Nanoimprinted 2D‐Chiral Perovskite Nanocrystal Metasurfaces for Circularly Polarized Photoluminescence

The versatile hybrid perovskite nanocrystals (NCs) are one of the most promising materials for optoelectronics by virtue of their tunable bandgaps and high photoluminescence (PL) quantum yields. However, their inherent crystalline chemical structure limits the chiroptical properties achievable with the material. The production of chiral perovskites has become an active field of research for its promising applications in optics, chemistry, or biology. Typically, chiral halide perovskites are obtained by the incorporation of different chiral moieties in the material. Unfortunately, these chemically modified perovskites have demonstrated moderate values of chiral PL so far. Here, a general and scalable approach is introduced to produce chiral PL from arbitrary nanoemitters assembled into 2D‐chiral metasurfaces. The fabrication via nanoimprinting lithography employs elastomeric molds engraved with chiral motifs covering millimeter areas that are used to pattern two types of unmodified colloidal perovskite NC inks: green‐emissive CsPbBr3 and red‐emissive CsPbBr1I2. The perovskite 2D‐metasurfaces exhibit remarkable PL dissymmetry factors (glum) of 0.16 that can be further improved up to glum of 0.3 by adding a high‐refractive‐index coating on the metasurfaces. This scalable approach to produce chiral photoluminescent thin films paves the way for the seamless production of bright chiral light sources for upcoming optoelectronic applications.

DOI: 10.1002/adma.202210477 encrypted transmission, [8] and spintronics, [9] photoelectric devices, and chiroptical materials. Traditionally, CP light is produced from unpolarized light by a combination of linear polarizers and quarter wave-plates or phase-shifting mirrors. These optical components limit device miniaturization, high-speed operation, and cause severe brightness losses due to undesired light trapping. Said limitations can be overcome with the use of luminescent chiral materials. Unfortunately, chiral light emitters are very scarce in nature and resorting to chemical modification of existing luminescent materials is a challenging task that has resulted in limited fractions of circularly polarized luminescence (CPL) so far. [10] Over the last few years, halide perovskite nanocrystals (NCs) have emerged as one of the most efficient materials for optoelectronics because of a high photoluminescence (PL) quantum yield, tunable emission across the entire visible spectrum, and highly scalable colloidal synthesis. [11][12][13] The defect-tolerant nature of bromide and iodide halide perovskites enables the high PL quantum yield without the need of high bandgap passivation shells on their surfaces. [14,15] The ambitious next step in the development of metal halide perovskites is to endow them with chiral properties. [16] To this end, the synthetic versatility of metal halide perovskites is exploited for the incorporation of chiral organic molecules, either a bulky organic chiral cation inside the perovskite structure, [17] a chiral molecule embedded between the layers of 2D-perovskites, [9,18,19] or a chiral capping ligand in NCs. [20] Despite great efforts, chemical modification has provided limited values of CPL [21] at room temperature with g lum values on the order of 10 −3 which are insufficient for practical applications. [21] The luminescence dissymmetry factor g lum quantifies the fraction of CP light emitted for each (right and left) handedness and it is defined as: where I L-CP and I R-CP are the emitted PL intensity for left circularly polarized (L-CP) and right circularly polarized (R-CP) The versatile hybrid perovskite nanocrystals (NCs) are one of the most promising materials for optoelectronics by virtue of their tunable bandgaps and high photoluminescence (PL) quantum yields. However, their inherent crystalline chemical structure limits the chiroptical properties achievable with the material. The production of chiral perovskites has become an active field of research for its promising applications in optics, chemistry, or biology. Typically, chiral halide perovskites are obtained by the incorporation of different chiral moieties in the material. Unfortunately, these chemically modified perovskites have demonstrated moderate values of chiral PL so far. Here, a general and scalable approach is introduced to produce chiral PL from arbitrary nanoemitters assembled into 2D-chiral metasurfaces. The fabrication via nanoimprinting lithography employs elastomeric molds engraved with chiral motifs covering millimeter areas that are used to pattern two types of unmodified colloidal perovskite NC inks: green-emissive CsPbBr 3 and red-emissive CsPbBr 1 I 2 . The perovskite 2D-metasurfaces exhibit remarkable PL dissymmetry factors (g lum ) of 0.16 that can be further improved up to g lum of 0.3 by adding a high-refractive-index coating on the metasurfaces. This scalable approach to produce chiral photoluminescent thin films paves the way for the seamless production of bright chiral light sources for upcoming optoelectronic applications.

Introduction
Circularly polarized (CP) light is used in a vast number of photonic technologies, including 3D-imaging, [1,2] biosensing, [3,4] bioimaging, [5] photocatalysts for asymmetrical synthesis, [6,7] www.advmat.de www.advancedsciencenews.com light, respectively. It is worth noting that g lum refers to PL and should not be confused with another typical magnitude used in the characterization of chiral materials, the dissymmetry factor (g), which quantifies the amount of each polarization that is absorbed by the material (also known as circular dichroism [CD]). A clear description of each magnitude can be found in Section S1, Supporting Information.
Alternative strategies to chemical or structural modification of the perovskites include the co-assembly of achiral perovskites with a chiral gelator to obtain a chiral arrangement, [22] the injection of spin-polarized charge carriers, [23] or directly encapsulating the materials within a liquid crystals acting as a polarization filter. [24] In addition, it is also possible to engineer the optical response of the perovskites to obtain CP light by coupling them to a photonic architecture that exhibits a chiral behavior. [25][26][27] Similar approaches have been previously applied in semiconductors such as galium nitride nanolasers, [28] InAs quantum dots (QDs) [29,30] as well as the coupling between plasmonic chiral metasurfaces with colloidal CdSe/ZnS QDs [31] to produce high fractions of CPL thanks to specific chiral electromagnetic (EM) modes with high dissymmetry factors. One way to endow chiral properties to an emitter is to place it in the vicinity of a chiral metasurface, where chirality is transferred from the external photonic architecture to the light source via resonant coupling. A different approach free from the introduction of external elements is to directly arrange the emitters into a chiral object capable of sustaining chiral resonances that affect the light produced from the constituting emitters. In the case of in MAPbI 3 , the transference of chirality from a nanostructure engraved in the material by electron beam lithography resulted in a remarkable CD, a differential light absorption for each circular polarization [32] but no chiral PL was reported therein. In the case of perovskite NCs, inorganic silica right (or left) handed nanohelices have been used as chiral templates to induce optically active properties to CsPbBr 3 NCs grafted on their surfaces, reaching maximum g lum values of 6.9 × 10 −3 at 517.5 nm. [33] Resorting to nanophotonic structures is promising alternative to induce a chiral character in any active media, [25,27] however it comes with several setbacks such as the cumbersome lithographic steps (especially if chiral objects are to be designed) or the typically small areas fabricated which undermine their implementation in actual applications.
Nanoimprinting lithography is an exciting alternative for the fabrication of nanostructures with high resolution, while being scalable, low cost, and free of cleanroom restrictions. [34][35][36] Nanoimprinting lithography uses pre-patterned molds to induce the ordering of a colloidal material such as perovskite NCs without recurring to etching processes nor external scaffolds via a template-induced self-assembling process. [37] This soft lithography derived approach has been previously applied to generate periodic arrangements of colloidal gold nanoparticles, [38] PbTe NCs, [39] and colloidal perovskite NCs [40] to control and enhance their optical properties through lattice-related plasmonic/photonic resonances.
In this work, we demonstrate the fabrication of large area CPL-active 2D-chiral perovskite-metasurfaces by template-induced self-assembly of halide perovskite NCs using pre-patterned elastomeric stamps. Two types of non-chiral perovskite inks, green-emissive CsPbBr 3 and red-emissive CsPbBr 1 I 2 NCs are used to obtain color tunable CP light. The single-step fabricated perovskite 2D-chiral metasurfaces exhibit CPL with a maximum g lum of 0.16 at RT within the visible region. These g lum values increase up to 0.31 for both perovskite inks by coating the metasurfaces with a high-refractive-index titanium dioxide (TiO 2 ) layer that increases the light extraction efficiency. In addition, each side of the 2D-chiral metasurface emits light with opposed handedness, a characteristic inherited from the photonic behavior from the metasurface. The CPL collected from each side of the metasurface is analyzed and explained via numerical simulations of the chiral near-fields present in the architectures. Our results underpin templateinduced self-assembly as a versatile and scalable technique that enables control on the optical properties of the nanostructured inks.

Results and Discussion
Two types of perovskite nanocube inks (CsPbBr 3 and CsPbBr 1 I 2 NCs) were synthesized by ligand-assisted tip-ultrasonication and halide exchange ( Figure S2, Supporting Information). The CsPbBr 3 NCs, exhibit green PL with a peak maximum at 519 nm (Figure 1a), while the CsPbBr 1 I 2 NCs emit red PL with a peak maximum at 645 nm ( Figure 1b). The prepared CsPbBr 3 and CsPbBr 1 I 2 NCs are nearly monodisperse with average size of 10.7 and 12 nm, respectively ( Figure S2, Supporting Information). Both colloidal dispersions were purified by centrifugation to obtain NC inks in hexane at a 20 and 80 mg mL −1 concentration for green and red NCs, respectively. The CPL-active chiral nanostructures were fabricated via template-induced self-assembly of halide perovskite NC inks using three different pre-patterned poly(dimethylsiloxane) (PDMS) molds with 500 nm gammadion-like structures (9 mm 2 patterned areas) with left (L), right (R), and racemic (O) orientations in a square array with a periodicity of 600 nm (structure dimensions in Figure 1c). The gammadion geometry was chosen mainly due to the abundance of previous literature, [28][29][30]32,[41][42][43][44][45][46] which facilitates the comparison between the optical properties of the newly produced nanostructures by nanoimprinting lithography and previous results from structures produced by electronic lithography. Second, the C4 symmetry of the array helps minimizing the contribution to the PL from linearly polarized light which results in elliptical polarization. Finally, the gammadion was one of the simplest geometries that could be used for template-induced self-assembly exhibiting a chiral character with a fourfold symmetry (a more detailed explanation of this choice in geometry can be found in Section S3, Supporting Information).
The fabrication process, summarized in Figure 1d, begins with 0.7 µL of the colloidal solution deposited on a cleaned glass coverslip and immediately covered with the pre-patterned stamp. After the solvent is evaporated (5 min), the stamp is removed, leaving the nanostructured perovskite NC film behind. The high quality of the resulting perovskite 2D-chiral metasurfaces with R (clockwise orientation), L (counterclockwise orientation), and O (racemic mixture of R and L gammadions) enantiomers was confirmed through scanning electron microscopy (SEM) imaging (Figure 1e-g and Figure S3, www.advmat.de www.advancedsciencenews.com Supporting Information) in which the constituent perovskite nanocubes can be distinguished ( Figure 1h). The racemic pattern acts as control to ensure that the origin of the optical chiral properties arise from the specific enantiomeric features of the metasurface.
One of the key properties of 2D-chiral metasurfaces is the inversion symmetry. [25,29,30,47] Unlike 3D-chiral objects such as helices, the gammadion shapes are considered to be 2D-chiral, meaning that the handedness of the object changes when inspected from the top or from the bottom. [42,43] This change in chiroptical response is corroborated by the chiral nearfields with opposed handedness appearing at each side of the structure when light impinges from one or the other side as illustrated in Figure 2. To evaluate the intrinsic chirality of the resulting perovskite 2D-chiral metasurfaces and their EM nearfields [48] at each side of the gammadion, we resort to the optical chirality factor C. This magnitude is defined as: where ε 0 is the vacuum dielectric permittivity, ω is the angular frequency of the EM wave, * E  is the conjugated electric field, and B  is the magnetic field. This optical chirality factor is often normalized to the reference value. In this work, the optical chirality factor is normalized to the R-CP light in vacuum as follows (more details in Section S5, Supporting Information): As the normalization is carried out using R-CP, the sign of the magnitude denotes the handedness of the polarization state of light, that is, positive values of C  denote predominant R-CP, whereas negative ones correspond to higher contribution of L-CP light. All the normalized C  values presented herein are evaluated using Equation (3). The values of C  vary from −1 for perfectly L-CP light up to 1 for R-CP light. Near-field values of | | 1  C > are considered to be super-chiral [49,50] and can be found in our structure when directly illuminated with CP light ( Figures S4-S7, Supporting Information). Near-field values of | | 1  C > imply that the EM near-field distribution shows a greater optical chirality than a perfectly circularly polarized plane wave. However, the aim of this work is to understand how the gammadion geometry can transfer its structural chirality to unpolarized light (with original 0  = C ), when a beam impinges from outside or when the light is emitted within the structure, rendering values of  www.advmat.de www.advancedsciencenews.com When unpolarized light propagating in z < 0 direction impinges on a single L-gammadion structure suspended in air placed between z = 0 and z = 120 nm (Figure 2a), the integrated  C factor along the propagation direction of the wave changes its value from negative (L-CP) to positive (R-CP) as it travels through the gammadion (Figure 2b, solid line). The simulated spatial distribution of the near-field in an XY plane for the reflected wave located 10 nm above (z = 130 nm, Figure 2c) or for the transmitted wave below (z = −10 nm, Figure 2d) the L-gammadion shows opposite values of  C factor, illustrating the symmetry inversion in these 2D metasurfaces. The  C factors are reversed when the light impinges from the bottom to z > 0, hence effectively encountering an R-gammadion (Figure 2b, dashed line). For a given propagation direction, the dissymmetry observed in  C at each side of the structure is only due to its higher transmittance coefficient compared to less intense reflected EM fields. The presence of the substrate in our experiments generates a refractive index mismatch between the air (n = 1) and the glass (Figure 2e) therefore breaking the symmetry inversion of the system. [27,42,43] This asymmetry in surrounding media effectively results in a quasi-2D chiral system in which different magnitudes of  C factor are found on each side of the structure, as observed comparing Figure 2a-h.
Next, we considered the situation in which the light is generated within the gammadion, in an attempt to predict qualitatively the effects of the nanostructuration on the PL from the perovskites. We calculated the values of  C at each side of the structure for light produced from a series of randomly oriented out-of-phase dipoles placed within the L-gammadion volume. In the case of an L-gammadion suspended in air (Figure 2i,j), the finite difference time domain (FDTD) simulation shows opposite values of the chirality factor  C on each side of the structure. As a result, an L-gammadion metasurface surrounded by air preferentially emits L-CP light toward the upper side and R-CP toward the bottom side with the same intensity and specificity ( Figure 2j). The presence of a glass substrate affects the 2D-chiral emission character of the gammadion, as presented in Figure 2k,l. [42,43] Opposite handedness is still observed at each side of the L-gammadion, but the refractive index mismatch renders higher  C values on the glass side (Figure 2k,l). This Adv. Mater. 2023, 35, 2210477 Figure 2. Optical chirality C inversion and dissymmetry for both sides of L-gammadion at the emission wavelength (520 nm). a,b,e,f) Schematic illustrations of an L-gammadion transferring optical chirality to unpolarized light and the calculated optical chirality factor along the propagation axis for a system with (first row) (a,b) and without (second row) (e,f) inversion symmetry along the axis. c,d,g,h,) Local optical chirality factor in the symmetric and non-symmetric system at 10 nm distance from the gammadion unit for the transmitted (z = 130 nm) (c,g) and reflected (z = −10 nm) (d,h) light. i,k) Schematic illustration of an L-gammadion emitting both polarizations for a system with (i) and without (k) inversion symmetry along the axis depending on the surface of emission measurement. j,l) Normalized optical chirality factor computed by FDTD simulations for randomly out of phase dipole cloud placed within the L-gammadion volume polarizations for a system with (j) and without (l) inversion symmetry www.advmat.de www.advancedsciencenews.com analysis indicates that a full optical characterization of the CPL from the 2D metasurfaces must consider light emitted from both sides of the structure, where higher luminescence dissymmetry factor g lum is expected for light emitted on the glass side.
We move now to the experimental characterization of the metasurfaces. PL from L-, O-, and R-metasurfaces made from CsPbBr 3 NCs was collected from both the air and the glass sides and the results are presented in Figure 3. A complete Adv. Mater. 2023, 35, 2210477   Figure 3. Chiral PL of green CsPbBr 3 NCs and red CsPbBr 1 I 2 perovskite NC. a-c,e-g) PL spectra measured from air (first row) and from the glass (second row) for green CsPbBr 3 NCs metasurfaces of L-gammadions (a,e), racemic mixture (b,f), and R-gammadions (c,g). d,h) g lum of metasurfaces comprising L-gammadions (blue), R-gammadions (red), and racemic mixture (black) from air (d) and from glass (h). i-k,m-o) PL measurements from air (third row) and from the glass (fourth row) for red CsPbBr 1 I 2 NCs metasurfaces of L-gammadion (i,m), racemic mixture (j,n), and R-gammadion (k,o). l,p) g lum for metasurfaces of the L-gammadion (red), racemic mixture (black), and R-gammadion (blue).
www.advmat.de www.advancedsciencenews.com description of the optical setup used to optically characterize each side of the 2D-chiral metasurfaces along with flat thin films (used as controls) made from both types of perovskite inks can be found in the Experimental Section. As predicted by the simulations in Figure 2, CPL with opposite handedness is preferably generated from each side of the metasurface but with a higher chiral dissymmetry factor value at the glass side. When measured from the air side, L-and R-gammadion metasurfaces displayed L-CP and R-CP preferential emissions with maximum g lum values of 0.06 and −0.04, respectively. Instead, glass side measurements of L-and R-enantiomeric surfaces displayed R-CP and L-CP preferential emissions, respectively, with maximum g lum values of −0.12 and 0.16. As expected, no selective CPL was displayed by the racemic mixtures (Figure 2b,f) nor unpatterned NCs films ( Figure S12a, Supporting Information) indicating that the CPL observed is originated by the chiral metasurface geometry. The values of polarized PL (g lum ) obtained in our metasurfaces represent a remarkable enhancement of two orders of magnitude over most previous works in perovskite only thin films (g lum around 10 −3 ). [21] The experimental and FDTD simulated CD of the metasurfaces ( Figure S9, Supporting Information), reported CD values of ΔT/T = ±0.005 at the emission wavelength of 520 nm for both enantiomeric CsPbBr 3 metasurfaces while no ΔT/T was observed from the racemic sample ( Figure S11, Supporting Information).
Producing circularly polarized PL via chiral shape patterning, opens up novel opportunities to obtain CPL from a variety of nanoemitters. To illustrate the versatility of our fabrication procedure, we fabricated chiral metasurfaces using a different perovskite NC ink with composition CsPbBr 1 I 2 , hence moving to the red part of the visible spectrum. In the last two rows of Figure 3 we present the chiral PL obtained from the metasurfaces made from CsPbBr 1 I 2 NCs inks with a maximum emission at 645 nm ( Figure 1). CsPbBr 1 I 2 NCs gammadion arrays reached maximum g lum values of −0.13 and 0.12 when measured from the glass substrate for the L-and R-enantiomers, respectively (Figure 3m,o,p). Similarly to the green CsPbBr 3 NCs, CPL from the air side was lower that from the glass side, reaching values of g lum = ±0.04 (Figure 3i,k,l). There was no selective CPL displayed by neither racemic mixtures (Figure 3j,n) nor unpatterned NCs films ( Figure S12b, Supporting Information). The versatility presented herein paves the way to fabricate CP-active halide perovskite NCs chiral structures directly with unmodified or intrinsically achiral perovskites NCs, such as the ones proposed by Joseph M. Luther and D. H. Waldeck, [20,51] and can be extended to wide variety of nanometric colloidal emitters.
At this point, we believe that the values of g lum obtained in our metasurfaces are limited by several structural factors such as: feature height (120 nm), geometry of the chiral motifs, and refractive index of the CsPbBr 3 and CsPbBr 1 I 2 NCs (in our simulations comprised between 1.7 and 2, as obtained by ellipsometry and modeled with modified harmonic oscillators, [52] see Section S6, Supporting Information). New designs of chiral geometries supporting stronger chiral resonances or the use of active materials with higher refractive index would definitely increase the optical chirality fields sustained by the metasurface, thus increasing the fraction of CPL produced. To illustrate this later point, we simply coated the previously studied perovskite metasurfaces with a high-refractive-index coating (TiO 2 ). High-refractive-index materials have been widely used to maximize the light-matter interaction in optically active nanostructures, [53][54][55] as it is possible to achieve strong EM resonances avoiding the optical losses presented by their homologue plasmonic counterparts. [56,57] Here, we thermally evaporated 75 nm of TiO 2 , a high-refractive-index material with n = 2.4 and no absorption in the visible range, onto the perovskite metasurfaces. The effects of the TiO 2 coating on the CsPbBr 3 metasurfaces are shown in Figure 4. Higher g lum values are obtained at the peak emission wavelength, going up to 0.31 when measured from the air side of the metasurfaces, reaching also values of 0.1 when inspected from the substrate side ( Figure 4e). Now, the highest values of CPL are obtained from the air side, where the higher refractive index coating helps the light extraction from the metasurface for both perovskite compositions (Figure 4b,d and Figure S14, Supporting Information). The preferential CPL from the air side of the titaniacoated metasurface correlates well with predicted  C by FDTD simulations shown in Figure 4c calculated from a random distribution of dipoles as in Figure 2h. Therefore, the TiO 2 layer renders our metasurface with PL emissions with higher CPL selectivity and g lum factors to the air side. This result serves as an example of how the specific CPL and directionality can be effectively tuned by designing the specific layers composing the 2D-chiral metasurface, paving the way for further values of g lum .
The TiO 2 -coated gammadion metasurfaces show sharper and more pronounced extinction peaks, a dramatic increase in the transmission CD compared to the original nanostructured array and finally, stronger chiral emission. In Figure 5, we summarize the transmission CD and CPL measurements for the titania-coated gammadions fabricated with green CsPbBr 3 (Figure 5a-h) and red CsPbBr 1 I 2 NCs (Figure 5i-p). In this case, both the CD and PL experiments were performed impinging light from the glass side as now emitted light is preferentially out-coupled from the air side with the TiO 2 . Therefore, L-gammadions display preferential L-CP emission and R-gammadions preferential R-CP emission. In both NCs, ΔT/T and g lum values present a strong spectral correlation.
For green CsPbBr 3 NCs, experimental CD values of ΔT/T = 0.07 were obtained at the emission wavelength of λ = 520 nm (Figure 5a-d and Figure S17, Supporting Information), being twelve times higher than the ones displayed by metasurfaces without TiO 2 coating in Figure S11, Supporting Information. In addition, highly preferential CPL was shown for both enantiomers, reaching g lum values of ±0.31 (Figure 5e-h). Similar CD results were obtained with red CsPbBr 1 I 2 NCs, reaching maximum ΔT/T values of 0.042 (Figure 5i-l and Figure S17, Supporting Information). Interestingly, in this case there is a strong inversion in the handedness behavior of the structure due to a lattice resonance supported by the gammadion array (of 600 nm period) which in this case overlaps with the emission band of the CsPbBr 1 I 2 NCs ( Figure S16, Supporting Information). Therefore, the R-gammadion array (Figure 5o) shows both a maximum R-CP preferential emission at 645 nm with g lum values of +0.1 and a maximum of L-CP preferential emission at 680 nm with g lum values of −0.30 (Figure 5m-p). This last result encourages the www.advmat.de www.advancedsciencenews.com use of lattice resonances supported by chiral motif arrays to further improve the optical chirality values of a metasurface. [58] The high g lum values obtained for both types of perovskite NCs illustrate the versatility of our approach to produce CPL from an unmodified achiral emitter. NCs chiral structures can be complemented and enhanced with the seamless addition of high-refractive-index materials on the structure. This leads to the conclusion that with a rational design on the chiral nanostructure, the materials composing it and targeting one specific emission wavelength (more details in Section S9, Supporting Information), one can obtain even higher g lum values with template-induced self-assembly of halide perovskite NCs.

Conclusions
We have demonstrated large area CPL-active halide perovskite NCs chiral structures using template-induced self-assembly. We produced 2D-chiral metasurfaces by nanostructuring two types of non-chiral halide perovskite NCs, CsPbBr 3 and CsPbBr 1 I 2 into gammadion arrays. This low cost and scalable fabrication approach can be seamlessly applied to other kind of emitters, including those with intrinsic chirality, to directly obtain circularly polarized light with relevant g lum values. In this work, the fabricated 2D-chiral metasurfaces rendered the perovskite NCs with CPL selectivities reaching g lum values up to 0.16 at RT. Opposed handedness CPL is observed at each side of the 2D chiral metasurface, which is corroborated numerically by analyzing the optical chirality of the near-fields generated at top and bottom sides of the structure in air or glass supported. Finally, we further improved the chiral emission properties of the metasurface by adding a thin film of a transparent highrefractive-index material atop. This coating rendered maximum g lum factors of 0.31 and demonstrated the ability to design and optimize the preferential chiral response by engineering the layers composing the metasurface. We believe that the work presented herein will contribute to the development of large scale selective CPL sources with the most efficient common light emitters that can be combined with chemical modification to achieve even greater CPL values with great potential for displays and quantum technologies.
Preparation of CsPbBr 3   www.advmat.de www.advancedsciencenews.com indicated the formation of perovskite NCs. The solution was purified by centrifugation (8000 rpm, 10 min) and the sediment was redispersed in 25 mL of hexane. Finally, the solution was centrifuged again (5000 rpm, 8 min) in order to remove the large particles. The supernatant was collected and used for the self-assembly as well as to obtain CsPbBr 1 I 2 NCs by halide exchange using PbI 2 solution.
Preparation of the PbI 2 Solution: 4 mmol of PbI 2 , 4 mL of oleylamine, 4 mL of oleic acid, and 100 mL of hexane were loaded in a 250 mL Pyrex bottle. The solution was heated-up until 60 °C under vigorous stirring until the salt was completely dissolved. The PbI 2 solution in hexane was employed as iodide source for the synthesis of CsPbBr 1 I 2 perovskite NCs.

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Adv. Mater. 2023, 35, 2210477 Synthesis of CsPbBr 1 I 2 Perovskite NCs: CsPbBr 1 I 2 NCs were synthesized by halide exchange reaction. In a typical synthesis, 25 mL of the CsPbBr 3 NCs colloidal dispersion previously synthesized were loaded in a 250 mL Pyrex bottle. The solution was heated-up to 40 °C and then PbI 2 solution was consecutively added under vigorous stirring until the PL tuned to 645 nm. The reaction was monitored by PL observing a red-shift in the PL emission that indicated the halide exchange.
Preparation of the Patterned Stamps: Preparation of the Original Master Structures: The original silicon masters (purchased from CONSCIENCE, Sweden) were constituted of arrays of gammadion shapes with 500 nm width and 120 nm depth, disposed in a squared arrangement forming a lattice of 600 nm pitch in 3 × 3 mm 2 areas. The masters were silanized with an anti-sticking layer of perfluorooctyl-trichlorosilane to prevent the adhesion of the silicones and resistance during replication. The silanization took place through chemical vapor deposition, leaving the masters for 30 min in a desiccator under vacuum together with 4 µL of perfluorooctyl-trichlorosilane. The substrates were rinsed with acetone and heated to 150 °C for 30 min to remove unreacted silane.
Preparation of the Working Masters: Intermediate masters were negative replicas of the original masters, used to obtain a final replica in PDMS of the original hole arrays. These negative hard molds were prepared using UV-nanoimprinting. Specifically, a drop of Ormostamp, a photosensitive resist, was placed directly on top of the silanized silicon master. Then, a cleaned glass slide was gently pressed on top making sure no bubbles remained trapped between the slide and the Ormostamp. The photoresist was then cross-linked and hardened under UV lamp for 5 min. To demold the Ormostamp working master, the substrates were placed on a hot plate at 160 °C. The difference in thermal expansion between photoresist and silicon induced the detachment of the Ormostamp mold. Finally, the working masters were silanized using isopropanol as solvent instead of acetone, to avoid the detachment of the photoresist layer from the glass slide.
Preparation of the PDMS Molds: Hybrid hard/soft PDMS molds were needed to ensure the correct replica of the working masters avoiding structural collapsing during demolding. In particular, they were composite molds where the structured layer (with a thickness of few micrometers) was made of hPDMS, while the backbone (several millimeters) was made of standard soft PDMS. To prepare the hPDMs mixture, 1.7 g of 7-8% vinylmethylsiloxane was mixed with 0.05 g of 1,3,5,7 tetracetylcyclosilane, 9 µL of Pt catalyst, 0.5 g of 25-35% hydroxyl siloxane, and 1.5 g of toluene in this order. All the additions needed to take place under vigorous stirring. After 5 min of stirring, the obtained mixture could be used for ≈20 min, before it solidifies after toluene evaporation. While it was still liquid, the mixture was drop cast onto the master and spread over the surface using an air gun. This step ensures that the mixture covers entirely all the structures and leaves no microbubbles trapped. After repeating this last step two or three times for each working master, the substrates were left 1 h at room temperature to ensure complete evaporation of the toluene, and 1 more hour at 60 °C to cure the hPDMS. Next, the backbone was prepared by standard PDMS protocols. A 10:1 mixture of the monomer and curing agent were mixed vigorously and left degassing for approximately an hour. Then, the mixture was gently poured onto the samples with the cured hPDMS. Finally, the polymer was cured at 100 °C for another hour. Once the PDMS was cured, it was manually demolded from the master.
Preparation of the Colloidal Nanoparticle Assemblies: The various colloidal solutions of halide perovskite NCs were prepared for templateinduced self-assembly by centrifugation. Specifically, the solutions were cleaned from the excess surfactant by centrifuging at 15 000 rpm and concentrated in hexane 20 and 80 mg mL −1 concentration for CsPbBr 3 and CsPbBr 1 I 2 , respectively.
Preparation of the Substrates for the Assembly: Borosilicate microscope coverslips (Menzel,#2) were used as substrates. The substrate preparation protocol comprised cleaning with sonication in acetone, Hellmanex III (3%) solution, isopropanol, and last a hydrophilization with NaOH for 10 min each step, with water rinsing in between.
Template-Induced Assembly: Perovskite NC metasurfaces were prepared by depositing a 0.7 µL drop of halide perovskite NCs on top of a hydrophilized coverslip glass. The drop was then covered with a prepatterned PDMS mold and left for 5 min. After drying, the mold was removed, leaving the perovskite nanostructured film on the coverslip substrate.
High-Refractive-Index Coating: 75 nm of TiO 2 were deposited on top of the structured perovskite gammadions arrays by electron beam deposition (ATC-8E Orion from AJA International Inc. with up to 10 kV HV source).
Optical Characterization: UV-Vis and Photoluminescence Spectroscopy: UV-vis extinction spectra were obtained using a Cary-60 UV-vis spectrophotometer (Agilent). PL spectra were obtained with a Cary Eclipse Fluorescence Spectrophotometer (Agilent). Quartz cuvettes with an optical path length of 1 cm were used for both optical analyses.
Ellipsometry: The optical constants of the perovskites were determined from measurements using a GES5E ellipsometer from SOPRALAB with a spectral range from 1.2 to 5.5 eV. All regression analyses were performed using an in-house code.
Transmittance Circular Dichroism: The optical measurements were carried out in a custom-made optical set up. A white tungsten halogen lamp (Ocean Optics, HL-2000-HP, FL, USA) corrected with two filters in the UV and NIR (Edmund Optics, SCHOTT BG64, and Thorlabs, SRF11) was coupled to protected silver reflective collimator (RC08SMA-P01, Thorlabs) as light source injection. The light collection consisted of another protected silver reflective collimator coupled to a spectrometer (Ocean Optics, QEPro-FL) by an optical fiber. The collimated white light beam was sent through a Glan-Thompson Calcite Polarizer (GTH10M, Thorlabs) mounted on a stepper motor rotation mount (K10CR1/M, Thorlabs). The linearly polarized light obtained was directed to a super achromatic quarter wave-plate (SAQWP05M-700, Thorlabs) mounted at ±π/4 compared to the polarization direction on a rotation mount (ELL14, Thorlabs) to obtain a circularly polarized light beam. All the optical elements were controlled automatically by custom software (LabView NXG) to ensure the reproducibility of the measurements. The illumination area was controlled by a pinhole (SM1D12). Two dualposition sliders (ELL6, Thorlabs) were used to place a shutter and the sample in the beam path. The first one was used to measure the dark current of the spectrometer, whereas the second one was used to place the sample in the light beam. The sample was placed between a pair of 4× objectives (NA = 0.1).
Chiral Emission Characterization: Two laser sources (NPL41B, Thorlabs and Crylas FDSS 532-150) with peak emission at 405 and 532 nm were used to excite CsPbBr 3 and CsPbBr 1 I 2 NCs, respectively. The PL obtained from the samples was collected through a 4× objective with 0.1 NA and collimated to a super achromatic quarter-wave plate and a Glan-Thomson polarizer. By means of a trigger controlled externally by an Arduino board (Arduino Uno), a fixed number of laser pulses were sent onto the sample to ensure both polarizations were given the same amount of energy in the excitation process. The laser signals were optically filtered using longpass band filters (FELH0450, Thorlabs for 405 nm laser, BLP01-532R-25, Semrock for the 532 nm laser line).
FDTD Simulations: General Considerations: FDTD simulations were performed using a commercial software (Lumerical Inc. by Ansys). The simulation reproduced a single 120 nm height and 500 nm-long gammadion-like structure with 100 nm arms (width) composed of a material whose optical parameters were obtained from ellipsometry measurements ( Figure S8, Supporting Information) for each type of perovskite NCs. Perfectly matched layers (PMLs) were set in all x, y, and z directions. In the case of the z-direction, the PML was placed at a z that was at least half of the maximum wavelength ensure the absorption of the light source in the propagation axis avoiding back reflections and interference in the simulation region. The same PML boundary conditions were used in the x and y directions for unpolarized excitation and dipole clouds simulations.
Transmittance Characterization: Two orthogonal plane wave sources with a spectral range in the visible-NIR in the x and y direction with a phase difference of ±π/2 were used as R-CP and L-CP input light. A pair of 2D z-normal power monitors were placed to measure the transmittance and reflectance spectra for both polarizations. In the x www.advmat.de www.advancedsciencenews.com Adv. Mater. 2023, 35,2210477 and y directions, periodic boundary conditions were used to simulate the 2D-chiral photonic metasurface.
Unpolarized Excitation: A pair of simulations with linearly polarized light at 45° and 135° to ensure the polarization does not follow any symmetry direction of the gammadion were done. These responses were added incoherently to simulate the optical response of the structure for unpolarized light. The source used was centered at 520 nm with FWHM of 40 nm, keeping the spectral region of interest for CsPbBr 3 NCs. A 3D-volume field profile monitor was used to record the complex EM fields to compute the optical chirality parameter C  within the region of interest −500 nm < z < 500 nm at the emission frequency of 520 nm. The optical chirality was integrated along the propagation axis in the XY plane in longitudinal steps of 5 nm. The optical chirality parameter was normalized to a R-CP wave in vacuum using Equation (3).
Dipolar Emission: A hundred dipoles emitting at 520 nm with a FWHM of 40 nm were placed all over a single gammadion volume with random orientations and phase to ensure the incoherent interaction between the sources. The PML boundary conditions in the x and y directions ensured that there was no interaction between a dipole and its mirror images outside the unit cell, giving the results of a single gammadion emitter. The same 3D volume field profile monitor used for unpolarized excitation was used here again to compute the optical chirality along the propagation axis. The absolute value was re-normalized, as the dipole cloud did not directly relate to a plane wave source and Equation (3) did not have a direct translation to this system.

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