Highly Stable CsPbI3 Perovskite Quantum Dots Enabled by Single SiO2 Coating toward Down-Conversion Light-Emitting Diodes

Abstract

All-inorganic CsPbI₃ perovskite quantum dots (PeQDs) have sparked widespread research due to their excellent optoelectronic properties and facile synthesis. However, attaining highly stable CsPbI₃ perovskite quantum dots (PeQDs) against heat and polar solvents still remains a challenge and hinders any further practical application. Here, by exploiting (3-aminopropyl) triethoxysilane (APTES) as the sole silica (SiO₂) precursor, we report a one-step in situ synthesis of single SiO₂-coated CsPbI₃ (SiO₂-CsPbI₃) PeQDs, namely that one SiO₂ particle only contains one CsPbI₃ PeQD particle. The obtained SiO₂-CsPbI₃ PeQDs are cubic in shape, have a more uniform size distribution, and possess narrow emission, with near unit photoluminescence quantum yields of up to 97.5%. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirm the formation of SiO₂ through the hydrolysis of APTES on the CsPbI₃ PeQDs surface. Furthermore, they have a significantly improved stability against storage, heat, and ethanol. By combining purple-emission GaN light-emitting diodes, the SiO₂-CsPbI₃ PeQDs were successfully employed as down-conversion emitters and exhibited considerable enhanced luminous performance and excellent stability, demonstrating their promising future in the practical application of solid-state lighting fields.

1. Introduction

All-inorganic cesium lead halide (CsPbX₃, X = Cl⁻, Br⁻, I⁻, or their mixture) perovskite quantum dots (PeQDs) have many applications in light-emitting devices [1,2,3,4], solar cells [5,6,7], lasers [8,9,10], and photodetectors [11] due to their excellent optoelectronic properties, such as cost effectiveness [12], facile synthesis [13], tunable emissions by particle size and composition [14], high photoluminescence quantum yield (PLQY) [15,16,17], and narrow emission linewidth [18]. In particular, the CsPbI₃ PeQDs have been intensively studied as down-conversion emitters in light-emitting diodes (LED) due to their high color purity and wide color gamut [19]. Unfortunately, CsPbI₃ PeQDs are vulnerable to phase transfer from photoactive black (α, β, γ) phase [20] into the nonphotoactive “yellow” (δ) phase [21,22], reducing their stability and thus limiting their practical application [23,24]. In principle, the instability is chiefly due to the fact that the commonly used capping ligands, oleylamine (OAm) and oleic acid (OA), are highly dynamic and could be easily detached when exposed to external environments including air, heat, and polar solvents [25,26].

In order to enhance the stability, considerable research efforts have been made, with various strategies developed such as elemental doping [27,28] and capping ligand engineering [29,30,31]. Specifically, Bi et al. doped smaller-sized Zn²⁺ into CsPbI₃ PeQDs to obtain CsPb_1−xZn_xI₃ PeQDs, which exhibits improved storage stability arising from increased lattice formation energy due to lattice contraction by Zn²⁺ doping [32]. Through a post-treatment method, Pan et al. used the bidentate ligand 2,2′-iminodibenzoic acid to effectively passivate the surface defects on CsPbI₃ PeQDs, which resulted in a highly efficient photoluminescence (PL) and much improved storage stability of 15 d [33]. Chen et al. obtained CsPbI₃ PeQDs with excellent storage of up to 180 d, which was achieved by partially replacing the long-chain OAm and OA by octylamine and octanoic acid owing to their stronger adsorption energy [34]. However, both strategies failed to resolve the instability against heat and polar solvents.

An alternative method to improve stability against heat and polar solvents is the coating technique of oxides, such as TiO₂ [35], AlO_x [36], and SiO₂ [37], on CsPbX₃ PeQDs. Among them, SiO₂ is an ideal choice due to its low cost, strong controllability, and high compatibility [38,39,40]. Song et al. reported the realization of monodispersed, SiO₂-coated, blue-emission CsPbBr₃ PeQDs through a low-temperature synthesis which used tetramethoxysilane (TMOS) as its silica precursor [41]. Through facile reverse microemulsion, our group employed tetraethoxysilane (TEOS) to produce ultrathin, core-shell structured, SiO₂-coated, Mn²⁺-doped CsPbBr₃ PeQDs in order to improve their heat and water stability [42]. (3-aminopropyl)-triethoxysilane (APTES) was also used in a synthesis of SiO₂-coated CsPbBr₃ PeQDs to improve heat and ethanol stability [43]. Most of the coating technology focused on CsPbBr₃ PeQDs, and only mere publications of SiO₂ coating were extended to CsPbI₃ PeQDs due to the fact that the latter is more prone to instability against heat and polar solvents than the former. Lin et al. combined the high-temperature sintering method and atomic layer deposition technique to produce SiO₂/AlO_x-coated CsPbI₃ PeQDs with increased stability against heat and polar solvents [44]. Yang et al. synthesized a micro-sized SiO₂ matrix to wrap both CsPbI₃ PeQDs and other emitters using APTES with enhanced storage stability in air [45]. However, these proposals are complex and time-consuming, and always require a polar catalyst, which results in the easy composition of PeQDs. Furthermore, one micro-sized SiO₂ particle contains more than one CsPbI₃ PeQD particle, which inevitably drops the PLQY and causes low dispersibility of the PeQD solution. Therefore, it remains highly necessary to propose a one-step method to in situ synthesize single SiO₂-coated CsPbI₃ PeQDs free of the necessity of a catalyst.

In this study, we report a one-step in situ synthesis of single SiO₂-coated CsPbI₃ PeQDs, which is achieved through a modified hot injection method using APTES as the sole SiO₂ precursor without any catalyst. The obtained SiO₂-coated CsPbI₃ (short for SiO₂-CsPbI₃ hereafter) PeQDs are cubic in shape with an average size of 10.07 ± 0.93 nm and a more uniform size distribution. They have near unit PLQY of up to 97.5%, higher than that of OAm-CsPbI₃ PeQDs (89.6%). Moreover, the SiO₂-CsPbI₃ PeQDs exhibit a much-improved stability against storage, heat, and ethanol. Finally, the SiO₂-CsPbI₃ PeQDs were used as down-conversion emitters of GaN LED chips, which also showed significantly enhanced luminous performance and stability.

2. Experimental Section

2.1. Materials

Lead (II) iodide (PbI₂, 99.9985%) was purchased from Alfa (Heysham, UK). Iodine (I₂, 99.8%), (3-Aminopropyl) triethoxysilane (APTES, 99%), oleylamine (OAm, 80–90%), methyl acetate (≥99%), hexane (97%), 1-octadecene (ODE, >90.0%), and polymethylmethacrylate (PMMA, 99%) were purchased from Macklin (Shanghai, China). Cesium carbonate (Cs₂CO₃, 99%), oleic acid (OA, 90%), toluene (≥99.5), and absolute ethanol (99.7%) were purchased from Aladdin (Shanghai, China). All chemicals were used without any further purification.

2.2. Preparation of Cs-Oleate Precursor

The Cs-oleate precursor solution was prepared by mixing Cs₂CO₃ (3.7 mmol) with 5 mL of OA and 50 mL ODE in a three-necked round-bottom flask with stirring. The reaction mixture was degassed under stirring at room temperature and then heated to 120 °C. This temperature was maintained for 30 min before being cooled down and stored in a vial for further usage.

2.3. Synthesis of the SiO₂- and OAm-CsPbI₃ PeQDs

The SiO₂-CsPbI₃ PeQDs were synthesized using a modified hot injection method. Typically, PbI₂ (2 mmol), I₂ (1 mmol), OA (1 mL), OAm (2 mL), and 50 mL of toluene are taken in a three-necked round-bottom flask. After being dried under vacuum for 5 min, the mixture was heated to 105 °C under N₂ protection, followed by the injection of dried APTES (0, 2, 4, and 6 mmol). After obtaining a clear yellow solution, 6 mL of preheated Cs-oleate precursor at 120 °C was quickly injected. The reaction was stopped within 10 s by immersing the reaction flask in an ice bath. The crude solution was directly washed by adding 3 times methyl acetate, then centrifuged at 10,000× g rpm for 1 min, and then dispersed in 20 mL hexane or toluene for further use. For OAm-CsPbI₃ PeQDs, the synthesis is carried out in a similar way, except for the addition of APTES (0 mmol).

2.4. Fabrication of Down-Conversion LED

PMMA were dispersed in toluene at 10 g/L while being heating at 100 °C for 2 h. The CsPbI₃ PeQDs in hexane at 10 g/L were mixed with the PMMA solution. The mixture was then dropped onto a purple LED chip (395 nm), followed by heating at 100 °C for 1 h to remove the toluene.

2.5. Characterization

Transmission electron microscopy (TEM) was performed using a thermal field emission transmission electron microscope (Tecnai G2 F30 S-TWIN, FEI, Hillsboro, OR, USA). X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra were measured using an X-ray diffractometer (XRD-7000, Shimadzu, Kyoto, Japan) with a Cu Kr radiation (λ = 1.5405 Å) and FTIR spectrometer (670-IR, Varian, Palo Alto, CA, USA), respectively. X-ray photoelectron spectroscopy (XPS) spectra were obtained using an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). Ultraviolet-visible (UV-vis) absorption and PL spectra were recorded using an ultraviolet-visible spectrophotometer (Cary 60, Agilent Technologies, Santa Clara, CA, USA) and fluorescence spectrophotometer (F4600, Hitachi, Tokyo, Japan), respectively. The time-resolved PL decay curves were measured with a spectrometer (FLS980, Edinburgh, Edinburgh, UK). The PLQY was measured using our customized system which is composed of a spectrometer (QEPRO, Ocean optics, Tianjin, China) and an integrating sphere. The electroluminescence (EL) spectra and optical properties of the LED devices were tested using an analyzer system with an integrating sphere (HPCS6500, HOPOO, Hangzhou, China).

3. Results and Discussion

To enhance stability, single silica-coated CsPbI₃ PeQDs (SiO₂-CsPbI₃ PeQDs) were designed and synthesized through a hot injection method using APTES as the SiO₂ source. The synthesis procedures and chemical structures of APTES, OAm, and OA are shown in Figure 1. Specifically, the APTES was injected into the mixture of PbI₂, I₂, OAm, and OA precursor in toluene solution at 105 °C under N₂ protection and maintained at this temperature for 5 min to remove oxygen and moisture. After the preheated Cs-oleate precursor was injected into the above mixture to initiate nucleation and growth, the SiO₂-CsPbI₃ PeQDs were obtained by purification. By changing the feeding content of APTES from 0, to 2, to 4, to 6 mmol, the experimental conditions were optimized; and the SiO₂-CsPbI₃ PeQDs refer to that with 4 mol APTES precursor without specifical notes. Through the interaction between APTES and the CsPbI₃ PeQD surface, APTES was hydrolyzed to form SiO₂, which was expected to improve both the optical properties and the stability. We did not add extra water into the reaction mixture, but some water nonetheless remained in the three-necked flask or toluene after drying. Furthermore, during the transfer process, but after the hot injection, the solution came into contact with air, leading to APTES hydrolysis.

To investigate the influence of SiO₂ coating on the morphology of CsPbI₃ PeQDs, TEM technology was used. Figure 2a shows the TEM of OAm-CsPbI₃ PeQDs, which indicates that the shape of OAm-CsPbI₃ PeQDs is cubic, with an average side length of 6.31 ± 1.21 nm, which was found by counting 120 particles (Figure S1a in Supplementary Materials). The shape of the SiO₂-CsPbI₃ PeQDs is also cubic, without aggregation, with a more uniform size distribution and an average side length of 10.07 ± 0.93 nm (Figure S1b in Supplementary Materials), which is larger than that of OAm, indicating that the SiO₂ coating contributes to an increase in the size of the CsPbI₃ PeQDs, which is due to the changed acid−base interaction caused by the addition of APTES [38,46]. Surprisingly, TEM results also verify that one SiO₂ particle contains only one CsPbI₃ PeQD particle, which is beneficial to the maximum of PLQY. High-resolution TEM (HRTEM) images of the OAm- and SiO₂-CsPbI₃ PeQDs were obtained and are displayed in Figure 2c,d, respectively. It can be seen that both interplanar distances are 0.58 nm, signifying their good crystallinity and that the attaching of SiO₂ onto CsPbI₃ PeQDs exerts no effect on the crystallinity.

In order to further evaluate the effect of SiO₂ on the crystallinity, XRD technology was employed on CsPbI₃ PeQDs. Figure 3a shows the XRD patterns of the OAm- (black line) and SiO₂- (red line) CsPbI₃ PeQD powders on glass substrates by drop-casting the inks. Both the CsPbI₃ PeQDs correspond well to the α-phase CsPbI₃, and the diffraction peaks at 14.3°, 20.2°, 24.7°, 28.8°, 32.4°, 35.6°, and 40.1° can be attributed to the (100), (110), (111), (200), (210), (211), and (220) planes, respectively, which is indicative of good crystallinity. Specifically, according to Bragg’s law, the interplanar distance of the (100) plane is 0.58 nm, which is in agreement with the HRTEM results. Furthermore, the diffraction peaks of SiO₂-CsPbI₃ PeQDs at 14.3° and 28.8° are higher and narrower than the OAm ones, indicating the larger size of the SiO₂-CsPbI₃ PeQDs, which is consistent with the size results from the TEM images. Both TEM and XRD data show that SiO₂ has no effect on the crystal structure and morphology of CsPbI₃ PeQDs but does increase size.

To examine the capping ligand of the CsPbI₃ PeQDs, FTIR technology was used. Figure 3b shows the FTIR spectra of the OAm- (black line) and SiO₂⁻ (red line) CsPbI₃ PeQD powders. For the OAm-CsPbI₃ PeQDs, the strong absorption bands centered at 2923 and 2854 cm⁻¹ are related to the asymmetric and symmetric tensile vibration of −CH₂ [47], while the weak absorption peaks at 1710 and 1640 cm⁻¹ correspond to the stretching vibrations of C=O and bending vibrations of −NH₂ groups, respectively. By checking the chemical structures of capping ligands (Figure 1c), we found that the C=O and −NH₂ groups are exclusive of OAm and OA, respectively. This indicates that both OAm and OA exist on the surface of CsPbI₃ PeQDs. For the SiO₂-CsPbI₃ PeQDs, the bending vibrations of −NH₂ group strengthens, indicating the occurrence of APTES on CsPbI₃ PeQDs (see chemical structure of APTES in Figure 1c). Furthermore, new absorption peaks at 1124 cm⁻¹ and 1037 cm⁻¹ appear and can be assigned to the vibrations of Si-O-Si and Si-O-C groups, respectively, proving the hydrolysis of APTES into SiO₂ on CsPbI₃ PeQDs [48]. This signifies that, for the SiO₂-CsPbI₃ PeQDs, the capping ligands are OAm, OA, and APTES.

To further confirm the existence of SiO₂, X-ray photoelectron spectroscopy (XPS) technology was used on the CsPbI₃ PeQDs powders. In the XPS full spectra (Figure S2, Supplementary Materials), it can be seen that the signals of Cs 3d, Pb 4f, I 3d, C, N, and O appear on both the OAm- and SiO₂-CsPbI₃ PeQDs, but the Si 2p signal only appears on the SiO₂-CsPbI₃ PeQDs (high-resolution XPS spectra in Figure 3c), which is consistent with the EDX results in Figure S4 (Supplementary Materials). This indicates that the capping ligands for OAm-CsPbI₃ PeQDs are OAm and OA only, but the capping ligands for the SiO₂-CsPbI₃ PeQD surface are OAm, OA, and APTES, which is consistent with the FTIR results. Furthermore, the ratios of N:Pb and O:Pb increases (Figure 3d), which confirms the appearance of APTES. In addition, the ratio of I:Pb increases from 3.73 for OAm-CsPbI₃ PeQDs to 4.39 for SiO₂-CsPbI₃ PeQDs, signifying that a richer environment is provided by the inclusion of an SiO₂ coating, which indicates a more effective defect passivation and thus higher PLQY [49].

Based on the above results, we anticipate that the extra SiO₂ coating will endow the CsPbI₃ PeQDs with highly efficient emission. To verify this point, the UV-vis absorption and PL spectra were obtained with different APTES (Figure S5 in Supplementary Materials). From these results, we observed that with the increase of APTES from 0 to 6 mmol, the PLQY first enhances and then decreases with the maximum PLQY obtained at 4 mmol APTES. Furthermore, during the whole process, the UV-vis absorption and PL spectra shift to long wavelength. Specifically, the UV-vis absorption of OAm-CsPbI₃ PeQDs (0 mmol APTES precursor) and SiO₂-CsPbI₃ PeQDs (4 mmol APTES precursor) in Figure 4a shows that absorption red shifts from 593 nm to 610 nm due to the SiO₂ coating. The PL spectra in Figure 4b also shows a red shift from 668 nm to 678 nm due to the SiO₂ coating. Both the red shift in UV-vis and PL spectra signify the increase in particle size caused by the SiO₂ coating, which agrees with the TEM results. Importantly, the full width at half maximum of the SiO₂-CsPbI₃ PeQDs considerably reduces from 40 nm to 28 nm for the OAm-CsPbI₃ PeQDs, which could be ascribed to the more uniform size distribution of the CsPbI₃ PeQDs due to the SiO₂ coating. Expectedly, the PLQY increases from 89.6% for the OAm-CsPbI₃ PeQDs to 97.5% for the SiO₂-CsPbI₃ PeQDs, as is clear from the improved PL from the UV light photographs (insets in Figure 4b). Together, these results verify that SiO₂ is more effective in passivating surface defects and thus enhancing radiative emission.

To further uncover the origin of the PL enhancement, time-resolved PL (TRPL) spectra were obtained, as displayed in Figure 4c. It can be well described by a biexponential fitting Equation (1). The average lifetimes (τ_avg) were calculated using Equation (2) and the fitting data as shown in Table 1. Short-lived τ₁ is related to the recombination by defects and traps, and long-lived τ₂ is related to the radiative recombination, while A₁ and A₂ are the fractional contributions of the decay components [50]. Comparing with OAm-CsPbI₃ PeQDs, the SiO₂-CsPbI₃ PeQDs have a lower A₁ and a higher A₂, which proves that nonradiative decay is suppressed by the SiO₂ coating, which is beneficial for the recombination of the generated excitons and results in a high PLQY [51]. The radiative and nonradiative components of the total PL decay were further determined using Equations (3) and (4) [52]. The nonradiative decay rate (k_nr) decreases by 76.6% from 0.0047 s⁻¹ for OAm-CsPbI₃ PeQDs to 0.0011 s⁻¹ for SiO₂-CsPbI₃ PeQDs, while the decay rate (k_r) increases by 11.5%, from 0.0312 s⁻¹ for OAm-CsPbI₃ PeQDs to 0.0348 s⁻¹ for SiO₂-CsPbI₃ PeQDs. This result (increase in k_r and decrease in k_nr) clearly confirms that the SiO₂ coating on CsPbI₃ PeQDs enables the effective passivation of surface defects and thus enhances the PLQY [51].

(1)${\mathrm{I}\left(\mathrm{t}\right)=\mathrm{A}}_1\exp \left(-t/{\tau }_1\right){+\mathrm{A}}_2\exp \left(-t/{\tau }_2\right)$

(2)${\tau }_{\mathrm{avg} }=\frac{{\mathrm{A}}_1{\tau }_1^2{+\mathrm{A}}_2{\tau }_2^2}{{\mathrm{A}}_1{\tau }_1{+\mathrm{A}}_2{\tau }_1}$

(3)$\frac{1}{k_{\mathrm{r}}}=\frac{{\tau }_{\mathrm{avg}}}{\mathrm{PLQY}}$

(4)$\frac{1}{k_{\mathrm{nr}}}=\frac{{\tau }_{\mathrm{avg}}}{1-\mathrm{PLQY}}$

Subsequently, we investigated the stability of CsPbI₃ PeQDs with 0 and 4 mmol APTES precursor against air, heat, and polar solvent. Figure 5a shows the XRD patterns of the OAm- and SiO₂-CsPbI₃ PeQD films on glass substrates before and after 7-day storage under ambient air atmosphere. From the XRD pattern of OAm-CsPbI₃ PeQDs, it can be seen that, after 7-day storage under ambient atmosphere, the symmetry is reduced, with the profiles becoming broader, and, importantly, extra diffraction peaks at 22.8, 26.5, and 37.7°, which are marked with a green star () and are assigned to yellow phase (δ) CsPbI₃ [44,53]. This indicates the occurrence of the yellow phase (δ) for the OAm-CsPbI₃ PeQDs, which is also evidenced by the yellow-colored OAm-CsPbI₃ PeQD film (inset in Figure 5a). In contrast, as expected, the XRD pattern of the SiO₂-CsPbI₃ PeQDs remains completely unchanged with high and sharp peaks (α-phase CsPbI₃), as can also be seen from the dark red SiO₂-CsPbI₃ PeQD film (inset in Figure 5a). This verifies that the storage stability of CsPbI₃ PeQDs is improved by a SiO₂ coating.

Figure 5b,c show the evolutional PL spectra of OAm- and SiO₂-CsPbI₃ PeQD solutions under continuous heating at 100 °C in air, respectively. For the OAm-CsPbI₃ PeQDs, it can be seen that when the temperature is raised to 100 °C, the PL intensity decreases immediately and continues to do so as time goes on, with a red shift of 4 nm. After 60 min heating, the PL intensity decreases to 36% of the initial value (Figure 5d). This can be also be observed visually, since the color of the OAm-CsPbI₃ PeQD solution changes from bright red at room temperature to dark brown at 100 °C (insets in Figure 5b), which can be attributed to the detachment of dynamic capping ligands which introduces more defect-relative nonradiative recombination and leads to the growth of PeQDs [54]. As expected, the PL intensity of the SiO₂-CsPbI₃ PeQDs was almost unchanged in the first 30 min heating at 100 °C and remained at approximately 87% of the initial intensity after 60 min heating (Figure 5c,d). Furthermore, there was no visible shift in PL spectra. This is also evidenced by the unchanged color of the PeQD solution during the heating (insets in Figure 5c). These results indicate that the SiO₂-CsPbI₃ PeQDs exhibit a significantly enhanced stability against heating due to the protection of the SiO₂ coating.

To evaluate the effect of the SiO₂ coating on the stability of the CsPbI₃ PeQDs against polar solvents, the CsPbI₃ PeQD solutions in hexane were diluted by ethanol (with hexane:ethanol = 10:1). The corresponding evolutional PL spectra were obtained and are shown in Figure 6. It can be shown that after the addition of ethanol, the OAm-CsPbI₃ PeQDs show a significant drop in PL intensity within the initial 5 min and become almost completely quenched at 20 min (Figure 6a,c). At the same time, the corresponding photographs of the OAm-CsPbI₃ PeQD solutions were taken and are shown in Figure 6d. These images show that the OAm-CsPbI₃ PeQDs gradually become light in color and turbid as time increases, and finally turn from a dark red color into a yellow color that does not emit PL any longer (Figure 6e), indicating the occurrence of a non-fluorescent yellow phase, which is consistent with the PL results and previous studies [17]. In great contrast, the SiO₂-CsPbI₃ PeQDs only show a marginal decrease in PL intensity within the first 5 min and gradually decrease (Figure 6b,c) thereafter. At 40 min, the PL intensity still maintains 79% of the initial value. During the whole process, for the SiO₂-CsPbI₃ PeQDs, the daylight photographs show no visible change, and the UV photographs indicate a slight decrease in emission (Figure 6d,e), in line with the PL results. This unequivocally shows that the stability of the SiO₂-CsPbI₃ PeQDs against polar solvent is greatly enhanced. Therefore, the SiO₂-CsPbI₃ PeQDs have exhibited optical properties and outstanding stability against air, heat, and polar solvent due to the encapsulation of SiO₂ through the hydrolysis of APTES.

With the excellent optoelectronic properties and great stability in mind, we anticipate that the SiO₂-CsPbI₃ PeQDs could be ideal candidates as down-conversion emitters. To verify this point, we mixed CsPbI₃ PeQDs and PMMA, which were dropped onto the purple LED chip (395 nm). Figure 7a,b show the EL spectra of the OAm- and SiO₂-CsPbI₃ PeQDs under different driving currents (20–200 mA), respectively. It can be seen that the EL intensity increases with driving current, with the corresponding EL spectra remaining intact and below the upper limit. We therefore set up a driving current of 50 mA to investigate the stability under continuous power on, and the evolutional EL spectra were recorded. Figure 7c,d show the evolutional EL spectra of the OAm- and SiO₂-CsPbI₃ PeQDs under a driving current of 50 mA, respectively. These spectra show that as time increases, both the EL intensities decrease gradually over a period of 120 min. For the OAm-CsPbI₃ PeQDs, EL intensity dropped significantly to 38% of the initial value, while for the SiO₂-CsPbI₃ PeQDs, it only fell to 77% of the initial value. These results verify that the SiO₂-CsPbI₃ PeQDs have excellent stability in practical applications, which coincides with the above results of the enhanced stability against air, heat, and polar solvent.

4. Conclusions

In summary, we synthesized single SiO₂-coated CsPbI₃ PeQDs through a one-step hot injection method which exploited hydrolysis of a sole silica precursor of APTES without any catalyst. We used TEM, XRD, FTIR, and XPS to study the effect of SiO₂ on CsPbI₃ PeQD morphology and composition. These characterizations indicated that the SiO₂ coating was formed on the CsPbI₃ PeQD surface, with OAm and OA as ligands through hydrolysis of APTES, which contributed to the increase in size of the CsPbI₃ PeQD. Using UV-vis absorption and PL spectra, the content of the APTES was optimized and the champion PLQY increased to 97.5% for the SiO₂-CsPbI₃ PeQDs, which was higher than that of OAm-CsPbI₃ PeQDs (89.6%). Due to the SiO₂ coating, the SiO₂-CsPbI₃ PeQDs exhibited significantly improved stability against air, heat, and ethanol. To further confirm their stability from a practical application point of view, we employed the SiO₂-CsPbI₃ PeQDs as down-conversion emitters of a GaN LED, which also yielded satisfactory results. We hope this study will provide a new method for preparing inorganic CsPbI₃ PeQDs with highly optical properties and excellent stability, promoting their commercial development in lighting and display.

Author Contributions

Z.P.: methodology, investigation, writing—original draft. X.Z., T.X. and Q.X.: formal analysis, investigation. H.C. and F.X.: resources. H.L. and J.W.: investigation. Y.L.: conceptualization, methodology, writing—review and editing, supervision. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Available on request.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

Figure 1. Schematic illustration for the SiO2-CsPbI3 PeQDs: (a) synthesis procedures; (b) hydrolysis diagram; and (c) chemical structures of the capping ligands.

Figure 2. TEM images of the (a) OAm- and (b) SiO2-CsPbI3 PeQDs. HRTEM images of the (c) OAm- and (d) SiO2-CsPbI3 PeQDs.

Figure 3. Characterizations for the OAm− and SiO2−CsPbI3 PeQDs: (a) XRD patterns; (b) FTIR spectra; (c) high-resolution XPS spectra of Si 2p; and (d) atomic ratios of I, N, and Si to Pb.

Figure 4. (a) UV-vis absorption spectra; (b) PL spectra; and (c) the PL decay curves of the OAm-CsPbI3 and SiO2-CsPbI3 PeQDs. The insets show the (left) day and (right) UV light photographs of both CsPbI3 PeQDs.

Figure 5. (a) XRD patterns of the CsPbI3 PeQD films before and after 7-day storage under ambient air atmosphere (insets show the photographs under daylight irradiation); green stars ([Image omitted. Please see PDF.]) are used to mark yellow phase (δ) CsPbI3. The evolutional PL spectra of (b) OAm- and (c) SiO2-CsPbI3 PeQD solutions under continuous heating at 100 °C in air, and (d) showing the corresponding normalized PL.

Figure 6. The evolutional PL spectra of (a) OAm- and (b) SiO2-CsPbI3 PeQD solutions (with hexane:ethanol = 10:1), (c) corresponding normalized PL intensity, (d) daylight photographs taken at different times, and (e) UV light photographs taken at 0 and 40 min.

Figure 7. The EL spectra of (a) OAm- and (b) SiO2-CsPbI3 PeQD LED under different driving currents (20–200 mA). The evolutional EL spectra of (c) OAm- and (d) SiO2-CsPbI3 PeQD LED under a driving current of 50 mA, with insets showing the LED device with current off (bottom) and on (upper), and (e) showing the corresponding normalized EL intensities.

Table 1

Fitting and calculated results for the CsPbI₃ PeQDs based on the TRPL in Figure 4c and PLQY.

CsPbI3 PeQDs	τ1 (ns)	${\mathbf{A}}_{\mathbf{1}} \mathbf{(}\mathbf{\%}\mathbf{)}$	τ2 (ns)	${\mathbf{A}}_{\mathbf{2} }(\%)$	kr (s−1)	knr (s−1)	τavg (ns)
OAm	21.64	87.86	48.27	12.14	0.0312	0.0047	27.91
SiO2	22.32	81.19	41.12	18.81	0.0348	0.0011	27.94

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app13137529/s1. Figure S1. The size distribution statistics of the (a) OAm- and (b) SiO₂-CsPbI₃ PeQDs; Figure S2. XPS full spectra of the OAm- and SiO₂-CsPbI₃ PeQDs; Figure S3. High-resolution XPS spectra of (a) Cs 3d, (b) Pb 4f, (c) I 3d, (d) C 1s, (e) O 1s of the OAm- and SiO₂-CsPbI₃ PeQDs; Figure S4. The EDX spectra of the (a) OAm- and (b) SiO₂-CsPbI₃ PeQDs; Figure S5. The normalized absorption and PL spectra of adding APTES from 0 to 6 mmoL.

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