-
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Vis absorption spectra are almost identical (Figure 4(a)) indicating that the band edges position are independent on dopants amount added and so do the band gap energy (using well-known Tauc’s equation [31]) which was fixed at about 2.95 eV. The optimized
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Ag/(Zn+Cd) molar ratio, for fixed Ni/(Zn+Cd) molar ratio of 0.66% (see Figure S4), was found to be 0.50%. Finally, as a very important result, one can immediately find out that the 12
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present work suggest a simple synthetic method to prepare tunable emission without any kind of size and host composition alteration from 539 (in Ag-doped ZnCdS/ZnS QDs) to 588 nm
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(in Ni-doped ZnCdS/ZnS QDs). It can be achieved just by using different dopant ions in their
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highest concentrations.
Figure 4: Effect of Ag/(Zn+Cd) molar ratio on: (a) UV-Vis absorption and normalized PL spectra of dualdoped Ag,Ni:ZnCdS/ZnS core/shell QDs and (b) intensity (circles) and position (squares) of the emission peaks.
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3.2.2. Influence of refluxing temperature. Due to the very strong dependency of doping efficiency to preparation temperature, three different refluxing temperatures of 45, 75 and 110 °C were used to find the best one (while all other parameters were kept constant). As is shown in Figure 5(a), with increasing of refluxing temperature the absorption edge position
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shifts clearly to longer wavelengths, which means that the higher refluxing temperature makes a facile condition for further growth of the QDs and subsequent red shift in absorption spectra and reduction of the band gap energy. The calculated band gap energy of the dual-
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doped Ag,Ni:ZnCdS/ZnS core/shell QDs at different temperatures of 45, 75 and 110 °C are
3.3, 3.15 and 2.95 eV, respectively. 13
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Since, nature of emission in both Ni and Ag centers is related to band edges position (Scheme 1) [21], reduction of the band gap energy is associated with obvious red shift in
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dopant-related emission peak from 500 nm to 554 nm (Figure 5(b)), because the conduction
band moves down and dopant states probably remain unchanged. As is seen in Figure 5(c), intensity of the emission peaks is highly dependent on reaction temperature. There are two
observations in this regard. The intensity of the dopant emission at 45 °C is considerable which can be an indication over suitable efficiency of the present method even at low
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temperatures. Moreover, the higher emission intensity is obtained at higher temperatures. A possible interpretation can be that the doping process is a multiple process. Dopant ions
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should be initially adsorbed on the surface of the QDs for sufficient time, and then it would be incorporated into the host matrix followed by lattice diffusion [32]. Although, each step has its own critical temperature, however, the overall effect of the sufficiently higher
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temperatures is to improve the possibility of internal doping, easier diffusion of dopant ions into lattice structure and ensuing improvement of emission possibility through dopant-related
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radiation centers.
Figure 5: Effect of refluxing temperature on (a) UV-Vis absorption spectra, (b) normalized PL spectra of
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dual-doped Ag,Ni:ZnCdS/ZnS core/shell QDs, and (c) intensity (circles) and position (squares) of the emission peaks.
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3.2.3. Effect of core and shell solutions pH. Figure 6(a) shows UV-Vis and normalized PL
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spectra of dual-doped Ag,Ni:ZnCdS/ZnS core/shell QDs prepared at different pH values
(while other experimental conditions are kept same). It can be seen that the absorption band shifts from 397 to 416 nm by adjustment of the core’s solution pH from 8.6 to 12. This
demonstrates the faster growth of the QDs in strong alkaline matrices [33]. As analyzed by Yang’s group, the high pH values are associated with low surface potential [34], which
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results in faster growth rate of the QDs. The measured band gap energy is also decreased
from 3.05 to 2.83 eV, by increasing of the solution pH. The PL emission results of the Ag,Ni:ZnCdS/ZnS QDs (Figure 6(b) and (c)), showed that despite the clear red-shift which
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has to be observed due to reduction of the band gap, the intensity of dopant emission peak is also improved until pH=11.4. This increasing in intensity of emission peak at high pH values
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can be related to better deprotonation of thiol groups and their subsequent covalent bond with cations and passivation of dangling bonds at the surface of the QDs. However, with further increasing of precursor solution pH (up to 12), the PL intensity decreases meaningfully, due
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to aggregation effects and instability of the QDs at very high pH values [33].
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Figure 6: (a) UV-Vis absorption spectra, (b) normalized PL spectra, and (c) evolution of intensity (circles) and position (squares) of the emission peaks for Ag,Ni:ZnCdS/ZnS core/shell QDs prepared at different pH values. 15
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After refluxing of the Ag,Ni:ZnCdS core QDs for 90 min, desired amount of the freshly-prepared ZnS shell precursor solution was adjusted at three different pH values of 2.5,
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8.7, 11.3 (whereas other experimental variables were kept as fixed) and subsequently used for shell deposition. As has been shown in Figure 7, the high pH values for shell precursor lead to slight red shift of the both absorption and emission spectra. With increasing of shell’s solution pH, formation of the ZnS nanostructures is facilitated. Hence, injecting of ZnS shell
with high pH values may result in two happenings; providing the thicker shell of ZnS around
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core QDs and better surface passivation. The first one leads to small growth of QDs
appearing in small red-shift of the absorption/emission spectra and decreasing of the band
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gap energy from 3 eV to 2.9 eV. The second one, however, leads to more effective passivation of surface dangling bonds, surface trap states and separating the dopant ions from surface of the core QDs. It can eliminate the non-radiative centers and increase the PL
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intensity (Figure 7(b)).
Figure 7: (a) UV-Vis absorption and normalized PL spectra and (b) evolution of intensity (circles) and
position (squares) of the emission peaks for Ag,Ni:ZnCdS/ZnS core/shell QDs prepared at different pH
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values of injected shell precursor solution.
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3.2.4. Effect of core and shell precursors concentration. To analyze the effect of core concentration on optical properties of the dual-doped QDs, the amounts of (Zn2++Cd2+) ions
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has been adjusted at 0.06, 0.12 and 0.24 mmol, where the amounts of other precursors
changed according to their molar ratios. Based on absorption and emission data (Figure 8),
the overall red shift of the absorption edge is associated with growth of the QDs at higher concentrations and as mentioned before, it reduces the energy of the emitted color. However,
this faster growth may results in higher degree of defects and ensuing increasing of non-
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radiative recombination in QDs. Figure 8(b) shows that, at the concentration of 0.24 mmol,
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the emission intensity decreases about four times compared to which for 0.12 mmol.
Figure 8: (a) UV-Vis absorption and normalized PL spectra and (b) evolution of (circles) and position (squares) of the emission peaks for the Ag,Ni:ZnCdS/ZnS core/shell QDs prepared at different concentration of core precursor solution.
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To minimize the density of the surface defects, here, we use different amounts of ZnS
shell at pH=11.3. In the present work, we simply introduced different amounts of ZnS shell precursor (0.07, 0.14, 0.28 and 0.42 mmol) and let it refluxed for extra 90 min at the 110 °C.
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Figure 9(a) shows that the absorption edge of the QDs shifted to longer wavelength with addition of more ZnS shell precursor solution. This shift indicates the productive formation of 17
AUTHOR SUBMITTED MANUSCRIPT - NANO-115426.R1
the shell around the core QDs which is associated with growth of the core/shell QDs and reduction of their band gap energy (from 3.05 eV to 2.75 eV). Upon addition of the shell
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precursor up to concentration of 0.28 mmol, the intensity of the emission peak exerts an interesting enhancement as is seen in Figure 9(c). In this case, the additional ZnS shell with higher band gap energy can bury the dopant elements and make an extra force (potential
barrier) on them to be internally positioned. Indeed, it can effectively confine the carriers to locate inside of core structure. However, the ensuing decreasing of the emission intensity at
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higher concentration of shell precursor (0.42 mmol) can be attributed to formation of higher
density of surface defects at the interfacial of the core and shell structures as the thickness of
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shell over Mn-doped ZnSe NCs [35].
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the shell increases. Zeng et al., have reported exactly the same alteration by addition of ZnS
Figure 9: (a) UV-Vis absorption spectra, (b) PL spectra, and (c) evolution of intensity (circles) and
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position (squares) of the emission peaks for Ag,Ni:ZnCdS/ZnS core/shell QDs at different amounts of injected ZnS shell precursor solution.
After optimization the mentioned parameters, the quantum yields (QYs) of the doped-
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core and -core/shell QDs have been calculated as illustrated in the Supporting Information. It was found that the QY of the Ag,Ni;ZnCdS QDs increases from 11.6 to 30.2, by ZnS shell 18
Page 19 of 24
overcoating which implies their tremendous possibility to be used in optoelectronic and
3.3.
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biological applications. Chemical and optical stability of the QDs. According to the literature method [36],
we also investigated the tolerance of both dual-doped core and core/shell QDs against chemi-
oxidization by H2O2. As can be seen in Figure 10, by increasing of the etching time, the
impressionability of the QDs without shell is higher than that of with shell. Indeed, the PL intensity of the dual-doped core QDs decreased until 102 min while this time is about 50 min
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for core/shell QDs. The presence of the additional shell with smaller atomic radius elements,
leads to improvement of the chemical stability by formation of more compressed (packed)
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ligands at the surface of the QDs [37]. In spite of this, the overall effect of chemical corrosion on both of investigated specimens is small and after 60 min, the PL intensity reaches 53% and 33% of the initial intensity, for dual-doped core/shell and core QDs, respectively. The
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enormous chemical stability of the QDs demonstrates that the emission centers are related to dopant ions which probably located far from the surface of the QDs. This can be an additional indication for successful internal doping of the Ag+ and Ni2+ ions into the lattice
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structure of the ZnCdS/ZnS QDs.
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Figure 10: Temporal evolution of PL spectra of Ag,Ni:ZnCdS QDs (a) without shell, (b) with ZnS shell, and (c) comparison of their chemical stability for serial times. 19
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The storage stability test on both of the core only doped QDs and core/shell QDs was also done at different time intervals. The as-prepared QDs have been subjected to visible
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light, at room temperature for 28 days. As can be seen in Figure 11, both of the core and core/shell QDs show high resistance versus photo-bleaching at room temperature. From
Figure 11(a) and (b), it is clear that the PL emission intensity of the Ag,Ni-doped ZnCdS QDs has been completely quenched after 28 days. While the PL intensity of the core/shell QDs
remains at about 40% of the initial value at this time period (Figure 11(c)). This remarkable
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feature would demonstrate the success of the doped QDs preparation method in both of the
synthetic challenges; internal diffusion of the dopant ions into the host matrix and providing
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an efficient additional shell of ZnS with higher band gap energy where both of them prohibit the dopant emission centers from the surface of the QDs. As can be concluded the colloidal and optical properties of the as-prepared QDs are high enough to be used in optoelectronic
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devices and biological systems.
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Figure 11: Temporal evolution of PL spectra of Ag,Ni:ZnCdS QDs (a) without shell and (b) with ZnS shell, and (c) comparison of their optical stability for different time periods under day light at room
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Page 20 of 24
temperature.
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3.4.
Upscaling. Due to undeniable importance of the high quality QDs from both
scientific and industrial perspectives, here we show the capability of the suggested method to
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be used for industrial applications in light emitting devices [38]. The mentioned preparation
method of ZnCdS/ZnS core/shell QDs was extended by simple increasing of the both
precursors amount to ten times larger and solution volume to about 1 L, so that except the
dopant amount, all of the experimental conditions such as; total molarity of the solution, the molar ratios of precursors, pH of solution and refluxing time and temperatures has been
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selected in their optimized values. Figure 12 shows the digital images of the as-prepared ZnCdS/ZnS core/shell QDs at large volume of about 1 L under visible and UV lights. As can
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be seen, the up-scaled preparation method retains the high emission quality of the QDs. The PL emission spectrum of the corresponding solution has been also exhibited at the inset of
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Figure 12(b) to demonstrate our assertion.
Figure 12: Photographs of the as-prepared core/shell QDs in large scale under (a) room light and (b) UV
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light. Inset of (b) shows the corresponding PL emission spectra.
21
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4.
Conclusions Here, we have proposed a technologically-favored preparation method for dual-doped
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ZnCdS QDs with small amount of Ag+ and Ni2+ ions as impurities, NAC as capping agent
and ZnS shell. The possible correlation between two dopant ions showed that they should be
localized at different positions (energies) at the band gap region. A series of experiments showed that the best PL emission results are achieved at; Ag/(Zn+Cd) and Ni/(Zn+Cd) molar
ratios of 0.5% and 0.66%, respectively, refluxing temperature of 110 °C, solutions pH of
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about 11.4, core and shell precursor concentrations of 0.12 and 0.28 mmol, respectively. As a very important factor, the presence of the extra ZnS shell in our route has three advantages
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which are highly promising for their possible applications. Increasing of emission intensity, reduction of the possible cytotoxicity of the water-dispersible QDs and enhancement of their optical and chemical durability are all of these outstanding advantages. Simplicity, relative
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fastness, environmental/user friendliness of the precursors with outstanding durability (against chemical and optical corrosions) and high enough emission intensity of the QDs even at large-scale preparation, are all the demanding advantageous for industrial and medical usages (specially as bio-probes for tissue and cell imaging) which we collect in the
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present proposed route to prepare the dual-doped Ag,Ni:ZnCdS/ZnS QDs in aqueous media.
Conflict of interest: The authors declare that they have no conflict of interest.
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References
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[22] Jana S., Srivastava B. B., Jana S., Bose R., and Pradhan N., 2012, J. Phys. Chem. Lett. 3, 2535–2540. 23
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ACCEPTED MANUSCRIPT
Facile, one-pot and scalable synthesis of highly emissive aqueousbased Ag,Ni:ZnCdS/ZnS core/shell quantum dots with high chemical and optical stability To cite this article before publication: Reza Sahraei et al 2017 Nanotechnology in press https://doi.org/10.1088/1361-6528/aa92b2
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Facile, one-pot and scalable synthesis of highly emissive aqueousbased Ag,Ni:ZnCdS/ZnS core/shell quantum dots with high
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chemical and optical stability
Reza Sahraeia*, Ehsan Soheylia,b, Zahra Farajia, Mohammad Soleiman-Beigia a
Department of Chemistry, Faculty of Science, University of Ilam, 65315-516, Ilam, Iran b
Department of Physics, Faculty of Science, University of Ilam, 65315-516, Ilam, Iran *Corresponding author. Tel/ fax: +98 841 222 7022
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E-mail address: [email protected] & [email protected]
Abstract
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We report here a one pot, mild and low cost aqueous-based synthetic route for preparation of colloidally stable and highly luminescent dual-doped Ag,Ni:ZnCdS/ZnS core/shell quantum dots (QDs). The pure dopant emission of the Ni-doped core/shell quantum dots was found to
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be highly effected at the presence of second dopant ion (Ag+). Results showed that the PL emission intensity increases while its peak position experiences an obvious blue shift with increasing the content of Ag+ ions. Regarding the optical observations, we simply provide a scheme for absorption-recombination processes of the carriers through impurity centers. To
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obtain an optimum conditions with better emission characteristic, we also study the effect of different reaction parameters such as: refluxing temperature, core and shell solutions pH, molar ratio of the dopant ions (Ni:(Zn+Cd) and Ag:(Zn+Cd)), and concentration of the core and shell precursors. Nonetheless, the most effective parameter is the presence of the ZnS
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shell with suitable amount to eliminate the surface trap states and enhance their emission intensity. It can also, improve the bio-compatibility of the prepared QDs by restricting the Cd2+ toxic ions inside the core of the QDs. The present suggested route was also yielded to
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remarkable optical and chemical stability of the colloidal QDs which introduce them as a decent kind of nano-scale structures for light emitting applications, especially in the 1
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biological technologies. The suggested process has also this interesting potential to be scaledup while remaining the emission characteristics and structural quality which is inevitable for
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industrial applications in optoelectronic devices.
Keywords: Aqueous-based synthesis; ZnCdS quantum dots; dual doping; optical properties; chemical stability; up-scaling
Introduction
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1.
The advent of quantum mechanics at the beginning of the 20th century and finding the
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possible ways through chemical preparation of the extremely low size/dimension structures at the recent three decades has opened a very interesting window to achieve more applicable materials with peculiar behaviors. One of these wonderful structures, is quantum dots (QDs);
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a zero dimensional systems with semiconducting electronic nature [1]. The fabulous sizedependent optical properties can be observed in such materials as their size reaches to their corresponding excitonic Bohr radius [2] which leads to remarkable applications of colloidal QDs in optoelectronic devices, biological systems, photo-catalysis, chemical probes and etc
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[3-9]. The diverse nanostructures of binary, ternary, alloy and core/shell QDs have been synthesized using different elements, ligands and in various media [10-13]. Nonetheless, creation of desired characteristics, improvement of the intrinsic properties (such as longer excited state lifetimes, and enhanced thermal and chemical stability) and specifically
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eradication of the re-absorption in emitting-required applications have been led to tendency of the researchers to doped-QDs (d-dots) [14, 15]. Pioneer researchers have been concentrated on the preparation of high-quality d-dots
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in organic solvents [16, 17]. But, direct synthesis of the d-dots in aqueous media using low
temperatures and non-toxic precursors can be a good alternative to overcome the difficulties 2
Page 3 of 24
of the organometallic methods [18]. As presented by Peng’s group [19], internally doped
be employed in direct aqueous-based methods.
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nanostructures can be achieved by separation of doping and growth processes which can also
As suggested by Zhang and co-workers as well as Jana and co-workers, ZnCdS alloys
with zinc blend structure can be a suitable host structure for doping different types of
transition metal ions [20, 21] ]. Nickel is a transition metal element which has been rarely employed as dopant element in direct preparation of high quality d-dots in aqueous media. A
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simultaneous enhancement in optical, electrical and magnetic properties of the core structure
(by insertion of Ni2+ ions) is an indication over multifunctional utilization of such d-dots
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which has to be studied as they deserve [22]. However, the presence of the second dopant ion may have different effects on emission properties of the d-dots. As showed by Cui’s group, introduction of Ag+ dopant ion leads to quenching of dopant-related emission in Cu:ZnSe
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QDs while it improves that of Mn:ZnSe QDs [23]. Pradhan’s group also studied the variation of the emission color for Cu, Mn-doped CdZnS NCs, as the band edge positions of the host matrix was changing [24]. They realized from their hot injection-based method that the allowed transition would be occurred in selective way which involves just one dopant-related
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state. In a systematic investigation, Huang et. Al [25], also reported the nontoxic Ag- and Mn-doped ZnInS/ZnS dual-emission QDs with simultaneous tunable emission wavelengths. However, they also use a high temperature route of about 230 °C in organic solvents. They reported that the ratio-metric of Ag and Mn dual emissions can be tuned by controlling Ag–
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Mn and Mn–Mn dopant coupling.
In order to achieve high quantum yield emission, passivation of the surface states is
necessary which can be obtained by over coating of the core nuclease with an inorganic shell
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of the wider band gap semiconductor. The Schneider’s group prepared high quality ZnSe:Mn/ZnO core/shell QDs by aqueous route [26]. The results were very interesting but, 3
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they had to use more than 16 h refluxing temperature to obtain ZnSe:Mn core QDs. Recently, Levchuk et. Al [27], developed a one-pot, two-step synthetic route for highly luminescent
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Mn-doped ZnxCd1-xS/ZnS core/shell NCs in organic medium and at 230 °C. They also scaled up their industrially compatible method and used the obtained NCs for fabrication of downshifting layers in monocrystalline silicon-based solar cells.
Herein we present, for a first time, a growth doping strategy for cost-effective and
one-pot synthesis of water soluble dual-doped Ag,Ni:ZnCdS/ZnS core/shell QDs using N-
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acetyl-L-cysteine as a green stabilizer. Since the dopant emission is not an accidental phenomenon and strongly depends on the kind of the host matrix, its quality and dopant
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itself, beside of the successful preparation of QDs, our attention will be paid to possible correlation effect between the two different dopant ions and finding the best conditions to achieve better emission characters. Regarding the necessity of the resistance of the specimens
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against optical and chemical corrosions, their stabilities is examined and in order to prove the feasibility of the present method for industrial usage, the large-scale preparation of the high quality core/shell QDs is also introduced. Due to the fastness and simplicity of the present method, very cheapness and biocompatibility of the precursors, specially capping agent and
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high emission intensity [28], the present core/shell QDs have this potential to be used in biolabeling, solar cells and light emitting devices.
2.
Experimental
2.1.
Chemical
Merck’s
chemicals
of
zinc
sulfate
heptahydrate
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Materials.
(ZnSO4.7H2O, 99-103%), cadmium acetate dehydrate (Cd(OAc)2.2H2O, +98%), N-acetyl-Lcysteine (NAC, 99%), silver sulfate (Ag2SO4, +99.5%), nickel acetate tetrahydrate (Ni(OAc)2.4H2O, 99%), Thiourea (TU, +99%), H2O2 (30%), NaOH (+99%), 2-propanol
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(+99.8%) and ethanol (+99.9%) were used in the present work. Extra pure sodium sulfide
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hydrate (Na2S.H2O, 60-63%) was purchased from Acros Organics. Deionized water was also utilized, as solvent, in all experiments. All of the precursor materials were used without any
2.2.
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purification.
Synthesis of Ag,Ni:ZnCdS/ZnS core/shell QDs. Water soluble ZnCdS QDs with
small amount of two dopant elements and additional inorganic shell of ZnS have been prepared using growth doping strategy. Namely, for preparation of Ag,Ni:ZnCdS core QDs,
equal amounts of Cd and Zn precursors were added to the mixture of NAC and 87 mL
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deionized water. Then, desired molar ratio of Ag+ and Ni2+ precursors were added to solution and it was adjusted to specific pH by using 1 molL-1 (M) NaOH. After a while, the solution
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was transferred to three-necked flask and appropriate amount of a 0.1 M Na2S stock solution was injected to solution as fast as possible we can, under vigorous stirring. The total volume of the solution was close to 100 mL. Next, the flask was heated up to desired temperature and
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refluxed for 90 min. The reagents molar ratio of Zn:Cd:NAC:S was taken as 1:1:5:3 for different amounts of dopant ions. Actually, the amount of Ni2+ ions was firstly justified in Nidoped ZnCdS/ZnS core/shell QDs and then at the optimized amount of Ni2+, the second dopant ion (Ag+) was added at different amounts. In order to investigate the effect of core
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precursors concentration, the amounts of (Zn2+ + Cd2+) ions has been adjusted at 0.06, 0.12 and 0.24 mmol, where the amounts of other precursors varied based on their molar ratios. Over coating of the core QDs with ZnS shell was also carried out as follows; firstly, Zn2+ and TU precursors were respectively added to a 20 mL of NAC-containing deionized water at
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molar ratios of 1:1.5:1 (Zn:NAC:TU) and the solution was brought to specific pH values by 1 M NaOH. Then, desired amounts of the ZnS shell precursor solution (0.07, 0.14, 0.28 and 0.42 mmol) was injected in two steps into the stirring Ag,Ni:ZnCdS core QDs at its refluxing
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temperature. Following extra 90 min, for efficient shelling, the heater and stirrer were turned off and flask was cooled down to room temperature. All of the optical measurements were 5
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carried out immediately after this step. To obtain a suitable powder of the specimens, the asprepared QDs were precipitated by initial combination with acetone (1:1 volume ratio) and
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ensuing centrifugation at 4500 rpm for 10 min. After that, the precipitates were washed by mixture of ethanol and deionized water for several times and then dried overnight at 40 °C.
As one can see in Scheme S1 (See Supporting Information), the suggested strategy is really facile and one-pot/two steps method which all of its procedures are carried out at one three-
necked flask. The measurement details and instruments used have been mentioned in the
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Supporting Information. Results and Discussion
3.1.
Structural Characterizations. The XRD patterns of the Ag,Ni:ZnCdS QDs before
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3.
and after covering by ZnS shell are shown in Figure 1 which are broadened because of the nano-scale size of the structures. As can be seen, the peaks position of the core QDs has been
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located between those for ZnS and CdS ones with cubic zinc blend phase. It means that the prepared specimens have been crystallized in alloy form. However, by over coating of the core with ZnS shell, the peaks positions are shifted toward larger angles correspond to ZnS cubic structure. Interestingly, there are no any traces of the Ag2S and NiS-related peaks
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which shows that the small amount of silver and nickel ions are just introduced as dopant ions. The slight increasing of the first peak intensity after shell overcoating also can be
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attributed to increasing of the average size of the QDs.
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6
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Figure 1: XRD patterns of the dual doped core and core/shell QDs.
Figure 2 shows TEM images of the prepared QDs. Both core and core/shell structures are nearly mono-disperse QDs with average sizes of 2.9±0.2 and 4.9±0.4 nm, respectively.
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Since, the excitonic Bohr radius of the ZnCdS structure with similar amount of Cd+2 and Zn+2 ions is about 2.6 nm [29], the strong quantum confinement effect has to be presented at the present dual-doped QDs. Also, based on the dynamic light scattering measurements, the size and size distribution of the QDs was investigated (See Figure S1). The average size of the
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core and core/shell QDs were around 3.1±0.1 and 4.8±0.3 nm, respectively.
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Figure 2: TEM images of, (a) Ag,Ni:ZnCdS QDs and (b) Ag,Ni:ZnCdS/ZnS core/shell QDs. 7
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Figure S2 shows the FT-IR spectra of pure NAC and NAC-capped Ag,Ni:ZnCdS core QDs before and after covering by ZnS shell. As is seen, the infrared spectra of all samples are
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quite similar to that of NAC, except in one case. The characteristic IR absorption bond of thiol group (2570 cm-1) in pure NAC powder has been completely disappeared in both types of prepared core and core/shell QDs. This observation shows that the deprotonated thiol
terminals of the NAC molecules form the metal-thiol complexes with unreacted cations at the surface of the QDs and effectively passivate them.
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The elemental analysis for dual-doped ZnCdS QDs before and after covering by ZnS shell has been showed in Figure S3. The presence of the Zn, Cd and S elements are clear. It
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can be also observed that the amount of Zn increases after shell injection. There are no any traces regarding Ag or Ni species. It means that the used-amount of these ions are as low as which cannot be detected by EDX analysis. To ensure the dopants collaboration into lattice
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structure of the QDs, ICP-AES measurements were carried out on Ag,Ni:ZnCdS QDs at different Ag+ concentrations. Based on these measurements, the nominal Ni:(Cd+Zn) and Ag:(Cd+Zn) molar ratios used in the synthesis and the real Ni:(Cd+Zn) and Ag:(Cd+Zn) molar ratios incorporated into the lattice structure of the QDs were determined and given in
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Table 1.
Table 1: Nominal Ni:(Cd+Zn) and Ag:(Cd+Zn) molar ratios used in the synthesis and the real Ni:(Cd+Zn) and Ag:(Cd+Zn) molar ratios in Ag,Ni:ZnCdS core QDs samples determined by ICP-AES measurements.
nominal Ni:(Cd+Zn) molar ratio
ICP results of Ag:(Cd+Zn) molar ratio
ICP results of Ni:(Cd+Zn) molar ratio
0.16:100
0.66:100
0.15:100
0.42:100
0.33:100
0.66:100
0.30:100
0.43:100
0.50:100
0.66:100
0.48:100
0.45:100
0.66:100
0.66:100
0.65:100
0.44:100
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nominal Ag:(Cd+Zn) molar ratio
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It can be found that the real Ag:(Cd+Zn) molar ratios in actual samples are quite similar to the nominal values, while the real Ni:(Cd+Zn) molar ratio is lower than the
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nominal values. This can be attributed to enormous difference in the solubility products of Ag2S (Ksp=3×10-50) and NiS (Ksp=1×10-21). It also explains the difficulty of the coincorporation of Ag+ and Ni2+ ions into the host matrix. 3.2.
Optical properties. As described in Scheme S1, the Ag,Ni:ZnCdS/ZnS core/shell
QDs have been prepared in two steps. Firstly, the NAC-capped ZnCdS cores QDs with
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suitable amount of Ag and Ni ions have been synthesized and then, they were over-coated
with additional ZnS shell. The presence of the strong quantum confinement effect in as-
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prepared QDs can be concluded based on a considerable blue-shift of the absorption edge, compared to band edge position of the corresponding bulk structure (from 540 to 405 nm). However, there are several factors which can be highly effective on emission characters of
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the prepared QDs. Refluxing temperature, the solution pH of core and shell precursors, concentration of core and shell precursors, effect of dopant ions and their concentrations on optical properties of QDs are all of the factors which we are going to report and discuss about their influence.
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3.2.1. Influence of the dopant ions. UV-Vis and normalized PL spectra of undoped, Nidoped, Ag-doped and Ag,Ni-dual-doped ZnCdS/ZnS core/shell QDs are shown in Figure 3(a). The ZnCdS/ZnS QDs were prepared at core and shell precursors concentrations of 0.12 (amount of (Zn2++Cd2+) ions) and 0.28 (amount of Zn2+ ions) mmol, respectively. The molar
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ratios of 1:1:5:3, for Cd:Zn:NAC:Na2S in the core reaction mixture and 1:1.5:1 for Zn:NAC:TU in shell precursor were used. The pH of core solution was 11.4 and that of injected ZnS shell was about 11.3. The refluxing time and temperature of both core and shell
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formation were also set at 90 min and 110 °C, respectively. As it has been shown in Figure 3(a), the undoped ZnCdS/ZnS QDs have an absorption edge around 405 nm and with 9
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relatively large stokes shift, a prominent PL emission peak at about 562 nm. This remarkable energy difference between two spectra indicates that the emission should not be happened
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through excitonic states of the ZnCdS. Therefore, this broad peak can be attributed to trap
states emission (a very weak peak at 420 nm is also observed due to excitonic recombination). Despites the core over-coating with higher band gap ZnS shell, the intense trap-related peak is still observed that has been previously attributed to intrinsic trap sites of
such structures for hole carriers [30]. However, by introduction of small amount of Ni2+
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dopant ions (Ni/(Zn+Cd) molar ratio of 0.66%), the emission peak depicts a red-shift to 576 nm while its absorption edge doesn’t change, remarkably. This red-shift is probably because Ni2+-related levels in band gap region are located at higher energies rather than trap states
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which results in emission at longer wavelengths. As Figure 3 shows, when we used Ag+ ions (Ag/(Zn+Cd) molar ratio of 0.5%) instead of Ni2+ ions, in spite of the unaltered absorption
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spectrum, the dopant-related emission peak experiences a clear blue shift with higher intensity relative to both of undoped and Ni-doped QDs. However, co-existence of the both dopant ions has interesting results. It causes an meaningful red-shift in emission spectrum regarding that of Ag+-doped QDs which has to be due to the lower energy of Ag+ states relative to Ni2+ ones, at forbidden band gap region. This kind of behavior has been reported
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by Jana et. al [21], in high temperature prepared dual-doped ZnCdS NCs. These observations in absorption and emission spectra can be illustrated as follows, respectively; the band edges position of the present Ni-doped QDs are highly dependent on size and composition of the
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samples and in the case of fixed reaction conditions (solution pH, refluxing time and temperature, precursor’s molar ratio and concentration), band gap of the specimens remains unchanged even when different kinds of impurities are used, which is confirmed by approximately similar absorption spectra for all of as-prepared core/shell QDs in Figure 3(a).
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On the other hand, the emission peak position changes considerably, due to a chemical
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competition between Ag+ ions (as a soft acid) and Ni2+ ions (as a borderline acid) to react with S2- ions (as a soft base). This intrinsic characteristic, leads to significantly higher
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reactivity of the silver ions thorough S2- ions, rather than nickel ions (solubility product of
Ag2S; Ksp=3×10-50 compared to NiS; Ksp=1×10-21). Hence, as previously determined by ICPAES measurements (Table 1), in a fixed amount of S2- ions, the Ag2S specimens are formed more easily which yields to observation of the Ag+-related emission and due to the lower
energy of Ag+ states, the blue shift in emission spectrum is comes to pass. Based on optical
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observations, we suggest a scheme for possible absorption-recombination processes in the
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present Ag,Ni:ZnCdS/ZnS core/shell QDs (Scheme 1).
Figure 3: (a) UV-Vis absorption and normalized PL spectra of undoped, Ni-doped, Ag-doped and dualdoped (Ni and Ag) ZnCdS/ZnS core/shell QDs. Weak excitonic peak has been indicated by dot-line. (b)
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Evolution of intensity (circles) and position (squares) of the emission peaks in different samples.
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Scheme 1: Schematic diagram for the position of Ag+, trap and Ni2+ states and their possible
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recombination process in the Ag,Ni:ZnCdS/ZnS core/shell QDs.
In order to study the effect of the second dopant ions (Ag+) concentration on PL
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emission properties of Ag,Ni:ZnCdS/ZnS core/shell QDs, a series of samples with different Ag/(Cd+Zn) molar ratios of 0.00%, 0.16%, 0.33%, 0.50%, and 0.66% were prepared while keeping the other experimental variables fixed. As shown in Figure 4, with increasing of Ag/(Zn+Cd) molar ratio, the dopant emission intensity increases until it reaches to its maximum at Ag/(Zn+Cd) molar ratio of 0.50%, whilst the position of PL emission peaks of
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the Ag,Ni:ZnCdS/ZnS core/shell QDs is shifted to shorter wavelengths (from 585 nm to 545 nm). However, with further increasing in Ag/(Zn+Cd) molar ratio (0.66%), the PL intensity decreased, where quenching phenomenon is appeared. It can be attributed to energy transfer between Ag dopant states in a non-radiative manner. Compared to this PL behavior, the UV-
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Vis absorption spectra are almost identical (Figure 4(a)) indicating that the band edges position are independent on dopants amount added and so do the band gap energy (using well-known Tauc’s equation [31]) which was fixed at about 2.95 eV. The optimized
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Ag/(Zn+Cd) molar ratio, for fixed Ni/(Zn+Cd) molar ratio of 0.66% (see Figure S4), was found to be 0.50%. Finally, as a very important result, one can immediately find out that the 12
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present work suggest a simple synthetic method to prepare tunable emission without any kind of size and host composition alteration from 539 (in Ag-doped ZnCdS/ZnS QDs) to 588 nm
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(in Ni-doped ZnCdS/ZnS QDs). It can be achieved just by using different dopant ions in their
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highest concentrations.
Figure 4: Effect of Ag/(Zn+Cd) molar ratio on: (a) UV-Vis absorption and normalized PL spectra of dualdoped Ag,Ni:ZnCdS/ZnS core/shell QDs and (b) intensity (circles) and position (squares) of the emission peaks.
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3.2.2. Influence of refluxing temperature. Due to the very strong dependency of doping efficiency to preparation temperature, three different refluxing temperatures of 45, 75 and 110 °C were used to find the best one (while all other parameters were kept constant). As is shown in Figure 5(a), with increasing of refluxing temperature the absorption edge position
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shifts clearly to longer wavelengths, which means that the higher refluxing temperature makes a facile condition for further growth of the QDs and subsequent red shift in absorption spectra and reduction of the band gap energy. The calculated band gap energy of the dual-
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doped Ag,Ni:ZnCdS/ZnS core/shell QDs at different temperatures of 45, 75 and 110 °C are
3.3, 3.15 and 2.95 eV, respectively. 13
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Since, nature of emission in both Ni and Ag centers is related to band edges position (Scheme 1) [21], reduction of the band gap energy is associated with obvious red shift in
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dopant-related emission peak from 500 nm to 554 nm (Figure 5(b)), because the conduction
band moves down and dopant states probably remain unchanged. As is seen in Figure 5(c), intensity of the emission peaks is highly dependent on reaction temperature. There are two
observations in this regard. The intensity of the dopant emission at 45 °C is considerable which can be an indication over suitable efficiency of the present method even at low
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temperatures. Moreover, the higher emission intensity is obtained at higher temperatures. A possible interpretation can be that the doping process is a multiple process. Dopant ions
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should be initially adsorbed on the surface of the QDs for sufficient time, and then it would be incorporated into the host matrix followed by lattice diffusion [32]. Although, each step has its own critical temperature, however, the overall effect of the sufficiently higher
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temperatures is to improve the possibility of internal doping, easier diffusion of dopant ions into lattice structure and ensuing improvement of emission possibility through dopant-related
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radiation centers.
Figure 5: Effect of refluxing temperature on (a) UV-Vis absorption spectra, (b) normalized PL spectra of
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dual-doped Ag,Ni:ZnCdS/ZnS core/shell QDs, and (c) intensity (circles) and position (squares) of the emission peaks.
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3.2.3. Effect of core and shell solutions pH. Figure 6(a) shows UV-Vis and normalized PL
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spectra of dual-doped Ag,Ni:ZnCdS/ZnS core/shell QDs prepared at different pH values
(while other experimental conditions are kept same). It can be seen that the absorption band shifts from 397 to 416 nm by adjustment of the core’s solution pH from 8.6 to 12. This
demonstrates the faster growth of the QDs in strong alkaline matrices [33]. As analyzed by Yang’s group, the high pH values are associated with low surface potential [34], which
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results in faster growth rate of the QDs. The measured band gap energy is also decreased
from 3.05 to 2.83 eV, by increasing of the solution pH. The PL emission results of the Ag,Ni:ZnCdS/ZnS QDs (Figure 6(b) and (c)), showed that despite the clear red-shift which
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has to be observed due to reduction of the band gap, the intensity of dopant emission peak is also improved until pH=11.4. This increasing in intensity of emission peak at high pH values
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can be related to better deprotonation of thiol groups and their subsequent covalent bond with cations and passivation of dangling bonds at the surface of the QDs. However, with further increasing of precursor solution pH (up to 12), the PL intensity decreases meaningfully, due
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to aggregation effects and instability of the QDs at very high pH values [33].
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Figure 6: (a) UV-Vis absorption spectra, (b) normalized PL spectra, and (c) evolution of intensity (circles) and position (squares) of the emission peaks for Ag,Ni:ZnCdS/ZnS core/shell QDs prepared at different pH values. 15
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After refluxing of the Ag,Ni:ZnCdS core QDs for 90 min, desired amount of the freshly-prepared ZnS shell precursor solution was adjusted at three different pH values of 2.5,
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8.7, 11.3 (whereas other experimental variables were kept as fixed) and subsequently used for shell deposition. As has been shown in Figure 7, the high pH values for shell precursor lead to slight red shift of the both absorption and emission spectra. With increasing of shell’s solution pH, formation of the ZnS nanostructures is facilitated. Hence, injecting of ZnS shell
with high pH values may result in two happenings; providing the thicker shell of ZnS around
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core QDs and better surface passivation. The first one leads to small growth of QDs
appearing in small red-shift of the absorption/emission spectra and decreasing of the band
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gap energy from 3 eV to 2.9 eV. The second one, however, leads to more effective passivation of surface dangling bonds, surface trap states and separating the dopant ions from surface of the core QDs. It can eliminate the non-radiative centers and increase the PL
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intensity (Figure 7(b)).
Figure 7: (a) UV-Vis absorption and normalized PL spectra and (b) evolution of intensity (circles) and
position (squares) of the emission peaks for Ag,Ni:ZnCdS/ZnS core/shell QDs prepared at different pH
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values of injected shell precursor solution.
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3.2.4. Effect of core and shell precursors concentration. To analyze the effect of core concentration on optical properties of the dual-doped QDs, the amounts of (Zn2++Cd2+) ions
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has been adjusted at 0.06, 0.12 and 0.24 mmol, where the amounts of other precursors
changed according to their molar ratios. Based on absorption and emission data (Figure 8),
the overall red shift of the absorption edge is associated with growth of the QDs at higher concentrations and as mentioned before, it reduces the energy of the emitted color. However,
this faster growth may results in higher degree of defects and ensuing increasing of non-
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radiative recombination in QDs. Figure 8(b) shows that, at the concentration of 0.24 mmol,
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the emission intensity decreases about four times compared to which for 0.12 mmol.
Figure 8: (a) UV-Vis absorption and normalized PL spectra and (b) evolution of (circles) and position (squares) of the emission peaks for the Ag,Ni:ZnCdS/ZnS core/shell QDs prepared at different concentration of core precursor solution.
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To minimize the density of the surface defects, here, we use different amounts of ZnS
shell at pH=11.3. In the present work, we simply introduced different amounts of ZnS shell precursor (0.07, 0.14, 0.28 and 0.42 mmol) and let it refluxed for extra 90 min at the 110 °C.
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Figure 9(a) shows that the absorption edge of the QDs shifted to longer wavelength with addition of more ZnS shell precursor solution. This shift indicates the productive formation of 17
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the shell around the core QDs which is associated with growth of the core/shell QDs and reduction of their band gap energy (from 3.05 eV to 2.75 eV). Upon addition of the shell
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precursor up to concentration of 0.28 mmol, the intensity of the emission peak exerts an interesting enhancement as is seen in Figure 9(c). In this case, the additional ZnS shell with higher band gap energy can bury the dopant elements and make an extra force (potential
barrier) on them to be internally positioned. Indeed, it can effectively confine the carriers to locate inside of core structure. However, the ensuing decreasing of the emission intensity at
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higher concentration of shell precursor (0.42 mmol) can be attributed to formation of higher
density of surface defects at the interfacial of the core and shell structures as the thickness of
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shell over Mn-doped ZnSe NCs [35].
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the shell increases. Zeng et al., have reported exactly the same alteration by addition of ZnS
Figure 9: (a) UV-Vis absorption spectra, (b) PL spectra, and (c) evolution of intensity (circles) and
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position (squares) of the emission peaks for Ag,Ni:ZnCdS/ZnS core/shell QDs at different amounts of injected ZnS shell precursor solution.
After optimization the mentioned parameters, the quantum yields (QYs) of the doped-
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core and -core/shell QDs have been calculated as illustrated in the Supporting Information. It was found that the QY of the Ag,Ni;ZnCdS QDs increases from 11.6 to 30.2, by ZnS shell 18
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overcoating which implies their tremendous possibility to be used in optoelectronic and
3.3.
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biological applications. Chemical and optical stability of the QDs. According to the literature method [36],
we also investigated the tolerance of both dual-doped core and core/shell QDs against chemi-
oxidization by H2O2. As can be seen in Figure 10, by increasing of the etching time, the
impressionability of the QDs without shell is higher than that of with shell. Indeed, the PL intensity of the dual-doped core QDs decreased until 102 min while this time is about 50 min
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for core/shell QDs. The presence of the additional shell with smaller atomic radius elements,
leads to improvement of the chemical stability by formation of more compressed (packed)
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ligands at the surface of the QDs [37]. In spite of this, the overall effect of chemical corrosion on both of investigated specimens is small and after 60 min, the PL intensity reaches 53% and 33% of the initial intensity, for dual-doped core/shell and core QDs, respectively. The
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enormous chemical stability of the QDs demonstrates that the emission centers are related to dopant ions which probably located far from the surface of the QDs. This can be an additional indication for successful internal doping of the Ag+ and Ni2+ ions into the lattice
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structure of the ZnCdS/ZnS QDs.
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Figure 10: Temporal evolution of PL spectra of Ag,Ni:ZnCdS QDs (a) without shell, (b) with ZnS shell, and (c) comparison of their chemical stability for serial times. 19
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The storage stability test on both of the core only doped QDs and core/shell QDs was also done at different time intervals. The as-prepared QDs have been subjected to visible
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light, at room temperature for 28 days. As can be seen in Figure 11, both of the core and core/shell QDs show high resistance versus photo-bleaching at room temperature. From
Figure 11(a) and (b), it is clear that the PL emission intensity of the Ag,Ni-doped ZnCdS QDs has been completely quenched after 28 days. While the PL intensity of the core/shell QDs
remains at about 40% of the initial value at this time period (Figure 11(c)). This remarkable
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feature would demonstrate the success of the doped QDs preparation method in both of the
synthetic challenges; internal diffusion of the dopant ions into the host matrix and providing
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an efficient additional shell of ZnS with higher band gap energy where both of them prohibit the dopant emission centers from the surface of the QDs. As can be concluded the colloidal and optical properties of the as-prepared QDs are high enough to be used in optoelectronic
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devices and biological systems.
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Figure 11: Temporal evolution of PL spectra of Ag,Ni:ZnCdS QDs (a) without shell and (b) with ZnS shell, and (c) comparison of their optical stability for different time periods under day light at room
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temperature.
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3.4.
Upscaling. Due to undeniable importance of the high quality QDs from both
scientific and industrial perspectives, here we show the capability of the suggested method to
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be used for industrial applications in light emitting devices [38]. The mentioned preparation
method of ZnCdS/ZnS core/shell QDs was extended by simple increasing of the both
precursors amount to ten times larger and solution volume to about 1 L, so that except the
dopant amount, all of the experimental conditions such as; total molarity of the solution, the molar ratios of precursors, pH of solution and refluxing time and temperatures has been
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selected in their optimized values. Figure 12 shows the digital images of the as-prepared ZnCdS/ZnS core/shell QDs at large volume of about 1 L under visible and UV lights. As can
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be seen, the up-scaled preparation method retains the high emission quality of the QDs. The PL emission spectrum of the corresponding solution has been also exhibited at the inset of
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Figure 12(b) to demonstrate our assertion.
Figure 12: Photographs of the as-prepared core/shell QDs in large scale under (a) room light and (b) UV
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AUTHOR SUBMITTED MANUSCRIPT - NANO-115426.R1
light. Inset of (b) shows the corresponding PL emission spectra.
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AUTHOR SUBMITTED MANUSCRIPT - NANO-115426.R1
4.
Conclusions Here, we have proposed a technologically-favored preparation method for dual-doped
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ZnCdS QDs with small amount of Ag+ and Ni2+ ions as impurities, NAC as capping agent
and ZnS shell. The possible correlation between two dopant ions showed that they should be
localized at different positions (energies) at the band gap region. A series of experiments showed that the best PL emission results are achieved at; Ag/(Zn+Cd) and Ni/(Zn+Cd) molar
ratios of 0.5% and 0.66%, respectively, refluxing temperature of 110 °C, solutions pH of
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about 11.4, core and shell precursor concentrations of 0.12 and 0.28 mmol, respectively. As a very important factor, the presence of the extra ZnS shell in our route has three advantages
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which are highly promising for their possible applications. Increasing of emission intensity, reduction of the possible cytotoxicity of the water-dispersible QDs and enhancement of their optical and chemical durability are all of these outstanding advantages. Simplicity, relative
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fastness, environmental/user friendliness of the precursors with outstanding durability (against chemical and optical corrosions) and high enough emission intensity of the QDs even at large-scale preparation, are all the demanding advantageous for industrial and medical usages (specially as bio-probes for tissue and cell imaging) which we collect in the
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present proposed route to prepare the dual-doped Ag,Ni:ZnCdS/ZnS QDs in aqueous media.
Conflict of interest: The authors declare that they have no conflict of interest.
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