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ZnS nanocrystals with high luminescence
Materials Chemistry and Physics 149-150 (2015) 437e444
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Facile synthesis and characterization of core/shell CueIneZneS/ZnS nanocrystals with high luminescence Weidong Xiang a, *, Xin Ma a, Le Luo b, Wen Cai c, Cuiping Xie b, Xiaojuan Liang a a
School of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, China College of Materials Science and Engineering, Tongji University, Shanghai 201804, China c Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China b
h i g h l i g h t s We report a simple one-pot two-step approach to synthesize core/shell CueZneIneS/ZnS nanocrystals (CZIS/ZnS NCs). The as-prepared CZIS/ZnS NCs were investigated by XRD, SAED, EDS, TEM and HR-TEM. The optical properties of the CZIS/ZnS NCs were measured by UVevis, PL spectra and QY. The photoluminescence mechanism of the CZIS/ZnS NCs was investigated by PL decay curves. The possible shell formation process of CZIS/ZnS NCs was proposed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2014 Received in revised form 24 October 2014 Accepted 26 October 2014 Available online 4 November 2014
CueZneIneS/ZnS (CZIS/ZnS) core/shell nanocrystals (NCs) with high quality have been successfully synthesized via a one-pot two-step method. The photoluminescent properties including emission spectra, photoluminescence quantum yield (PL QY) and PL decay time of the obtained CZIS/ZnS NCs could be tuned by controlling the growth process of ZnS shell. The effect of the amount of Zn added on the growth of ZnS shell was investigated. The shell growth temperature and time for CIZS/ZnS NCs were discussed in detail. A relatively high PL QY of 77% could be achieved under certain reaction condition after systematic study. The crystal structure, morphology, size distribution and chemical composition of CZIS/ZnS NCs were determined by X-ray Diffraction (XRD), selected-area electron diffraction (SAED), lowand high-resolution transmission spectra (TEM) and STEM-EDS, respectively. The photoluminescence mechanisms of the CZIS/ZnS NCs were discussed by time-resolved photoluminescence. © 2014 Elsevier B.V. All rights reserved.
Keywords: Semiconductor Nanostructures Chalcogenides Photoluminescence spectroscopy Chemical synthesis
1. Introduction In recent years, semiconductor nanocrystals (NCs) or quantum dots (QDs) have attracted increasing attention due to their unique optical properties, which show a good prospective application such as solar cells, biomedical labeling, light emitting diodes (LEDs), etc [1e6]. Recently, Sony has announced its latest QD-embedded flatscreen television for the first time [7] which made the study on QDs hotter than ever before. Preparation and characterization of IIeVI and IVeVI type QDs had been focused owning to their excellent optical and electrical properties. However, large scale applications of QDs are strongly restricted because of containing toxic heavy
* Corresponding author. E-mail address: [email protected] (W. Xiang). http://dx.doi.org/10.1016/j.matchemphys.2014.10.042 0254-0584/© 2014 Elsevier B.V. All rights reserved.
metals, cadmiums, lead, etc. [8,9]. In comparison with IIeVI or IVeVI type QDs, IeIIIeVI QDs are less toxic and more biocompatible. For instance, CuInS2 (CIS), a direct band gap semiconductor with high absorption coefficient is of great interest during the past five years. CIS QDs have a lot of advantages such as markedly low toxicity, large stokes shifts, long photoluminescence (PL) lifetime and sizetunable emissions. However, it has a severe drawback: the quantum yield (QY) is usually less than 10%, which is too low for further use [10e12]. Many great contributions has been made to solving this problem, for example, surface modification such as ZnS shell coating is an effective method [13,14]. An early example was given in 2009 by Xie et al. who had increased the CIS QY from 3 % to 30 % successfully through overcoating one monolayer of ZnS shell [12]. Zhang et al. prepared CueZneIneS (CZIS) by using acetate salts of corresponding metals as precursors in noncoordinating solvent in
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the presence of DDT. The plain CZIS core showed quite low QY (below 3%), but after ZnS shell growth, the maximum QY was increased to 56% [15]. Later, in 2012, Deng et al. synthesized CIS/ZnS QDs with roughly 3e4 monolayers of ZnS which had a relatively high QY reaching 80%, using acetate salts of the corresponding metals as starting materials [16]. Jang et al. reported highly luminescent double-ZnS-shell-capped CIS QDs with an extremely high QY of 92% and utilized them as color converter for the fabrication of a QD-based WLED. After covering CIS cores with two monolayers of ZnS shell, a notable blue-shift from 660 nm to 559 nm was observed in the PL emission spectrum [17]. Very recently, Guo et al. reported CZIS/ZnS QDs showing inhibitory blue-shift throughout ZnS shell growth process and proposed a hypothesis to explain this phenomenon; the maximum QY of the as-prepared QDs was 50% [18]. From the above, it could be seen that considerable progress has been made until now, however, some challenging problems still remain to be solved: (1) Although ZnS shell coating has been studied extensively, most of these reports are on the basis of ternary CIS cores while quite rare efforts could be seen in the quaternary CZIS cores. One of the possible reasons is that it is more difficult to prepare quaternary QDs because the metal precursors have different chemical reactivties, which increases the difficulty in making a balance between them so as to prevent unexpected nucleation or phase separation [12,19,20]. (2) Among most of previous studies, QY in the quaternary CZIS/ZnS system is not so high (usually below 60%) as that in the ternary CIS/ZnS system (above 75%) [21e24]. Further exploration in improving QY in the quaternary CZIS/ZnS system and analyzing factors influencing optical properties during shell growth process is still recommended. (3) In view of the precursors chosen, long-chain metal carboxylates were used predominantly in most of the high QY CZIS/ZnS or CIS/ZnS QDs. In contrast, inorganic metal salts such as CuCl and CuI attracted less attention because of higher chemical reactivity, which would lead to much faster nucleation and growth rates, and thus made process controlling more difficult [25e27]. Herein, quaternary CZIS QDs were synthesized by using inexpensive chloride salts of corresponding metals as precursor firstly and then they were covered with ZnS shell. Synthetic variables in the shell growth stage (i.e., shell growth temperature, Zn concentration and shell growth time) which would affect optical properties, especially QY were discussed systematically in this work. Furthermore, the changing mechanism of photoluminescence properties was analyzed by using PL decay curves. 2. Experimental 2.1. Materials Copper chloride (CuCl, 99.7%), indium chloride (InCl3$4H2O, 99.7%), zinc chloride (ZnCl2, 99.7%), n-dodecanethiol (DDT, 98%), oleic acid (OA, 98%), and sulfur powder (S, 99.5%) were purchased from Sinopharm. Oleylamine (OAm, 80e90 %) and 1-octadecene (ODE, 90%) were purchased from Alladdin. All chemicals were used without further purification.
0.8 mmol of sulfur powder dispersed in 2 ml oleylamine (OAm) was quickly injected into the reaction solution and the temperature dropped to 160 C, then the mixture solution was kept for 10 min at this temperature to allow the growth of CZIS core NCs. 2.3. Preparation of the Zn stock solution for shell growth For a typical synthetic reaction, Zn stock solution was prepared by mixing zinc chloride (0.2726 g, 2 mmol), OAm (1 ml) and ODE (4 ml) in a three-neck reaction flask. The reaction mixture was heated to 90 C under an argon atmosphere with magnetic stirring and kept at this temperature for 30 min. The flask was then heated to 150 C for another 10 min and a clear solution was formed. After that, the reaction mixture was kept at 50 C for further use. In this work, four different Zn amount, i.e., 2 mmol, 1 mmol, 0.5 mmol, and 0.25 mmol were conducted while the amount of OAm and ODE were fixed. 2.4. Synthesis of CZIS/ZnS core/shell NCs The synthesized CZIS core NCs growth solution was used directly without intermediate purification step. The ZnS coating was accomplished by injecting the Zn stock solution into the QD growth solution with a syringe. After Zn stock solution which contains 2 mmol of Zn2þ was injected into CZIS core NCs growth solution, the temperature was set as 240 C for 45 min for the shell growth in this work. 2.5. Characterization XRD measurements were performed on a Bruker D8 Advance Xray diffractometer (40 kV, 40 mA) with monochromatic Cu Ka radiation (l ¼ 1.5406 Å) and a scan rate of 5 min1 in the 2q range of 10e70 . X-ray diffraction samples were prepared by depositing the nanocrystals on a Si (100) wafer. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were carried out on a JEM-2010HR transmission electron microscope at an acceleration voltage of 200 kV. Field-emission scanning electron microscope (FESEM) images were taken on a JEOL-JSM-6700F scanning electron microscope (10 kV) equipped with an X-ray energy dispersive spectroscope (EDS) at an accelerating voltage of 20 kV. UVeVis absorption spectra were measured with a HITACHI U-4100 spectrophotometer. PL spectra, PL quantum yield and PL decay spectra were recorded with a Horiba Jobin Yvon Fluromax-4P spectrophotometer equipped with absolute quantum yield measurement apparatus and a time-correlated single-photocounting (TCSPC) spectrometer. In the PL decay spectra measurements, the probing wavelength for PL decay was set to the wavelength of the PL at its maximum intensity. Samples were excited at 450 nm by a pulsed xenon lamp at a 1 MHz repetition rate, and PL was detected by using a spectrally resolved, TCSPC spectrometer. 3. Results and discussions 3.1. Synthesis and characterizations of CZIS/ZnS core/shell structure
2.2. Synthesis of CZIS core NCs For the synthesis of CZIS NCs, CuCl (0.0040 g, 0.04 mmol), indium chloride (0.0587 g, 0.2 mmol), zinc chloride (0.0273 g, 0.2 mmol), oleic acid (0.06 ml), n-dodecanethiol (DDT, 0.25 ml) and 1-octadecene (ODE, 8 ml) were loaded into a three-neck reaction flask, which was then degassed at 90 C for 30 min and purged with argon. The flask was heated to 180 C for another 5 min. During the heating-up process, the solution remained clear while its color changed gradually from colorless to slight yellow. After that,
Bare core CZIS and its corresponding CZIS/ZnS core/shell NCs were prepared. In a typical synthesis process for ZnS shell growth, 2 mmol of Zn was injected into the crude solution containing CZIS core NCs, then the temperature was maintained at 240 C. Fig. 1a shows the XRD pattern of CZIS NCs and their corresponding core/ shell CZIS/ZnS NCs. The XRD peaks of CZIS NCs were quite broad, suggesting the obtained NCs were small in size and had poor crystallinity due to low synthesis temperature. Upon ZnS shelling, the patterns of the cubic lattice is maintained, but the diffraction
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Fig. 1. (a) XRD patterns and (b) Absorption, emission spectra of CZIS QDs before and after the coating of ZnS shell.
peaks become sharper and shift to larger angles, which is consistent with the smaller lattice constant for ZnS compared with CZIS. This could be attributed to the combined effect of a suitable ZnS shell formation on the CZIS core NCs and a slight cation exchange between CZIS cores and Zn2þ [8,28]. Fig. 1b shows the absorption and PL emission spectra of CZIS NCs before and after covering ZnS shell. A remarkable blue-shift of PL peak from 657 to 579 nm is found after the growth of ZnS shell, which is consistent with previous reports [29,30]. These reports suggested that the blue-shift originate from cation exchange occurred inside the core, which resulted in concomitant shift of absorption and PL spectra. Since ZnS has a wider band gap of 3.7 eV than CZIS core NCs, if it is that ZnS diffuses into core materials instead of ZnS shell formation, the band gap of the NCs will become larger, and thus lead to a similar blue-shift trend in both UVevis absorption edge and PL peak of QDs [31]. However, differences from the reports mentioned above, the shift in the absorption spectra here was much less pronounced in comparison with the large shift of PL spectra. It can be seen from the absorption spectra in Fig. 1b, although there is no obvious absorption peak, the CZIS/ZnS NCs have an absorption edge at around 560 nm, which shows only a slight blue-shift compared with that of CZIS NCs. Therefore, Zn diffusion becomes a secondary reason while core/shell structure makes greater contribution here. Furthermore, the ZnS overcoating offers a dramatic improvement on PL QY, making the fluorescence QY of CZIS NCs increase from 21.3 % to 77.0 %.
3.2. Influence of synthetic parameters in the shell growth stage on optical properties of CZIS/ZnS core/shell NCs As mentioned above, the optical properties especially PL QY of CZIS NCs was greatly improved after ZnS shell formation. It is known that CZIS/ZnS NCs belong to Type-1 QDs whose optical properties are strongly dependent on the synthetic parameters in the shell growth stage [33]. Therefore, we systematically discussed the influence of experimental variables in the shell growth stage on the optical properties (PL peak position and PL QY) of CZIS/ZnS QDs in this work, such as shell growth time and temperature, and Zn concentration. In the investigation of the influence of different experimental variables on the optical properties of the obtained core/shell NCs, all other experimental variables are kept the same in the experimental section and only the studied variable is changed. The procedures for synthesis of Zn stock solution (Fig. 2a) and CZIS core NCs and CZIS/ZnS core/shell NCs (Fig. 2b) could be seen in Fig. 2. 3.2.1. Effect of shell growth time After Zn stock solution containing 2 mmol of Zn2þ is injected into the crude CZIS core NCs for shell growth, aliquots were taken at different time intervals to monitor the reaction. Fig. 3 shows the evolution of absorption and PL spectra of CZIS/ZnS NCs with shell growth time. As can be seen from Fig. 3a, the absorption shoulders of all NC samples are located at similar wavelength, revealing that the band gap of crude core NCs undergo quite minor changes after
Fig. 2. Synthetic process for synthesis of (a) Zn stock solution; (b) CZIS NCs and CZIS/ZnS NCs.
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Fig. 3. (a) absorption and (b) emission spectra of CZIS/ZnS QDs prepared with different shell growth time.
Zn was injected into the core NCs, which confirms the formation of ZnS shell and makes cation exchange between Zn2þ, Cuþ and In3þ to be a secondary reason. As shown in the PL emission spectra in Fig. 3b, the PL peak blue-shift from 643 to 579 nm as the shell growth time increases from 30 s to 120 min. During the first 15 min, the PL spectra experience a 53 nm of blue-shift, which is 82.8% of the total blue-shift amount. In the following 30 min, the PL peak blue shifts further from 590 to 579 nm, finishing the whole blueshift process; and then remain unchanged with longer shell growth time. The blue-shift of PL spectra may be attributed to several factors: (i) minor cation exchange between Zn2þ and Cuþ or In3þ, especially in the first 15 min [28,29]; (ii) ZnS shell formation [32] through Zn2þ accumulation on the surface of core CZIS NCs following cation exchange [18]; (iii) size/shape homogenization after surface modification. It is known that the PL spectra of semiconductor NCs usually shows red-shifted characteristics when the size/shape of NCs is highly inhomogeneous [33,34]. The morphology and crystal structure of CZIS NCs and their corresponding core/shell NCs prepared with ZnS shell growth for 1 min, 15 min and 45 min were examined by TEM and XRD. The TEM images demonstrate that the uniformity of NCs is improved together with an increasing size by extending shell growth time (Table 1), which is in accordance with the phenomena in PL spectra, as shown in Fig. 4aed. An increasing of 0.38 nm is found after 45 min of shell growth, which means roughly one monolayer of ZnS shell is formed [7,12,16]. The SAED pattern of CZIS/ZnS with a 45 min of shell growth time (Fig. 4e) can be indexed to a facecentered cubic (FCC) phase, which is in accordance with the result of corresponding XRD [22]. As shown in Fig. 4g, XRD patterns of the samples indicate that they all have an FCC crystal structure. Besides, XRD peaks become stronger with the trend of shifting to larger angles when the shell growth time is prolonged, indicating better crystallinity of the samples and gradual increasing in the amount of Zn which may result from the slight cation exchange and Zn ions accumulation on the surface of core CZIS NCs [18,35]. High crystallinity of CZIS/ZnS NCs with ZnS shell coating could also be confirmed by HRTEM in Fig. 4f, where clear lattice fringe throughout the particle could be seen. It is noted that the distance between adjacent lattice fringes is 3.13 Å, fitting well with the interplanar distance of (111) plane of cubic phase ZnS (JCPDS 659585). As mentioned above, being one of the most important optical properties of NCs, QY is affected by shell growth time. Table 2 list the QY value of all samples as a function of shell growth time. PL QY
of the bare CZIS cores is only 21.3%. However, it begins to increase significantly at the same time when ZnS shell start forming. After 15 min of shell growth, PL QY of the NCs reaches the maximum of 77% and then drops to 63.0% after 30 min. In order to explain this phenomenon, further understanding about the optical mechanism that controls QY is necessary. Therefore, PL decay curves were measured, as shown in Fig. 5. The PL curves of all samples can be fitted well by a triexponential function (1):
IðtÞ ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ þ A3 expðt=t3 Þ
(1)
Where ti represents the decay time of PL emission; Ai represents the relative weight of the decay components at t ¼ 0 [27,36]. As shown in Table 2, the relative weight A1 of fast decay component t1 keeps decreasing from 25.7 % to 9.3 % at the first 15 min, and then increases to 11.1% in the next 30 min. This variation trend is just the opposite of QY. According to the previous research, the fast decay component t1 is related to nonradiative recombination associated with surface defects [10,30,37,38]. Bare core CZIS NCs contain plenty of surface dangling bonds which can act as trap states for charge carriers. However, the formation of ZnS shell provides an efficient passivation of the surface trap states (Fig. 6), thus giving rise to a strongly enhanced QY which is seen at the first 15 min [33]. The decrease of QY when the shell growth time is further prolonged might be related to the following reasons: (i) more defects are induced due to an overlong shell growth time. According to classical nucleation theory, the growth of NCs is a dynamic equilibrium between the monomers and nucleus and is a diffusion-controlled process [39,40,42]. An increasing in temperature for further shell growth (refer to experimental section) accelerate the growth rate of NCs. This accelerated growth rate leads to more defects when the reaction time is long enough although it make contributions to shell formation at the beginning; (ii) when the reaction time is furthermore prolonged, the ZnS shell becomes too thick (confirmed by the increase in NC size from
Table 1 Composition (from EDS), average size (from TEM) and emission wavelength (from PL spectra) of CZIS/ZnS NCs prepared with different shell growth time. Shell growth time (min)
Composition
Size (nm)
0 (CZIS core) 1 15 45
Cu7.70Zn1.83In0.86S2 Cu5.21Zn3.27In0.80S2 Cu2.11Zn3.47In0.82S2 Cu1.42Zn3.80In0.86S2
2.69 2.74 2.98 3.07
± ± ± ±
0.81 0.65 0.54 0.40
l (nm) 657 641 590 579
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Fig. 4. (a) TEM image of CZIS core NCs and CZIS/ZnS NCs with shell growth time of (b) 1 min; (c) 15 min; (d) 45 min; (e) SAED pattern and (f) HRTEM image of CZIS/ZnS NCs with shell growth time of 45 min; (g) the corresponding XRD patterns of CZIS/ZnS NCs with different shell growth time.
Table 1). With increasing of shell thickness, the interface strain accumulates dramatically because lattice mismatch between the CZIS core and ZnS shell material, thus resulting in the deterioration of QY of the samples [32,35,41]. The chemical structure of CZIS/ZnS NCs prepared with different shell growth time was determined by STEM-EDS. As listed in Table 1, the indium content almost remain unchanged while a significant decrease in copper content together with an increase in zinc content can be easily noticed when the shell growth time is prolonged. The increase in Zn content probably is attributed to the following two factors. Firstly, it could be a result of slight cation exchange between Zn2þ and Cuþ or In2þ. It is known that the growth of NCs is a diffusion-controlled growth process which allows cation exchange in nanometer-sized particles because of effective reaction barrier [27,28,30,42]. The preferential exchange
between Zn2þ and Cuþ instead of Zn2þ and In3þ may be due to the fact that CueS bond is much weaker than that of IneS bond and Zn substitution of Cu was shown to be energetically favorable [11,30,43]. Secondly, it could be a result of Zn2þ occupation on the surface of core materials. As Zn2þ injected in the solution is excessive for shell growth, more and more Zn2þ ions accumulate on the CZIS core NCs with increasing of reaction time, leading to further deposition of ZnS shell on the surface of core NCs [18]. The additional ZnS shell can also be convinced by the increase in the size of NCs (Table 1). Average diameter of CZIS NCs changes from 2.69 to 3.07 nm after 45 min of treatment, suggesting that roughly one monolayer of ZnS shell was formed during shell formation process. Fig. 6 is given to further illustrate the whole cation exchange and Zn accumulation process.
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Table 2 QY and fitting results of PL curves of CZIS/ZnS NCs prepared with different shell growth time, derived from Eq. (1). Shell growth time (min)
QY (%) A1 (%) t1(ns) A2 (%) t2(ns) A3 (%) t3(ns) tav(ns)
0 (core) 1 5 15 45
21.3 51.1 63.3 77.0 63.0
25.7 14.5 9.7 9.3 11.1
25.3 31.2 32.5 39.5 19.9
57.2 61.1 64.7 66.1 64.8
150.8 176.4 178.2 180.7 171.0
15.0 20.8 22.1 21.1 21.0
673.7 607.4 573.8 564.8 550.8
419.2 401.8 380.8 372.0 361.6
Fig. 7. XRD patterns of CZIS/ZnS NCs obtained by adding 0.25 mmol, 0.5 mmol, 1 mmol, and 2 mmol of Zn in CZIS core NCs for the growth of ZnS shell.
Fig. 5. PL lifetime of decays of CZIS/ZnS samples with different shell growth time.
the increasing of Zn amount for shell growth from 0.25 to 2 mmol, indicating the decrease of lattice constants of the NCs [7,35,46]. When the amount of Zn added in the second stage for shell growth increased from 0.25 mmol to 2 mmol, the PL spectra witnessed a systematic blue-shift from 674 to 579 nm (Fig. 8) while the PL QY increased from 13.6 % to 63.0 % (Fig. 9a). The blue shift of PL spectra in combination with the increasing of QY may be the result of the following two reasons: (i) As feed amount of Zn for shell growth increases, Zn concentration in the solution becomes higher, thus leading to accelerated the interdiffusion of Zn ions into CZIS core NCs [39,40,42], which is in accordance with EDS results shown in Table 1; (ii) A denser ZnS shell is formed. Higher Zn concentration in the solution contributes to ion accumulation on the surface of CZIS cores (Fig. 6), which leads to further deposition of ZnS shell and provides a more efficient passivation of the surface trap states, and this can be confirmed by the fitting results of PL curves (Fig. 10). 3.2.3. Effect of shell growth temperature It is found that PL QY and PL peak position of the resulting CZIS/ ZnS NCs is strongly dependent on the shell growth temperature. As shown in Fig. 9b, QY of CZIS/ZnS NCs rise from 22.6 % to 63.0 % when
Fig. 6. ZnS shell formation process that includes elimination of dangling bonds, slight cation exchange between Zn2þ, Cuþ (In3þ) and Zn occupation on the surface of CZIS cores. Cation exchange between Zn2þ and Cuþ instead of Zn2þ and In3þ is preferred because CueS bond is weaker than IneS bond.
3.2.2. Effect of Zn amount added in the shell growth stage Preparation of CZIS/ZnS NCs is based on the experimental section. The only difference between all the samples is that different amount of Zn is used for shell growth (Fig. 2). The phase purities of CZIS/ZnS NCs synthesized with 2 mmol, 1 mmol, 0.5 mmol, 0.25 mmol of Zn added in the shell growth stage were confirmed by XRD (Fig. 7). The characteristic peaks of all the samples were located between the corresponding distinct peaks of Cu0.4In0.4Zn0.2S (JCPDS 47-1370) and ZnS (JCPSD 65-9585), which implies the ZnS shell formation on CZIS NCs [44,45]. Furthermore, the diffraction peaks gradually shifted towards larger angles with
Fig. 8. Absorption and emission spectra of CZIS/ZnS NCs prepared with different Zn amount at shell growth stage.
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Fig. 9. PL emission wavelength and QY of CZIS/ZnS NCs obtained (a) by adding different amount of Zn (0.25 mmol, 0.5 mmol, 1 mmol, and 2 mmol) in the shell growth stage; (b) at different shell growth temperature (130 C, 190 C, 240 C, 280 C).
the shell growth temperature increased from 130 to 240 C. Meanwhile, PL peak shows a blue-shift from 644 to 579 nm. According to the previous investigation [33,39], the formation of NCs from the precursor is endothermic, the elevated shell growth temperature leads to a more thoroughly shell formation reaction and a higher extent of Zn interdiffusion (Fig. 11), giving rise to an enhanced QY. However, as mentioned above, exceeding temperature may induce more defects, thus causing the deterioration of optical properties, especially the QY of NCs (Fig. 9b) when the temperature increased from 240 to 280 C. It is also found that PL peak undergo a slight red shift during the process, which might be the result of an increased size of core NCs [12]. Phase purity of the as-prepared CZIS/ZnS NCs prepared with different reaction temperature is also confirmed by XRD. As shown in Fig. 12, the CZIS/ZnS NCs have similar diffraction patterns although they are prepared with different shell growth temperature. Furthermore, the continuous peak-shift revealed that no phase separation or separated nucleation occurred in the synthetic process. Fig. 10. PL lifetime of decays of CZIS/ZnS samples with different Zn amount at shell growth stage.
4. Conclusions In summary, a facile approach for the synthesis of core/shell CZIS/ZnS NCs with highest QY of 77% is reported. The influences of
Fig. 11. The ratio between Zn: (Cu þ In) as a function of shell growth temperature.
Fig. 12. XRD patterns of CZIS/ZnS NCs obtained under different shell growth temperature.
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synthetic parameters including shell growth time and temperature, and Zn amount for shell growth stage on optical properties are studied systematically. It is found that there is a remarkable blueshift of PL peak from 657 to 579 nm and the QY of CZIS NCs increases from 21.3 % to 63.0 % after the growth of ZnS shell. As the shell growth time increases from 30 min to 120 min, the PL peak shifts from 643 to 579 nm and PL QY of the NCs reaches the maximum of 77% after 15 min of shell growth. When the shell growth temperature increases from 130 to 240 C, the QY rises from 22.6 % to 63.0 % and PL peaks shifts from 644 to 579 nm. When the amount of Zn increases from 0.25 to 2.00 mmol, the PL spectra shift from 674 to 579 nm and the PL QY increase from 13.6% to 63.0%. It is also suggested that ZnS shell formation offer an efficient way to eliminate nonradiative channels associated with dangling bonds and surface traps. And the shell formation process of our NCs is a combination of slight cation exchange between Zn2þ and Cuþ (In3þ) with Zn2þ ions occupation on the surface of core CZIS NCs. Acknowledgments The authors acknowledge the financial support from National Natural Sciences Foundation of China (Grant nos. 50972107 and 51272059) and Key foundation of Zhejiang Province Key Technology Innovation Team (2009R50010).
[19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
References
[29]
[1] J.M. Caruge, J.E. Halpert, V. Wood, V. Bulovic, M.G. Bawendi, Colloidal quantum-dot light-emitting diodes with metal-oxide charge transport layers, Nat. Photonics 2 (2008) 247. [2] D.V. Talapin, J.S. Lee, M.V. Kovalenko, E.V. Shevchenko, Prospects of colloidal nanocrystals for electronic and optoelectronic applications, Chem. Rev. 110 (2010) 389. [3] Y. Shirasaki, G.J. Supran, M.G. Bawendi, V. Bulovic, Emergence of colloidal quantum-dot light-emitting technologies, Nat. Photonics 7 (2013) 13. [4] J.S. Zhong, Q.Y. Wang, X.H. Xu, Photodeposition of CdS nanoparticles sensitized TiO2 nanotube arrays for enhanced photoelectrochemical performance, J. Electrochem. Soc. 161 (2014) H656. [5] Z. Pan, H. Zhang, K. Cheng, Y. Hou, J. Hua, X. Zhong, Highly efficient inverted type-I CdS/CdSe core/shell structure QD-sensitized solar cells, ACS Nano 6 (2012) 3982. [6] D.Q. Chen, P. Huang, Highly intense upconversion luminescence in Yb/Er: NaGdF4@NaYF4 core-shell nanocrystals with complete shell enclosure on core, Dalton Trans. 43 (2014) 11299. [7] K. Bourzac, Quantum dots go on display, Nature 493 (2013) 283. [8] D. Chen, F. Zhao, H. Qi, M. Rutherford, X. Peng, Bright and stable purple/blue emitting CdS/ZnS core/shell nanocrystals grown by thermal cycling using a single-source precursor, Chem. Mater. 22 (2010) 1437. [9] X. Wang, J. Zhuang, Q. Peng, Y. Li, A general strategy for nanocrystals synthesis, Nature 437 (2005) 121. [10] T. Pons, E. Pic, N. Lequeux, E. Cassette, L. Bezdetnaya, F. Guillemin, F. Marchal, B. Dubertret, Cadmium-free CuInS2/ZnS quantum dots for sentinel lymph node imaging with reduced toxicity, ACS Nano 4 (2010) 2531. [11] L. Li, A. Pandey, D.J. Werder, B.P. Khanal, J.M. Pietryga, V.I. Klimov, Efficient synthesis of highly luminescent copper indium sulfide-based core/shell nanocrystals with surprisingly long-lived emission, J. Am. Chem. Soc. 133 (2011) 1176. [12] R. Xie, M. Rutherford, X. Peng, Formation of high-quality I-III-VI semiconductor nanocrystals by tuning reactive reactivity of cationic precursors, J. Am. Chem. Soc. 131 (2009) 5691. [13] J. Kolny-Olesiak, H. Weller, Synthesis and application of colloidal CuInS2 semiconductor nanocrystals, ACS Appl. Mater. Interfaces 5 (2013) 12221. [14] H. Zhong, Z. Bai, B. Zou, Tuning the luminescence properties of colloidal I-III-VI semiconductor nanocrystals for optoelectronics and biotechnology applications, J. Phys. Chem. Lett. 3 (2012) 3167. [15] W. Zhang, X. Zhong, Facile synthesis of ZnS- CuInS2-alloyed nanocrystals for a color-tunable fluochrome and photocatalyst, Inorg. Chem. 50 (2011) 4065. [16] D. Deng, Y. Chen, J. Cao, J. Tian, Z. Qian, S. Achilefu, Y. Gu, High-quality CuInS2/ ZnS quantum dots for in vitro and in vivo bioimaging, Chem. Mater. 24 (2012) 3029. [17] E.P. Jang, W.-S. Song, K.-H. Lee, H. Yang, Preparation of a photo-degradationresistant quantum dot-polymer composite plate for use in the fabrication of a high-stability white-light-emitting diode, Nanotechnology 24 (2013) 045607. [18] W. Guo, N. Chen, Y. Tu, C. Dong, B. Zhang, C. Hu, J. Chang, Synthesis of Zn-CuIn-S/ZnS core/shell quantum dots with inhibited blue-shift
[30] [31] [32]
[33] [34]
[35]
[36]
[37] [38]
[39]
[40]
[41]
[42] [43]
[44]
[45]
[46]
photoluminescence and applications for tumor targeted bioimaging, Theranostics 3 (2013) 99. Z.B. Zhuang, Q. Peng, B.C. Zhang, Y. Li, Controllable synthesis of Cu2S nanocrystals and their assembly into a supperlattice, J. Am. Chem. Soc. 130 (2008) 10482. L.S. Li, N. Pradhan, Y. Wang, X. Peng, Hight quality ZnSe and ZnS nanocrystals formed by activating zinc carboxylate precursors, Nano Lett. 4 (2004) 2261. S. Li, Y. Chen, L. Huang, D. Pan, Simple continuous-flow synthesis of Cu-In-ZnS/ZnS and Ag-In-Zn-S/ZnS core/shell quantum dots, Nanotechnology 24 (2013) 395705. H. Shen, H. Yuan, W. Feng, X. Bai, C. Zhou, H. Wang, T. Lu, Z. Qin, L. Ma, L.S. Li, Facile synthesis of high-quality CuInZnxS2þx core/shell nanocrystals and their application for detection of C-reactive protein, J. Mater. Chem. 22 (2012) 18623. W. Song, H. Yang, Efficient white-light-emitting diodes fabricated from highly fluorescent copper indium sulfide core/shell quantum dots, Chem. Mater. 24 (2012) 1961. B. Chen, Q. Zhou, J. Li, F. Zhang, R. Liu, H. Zhong, B. Zou, Red-emissive CuInS2based nanocrystals: a potential phosphor for warm white light-emitting diodes, Opt. Express 21 (2013) 10105. R. Zhang, P. Yang, Y. Wang, Facile synthesis of CuInS2-ZnS quantum dots with highly near-infrared photoluminescence via phosphor-free process, J. Nanopart Res. 15 (2013) 1910. K. Yu, P. Ng, J. Ouyang, M.B. Zaman, A. Abulrob, T.N. Baral, D. Fatehi, Z.J. Jakubek, D. Kingston, X. Wu, X. Liu, C. Hebert, D.M. Leek, D.M. Whitfield, Low-temperature approach to highly emissive copper indium sulfide colloidal nanocrystals and their bioimaging application, ACS Appl. Mater. Interfaces 5 (2013) 2870. W. Xiang, H. Yang, X. Liang, J. Zhong, J. Wang, L. Luo, C. Xie, Direct synthesis of highly luminescent Cu-Zn-In-S quaternary nanocrystals with tunable photoluminescence spectra and decay times, J. Mater. Chem. C 1 (2013) 2014. J. Zhang, R. Xie, W. Yang, A simple route for highly luminescent quaternary Cu-Zn-In-S nanocrystals emitters, Chem. Mater. 23 (2011) 3357. J. Park, S.-W. Kim, CuInS2/ZnS core/shell quantum dots by cation exchange and their blue-shifted photoluminescence, J. Mater. Chem. C 21 (2011) 3745. L.D. Trizio, M. Prato, A. Genovese, A. Casu, M. Povia, R. Simonutti, J.P. Marcelo, C. D'Andrea, F. Tassone, L. Manna, Chem. Mater. 24 (2012) 2400. J. Feng, M. Sun, F. Yang, X. Yang, A facile approach to synthesize high-quality ZnxCuyInS1.5þxþ0.5y nanocrystals emitters, Chem. Commun. 47 (2011) 6422. R.K. Capek, K. Lambert, D. Dorfs, Synthesis of extremely small CdSe and bright blue luminescent CdSe/ZnS nanoparticles by a prefocused hot-injection approach, Chem. Mater. 21 (2009) 1743. P. Reiss, M. Protiere, L. Li, Core/shell semiconductor nanocrystals, Small 5 (2009) 154. J.S. Zhong, J.R. Xu, Q.Y. Wang, Nitrogen and vanadium Co-doped TiO2 mesosponge layers for enhancement in visible photocatalytic activity, Appl. Surf. Sci. 315 (2014) 131. R. Xie, U. Kolb, J. Li, T. Basche, A. Mews, Synthesis and characterization of highly luminescent CdSe-core-CdS/Zn0.5Cd0.5/ZnS multishell nanocrystals, J. Am. Chem. Soc. 127 (2005) 7480. H. Zhong, Y. Zhou, M. Ye, Y. He, J. Ye, C. He, C. Yang, Y. Li, Controllable synthesis and optical properties of colloidal ternary chalcogenide CuInS2 nanocrystals, Chem. Mater. 20 (2008) 6434. X. Tang, W.B.A. Ho, J.M. Xue, Synthesis of Zn-doped AgInS2 nanocrystals and their fluorescence properties, J. Phys. Chem. C 116 (2012) 9769. T.K.C. Tran, Q.P. Le, Q.L. Nguyen, L. Li, Time-resolved photoluminescence study of CuInS2/ZnS nanocrystals, Adv. Nat. Sci. Nanosci. Nanotechnol. 1 (2010) 025007. C. de M. Donega, P. Liljeroth, D. Vanmaekelbergh, Physicochemical evaluation of hot-injection method, a synthesis route for monodisperse nanocrystals, Small 1 (2005) 1152. S.G. Kwon, Y. Piao, J. Park, S. Angappane, Y. Jo, N.-M. Wang, J.-G. Park, T. Hyeon, Kinetics of monodisperse iron oxide nanocrystals formation by “heating-up” process, J. Am. Chem. Soc. 129 (2007) 12571. X. Wang, R. Liu, T. Wang, B. Wang, Y. Xu, H. Wang, Dual roles of ZnS thin layers in significant photocurrent enhancement of ZnO/CdTe nanocable arrays photoanode, ACS Appl. Mater. Interfaces 5 (2013) 3312. J. Park, J. Joo, S.G. Kwon, Y. Jang, T. Hyeon, Synthesis of monodisperse spherical nanocrystals, Angew. Chem. Int. Ed. 46 (2007) 4630. T. Yamamoto, I.V. Luck, R. Scheer, H. Katayama-Yoshida, Differences in the electronic structure and compensation mechanism between n-type Zn- and Cd-doped CuInS2 crystals, Physca B 274 (1999) 927. T. Torimoto, T. Adachi, K. Okazaki, M. Sakuraoka, T. Shibayama, B. Ohtani, A. Kudo, S. Kuwabata, Facile synthesis of ZnS-AgInS2 solid solution nanoparticles for a color-adjustable luminphore, J. Am. Chem. Soc. 129 (2007) 12388. Y.K. Kim, S.-H. Ahn, K. Chung, Y.-S. Cho, C.-J. Choi, The photoluminescence of CuInS2 nanocrystals: effect of non-stoichiometry and surface modification, J. Mater. Chem. 22 (2012) 1516. H. Shen, H. Wang, X. Li, J. Niu, H. Wang, X. Chen, L. Li, Phosphine-free synthesis of high quality ZnSe, ZnSe/ZnS, and Cu-, Mn-doped ZnSe nanocrystals, Dalton Trans. (2009) 10534.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Facile synthesis and characterization of core/shell CueIneZneS/ZnS nanocrystals with high luminescence Weidong Xiang a, *, Xin Ma a, Le Luo b, Wen Cai c, Cuiping Xie b, Xiaojuan Liang a a
School of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, China College of Materials Science and Engineering, Tongji University, Shanghai 201804, China c Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China b
h i g h l i g h t s We report a simple one-pot two-step approach to synthesize core/shell CueZneIneS/ZnS nanocrystals (CZIS/ZnS NCs). The as-prepared CZIS/ZnS NCs were investigated by XRD, SAED, EDS, TEM and HR-TEM. The optical properties of the CZIS/ZnS NCs were measured by UVevis, PL spectra and QY. The photoluminescence mechanism of the CZIS/ZnS NCs was investigated by PL decay curves. The possible shell formation process of CZIS/ZnS NCs was proposed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2014 Received in revised form 24 October 2014 Accepted 26 October 2014 Available online 4 November 2014
CueZneIneS/ZnS (CZIS/ZnS) core/shell nanocrystals (NCs) with high quality have been successfully synthesized via a one-pot two-step method. The photoluminescent properties including emission spectra, photoluminescence quantum yield (PL QY) and PL decay time of the obtained CZIS/ZnS NCs could be tuned by controlling the growth process of ZnS shell. The effect of the amount of Zn added on the growth of ZnS shell was investigated. The shell growth temperature and time for CIZS/ZnS NCs were discussed in detail. A relatively high PL QY of 77% could be achieved under certain reaction condition after systematic study. The crystal structure, morphology, size distribution and chemical composition of CZIS/ZnS NCs were determined by X-ray Diffraction (XRD), selected-area electron diffraction (SAED), lowand high-resolution transmission spectra (TEM) and STEM-EDS, respectively. The photoluminescence mechanisms of the CZIS/ZnS NCs were discussed by time-resolved photoluminescence. © 2014 Elsevier B.V. All rights reserved.
Keywords: Semiconductor Nanostructures Chalcogenides Photoluminescence spectroscopy Chemical synthesis
1. Introduction In recent years, semiconductor nanocrystals (NCs) or quantum dots (QDs) have attracted increasing attention due to their unique optical properties, which show a good prospective application such as solar cells, biomedical labeling, light emitting diodes (LEDs), etc [1e6]. Recently, Sony has announced its latest QD-embedded flatscreen television for the first time [7] which made the study on QDs hotter than ever before. Preparation and characterization of IIeVI and IVeVI type QDs had been focused owning to their excellent optical and electrical properties. However, large scale applications of QDs are strongly restricted because of containing toxic heavy
* Corresponding author. E-mail address: [email protected] (W. Xiang). http://dx.doi.org/10.1016/j.matchemphys.2014.10.042 0254-0584/© 2014 Elsevier B.V. All rights reserved.
metals, cadmiums, lead, etc. [8,9]. In comparison with IIeVI or IVeVI type QDs, IeIIIeVI QDs are less toxic and more biocompatible. For instance, CuInS2 (CIS), a direct band gap semiconductor with high absorption coefficient is of great interest during the past five years. CIS QDs have a lot of advantages such as markedly low toxicity, large stokes shifts, long photoluminescence (PL) lifetime and sizetunable emissions. However, it has a severe drawback: the quantum yield (QY) is usually less than 10%, which is too low for further use [10e12]. Many great contributions has been made to solving this problem, for example, surface modification such as ZnS shell coating is an effective method [13,14]. An early example was given in 2009 by Xie et al. who had increased the CIS QY from 3 % to 30 % successfully through overcoating one monolayer of ZnS shell [12]. Zhang et al. prepared CueZneIneS (CZIS) by using acetate salts of corresponding metals as precursors in noncoordinating solvent in
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the presence of DDT. The plain CZIS core showed quite low QY (below 3%), but after ZnS shell growth, the maximum QY was increased to 56% [15]. Later, in 2012, Deng et al. synthesized CIS/ZnS QDs with roughly 3e4 monolayers of ZnS which had a relatively high QY reaching 80%, using acetate salts of the corresponding metals as starting materials [16]. Jang et al. reported highly luminescent double-ZnS-shell-capped CIS QDs with an extremely high QY of 92% and utilized them as color converter for the fabrication of a QD-based WLED. After covering CIS cores with two monolayers of ZnS shell, a notable blue-shift from 660 nm to 559 nm was observed in the PL emission spectrum [17]. Very recently, Guo et al. reported CZIS/ZnS QDs showing inhibitory blue-shift throughout ZnS shell growth process and proposed a hypothesis to explain this phenomenon; the maximum QY of the as-prepared QDs was 50% [18]. From the above, it could be seen that considerable progress has been made until now, however, some challenging problems still remain to be solved: (1) Although ZnS shell coating has been studied extensively, most of these reports are on the basis of ternary CIS cores while quite rare efforts could be seen in the quaternary CZIS cores. One of the possible reasons is that it is more difficult to prepare quaternary QDs because the metal precursors have different chemical reactivties, which increases the difficulty in making a balance between them so as to prevent unexpected nucleation or phase separation [12,19,20]. (2) Among most of previous studies, QY in the quaternary CZIS/ZnS system is not so high (usually below 60%) as that in the ternary CIS/ZnS system (above 75%) [21e24]. Further exploration in improving QY in the quaternary CZIS/ZnS system and analyzing factors influencing optical properties during shell growth process is still recommended. (3) In view of the precursors chosen, long-chain metal carboxylates were used predominantly in most of the high QY CZIS/ZnS or CIS/ZnS QDs. In contrast, inorganic metal salts such as CuCl and CuI attracted less attention because of higher chemical reactivity, which would lead to much faster nucleation and growth rates, and thus made process controlling more difficult [25e27]. Herein, quaternary CZIS QDs were synthesized by using inexpensive chloride salts of corresponding metals as precursor firstly and then they were covered with ZnS shell. Synthetic variables in the shell growth stage (i.e., shell growth temperature, Zn concentration and shell growth time) which would affect optical properties, especially QY were discussed systematically in this work. Furthermore, the changing mechanism of photoluminescence properties was analyzed by using PL decay curves. 2. Experimental 2.1. Materials Copper chloride (CuCl, 99.7%), indium chloride (InCl3$4H2O, 99.7%), zinc chloride (ZnCl2, 99.7%), n-dodecanethiol (DDT, 98%), oleic acid (OA, 98%), and sulfur powder (S, 99.5%) were purchased from Sinopharm. Oleylamine (OAm, 80e90 %) and 1-octadecene (ODE, 90%) were purchased from Alladdin. All chemicals were used without further purification.
0.8 mmol of sulfur powder dispersed in 2 ml oleylamine (OAm) was quickly injected into the reaction solution and the temperature dropped to 160 C, then the mixture solution was kept for 10 min at this temperature to allow the growth of CZIS core NCs. 2.3. Preparation of the Zn stock solution for shell growth For a typical synthetic reaction, Zn stock solution was prepared by mixing zinc chloride (0.2726 g, 2 mmol), OAm (1 ml) and ODE (4 ml) in a three-neck reaction flask. The reaction mixture was heated to 90 C under an argon atmosphere with magnetic stirring and kept at this temperature for 30 min. The flask was then heated to 150 C for another 10 min and a clear solution was formed. After that, the reaction mixture was kept at 50 C for further use. In this work, four different Zn amount, i.e., 2 mmol, 1 mmol, 0.5 mmol, and 0.25 mmol were conducted while the amount of OAm and ODE were fixed. 2.4. Synthesis of CZIS/ZnS core/shell NCs The synthesized CZIS core NCs growth solution was used directly without intermediate purification step. The ZnS coating was accomplished by injecting the Zn stock solution into the QD growth solution with a syringe. After Zn stock solution which contains 2 mmol of Zn2þ was injected into CZIS core NCs growth solution, the temperature was set as 240 C for 45 min for the shell growth in this work. 2.5. Characterization XRD measurements were performed on a Bruker D8 Advance Xray diffractometer (40 kV, 40 mA) with monochromatic Cu Ka radiation (l ¼ 1.5406 Å) and a scan rate of 5 min1 in the 2q range of 10e70 . X-ray diffraction samples were prepared by depositing the nanocrystals on a Si (100) wafer. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were carried out on a JEM-2010HR transmission electron microscope at an acceleration voltage of 200 kV. Field-emission scanning electron microscope (FESEM) images were taken on a JEOL-JSM-6700F scanning electron microscope (10 kV) equipped with an X-ray energy dispersive spectroscope (EDS) at an accelerating voltage of 20 kV. UVeVis absorption spectra were measured with a HITACHI U-4100 spectrophotometer. PL spectra, PL quantum yield and PL decay spectra were recorded with a Horiba Jobin Yvon Fluromax-4P spectrophotometer equipped with absolute quantum yield measurement apparatus and a time-correlated single-photocounting (TCSPC) spectrometer. In the PL decay spectra measurements, the probing wavelength for PL decay was set to the wavelength of the PL at its maximum intensity. Samples were excited at 450 nm by a pulsed xenon lamp at a 1 MHz repetition rate, and PL was detected by using a spectrally resolved, TCSPC spectrometer. 3. Results and discussions 3.1. Synthesis and characterizations of CZIS/ZnS core/shell structure
2.2. Synthesis of CZIS core NCs For the synthesis of CZIS NCs, CuCl (0.0040 g, 0.04 mmol), indium chloride (0.0587 g, 0.2 mmol), zinc chloride (0.0273 g, 0.2 mmol), oleic acid (0.06 ml), n-dodecanethiol (DDT, 0.25 ml) and 1-octadecene (ODE, 8 ml) were loaded into a three-neck reaction flask, which was then degassed at 90 C for 30 min and purged with argon. The flask was heated to 180 C for another 5 min. During the heating-up process, the solution remained clear while its color changed gradually from colorless to slight yellow. After that,
Bare core CZIS and its corresponding CZIS/ZnS core/shell NCs were prepared. In a typical synthesis process for ZnS shell growth, 2 mmol of Zn was injected into the crude solution containing CZIS core NCs, then the temperature was maintained at 240 C. Fig. 1a shows the XRD pattern of CZIS NCs and their corresponding core/ shell CZIS/ZnS NCs. The XRD peaks of CZIS NCs were quite broad, suggesting the obtained NCs were small in size and had poor crystallinity due to low synthesis temperature. Upon ZnS shelling, the patterns of the cubic lattice is maintained, but the diffraction
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Fig. 1. (a) XRD patterns and (b) Absorption, emission spectra of CZIS QDs before and after the coating of ZnS shell.
peaks become sharper and shift to larger angles, which is consistent with the smaller lattice constant for ZnS compared with CZIS. This could be attributed to the combined effect of a suitable ZnS shell formation on the CZIS core NCs and a slight cation exchange between CZIS cores and Zn2þ [8,28]. Fig. 1b shows the absorption and PL emission spectra of CZIS NCs before and after covering ZnS shell. A remarkable blue-shift of PL peak from 657 to 579 nm is found after the growth of ZnS shell, which is consistent with previous reports [29,30]. These reports suggested that the blue-shift originate from cation exchange occurred inside the core, which resulted in concomitant shift of absorption and PL spectra. Since ZnS has a wider band gap of 3.7 eV than CZIS core NCs, if it is that ZnS diffuses into core materials instead of ZnS shell formation, the band gap of the NCs will become larger, and thus lead to a similar blue-shift trend in both UVevis absorption edge and PL peak of QDs [31]. However, differences from the reports mentioned above, the shift in the absorption spectra here was much less pronounced in comparison with the large shift of PL spectra. It can be seen from the absorption spectra in Fig. 1b, although there is no obvious absorption peak, the CZIS/ZnS NCs have an absorption edge at around 560 nm, which shows only a slight blue-shift compared with that of CZIS NCs. Therefore, Zn diffusion becomes a secondary reason while core/shell structure makes greater contribution here. Furthermore, the ZnS overcoating offers a dramatic improvement on PL QY, making the fluorescence QY of CZIS NCs increase from 21.3 % to 77.0 %.
3.2. Influence of synthetic parameters in the shell growth stage on optical properties of CZIS/ZnS core/shell NCs As mentioned above, the optical properties especially PL QY of CZIS NCs was greatly improved after ZnS shell formation. It is known that CZIS/ZnS NCs belong to Type-1 QDs whose optical properties are strongly dependent on the synthetic parameters in the shell growth stage [33]. Therefore, we systematically discussed the influence of experimental variables in the shell growth stage on the optical properties (PL peak position and PL QY) of CZIS/ZnS QDs in this work, such as shell growth time and temperature, and Zn concentration. In the investigation of the influence of different experimental variables on the optical properties of the obtained core/shell NCs, all other experimental variables are kept the same in the experimental section and only the studied variable is changed. The procedures for synthesis of Zn stock solution (Fig. 2a) and CZIS core NCs and CZIS/ZnS core/shell NCs (Fig. 2b) could be seen in Fig. 2. 3.2.1. Effect of shell growth time After Zn stock solution containing 2 mmol of Zn2þ is injected into the crude CZIS core NCs for shell growth, aliquots were taken at different time intervals to monitor the reaction. Fig. 3 shows the evolution of absorption and PL spectra of CZIS/ZnS NCs with shell growth time. As can be seen from Fig. 3a, the absorption shoulders of all NC samples are located at similar wavelength, revealing that the band gap of crude core NCs undergo quite minor changes after
Fig. 2. Synthetic process for synthesis of (a) Zn stock solution; (b) CZIS NCs and CZIS/ZnS NCs.
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Fig. 3. (a) absorption and (b) emission spectra of CZIS/ZnS QDs prepared with different shell growth time.
Zn was injected into the core NCs, which confirms the formation of ZnS shell and makes cation exchange between Zn2þ, Cuþ and In3þ to be a secondary reason. As shown in the PL emission spectra in Fig. 3b, the PL peak blue-shift from 643 to 579 nm as the shell growth time increases from 30 s to 120 min. During the first 15 min, the PL spectra experience a 53 nm of blue-shift, which is 82.8% of the total blue-shift amount. In the following 30 min, the PL peak blue shifts further from 590 to 579 nm, finishing the whole blueshift process; and then remain unchanged with longer shell growth time. The blue-shift of PL spectra may be attributed to several factors: (i) minor cation exchange between Zn2þ and Cuþ or In3þ, especially in the first 15 min [28,29]; (ii) ZnS shell formation [32] through Zn2þ accumulation on the surface of core CZIS NCs following cation exchange [18]; (iii) size/shape homogenization after surface modification. It is known that the PL spectra of semiconductor NCs usually shows red-shifted characteristics when the size/shape of NCs is highly inhomogeneous [33,34]. The morphology and crystal structure of CZIS NCs and their corresponding core/shell NCs prepared with ZnS shell growth for 1 min, 15 min and 45 min were examined by TEM and XRD. The TEM images demonstrate that the uniformity of NCs is improved together with an increasing size by extending shell growth time (Table 1), which is in accordance with the phenomena in PL spectra, as shown in Fig. 4aed. An increasing of 0.38 nm is found after 45 min of shell growth, which means roughly one monolayer of ZnS shell is formed [7,12,16]. The SAED pattern of CZIS/ZnS with a 45 min of shell growth time (Fig. 4e) can be indexed to a facecentered cubic (FCC) phase, which is in accordance with the result of corresponding XRD [22]. As shown in Fig. 4g, XRD patterns of the samples indicate that they all have an FCC crystal structure. Besides, XRD peaks become stronger with the trend of shifting to larger angles when the shell growth time is prolonged, indicating better crystallinity of the samples and gradual increasing in the amount of Zn which may result from the slight cation exchange and Zn ions accumulation on the surface of core CZIS NCs [18,35]. High crystallinity of CZIS/ZnS NCs with ZnS shell coating could also be confirmed by HRTEM in Fig. 4f, where clear lattice fringe throughout the particle could be seen. It is noted that the distance between adjacent lattice fringes is 3.13 Å, fitting well with the interplanar distance of (111) plane of cubic phase ZnS (JCPDS 659585). As mentioned above, being one of the most important optical properties of NCs, QY is affected by shell growth time. Table 2 list the QY value of all samples as a function of shell growth time. PL QY
of the bare CZIS cores is only 21.3%. However, it begins to increase significantly at the same time when ZnS shell start forming. After 15 min of shell growth, PL QY of the NCs reaches the maximum of 77% and then drops to 63.0% after 30 min. In order to explain this phenomenon, further understanding about the optical mechanism that controls QY is necessary. Therefore, PL decay curves were measured, as shown in Fig. 5. The PL curves of all samples can be fitted well by a triexponential function (1):
IðtÞ ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ þ A3 expðt=t3 Þ
(1)
Where ti represents the decay time of PL emission; Ai represents the relative weight of the decay components at t ¼ 0 [27,36]. As shown in Table 2, the relative weight A1 of fast decay component t1 keeps decreasing from 25.7 % to 9.3 % at the first 15 min, and then increases to 11.1% in the next 30 min. This variation trend is just the opposite of QY. According to the previous research, the fast decay component t1 is related to nonradiative recombination associated with surface defects [10,30,37,38]. Bare core CZIS NCs contain plenty of surface dangling bonds which can act as trap states for charge carriers. However, the formation of ZnS shell provides an efficient passivation of the surface trap states (Fig. 6), thus giving rise to a strongly enhanced QY which is seen at the first 15 min [33]. The decrease of QY when the shell growth time is further prolonged might be related to the following reasons: (i) more defects are induced due to an overlong shell growth time. According to classical nucleation theory, the growth of NCs is a dynamic equilibrium between the monomers and nucleus and is a diffusion-controlled process [39,40,42]. An increasing in temperature for further shell growth (refer to experimental section) accelerate the growth rate of NCs. This accelerated growth rate leads to more defects when the reaction time is long enough although it make contributions to shell formation at the beginning; (ii) when the reaction time is furthermore prolonged, the ZnS shell becomes too thick (confirmed by the increase in NC size from
Table 1 Composition (from EDS), average size (from TEM) and emission wavelength (from PL spectra) of CZIS/ZnS NCs prepared with different shell growth time. Shell growth time (min)
Composition
Size (nm)
0 (CZIS core) 1 15 45
Cu7.70Zn1.83In0.86S2 Cu5.21Zn3.27In0.80S2 Cu2.11Zn3.47In0.82S2 Cu1.42Zn3.80In0.86S2
2.69 2.74 2.98 3.07
± ± ± ±
0.81 0.65 0.54 0.40
l (nm) 657 641 590 579
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Fig. 4. (a) TEM image of CZIS core NCs and CZIS/ZnS NCs with shell growth time of (b) 1 min; (c) 15 min; (d) 45 min; (e) SAED pattern and (f) HRTEM image of CZIS/ZnS NCs with shell growth time of 45 min; (g) the corresponding XRD patterns of CZIS/ZnS NCs with different shell growth time.
Table 1). With increasing of shell thickness, the interface strain accumulates dramatically because lattice mismatch between the CZIS core and ZnS shell material, thus resulting in the deterioration of QY of the samples [32,35,41]. The chemical structure of CZIS/ZnS NCs prepared with different shell growth time was determined by STEM-EDS. As listed in Table 1, the indium content almost remain unchanged while a significant decrease in copper content together with an increase in zinc content can be easily noticed when the shell growth time is prolonged. The increase in Zn content probably is attributed to the following two factors. Firstly, it could be a result of slight cation exchange between Zn2þ and Cuþ or In2þ. It is known that the growth of NCs is a diffusion-controlled growth process which allows cation exchange in nanometer-sized particles because of effective reaction barrier [27,28,30,42]. The preferential exchange
between Zn2þ and Cuþ instead of Zn2þ and In3þ may be due to the fact that CueS bond is much weaker than that of IneS bond and Zn substitution of Cu was shown to be energetically favorable [11,30,43]. Secondly, it could be a result of Zn2þ occupation on the surface of core materials. As Zn2þ injected in the solution is excessive for shell growth, more and more Zn2þ ions accumulate on the CZIS core NCs with increasing of reaction time, leading to further deposition of ZnS shell on the surface of core NCs [18]. The additional ZnS shell can also be convinced by the increase in the size of NCs (Table 1). Average diameter of CZIS NCs changes from 2.69 to 3.07 nm after 45 min of treatment, suggesting that roughly one monolayer of ZnS shell was formed during shell formation process. Fig. 6 is given to further illustrate the whole cation exchange and Zn accumulation process.
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Table 2 QY and fitting results of PL curves of CZIS/ZnS NCs prepared with different shell growth time, derived from Eq. (1). Shell growth time (min)
QY (%) A1 (%) t1(ns) A2 (%) t2(ns) A3 (%) t3(ns) tav(ns)
0 (core) 1 5 15 45
21.3 51.1 63.3 77.0 63.0
25.7 14.5 9.7 9.3 11.1
25.3 31.2 32.5 39.5 19.9
57.2 61.1 64.7 66.1 64.8
150.8 176.4 178.2 180.7 171.0
15.0 20.8 22.1 21.1 21.0
673.7 607.4 573.8 564.8 550.8
419.2 401.8 380.8 372.0 361.6
Fig. 7. XRD patterns of CZIS/ZnS NCs obtained by adding 0.25 mmol, 0.5 mmol, 1 mmol, and 2 mmol of Zn in CZIS core NCs for the growth of ZnS shell.
Fig. 5. PL lifetime of decays of CZIS/ZnS samples with different shell growth time.
the increasing of Zn amount for shell growth from 0.25 to 2 mmol, indicating the decrease of lattice constants of the NCs [7,35,46]. When the amount of Zn added in the second stage for shell growth increased from 0.25 mmol to 2 mmol, the PL spectra witnessed a systematic blue-shift from 674 to 579 nm (Fig. 8) while the PL QY increased from 13.6 % to 63.0 % (Fig. 9a). The blue shift of PL spectra in combination with the increasing of QY may be the result of the following two reasons: (i) As feed amount of Zn for shell growth increases, Zn concentration in the solution becomes higher, thus leading to accelerated the interdiffusion of Zn ions into CZIS core NCs [39,40,42], which is in accordance with EDS results shown in Table 1; (ii) A denser ZnS shell is formed. Higher Zn concentration in the solution contributes to ion accumulation on the surface of CZIS cores (Fig. 6), which leads to further deposition of ZnS shell and provides a more efficient passivation of the surface trap states, and this can be confirmed by the fitting results of PL curves (Fig. 10). 3.2.3. Effect of shell growth temperature It is found that PL QY and PL peak position of the resulting CZIS/ ZnS NCs is strongly dependent on the shell growth temperature. As shown in Fig. 9b, QY of CZIS/ZnS NCs rise from 22.6 % to 63.0 % when
Fig. 6. ZnS shell formation process that includes elimination of dangling bonds, slight cation exchange between Zn2þ, Cuþ (In3þ) and Zn occupation on the surface of CZIS cores. Cation exchange between Zn2þ and Cuþ instead of Zn2þ and In3þ is preferred because CueS bond is weaker than IneS bond.
3.2.2. Effect of Zn amount added in the shell growth stage Preparation of CZIS/ZnS NCs is based on the experimental section. The only difference between all the samples is that different amount of Zn is used for shell growth (Fig. 2). The phase purities of CZIS/ZnS NCs synthesized with 2 mmol, 1 mmol, 0.5 mmol, 0.25 mmol of Zn added in the shell growth stage were confirmed by XRD (Fig. 7). The characteristic peaks of all the samples were located between the corresponding distinct peaks of Cu0.4In0.4Zn0.2S (JCPDS 47-1370) and ZnS (JCPSD 65-9585), which implies the ZnS shell formation on CZIS NCs [44,45]. Furthermore, the diffraction peaks gradually shifted towards larger angles with
Fig. 8. Absorption and emission spectra of CZIS/ZnS NCs prepared with different Zn amount at shell growth stage.
W. Xiang et al. / Materials Chemistry and Physics 149-150 (2015) 437e444
443
Fig. 9. PL emission wavelength and QY of CZIS/ZnS NCs obtained (a) by adding different amount of Zn (0.25 mmol, 0.5 mmol, 1 mmol, and 2 mmol) in the shell growth stage; (b) at different shell growth temperature (130 C, 190 C, 240 C, 280 C).
the shell growth temperature increased from 130 to 240 C. Meanwhile, PL peak shows a blue-shift from 644 to 579 nm. According to the previous investigation [33,39], the formation of NCs from the precursor is endothermic, the elevated shell growth temperature leads to a more thoroughly shell formation reaction and a higher extent of Zn interdiffusion (Fig. 11), giving rise to an enhanced QY. However, as mentioned above, exceeding temperature may induce more defects, thus causing the deterioration of optical properties, especially the QY of NCs (Fig. 9b) when the temperature increased from 240 to 280 C. It is also found that PL peak undergo a slight red shift during the process, which might be the result of an increased size of core NCs [12]. Phase purity of the as-prepared CZIS/ZnS NCs prepared with different reaction temperature is also confirmed by XRD. As shown in Fig. 12, the CZIS/ZnS NCs have similar diffraction patterns although they are prepared with different shell growth temperature. Furthermore, the continuous peak-shift revealed that no phase separation or separated nucleation occurred in the synthetic process. Fig. 10. PL lifetime of decays of CZIS/ZnS samples with different Zn amount at shell growth stage.
4. Conclusions In summary, a facile approach for the synthesis of core/shell CZIS/ZnS NCs with highest QY of 77% is reported. The influences of
Fig. 11. The ratio between Zn: (Cu þ In) as a function of shell growth temperature.
Fig. 12. XRD patterns of CZIS/ZnS NCs obtained under different shell growth temperature.
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synthetic parameters including shell growth time and temperature, and Zn amount for shell growth stage on optical properties are studied systematically. It is found that there is a remarkable blueshift of PL peak from 657 to 579 nm and the QY of CZIS NCs increases from 21.3 % to 63.0 % after the growth of ZnS shell. As the shell growth time increases from 30 min to 120 min, the PL peak shifts from 643 to 579 nm and PL QY of the NCs reaches the maximum of 77% after 15 min of shell growth. When the shell growth temperature increases from 130 to 240 C, the QY rises from 22.6 % to 63.0 % and PL peaks shifts from 644 to 579 nm. When the amount of Zn increases from 0.25 to 2.00 mmol, the PL spectra shift from 674 to 579 nm and the PL QY increase from 13.6% to 63.0%. It is also suggested that ZnS shell formation offer an efficient way to eliminate nonradiative channels associated with dangling bonds and surface traps. And the shell formation process of our NCs is a combination of slight cation exchange between Zn2þ and Cuþ (In3þ) with Zn2þ ions occupation on the surface of core CZIS NCs. Acknowledgments The authors acknowledge the financial support from National Natural Sciences Foundation of China (Grant nos. 50972107 and 51272059) and Key foundation of Zhejiang Province Key Technology Innovation Team (2009R50010).
[19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
References
[29]
[1] J.M. Caruge, J.E. Halpert, V. Wood, V. Bulovic, M.G. Bawendi, Colloidal quantum-dot light-emitting diodes with metal-oxide charge transport layers, Nat. Photonics 2 (2008) 247. [2] D.V. Talapin, J.S. Lee, M.V. Kovalenko, E.V. Shevchenko, Prospects of colloidal nanocrystals for electronic and optoelectronic applications, Chem. Rev. 110 (2010) 389. [3] Y. Shirasaki, G.J. Supran, M.G. Bawendi, V. Bulovic, Emergence of colloidal quantum-dot light-emitting technologies, Nat. Photonics 7 (2013) 13. [4] J.S. Zhong, Q.Y. Wang, X.H. Xu, Photodeposition of CdS nanoparticles sensitized TiO2 nanotube arrays for enhanced photoelectrochemical performance, J. Electrochem. Soc. 161 (2014) H656. [5] Z. Pan, H. Zhang, K. Cheng, Y. Hou, J. Hua, X. Zhong, Highly efficient inverted type-I CdS/CdSe core/shell structure QD-sensitized solar cells, ACS Nano 6 (2012) 3982. [6] D.Q. Chen, P. Huang, Highly intense upconversion luminescence in Yb/Er: NaGdF4@NaYF4 core-shell nanocrystals with complete shell enclosure on core, Dalton Trans. 43 (2014) 11299. [7] K. Bourzac, Quantum dots go on display, Nature 493 (2013) 283. [8] D. Chen, F. Zhao, H. Qi, M. Rutherford, X. Peng, Bright and stable purple/blue emitting CdS/ZnS core/shell nanocrystals grown by thermal cycling using a single-source precursor, Chem. Mater. 22 (2010) 1437. [9] X. Wang, J. Zhuang, Q. Peng, Y. Li, A general strategy for nanocrystals synthesis, Nature 437 (2005) 121. [10] T. Pons, E. Pic, N. Lequeux, E. Cassette, L. Bezdetnaya, F. Guillemin, F. Marchal, B. Dubertret, Cadmium-free CuInS2/ZnS quantum dots for sentinel lymph node imaging with reduced toxicity, ACS Nano 4 (2010) 2531. [11] L. Li, A. Pandey, D.J. Werder, B.P. Khanal, J.M. Pietryga, V.I. Klimov, Efficient synthesis of highly luminescent copper indium sulfide-based core/shell nanocrystals with surprisingly long-lived emission, J. Am. Chem. Soc. 133 (2011) 1176. [12] R. Xie, M. Rutherford, X. Peng, Formation of high-quality I-III-VI semiconductor nanocrystals by tuning reactive reactivity of cationic precursors, J. Am. Chem. Soc. 131 (2009) 5691. [13] J. Kolny-Olesiak, H. Weller, Synthesis and application of colloidal CuInS2 semiconductor nanocrystals, ACS Appl. Mater. Interfaces 5 (2013) 12221. [14] H. Zhong, Z. Bai, B. Zou, Tuning the luminescence properties of colloidal I-III-VI semiconductor nanocrystals for optoelectronics and biotechnology applications, J. Phys. Chem. Lett. 3 (2012) 3167. [15] W. Zhang, X. Zhong, Facile synthesis of ZnS- CuInS2-alloyed nanocrystals for a color-tunable fluochrome and photocatalyst, Inorg. Chem. 50 (2011) 4065. [16] D. Deng, Y. Chen, J. Cao, J. Tian, Z. Qian, S. Achilefu, Y. Gu, High-quality CuInS2/ ZnS quantum dots for in vitro and in vivo bioimaging, Chem. Mater. 24 (2012) 3029. [17] E.P. Jang, W.-S. Song, K.-H. Lee, H. Yang, Preparation of a photo-degradationresistant quantum dot-polymer composite plate for use in the fabrication of a high-stability white-light-emitting diode, Nanotechnology 24 (2013) 045607. [18] W. Guo, N. Chen, Y. Tu, C. Dong, B. Zhang, C. Hu, J. Chang, Synthesis of Zn-CuIn-S/ZnS core/shell quantum dots with inhibited blue-shift
[30] [31] [32]
[33] [34]
[35]
[36]
[37] [38]
[39]
[40]
[41]
[42] [43]
[44]
[45]
[46]
photoluminescence and applications for tumor targeted bioimaging, Theranostics 3 (2013) 99. Z.B. Zhuang, Q. Peng, B.C. Zhang, Y. Li, Controllable synthesis of Cu2S nanocrystals and their assembly into a supperlattice, J. Am. Chem. Soc. 130 (2008) 10482. L.S. Li, N. Pradhan, Y. Wang, X. Peng, Hight quality ZnSe and ZnS nanocrystals formed by activating zinc carboxylate precursors, Nano Lett. 4 (2004) 2261. S. Li, Y. Chen, L. Huang, D. Pan, Simple continuous-flow synthesis of Cu-In-ZnS/ZnS and Ag-In-Zn-S/ZnS core/shell quantum dots, Nanotechnology 24 (2013) 395705. H. Shen, H. Yuan, W. Feng, X. Bai, C. Zhou, H. Wang, T. Lu, Z. Qin, L. Ma, L.S. Li, Facile synthesis of high-quality CuInZnxS2þx core/shell nanocrystals and their application for detection of C-reactive protein, J. Mater. Chem. 22 (2012) 18623. W. Song, H. Yang, Efficient white-light-emitting diodes fabricated from highly fluorescent copper indium sulfide core/shell quantum dots, Chem. Mater. 24 (2012) 1961. B. Chen, Q. Zhou, J. Li, F. Zhang, R. Liu, H. Zhong, B. Zou, Red-emissive CuInS2based nanocrystals: a potential phosphor for warm white light-emitting diodes, Opt. Express 21 (2013) 10105. R. Zhang, P. Yang, Y. Wang, Facile synthesis of CuInS2-ZnS quantum dots with highly near-infrared photoluminescence via phosphor-free process, J. Nanopart Res. 15 (2013) 1910. K. Yu, P. Ng, J. Ouyang, M.B. Zaman, A. Abulrob, T.N. Baral, D. Fatehi, Z.J. Jakubek, D. Kingston, X. Wu, X. Liu, C. Hebert, D.M. Leek, D.M. Whitfield, Low-temperature approach to highly emissive copper indium sulfide colloidal nanocrystals and their bioimaging application, ACS Appl. Mater. Interfaces 5 (2013) 2870. W. Xiang, H. Yang, X. Liang, J. Zhong, J. Wang, L. Luo, C. Xie, Direct synthesis of highly luminescent Cu-Zn-In-S quaternary nanocrystals with tunable photoluminescence spectra and decay times, J. Mater. Chem. C 1 (2013) 2014. J. Zhang, R. Xie, W. Yang, A simple route for highly luminescent quaternary Cu-Zn-In-S nanocrystals emitters, Chem. Mater. 23 (2011) 3357. J. Park, S.-W. Kim, CuInS2/ZnS core/shell quantum dots by cation exchange and their blue-shifted photoluminescence, J. Mater. Chem. C 21 (2011) 3745. L.D. Trizio, M. Prato, A. Genovese, A. Casu, M. Povia, R. Simonutti, J.P. Marcelo, C. D'Andrea, F. Tassone, L. Manna, Chem. Mater. 24 (2012) 2400. J. Feng, M. Sun, F. Yang, X. Yang, A facile approach to synthesize high-quality ZnxCuyInS1.5þxþ0.5y nanocrystals emitters, Chem. Commun. 47 (2011) 6422. R.K. Capek, K. Lambert, D. Dorfs, Synthesis of extremely small CdSe and bright blue luminescent CdSe/ZnS nanoparticles by a prefocused hot-injection approach, Chem. Mater. 21 (2009) 1743. P. Reiss, M. Protiere, L. Li, Core/shell semiconductor nanocrystals, Small 5 (2009) 154. J.S. Zhong, J.R. Xu, Q.Y. Wang, Nitrogen and vanadium Co-doped TiO2 mesosponge layers for enhancement in visible photocatalytic activity, Appl. Surf. Sci. 315 (2014) 131. R. Xie, U. Kolb, J. Li, T. Basche, A. Mews, Synthesis and characterization of highly luminescent CdSe-core-CdS/Zn0.5Cd0.5/ZnS multishell nanocrystals, J. Am. Chem. Soc. 127 (2005) 7480. H. Zhong, Y. Zhou, M. Ye, Y. He, J. Ye, C. He, C. Yang, Y. Li, Controllable synthesis and optical properties of colloidal ternary chalcogenide CuInS2 nanocrystals, Chem. Mater. 20 (2008) 6434. X. Tang, W.B.A. Ho, J.M. Xue, Synthesis of Zn-doped AgInS2 nanocrystals and their fluorescence properties, J. Phys. Chem. C 116 (2012) 9769. T.K.C. Tran, Q.P. Le, Q.L. Nguyen, L. Li, Time-resolved photoluminescence study of CuInS2/ZnS nanocrystals, Adv. Nat. Sci. Nanosci. Nanotechnol. 1 (2010) 025007. C. de M. Donega, P. Liljeroth, D. Vanmaekelbergh, Physicochemical evaluation of hot-injection method, a synthesis route for monodisperse nanocrystals, Small 1 (2005) 1152. S.G. Kwon, Y. Piao, J. Park, S. Angappane, Y. Jo, N.-M. Wang, J.-G. Park, T. Hyeon, Kinetics of monodisperse iron oxide nanocrystals formation by “heating-up” process, J. Am. Chem. Soc. 129 (2007) 12571. X. Wang, R. Liu, T. Wang, B. Wang, Y. Xu, H. Wang, Dual roles of ZnS thin layers in significant photocurrent enhancement of ZnO/CdTe nanocable arrays photoanode, ACS Appl. Mater. Interfaces 5 (2013) 3312. J. Park, J. Joo, S.G. Kwon, Y. Jang, T. Hyeon, Synthesis of monodisperse spherical nanocrystals, Angew. Chem. Int. Ed. 46 (2007) 4630. T. Yamamoto, I.V. Luck, R. Scheer, H. Katayama-Yoshida, Differences in the electronic structure and compensation mechanism between n-type Zn- and Cd-doped CuInS2 crystals, Physca B 274 (1999) 927. T. Torimoto, T. Adachi, K. Okazaki, M. Sakuraoka, T. Shibayama, B. Ohtani, A. Kudo, S. Kuwabata, Facile synthesis of ZnS-AgInS2 solid solution nanoparticles for a color-adjustable luminphore, J. Am. Chem. Soc. 129 (2007) 12388. Y.K. Kim, S.-H. Ahn, K. Chung, Y.-S. Cho, C.-J. Choi, The photoluminescence of CuInS2 nanocrystals: effect of non-stoichiometry and surface modification, J. Mater. Chem. 22 (2012) 1516. H. Shen, H. Wang, X. Li, J. Niu, H. Wang, X. Chen, L. Li, Phosphine-free synthesis of high quality ZnSe, ZnSe/ZnS, and Cu-, Mn-doped ZnSe nanocrystals, Dalton Trans. (2009) 10534.