A facile method for the synthesis of CuInS₂–ZnS quantum dots with tunable photoluminescent properties

Abstract

CuInS₂ quantum dots were synthesized via a solvothermal approach by heating up a mixture of the corresponding metal precursors and dodecanethiol in octadecene. By coating ZnS shells around the CuInS₂ quantum dot cores directly in a pristine reaction solution, we successfully synthesized CuInS₂–ZnS quantum dots with good monodispersion, spherical morphology and a diameter of 7 nm on average. The absorption and PL emission spectra of the CuInS₂ QDs were controllable from 600 nm to 550 nm and from 670 nm to 600 nm, respectively. With the growth of ZnS, the percentage of internal defect recombination increased, which lead to the enhancement of PL intensity of CuInS₂–ZnS QDs. The PL intensity and peak position of CuInS₂ and CuInS₂–ZnS QDs could also be tuned by varying the excitation wavelength. Besides, the reaction temperature and time play important roles on the photoluminescent properties of the CuInS₂–ZnS quantum dots. This line of research finds feasible approaches to prepare CuInS₂–ZnS quantum dots with great controllable photoluminescent properties. The CuInS₂–ZnS quantum dots could greatly contribute to the development of quantum dot light-emitting diode (QLEDs) devices and other potential applications due to their non-toxicity and excellent optical properties.

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Introduction

Over the past two decades, colloidal luminescent semiconductor nanocrystals, which are also called quantum dots (QDs), have been exclusively studied due to their unique optical properties and wide applications in photovoltaics,^1–7 light emitting diodes,^8–14 biomedical labelling^15,16 and so on. Binary II–VI compounds, mainly cadmium chalcogenide-based QDs, have been studied intensively to tune their photoluminescence (PL) in the visible spectrum by changing their sizes and compositions.^17–22 Unfortunately, the intrinsic toxicity of cadmium impedes the practical application of these QDs, so it is urgent to develop cadmium-free QDs to reach non-toxic luminescence, especially in the fields of biological and medical applications.

In recent years, I–III–VI compounds-based QDs, especially CuInS₂ QDs, have gained increasing attention due to their nontoxicity, which makes the CuInS₂ QDs promising candidates for energy^23–25 and healthcare fields.²⁶ However when it comes to the practical application, there are still lots of problems need to be solved, such as large-scale production of CuInS₂ QDs and precise control of their optical properties. Up to now, much work has been done to prepare CuInS₂ QDs, tune PL behaviour and enhance fluorescence quantum yield (QY). For example, Zhong et al. synthesized spheric, fluorescence-tunable CuInS₂ QDs by using thermolysis of a mixed solution of Cu(OAc), In(OAc)₃ and dodecanethiol (DDT) in noncoordinating solvent 1-octadecene (ODE) in gram-scale.²⁷ Li et al. used a “core–shell” structure to increase the QY of the CuInS₂ QDs up to 10-fold with a blue shift of PL peak position by using ZnS as the shell material.²⁸ Zhang et al. prepared Cu–Zn–In–S alloyed QDs and widened the range of PL spectrum from UV-Vis to NIR through changing the Cu : Zn ratios from 1 : 20 to 2 : 1.²⁹ Zhang et al. developed a green method for the one-pot direct synthesis of hydrophilic CuInS₂ and CuInS₂–ZnS QDs by employing N,N-dimethylformamide (DMF) as a solvent.³⁰

Herein, we successfully synthesized CuInS₂ quantum dots with good monodispersion by heating up a mixture of the corresponding metal precursors and dodecanethiol in octadecene. Subsequently, CuInS₂–ZnS QDs were obtained by coating the as-prepared CuInS₂ quantum dots with the ZnS shell. Such CuInS₂–ZnS QDs show a remarkable blue shift of the PL emission and UV-Vis absorption and a great enhancement of the luminescent intensity. On this basis, we further studied the PL behaviour of the obtained QDs under different excitation wavelengths and the influence of experimental parameters, such as reaction temperature and time, on the optical properties of the core–shell CuInS₂–ZnS QDs. This work leads us to feasible approaches to tune the optical properties of CuInS₂–ZnS QDs. Due to their excellent properties and non-toxicity, CuInS₂–ZnS QDs are one of great candidates for the applications in such optical devices as QLEDs.

Experimental section

Materials

Zinc stearate (ZnSt₂, 86–87.5%), 1-octadecene (ODE, 90%), 1-dodecanethiol (DDT, 98%) were purchased from Alfa Aesar, cuprous acetate (Cu(OAc), 99%) from Strem Chemicals, indium acetate (In(OAc)₃, 99.99%) and oleylamine (OAm, 80–90%) from Acros Organics, and oleic acid (OA, AR) from Xilong Chemical Co. Ltd., China. All the above chemicals were used as received.

Preparation of stock solution

The zinc solution for the ZnS shell growth was prepared by dissolving 1.27 g (2 mmol) of ZnSt₂ in a mixture of 8 mL ODE and 2 mL OAm at 120 °C under flowing argon. The obtained zinc stock solution was stored for the following use.

Synthesis of CuInS₂ and CuInS₂–ZnS QDs

In a typical procedure, Cu(OAc) (0.245 g, 2 mmol), In(OAc)₃ (0.584 g, 2 mmol) together with 10 mL DDT and 50 mL ODE were loaded in a 250 mL flask at room temperature. The mixture was then heated up under argon flow, in which 2 mL OA was dropped in fast at 90 °C. It was further heated to 220 °C and kept at this temperature for 1 hour allowing for CuInS₂ QDs to grow. The as-prepared QDs were precipitated by adding hexane and excessive acetone into the sample and purified by repeating centrifugation and decantation cycles.

Growth of the ZnS shell around the CuInS₂ core template was carried out in pristine Cu–In–S reaction mixture. The reaction system was cooled down to 120 °C first, where the zinc stock solution was injected subsequently. After that, the temperature of this mixture was raised to 240 °C for CuInS₂–ZnS QDs growth. Aliquots of the sample were taken out at different time intervals. Purification of CuInS₂–ZnS QDs was similar to that of CuInS₂ QDs as described above.

Characterization

PL spectra were recorded on a Hitachi F-7000 fluorescence spectrometer. UV-Vis spectra were achieved on a Shimadzu UV-Vis-NIR spectrophotometer. PL dynamics were measured on Edinburgh FLS920. Transmission electron microscopy (TEM) images were acquired using a Tecnai F30 S-TWIN transmission electron microscope. The TEM samples were obtained by depositing a drop of QDs dispersion in hexane onto molybdenum grids. Powder X-ray diffraction (XRD) was performed using wide-angle X-ray diffraction on a Bruker D8 Advance X-ray powder diffractometer. XRD samples were prepared by depositing QDs powder on a piece of Si (100) wafer. X-ray photoelectron spectroscopy (XPS) was carried out on an Escalab 250Xi X-ray photoelectron spectrometer.

Results and discussion

Morphology and structure of CuInS₂ and CuInS₂–ZnS QDs

The CuInS₂ QDs were prepared by heating up a mixture of corresponding metal precursors and dodecanethiol in octadecene media. This facile method shows high reproducibility and scale-up capability.^27,28 Here, DDT is served as not only the anionic precursor, but also the ligand and solvent, which can eliminate the surface trap-states,³¹ allow for nearly complete consumption of cationic precursors, and minimize the waste of reagents. The CuInS₂ QDs were synthesized to demonstrate this facile approach in preparing high-quality CuInS₂ QDs, and the detailed procedures were described in the Experimental section. Morphology and structure characterization of CuInS₂ and CuInS₂–ZnS QDs were performed using XRD and TEM (Fig. 1). The wide-angle XRD patterns of the obtained QDs showed the characteristic peaks of the tetragonal chalcopyrite structure. After being coated by ZnS shell, the QDs maintained in the tetragonal structure but the diffraction peaks shifted to larger angles, which is consistent with the smaller lattice constant for ZnS compared to CuInS₂. The TEM images demonstrate that the as-prepared QDs are of near-spherical shape and have a narrow size distribution. Fig. 1b and c show that the average diameter of CuInS₂ and CuInS₂–ZnS QDs is nearly 3.0 ± 0.3 nm and 7.0 ± 0.2 nm, respectively.

Fig. 1 (a) XRD patterns of CuInS₂ and CuInS₂–ZnS QDs. (b) TEM and HRTEM images of CuInS₂ QDs, (c) TEM and HRTEM images of CuInS₂–ZnS QDs.

To investigate the compositions and valence states of the as-prepared QDs, EDS and XPS analysis were carried out (shown in Tables 1 and 2 and Fig. 2). The element composition of CuInS₂ QDs is CuIn_0.95S_2.13, in which the larger S content is probably owing to the capping thiol ligands existed on the QD surface. After the growth of ZnS shell, the element composition is CuIn_0.84Zn_1.32S_2.89. It is noted that compared to the nominal Cu/In/Zn ratio (1/1/1), the real value of Zn is relatively higher, which might be attributed to a partial cationic exchange.³² The binding energy of XPS narrow scans on Cu 2p, In 3d, Zn 2p and S 2p peaks of the as-prepared QDs demonstrated that the chemical states were Cu⁺, In³⁺, Zn²⁺ and S²⁻, respectively.

Table 1 EDX analysis of CuInS₂ QDs

Element	Weight/%	Atomic/%	Uncert./%
Cu	26.33	24.47	0.43
In	45.30	23.29	1.13
S	28.36	52.23	0.39

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Table 2 EDX analysis of CuInS₂–ZnS QDs

Element	Weight/%	Atomic/%	Uncert./%
Cu	18.76	16.54	0.46
In	28.43	13.87	1.26
Zn	25.47	21.83	0.54
S	27.31	47.74	0.45

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Fig. 2 Electronic binding energy of XPS narrow scan spectra of (a) Cu 2p, (b) In 3d, (c) Zn 2p and (d) S 2p, respectively.

Influence of ZnS shell on the optical properties of CuInS₂ QDs

The as prepared CuInS₂ and CuInS₂–ZnS QDs exhibit interesting optical properties. Fig. 3a shows the typical UV-Vis absorption spectra and PL spectra of CuInS₂ QDs. The absorption spectra show a broad shoulder with a trail in the long-wavelength direction. Being different from the binary II–VI QDs, the absorption spectra of ternary CuInS₂ QDs has no well-defined exciton absorption peaks. Two reasons why this phenomenon occurred were stated in previous publications.²⁷ On the one hand, the size/shape inhomogeneity of the ensemble can be associated with the influence of quantum confinement. On the other hand, the irregular composition distribution among different nanocrystals could result in continuous distribution of the band width. Although there is no obvious absorption peak, the CuInS₂ QDs still have an absorption edge at around 600 nm which is significantly blue-shifted from that of bulk CuInS₂ (810 nm) and is consistent with the expected quantum confinement effect. The PL spectrum shown in Fig. 3a exhibits a typical emission peak at around 668 nm with a full width at half-maximum (fwhm) of ∼100 nm, and the apparent Stokes shift is ∼120 nm. The large Stokes shift decreases the self-absorption of QDs and ensures their practical application in QLEDs.

Fig. 3 UV-Vis/PL spectra of (a) CuInS₂ and (b) CuInS₂–ZnS QDs, comparison of (c) UV/Vis and (d) PL of CuInS₂ and CuInS₂–ZnS QDs ZnS growth time is 20 min. (e) is normalized corresponding to (d).

Further injection of zinc precursor to the Cu–In–S reaction mixture led to the formation of a CuInS₂–ZnS core–shell nanostructure. The typical UV-Vis absorption spectra and PL spectra are shown in Fig. 3b. After being coated with ZnS shell, the UV-Vis absorption spectra and PL spectra both show an evident blue-shift (shown in Fig. 3c–e) compared to the CuInS₂ QDs. No obvious absorption peak can be found, and the absorption edge is at around 550 nm with a 50 nm shift compared to as prepared CuInS₂ QDs. Meanwhile, the PL spectrum of the CuInS₂–ZnS QDs shown in Fig. 3b exhibits a typical emission peak at around 605 nm which has blue-shift about 60 nm compared to CuInS₂ QDs (shown in Fig. 3e) with a full width at half-maximum (fwhm) of ∼89 nm, and the apparent Stokes shift is nearly 110 nm. The decreased fwhm indicates a greater monochromaticity of QDs which makes them promising materials for QLEDs and displays. Besides, it can be found that the PL intensity enhanced greatly after the growth of ZnS shell (Fig. 3d). Recently, it has been reported that the core–shell structure can enhance the luminescent efficiency for some reasons.³¹ The type I band alignment makes both carriers (electrons and holes) localize in the core, which increases the rate of electron–hole recombination process. Furthermore, ZnS shell can eliminate surface trap-states of the CuInS₂ core, which diminished the non-radiative electron–hole recombination process and improved the thermal and chemical stability of QDs. The blue-shift of the PL peak position after being coated with ZnS shell indicates that further incorporation of zinc into the core material can result in an increase of the band gap energy.^33–35

The radiative lifetime of PL emission is another important optical property of semiconductor nanocrystals that needs to be further studied. Different radiative lifetimes may correspond to different electron–hole recombination mechanisms. To demonstrate the two recombination pathways,²⁸ time-resolved laser technique was used to study the lifetime of PL emission. Fig. 4 shows the PL decay curves of the CuInS₂ and CuInS₂–ZnS QDs. The PL decay curves can be well fitted by a double-exponential function:

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂).

where τ represents the decay time of the PL emission. A represents the amplitude of the decay components at t = 0, relative data are listed in Table 3.

Fig. 4 PL decay curves of the (a) CuInS₂ and (b) CuInS₂–ZnS QDs.

Table 3 Decay times and amplitude constant ratios of the PL emission for CuInS₂ and CuInS₂–ZnS QDs

Sample	Wavelength/nm	τ1/ns	A1/%	τ2/ns	A2/%
CuInS2	650	14.768	21.486	80.549	78.514
CuInS2–ZnS	600	5.613	5.12	52.93	94.88

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Influence of the excitation wavelength on the optical properties of CuInS₂–ZnS QDs

It was also found that the optical properties of the prepared CuInS₂ and CuInS₂–ZnS QDs could be affected by the change of excitation wavelength, as shown in Fig. 5. The PL intensity increased first, and then decreased with increasing excitation wavelengths from 550 nm to 630 nm. The maximum of PL intensity was obtained at the excitation wavelength of 590 nm. This phenomenon could be attributed to the different absorption intensities at different excitation wavelengths. The UV-Vis absorption spectrum of CuInS₂ QDs above tells us the exciton absorption peak is around 570 nm, which agrees well with the photoluminescent behaviour of the QDs. As the excitation wavelength red shifted, the PL peak also had a red-shift from about 665 nm to 685 nm. There are two recombination pathways observed in CuInS₂ QDs that involve an electron in the quantized conduction band state and a hole trapped at either a surface defect which is the non-radiative recombination process or an internal defect which is the radiative recombination process.²⁸ Since the internal defect state is fixed, the larger excitation wavelength, the lower quantized conduction band state electrons are in, which leads to release of photons with smaller energies when electron and hole recombine. That is why the PL emission peak red shifted along with the excitation wavelength. Meanwhile, similar phenomenon was found in CuInS₂–ZnS QDs, the largest PL intensity was observed under the excitation of 530 nm which is also around the corresponding UV-Vis absorption band and the PL emission peak moved from about 604 nm to 630 nm when the excitation wavelength ranged from 510 nm to 590 nm.

Fig. 5 PL spectra of CuInS₂ and CuInS₂–ZnS QDs with different excitation wavelength, (b), (d) is normalized corresponding to (a), (c).

Influence of the reaction temperature and time on the optical properties of CuInS₂–ZnS QDs

The PL peak position and intensity of the resulting CuInS₂–ZnS QDs were also found to be dependent on the preparation parameters such as the reaction temperature and time. Therefore, we studied the influences of these two variables on the optical properties of the obtained QDs while maintaining all the other experimental variables as described in the Experimental section.

Reaction temperature plays a key role on the growth process, including surface adsorption, lattice incorporation, lattice diffusion, and etc.^36,37 Fig. 6a and b show the PL spectra of the samples with different reaction temperatures. It could be noticed that with the reaction temperature increasing, the PL intensity increased initially and declined afterwards while the temperature exceeded 230 °C. The PL peak kept a blue-shift from about 616 nm to 580 nm as the reaction temperature increased from 210 °C to 240 °C. This blue-shift indicated a further incorporation of zinc into the core material, so it can be noted that increasing the reaction temperature could result in the precipitate during the growth of ZnS shell. The PL intensity enhancement could be ascribed to the core–shell structure mentioned above, for the ZnS shell will continue to grow with the increasing reaction temperature. Whereas the declination of PL intensity at higher reaction temperature might be attributed to the destruction of DDT ligands²⁹ on the QDs surface.

Fig. 6 (a) PL spectra of CuInS₂–ZnS QDs with different reaction temperature and a fixed reaction time of 20 min. (b) Normalized PL spectra corresponding to (a). (c) PL spectra of CuInS₂–ZnS QDs with different reaction time and a fixed reaction temperature of 240 °C. (d) Normalized PL spectra corresponding to (c).

The influence of the reaction time on the PL spectra of the samples is shown in Fig. 6c and d. It can be noticed that, as the reaction time prolonged, the PL intensity increased firstly, and then declined. The largest PL intensity was obtained at the reaction time of 60 min. The whole process was accompanied by a light blue-shift of the PL emission peak position, which is similar to that of the reaction temperature.

Conclusions

High quality CuInS₂ QDs with an average size of ∼3 nm were prepared using a solvothermal approach, and CuInS₂–ZnS QDs with an average size of ∼7 nm were synthesized by the growth of ZnS shell on the CuInS₂ core. The as-prepared CuInS₂ and CuInS₂–ZnS nanocrystals were in a tetragonal crystalline structure. The measurement results indicated that, the absorption and PL emission spectra of the CuInS₂ QDs were controllable from 600 nm to 550 nm and from 670 nm to 600 nm, respectively. The PL decay curve of the CuInS₂ nanoparticles had a bi-exponential characteristic which implied that surface defect recombination process and internal defect recombination process were both involved in the PL emission of CuInS₂ QDs. With the growth of ZnS, the percentage of internal defect recombination increased which lead to the enhancement of PL intensity. The PL intensity and peak position of CuInS₂ and CuInS₂–ZnS QDs could also be tuned by varying the excitation wavelengths. The fluorescence of CuInS₂ nanocrystals could be achieved from 665 nm to 685 nm when excitation wavelength changed from 550 nm to 630 nm. Meanwhile, for CuInS₂–ZnS nanocrystals, the fluorescence could be achieved from 600 nm to 630 nm with the excitation wavelength changing from 510 nm to 590 nm. Furthermore, reaction temperature and time also played key roles on the optical properties of CuInS₂–ZnS QDs. As reaction temperature increased, the PL peak position blue shifted from about 625 nm to 585 nm. And the PL intensity increased firstly, then decreased slightly. The reaction time had a similar effect trend on the PL spectra of the samples. Overall, the cadmium-free CuInS₂ and CuInS₂–ZnS QDs demonstrate great potential for practical applications in QLEDs and bio-labelling due to their non-toxicity and excellent optical properties.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

H. W. acknowledges Prof. Quanlin Liu and Dr Zhen Song at USTB for PL spectra measurement. The authors are grateful for the financial support from the National Natural Science Foundation of China (51101013) and the Fundamental Research Funds for the Central Universities (FRF-TP-14-012A2, FRF-TP-15-007A3).

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Footnote

† Y. H. Jia and H. C. Wang contributed this paper equally.

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