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High Optical Energy Storage and Two-Photon Luminescence from Solution-Processed Perovskite-Polystyrene Composite Microresonators Mari Annadhasan, Uppari Venkataramudu, Nikolai V. Mitetelo, Evgeniy Mamonov, Chakradhar Sahoo, Sri Ram Gopal Naraharisetty, Tatiana V. Murzina, and Rajadurai Chandrasekar ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01459 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 3, 2018

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ACS Applied Energy Materials

High Optical Energy Storage and Two-Photon Luminescence from Solution-Processed PerovskitePolystyrene Composite Microresonators Mari Annadhasan,† Uppari Venkataramudu,† Nikolai V. Mitetelo, ‡ Evgeniy A. Mamonov, ‡ Chakradhar Sahoo,£ Sri Ram Gopal Naraharisetty,£ Tatiana V. Murzina‡ and Rajadurai Chandrasekar†,* †School

of Chemistry and £ School of Physics, University of Hyderabad, Prof. C. R. Rao Road,

Gachibowli, Hyderabad 500046, India ‡Division

of Quantum Electronics, Department of Physics, M. V. Lomonosov Moscow State

University, Moscow 119991, Russia Tel: +91(40)23134824; Fax: +91(40)23134800 E-mail: [email protected] KEYWORDS: polymer microspheres, perovskite, optical microresonators, two-photon emission, non-linear optical properties ABSTRACT Optical green emitting microresonators with high values of nonlinearity are desired for high optical up-conversion energy storage and lasing applications. Here we report on the synthesis of benzylammonium lead iodide (BALI) perovskite microcrystals made via anti-solvent diffusion 1 ACS Paragon Plus Environment

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method. The use of polystyrene (PS) matrix helps the growth of environmentally stable BALI nanocrystals as a result of polymer capping. Optimization of the ratio of BALI-PS during the selfassembly process leads to the formation of defect-free composite microspheres. Further, these BALI crystals act as an active material within the microsphere resonators that display whispering gallery modes (WGM). These WGMs are responsible for high optical storage energy of resonators evidenced by their Q-factor as high as ~1180. Two-dimensional finite difference time-domain calculation shows the concentration of the electric field near the microspheres boundary within a small mode volume of 1.83 µm3. Remarkably, BALI-PS composite microresonators and their neat crystals exhibit brilliant two-photon luminescence upon excitation by the infrared fundamental radiation. This simplistic microresonators fabrication technique provides a route towards low-cost non-linear optical microlasers and other optical energy devices useful for various applications.

1. INTRODUCTION

In recent years, solution-based preparation and processing technologies of organic-inorganic hybrid perovskites have attracted significant attention due to their low-cost and captivating applications in optoelectronic, photovoltaic devices and microlasers.1-3 The most explored perovskites till date include methylammonium lead halides (MAPbX3), tin halides (MASnX3), and CsSnX3 (where, X = Cl-, Br- and I-). Specifically, lead halide compounds have suitable optical band gaps, and they have been used as resourceful emissive species in the light emitting diodes and low thresholds microlasers.4-8 However, the poor stability of these materials against moisture limits their applications in various fields. The basic crystal structure of these perovskites (ABX3) tends to hydrolyze in water that destroys the lattice structure. To overcome this drawback, aromatic amine or larger amine groups are introduced to create two dimensional (2D) perovskite 2 ACS Paragon Plus Environment

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structures.9,10 It is expected that these amine groups arranged in layered structure increase the van der Waals interaction between the organic moieties providing high structural stability.9 Moreover, the optical band gap of these hybrid materials can be tuned over a wide range of energy by varying the organic ammonium cations or halogens or by controlling the dimensionality of inorganic cations. These chemical adjustments tend to affect the charge carrier mobility, carrier diffusion length in addition to band gap energies.11 Various one dimensional (1D) to three dimensional (3D) perovskites were prepared depending on the structure of organic ammonium cations.12-13 Specifically, 2D structures provide modified optical and structural behavior and hence they are expected to display wider band gaps and narrow photoluminescence (PL) at room temperature compared to that of 3D structures.14 Particularly, to date, a vast number of articles are available, which report on the one-photon pumped optical luminescence of neat MAPbX3 based compounds.15,16 On the other hand, materials with non-linear optical (NLO) response such as two-photon luminescence (TPL), second harmonic generation and optical rectification are of increasing interest.17 For example, Sargent et.al. demonstrated two-photon absorption (TPA) in MAPbBr3 perovskite single crystals.16 Change in the organic cation also leads to dramatic changes in the formation of perovskites in different dimensions with varied optoelectronic properties. Organic-inorganic perovskites which are sensitive to humidity and heat can be stabilized within the PS matrix.18 Further, PS is also known to form stable microspheres during solventassisted self-assembly which are suitable to support optical WGMs.19-20 These PS microspheres, due to their surface smoothness, are also known to increase the optical energy storage by (minimizing light scattering) efficiently trapping the light thus providing WGM resonators with high Q. The parameter Q can be estimated from the ratio Q = /FWHM, where  is the wavelength 3 ACS Paragon Plus Environment


of the peak and FWHM is the full-width-at-half-maximum of the peak. The direct relationship between high Q-resonators and their high energy storage (in other words, high photon lifetime, τp) can be expressed as Q = ω τp, where ω is the angular frequency of the photon.21 Therefore, we envisioned stabilizing unstable perovskites such as benzyl ammonium lead halide (BALX3) within spherical PS microspherical resonators generating PL, including strong TPL. To our knowledge, only the synthesis of BALX3 is mainly focused in the literature,22-24 its stabilization within PS WGM resonators, and NLO properties are not explored. Scheme 1. (a) Schematic illustration of BALI perovskite microcrystal growth on a glass slide. (b) Fabrication of BALI-PS composite microspherical WGMRs via self-assembly method.

In this study, we present the first synthesis of green emitting 2D benzyl ammonium lead iodide (BALI) perovskite microcrystals by facile one-step solution self-assembly method. A comparative photonic property of pure BALI microcrystals and BALI stabilized within PS 4 ACS Paragon Plus Environment

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microspheres is presented. Interestingly, BALI-PS composite microspheres exhibit enhanced PL due to WGM optical resonance with a high Q-factor of 1180. The finite difference time-domain (FDTD) calculation reveals the concentration of optical field close to the boundary of the spherical structure due to WGM resonance. Two-photon pumped photoluminescence (TPL) studies confirm high values of second-order nonlinearity of BALI based microstructures. These results show that the BALI perovskite can act as multi-photon (infra-red) pumped optical gain media and provide a new platform for perovskite-based NLO photonic devices. 2. EXPERIMENTAL SECTION 2.1 Materials. Benzylamine (99.5%), hydrogen iodide (HI) (48 wt% in water), Pb(OAc)23H2O (≥99.99%) and polystyrene (PS) beads (Mw~ 28 kDa) were purchased from Sigma Aldrich. HPLC grade methanol, isopropanol, N,N-dimethylformamide (DMF), diethyl ether, dichloromethane (DCM), and tetrahydrofuran (THF) were purchased from Merck and used as such. 2.2 Preparation of benzyl ammonium iodide (BAI). The synthetic protocol of BAI is similar to benzyl ammonium bromide reported earlier.24 In brief, initially, 5 mL of benzylamine was added to 15 mL of methanol (AR) kept at 0 °C. Then 5 mL of HI was dropped into the mixture and stirred for 1h, after that it was transferred to room temperature and stirred for another 2 h. The solvent retained in the mixture was evaporated by a rotary evaporator at 60 °C, leaving with the white crude product. White flake crystal of C6H5CH2NH3I (BAI) was finally obtained by recrystallizing the as-obtained crude products in a mixture of diethyl ether and methanol. 2.3 Preparation of benzyl ammonium lead iodide (BALI). At first, a thin film of Pb(OAc)2 was prepared on a glass slide by spin-coating of 20 µL solution of Pb(OAc)2 in DMF (100 mg/mL). The film was dried on a hot plate for 30 min at 60°C. BALI bulk crystals were synthesized by 5 ACS Paragon Plus Environment


placing the Pb precursor into BAI solution in IPA with concentrations 500 mg/mL at room temperature. The Pb precursor coated side of the slide was kept in such a way that it faces the solution in a beaker and left undisturbed for about 24 h. Further, a similar procedure was repeated by placing the Pb precursor-coated side facing up in the solution of BAI (50 mg/mL) in IPA. The glass plates were washed using IPA to remove any residual solution on the substrate. The bulk crystals of BALI were analyzed with different spectroscopic techniques. 2.4 Synthesis and growth of BALI microstructure (Anti-solvent Method).26 Initially, the precursor solutions of BAI (0.2 M) and Pb(OAc)2·3H2O (0.2 M) in DMF were prepared. Prior to the synthesis, equal volumes of these precursor solutions were mixed thoroughly to give a stock solution of C6H5CH2NH3PbI3 perovskite. In order to obtain microcrystals, 20 µL of the stock solution was drop casted on a 1×1 cm2 cover-slip. This slide was placed on a glass stage inside the beaker (see Scheme 1a) containing 25 mL of DCM leveled below the glass stage, and it was sealed with a porous parafilm to control the evaporation rate of both precursor solution and DCM. Here DCM acts as a poor solvent for C6H5CH2NH3PbI3 microstructures, while the former is miscible with DMF. As DCM is highly volatile, its vapor diffuses into the stock solution, induces the nucleation,25,26 which subsequently grows into long needle-like microcrystals (Scheme 1a). The single-crystal x-ray structure of the obtained microcrystals confirmed the 2D layered structure of BALI which is identical to the structure reported earlier (Figure S1).27 2.5 Fabrication of PS-BALI composite microspheres. PS beads (25 mg) were dissolved in THF (4.0 mL) and sonicated for a minute to dissolve the polymer. BALI incorporated PS microspheres were realized by the addition of 20 µL of 0.05 M BALI in DMF to the PS solution, it was mixed thoroughly and kept for 5 min. To this mixture, 1.0 mL of deionized water was added and left undisturbed for about 10 min to ensure the growth of high-quality microspheres and the growth of 6 ACS Paragon Plus Environment

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nanocrystals of BALI with PS matrix. Afterward, 100 µL of this mixture was drop-casted on a clean glass coverslip and the solvent was evaporated to get PS-BALI microspheres (Scheme 1b). In the case of BALI coated PS microspheres, 5L to 20 L of BALI solution was added after the formation of microspheres in THF:H2O (4:1). 2.6. Instrumentation. The solid-state absorbance spectra were collected using a Shimadzu UV3600 spectrometer in a diffuse reflectance UV−visible (DR−UV−vis) mode. The reflectance spectra were converted to an absorbance spectrum using the Kubelka−Munk function. The optical spectra of single microrod and microsphere particles were recorded on a WI-Tec alpha 300 AR confocal spectrometer equipped with a Peltier-cooled CCD detector. An Ar+ 488 nm CW laser was used as an excitation source. Size and morphology of the BALI-PS microspheres were examined by using a Zeiss field-emission scanning electron microscope (FESEM) operating at 3 kV and Transmission electron microscope (TEM) measurements were performed on a Tecnai G2 FEI F12 instrument operating at an accelerating voltage of 200 kV. All measurements were performed at ambient conditions. 2.7 Two-photon luminescence (TPL) measurements. NLO studies of microcrystals were performed on a two-photon spectroscopy set-up based on a femtosecond Ti:sapphire laser having a wavelength of 800 nm, pulse duration of 100 fs, and repetition rate of 1 kHz. The laser beam was focused by a lens of focal length 50 cm which produces a spot diameter of 127 μm. The sample was exposed at the focus for the generation of TPL, and the TPL signal was collected by a spectrometer in transmission geometry. Typically, as the BALI absorption spectrum maximum is found in the wavelength range of 250–580 nm, wavelength of 800nm was selected for two-photon excitation. For fundamental wavelength dependent TPL intensity studies, a pulse duration of 60 fs, repetition rate 80 MHz, mean power 3–200 mW were applied using a tunable laser from 740 7 ACS Paragon Plus Environment


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nm up to 890 nm wavelengths. The laser beam passed through a dichroic mirror and was focused on the surface of the sample by an objective Leica PL FLUOTAR L 63x with the numerical aperture of 0.7. The TPL signal reflected from the sample was collected by the same objective and detected either by a photomultiplier or by a home-made spectrometer based on the Peltier-cooled CCD matrix Hamamatsu S10141-1107S-01. The peak power was about 0.3–30 kW cm-2. 2.8 Laser confocal optical microscope set-up. The optical spectra of single microspheres were recorded on a WI-Tec alpha 300 AR confocal spectrometer equipped with a Peltier-cooled CCD detector. Using 300 grooves/mm grating BLZ = 700 nm, the accumulation and integration time were typically 10 s and 1 s, respectively. Ten accumulations were performed for acquiring a single spectrum. A 488 nm (Ar+) CW lasers were used as a one-photon excitation source.

All

measurements were performed at ambient conditions. A 150× objective (0.95 NA) was used for all the measurements. Laser power was estimated using THORLabs power meter.

3. RESULTS AND DISCUSSION 3.1 Spectroscopic and microscopic studies of BALI. The solid-state UV-visible absorption spectrum of pure BALI shows a continuous spectrum with a band-edge at ~560 nm, which arises as a result of exciton contribution.28 Therefore the sample was excited by a xenon lamp with the wavelength range from 450 nm to 500 nm. BALI shows PL with a maximum intensity at 538 nm (2.30 eV) with a full width at half maximum (FWHM) of ∼30 nm (Figure 1a). The corresponding optical band gap calculated from absorbance spectrum of BALI perovskite was about 2.18 eV.13 Figure 1b,c shows a digital photograph of BALI thin film at the ambient light and under UV light (365 nm) excitation, respectively. At ambient temperature, 8 ACS Paragon Plus Environment

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the BALI thin layer appeared in orange color, however when the same sample was illuminated with UV light (365 nm) it exhibited a bright green emission.

Figure. 1. (a) UV-visible absorbance and PL spectra (ex = 488 nm; Ar+ laser) of BALI at room temperature. The green filled band exhibits PL band of BALI thin film. The red line with circles shows the PL spectrum of BALI microcrystal. (b) Photographs of BALI thin film at ambient light and (c) under UV-365 nm light. (d) PL image of BALI microcrystal when excited with 488 nm Ar+ laser. The FESEM analysis showed that the microstructures are nearly monodispersed parallelepipeds (Figure 2a) with length and width in the range of 50 m and 5m, respectively. A close-up view exposed the presence of nano-sized (diameter~20-50 nm) pores in the microrods (Figure 2b). The microrods were further subjected to TEM studies (Figure 2c) and the width of the microrod was found to be 5 ± 2 m. Selected area electron diffraction (SAED) pattern was obtained by directing the electron beam perpendicular to the surface of a single microrod as shown in Figure. 2c inset. 9 ACS Paragon Plus Environment


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Figure 2. (a, b) FESEM images and (c) TEM image of BALI perovskite microrods. The insets (b) and (c) show a color-coded magnified view of the porous structure of a microrod and its SAED pattern, respectively), (d) EDS spectrum of a micro-rod and the inset shows the percentage of elements present within micro-rod. (e) elemental mapping of a single microrod. The hexagonal symmetry of the SAED pattern clearly established the single crystalline nature of BALI microrods. The energy dispersive spectrum (EDS) clearly confirmed the composition of the microrods by revealing the presence of Pb, I, C and N elements (Figure 2d). Further, the presence of these elements within the microstructures was verified by conducting elemental mapping on a single microrod (Figure 2e). To probe the waveguiding propensity of the 1D microrods, single-particle experiments were carried out using a 488 nm (Ar+) continuous wave (CW) laser as an excitation source. When 10 ACS Paragon Plus Environment

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a bulk of the single microrod was excited with a 20x objective with the laser spot size of 6 m, it showed a sharp emission peak maximum at ~538 nm with a reduced FWHM of ∼21 nm compared to those of a thin film (see the red line in Figure 1). Such a focused laser excitation of one of the micro-rods ends displayed a bright localized green emission (Figure S2b,c), while excitation at the center of the microrod exhibited a localized bright green emission (Figure S2d). The absence of waveguiding property along the microrods indicated the extended nature of the nanopores (order of microns). These nano-pores lead to a poor light confinement or, in other words, too high optical loss of the propagating light via scattering. 3.2 Spectroscopic and microscopic studies of BALI-PS. Since the composed BALI microrods are very sensitive to moisture, to make use of the perovskite as an active material for photonic device applications, we utilized PS as a matrix to stabilize BALI as the former forms mechanically stable microspheres suitable for the resonator and lasing applications.29 An earlier study in this context showed that coating of MAPbI3 on the surface of the silicon microspheres exhibiting lasing action.30 However, an expensive atomic layer deposition method was utilized for coating the perovskite on the surface of silicon spheres30 and the stability of the perovskite coated on silicon microsphere was not fully addressed. Hence we have intended two experiments to understand the stability of BALI-PS composite: (i) BALI coated on the surface of the PS micro-sphere (see Figure 3) and (ii) BALI mixed with PS solution to ensure the BALI within the PS microsphere matrix. In the first approach, PS microspheres were coated with BALI, where the BALI was exposed to moisture. As perovskites are highly sensitive to moisture, they tend to decompose in a shorter time, hence they show less stability. In the second experiment, the BALI and PS were mixed (20 L in DMF: 4 mL in THF) together then water (1 11 ACS Paragon Plus Environment


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mL) was added to aggregate PS to form microspheres. The solution was left undisturbed for 20 min and it was drop casted on a glass slide and the solvents were evaporated at room temperature. Examination of the sample under a confocal microscope exhibited circular structures demonstrating the formation of microspheres. These BALI within the PS matrix show high stability compared to the former one, as it has been embedded within the polymer matrix it prevents the degradation of BALI. The mechanism of formation of PS-BALI composite microspheres can be explained as follows: During the addition of water (1 mL) to DMF/THF solution (4 mL) containing BALI +PS, due to hydrophobic effect, PS instantly self-assembled by capping some BALI precursor. Further, the slow solvents evaporation from the coverslip facilitated the selfassembly of PS into microspheres and nano-crystallization of BALI within them. To further confirm the shape, size and surface smoothness of PS-BALI microspheres, FESEM analysis was carried out (Figure 3). The FESEM micrographs of PS microspheres blended with BALI (20 L) clearly elucidated the presence of polydispersed PS-BALI microspheres with a smooth surface, which is essential for a microsphere to act as a micro-resonator (Figure 3a(i)). The TEM images of a micro-sphere also exhibited a dark contrast, which confirmed its defect-free smooth surface as shown in Figure 3a(ii). Similarly, the FESEM micrographs of BALI (5 L) coated on PS micro-spheres showed nearly smooth surface (Figure 3b(i)). However, when the concentration of the BALI in isopropanol (IPA) and DMF were increased from 10 µL to 20 µL, concave (~100 nm to 1 m) shaped deformations were formed on the surface of these microspheres producing golf ball-like surface features (Figure 3b (iii -viii). These deformations could be due to microsolvent droplets which were attached to the surface of growing PS microspheres. High concentration of the BALI increases the solvent volume, which leads to the formation of golf balllike structures. 12 ACS Paragon Plus Environment

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Figure 3. (a) FESEM (i) and TEM (ii) images, SAED pattern (iii) and EDS elemental map (iv-vii) of PS micro-spheres blended with BALI. (b) FESEM (i, iii, v, vii), TEM (ii, iv, vi, viii) images and SAED pattern (iii) of PS microspheres coated with BALI at different concentrations. Similarly, BALI coated PS microspheres showed defect-free surfaces. The chemical composition of these microspheres was further confirmed by FESEM and EDS studies. The EDS elemental mapping of a single PS-BALI microsphere clearly showed the presence of C, N, Pb and I elements confirming that the BALI was incorporated into PS microsphere (Figure 3a (iv-viii)). The TEM micrographs also elucidated that the deformations are only on the surface (Figure 3b (iii-viii)). Further, the confocal microscopy, FESEM, and TEM clearly indicate the polydispersed nature of 13 ACS Paragon Plus Environment


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the microspheres with the size distribution in the range of 2-8 m. The SAED pattern showed no diffraction spots from the microspheres indicating their amorphous nature (Figure 3a(iii)). 3.3. Single-particle micro-spectroscopy studies and numerical calculation of microspheres. The photonic properties of microspheres were verified by single-particle confocal fluorescence microscopic studies. When the edge of a BALI-PS microsphere was excited with a CW laser (Ar+ 488 nm; power: 10 mW; objective: 150x) beam, the microspheres displayed a bright green emission. The band width of the emission was in the range of ~500-750 nm which is similar to BALI microcrystals (Figure 4). The later result also confirmed the (nano) crystallization of BALI within PS microspheres during the self-assembly. Further, a series of distinct pairs of sharp peaks were observed in the emission spectrum (from 500 nm to 750 nm) collected from the edge of the microsphere. This clearly indicated the existence of WGM resonances as a result of multiple total internal reflections of the emitted light at the microspheres surface and air (n = 1) interface.19-21 These pairs of peaks are known as transverse magnetic (TM) and electric (TE) modes, which arise due to the splitting of the TM/TE degeneracy.27-28, 31,32 The appearance of both TE/TM modes indicated the unpolarized nature of the emission. Another interesting feature of the WGM spectrum is the near absence of PL background, which points toward high Q-resonators.21,33 A twodimensional finite difference time-domain (FDTD) method was employed to simulate the WGM spectrum (for radial mode number, r =1; polar and azimuthal mode number, m=l) and the electric field distribution within the resonator. From the calculation, the pair of peaks such as TM and TE was identified as azimuthal mode numbers from TM18-TM27 and TE18-TE27, respectively (Figure 4c). The simulated electric field distribution (for TE23) within the micro-resonator showed the localization of an intense electric field around its periphery (Figure 4d). The estimated TE23 mode volume of the microspheres is 1.83 µm3. 14 ACS Paragon Plus Environment

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Figure 4. (a) and (b) show the bright field and PL images of a BALI-PS composite microresonator excited with CW laser (488 nm). (c) Single-particle PL spectrum of microspherical WGM resonator. (d) FDTD numerical simulation of a microspherical resonator displaying the electric field distribution for TE2,3 and the corresponding mode volume 1.83 µm3. Similar single-particle PL studies on microspheres with concave surface deformation showed a broad PL spectrum centered at 540 nm without any WGMs (Figure S3). Moreover, the WGM parameters of individual microspheres with different sizes were also investigated to gain more insight about the effect of cavity size on field confinement, free spectral range (FSR) and Qfactor. Figure 5 presents the PL spectra and PL image (insets) of three distinct microspheres with different diameters (D) under excitation with 488 nm CW laser. The FSR values obtained for variously sized microspheres are plotted in Figure 5d.

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Figure 5. Single-particle PL spectra of BALI-PS microsphere with different size (a-c). The insets show its corresponding PL images of microspheres. Plots of (d) FSR values versus 1/D with linear base fitted line and (e) Q-factor versus particle diameter. Unambiguously, the FSR values were found to decrease with increase in the microsphere D as per the relation, FSR = λ2/π D neff, where λ is the wavelength of light and neff is the effective refractive index. Microsphere with D of 6.6 µm showed lower FSR value i.e., 18.3 nm (Figure 5a), however, when the value of D ~ 4.4 and 3.2 µm, the FSR values increased to 25.4 and 31 nm, respectively (Figure 5a-c). The Q-factor is a vital parameter for any kind of optical resonator to describe the stored optical energy. For BALI-PS microresonators Q improved with the increase of the cavity D 16 ACS Paragon Plus Environment

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since the surface scattering losses of the circular resonator drop exponentially with their size. The maximum Q-factor achieved for the microsphere was about 1180 (λ= 552 nm; FWHM = 0.54 nm) (Figure 5e), which corroborated well with the absence of PL background in the WGM spectrum. 3.4. NLO studies To explore the NLO property of BALI, experiments were carried out using high intense femtosecond (fs) pulse laser field and compared with OPL. An ultrafast Ti:Sapphire laser system with the fundamental wavelength of 800 nm, a pulse width of 100 fs was used for the excitation. BALI crystals excited with this laser radiation showed a green TPL centered at 553 nm in contrast to one photon-pumped bright green emission observed at 537 nm (Figure 6). This result clearly indicated that the BALI crystal is a multi-photon active material. A slight red shift in the TPL spectrum of BALI with respect to OPL spectrum could be due to the reabsorption effects. Driven by the emanating TPL from BALI, experiments were carried out with different pump power to study the dependence of TPL intensity of the material. At low (8 µJ/cm2) pump fluence, a very weak TPL intensity was observed. However, an increase in the excitation intensity leads to an increase in the TPL intensity. The dependence of the TPL intensity on the fundamental radiation power corresponds to the quadratic power law (Figure 6 inset). In order to indirectly probe the TPA range, the sample was excited at the different fundamental wavelength (760 nm to 860 nm) and 3D excitation-emission dependence was measured, which is shown in Figure 7a. The NLO response from BALI crystals was obtained in the range of about 540 nm to 590 nm. The intensity distribution shows maximum around 565 nm, with its amplitude weakly depending on the pump or fundamental wavelength in the range from 760 nm to 860 nm (Figure 7a).



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Figure 6. One- and two-photon luminescence spectra of BALI crystal. The inset shows double logarithmic power vs intensity plot for TPL, the red line is the linear power fit, digital photographs of OPL and TPL excited luminescence of BALI crystals. Similarly, a bunch of PS-BALI microsphere film of the thickness about 20 m (Figure 7b) also emitted two-photon pumped luminescence. The TPL spectra collected with different pump energies are shown in Figure 7c. Here, WGM signatures are not seen in the TPL since several microspheres with different diameters are illuminated simultaneously and the overall spectrum is averaged over their spectra. The intensity of the TPL spectrum increased as a square of the pump pulse energy of up to ~15.4 mJ/cm2. Upon additional increase of the pump power, a sudden decrease of the TPL intensity was observed (Figure 7c inset), which could be due to the radiation damage of the sample.34 Thus the laser damage threshold of the PS-BALI microspheres is 15 mJ/cm2.


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Figure 7. a) Emission – excitation spectrum of the TPL intensity for BALI crystal. b) FESEM cross-sectional image of PS-BALI microspheres deposited on a glass slide. c) TPL intensity of PSBALI microspheres with different fundamental pump power. The inset shows power versus intensity plot. 4. CONCLUSION In summary, we have demonstrated the preparation of 2D layered BALI microcrystals on glass slides by an anti-solvent diffusion method using DCM as anti-solvent. For the first time, the green emitting BALI perovskite was incorporated within PS to produce an air-stable high-Q WGM resonator with the Q-factor as high as ∼1.18103. FDTD calculation demonstrated the localization of optical field near the microsphere boundary with the mode volume as small as 1.83 µm3 for TE23 mode. Additionally, for the first time the intrinsic TPL property of 2D BALI crystal was demonstrated and interestingly the crystal provided access to TPL in a broad range of the fundamental excitation wavelengths from 760 nm – 860 nm. Importantly, a film of high Q-factor PS-BALI microsphere also displayed intense two-photon pumped emission to a smaller pump fluence in comparison to homogeneous thin film. The PS-BALI microsphere film was stable up to 19 ACS Paragon Plus Environment


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the pump fluence of ~15.4 mJ/cm2. The NLO property of this 2D perovskite paves way for the development of new functional optoelectronic devices for direct practical applications. Supporting Information. The PL spectra of single microrod, bright and PL field images of microrods, microsphere with deformation and its PL spectrum. Acknowledgments MA and UR thank UGC Kothari post-doctoral and CSIR-New Delhi for the fellowships, respectively. TM and RC acknowledge research funds from RSF-Moscow (Grant No. 16-4202024) and DST-New Delhi (Grant No. DST-RSF-P(05)), respectively. NM acknowledges financial support from the Foundation for the advancement of theoretical physics and mathematics "BASIS". RC also acknowledges financial support from DST PURSE Phase 2. References 1. Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319. 2. Gao, P.; Gratzel M.; Nazeeruddin, M. K. Organohalide Lead Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 2448−2463. 3. Zhang, H.; Liao, Q.; Wang, X.; Yao, J.; Fu, H. Water‐Resistant Perovskite Polygonal Microdisks Laser in Flexible Photonics Devices. Adv. Opt. Mater. 2016, 4, 1718−1725. 4. Liao, Q.; Hu, K.; Zhang, H.; Wang, X.; Yao J.; Fu, H. Perovskite Microdisk Microlasers SelfAssembled from Solution. Adv. Mater. 2015, 27, 3405−3410. 5. Veldhuis, S. A.; Boix, P. P.; Yantara, N.; Li, M.; Sum, T. C.; Mathews, N.; Mhaisalkar, S. G. Perovskite Materials for Light-Emitting Diodes and Lasers. Adv. Mater. 2016, 28, 6804−6834. 20 ACS Paragon Plus Environment

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