Room-Temperature Synthesis of Air-Stable Near-Infrared Emission in FAPbI3 Nanoparticles Embedded in Silica

Hybrid organic−inorganic and all-inorganic metal halide perovskite nanoparticles (PNPs) have shown their excellent characteristics for optoelectronic applications. We report an atmospheric process to embed formamidinium CH(NH2)2PbI3 (FAPbI3) PNPs in silica protective layer at room temperature (approximately 26 °C) employing (3-aminopropyl) triethoxysilane (APTES). The resulting perovskite nanocomposite (PNCs) achieved a high photoluminescence (PL) quantum yield of 58.0% and good stability under atmospheric moisture conditions. Moreover, the PNCs showed high PL intensity over 1 month of storage (approximately 26 °C) and more than 380 min of PNCs solutions in DI water. The studied near-infrared (NIR) light-emitting diode (LED) combined a NIR-emitting PNCs coating and a blue InGaN-based chip that exhibited a 788 nm electroluminescence spectrum of NIR-LEDs under 2.6 V. This may be a powerful tool to track of muscle and disabled patients in the detection of a blood vessel.

The PNPs were synthesized by a typical hot injection process and a post treatment for encapsulation, which exhibits a low throughput. Sun and coworkers used a one-pot method to prepare silica-coated CsPbX 3 (X = Cl, Br and I) PNP, which added a little number of APTES during the hot injection process. This is an easy and effective method to improve stability [34]. Organic-inorganic CH 3 NH 3 PbBr 3 PNPs were also prepared in a facile roomtemperature one-pot method employing (3-aminopropyl) trimethoxysilane (APTMS) [37], which ensures high luminescence and stability using an easy and rapid strategy. It is highly desirous to develop a near-infrared (NIR) light for the tracking of muscle or disabled patients in the detection of blood vessel, because 650-950 nm wavelengths in NIR are less significantly absorbed by human skin, and can therefore penetrate deeper into the body [38]. Therefore, a one-pot method is necessary for silica-wrapped NIR FAPbI 3 PNPs at room temperature in open air.
Herein, a fast, simple, and efficiency strategy to synthesize high-stability PNPs embedded into silica by air synthesis at room temperature was demonstrated. The perovskite nanocomposites (PNCs) were prepared via a APTES hydrolysis encapsulation strategy. The NIR PNCs was very stable in several rigorous conditions, such as storing in the humid air and ultrasonication in water. In addition, NIR-LED devices were also prepared by FAPbI 3 PNCs as the light-conversion materials coated on the commercial blue InGaN chip. The PNCs exhibits well moisture-resistant and air stability with a long operating lifetime compared to FAPbI 3 PNPs.

Air-Synthesis of NIR-FAPbI 3 PNPs and PNCs
First, 0.1 mmol of formamidine acetate (99%) was dissolved in 10 mL OCTA and stirred 10 min at room temperature (25 • C) in open air for preparation FA precursor as the first step. Then, 0.1 mmol of lead (II) iodide (PbI 2 , 99.999%) were dissolved in a mixture of 10 mL of toluene (98%), 0.8 mL of oleic acid (OA, 90%), 1.2 mL of oleylamine (OAM, 90%), and 1 mL of APTES (99%) at room temperature in the air under stirring for 1 h until PbI 2 was completely dissolved. Subsequently, 2 mL of FA precursor solution was added into the mixture and vigorously stirred for 30 min. The mixture solution was added to hexane (95%) and centrifuged at 9000 rpm for 5 min and the hexane was used to disperse the precipitates. After the second centrifugation, the powders of the NIR-FAPbI 3 PNCs can be obtained by removing the hexane under the airflow at room temperature.

Results and Discussion
PNCs were obtained through the air synthesis at room temperature. The simple reaction system, PbI 2 , OA, OAM, toluene, and APTES in one pot, was stirred 30 min at room temperature (28 • C) in open air ( Figure 1). The FA precursor was then rapidly injected into the mixture, and the colorless solution turned dark red immediately, which indicates the constitution of FAPbI 3 PNCs (Video S1, Supporting Information). The APTES molecule provides Si-O bonds which generate the Si-O-Si ligands through hydrolysis and dehydration in the reaction to package PNPs. This protects PNPs from environmental factors [39][40][41]. Therefore, to verify Si-O-Si ligands on the surface of PNCs, a FTIR spectrum was used to prove the silica wrapping ( Figure 2). The absorption peak at 914 and 1108 cm −1 can be observed in the FAPbI 3 PNCs sample, which is attributed to Si-OH bonds caused by the hydrolysis condensation of APTES and asymmetrical Si-O-Si groups, respectively. These two peaks at 914 and 1108 cm −1 indicate that APTES is well bonded to FAPbI 3 PNCs. In addition, there is a strong stretching vibration at 1710 cm −1 due to C=N from FA + . The C-H stretching vibrations of CH 2 and CH 3 were detected from 2800 to 3000 cm −1 [41][42][43]. In order to verify that the PNPs embedded in silica, and confirm the real PNCs structure, the morphological features of the PNCs were observed by TEM. HRTEM images ( Figure 3a) show that the as-synthesized FAPbI 3 PNPs have a cubic shape. Figure 3b shows the HRTEM image of the as-prepared FAPbI 3 PNCs; the PNPs embedded into a shapeless material can be clearly seen, which suggests the presence of SiO 2 materials. These SiO 2 shells protect the PNPs from the influence of atmospheric moisture and oxygen [29,30]. The particle sizes have provided in Figure S1. Si and O elements can be detected by Energy dispersive spectroscopy (EDS) of Figure 3b (Figure S2), which is the evidence for the silica presence. The particle sizes of PNPs and PNCs were established to be 16.8 and 10.6 nm, respectively. The smaller size of PNCs may be due to the fact that the Si-O-Si ligands inhibit contacts between FAPbI 3 , leading to limited particle growth. Similar results were observed in X-ray diffraction (XRD) patterns, as shown in Figure S3. Both samples only showed the cubic phase of FAPbI 3, indicating amorphous SiO2. Compared with PNPs, PNCs exhibited weaker XRD intensity, which was attributed to smaller particle size and lower perovskite particle density in the powder. Meanwhile, compared with air, the higher refractive index of SiO 2 can enhance the light extraction from PNCs.   Figure 4a shows that the larger grain size (approximately a few hundred nanometers) in the PNP powders is much greater than the TEM observation, which indicates that the PNPs aggregate without SiO 2 protection. The larger particles in Figure 4b were attributed to the SiO 2 matrix growth and network covalent solid of SiO 2 . Thus, the abovementioned results evidence that the PNPs and PNCs can be obtained using our simple room-temperature synthesis method. The PL spectrum of 0.25 mL APTES exhibits a narrow symmetric emission band with a peak at 795 nm, with a longer wavelength because of the scattering effect of large particles, as shown in Figure 5. However, an inadequate number of ligands leads to low PLQY (ca. 23%). When the APTES concentration increases to 0.5 mL, the highest PLQY (58.0 %) was obtained with a slight blue-shift emission. Although the emission could be further blue-shifted, the PLQY of NCs reduced. It is known that with high ligand concentrations, the rate of the reactive molecules' delivery through the silica-wrapped layer becomes slower due to the steric hindrance of Si-O-Si, resulting in smaller particles and the reduced PLQY [38]. Figure 5c shows the as-prepared PNPs and PNCs powders. To confirm that PNCs effectively blocks moisture and oxygen in the atmosphere, the PL spectra of the respective powders stored at approximately 26 • C with a relative humidity of approximately 75 % were measured for the different storage times. The PL intensities of the FAPbI 3 PNPs showed an obvious decay after 16 days, which is in agreement with previous reports [34,39,43], as shown in Figure 6a. In contrast, Figure 6b exhibits a slow decrease in PL intensity which suggests a good stability in the moist air for the FAPbI 3 PNCs. Furthermore, the water stability of FAPbI 3 PNCs was recorded by 1 mL of FAPbI 3 PNP and PNC solutions injecting to 2 mL of DI water. Figure 6c shows the PL intensities of FAPbI 3 PNP and PNC solutions in DI water; the dark red fluorescence of FAPbI 3 PNPs solution decayed swiftly after 16 min in DI water. However, the FAPbI 3 PNC solution still showed dark red light in the DI water even after 32 min, as shown in Figure 6c. It also remained 25% of initial PL intensity after 384 min. In contrast, the FAPbI 3 PNCs, revealed better water stability for the FAPbI 3 PNCs. The NIR FAPbI 3 PNCs powder was coated on blue InGaN chip (wavelength = 455 nm) and NIR-LEDs were fabricated, as displayed in Figure 7a. Figure 7b shows a typical EL spectrum of NIR LEDs located at 788 nm under 2.6 V, indicating NIR emission. This may have a potential as a NIR light source to detect a blood vessel. Our results indicate that moisture-resistant and air-stability FAPbI 3 PNCs synthesis at room temperature is a promising material in bio-optoelectronic devices.

Conclusions
In conclusion, we successfully synthesized FAPbI 3 embedded into silica at room temperature in open air by a facile method. The air-synthesized PNCs at room temperature treatments still display high stability under ambient exposure and a narrow emission in the PL spectra. In particular, the SiO 2 protective layer provides high PL intensity after 32 days of storage atmosphere (28 • C) and stability in DI water. The NIR-LEDs based on the NIR-emitting FAPbI 3 PNCs powder coated on the blue LED have a 788 nm EL spectra.
We hope our results can be further applied in biomedical lighting applications and devices based.