RESEARCH ARTICLE www.advmatinterfaces.de Amplified Photoluminescence of CsPbX3 Perovskites Confined in Silica Film with a Chiral Nematic Structure Marija Knezevic, Thi-Hieu Hoang, Cong Wang, Masa Johar, Alba Garzón Manjón, David Llorens Rauret, Christina Scheu, Marie Erard, David Berardan, Jordi Arbiol, Christophe Colbeau-Justin, and Mohamed Nawfal Ghazzal* synthesis or selective substitution of Metal halide perovskites (MHPs, CsPbX3: X = Cl, Br, I) have advanced the the atoms, i.e., CsPbBr3-y Xy (X = Cl, field of optoelectronic devices due to their remarkable light-emitting I).[5] Compared to other semiconductors, capabilities, stemming from the large overlap between their emission and MHPs exhibit unusual defect-tolerant absorption spectra, offering the possibility to reabsorb their own emitted photophysics,[6] which can be rational- ized by the presence of defect energy photons. Herein, a straightforward method is reported to confine CsPbBr3 levels localized either within the va- into mesoporous silica films with a chiral nematic structure, allowing the lence band (VB) or conduction band amplification of the photoluminescence (PL). The simple room temperature (CB) instead of being deep within the ligand-free synthesis allows facile growth of CsPbBr3 in silica photonic films, bandgap. Still, some defects, such as in which the Bragg peak position can be tuned from the UV to the visible undesirable cation or anion vacancies, remain present, and their energy lev- range. The perovskite/silica films demonstrate a remarkable improvement in els fall within the CB and VB, nega- PL intensity and lifetime compared to the as-synthesized non-confined tively affecting the PL properties. Sev- perovskite nanocrystals (NCs) due to the overlap of the Bragg peak position of eral strategies have been proposed to ad- the chiral nematic photonic films and CsPbBr3 absorption band. Such a PL vance the MHP-based emissive technol- enhancement stems from the slow photon effect induced at blue and red ogy, significantly improving the photo- Bragg peak edges that facilitates the photon recycling of the emitted photons. luminescent quantum yield (PLQY).[7,8] Some suggested compositional engineer- This innovative approach offers a new way to fabricate highly emissive and ing, crystal size tuning,(9) direct spin- long-lived photoluminescent films at ambient conditions, potentially coating of colloidal perovskite NCs,[10] advancing perovskite utilization in light-emitting devices. constructing a quasi-core/shell structure that combines a high PLQY with bal- anced charge injection, surface passiva- tion to reduce the charge trap density,[11] 1. Introduction the introduction of functional molecules, metal ions, and halogen ions into the bulk or onto the surface of MHPs.[12] MHPs have attracted ever-increasing attention for their applica- Another paramount feature that has advanced MHPs in the tion in light-harvesting materials.[1,2] MHPs exhibit remarkable field of optoelectronic devices, leading to their remarkable light- charge transport properties,[3] long carrier lifetimes,[4] and low emitting capabilities, stems from the large overlap between their excitation binding energy.[5] Additionally, the electronic and opti- emission and absorption spectra, offering the possibility to re- cal properties are easily tuned by either controlling the chemical absorb their own emitted photons, enhancing the PL intensity. M. Knezevic, T.-H. Hoang, C. Wang, M. Johar, M. Erard, A. G. Manjón, D. L. Rauret, J. Arbiol C. Colbeau-Justin, M. N. Ghazzal Catalan Institute of Nanoscience and Nanotechnology (ICN2) Université Paris-Saclay CSIC and BIST Institut de Chimie Physique Campus UAB, Bellaterra, Barcelona 08193, Catalonia Spain UMR 8000 CNRS, Orsay 91405, France C. Scheu E-mail: mohamed-nawfal.ghazzal@universite-paris-saclay.fr Max-Planck-Institut für Eisenforschung GmbH Max-Planck-Strasse 1, 402bu37 Düsseldorf, Germany The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202300636 D. Berardan ICMMO © 2023 The Authors. Advanced Materials Interfaces published by UMR 8182 CNRS, Université Paris-Saclay, Orsay F-91405, France Wiley-VCH GmbH. This is an open access article under the terms of the J. Arbiol Creative Commons Attribution License, which permits use, distribution ICREA and reproduction in any medium, provided the original work is properly Pg. Lluís Companys 23, Barcelona 08010, Catalonia Spain cited. DOI: 10.1002/admi.202300636 Adv. Mater. Interfaces 2023, 2300636 2300636 (1 of 9) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300636 by Readcube (Labtiva Inc.), Wiley Online Library on [19/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 1. a) Schematic diagram of the template-synthesis of CsPbBr3 confined in the mesoporous silica film with a chiral nematic structure. b) Images and reflectance spectra of silica films obtained with variable CNCs/TMOS ratio and diffuse reflectance spectrum of CsPbBr3 NCs. c) Absorption spectra of CNS0.4 /CsPbBr3 , CNS0.8 /CsPbBr3 , and CNS1.0 /CsPbBr3 films. These properties make MHPs the primary choice material for sion and absorption spectra of the MHPs is expected to maximize next-generation color display applications.[13] An alternative to the PR (reabsorption–reemission events) and improve the PL in- enhance the PL properties involves the photon recycling (PR) ef- tensity and lifetime. fect in textured films.[14,15] This process directly affects the PLQY As proof of our concept, we confined the low emissive MHPs since emitted photons undergo successive events involving the into the mesoporous silica films with a chiral nematic structure reabsorption of radiatively emitted photons, resulting in photo- using precipitation. Compared to the as-synthesized MHPs NCs, generated charge recombination before eventually escaping from the CsPbBr3 @SiO2 films demonstrate higher PL intensity and the film, enhancing the PL lifetime.[14,16] The PR process was only longer PL lifetime. More importantly, the improvement of PL obtained by exploiting the light scattered at the film surface and properties matches the Bragg peak position of the chiral nematic interface structure.[14] Therefore, there is still a need to explore a photonic films, demonstrating the slow photon effect at their blue substantially structured cavity and its effect on the PL of MHPs. and red edges, facilitating the reabsorption of the emitted pho- In this work, we developed a straightforward strategy enabling tons leading to the PL amplification of MHPs. This innovative the confinement of MHPs in a periodically arranged mesoporous approach provides an original way to fabricate highly emissive silica film, so-called photonic crystals (PCs). The aim is to im- and long-lived photoluminescent films. prove the stability of MHP in an aqueous solution and simultane- ously the quantum utilization of incident light. Up to now, meso- 2. Results and Discussion porous silica materials have been used to enhance the stability of MHPs and induce quantum confinement.[6,17,18] However, no Cellulose nanocrystals (CNCs) are rod-like materials that can self- attention has been paid to the effect of structuring the silica ma- assemble into the chiral nematic structure during water evapora- terial on improving the PL of MHPs. The basics of the extraor- tion and give rise to porous PCs after calcination (CNC removal) dinary properties of the PCs, which could be found in various (Figure 1a).[22] The transfer of chiral nematic structure to the sil- species in nature, were proposed by Yablonovitch.[19,20] The PCs ica films after removing CNCs endows them with their unique exhibit a photonic bandgap that inhibits light propagation when chiral photonic properties. The pure chiral nematic silica (CNS) interacting with electromagnetic radiation (forbidden wavelength films show strong iridescent color and intense Bragg peak re- reflected by PCs). More importantly, the group velocity of light is flection. The wavelength maximum of the Bragg reflection was expressed in Equation (1) controlled by finely tuning the amount of tetramethyl orthosili- cate (TMOS) in the stock solution (0.4–1 mL), labeled as CNS0.4 , ⃗ g = 𝜕𝜔 V (1) CNS0.8 , and CNS1.0 . As the TMOS amount increases, the color 𝜕 k⃗ of the films changes from blue to green to yellow, as revealed by UV–vis–NIR spectroscopy (Figure 1b). This corresponds to where 𝜔 the pulsation and k the wave vector, tends to zero near the red shift of the maximum wavelength of the Bragg reflection the blue and red edges of the photonic bandgap.[21] Slowing down peak. The maximum wavelength reflected by the films (𝜆max ) is the photons combined with the large overlap between the emis- related to the effective refractive index “neff ”, the helical pitch “P”, Adv. Mater. Interfaces 2023, 2300636 2300636 (2 of 9) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300636 by Readcube (Labtiva Inc.), Wiley Online Library on [19/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 2. Cross-sectional images of a range of silica films obtained at different ratios of CNC/TMOS a,b) CNS0.4 /CsPbBr3 , c,d) CNS0.8 /CsPbBr3 , and e,f) CNS1.0 /CsPbBr3. and the angle between the film axis and the incident light, accord- stand the optical, morphological, and compositional properties ing to the following equation:[23] of the composite films. Figure 1c displays the UV–vis spec- tra of different silica films containing MHPs nanoparticles. A 𝜆max = neff P sin(𝜃) (2) broad absorbance band was observed starting at 525 nm, typ- ical of the direct optical bandgap of CsPbBr3 perovskites. The Assuming that the refractive index of CNS remains unchanged, bandgap of CsPbBr3 NCs, CNS0.4 /CsPbBr3 , CNS0.8 /CsPbBr3 , the redshift arises from the pitch “P” increase.[24] This will be and CNS1.0 /CsPbBr3 films was estimated from the Kubelka– discussed further in the following morphological analysis of the Munk modified function as 2.23, 2.37, 2.36, and 2.36 eV, respec- film cross-section. tively (Figure S3, Supporting Information). It is worth noting that We synthesized low emissive cesium lead halide perovskite the absorbance band edge of the MHPs confined in mesoporous CsPbBr3 NCs using a simple precipitation method at am- silica films is blue-shifted for 0.13 eV. The observed shift indicates bient conditions (Figure S1, Supporting Information). This the downsizing of MHPs grown in the silica pores. method leads to almost phase-pure CsPbBr3 NCs with only The SEM observations of the silica films cross-section obtained trace amounts of CsPb2 Br5 . CsPbBr3 NCs display intense diffrac- at different ratios of CNC/TMOS after calcination and MHPs con- tion peaks corresponding to the well-crystallized orthorhombic finement are shown in Figure 2. The films show layered mor- CsPbBr3 NCs (Figure S2, Supporting Information). Lattice pa- phology throughout their thickness, which is the typical finger- rameters of CsPbBr3 NCs were determined to be a = 8.2570 Å, print of the chiral nematic structure. The twist of the silica rods b = 11.7629 Å, and c = 8.2121 Å (±0.001 Å). We used an over a defined helical axe is further confirmed at higher magni- identical method for confining CsPbBr3 in the silica meso- fication (Figure S4, Supporting Information). This demonstrates pores films, obtaining, CNS0.4 /CsPbBr3 , CNS0.8 /CsPbBr3 , and that the isotropic liquid crystalline organization of CNCs is suc- CNS1.0 /CsPbBr3 . We used UV–vis–NIR spectroscopy, scanning cessfully transferred into the inorganic silica films. It is worth electron microscopy (SEM), high-resolution transmission elec- noting that the pitch can be modified by controlling the concen- tron microscopy (HRTEM), high-angle annular dark field scan- tration of the silica precursor, and an increase in the P is observed ning transmission electron microscopy (HAADF-STEM), and as the CNC/TMOS ratio increases. The average value of the P es- energy-dispersive X-ray spectroscopy (EDS) to deeply under- timated from SEM images is 215, 343, and 483 nm for CNS0.4 , Adv. Mater. Interfaces 2023, 2300636 2300636 (3 of 9) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300636 by Readcube (Labtiva Inc.), Wiley Online Library on [19/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 3. a) General view TEM images of CNS0.8 /CsPbBr3 , b) HRTEM image of spherical CsPbBr3 NCs in CNS0.4 , and FFT of the squared area in red. The top right image is a magnified detail of the area in red. c) XRD patterns of CNS and CNS/MHP films. d) STEM-EDS compositional mapping of CNS0.4 /CsPbBr3. CNS0.8 , and CNS1.0 , respectively, which is consistent with the red- confirming the chiral nematic organization of the photonic film. shift observed in the reflection spectra. As estimated from the TEM images, the silica pore size has The morphology of the CNS0.8 /CsPbBr3 film was further an- an average width of ≈8 nm, corresponding to the diameter of alyzed by TEM, and the results are shown in Figure 3a,b. The CNC rods, in agreement with reported studies.[24–26] Further films are porous and exhibit locally aligned rods separated by investigations of the films by HR-TEM indicate the CsPbBr3 mesopores. The localized fragments of silica rods appeared to inclusion in the film. Most of the nanoparticles are spheri- be twisting around the helical axis perpendicular to the plane, cal (Figure 3b) and in the pore channels (Figures S5 and S6, Adv. Mater. Interfaces 2023, 2300636 2300636 (4 of 9) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300636 by Readcube (Labtiva Inc.), Wiley Online Library on [19/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Table 1. Lattice parameters estimation of CsPbBr3 nanoparticles grown in Such a redshift, also observed for perovskite assembled into 2D CNS films, obtained from XRD data. photonic supercrystals,[27] underlines a strong degree of coupling phenomenon between confined MHPs nanoparticles, amplified Samples a b c by the silica photonic structure and the slow photon effect. A CNS0.4 /CsPbBr3 8.256 11.760 8.199 further increase in the amount of silica leads to the redshift of CNS0.8 /CsPbBr3 8.261 11.763 8.202 the Bragg peak maximum to a higher wavelength and results in CNS1.0 /CsPbBr3 8.256 11.755 8.194 a quenched PL intensity. The PL lifetime was probed by time- resolved fluorescence lifetime imaging (TRPL) at 440 nm excita- tion. The PL imaging demonstrated a significant change in the PL decay upon embedding MHP in the photonic films (Figure 4c). Supporting Information). The diffraction pattern obtained from The PL lifetime was remarkably enhanced when MHPs were FFT indicated crystalline CsPbBr3 . The average size distribu- grown into silica films with photonic properties. The PL lifetime tion of spherical MHPs in CNS0.8 /CsPbBr3 was estimated to is remarkably improved in the case of CNS0.4 and CNS0.8 . It is be 6.7 ± 1.2 nm (Figure S7a, Supporting Information). The four times higher compared to the solely CsPbBr3 NCs due to FFT diffraction pattern demonstrated crystalline CsPbBr3 . The the significant overlap between the blue (CNS0.4 ) and red edge mesoporous films behave as templates guiding the confined of the Bragg peak (CNS0.8 ), and CsPbBr3 absorption spectral line growth of perovskites, leading to the downsizing behavior. The (7.8 and 8.4 ns for CNS0.4 /CsPbBr3 and CNS0.8 /CsPbBr3 , respec- morphology of the CNS0.4 /CsPbBr3 is similar to the morphol- tively) (Table 2). However, the PL average lifetime increases from ogy of CNS0.8 /CsPbBr3, with an average particle size of 6.7 ± 2.8 ns for CsPbBr3 NCs to 3.1 ns for CNS1.0 /CsPbBr3 films. Such 1.4 nm (Figure S7b, Supporting Information). Furthermore, we a nonsignificant PL improvement stems from the mismatch be- observed a limited amount of non-confined cubic CsPbBr3 NCs tween CNS1.0 blue-edge of the Bragg peak and CsPbBr3 absorp- on the surface of CNS0.4 /CsPbBr3 and CNS0.8 /CsPbBr3 films, tion. The enhanced and long-lived PL of MHPs confined in CNS with an average size of 28.3 ± 5.4 and 69.7 ± 11.4 nm, re- films relies on the photonic bandgap position. The absorbance spectively. In CNS1.0 /CsPbBr3 films, several spherical NPs were band of MHPs coincides with the blue edge of the Bragg peak observed along the pores (Figure S8, Supporting Information); position of CNS0.8 and the red edge of the Bragg peak of CNS0.4 however, the surface of silica films is dominated by CsPbBr3 film. The perfect match between the absorbance band of the nanocubes with an average size of 27.6 ± 6.7 nm. MHPs and the blue and red edge of the silica PC promotes The XRD patterns of silica mesopores photonic films con- the light scattering inside the films, slowing the speed of light. taining MHPs nanoparticles (CNS0.4 /CsPbBr3 , CNS0.8 /CsPbBr3 , This paramount feature increases the photon retention inside the and CNS1.0 /CsPbBr3 ) exhibit typical patterns of the orthorhom- structure and the perovskite absorbance factor. In other words, bic CsPbBr3 , in agreement with HRTEM observations. In MHPs have more time to absorb photons, lowering the number contrast, native silica film shows a broad band from 15° to of non-absorbed photons. As a photon is emitted from the MHPs, 25°characteristic of the amorphous phase (Figure 3c). The lattice it experiences a slow photon itself, since the emission wave- parameters of orthormobic CsPbBr3 nanoparticles incorporated length matches with either the blue or red edge of the photonic in CNS films are given in Table 1. A secondary phase due to a mi- gap while being trapped in the photonic cavity. The surrounding nor amount of unreacted CsBr was observed with a weak diffrac- MHP nanoparticles can reabsorb the emitted light, leading to the tion peak around 28.7°. The EDS mapping analysis (Figure 3d, multiple reabsorption/reemission processes before leaving the Figure S9, Supporting information) confirmed the overlap of film (Figure 4d). Such a phenomenon has been recently demon- the silica mesoporous films with the localization Cs, Pb, and Br strated in surface-structured MHPs films.[28] As the Bragg posi- atoms, proving the formation of CsPbBr3 nanoparticles inside tion is shifted to a higher wavelength, the range of these edges the pores of CNS. no longer matches the absorbance band of MHPs leaving only a The optical properties of the photonic films with MHP limited effect on the PL intensity and lifetime. In addition, since nanoparticles were investigated, and the results are shown in MHPs exhibit large overlaps between their emission and absorp- Figure 4. The photographs of as-synthesized CsPbBr3 NCs tion spectra, reabsorption of the emitted light is expected to occur and CNS/CsPbBr3 films are given in Figure 4a. To demon- and induce PR. strate the amplified emission we evaluated the steady-state and The films, kept without further protection, were exposed to time-dependent PL performances of the CsPbBr3 confined in the ambient environment at room temperature for 30 days, films with a variable Bragg reflection wavelength maximum. and a daily PL intensity was measured to assess their stability Visual observations under daylight and under UV excitation (Figure 4e). PL intensity starts decreasing for CNS0.8 /CsPbBr3 (254 nm) indicate weak luminescence of the CsPbBr3 powder. compared to CNS0.4 /CsPbBr3 , while the PL intensity for the However, once the perovskite is confined into nanostructured CNS1.0 /CsPbBr3 film remains low but relatively stable over time. CNS films, a bright emission is obtained for CNS0.4 /CsPbBr3 The PL properties of confined MHPs showed remarkable stabil- and CNS0.8 /CsPbBr3 films (Figure 4a). The spontaneous UV- ity over time under an ambient atmosphere, with high PL inten- activated PL decreases significantly for CNS1.0 /CsPbBr3 . These sity, due to the limited chemical interaction with ambient oxy- observations were confirmed by steady-state PL spectroscopy gen and moisture.[29] It is known that CsPbBr3 NCs are very measurements (Figure 4b). A remarkable increase in PL inten- sensitive to moisture that makes them susceptible to degrada- sity is observed, up to ten fold,[ when the amount of silica precur- tion and PL quenching.[30] To preserve long-term high emis- sor increases, reaching its maximal value for CNS0.8 /CsPbBr3 . sion intensity, we grafted a hydrophobic layer of fluorinated In addition, the maximum wavelength is red-shifted to 519 nm. alkyl silane, i.e., tridecafluorooctyl triethoxysilane (FAS) at the Adv. Mater. Interfaces 2023, 2300636 2300636 (5 of 9) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300636 by Readcube (Labtiva Inc.), Wiley Online Library on [19/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 4. a) From left to right, photographs under ambient and UV illumination of non-confined CsPbBr3 NCs and CNS0.4 /CsPbBr3 , CNS0.8 /CsPbBr3 , and CNS1.0 /CsPbBr3 films. b) Emission spectra of CNS0.4 /CsPbBr3 , CNS0.8 /CsPbBr3 , and CNS1.0 /CsPbBr3 . c) TRPL lifetime decays obtained at 440 nm excitation. d) Schematic representation of PR in the CNS films containing CsPbBr3 NCs. e) PL evolution over time under ambient atmosphere. surface of CNS0.8 /CsPbBr3 films to improve their stability against time intervals (5–30 min), resulting in the bandgap tuning, with moisture. Upon immersion of CNS0.8 /CsPbBr3 films in water, it emission maximum ranging from 450 to 580 nm (Figure S12a, demonstrated a color change of the sample due to water infiltra- Supporting Information). Under ambient conditions, the anion- tion into the porous silica structure and the modification of the exchanged films kept identical color for more than a month un- refractive index of the surrounding media, with blue substantial less being ground. Nevertheless, the XRD patterns of these sam- luminescence followed by immediate PL quenching and struc- ples could not provide clear phase distinction due to substantial tural degradation. The grafting of FAS hydrophobic molecule al- amorphous phase contribution. It seems that CNS0.8 /CsPbBr3-y Iy lows the film to maintain its greenish PL, attesting to the inhi- consists of mixed Br/I orthorhombic perovskite; however, small bition of water diffusion into the films (Figure S10, Supporting and broad peaks cannot be resolved with sufficient certainty. In- Information). terestingly, complete anion substitution with PbI2 resulted in yel- To extend our study to the broader spectral range, we inves- low nonluminescent films. Further investigation of anion sub- tigated MHPs optical properties upon anion substitution. Inter- stitution with Cl demonstrated dramatic structural modification, estingly, our room temperature synthesis resulted in both lumi- i.e., CNS0.8 /CsPbBr3-y Cly predominantly consists of an unknown nescent CNS0.8 /CsPbBr3-y Cly and CNS0.8 /CsPbBr3-y Iy films upon non-perovskite phase, which could correspond to CsPb2 Cl5 , since 5-min-long immersion in PbX2 concentrated solution (X = Cl, I) longer immersion for up to 30 min, results in the predomi- (Figure 5a). The anion-exchange process gives a simple substitu- nant CsPb2 Cl5 non-perovskite phase (Figure S12b, Supporting tional technique to modify the composition and emission posi- Information). The presence of iodide and chloride was also con- tion in visible spectrum areas perceptible to the human eye.[31] firmed by EDS (Figures S13–S15, Supporting Information). Con- The bandgap of CNS0.8 /CsPbBr3-y Cly and CNS0.8 /CsPbBr3-y Iy sequently, TRPL revealed considerable fluorescence quenching films were estimated from the Kubelka–Munk modified func- upon anion substitution of bromide with iodide and chloride tion to be 2.7 and 2.2 eV, respectively (Figure S11, Supporting (Figure 5b). The average lifetime decreased to 0.78 and 0.75 ns for Information). However, the mixed halide films demonstrate an CNS0.8 /CsPbBr3-y Cly and CNS0.8 /CsPbBr3-y Iy , respectively. Upon emission band of ≈520 nm, which can be explained by halide anion substitution, the contribution of short kinetic components migration and incomplete substitution of bromide. It is possi- (𝜏 3 ) becomes significant (Table S1, Supporting Information). The ble to control the rate of anion substitution by immersing films contribution of 𝜏 4 cannot be assigned with certainty since the in- in a concentrated solution of PbX2 (X = Cl, I) during different strument response function is of the same order of magnitude. Adv. Mater. Interfaces 2023, 2300636 2300636 (6 of 9) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300636 by Readcube (Labtiva Inc.), Wiley Online Library on [19/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Table 2. The PL multiexponential lifetime decays with decay fractions cent materials. Further developments are needed to incorporate of CNS0.4 /CsPbBr3 , CNS0.8 /CsPbBr3 , CNS1.0 /CsPbBr3 , and CsPbBr3 CNSx /CsPbBr3 in a white light-emitting diode and improve their nanopowder. stability in harsh media and at higher temperatures. In addi- tion, the synthesis optimization of mixed halide perovskites, i.e., Samples 𝝉 1 [ns] 𝝉 2 [ns] 𝝉 3 [ns] 𝝉 average [ns] CsPbBr3-y Iy and CsPbBr3-y Cly , incorporated in CNS films, with CNS0.4 /CsPbBr3 14 ± 0.3 2.8 ± 0.1 0.46 ± 0.02 7.8 ± 0.1 improved structural and optical properties, would offer more flex- 51% 24% 25% ibility in PL coloration of the films. CNS0.8 /CsPbBr3 18 ± 0.8 3.8 ± 0.1 0.46 ± 0.01 8.4± 0.3 40% 30% 30% CNS1.0 /CsPbBr3 9.9 ± 0.6 2.4 ± 0.1 0.41 ± 0.01 3.1 ±0.1 4. Experimental Section 22% 32% 46% Materials: Lead iodide (PbI2 , ≥99.0%), lead chloride (PbCl2 , ≥99.0%), CsPbBr3 9.2 ± 0.9 1.5 ± 0.3 0.25 ± 0.3 2.8 ± 0.2 and lead bromide (PbBr2 , ≥98.0%) were purchased from Acros Organic. 25% 25% 50% Cesium bromide (CsBr, 99%) was purchased from Alfa Aesar, while dimethyl sulfoxide (DMSO, ≥99.0%), toluene (99.8%), hexane (95%), and TMOS (≥99.0%) were purchased from Sigma–Aldrich. CNCs (purchased The anion substitution introduces trapping centers that result from the University of MAINE) were used without further purification. All chemicals used were analytical grade and were directly used without fur- in dominant non-radiative decay paths, leading to significant PL ther purification. quenching. These results demonstrate the possibility of a room- Elaboration of SiO2 Photonic Films with a Chiral Nematic Structure: The temperature anion substitution in CNS0.8 /CsPbBr3 films. How- procedure used for the film elaboration was adapted from previously re- ever, further studies are necessary to improve their crystallinity, ported methods.[22,25] Adequate amount (0.4, 0.8, and 1.0 mL) of TMOS PL lifetime, and the optimization of optical bandgap overlap with was added to 5.0 mL CNCs aqueous suspension (4 wt%, pH = 2.4) pre- the photonic bandgap of CNS films. viously sonicated during 1 h in glass vials. The suspension was stirred for 15 min at room temperature. Subsequently, the CNCs/TMOS mixtures were transferred to polystyrene petri dishes and left to dry at room tem- perature without any control of the environment atmosphere. CNC/silica 3. Conclusion hybrid films with different colors and helical pitch values were obtained after 24 h. The biotemplate (CNCs) was removed by calcination under air A series of CNSx /CsPbBr3 (x = 0.4, 0.8, 1.0) photonic films at 540 °C for 4 h at a heating rate of 2 °C min−1 to generate mesopores were successfully synthesized using an evaporation-induced self- CNSs films. assembly process and precipitation method. The results show Synthesis of CNS/CsPbBr3 Films: CsPbBr3 precursor solution was pre- that CNSx /CsPbBr3 films displayed extraordinary bright green pared by dissolving 0.4 mmol PbBr2 and 0.4 mmol CsBr in 1 mL DMSO fluorescence. Owing to the significant overlap of CsPbBr3 ab- under sonication for 1 h. Then, the silica films with controlled photonic sorption spectral lines and CNS0.8 (CNS0.4 ) blue (red) photonic properties (CNS0.4 , CNS0.8 , and CNS1.0 ) were immersed in-stock solu- edge, a slow photonic effect was induced that directly influ- tion for 12 h at ambient conditions. Subsequently, CNS/CsPbBr3 films were taken out, washed several times with toluene and hexane, and enced PL through the PR effect, together with the downsizing dried at room temperature, yielding CNS0.4 /CsPbBr3 , CNS0.8 /CsPbBr3 , effect of CsPbBr3 within CNS pores. Such heterostructure ex- and CNS1.0 /CsPbBr3 films. In comparison, CsPbBr3 NCs were obtained periences quite good stability under ambient conditions, while following the identical procedure. The equimolar amount (0.4 mmol) of coating with a hydrophobic molecular layer allows direct im- PbBr2 and CsBr was mixed and dissolved in DMSO (1 mL). The obtained mersion of CNSx /CsPbBr3 in water, demonstrating ephemeral solution was drop-cast on a glass and dried under ambient conditions. PL. Using textured active films is an inexpensive way of synthe- Upon evaporation of DMSO, CsPbBr3 NCs were formed, collected, and ground. sizing highly luminescent quantum-confined CsPbBr3 NCs that Synthesis of CNS/CsPbBr3-y Xy (X = Cl, I) Hybrid Films: The anion could improve PL for photonic and light-emitting applications. substitution is achieved by a simple partial exchange of Br atom with Incorporating CsPbBr3 into the photonic films with a chiral ne- Cl and I atoms. Stock solutions of PbCl2 and PbI2 were prepared matic structure is a step forward in generating highly lumines- by dissolving 0.4 mmol PbCl2 and 0.4 mmol PbI2 in 1 mL DMSO Figure 5. a) Absorption and emission spectra of CNS0.8 /CsPbBr3-y Xy (X = Cl, I) with their corresponding sample photographs under ambient and UV illumination. b) TRPL lifetime decays of CsPbBr3 nanopowder, CNS0.8 /CsPbBr3 , CNS0.8 /CsPbBr3-y Cly , and CNS0.8 /CsPbBr3-y Iy films. Adv. Mater. Interfaces 2023, 2300636 2300636 (7 of 9) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300636 by Readcube (Labtiva Inc.), Wiley Online Library on [19/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de under sonication for 1 h. Previously prepared film CNC/CsPbBr3 was Acknowledgements directly immersed in a solution of either PbI2 (0.4 mmol) or PbCl2 (0.4 mmol) stock solution, and the substitution rate was controlled by MK acknowledges the French « Ministère de l’enseignement supérieur immersion time (30 min). After anion substitution, films were taken et de la recherche» (MESR) for the PhD grant. MNG thanks the public out, washed several times with toluene and hexane, and dried at room grant overseen by the French National Research Agency (ANR), through temperature. the IngenCat project (ANR-20-CE43-0014) and NEXTCCUS project as part Synthesis of CNS0.8 /CsPbBr3 Films Coated with Hydrophobic Molecu- of the ERANET-ACT3 call program for the financial support. The au- lar Layer: A solution of tridecafluorooctyl triethoxysilane (FAS) was pre- thors thank F. Brisset for his valuable help in performing SEM and TEM pared by dissolving FAS (200 μL) in hexane (800 μL). CNS0.8 /CsPbBr3 analysis. ICN2 is supported by the Severo Ochoa program from Span- films were immersed in the as-prepared solution for 2 h. After dry- ish MCIN / AEI (Grant no.: CEX2021-001214-S) and is funded by the ing, the stability of the films was evaluated by immersing films in pure CERCA Programme from the Generalitat de Catalunya. ICN2 acknowl- water. edges funding from Generalitat de Catalunya 2021SGR00457. MCIN sup- Characterization Techniques: Diffusion reflectance spectra (DRS) were ported this study with funding from European Union NextGenerationEU recorded using UV–vis–NIR Cary 5000 UV–vis–NIR spectrophotometer (PRTR-C17.I1) and Generalitat de Catalunya. The authors thank the sup- (Agilent Technologies), equipped with an integrating sphere for diffuse port from the project NANOGEN (PID2020-116093RB-C43), funded by and total reflection measurements. The maximum reflectance was set MCIN/ AEI/10.13039/501100011033/ and by “ERDF A way of making to 100% using BaSO4 as a reference in the 200–1100 nm wavelength Europe,” by the “European Union.” AGM acknowledges the RyC Pro- range. gramme by MCIN/AEI and Ayuda RYC2021-033479- I financiada por The PL emission spectra were recorded using Fluorolog 3 HORIBA MCIN/AEI /10.13039/501100011033 y por la Unión Europea NextGener- fluorimeter equipped with a solid sample holder. All samples were ex- ationEU/PRTR. cited at 400 nm, and emission was monitored in the 450–700 nm range. X-ray diffraction (XRD) measurements were carried out by room tem- Conflict of Interest perature powder X-ray diffraction (P-XRD). Patterns were recorded by a Panalytical X’Pert diffractometer with a Ge (111) incident monochroma- The authors declare no conflict of interest. tor (Cu K𝛼 radiation) and an X’celerator detector. The scanning electron microscopy (SEM) cross-section images were acquired with a ZEISS Supra 55VP FEG-SEM with an operating voltage of Data Availability Statement 2 kV and a working distance of 3–5.7 mm. The data that support the findings of this study are available from the Transmission electron microscopy (TEM). The TEM images were ob- corresponding author upon reasonable request. tained using a JEOL JEM 2100Plus, operating at 200 kV, and a Tecnai F20 field emission gun (FEG) microscope at 200 kV. The size of synthe- sized nanostructures was determined by Gatan Digital Micrograph soft- ware. The samples were crashed in small pieces, dispersed in hexane Keywords by stirring and deposited on the lacey carbon grids. The sonication was chiral nematic silica, perovskites, photoluminescence, photon recycling avoided for the grid preparation to limit the degradation/dissolution of perovskites. The perovskite can be destroyed under the beam and during Received: July 26, 2023 the image acquisition. Therefore, the acquisition time is only less than Revised: September 28, 2023 10 min. Published online: The high-angle annular dark field scanning TEM (HAADF-STEM). TEM analysis was conducted using a Titan Themis 60-300 X-FEG microscope from Thermo Fischer Scientific. The microscope was equipped with a probe corrector and was operated at 300 kV. High-angle annular dark field (HAADF) images, with an angular range of 72–352 mrad, and EDS [1] K. Lin, J. Xing, L. N. Quan, F. P. G. De Arquer, X. 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