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Tunable Hybrid Fano Resonances in Halide Perovskite Nanoparticles Ekaterina Tiguntseva, Denis G. Baranov, Anatoly Pushkarev, Battulga Munkhbat, Filipp E Komissarenko, Marius Franckevicius, Anvar A. Zakhidov, Timur Shegai, Yuri S. Kivshar, and Sergey V. Makarov Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01912 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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Tunable Hybrid Fano Resonances in Halide Perovskite Nanoparticles Ekaterina Y. Tiguntseva,† Denis G. Baranov,∗,‡,† Anatoly P. Pushkarev,† Battulga Munkhbat,‡ Filipp Komissarenko,† Marius Franckeviˇcius,¶ Anvar A. Zakhidov,§,† Timur Shegai,‡ Yuri S. Kivshar,k,† and Sergey V. Makarov∗,† †ITMO University, St. Petersburg 197101, Russia ‡Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden ¶Center for Physical Sciences and Technology, LT-10257 Vilnius, Lithuania §University of Texas at Dallas, Richardson TX 75080, USA kNonlinear Physics Centre, Australian National University, Canberra ACT 2601, Australia E-mail: [email protected]; [email protected]

Abstract Halide perovskites are known to support excitons at room temperatures with high quantum yield of luminescence that make them attractive for all-dielectric resonant nanophotonics and meta-optics. Here we report the observation of broadly tunable Fano resonances in halide perovskite nanoparticles originating from the coupling of excitons to the Mie resonances excited in the nanoparticles. Signatures of the photonexciton (’hybrid’) Fano resonances are observed in dark-field spectra of isolated nanoparticles, and also in the extinction spectra of aperiodic lattices of such nanoparticles. In the latter case, chemical tunability of the exciton resonance allows reversible tuning of the Fano resonance across the 100 nm bandwidth in the visible frequency range, providing a novel approach to control optical properties of perovskite nanostructures. The

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proposed method of chemical tuning paves the way to an efficient control of emission properties of on-chip-integrated light-emitting nanoantennas.

Keywords Fano resonance, halide perovskites, excitons, nanophotonics, Mie resonances Recently emerged novel platform for the study of light-matter interaction is associated with the physics of halide perovskites, which are compounds of the form ABX3 , where A stands for organic (e.g., CH3 NH3 =MA) or inorganic (e.g., Cs) cations, B is usually lead (Pb), and X is replaced by I, Br, or Cl. 1 Halide perovskites represent a class of intriguing dielectric materials with pronounced exciton resonances at room temperature, and high quantum yield. 2 Emission wavelengths of hybrid perovskites can be varied gradually over the entire visible spectrum (400–800 nm) by replacing or mixing the anion compounds (I, Br, or Cl) during the material synthesis process. These advantages, combined with lowcost fabrication methods, allow for creating various perovskite-based active nanophotonic structures and metadevices, 3 such as metasurfaces, 4,5 two-dimensional photonic crystals, 6,7 nanoantennas, 8 nanowaveguides and Fabry-Perot resonators, 9 as well as whispering-gallerymode resonators. 10 Coupling of an exciton to a resonant nanostructure may significantly alter the optical response of the entire system. When a narrow exciton resonance of any material couples to a broader cavity resonant mode, one can expect interference effects manifested in the characteristic Fano lineshape in the spectrum. 11–14 Such a hybrid Fano resonance is a ubiquitous phenomenon encountered whenever a single resonance of a system interferes with either a non-resonant background or another broader mode. 15 Coherent coupling of an exciton with a cavity mode has been observed in various configurations, involving organic molecules, 11,16–19 semiconductors, 20 carbon nanotubes, 21 and transition metal dichalcogenides. 22,23 However, the Fano regime in an optically resonant perovskite nanostructure, which would enable novel

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opportunities owing to unique advantages of this class of materials, has not been explored. Recent observation of perovskite nanoantennas exhibiting enhanced photoluminescence due to intrinsic Mie resonances 8 suggests that one can expect interference between exciton and Mie resonance even in an isolated perovskite nanoparticle, as sketched in Fig. 1(a). Here, we report on the first experimental observation of hybrid Fano resonances in isolated perovskite nanoparticles and two-dimensional aperiodic structures of such nanoparticles. The Fano resonances originate from a coupling of perovskite excitons to the geometry-driven Mie resonances, and they are observed in the dark-field scattering and extinction spectra. The unique material properties of the perovskite nanoparticles allow to tune chemically their excitonic states over 100 nm frequency range in a reversible manner. Our findings may have far reaching implications for a design of perovskite-based optoelectronic devices with optimized optical characteristics as well as on-chip integrated reconfigurable light-emitting nanoantennas.

Results and Discussion Fano resonance in single perovskite nanoparticles. First, we inspect light scattering by a single spherical perovskite nanoparticle in free space within the framework of Mie theory. 25 Due to moderately high background refractive index of lead tribromide perovskites (e.g., MAPbBr3 or CsPbBr3 ), such nanoparticles exhibit prominent Mie resonances. 8 Furthermore, since they possess pronounced exciton state with binding energy more than 70 meV 26 (Fig. 1(b)), it is expected that coherent coupling of Mie modes of the particle to the excitons of perovskite 27 will result in a pronounced Fano resonance. To confirm this, we adopted the permittivity of bulk MAPbBr3 from Ref. 24 and calculated spectra of scattering efficiency, Qscat = σscat /πr2 of spherical nanoparticles versus particle radius r. The results shown in Fig. 1(c) indeed exhibit an asymmetric behavior inherent for Fano resonance close to the exciton resonance of perovskite at 520 nm correlated with the absorption peak. At

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Figure 1: Coupling of the exciton and Mie resonances in a perovskite nanoparticle. (a) Schematic illustration of the coupling between the Mie resonance of a perovskite nanoparticle and the exciton resonance of MAPbBr3 material. (b) Real and imaginary parts of MAPbBr3 refractive index at room temperature adopted from Ref. 24. (c) Analytically calculated scattering efficiency Qscat of a spherical MAPbBr3 nanoparticle in free space. Overlaid lines depict approximate dispersion of the electric dipole, magnetic dipole, and magnetic quadrupole resonances of the particle. The arrow indicates position of MAPbBr3 exciton. (d) Analytically calculated backward scattering for the same system. Overlaid lines depict approximate dispersion of the first and second Kerker conditions, when intensity of back scattered light is minimal or maximal, respectively.

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shorter wavelengths, the scattering efficiency is always suppressed: Mie resonances become overdamped due to higher optical losses in perovskite above the bandgap energy. Besides the dip in the scattering efficiency, the system exhibits another kind of asymmetric dip in the backward scattering intensity, as depicted in Fig. 1(d). This dip corresponds to the first Kerker condition, 28 manifested as destructive interference between the electric and magnetic dipoles in the backward direction, and it has a linear-like dispersion, as shown in Fig. 1(d), in contrast to the Mie-exciton dip whose spectral position does not change with nanoparticle radius. In order to demonstrate experimentally the hybrid Mie-exciton Fano resonances in individual nanostructures, we fabricate MAPbBr3 nanoparticles by laser ablation 29 of a perovskite thin film (for details, see Methods). The resulting nanoparticles have spheroidal shape and were dispersed with particle-to-particle distances of several microns, allowing to measure the optical properties from individual particles (see Fig. 2(b)). The particles sizes range mostly from 50 nm up to 400 nm, as revealed by scanning electron microscopy (SEM), as shown in Fig. 2(d). Our optical measurements (see Methods for details) showed that the spectral position of the photoluminescence (PL) peak from each nanoparticle does not depend on the particle size, is centered at 539 nm, and has a line width of around 20 nm, as shown in Fig. 2(a). In Fig. 2(d), we present several experimental optical dark-field (DF) spectra collected from single MAPbBr3 nanoparticles of different radius placed on a glass substrate as shown in Fig. 2(c) (for details, see Methods). All spectra exhibit a clear dip at the exciton resonance wavelength, which is the result of coherent coupling between the exciton and Mie resonances of the spherical particles. To fit the spectra, we performed numerical simulations of light scattering by a spherical MAPbBr3 nanoparticle on a glass substrate in COMSOL (for details, see Methods). Fitting the DF spectra with the numerical results along with the corresponding SEM images allow us to estimate the radii of the nanoparticles. Slight discrepancy between experimental and numerically calculated spectra can be attributed to nonsphericity of the

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Figure 2: Coherent Mie-exciton coupling in single MAPbBr3 nanoparticles. (a) PL spectra of a single MAPbBr3 nanoparticle. (b) The dark-field optical image (left) and optical image of a single MAPbBr3 nanoparticle emitting light under cw UV excitation (right). Scale bars in the photos are 1 µm. (c) Scheme of the dark-field optical spectroscopy of single nanoparticles. (d) Measured dark-field spectra of MAPbBr3 nanoparticles (solid) fitted with the simulated spectra (dashed). Inset: SEM images of MAPbBr3 nanoparticles and their diameters. Scale bar in the SEM images is 200 nm.

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particles, as shown in Fig. 2(d). Nevertheless, despite the size and shape variations in our experiments, the Fano dip always appears at the exciton wavelength for the particles with diameters at least up to D ≈180 nm. Tunable Fano resonances in nanoparticle structures. Although our measurements on single nanoparticles essentially reveal Fano resonance features, achieving such regime in large clusters of perovskite nanoparticles is much more attractive from the perspective of optoelectronic 30 and sensing applications. 31 In addition, a possibility to tune the spectral position of this resonance would be highly prospective for matching the Fano resonance with various molecular transitions and optical resonances of other nanostructures. In order to study the feasibility of the Fano resonance in macroscopic lattices of perovskite nanoparticles and ways of its tunability, we fabricate a layer of randomly distributed perovskite nanoparticles, see Fig. 3(a), with the same composition, where the organic cation MA is replaced by an inorganic one (cesium, Cs), with similar excitonic, optical, and electronic properties. 32 This composition (CsPbBr3 ) is more stable to environmental conditions and to light-induced ions movement (segregation effect). 33 On the other hand, MAPbBr3 in more preferable for the laser ablation to create well-separated and large enough nanoparticles supporting Mie resonances, but this material is not suitable for chemical tuning by the anion exchange (see Secs. S5,S6 of Supporting information). To produce aperiodic lattices of 80–600 nm halide perovskite nanoparticles (Fig. 3(a,b)), the strategy of dispersion of the material in the polymer matrix (e.g. polyethylene oxide (PEO), polyvinylpyrrolidone (PVP) or poly(9-vinylcarbazole) (PVK)) is adopted (for details, see Methods). Mixing 70 000 Mw PEO in DMF (10 mg/ml) and CsPbBr3 in DMSO (137 mg/ml) solutions in 1:3 mass ratio affords the formation of a thin film containing nanoparticles covered by a polymer layer (less than 50 nm, Ref. 30) after its spin-casting and annealing at 110o C for 3 min. The choice of PEO is dictated by its ability to act as a solid electrolyte facilitating the ionic transport and subsequent charge carriers injection inside the luminescent layer in efficient pero-LEDs. 30,34

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Figure 3: Chemical doping of perovskite nanoparticles. (a) SEM image with a tilted (45 degrees) view and (b) size distribution of chemically synthesized perovskite nanoparticles (bars) with normal fit of this distribution (curve). (c) Schematic illustration of extinction measurements: a cluster of perovskite nanoparticles of various radii placed on a glass substrate is normally illuminated from top. Bottom: illustration of chemical doping of perovskite particles. Br atoms in the atomic lattice of CsPbBr3 are being replaced with Cl under exposure to dense vapor of hydrochloric acid. (d,e) Real and imaginary parts of refractive indices of smooth CsPbBr3 films exposed to HCl vapor for various durations measured via ellipsometry. The arrows indicate the direction of tuning.

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Tuning spectral position of the exciton is usually achieved via replacement of bromine (Br) atoms with chlorine (Cl) atoms in the atomic lattice of CsPbBr3 during material synthesis. However, this process is complicated by a very poor solubility of CsCl in organic solvents. Therefore, we exploit a novel approach of chemical vapor anion exchange (CVAE) to form mixed halide perovskites, which is schematically shown in Fig. 3(c). Complex refractive indices of the perovskite films before and after CVAE process measured by ellipsometry are presented in Fig. 3(d,e), revealing dramatic tuning of the exciton resonance position along with significant modification of the refractive index in the entire spectral range. The intermediate phases of the aperiodic lattices of perovskite nanoparticles are obtained via exposing CsPbBr3 samples to HCl vapor for 7, 18, 60, 152, and 300 s at 120o C (for details, see Methods), yielding CsPbBr3−x Clx (0 < x ≤ 3) nanoparticles that exhibit PL peaks at 503, 485, 466, 445, and 423 nm, respectively, see Fig. 4(a). To evaluate the reliability of the obtained CsPbBr3−x Clx (0 ≤ x ≤ 3) materials for the long term photonic applications, we implement stability tests for continuous films. The samples are aged in the ambient air under exposure to 80 mW cm−2 halogen lamp light. The films stability is monitored by measuring the value of peak optical absorption at the exciton wavelength, which is changing over time. Figure 4(b) shows a very slow decrease of this value by several percents upon aging the perovskite films during 100 hours. Since higher chlorine content in the perovskite structure increases the material band gap, the nanoparticles films with more chlorine content absorbed less incident light from the halogen lamp and demonstrated reduced photobleaching rates in comparison with that of PEO:CsPbBr3 film. Experimentally measured extinction spectra (−logT) of the aperiodic lattices for different doping concentrations shown in Fig. 4(c) exhibit pronounced asymmetric dips near the exciton wavelengths, indicating that Fano resonance survives for extremely broad size distribution of nanoparticles. Expectedly, this feature of the extinction spectrum experiences shift from 540 nm down to 420 nm upon increasing exposure of perovskite nanoparticles to HCl. Since the PL emission wavelength (λP L ) monotonically changes with concentration

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Figure 4: Chemical tuning of the Fano resonance in an aperiodic lattice of nanoparticles. (a) Experimentally measured spectral positions of PL peak (blue squares), Fano resonance (red triangles), and their bi-exponential fitting for a two-dimensional structure of perovskite nanoparticles chemically doped different time in HCl vapor. (b) Stability of CsPbBr3−x Clx thin films: peak absorption by the nanoparticles structures vs. time under exposure to a halogen light source. (c) Measured extinction spectra of perovskite nanoparticles with chemically tuned anion. Lorentzian-like curves around each spectrum show the corresponding measured PL spectrum for each doping concentration. (d) Theoretically calculated extinction spectra of a cluster of chemically tuned perovskite nanoparticles with the size distribution from Fig. 3(b). The curves are shifted vertically for clarity.

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of chlorine in the crystal lattice, the kinetics of the doping process is well described by a bi-exponential law: λP L (t) = α1 exp(−t/τ1 ) + α2 exp(−t/τ2 ) + β. We believe the fast exponent (τ1 =10.94 s) specifies the chlorine doping of the initial tetragonal phase of CsPbBr3 that is formed in the 88–130o C temperature range. 35 In this case, the bromine substitution occurs easily because of the large lattice constants (a = b = 8.259˚ A, c = 5.897˚ A) and relatively low lattice energy. On the contrary, slow exponent (τ2 =268.61 s) describes the further chemical modification of the cubic phase of formed CsPbBr3−x Clx material possessing more dense lattice (a = b = c < 5.874˚ A) with energy sufficient to resist its structural transformation. The other fitting parameters are the following: α1 = 41.39 nm, α2 = 89.86 nm, β = 394.2 nm. The position of the Fano dip in the spectra, depending on duration of exposure to HCl vapor, can be also described by the bi-exponential behavior with two exponents having short τ1 =9.82 s and long τ2 =215.34 s time constants. The other fitting parameters for the Fano dip are the following: α1 =39.87 nm, α2 =82.72 nm, β=393.7 nm. In order to understand the origin of the Fano resonance in these spectra, we calculate extinction by a lattice of perovskite nanoparticles with the use of an analytical model. Extinction of light propagating through a layer of particles with volume density N is caused by absorption and diffuse scattering by the particles. The extinction coefficient of the medium therefore is given by κ = 4πN σext with σext being the extinction cross section of the particles calculated with the Mie theory 25 and the permittivities obtained from the ellipsometry measurements shown in Fig. 4(d,e). Since reflection from a polymer film is negligible, transmission through the film may be evaluated as T = e−κL with L being the film thickness. To account for variation of the particles sizes, we calculated the averaged extinction cross R R section σ ¯ = f (R) σext dR f (R) dR with f (R) being the statistical distribution of the nanoparticles radii. According to the observed distribution of the nanoparticles sizes, see Fig. 3(b), we assume the normal distribution of radii with the mean value of 160 nm and standard deviation of 60 nm.

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The resulting calculated extinction spectra (−logT) in Fig. 4(d) show reasonable agreement with the experimental data in Fig.4(c), reproducing spectral positions of the Fano dip. Discrepancy in the overall shape of the spectra may originate from the presence of the glass substrate and non-sphericity of the particles, which shift Mie resonances. Nevertheless, the modeling reveals the crucial contribution of the scattering part to the total extinction spectra, resulting in appearance of a dip in extinction at the spectral position of the material absorbance maximum (see comparison of absorption and scattering efficiencies of single nanoparticles in Supporting information). The absorption maximum corresponding to the exciton state gets ”hidden” into the Fano feature of the extinction spectrum. This result is important for correct interpretation of experimentally measured transmittance spectra in standard commercial spectrophotometers from very non-homogeneous perovskite films.

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Figure 5: Reversible chemical tuning of the Fano resonances. (a) Measured extinction spectra of a lattice of perovskite nanoparticles with different compositions tuned via the chemical vapor anion exchange method. Arrows indicate the tuning process in forward direction by exposing the initial CsPbBr3 compound to HCl acid vapor for 300 seconds, and in backward direction by exposing the resulting material to HBr vapor for the same amount of time. (b) Similar extinction spectra for smaller exposure intervals, resulting in intermediate compounds. The presented technique of perovskite nanoparticles tuning via chemical doping is com12


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pletely reversible. Figure 5 demonstrates that after exposing clusters of CsPbBr3 nanoparticles to HCl vapor for 300 s (for details, see Methods), and then for the same time to HBr vapor, extinction spectrum returns exactly to its original state. This approach potentially allows for multiple and precise tuning of resonant properties of a perovskite nanoparticle, which might be integrated into some specific photonic circuit or nanophotonic design. Such a direct in-place doping and broad spectral reconfiguration of light-emitting nanoantennas cannot be achieved easily with the conventional semiconductor technologies, making tunable halide perovskite indispensable in such kind of applications. Finally, it is interesting to discuss the feasibility of reaching the strong light-matter coupling regime between excitons and Mie resonances in a single perovskite nanoparticle. With an increase of the coupling strength, coherent coupling of two resonances may switch to the strong coupling regime, characterized by the presence of mixed exciton-polariton modes in the system spectrum separated by the vacuum Rabi splitting. 36–38 Exciton-polaritons have recently been observed in micrometer-scale perovskite nanowires; 39,40 however, Rabi splitting was only detected in the dispersion of the waveguide mode, as opposed to splitting of individual resonant modes. In our case, the strength of the coupling may be hindered by the insufficient background refractive index of perovskites (≈ 2), which lowers the Q−factor and increases the mode volume of the Mie modes. Potentially, alternative geometries, such as a particle on a mirror, an oligomer, or a chain of nanoparticles, can provide a more viable platform for realization of the strong coupling regime with a nanophotonic perovskite structure.

Conclusion We have observed a novel type of Fano resonance in isolated Mie-resonant halide perovskite nanoparticles and also in disordered lattices of such nanoparticles. The Fano resonance has been recorded as an asymmetric dip in the scattering spectra being originated from destruc-

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tive interference between the exciton resonance and the Mie dipole mode of a nanoparticle. Furthermore, the Fano resonances in disordered structures of CsPb(Br1−x Clx )3 nanoparticles have been tuned controllably over 100 nm in the visible frequency range by the chemicalvapor-anion exchange. The proposed method of reversible tunability of the Fano resonances can be useful for perovskite-based light-emitting nanoantennas integrated into a photonic circuitry. Also, tunable Fano resonances can be employed for coloring or light outcoupling in perovskite-based optoelectronic devices, especially for multilayer and multicolor architectures for white-light emitting metadevices.

Methods Fabrication of individual MAPbBr3 nanoparticles by laser ablation. For fabrication of nanoparticles we use the laser printing method. 29 In this approach, nanoparticles are fabricated from a perovskite thin film in the forward-transfer geometry, when the receiving substrate is placed under the film with a spacing of 50 µm. A solution of perovskite precursor (MAPbBr3 ) is prepared in a N2 -filled dry-box as follows: methylammonium bromide (MABr) in γbutyrolactone with dimethyl sulfoxide (GBL:DMSO) at the concentration of 1.5 M is used to dissolve 1.5 M of lead bromine (PbBr2 ) in DMF. The solution is stirred and heated (70◦ C) overnight and used after filtration through 0.45 µm PTFE syringe filter. A perovskite layer deposition is performed by a solvent-engineering technique inside the N2 -filled dry-box, including two step spin-coating process. 41 At the first step, a solution precursor MAPbBr3 is deposited at rotation speed 1000 rpm. The second step is the dripping 200 µl of the toluene at 3000 rpm, which does not dissolve perovskite during the film formation. Each film is annealed at 100◦ C for 10 min. 41 Glass substrates are washed by sonication in deionized water, toluene, acetone, and isopropanol, consequently. Despite low cost and simplicity, this method allows for the formation of high-quality films emitting light with high quantum yield (up to 70 %). 42 The perovskite nanoparticles are fabricated by employing Yb+3 femtosecond

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laser pulses at λ = 1050 nm with energy around 50 nJ focused by 10× objective (NA=0.25). According to our data, size of the nanoparticles was in the range of 50–500 nm, their shape is quasi-spherical with some facets, whereas their position on a receiving substrate is random (for details, see Supporting information and Ref. 8). Optical properties of individual nanoparticles. We study optical resonances of the fabricated perovskite nanoparticles deposited on a silica glass substrate, by using confocal darkfield optical spectroscopy. The nanoparticles are excited at an oblique angle (65 degrees with respect to the normal of the surface) by linearly polarized light from a halogen lamp (HL-2000-FHSA) through a weakly-focusing objective (Mitutoyo M Plan Apo NIR, 10×, NA = 0.28). Scattered light is collected from the top by an 50× objective (Mitutoyo M Plan APO NIR, NA = 0.42), sent to Horiba LabRam HR spectrometer and projected onto a thermoelectrically cooled charge-coupled device (CCD, Andor DU 420A-OE 325) with a 150-g/mm diffraction grating. Fabrication of clusters of CsPbBr3 perovskite nanoparticles. Lead(II) bromide (99.999%), cesium bromide (99.999%) and N,N-dimethylformamide (DMF, anhydrous, 99.8%) are purchased from Sigma-Aldrich. Dimethyl sulfoxide (DMSO, anhydrous, 99.8%) and poly(ethylene oxide) (average Mw 70 000) are purchased from Alfa Aesar and Agilent, respectively. All materials are used as received. Perovskite solution is prepared by dissolving PbBr2 (73.4 mg, 0.2 mmol) and CsBr (63.6 mg, 0.3 mmol) in 2 ml of DMSO. An excess of cesium bromide is taken to fully convert lead bromide into the perovskite. The solution is stirred for 1 h without heating. PEO (30 mg) is dissolved in 3 ml of DMF by stirring overnight at 70 o C. 170 mg of the perovskite solution is mixed with 510 mg of the PEO solution to give perovskite-polymer ink. The ink is stirred at 120 o C for 30 min and then filtered by using 0.45 µm syringe filter with PTFE membrane. Fused silica substrates are cleaned subsequently with detergent water, acetone, and isopropanol for 5 min with sonication, then treated with oxygen plasma at 100 W power for 3 min. The organic-inorganic composite films are spin-casted onto the

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substrates at 1500 rpm for 1 min and further annealed at 110 o C for 3 min on a hotplate. Solution preparation and film deposition are carried out inside N2 filled glovebox with both O2 and H2 O level not exceeding 1 ppm. Chemical vapor anion exchange method. Doping of the CsPbBr3 :PEO films by chlorine is conducted outside the glovebox at ≈30% humidity. The sample is placed in a glass Petri dish bottom and preheated on the hotplate at 120 o C for 2 min before the treatment. A glass low form weighing bottle (40×25 mm, 25 ml) is preheated in an oven at 120 o C as well. Then 3 mg of HCl azeotrope (37%, Vecton) onto a glass substrate is put near the sample, and all the system is encapsulated by the hot glass weighing bottle. After specific lapse of time the system is dismantled and the sample is heated up to 200o C at 40o C deg/min rate to eliminate possible structural defects of the crystal lattice. To confirm the reversibility of the anion exchange in the perovskite crystal lattice CsPbCl3 :PEO film obtained is treated in a similar manner by 3 mg of HBr azeotrope (40%, Vecton). All the samples are cooled down to room temperature before the measurements. Optical degradation measurements. Optical degradation studies of perovskite films were undertaken while measuring integrated absorption of the samples at different times with AvaSpec-HS1024x58/122 fiber optic spectrometer (Avantes). For photodegradation studies, samples were continuously illuminated with halogen lamp which was collimated and focused on sample producing power density of about 80 mW cm−2 . Ellipsometry. Ellipsometry measurements were carried out in air using J.A. Woollam M2000 ellipsometer over the wavelength range of 245–1000 nm with a step of 1 nm. Incidence angles from 65o to 75o relative to the surface normal were scanned with a step of 5o for reflection ellipsometry. The analysis of ellipsometric data was performed using the software WVASE-32 (J.A. Woolam Co.). Thicknesses of perovskite films on silicon substrates were determined by P-7 Stylus Profiler (KLA Tencor). Using the obtained perovskite film thickness, the optical constants (refractive index n and extinction coefficient k) were extracted from the ellipsometric data using the point-by-point method.

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Shape and size characterization. The fabricated nanoparticles are visualized and inspected using a field-emission scanning electron microscopy (SEM, JEOL, Cross Beam 1540 XB). Optical characterization of lattices of perovskite nanoparticles. All samples with nanoparticles clusters are visualized in an optical microscope (Axio Imager.A2m, Carl Zeiss) with different objectives (×10, ×20, ×50) under illumination by a halogen light source (HAL 300) in reflection mode and by 365-nm line of a mercury lamp (HBO 100) in a luminescent mode (see images in Supporting information). Extinction spectra are measured in a UV-VIS-NIR spectrophotometer (Shimadzu UV-2600). Numerical simulations of dark field scattering. Simulations are performed with the use of commercial software COMSOL. Permittivity of bulk MAPbBr3 is adapted from Ref. 24. Particles are placed on a glass substrate and illuminated by an s-polarized total field/scattered field source incident at an angle of 65 degrees. Scattered light is collected by a power monitor with NA≈ 0.4 corresponding to the objective used in the experiment.

Authors information Corresponding Authors *E-mail (D.G.B.): [email protected]; *E-mail (S.V.M.): [email protected].

Notes The authors declare no competing financial interests.

Supporting Information Available Information on size distribution of nanoparticles, multipole decomposition of electromagnetic field for spherical nanoparticles, scattering and absorption efficiencies, information on 17


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chemical tuning of MAPbBr3 nanoparticles, laser fabrication of CsPbBr3 nanoparticles, and morphology characterization of MAPbBr3 and CsPbBr3 films used for laser ablation.

Acknowledgement This work was supported by the Ministry of Education and Science of the Russian Federation (project 16.8939.2017/8.9, for the sample preparation) and the Russian Science Foundation (project 17-73-20336, for the optical characterization). D.G.B. and T.S. acknowledge a support from the Knut and Alice Wallenberg Foundation. M.F. acknowledges a funding from the European Social Fund according to the activity ‘Improvement of researchers’ qualification by implementing world-class R&D projects of Measure No. 09.3.3-LMT-K-712-01-0031. Y.K. acknowledges a support from the Strategic Fund of the Australian National University. A.Z. acknowledges a partial support from the Welch Foundation (grant AT 16-17).

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Graphical TOC Entry Chemical doping

Tuning

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Tunable Hybrid Fano Resonances in Halide ... - ACS Publications

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