Enhanced Long‐Term Luminescent Stability through Near‐Single‐Dot Passivation and Encapsulation of Perovskite Quantum Dots for Printable Photonics

Metal halide perovskites quantum dots (QDs) stand at the forefront of multifarious photonic applications, including micro‐light‐emitting diode and further augmented reality, virtual reality, and other novel display, lighting technologies. Barriers to applications, however, lie in their toxicity of lead, instability to light, moisture and heat, and processability at the nanoscale‐particle level. Herein, a simple and versatile postprocessing approach is reported for the near‐single‐dot passivation and encapsulation of representative lead‐free double perovskite Cs2Ag0.4Na0.6InCl6:Bi through liquid‐phase processing of perhydropolysilazane and quantum dots colloid with controllable hydrolysis curing. The conventional unstable oleylamine and oleic acid ligands are replaced by ‐NCl bonding on the surface of nanocrystal, accompanied by the resulting compact and robust silica layer without compromising the optical properties of the quantum dots. With the near‐single‐dot protection, the quantum dots do not show fluorescence quenching even when stored for more than 90 days and exhibit remarkably improved stability against heat, ultraviolet irradiation and humidity compared to the raw quantum dots. The strategy offers a versatile way of creating nanoscale‐particle level protection of luminescent quantum dots, and can be universally compatible with solution‐based patterning techniques and photonics applications where quantum dots are used.

strategies. As silicon material has strong durability and low cost, many reports have demonstrated that the stability of lead halide perovskite can be improved by silica coating. [18][19][20] In previous reports, their strategies for generating core@shell protective structures often present problems such as high reactant requirements, complex synthesis methods, and large aggregate sizes of the products, which is not conducive to the formation of stable colloidal state and solution-based printing. Therefore, a facile surface passivation and effective encapsulation of nanocrystals are very necessary, especially for the near-single-dot passivation and encapsulation of QDs, which will ensure a stable colloidal state of QDs and provide a high degree of flexibility and high pixel densities for large-scale printable miniature patterns and microlight sources.
Here, we demonstrate a simple and versatile approach for the near-single-dot passivation and encapsulation of lead-free double perovskite Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs with high stability and solution operability. Specifically, benefiting from the controllable hydrolysis of perhydropolysilazane (PHPS) and customizable solidification process, the unique -(SiH 2 ─NH)backbone of PHPS can be effectively evolved to -N─Cl bonds within the QDs surface, replacing the original unstable OA and OAm ligands with passivation effect. [21] As the hydrolysis of PHPS proceeds, a dense and homogeneous silica encapsulation can be achieved in a way of near-single-dot. Subsequent optical measurements revealed that the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 composites perfectly resisted the damage caused by extreme environments such as high temperature, humidity, and UV radiation, which can be ascribed to the protection of external structure. Finally, delicate hand-painted pattern and broadband yellow-orange LED covering the whole absorption curve for plant growth were demonstrated with Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 as the luminescent material on the n-UV chip, which manifested very flexible operability in the way of solution drawing and printing without complex encapsulation. This work offers a versatile way of creating nanoscale-particle level protection of luminescent QDs in a cost-effective and nondestructive manner, and could be universally compatible with solution-based patterning techniques, especially for micro-LED, mini-LED, and other novel photonics technologies.

Results and Discussion
As shown in Figure 1a, the crystal structure of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs is same as A 2 B(I)B(III)X 6 double perovskites with a 3D network of alternating [B þ X 6 ] and [B 3þ X 6 ] split-angle octahedra. The methods for one-pot synthesizing of the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs are shown in Supporting Information. The X-ray diffraction (XRD) pattern of the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs can be well indexed with the Cs 2 AgInCl 6 phase (Figure 1b), indicating a high phase purity. [22] The XRD pattern of uncoated Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs indicates that the QDs were crystallized in a cubic Fm3m double perovskites structure, compatible with the sample of Cs 2 AgInCl 6 (ICSD number 11524), without the presence of secondary phases. Meanwhile, according to previous literature arguments, with the increase of Na content, the (111) diffraction peak of Cs 2 AgInCl 6 shifts to a lower 2θ angle and the intensity of the (222) diffraction peak increases significantly. The same situation is also found in the spectrum of our synthesized Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi. [23] In the synthesis of perovskite QDs, OA and OAm are the most popular and successful ligands, and have been widely used. [4,15] The nuclear magnetic resonance (NMR) characterization in relevant literature demonstrates that the presence of OA ligands in the final product does not indicate the existence of chemical bonds between the OA ligand and the surface of the QDs, but the Pb-OA complex. [14,16] OAm ligands act as a passivation binding to the surface chloride through a hydrogen bridge or otherwise electrostatic interactions. [13,17] Therefore, the OA and OAm ligands are in dynamic equilibrium, with some of them detaching from the QDs and some others reconnecting to the surface at the same time. [13,24] The loose ligands had difficulties in long-standing encapsulating the QDs by isolating and purifying colloids with normal methods, namely, by repetitive ultracentrifugation with a nonsolvent followed by redispersion in a pure solvent. In order to optimize the optical stability of the QDs, in this work, PHPS with a unique -(SiH 2 ─NH)backbone was introduced into the QDs solution, which could produce -N─Cl bonds within the QDs surface to replace the original unstable ligands as the hydrolysis of PHPS proceeding. [21,25] And resulting external compact and robust silica layer of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs at the nanoscale-particle level was achieved. Figure 1c illustrates the ligand exchange and the formation of an external silica layer on the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs surface. During the ultrasound and washing process, the surface ligands of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs are heavily depleted. With the entry of water vapor from air, hydrophobic OA ligands are also lost in large quantities. At this time, PHPS was added to the QDs solution, where PHPS could be stably attached to the surface of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs through -N─Cl bond. As hydrolysis reactions of PHPS proceeded, cross-linked Si─O bonds were formed like hydrolysis Equation (1) and (2). [21,25,26] During curing, owing to the massive elimination of hydrogen and ammonia in hydrolysis, lots of growth sites were presented on the surface binding point. As hydrolysis continued, the -Si─O─Sibonds at the growth sites were further developed, producing an external compact and robust silica layer of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs at the nanoscale-particle level.
From XRD patterns before and after treatment with PHPS, we can see that the XRD results of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 and Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi are similar, and the positions of the main diffraction peaks do not change obviously, thus indicating that the addition of PHPS hardly affects the crystal phase of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi. Furthermore, we compare the XRD patterns of bare and PHPS-treated QDs after storage under the same conditions for 20 days. As shown in Figure S1, Supporting Information, the XRD signals of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs become weak when the storage time is extended from 1 to 20 days, suggesting a decrease in crystallinity. On the contrary, the intensity of the diffraction peak of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 decreases insignificantly, which reflects the advantage of the surface passivation and protection of silicon oxide coating.
Compared with the traditional alkoxide-derived methods, using PHPS to generate SiO 2 coating on QDs is simpler. Moreover, the high density of PHPS-derived near-single-dot silicon oxide capping layers provides higher mechanical and chemical durability. [21] In addition, ligand-deficient QDs were believed to form new chemical bonds in crystal surfaces by the process of hydrolysis and curing of PHPS. To explore the potential mechanism of PHPS passivation for Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs, we examined X-ray photoelectron spectroscopy (XPS) using an equivalent amount of dry powder for each test of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi and Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 . As shown in Figure 2a, the increased strength of the O 1s peak and the Si 2p peak (red curve) in the survey scan indicates effective formation of SiO 2 encapsulation on the surface of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi after PHPS treatment. In Figure 2b,c, it is interesting to find that the O 1s spectrum of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 is obviously different from that of the naked QDs. The oxygen atom possesses two chemical environments on the surface of Cs 2 Ag 0.4 Na 0.6 InCl 6 : Bi QDs (Figure 2b), with significantly higher and lower binding energy regions corresponding to the C─O and C═O bindings, respectively. However, for the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 capsules (Figure 2c), the O 1s can be deconvolved into two peaks at 532.0 and 533.0 eV, which correspond to C─O bond and Si─O bond, respectively. And the strength of the C─O bond at 532 eV is significantly reduced, which verifies the absence of the OA ligand. [27] It has been reported that Pb 2þ in lead-based QDs had a higher binding energy region of Pb-halogen elements and a lower  binding energy region of Pb-OA, which is consistent with the results of In 3þ in Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs. [28] Before adding PHPS, as shown in Figure 2d about In 3d peak fitting, the octahedral coordination bond In-Cl and In-OA complex are all demonstrated. But with the PHPS introduction (Figure 2e), the peak of In-OA disappears, further verifying the deficiency of OA ligand in the nanocrystal surface. Furthermore, before and after the treatment with PHPS to the QDs, we compared the XPS spectra of Cl (Figure 2f,g). Besides the octahedron of the original Cl connected to In/Bi, Ag/Na, a peak of Cl─N binding enables the Cl 2p to form a broad peak. [29,30] Si─N bonding is also corroborated in the 2p peak of Si (Figure 2h), which confirms our previous conjecture concerning the passivating effect of PHPS. [25] In order to ensure the best luminescence performance and stability of the final product, the best mixing ratio of QDs and PHPS and the optimal hydrolysis conditions of PHPS should be determined. After mixing Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs hexane solution (1000 μL, 0.1 mmol QDs) with different volumes of PHPS solution, it was found that a fourfold volume of 20 wt% PHPS solution can produce appropriate silica which can ensure well photopermeability ( Figure S2e, Supporting Information), simultaneously guaranteeing enough silica matrix for encapsulating and protecting QDs at a nanoscale-particle level without compromising the optical properties of the QDs. The TEM pictures of QDs with different volumes of PHPS are shown in Figure S2, Supporting Information. To explore the suitable conditions, the Fourier transform infrared spectroscopy (FTIR) spectra of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 were examined by varying its curing temperature and time ( Figure S3, Supporting Information). It was found that the Si─H bond strength decreased and Si─O bond strength increased significantly as the curing temperature reaching to 80°C and the time reaching to 12 h, which indicates the completion of PHPS hydrolysis with the resulting robust SiO 2 . [25] The material characterization and thermal, optical tests in the following works can verify that the proper mixture of PHPS and the QDs solution not only does not damage the material's own properties, but also improves the raw material's stability by SiO 2 capping. The typical transmission electron microscopy (TEM) image of bare Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs and the corresponding high resolution TEM (HRTEM) image are shown in Figure 3a,b, respectively. The QDs present a cuboidal shape with an average size of 13 AE 2.9 nm. The good crystallinity of Cs 2 Ag 0.4 Na 0.6 InCl 6 : Bi QDs can be verified by HRTEM images. The highly crystalline lattice fringes with an interplanar spacing of 0.376 nm can be assigned to the (022) planes of the bulk Cs 2 AgInCl 6 . [22] Figure 3d shows the TEM image of monodisperse Cs 2 Ag 0.4 Na 0.6 InCl 6 : Bi@SiO 2 QDs. The particles capped by the amorphous material can be clearly seen in this image, which indicates the presence of SiO 2 layers at the nanoscale-particle level. To clearly resolve whether the crystal structure of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 QDs is altered, the HRTEM image ( Figure 3e) with an obviously defined core@shell structure was obtained. The crystal lattice fringes of QDs in Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 complex are clear, and the crystal plane spacing of QDs is also 0.376 nm which is consistent with that of bare Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi. Moreover, the introduction of PHPS does not cause damage to the morphology of the QDs, as shown in Figure 3d, where the cubic block QDs keep their morphology unchanged and are wrapped in a way of near-single-dot with a layer of transparent material (SiO 2 ) outside. When the content of PHPS is too much, the cubic shape of QDs remains unchanged as shown in Figure S2d, Supporting Information, only the newly formed outer coating has changed the appearance.   which is due to the aggregation of QDs caused by centrifugal washing, drying, grinding, and other processes. As for Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 particles, Figure 3f shows that the QDs are capped in the silica matrix with perceptible graininess, which is agreed with the TEM results of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 . Moreover, the elemental composition measurement performed by EDS pattern clearly showed that in addition to the signals of the elements Cs, Na, Ag, In, Bi, and Cl, the signals of the Si and O elements were also detected, confirming the formation of the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 QDs as shown in Figure S4a, Supporting Information. The EDS elemental mapping of the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 QDs in Figure S4b, Supporting Information, showed that the Si and O elements are uniformly distributed in selected regions, which also indicates that the perovskite QDs are encapsulated in the silica.
To further study the crystal structure of SiO 2 -coated Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs, we performed FTIR and Raman spectra were used to verifying the role of PHPS in forming a SiO 2 coating on the surface of the material. The Raman spectrum of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi perovskite shown in Figure 4a (black curve) indicates three major vibrations at approximately 144.9, 241.9, and 296.9 cm À1 , which can be assigned to T 2g , E g , and A 1g modes of the QDs lattice, respectively. [31] Compared with Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs, the main Raman peak position (50-600 cm À1 ) of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 remains unchanged except for the emerging vibration peak at 510 cm À1 due to the newly formed SiO 2 coating, which confirmed that the introduction of PHPS does not lead to changes for crystal structural of QDs. [32,33] In addition, two typical Raman signal peaks of uncoated Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs at 1376 and 1585 cm À1 are attributed to the vibration of C─H and C═O bonds from the OA and OAm ligands (Figure 4b). [34] Due to the introduction of PHPS, the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 curve does not show the same strong peaks of C─H and C═O bonds, which is due to the surface substitution and is also seen in the next FTIR analysis. According to the molecular formula of the PHPS, the peak at 2190 and 3381 cm À1 of the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 curve can be clearly attributed to the vibration of the residual Si─H bond and N─H bond. [25] As shown in Figure 4c, the FTIR spectra of bare Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs show sharp peaks at 2927 and 2854 cm À1 , which are attributed to the C─H vibrations of OA and OAm. Meanwhile, the peaks around 1591 cm À1 can be attributed to the C─N vibrations of OAm. [35] For the curve of the PHPStreated Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 , there are Si─O vibrations at 1070 and 827 cm À1 , but the C─H bond and C─N vibrations in this curve are not obvious. These phenomena further verify the presence of cross-linked SiO 2 coating around the QD surface. [26] Figure 4d shows thermo gravimetric analysis (TGA) curves of the bare and surface treated Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi powder. For the bare nanoparticles, the weight loss curve is sharp between 50 and 200°C, while the curve of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 is almost constant. At temperatures below 200°C, the perovskite crystals did not undergo decomposition reactions to produce phase changes, so it can be ascribed that the mass deficit in this temperature range is due to the instability of OA and OAm ligands. [36] On the contrary, after surface replacement and protection, the QDs@SiO 2 composite structure is more stable, with almost no weight changes under 200°C.
The basic optical properties of bare Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi film and Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 film were assessed and  relevant results are shown in Figure 5a,b. The Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 composites exhibited slight redshifts of absorption (peak: 333-348 nm), photoluminescence excitation (PLE) (peak: 351 to 358 nm), and PL (peak: 611 to 618 nm) spectra, respectively. Their PL spectra (Figure 5a,b) show one broad emission from 400 to 850 nm, after the theoretical calculations, originating from self-trapped excitons (STEs) upon excitation at 365 nm with a large Stokes shift. [10] Meanwhile, the slight redshifts of absorption and emission, derived from the introduction of a silica oxide overlay via PHPS hydrolysis, allow the broadband emission spectrum to better cover the red and far-red bands needed for plant growth. Then, the PLQY of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs was measured to be 63.8%, and the PLQY of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 www.advancedsciencenews.com www.small-structures.com was also 58.3%. Further, time-resolved PL decay is measured under 365 nm excitation as shown in Figure S5, Supporting Information, and Table S1, Supporting Information. As we all know, PL decay of the perovskite material can be generally ascribed to the exciton radiative and nonradiative recombination process, where nonradiative recombination is usually due to surface defects and is used as short-lived in the lifetime fitting.
Herein, the PL decay curves of the two samples can be well fitted by Equation (1), and the average lifetimes (τ avg ) can be calculated by Equation (2) I ðtÞ ¼ A 1 expðÀt=τ 1 Þ þ A 2 expðÀt=τ 2 Þ (1) At room temperature, the percentage of shorter-life is decreased from 10.93% (Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi) to 9.58% (Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 ). The decrease in the percentage of short lifetimes indicates that less surface defects exist in QDs after PHPS treatment and then nonradiative recombination process was restrained, which corroborates our speculation on the passivation effect of PHPS.
Subsequently, the bare Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi film and Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 film were placed in the circumstance without any protective treatment (average temperature of 35°C and average humidity of 65% during the period) and the PL spectroscopy tests were performed every 7 days. As shown in Figure 5c and S6, Supporting Information, the PL intensity of the uncoated QDs film showed a gradual decay trend. In contrast, the coated QDs film did not show a significant decreasing trend even after an extension of 90 days in the same environment. The above data prove that the SiO 2 capping has improved the optical stability of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs. A similar protecting effect was also applied to CsPbBr 3 QDs ( Figure S7, Supporting Information), which shows a certain universality of our protocol. Moreover, when the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs hexane solution was mixed with quantitative PHPS solution by controlled ultrasound-assisted hydrolysis and curing reactions, the surface passivation and coating of nanocrystals have been formed and the product presents a stable colloidal state. As shown in Figure S8, Supporting Information, the colloid remains stable for 30 days without condensation, and the luminescence intensity was the initial intensity. The results showed that the near-singledot passivation and encapsulation of QDs showed good stability in the solvent, which is suitable for solution printing and the manipulation of miniature light sources and patterns.
To further examine the optical properties of the composites in extreme environments, the QDs with and without a protective layer of silicon oxide were subjected to UV irradiation (365 nm), heating (45 to 115°C), and water for tests. One irradiation treatment took 24 h and then the PL spectra of both samples were collected and compared. The PL intensity variation of the two samples is shown in Figure S9, Supporting Information, with increasing UV irradiation time (10 treatments for up to 240 h). To more directly compare the irradiation effect on PL performance, normalized PL integrated intensities of the two samples were extracted and plotted as a function of different irradiation time in Figure 5d. The emission intensity of bare Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi films showed a downward trend after 24 h irradiation, and was almost completely quenched at www.advancedsciencenews.com www.small-structures.com 240 h. According to previous reports, continuous UV irradiation causes surface ligands desorption and ion migration, which may lead to more defect formation in the halide perovskites.
In addition, long-term exposure to high-energy UV irradiation will also induce the decomposition of halide perovskites, which will further bring down the fluorescence properties of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs. [4] Thus, defect-assisted nonradiative recombination channels dominate the rapid quenching in bare Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi films. However, the PL degradation of the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 QDs is not significant with increasing irradiation time, and more than 85% of the initial intensity is preserved when the irradiation time reaches 240 h. Obviously, the passivated surface and coated silica shell can effectively prevent UV radiation hazards, thus improving the photostability of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs. Figure 5e and S10, Supporting Information, show the PL intensities of samples as a function of time after injection into deionized water. The Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi powders showed severe degradation and even fluorescence loss as increasing time for about 6 h. In contrast, the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 sample showed high resistance to water, and the fluorescence intensity after 6 h was measured to be just a slight decrease. Finally, the two samples were heated from 35 to 115°C and PL spectra were performed after a dwell time of 20 min per 10°C. According to the two different growth trends in Figure 5f and S11, Supporting Information, it can be seen that the fluorescence intensity of SiO 2 -coated Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi films gradually increased with increasing temperature in a regular ascending way, but no regular change was found for uncoated Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi. We speculate that the lattice distortion is enhanced with increasing temperature from 35 to 115°C, allowing more carriers to be bound to the octahedra of [Na(Ag)Cl 6 ] 5À and [Bi(In)Cl 6 ] 3À ( Figure S12, Supporting Information). The enhanced self-trapped states result in the rising luminescence intensity. On the other hand, the increase in temperature may lead to the shedding of ligands on the surface of bare Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi materials. Therefore, the fluorescence intensity of Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs could not show a regular increase, and the increase was significantly weaker compared to the PHPS-treated QDs.
Benefiting from the near-single-dots passivation and encapsulation of lead-free double perovskite Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs through liquid-phase mixing and curing, we have demonstrated a nondestructive direct patterning process of QDs pattern without compromising their optical characteristic (Figure 6a and S13, Supporting Information). As shown in Figure 6b, the delicate hand-painted pattern is a further indication of the flexible operability of the processed QDs ink. On the other hand, the excellent broadband emission covers main red and far-red absorption region of plant pigments, which plays key roles in plant cultivation light. For this purpose, we constructed a broadband-LED by combining the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi@SiO 2 as a downconversion emitting layer on a 385 nm LED-chip through solution drop coating without complex silicone encapsulation. As shown in Figure 6c,d, the luminescence spectra of the obtained LED can just cover the whole red (600-700 nm) and far-red (600-780 nm) range of plant pigments spectrum, which is essential to the photosynthesis and phototropism of plants. The corresponding digital photograph of the working LED is shown as an inset of Figure 6c, exhibiting a bright yellow-orange light.  Figure 6e. Compared with a conventional white light lamp (Figure 6f ), it has richer color and luster in Figure 6e. Along with cost-effective and large-scale synthesis, this nanoscale-particle level protection method of luminescent QDs could be universally compatible with solution-based patterning techniques and promotes a wider range of photonics applications.

Conclusion
In summary, benefiting from the controllable hydrolysis of PHPS and customizable solidification process, the unique -(SiH 2 ─NH)backbone of PHPS can be effectively evolved to form -N─Cl bonds within the Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi QDs surface, replacing the original unstable OA and OAm ligands with passivation effect. After enough hydrolysis of PHPS under suitable conditions (kept at 80°C for 12 h), a dense and homogeneous silicon oxide shell was finally formed in a near-singledot way. Due to the internal crystal structure preserved well, accompanied by surface passivation and encapsulation, the stability of Cs 2 Ag 0.4 Na 0.6 InC 6 :Bi@SiO 2 is significantly improved, regardless of exposure to humidity, UV irradiation, and thermal treatment. In addition, the luminescence properties of Cs 2 Ag 0.4 Na 0.6 InC 6 :Bi@SiO 2 composite were improved to well covering the whole absorption curve for plant growth. Finally, we processed QDs ink to get luminous patterns; meanwhile, a broadband LED was demonstrated with Cs 2 Ag 0.4 Na 0.6 InCl 6 : Bi@SiO 2 as the light conversion layer on the n-UV chip, which manifested very flexible operability in the way of solution drop coating without complex silicone encapsulation. This work provides inspiration for further applications of halide perovskite nanomaterials and also paves the way for solution-based QDs patterning techniques, especially for micro-LED, mini-LED, and other photonics applications.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.