Circularly Polarized Luminescence from Chiral Photonic Crystals of Caesium Lead Halide Perovskites

The generation and detection of circularly polarized light (CPL) are of importance in many modern technologies such as digital communications, machine vision, bio‐imaging, and 3D displays. In this study, inspired by the fact that various species of plants and animals can efficiently produce and/or perceive CPL with chiral photonic crystal structures, efforts are made to fabricate luminescent chiral photonic crystals with caesium lead halide perovskites, a class of solution‐processable semiconductors that have excellent luminescence properties and readily tunable electronic band gaps. Coupling between the emission frequency of the perovskite and the cavity resonance frequency of the photonic crystal structure can be established at any desired point in the visible region, giving rise to CPL output with a dissymmetry factor up to −0.34. These results suggest a new approach to chiral light‐emitting materials and may contribute to the development of semiconductors with ordered microstructures.


Introduction
Circularly polarized light (CPL), the electric field of which rotates around the direction of propagation, has unique applications in optical communications, [1][2][3] information storage, [4][5][6] displays, [7][8][9] sensing, [10][11][12] bio-imaging, [13][14][15][16][17] and asymmetric synthesis, [18][19][20][21] etc.The efficient production of circularly polarized light with a high degree of polarization is a prerequisite for the development of CPL-related optoelectronics.Traditionally, DOI: 10.1002/admi.202300576circularly polarized light could be generated when unpolarized light passes first through a linear polarizer and then through a quarter-wave retardation plate, but this process is accompanied by the loss of at least 50% of the input energy. [22]PL may also be directly emitted from luminescent materials with molecular or structural chirality, and higher efficiency might be achieved by this approach since the use of polarizers and wave retarders is avoided.Various types of CPL-emitting materials have been developed over the past three decades, such as organic fluorescent molecules with chiral centers, [23][24][25][26] conjugated polymers containing chiral repeating units, [27][28][29] light-emitting transition metal complexes with chiral ligands, [30][31][32] metalorganic frameworks incorporating chiral luminophores, [33][34][35] helical superstructures self-assembled by luminescent amphiphiles, [36][37][38][39][40] and so forth.Many of the reported methods lead to relatively low degrees of circular polarization, i.e., the absolute value of the dissymmetry factor (g lum = 2(I L −I R )/(I L +I R ), where I L and I R represent the intensities of left-and right-handed CPL, respectively) of the emitted light is usually smaller than 0.1. [41]44] Chen et al. observed nearly pure right-handed CPL emission from luminophore-doped cholesteric liquid crystal films, although handedness reversal occurred near the selective reflection wavelength. [45]Xu and coworkers recorded a dissymmetry factor of −0.68 for the right-handed CPL emission from a luminophoreincorporated chiral nematic film of cellulose nanocrystals. [46]hese findings suggest that the exploitation of chiral photonic crystal structures would be a promising approach to the efficient generation of CPL with large dissymmetry factors, while further improvements in quantum yields, brightness, the full width at half-maximum, the adjustability of emission wavelengths, as well as other parameters could be achieved by functionalization of the luminophores.
In this study, chiral photonic crystals with tunable photonic band gaps were fabricated with caesium lead halide perovskites (CsPbX 3 , X = Cl, Br, I, or their mixtures), an emerging class of solution-processable inorganic semiconducting materials that have excellent compositional diversity, readily adjustable energy band gaps, and high quantum yields. [47,48]Combining metal halide perovskites with chiral photonic crystals, either by covering a perovskite film with a cholesteric liquid crystal layer, [49,50] or by dispersing perovskite nanocrystals in a chiral nematic liquid crystalline matrix, [51] has proved to be a practical approach to circularly polarized emission at room temperature.In comparison, CPL emission from hybrid organic-inorganic perovskites containing chiral organic cations may only be observed at low temperatures, and the dissymmetry factors are usually small. [52]evertheless, metal halide perovskites themselves, in the form of a continuous phase instead of discrete nanocrystals or quantum dots, have hardly been endowed with chiral photonic crystal structures.We herein demonstrate the template-assisted fabrication of chiral photonic crystals of caesium lead halide perovskites, where the coupling between emission and cavity resonance frequencies could be achieved at a desired wavelength in the visible light region, thus enabling circularly polarized luminescence with a red, green, or blue color.These results add to the growing literature on the development of CPL-emitting materials, and may also help to design semiconductors with ordered microstructures.

Results and Discussion
Chiral photonic crystals formed by the self-assembly of cellulose nanocrystals (CNCs) in water, the photonic band gaps of which could be readily adjusted in the range from the near-ultraviolet to the near-infrared, [53][54][55] were selected as the source of structural asymmetry for chiral caesium lead halide perovskites.Due to the ionic nature and moisture-sensitivity of metal halide perovskites, the direct combination of perovskite precursors or nanocrystals with aqueous liquid crystalline dispersions of CNCs (i.e., the coassembly or soft-templating approach) may destroy both the perovskite structure and the liquid crystallinity.A straightforward solution is to protect perovskite nanocrystals from water erosion with waterproof polymers, [56] but high dissymmetry factors (e.g., |g lum | > 0.1) have not been achieved by this strategy.Moreover, co-assembly methods would generally lead to discrete perovskite nanocrystals embedded within an insulating matrix, while a continuous perovskite phase, which allows the transport of charge carriers between perovskite semiconductors and electrodes, is more suitable for the manufacture of electronic devices.Inspired by the template-synthesis of free-standing chiral nematic titanium dioxide films, [57] we hypothesized that it would be possible to transfer the chiral photonic crystal structures formed by CNCs to metal halide perovskites if rigid mesoporous templates of silica or polymers are used as intermediates, and in this way we investigated the fabrication as well as the circularly polarized luminescence of caesium lead halide perovskites with structural chirality (Figure 1).
In our experiments, cellulose nanocrystals were prepared by partial hydrolysis of natural cellulose in sulfuric acid (64 wt.% in water, 318-323 K, 60 minutes) according to the method reported by Gray in 1992. [58]After removing excess ions and other watersoluble impurities by ultracentrifugation and dialysis against deionized water, these rod-shaped nanoparticles, with lengths of 131 ± 4 nm, widths of 16 ± 2 nm (Figure S1, Supporting Information), and a zeta-potential of −64.7 mV, were able to form a stable colloidal dispersion in water and further self-organize into chiral nematic (cholesteric) liquid crystalline phases over a threshold concentration, [59] which could be observed as periodic birefringent bands between crossed linear polarizers using polarized optical microscopy (POM) (Figure 2A). [60]Composite films of CNCs and silica were obtained by drying a homogeneous mixture of tetramethoxysilane (0.1 mL) and an aqueous dispersion of cellulose nanocrystals (5.0 mL; 2 wt.%, pH = 2.5) under ambient conditions (Figure S2, Supporting Information).After calcination at 813 K for 5 h in air, cellulose was selectively removed from the composites, leaving behind mesoporous silica films with iridescent colors arising from their internal chiral photonic crystal structures (Figure 2B), [61] which appeared as left-handed helical conformations when examined by cross-sectional scanning electron microscopy (SEM) (Figure 2C).Mesoporosity of these silica films was characterized by nitrogen adsorption-desorption isotherms.In a typical experiment, the as-prepared films showed a BET specific surface area of 222.5 m 2 g −1 and an average pore width of 9.2 nm (Figure 2D), indicating the existence of empty channels that can be further filled with perovskite materials.
[64] In order to investigate the optical effects of the coupling between cavity resonance and emission wavelengths, dispersions of CNCs were probe-sonicated with different energy inputs before mixing with tetramethyl orthosilicate to finally give mesoporous silica films showing blue-, green-, and red-colored iridescence (denoted as Silica_B, Silica_G, and Silica_R) (Figure 3A-C). [65]These silica films, exhibiting fingerprint textures characteristic of chiral nematic liquid crystalline ordering when examined under POM (Figure 3D-F), showed ultraviolet-visible (UV-vis) transmission spectra peaking at 470 nm, 507 nm, and 647 nm (Figure 4A), as well as broadband circular dichroism (CD) signals peaking at 475 nm, 511 nm, and 637 nm (Figure 4B), which closely matched the emission wavelengths of CsPbClBr 2 , CsPbBr 3 , and CsPb 0.83 Zn 0.17 I 3 .Chiral photonic crystals of perovskites were then fabricated by template synthesis.In a representative procedure, a chiral nematic mesoporous silica film was first impregnated to incipient wetness with a dimethyl sulfoxide solution of caesium halide (0.5 mol L −1 ) and lead halide (0.5 mol L −1 ), after annealing at 353 K for 15 min in air, a layer of perovskite nanocrystals was deposited within the empty spaces previously occupied by cellulose nanocrystals in the silica matrix.The above process was repeated five times to ensure sufficient loading of perovskites into the mesoporous silica template, where the total mass of the perovskite/silica composite kept nearly constant after the third loading cycle.According to the results of a series of parallel experiments, after the incorporation of CsPbClBr 2 , CsPbBr 3 , and CsPb 0.83 Zn 0.17 I 3 perovskites, masses of the mesoporous silica templates increased by ≈8.0 ± 0.2%, 11.8 ± 0.4%, and 7.1 ± 0.3%, respectively (Table S1, Supporting Information).
The incorporation of perovskites in silica templates was confirmed by photoluminescence spectroscopy and powder X-ray diffraction (PXRD) analysis.Under ultraviolet irradiation at 365 nm, PVK_R/Silica_R, PVK_G/Silica_G, and PVK_B/Silica_B composites showed intense red, green, and blue luminescence (Figure 5A-C) with emission peaks at 645 nm, 512 nm, and 472 nm, respectively (Figure 4C; Figure S3, Supporting Infor-mation), where the photoluminescence quantum yields (PLQYs) were measured to be about 1.88%, 10.23%, and 2.05%.Photoluminescence decay measurements of PVK_G/Silica_G samples showed an average fluorescence lifetime of ≈2.1 ns (Figure S4, Supporting Information).Besides, to evaluate the brightness of these luminescent materials upon excitation, a PVK_G/Silica_G composite film was subjected to 405-nm ultraviolet irradiation.As the power of the incident ultraviolet light beam gradually increased, a maximum illuminance value of ≈57 lx was measured before the perovskite material was decomposed (Figures S5 and S6, Supporting Information).By comparing the experimentally collected PXRD spectra of the composites with the standard PXRD patterns of CsPbCl 3 , CsPbBr 3 , CsPbI 3 , and SiO 2 (Figure 5D; Figure S7, Supporting Information), the presence of perovskite crystalline lattices within silica matrices could be verified.
Preservation of chiral photonic crystal structures in perovskite/silica composite films was proved by microscopy techniques, where fingerprint textures of chiral nematic liquid crystalline phases were observed under POM (Figure 5E; Figure S8, Supporting Information), and left-handed helical ordering was directly imaged using cross-sectional SEM (Figure 5F,G; Figure S9, Supporting Information).Elemental mapping (performed with energy-dispersive X-ray spectroscopy) of a fresh crosssection of a CsPbBr 3 /silica composite film revealed the existence of lead (M1, 10.3 wt.%) across the thickness of the film (Figure S10, Supporting Information), indicating that the mesopores left by burned-out CNCs had been sufficiently filled with perovskite materials.However, perovskites may also form on the surfaces of silica templates.As revealed by SEM images, pure silica films showed clean and smooth surfaces with a large amount of elongated narrow pores (Figure S11A, Supporting Information).After incorporation of perovskites, the resultant CsPbBr 3 /silica composite films exhibited apparently rougher surfaces with a lot of irregularly-shaped nanoparticles as well as fewer but larger pores (Figure S11B-D, Supporting Information), which were probably caused by the growth of perovskite nanocrystals on the surfaces and inside the mesopores of the silica templates.Energydispersive X-ray spectroscopy further confirmed the existence of lead and bromine atoms on the surfaces of CsPbBr 3 /silica composites (Figure S11E-H, Supporting Information).These results suggest that it might be difficult to completely avoid the formation of excess perovskite materials on the surfaces of the templates outside the chiral nematic matrices.Since these nanoparticles can emit unpolarized light upon excitation, their existence would decrease the dissymmetry factors of the perovskite/silica composite films.
The phase-continuity of perovskites in perovskite/silica composites was further evaluated with electrical resistivity measurements performed using the four-terminal sensing method. [66]To minimize the potential influences of excess perovskite particles possibly existing on the surfaces of samples, the perovskite/silica films were carefully polished with abrasive paper before testing.Electrical resistivity of pure silica films was measured to be ≈59.8± 2 ohm m, while CsPbBr 3 /silica composites showed significantly lower resistivity of ≈13.4 ± 2 ohm m, suggesting that continuous semiconducting phases of CsPbBr 3 would have formed within the insulating mesoporous silica matrices.Although the electroluminescence properties of perovskite/silica composites were not examined in our research, considering the phase-continuity of perovskites in silica matrices, the fabrication of light-emitting diodes with these materials after reducing their thicknesses should be possible.
When unpolarized light (either externally applied or internally generated) passes through a chiral photonic crystal possessing left-handed helical ordering, the propagation of left-handed circularly polarized photons with frequencies inside the photonic band gap would be suppressed, resulting in the release of (partially) right-handed circularly polarized light. [46]In our experiments, the relatively narrow photoluminescence emission peaks of PVK_R/Silica_R, PVK_G/Silica_G, and PVK_B/Silica_B composites, which showed full widths at half-maximum (FWHM) of ≈57 nm, 25 nm, and 36 nm, located almost entirely within the broad photonic band gaps of Silica_R, Silica_G, and Silica_B templates, respectively.As revealed by circularly polarized luminescence spectroscopy, these composites were able to emit right-handed circularly polarized light with high dissymmetry factors of ≈−0.28 (PVK_R/Silica_R), −0.34 (PVK_G/Silica_G), and −0.27 (PVK_B/Silica_B) (Figure 4D-F), indicating the existence of strong interactions between chiral photonic crystal structures and photons produced by perovskites.On the contrary, when there was no overlap between the photonic band gap and the emission spectrum, remarkably lower dissymmetry factors of −0.09 and −0.11 were recorded for PVK_R/Silica_B and PVK_B/Silica_R composite films (Figure S12, Supporting Information), suggesting that in these situations the polarization of emitted light was not effectively affected by the surrounding chiral environments.The above observations agreed with the results reported by Xu et al., [46] and experimentally proved the feasibility of fabricating circularly polarized light sources with chiral photonic crystals of perovskites.Moreover, we also tried to incorporate cesium lead halide perovskites into chiral nematic mesoporous films of cellulose nanocrystals, a type of chiral photonic template that is more flexible than silica matrices. [67]The resultant chiral nematic perovskite/cellulose composite films exhibited circularly polarized photoluminescence with relatively high dissymmetry factors up to −0.6 (Figure S13, Supporting Information).The higher |g lum | values may arise from the more ordered internal structures of cellulose films as these materials were prepared at lower temperatures.However, compared with rigid silica templates, the helical pitches of cellulose photonic films would be changed by the perovskite nanocrystals formed within them, which made it more difficult to match the emission and cavity resonance wavelengths.

Conclusion
In conclusion, a simple template-synthesis method has been employed in this study to endow semiconducting caesium lead halide perovskites with chiral photonic crystal structures, where both the photonic and electronic energy band gaps could be readily adjusted to achieve the coupling between emission and cavity resonance frequencies, therefore enabling circularly polarized light emission at a desired wavelength in the visible region.Although it was difficult to obtain free-standing macroscopic perovskite photonic crystals by selectively removing mesoporous silica templates, the perovskites were able to form continuous phases as determined by electrical resistivity measurements.These results suggest a new approach to semiconducting chiral photonic crystals with tunable photonic and electronic band gaps, and may help to develop anisotropic luminescent materials for the generation and detection of polarized light.

Experimental Section
Preparation of Liquid Crystalline Dispersions of Cellulose Nanocrystals: Small pieces (≈10 mm x 10 mm in size) of qualitative filter paper were mix with a proper amount of water and smashed into a wet pulp in a food blender, and the wet pulp was then thoroughly dried at 323 K in the open air to give paper powder.In a representative hydrolysis experiment, 5.0 g of the dried paper powder was quickly mixed with 100.0 mL of sulfuric acid (64.0 wt.% in water, pre-heated to 318 K) under vigorous stirring, and the reaction mixture was stirred in the temperature range between 318 K and 323 K for 60 min.After completion of the hydrolysis process, the reaction mixture was poured into 500 mL of cold deionized water (≈278 K), and then the diluted reaction mixture was allowed to stand still for 12 h, during which time a pale-brown-to grey-colored cloudy layer would form at the bottom of the container, leaving a brown-colored clear supernatant solution above it.The supernatant was carefully decanted, and the remaining cloudy layer was centrifuged at 8000 rpm for about 15 min to give cellulose nanocrystals as solid precipitates.The precipitates were sealed in dialysis tubing (with a molecular weight cut-off of 14 000 daltons) and dialyzed against deionized water for 5 days to remove excess ions.After dialysis, a stable colloidal dispersion of cellulose nanocrystals in water was obtained.

Preparation of Chiral Nematic Mesoporous Silica Films:
In a typical experiment, 5.0 mL of an aqueous suspension of cellulose nanocrystals (≈2 wt.%, pH = 2.5) was tip-sonicated at 130 watts and 20-25 kilohertz for 2 min using a JY92-IIN ultrasonic homogenizer, which enabled CNCs to form chiral nematic structures by decreasing their aspect ratios.Afterwards, 0.1 mL of tetramethyl orthosilicate was added, and the mixture was bath-sonicated for 15 min to give a homogeneous dispersion, which was then poured into a polystyrene Petri dish (with a diameter of 60 mm).Upon standing in the open air at room temperature, the dispersion slowly dried into a solid film with iridescent colors.This film was first heated at 373 K for 2 h and then calcined at 813 K for 5 h in air, resulting in the formation of mesoporous silica films showing blue-colored iridescence.By changing the duration of tip-sonication to 3 and 4 min, mesoporous silica films with green-and red-colored iridescence could be finally obtained, respectively.
Preparation of Perovskite/Silica Composite Films: A chiral nematic mesoporous silica film was completely immersed in a dimethyl sulfoxide solution of caesium halide (0.5 mol L −1 ) and lead halide (0.5 mol L −1 ) for 15 min until the iridescent colors disappeared.The film was then lifted out of the perovskite solution with fine tweezers, carefully wiped with lens paper to remove excess liquid on its surface, and heated at 353 K for 15 min in air.During this annealing process, perovskite nanocrystals would be formed within the mesopores of the silica template.The above steps were repeated five times to ensure sufficient loading of perovskite materials into the mesoporous silica matrix.Finally, the surface of the perovskite/silica composite film was gently wiped with a cotton swab that had been wetted with dimethyl sulfoxide to remove perovskite nanocrystals formed on the surface of the film.Perovskite/cellulose composite films could be similarly prepared.
Preparation of Chiral Nematic Mesoporous Films of Cellulose Nanocrystals: In a typical procedure, 2.0 g of urea was homogeneously mixed with 10.0 g of formaldehyde (37-40 wt.% in water) by stirring.About 0.05 mL of hydrochloric acid (37 wt.% in water) was then added, and the resulting opaque mixture was heated at 373 K for 30 min until a clear solution of urea-formaldehyde polymer precursors was formed.Afterwards, 0.1 mL of the above urea-formaldehyde polymer precursor solution was added to 5.0 mL of an aqueous suspension of cellulose nanocrystals (2 wt.%, pH = 2.5), after stirring for 10 min at room temperature, the mixture was transferred to polystyrene Petri dishes (with a diameter of 60 mm) and dried into solid films upon standing at ambient conditions.These films were then heated at 393 K for 16 h, during which time urea-formaldehyde polymers were formed between cellulose nanocrystals.By immersing the composite films in a hot aqueous solution of potassium hydroxide (15 wt.%, 343 K) for 16 h, the urea-formaldehyde polymers were removed.The products were washed first with deionized water and then with ethanol, and finally dried from supercritical carbon dioxide to give mesoporous chiral nematic films of cellulose nanocrystals.

Figure 1 .
Figure 1.Schematic diagram illustrating the template-assisted fabrication of chiral photonic crystals of perovskites.

Figure 2 .
Figure 2. A) POM image (taken with crossed polarizers) showing the formation of chiral nematic liquid crystalline microphases in an aqueous dispersion of CNCs.B) Photograph showing the iridescent colors of chiral nematic mesoporous silica films.C) Chiral nematic microstructures in a mesoporous silica film revealed by cross-sectional SEM.D) Nitrogen adsorption/desorption isotherms of mesoporous silica films.Scale bars: 50 μm A), 10 mm B), and 1 μm C).