Since electroluminescent (EL) quantum dots (QDs) are considered a key component of the next-generation display, and large-scale production of environment-friendly QDs is required for their wide use in commercial displays. Therefore, several studies on non-cadmium QDs, such as indium phosphide (InP) QDs in the III-V category, graphene QDs, and copper indium sulfide (CuInS2) or silver indium sulfide (AgInS2) QDs in the I-III-VI2 category, have been conducted owing to their non-toxicity and good optical properties. Subsequently, significant results have been reported for green and red colors. However, for synthesis of blue QDs, pure blue emission in the range of 440-460 nm has been achieved with few materials. Among them, zinc selenide (ZnSe) is a promising candidate for synthesizing blue QDs. However, owing to the wide band gap (2.7 eV) of ZnSe, highly effective QDs were attained in the violet region (420-440 nm). Here, for the first time, we have synthesized ZnSe/ZnSexS1-x/ZnS QDs emitting at a wavelength of 444 nm with high photoluminescence quantum yield (PLQY) of 77.2%. Also, full width at half maximum (FWHM) of 23.3 nm ensured its excellent color purity. Use of a gradient intermediate shell of ZnSeS in the original ZnSe/ZnS QDs was the key factor behind this achievement. The intermediate gradient shell of ZnSeS around the core delocalizes the electrons, weakening the quantum confinement effect (QCE), hence rendering the emission color of the QDs tunable from violet to blue by manipulating the ratio of selenium (Se) and sulfur (S) in the composites. A blue emission peak centered at 452 nm was observed for the quantum dot light-emitting diodes (QD-LEDs) fabricated using the above-mentioned QDs, and an external quantum efficiency (EQE) of 5.32%, current efficiency of 1.51 cd/A, and power efficiency of 0.74 lm/W were reported. Furthermore, our fabricated device exhibited a maximum brightness of 3,754 cd/m2 and a half operational time (LT50) with 100 cd/m2 initial luminance of 1.27 h, which are the highest values of these parameters to be reported till date for a blue QD-LED fabricated using ZnSe core based QDs in pure blue region.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Quantum dots (QDs) have been extensively studied worldwide because of their remarkable optical characteristics, such as convenience in band gap tuning, narrow full width at half maximum (FWHM), high photoluminescence quantum yield (PLQY), and cost-efficiency in fabrication. Recently, colloidal quantum dot light-emitting diodes (QD-LEDs) based on QD nanoparticles have been attracting attention for application in electroluminescent displays owing to their high color purity, stability, cost efficiency, and solution-based applications [1–3]. Up to date, many EL devices using cadmium based QDs [4–6] and cesium lead bromide (CsPbBr3) perovskite QDs [7–9] have shown remarkable results owing to the outstanding optical properties of the QDs themselves, but potentially harmful elements on human health such as Cd and Pd were used.
To date, considerable efforts have been made for synthesizing environment-friendly green QDs for the commercialization of non-cadmium QD-based QD-LEDs [10–12]. For green and red color QDs, remarkable improvement in results in terms of high PLQY, narrow FWHM, and high stability has been reported by using indium phosphide (InP) core with further multi-shelling methods [13–15]. Employing these QDs to fabricate EL devices, external quantum efficiencies (EQEs) above the theoretical limit (∼20%) were achieved with various promising device properties, such as luminance, and current efficiencies, which were all similar to those obtained from EL devices fabricated using cadmium based QDs . However, among the three primary colors of blue, green, and red, blue emitters did not exhibit expected efficiency because the synthesis of blue QDs using non-cadmium materials had several limitations [17–19].
To synthesize environment-friendly blue QDs, ZnSe based QDs have shown significant potential. Although the wide band gap of 2.7 eV of ZnSe limits its use in the growth of the QDs emitting pure blue color, some researchers have successfully deposited a zinc sulfide (ZnS) shell around a ZnSe core to synthesize QDs emitting in the range from 400 nm to 450 nm . However, the synthesized QDs efficiently emitted only around 430 nm wavelength with the highest PLQY of 83% and with a color purity corresponding to an FWHM of 15 nm.
Another way of synthesizing a ZnSe core-based QD for true blue emission is to use an alloy of ZnSe and zinc telluride (ZnTe) as the core because ZnTe has a low band gap of 2.25 eV. . Additional shelling of ZnSe and ZnS around the ZnSeTe core resulted in the synthetization of QDs emitting at 441 nm with a PLQY of 70% and FWHM of 32 nm. When these QDs were used to fabricate QD-LEDs with conventional device structure, an EQE of 4.2%, luminance of 1,195 cd/m2, and current efficiency of 2.4 cd/A were obtained. However, we notice that the FWHM of the QDs with ZnSeTe alloy core is approximately twice as large as compared to that of the ZnSe core QDs, which results from the vulnerable heterogeneity of the core due to alloying. In addition, some types of defects may be formed due to the difference in the ionic radius between Se and Te causing a broader FWHM.
In this study, to maintain the high optical characteristics (PLQY, FWHM) values of the QDs and simultaneously, tune the emission wavelength from violet to blue, we synthesized ZnSe/ZnSexS1-x/ZnS QDs by adopting a gradient shell of ZnSeS. According to previous reports, for alloyed intermediate shells with different composition ratio of Se and S, the emitting wavelength of the QDs exhibited a red-shift tendency of as much as 30 nm with fixed QD sizes . Moreover, introducing a ZnSeS gradient shell in between a ZnSe core and a ZnS outer-most shell through well-passivated shelling methods for improved efficiencies of the QDs and their performances when used to fabricate QD-LEDs can possibly reduce the Auger recombination and non-radiative decay [23–24]. Therefore, synthesis of the intermediate ZnSeS gradient shell resulted in the fabrication of highly efficient and stable QDs while shifting the emission wavelength of the QDs from violet to true blue color.
2. Experimental section
Sulfur (S, 99.998%), zinc stearate (Zn(St)2, C36H70O4Zn, 99%), oleic acid (OA, C18H34O2, 99.99%), 1-octadecene (ODE, C18H36, 90%), 1-octanethiol (OT, C8H18S, 98%), zinc acetate dihydrate (Zn(ac)2, C4H6O4Zn·2H2O, 98%), n-hexane (C6H14, 99.8%), acetone (C3H6O, 99.9%), isopropanol (C3H8O, 99.7%), ethanol (C2H5OH, 99.5%), and chlorobenzene (C6H5Cl, 99.8%) were purchased from Sigma Aldrich. Selenium powder (Se, 99.5%) and trioctylphosphine (TOP, C24H51P, 90%) were obtained from Alfa Aesar and Acros Organics, respectively. tris(4-carbazoyl-9-ylphenyl)amine (TCTA), Molybdenum(VI) oxide (MoO3), and silver (Ag) were procured from OSM Co. Zinc magnesium oxide was purchased from NanoFix Co. All materials were used with no further purification.
2.2 Synthesis of ZnSe/ZnSeS/ZnS QDs
2.2.1 Synthesis of ZnSe core
For the synthesis of the ZnSe core, 2 mmol of Zn(St)2 and 1 mmol of Se with 20 mL of ODE were placed in a three-neck flask. This reaction was degassed at 120 °C overnight to get rid of all residues, such as oxygen and water. The reaction was further heated to 200 °C for 2 h to completely ionize the Zn and Se precursors. To initiate the growth of the ZnSe core, the temperature was raised to 280 °C and was maintained for 1 h. Further, Zn stock solution (2.5 mmol of Zn acetate, 2.5 mL of OA, and 5 mL of ODE) and Se stock solution (2.5 mmol of Se and 1 mL TOP) were injected into the flask at 280 °C, and the reaction mixture was stirred for 1 h.
2.2.2 Synthesis of ZnSeS shell
The intermediate ZnSe0.4S0.6 gradient shell was synthesized around the synthesized ZnSe core. Se precursor (0.4 mmol of Se in 0.5 mL TOP) and S precursor (0.6 mmol of S in 0.5 mL TOP) were injected at 280 °C to the reaction flask followed by injection of Zn precursor (0.75 mmol of Zn acetate, 0.75 mL of OA, and 1.5 mL ODE). For proper shelling, the temperature was elevated to 300 °C, and the reaction mixture was maintained for 30 min. The process was repeated once more; however, the injection temperature was raised to 320 °C this time.
2.2.3 Synthesis of ZnS shell
Synthesis of the outer-most ZnS shell was carried out around the synthesized ZnSe/ZnSe0.4S0.6 core/shell structure. Zn precursor (1 mmol of Zn acetate, 1 mL OA, and 1 mL ODE) and S precursor (1 mmol of S and 1 mL TOP) were sequentially injected at 320 °C upon the synthesized ZnSe/ZnSeS core/shell, and the reaction mixture was maintained for 30 min at 320 °C. Further, the temperature was lowered to 200 °C; the same amount of Zn precursor previously injected and S precursor (0.25 mL of OT) were further injected, and the reaction was kept at 200 °C for 1 h. The synthesized ZnSe/ZnSeS/ZnS core/shell/shell QDs were purified using acetone and were re-dispersed in hexane. This process was repeated three times.
2.3 Fabrication of QD-LEDs
QD-LEDs using ZnSe/ZnSeS/ZnS blue-emitting QDs were fabricated using a multi-layered inverted device structure. For the fabrication of QD-LED devices, ITO-coated glass substrates (25.4 × 25.4 mm2) were cleaned consecutively in ultrasonic baths of isopropanol/acetone mixture and isopropanol and subsequently, were exposed to UV/O3 for 15 min. All thin films made on ITO-coated glass substrates were fabricated under an inert atmosphere or high vacuum. The substrates were spin-coated with Zn85Mg15O NPs at 2,000 rpm for 60 s and were baked at 150 °C for 30 min. The QDs in hexane solution were spin-coated at 2000rpm for 30 s and were annealed at 60 °C for 10 min. Finally, the multilayer samples were transferred into a high vacuum chamber, and 40 nm of TCTA, 10 nm of MoO3, and 100 nm of silver (Ag) were consecutively thermally deposited at a base pressure of 1.0 × 10−6 torr.
2.4 Material and device characterization
The absorption and PL spectra were measured using an UV-1601PC (Dong-il SHIMADZU Co.) and an FP- 6500 (JASCO Co.) spectrophotometer, respectively. PLQY was measured using a C11347-11 Quantaurus-QY absolute PL QY spectrometer (Hamamatsu photonics). X-ray diffraction (XRD) profiles were obtained using an X-ray diffractometer (Rigaku, Ultima IV) with a monochromatic Cu Kα radiation source. Transmission electron microscopy (TEM) images of the quantum dots and of a cross section of the QD-LED device were acquired using an electron microscope (JEOL, JEM-ARM200F) operating at 200 keV. Energy dispersive spectrometry (EDS) was investigated by using a JEOL Dual SDD Type (Φ: 100 mm2, solid angle: 1.7 sr). Ultra-violet photoelectron spectroscopy (UPS) of the synthesized QDs were collected from Electron Spectroscopy for Chemical Analysis (AXIS SUPRA (Kratos, U.K)). All device performances, including EL spectra, were measured using a Spectra Scan PR655 (Photo Research) and a computer-controlled Keithley 2400 (Tektronix Co.). The emission area of all QD-LED devices was 2 × 2 mm2, and the luminance was measured only in the forward direction.
3. Results and discussion
3.1 Structural characterization and optical properties
Synthesis of the QDs consists of nucleation at low temperature and growth at higher temperature with multiple injection steps inbetween with increasing temperature [25–28]. As recorded in the experimental section, the experiment was conducted at a reaction temperature of 280 °C for 1 hour to synthesize the ZnSe core, but these conditions are not considered optimal Ostwald ripening conditions. In the next study, we plan to find the optimal Ostwald ripening condition by performing another ZnSe core synthesis conditions. In general, cores of the QDs are crystallized at higher temperature than that for the growth of the shells eliminating the possibility of secondary nucleation during the growth of the shells around the core. However, by using a lower amount of precursor concentrations injected during the growth of the shell compared to that injected during the core growth, secondary nucleation could be limited. Furthermore, using mildly active precursors such as Zn(OA)2 and OT as Zn and S precursors, respectively limits the concentration level beneath the super-saturation level causing non-secondary nucleation. In addition, injecting the shell precursors in periodic steps ensures a low concentration level of the injected shell precursors and stimulates homogenous growth of the shell . Efficient passivation of the shells around the core was achieved using this process. Further, in this study, we observed changes in the optical properties of the synthesized QDs upon varying the constituent ratio of the Se and S in the intermediate shell (ZnSeS).
As shown in Figs. 1(a) and 1(b), the absorbance and the PL spectra of ZnSe and ZnSe/ZnSeS/ZnS QDs were measured. As indicated in Fig. 1(a), any exciton absorption peak of ZnSe/ZnSeS/ZnS was not observed. Considering blue Cd-free QDs, many studies including our work had trouble distinguishing evident exciton absorption peak [21, supporting info #4]. However, the degree of red shift can be observed in the absorption and PL spectra, the main discussion of the ZnSeS gradient shell. As shown in Fig. 1 and listed in Table 1, with increase in the Se concentration at the cost of S concentration, a maximum red-shift of 13 nm (from 434 to 447 nm) is observed in the maximum PL wavelength. This may be caused by the delocalization of charge carriers into the shell as the gradient intermediate shell is introduced [22–23,29]. Subsequently, electrons would become more delocalized, whereas the holes would be confined mainly in the core region due to the heavy mass compared to the electrons in the QD’s lattice. This would lead to the weakening of the QCE, further, resulting in the red-shift in the absorbance and PL spectra. However, the reduction in QCE through delocalization of charge carriers appears to have reached a maximum in the QD with Se and S ratio of 0.6:0.4. This is because, as shown in Fig. 1(b) and the Table 1, no further red-shift were observed at the PL maximum peak positions of the QD with Se and S ratios of 0.6:0.4 and 0.8:0.2. Moreover, as indicated in Table 1, we observed that PLQY mostly increases when a gradient intermediate shell is introduced between the ZnSe core and the outer-most ZnS shell. This may be attributed to the alleviation of the significant lattice mismatch between the core ZnSe (a = 5.667 Å) and the shell ZnS (a = 5.405 Å) upon the introduction of the intermediate shell . Moreover, PLQY increases upon increasing the Se concentration to 0.4 and decreases with further increase in the Se concentration. However, as the Se proportion in the ZnSeS intermediate shell increases to a certain point, PLQY drops because of possible increase in nonradiative decay such as Auger recombination. Moreover, for Se concentration above 0.4, the PLQY loss may be attributed to the inhomogeneous deposition of the gradient ZnSeS shell around the core causing vacancies and interfacial defects between the core and the shell. Another important optical characteristic, FWHM, seems to compensate the red-shift of the absorbance and PL spectra as it becomes broader with the red-shift of the emission wavelength of the QDs (Fig. 1(b)). From Table 1, as the proportion of Se increases, broader PL emission is observed (from 16.7 to 30.5 nm). Similar to recent research using gradient alloyed shell , an increase in Se precursor concentration can lead to competition in ZnSeS shell growth and new ZnSe nucleation, which could cause heterogenous growth of the QD. In more detail, the probability of new ZnSe nucleation at an augmented Se ratio can increase because the shelling temperature of the gradient ZnSeS shell already exceeds the critical point temperature, thereby making the synthesis vulnerable to secondary nucleation of the ZnSe core. At this point, different sized ZnSe cores (ZnSe cores formed by primary nucleation and secondary nucleation) continue to grow in the further shelling process, making the final QD product heterogeneous with broadened FWHM. Narrowing of FWHM due to the alloying process  between the core and the shell atoms was not observed in our system. Herein, to further study the optical properties, crystallinity, and morphology, we selected the QD possessing a ratio of 0.4:0.6 between Se and S concentration in the intermediate shell (ZnSe/ZnSe0.4S0.6/ZnS) since it had the highest PLQY of 77.2% and the narrowest FWHM (22.3 nm) among the QDs having PLQY higher than 70% and emission wavelength in the range of pure blue.
To investigate the crystallinity and estimate the particle sizes of the synthesized QDs, TEM images and XRD patterns were acquired. In Fig. 2(a), the core, core/shell and core/shell/shell structured QDs are shown, and their sizes of approximately 6.9, 8.0, and 9.2 nm, respectively, are listed in Table 2. In Fig. 2(d), XRD patterns for the core, core/shell, core/shell/shell structured QDs are observed. Three distinctive peaks corresponding to (111), (220), and (311) are present indicating the zinc blende structure of ZnSe and ZnS. The peaks of the ZnSe/ZnSeS/ZnS QDs are shifted to a larger 2θ value compared to those of the ZnSe core and ZnSe/ZnSeS QDs because the outer-most ZnS shell is finally passivated in the ZnSe/ZnSeS/ZnS QDs. In addition, the peaks are more prominent in the ZnSe/ZnSeS/ZnS QDs since the shells are effectively passivated on the core implying significantly high crystallinity of the QDs with the core/shell/shell structure.
By analyzing the scanning transmission electron microscopy (STEM) images in Fig. 3(a), size distribution of the as-synthesized ZnSe/ZnSe0.4S0.6/ZnS QDs is confirmed to be homogenously spherical. In the overlay image of Zn, Se, and S in Fig. 3(a), the core/shell/shell structure of the QD is observed with ZnSe core in the center, ZnSeS gradient shell around the core, and the outer-most shell of ZnS since Se is not detected in the outer-most part of the QDs. In Fig. 3(b), from the EDS spectra, the atomic percentages of Zn, Se, and S in the ZnSe/ZnSe0.4S0.6/ZnS QDs are estimated to be 42.51%, 39.97% and 17.52% respectively.
To estimate the energy levels of the synthesized ZnSe/ZnSe0.4S0.6/ZnS QDs, Tauc plot band gap and UPS data were obtained. Tauc plot of the ZnSe/ZnSe0.4S0.6/ZnS QDs from the UV-vis absorption spectra is shown in Fig. 4(c). From the Tauc plot, the band gap (Eg) of the QDs is estimated to be 2.76 eV, which is in good correlation with the emission of the corresponding QD at 444 nm (Table 1). Figures 4(a) and 4(b) show the cut-off and onset UPS spectra, respectively. The cut-off energy (Ecut-off) and on set energy (Eon set) are estimated to be 19.12 and 4.56 eV, respectively. Using these values, the valence band maximum (VBM) and conduction band minimum (CBM) are calculated to be -6.64 and -3.88 eV, respectively (Table 3) .
Tauc plot of the ZnSe/ZnSe0.4S0.6/ZnS QDs from the UV-vis absorption spectra is shown in Fig. 4(c). From the Tauc plot, the Eg of the QDs is estimated to be 2.76 eV, which is in good correlation with the emission of the corresponding QD at 444 nm (Table 1).
3.2 Device performances
To use the synthesized ZnSe/ZnSe0.4S0.6/ZnS QDs in fabrication of QD-LED devices, we chose an inverted device structure, where indium tin oxide (ITO) and Ag act as the cathode and anode electrodes, respectively, because the VBM of the synthesized QDs is -6.64 eV, which is significantly lower compared to the highest occupied molecular orbital (HOMO) of several organic hole transport layers (HTLs). Employing a conventional device structure limits the selection of the HTL to well-known Poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenylamine) (TFB) and poly(9-vinylcarbazole) (PVK). TFB exhibits excellent hole mobility; however, its HOMO is approximately -5.3 eV implying that it has an energy barrier of approximately 1.3 eV with our QDs. Although PVK (-5.8 e V) has a lower HOMO than TFB, PVK possesses significantly low hole transporting ability, which leads to low device efficiency. Therefore, adopting the inverted device structure offered greater choice of HTLs. Among them, tris(4-carbazoyl-9-ylphenyl)amine (TCTA) was chosen in our device since it had high hole mobility and low HOMO (-5.9 eV). In Fig. 5(a), the device scheme is depicted. Here, ITO, 15% magnesium doped zinc oxide nanoparticles, TCTA, molybdenum oxide (MoO3), and silver (Ag) were used as the cathode, electron transport layer (ETL), HTL, hole injection layer (HIL), and anode, respectively. The thickness of the fabricated QD-LEDs is estimated using cross-sectional TEM image (Fig. 5(c)). The thickness target is as follows: ITO (150 nm)/ ETL (35 nm)/ EML (25 nm) / HTL (40 nm) / HIL (10 nm) / Ag (100 nm). Moreover, as shown in Fig. 5(d), the EDS elemental image taken from the cross-sectional TEM of the fabricated device confirmed that each layer was properly stacked with the appropriate layers.
In Fig. 5(b), the energy band diagram of the inverted QD-LEDs is shown. The reason of using 15% magnesium doped zinc oxide nanoparticles is evident in Fig. 5(c). Electrons move considerably faster than holes in QD-LEDs. Therefore, by using 15% magnesium doped zinc oxide nanoparticles, the energy barrier between the ITO and ETL is maximized resulting in slower electron mobility in the device.
Based on the QD-LED device structure shown in Fig. 5(a), the device characteristics are shown in Fig. 6, and obtained parameters are listed in Table 4. From the fabricated QD-LEDs, obtained maximum EQE, current efficiency (CE), power efficiency (PE), and luminance are 5.32%, 1.54 cd/A, 0.74 lm/W, and 3,754 cd/m2, respectively. The CIE color coordinates are (0.155, 0.025) with EL emission wavelength of 452 nm. The EQE of 5.32% is, so far, the highest among the reported values of EQE of ZnSe core-structured QD based QD-LEDs emitting near 450 nm. The reason for this relatively high device performance can be attributed to two factors. Firstly, Our QD achieved the best EQE by adopting a gradient ZnSeS shell placed between the ZnSe core and the outermost ZnS shell, providing an optimal lattice constant and high potential barrier. This can well suppress non-radiative energy transfer processes such as Auger resulting in high PLQY of QD and efficient device properties. Secondly, even though the hole transport barrier still exists between the valence energy level of QD (-6.64 eV) and the HOMO energy level of HTL (-5.83 eV), excellent hole transport mobility of TCTA (3×10−4 cm2 V−1 s−1)  played a beneficial role in achieving good device efficiencies, especially brightness of the fabricated device. Furthermore, luminance of 3,754 cd/m2 makes it the brightest emitting QD-LED among the QD-LEDs fabricated using ZnSe core based QDs. In Fig. 6(f), approximately 7 nm of red-shift is observed between the PL (444 nm) and EL (452 nm) emission wavelength primarily because of the quantum-confined Stark shift . Also, from the EL spectrum in Fig. 6(f), none of parasitic EL spectrum from the neighboring layers were observed, indicating that the recombination of holes and electrons were effectively processed within the QD. Turn on voltage (Von) of 4.01 V is significantly high (Table 4), which is attributed to the high charge injection barrier between the HOMO of the TCTA and VBM of the QDs. This problem results in lower device efficiency compared to the green and red QD-LEDs, where mostly non-cadmium containing InP/ZnSe/ZnS QDs are used as the EML. Another reason behind the unsatisfactory QD-LED performance is that the PLQY of the non-heavy metal containing QDs in the true blue region is considerably lower than that of the eco-friendly QDs in the green and red region.
With the initial luminance of 100 cd/m2, the operational lifetime measurement was conducted for half of relative luminance (LT50). From Fig. 7, LT50 is estimated to be approximately 1.27 h, which is listed in Table 5. It is the best performing QD-LED among QD-LEDs using non-heavy metal QDs as EML although for commercial of blue QD-LEDs, intensive research on operational device lifetime is still in need.
In this study, we successfully tuned the emission wavelength of the ZnSe core based QDs toward true blue color from the original violet by introducing an intermediate ZnSeS shell around the ZnSe core in ZnSe/ZnSeS/ZnS QDs with core/shell/shell structure. By varying the ratio between Se and S in the ZnSexS1-x shell, optical properties of PLQY, FWHM, and emission wavelength were manipulated. Among them, ZnSe/ZnSe0.4S0.6/ZnS QDs, which had the highest PLQY of 77.2%, were used in the fabrication of QD-LEDs. Inverted device structure of the QD-LEDs was employed to reduce the large difference in energy level of the HOMO of the HTL and VBM of the QDs. Our device showed remarkable efficiencies, especially in terms of brightness and EQE. To the best of our knowledge, this device exhibited the highest luminance of 3,754 cd/m2 and its EQE value of 5.32% is reported to be the highest among the reported devices emitting pure blue color (452 nm) using ZnSe as the core of the constituent QDs. In order to use the ZnSe core based QDs synthesized in this study as a display emitting layer, the material and device characteristics must be further improved. Nevertheless, this method of synthesizing non-heavy metal based QDs for pure blue emission will offer a viable option in future display through the use of eco-friendly inorganic materials as LEDs because QD materials can completely solve the chronical burn-in problem found in OLED displays, and can mass-produce displays on large substrates of the 8th generation or higher.
National Research Foundation of Korea (2019R1A2C1005784); Gyeonggi-do Regional Research Center (dankook 2016-B01).
The authors declare no conflicts of interest
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