Introduction

Semiconducting metal halide-based perovskite nanocrystals (NCs) have been studied extensively over the past several years due to their remarkable optoelectronic properties, low-cost solution processability, and scalable fabrication1,2,3,4,5,38,39,40,41. Recently, we reported the synthesis of lead-reduced CsPb1-xMgxX3 (up to x = 20%) NCs for the fabrication of the WLEDs42. Our experimental results showed that the as-synthesized NCs retained the superior optical properties and these NCs have been utilized to fabricate WLEDs with superior color quality. Partial replacement of Pb2+ using Zn2+ in lead halide perovskites and their potential applications in photovoltaic cells have been demonstrated43,44,45. Besides the reduction of lead content, the as-fabricated solar cells resulted in enhanced thermal stability and improved photovoltaic performance of the device structure. The partial substitution of the reduced B-site cations (compared to Pb2+) such as the transition metal ions can reduce the size of eight cubo-octahedral voids which has the potential to decrease all of the cubo-octahedral voids for A-site cations in the ABX3 lattice. This results in a stable perovskite phase with superior thermal stability38,45. Furthermore, Shen et al. observed that the application of Zn2+-alloyed CsPbI3 NCs as a light emitter resulted in an LED with a peak external quantum efficiency of 15.1%46. However, there is no report on the fabrication of WLEDs using Zn2+-alloyed CsZnxPb1-xX3 NCs as color conversion layers.

In this work, the comprehensive studies are carried out to investigate the effect of Zn2+-do** on the structural and optical properties of CsZnxPb1-xX3 NCs (x up to 15%). The ultimate goal of this work is to realize the low-cost, highly efficient, and environmental-friendly white light with superior color quality. The inclusion of Zn2+ in the CsZnxPb1-xX3 NCs results in a lattice contraction, however, the NCs maintain identical crystal structure and the morphology. The as-grown NCs exhibit strong photoluminescence (PL) emissions that are blue-shifted with increasing Zn2+ content because of the reduced size of unit cells while maintaining the high PLQY (>16% for CsZnxPb1-xCl3 and >80% for CsZnxPb1-xBr3). In addition, the tunable bandgap from 1.82 eV to 3.19 eV and tunable emission from 411 nm to 636 nm are obtained by engineering the halide compositions for 15% Zn2+ inclusion. Furthermore, the prototype WLEDs based on CsZnxPb1-xX3 NCs (for 15% Zn2+) are experimentally demonstrated by stacking the Zn2+-alloyed blue-, green-, yellow-, and red-emitting NCs on top of a UV LED. The WLEDs emit white light with a tunable CCT from warm to cold (2218–8335 K). In addition, an extra high CRI (maximized to 93) and high LER (268–318 lm·W−1) are achieved which is superior to the previous studies on WLEDs based on metal halide perovskites doped with other transition metals. Thus, this work opens up new prospects to engineer the properties of compositionally diverse lead-reduced halide perovskite NCs potentially to be used in lighting technology.

Results and Discussion

Effect of Zn2+-alloying on the Structural, Morphological, and Optical Properties of CsZnxPb1-xCl3 and CsZnxPb1-xBr3 NCs

The effect of the inclusion of Zn2+ on the structural property of CsPbCl3 and CsPbBr3 is investigated by measuring the XRD patterns. The XRD patterns of CsZnxPb1-xCl3 (Fig. 1a) and CsZnxPb1-xBr3 (Fig. 1b) for the different concentrations of Zn2+ confirm that the NCs have cubic structure and the phase is independent of Zn2+ content. The main diffraction peaks are at ∼15°, ∼22°, ∼30°, and ~38° corresponding to the (100), (110), (200), and (211) diffraction planes of the cubic crystal structure, respectively. However, there is some evidence that the structure of the CsPbCl3 and CsPbBr3 NCs exhibit an orthorhombic phase at the unit cell level47. The opposed orthorhombic subdomains of the NCs annihilate their superstructural peaks leading to a perceived cubic phase that is in reality pseudocubic. As shown in Fig. 1b, the diffraction peak corresponding to the angle ∼30° for CsZnxPb1-xBr3 appears more discrete with a gradual increase in the content of Zn2+. This is an indication of the presence of the orthorhombic structure due to the octahedral tilting47,48. Furthermore, the Zn2+-alloyed CsZnxPb1-xCl3 and CsZnxPb1-xBr3 NCs exhibit the diffraction peaks shifted towards a higher 2θ range. The shift corresponds to 0.174° and 0.276° for CsZnxPb1-xCl3 and CsZnxPb1-xBr3 NCs, respectively when the concentration of Zn2+ increases form 0% to 15% (Supplementary Fig. S1). The observed peak shift is due to the lattice contraction of as-synthesized NCs within the unit cell. The size of the lattice decreases from 5.632 Å to 5.601 Å for CsZnxPb1-xCl3 NCs and from 5.869 Å to 5.817 Å for CsZnxPb1-xBr3 NCs. The lattice contraction suggests that Pb2+ cations with a bigger ionic radius (119 pm) occupying the B-site of BX6 octahedra are being partially replaced by Zn2+ cations with a smaller ionic radius (74 pm) at the same lattice site of Pb2+. If Zn2+ atoms have resided in interstitial sites, we would have observed a lattice expansion with increasing Zn2+ content46. Thus, the incorporation of the smaller guest element Zn2+ in the octahedra of parent CsPbX3 perovskite structure results in the reduced size of the BX6 octahedra (Fig. 1c) that gives rise to the distortion and tilting of the BX6 octahedra. The octahedral tilting accompanied by the change in the lattice constant lead to the increased bandgap and blue-shifted PL emission at higher content of Zn2+ which will be discussed in the latter part. Interestingly, the XRD scans do not indicate the presence of the additional peaks even after the incorporation of guest cation Zn2+, signifying the absence of any crystalline impurities. Furthermore, the presence of a low angle (<10°) peak in the XRD patterns indicate the presence of superlattice structures. The intensity of the superstructural peaks increases with an increase in the content of Zn2+ which indicates the enhancement in the periodicity of the NCs.

Figure 1
figure 1

Structural and morphological properties of CsZnxPb1-xX3 NCs at a different content of Zn2+. (a,b) XRD patterns of CsZnxPb1-xCl3 and CsZnxPb1-xBr3 NCs. (c) Schematic overview of contraction of octahedral after partially replacing Pb2+ by smaller Zn2+ ions. (d,e) TEM images (scale bar, 50 nm) of the NCs. (f,g) Histograms showing size distribution of corresponding NCs.

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The morphology of the as-synthesized NCs is investigated using transmission electron microscopy (TEM). The TEM images (Fig. 1d,e) show the uniformly distributed nanocubes without a change in morphology even after the inclusion of Zn2+ in the parent NCs. However, the average side length of CsZnxPb1-xCl3 NCs decreased from ∼8.1 ± 1.4 nm to ∼7.5 ± 0.9 nm and that of CsZnxPb1-xBr3 decreased from ∼10.3 ± 1.7 nm to ∼8.41 ± 1.2 nm when the content of Zn2+ is increased from 0% to 15%. The decrease in particle size at high content of Zn2+ may be a result of increased halide ion uptake, which hinders the crystal growth42,46. The size distribution analysis of as-grown samples is illustrated in Fig. 1f,g. From these histograms, it can be concluded that the particle size distribution also decreases with increasing the content of Zn2+. Thus, from the XRD and TEM analysis, we found that the incorporation of Zn2+ results in the increase in periodicity and uniformity of the NCs with a decrease in NCs size.

The effect of the inclusion of Zn2+ on the optical properties of CsPbX3 perovskites is characterized by measuring PL and absorption spectra as shown in Fig. 2. The normalized PL and absorption spectra of CsZnxPb1-xCl3 and CsZnxPb1-xBr3 exhibit a modest blue-shift with increasing Zn2+ content in the host materials (Fig. 2a–d). Since both the Pb2+ and Zn2+ precursors contain the same halides, this paves a pathway for the inclusion of Zn2+ ions upon its introduction into the host CsPbX3 matrix and favors the change in the optical properties of Zn2+-alloyed NCs. As-grown CsZnxPb1−xCl3 NCs exhibit PL emission in the violet region with emission peak attenuated from 417 nm to 411 nm (excited at 300 nm), whereas CsZnxPb1-xBr3 NCs, show the green emission which is tuned from 522 nm to 517 nm (excited at 365 nm). This blue-shift in the optical properties is related to the slight decrease in the size of the NCs that can be ascribed to the weak quantum confinement effect. The full width at half-maximum (FWHM) of PL emissions of CsZnxPb1-xCl3 NCs is decreased from 13.4 nm to 12.8 nm and that of CsZnxPb1-xBr3 NCs is decreased from 16.9 nm to 15.5 nm when the content of Zn2+ is increased from 0% to 15%. The narrow FWHM shows that the obtained NCs are uniformly distributed and decreased FWHM indicates an increase in the uniformity in the size distribution of NCs. This observation is also in agreement with the TEM images. In addition, the reduced FWHM of PL emission is advantageous for the fabrication of energy-efficient LEDs with high color purity. The UV-visible absorption spectra show a well-defined absorption peak for all the samples

Figure 2
figure 2

Optical properties of CsZnxPb1-xX3 NCs at a different content of Zn2+. (a,b) PL emission spectra of CsZnxPb1-xCl3 and CsZnxPb1-xBr3 NCs, respectively. (c,d) UV-visible absorption spectra of the corresponding NCs. (e,f) Excitation spectra of the corresponding NCs.

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(independent of the halide) that indicate the larger excitonic binding energy of the NCs49. The key spectral features of the parent CsPbCl3 and CsPbBr3 NCs, such as the sharp optical absorption edge and well-defined absorption peaks are maintained even after the inclusion of Zn2+ (Fig. 2c,d). The absorption peak of CsZnxPb1-xCl3 observed at ∼394 nm is blue-shifted to ∼388 nm and that of CsZnxPb1-xBr3 is shifted from ∼485 nm to ∼480 nm. The corresponding absorption peak was examined to estimate the optical bandgap of the different composite of NCs. The bandgap of CsZnxPb1-xCl3 and CsZnxPb1-xBr3 NCs is tuned from 3.144 eV to 3.194 eV and 2.557 eV to 2.578 eV, respectively. The increase in bandgap is most likely due to the increasing electronegativity (χ) of the Zn2+ ions (χ ∼1.7) occupying the B-site in the ABX3 perovskite structure compared to the Pb2+ ions (χ ∼1.6), which reduces the dimensions for both the crystal lattice and the NC and is in agreement with the XRD and TEM analyses50. Furthermore, all the samples prepared with various Zn2+ content exhibit identical and broadband excitation spectra (Fig. 2e,f). The excitation spectra are measured by monitoring the corresponding peak emission wavelength. The broad spectral band suggests that these perovskite materials can be effectively excited at a broad range of wavelengths. The excitation spectrum is at a higher energy region compared to the corresponding emission energy spectrum that reflects the potential applications of these materials in conjunction with III-nitride based UV/blue LEDs to obtain white light through photon down-conversion. The dual spectral bands shown in the excitation spectra are related to a charge-separated bandgap state (at higher wavelength) and the charge transfer band (at lower wavelength)51. Furthermore, the observed linear relationship between the PL energy and the lattice vector (Fig. 3a,b) for the NCs with the different incorporated amounts of Zn2+ signifies that the effect of lattice contraction is dominant factors in altering the observed optical behavior of the as-synthesized NCs. In brief, the lattice contraction favors the stronger interactions between ‘B’ and ‘X’, which induces the antibonding combinations between Pb(6p) and Br(4p) resulting in a shift of the conduction band minimum to higher energies and, hence the bandgap of NCs increases39. A similar trend was observed on ABX3 NCs (with tunable blue-shifted PL emission) that were obtained by other compositional control3,20. Moreover, the as-grown NCs exhibit the high PLQYs as shown in Table 1. The PLQY of CsZnxPb1-xCl3 NCs at 0% Zn2+ is 19% and that at 15% Zn2+ is 16%. Similarly, PLQY of CsZnxPb1-xBr3 NCs at 0% Zn2+ is 93% and that at 15% Zn2+ is 92%. The PLQY of CsZnxPb1-xCl3 NCs is reduced by a higher value in comparison to CsZnxPb1-xBr3 NCs. The halide vacancies and Cs vacancies are the dominant defect states for CsPbCl3 and CsPbBr3 perovskite NCs. At the highest content of Zn2+ do**, the formation energy corresponding to these vacancies in the Cl-based NCs might have decreased by a larger amount in comparison to the Br-based NCs. Eventually, this results in an increase in the structural defects leading to a higher decrease in PLQY of the Cl-based NCs52,53. This demonstrates that Zn2+ has an impact on absolute PLQYs of alloyed NCs suggesting that the inclusion of Zn2+ altered the defect states of the host NCs.

Figure 3
figure 3

The variation of PL energy with lattice vector for different compositions of Zn2+, fitted with an error bar which is approximated using the Gaussian model fitting. (a) PL energy variation of CsZnxPb1-xCl3 NCs. (b) PL energy variation of CsZnxPb1-xBr3 NCs. The different points correspond to 0%, 3%, 5%, 10%, and 15% of Zn2+, respectively. The highest energy point corresponds to the highest content of Zn2+ and the lowest energy point corresponds to the lowest content of Zn2+.

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Table 1 The PLQYs of CsZnxPb1-xCl3 and CsZnxPb1-xBr3 at various Zn2+ content.
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Exploration of Zn2+-alloyed CsZnxPb1-xI3 Perovskites

The study of the effect of Zn2+ incorporation on the iodide-based perovskite NCs is carried out. XRD patterns (Supplementary Fig. S2) indicate that the samples prepared and stored in an ambient atmosphere decomposed significantly after some hours, similar to the study reported earlier54. This is ascribed to the degradation of the NCs from the black phase to the non-emissive yellow phase. However, the fresh samples prepared without and with the inclusion of Zn2+ show tunable PL emission (672–646 nm) with the FWHM in the range of 50–54 nm, Supplementary Fig. S3.

Tunable Emission from CsZn0.15Pb0.85X3 (X = Cl−, Br−, I−) NCs by Varying the Halide Compositions

All-inorganic halide perovskite NCs (15% Pb2+-reduced) with tunable emission from blue to red is synthesized through the compositional modulation of halide ions that results in a series of CsZn0.15Pb0.85(ClxBr1-x)3 and CsZn0.15Pb0.85(BryI1-y)3 NCs. As revealed by the XRD patterns (Fig. 4a) and the TEM images (Supplementary Fig. S4) these mixed-halide NCs confirm that they possess identical crystalline structure and morphology except for the slight changes in the dimension of the NCs. The PL emission and the corresponding UV-visible absorption measurements indicate that the mixed halides NCs experience a red-shift in their peak position on decreasing the ratio of Cl−/Br− and Br−/I− in CsZn0.15Pb0.85(ClxBr1-x)3 and CsZn0.15Pb0.85(BryI1-y)3 NCs respectively as shown in Fig. 4b. The PL emission is engineered across the significant portion of the visible spectrum (411–636 nm) with narrow FWHM (13–43 nm) and PLQY above 16%, which is summarized in Supplementary Table S1. The corresponding bandgap of the as-prepared NCs can be finely tuned from 3.19 eV to 1.82 eV. A linear absorption profile for the higher I− content in the CsZn0.15Pb0.85(BryI1-y)3 NCs indicates the formation of larger NCs which is in agreement with the TEM images provided in Supplementary Fig. S4.

Figure 4
figure 4

Structural and optical properties of mixed halides NCs at 15% Zn2+. (a) XRD patterns of CsZn0.15Pb0.85X3 NCs according to ‘X’ as- (i) Cl−, (ii) Cl−:Br− = 1:1, (iii) Cl−:Br− = 1:3, (iv) Br−, (v) Br−:I− = 1:1, and (vi) Br−:I− = 1:3. (b) The PL emission and the UV-visible absorption spectra corresponding to the as-synthesized NCs.

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Furthermore, the remarkable fluorescence quality of halide perovskite NCs such as narrower line-width and wider color gamut coincide with the requirements for high-quality white light generation. However, metal halide perovskite NCs having different emissions cannot directly be mixed to produce white light as cadmium-based NCs, because of the rapid anion exchange process for this material. To overcome this issue, Pathak et al. have reported the remote phosphor technique to obtain white light by stacking different color emitting NCs containing quartz substrates as the color conversion layers on top of the blue LEDs and obtained a CRI of 86 and a CCT of 5229 K55. In this structure, the gap maintained between the LED chips and the conversion layers protect the NCs from the direct effect of heat produced by the LEDs. This non-contact configuration is a reliable approach to avoid the heating effect on the NCs without disturbing the LED junction principle. Thus, this work stimulates us to study further about the applications of this technique. We proposed the tetradic phosphor technique to get white light by employing the four different color emitting NCs in conjunction with UV LED and succeed to observe a high value of CRI (up to 95.4) and LER (>300 lm·W−1) with tunable CCT from warm to cold hue56,57. The enhancement in the quality and optical property of light is attributed to the incorporation of an additional yellow phosphor component that bridges the energy gap between the green and red phosphor components.

Herein, we have used the lead-reduced CsZn0.15Pb0.85X3 NCs to fabricate the fourfold phosphor-based WLEDs employing a remote phosphor technique. For this configuration, we have prepared the thin films of individual color such as blue [λpeak ∼451 nm, CsZn0.15Pb0.85(Cl0.5Br0.5)3], green [λpeak ∼517 nm, CsZn0.15Pb0.85Br3], yellow [λpeak ∼576 nm, CsZn0.15Pb0.85(Br0.5I0.5)3], and red [λpeak ∼636 nm, CsZn0.15Pb0.85(Br0.25I0.75)3] that acts as color conversion layers. Then, we stacked them on top of the commercialized UV LED chip to obtain white light emission by injecting the current into UV LED chips (λpeak ∼405 nm, runs at 8 V and 10 mA). The tunable CCT was obtained by tuning the ratios of blue and green to yellow and red emissions to achieve the different spectral power distributions of white light as reported in our previous work56,58. Here we have engineered the amount of blue-emitting, green-emitting, yellow-emitting nanocrystals used in the fabrication of color-emitting thin films to achieve the different spectral power distributions with the goal of obtaining the white light with tunable CCT. For instance, for obtaining the warm white, we increased the yellow- and red-emitting NCs in comparison to the blue- and green-emitting NCs whereas for the cool white light, we increased the amount of blue- and green-emitting NCs in comparison to the other two NCs. The EL spectra of as-fabricated WLEDs are measured which are shown in Fig. 5a. No intermediate EL emission peak is detected, suggesting that the anion exchange reactions have been eliminated. Then, we have calculated the CCT, CRI, LER, and CIE coordinates based on the EL spectrum of individual WLEDs (shown in Table 2). The tunable CCT from warm to cold white (2218–8335 K) is achieved with high CRI (84–93) and LER (268–318 lm·W−1). The maximum CRI we obtained is 93 at a CCT of 2218 K and an LER of 286 lm·W−1. These CRI and LER are superior to the previous reports on WLEDs based on CsPbX3 NCs doped with other transition metal ions41,59. The CIE diagram is shown in Fig. 5b. The inset of Fig. 5b is the photographs of the as-fabricated WLEDs having different CCT at 8335 K, 6316 K, and 2510 K. The EL spectra (Fig. 5a) demonstrates that a relatively high- the integrated intensity of the yellow and red phosphors is suitable for warm white light whereas increasing the integrated intensity of blue and green phosphors cool the light hue. The warm white creates a relaxing and cozy atmosphere whereas the neutral and cold white produces energizing and uplifting effects on human health. Besides, the comparable high CRI and LER are due to the smaller FWHM of the emission. The high CRI of light source indicates that the human eyes can recognize the natural color of the object under the light source and the high value of LER demonstrates the high vision performance which will lead to fabricating the highly energy-efficient LEDs. The CIE coordinates of the corresponding white light are close to the Planckian locus (Fig. 5b), which suggests that the color temperature we obtained is pleasant for human eyes. In addition, one of the WLED (CCT of 5572 K) shows the CIE coordinates at (0.33, 0.36) is close to the CIE coordinates of standard neutral white light (0.33, 0.33). Thus, these remarkable optical properties of WLEDs based on Zn2+-alloyed all-inorganic lead halide perovskites make them promising for light-emitting devices as a replacement for conventional phosphors.

Figure 5
figure 5

Optical characterization of white light emission. (a) EL spectra of white light with a tunable CCT. (b) CIE diagram showing CIE coordinates of white light with different CCT. (Inset is the photographs of the devices at CCT: (i) 8335 K, (ii) 6316 K, and (iii) 2510 K of white light.)

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Table 2 Estimation of optical parameters of WLEDs.
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Conclusion

In summary, a series of high-quality Zn2+-alloyed CsZnxPb1-xX3 NCs are synthesized using the ZnX2 salts. Since both the Pb2+ and Zn2+ precursors contain the same halide, this paves a pathway for the inclusion of Zn2+ upon its introduction in the host CsPbX3 matrix. When the content of Zn2+ is increased from 0% to 15% in CsZnxPb1-xX3, the NCs retain the identical crystal structure and morphology except for the lattice contraction and the reduced NCs dimension. The lattice contraction and the reduced unit cell dimension of the NCs resulted in an increased bandgap and blue-shifted PL emission without much reduction in the PLQYs (>16% for CsZnxPb1-xCl3 and >80% for CsZnxPb1-xBr3). Moreover, owing to their outstanding optical properties, the CsZnxPb1-xX3 NCs exhibit potential for use in WLEDs as a color conversion layer. Experimentally, for the first time, the fourfold conversion layers (blue, green, yellow, and red) of CsZn0.15Pb0.85X3 NCs are used with UV LED chips to obtain white light having superior quality that leads to a tunable color hue (2218–8335 K) with CRI maximized to 93, and high LER in the range of 268–318 lm·W−1. Based on these results, we expect that these NCs are feasible for obtaining next-generation white light sources with remarkable optical properties and reduced environmental toxicity.

Materials and Methods

Synthesis of Zn2+-alloyed CsZnxPb1-xX3 NCs

We used the hot-injection method with a slight modification for the synthesis of CsZnxPb1-xX3 NCs20,42. Shortly, the preparation of the Cs-oleate precursor was performed by adding cesium carbonate (Cs2CO3, 0.243 g) to oleic acid (OA, 0.75 mL), and octadecene (ODE,12 mL) into 50 mL flask. The mixture was dried under vacuum for approximately 1 h at 120 °C and then heated under N2 to 150 °C until all Cs2CO3 had reacted with OA. Then, 0.188 mmol of PbX2 [PbBr2 (0.069 g), PbCl2 (0.052 g), and PbI2 (0.087 g)] or their mixtures (in desired ratio) and 0.188 mmol of ZnX2 [ZnBr2 (0.042 g), ZnCl2 (0.026 g), and ZnI2 (0.060 g)] were taken into 25 mL flask with ODE (5 mL), OA (0.5 mL), and oleylamine (0.5 mL) separately, and were dried using vacuum oven for 1 h at 120 °C. Note that the Trioctylphosphine (1 mL per 5 mL of ODE) was added into the aforementioned mixture for the synthesis of chloride-based NCs. These mixtures were stirred at 90 °C on a hot plate until the PbX2 and ZnX2 salts were completely dissolved. To the solution obtained by mixing the proportionate amount of PbX2 and ZnX2 precursors, the Cs-oleate solution (0.4 mL, preheated to 100 °C before injection) was quickly injected into the mixture. This was followed by the immediate transference of the mixture into the ice-water bath where the mixture was allowed to cool for a certain time (∼2 min). The cooling process results in the crude solution of the NCs that were subjected to centrifugation at 3500 rpm for 10 minutes. Then, the precipitated NCs were separated from the supernatant, thereby transferring the NCs in toluene for storage. In addition, the composition of halides was attenuated on CsZnxPb1-xX3 (15% Zn2+) to obtain the desired tunable emission. Finally, the colloidal solution of the samples was used for the characterization and device fabrication.

WLED fabrication

Commercially available GaN UV LED chips with the emission peak centered at 405 nm were purchased. The quartz substrates were ultrasonically cleaned in detergent, isopropyl alcohol, deionized water, and ethanol for 15 minutes in sequence and dried naturally. The CsZnxPb1-xX3 perovskite NCs active layers were spin-casted from their colloidal solution at 1000 rpm for 60 seconds for different cycles (10 µL/cycle) which is then dried at an ambient atmosphere without providing any external heat. The four different emissive layers (blue, green, yellow, and red) prepared in separate quartz substrates were stacked one above another in the hierarchy of their increasing energy order in conjunction with UV LED chip to obtain white light.

Characterization

The characterization techniques that we had performed were similar to our previous report42. In details, to study the structural properties of the samples, the XRD measurements were performed using A Rigaku SmartLab diffractometer with CuKα1 radiation, λ = 1.54 Å operated at 40 kV and 44 mA, and at an angular range (2θ) with a step of 0.01°. The TEM images recorded by a Hitachi H-7000 transmission microscope operated at 75 kV were used to study the morphology of the samples. The PL spectra were recorded using a spectro-fluorophotometer (Shimadzu, RF6000). UV-visible absorption spectra were obtained using a Varian Carry 50 Scan UV spectrophotometer. The PLQYs were measured using a QE-pro spectrometer (QEP02037, Ocean Optics) and an integrated sphere (819C-SF-6, Newport). The NCs were excited using a laser source at a wavelength of 350 nm for chloride-based NCs and 405 nm for bromide and iodide-based NCs for PLQY measurement. The measurement of EL spectra was carried out using a QE-pro spectrometer (QEP02037, Ocean Optics) in conjunction with 405 nm UV LED chip and Keithley 2450 source units operated at 8 V and 10 mA under forward bias.