Angewandte Research Articles Chemie How to cite: Angew. Chem. Int. Ed. 2020, 59, 17750 – 17756 Perovskite Nanocrystals Hot Paper International Edition: doi.org/10.1002/anie.202006152 German Edition: doi.org/10.1002/ange.202006152 Templated-Assembly of CsPbBr3 Perovskite Nanocrystals into 2D Photonic Supercrystals with Amplified Spontaneous Emission David Vila-Liarte+, Maximilian W. Feil+, Aurora Manzi, Juan Luis Garcia-Pomar, He Huang, Markus Dçblinger, Luis M Liz-Marzn, Jochen Feldmann, Lakshminarayana Polavarapu,* and Agustn Mihi* Abstract: Perovskite nanocrystals (NCs) have revolutionized nality.[1c, 3] We have witnessed tremendous research progress optoelectronic devices because of their versatile optical proper- regarding the shape and composition-controlled synthesis of ties. However, controlling and extending these functionalities perovskite NCs for tailoring their emission color, as well as often requires a light-management strategy involving addi- improving quantum efficiency.[3f, 4] Among all available com- tional processing steps. Herein, we introduce a simple ap- positions, Pb-based perovskite NCs stand out for their proach to shape perovskite nanocrystals (NC) into photonic stability and optical efficiency, even as a two-photon-pumped architectures that provide light management by directly shap- lasing medium.[5] Because of the interest in perovskite ing the active material. Pre-patterned polydimethylsiloxane nanocrystals as light sources, a light management strategy is (PDMS) templates are used for the template-induced self- often required. An appealing strategy to engineer the optical assembly of 10 nm CsPbBr3 perovskite NC colloids into large response of the material without recurring to changes in its area (1 cm2) 2D photonic crystals with tunable lattice spacing, composition comprises molding it into higher order architec- ranging from 400 nm up to several microns. The photonic tures. This approach has been previously applied to colloidal crystal arrangement facilitates efficient light coupling to the gold nanocrystals, self-assembled into 2D plasmonic super- nanocrystal layer, thereby increasing the electric field intensity crystals (SCs) exhibiting tunable lattice plasmon resonances, within the perovskite film. As a result, CsPbBr3 2D photonic but also to PbTe nanocrystals, for which the arrangement was crystals show amplified spontaneous emission (ASE) under modulated into a variety of motifs.[6] Self-assembly concepts lower optical excitation fluences in the near-IR, in contrast to have also been recently extended to colloidal perovskite equivalent flat NC films prepared using the same colloidal ink. NCs,[7] including the formation of colloidal SCs with distinct This improvement is attributed to the enhanced multi-photon optical properties.[4b, 8] Despite these early efforts, the self- absorption caused by light trapping in the photonic crystal. assembly of perovskite NCs into 2D SC arrays with subwa- velength patterns remains a daunting challenge, and therefore Introduction their optical properties remain unexplored. Photonic crystals (PhCs) are materials exhibiting a perio- Halide perovskite nanocrystals (NCs)[1] have emerged as dicity in the refractive index on the order of the wavelength of a new class of efficient and tunable light sources for various incident light.[9] Light propagation in these materials can be optical and photonic applications and have already shown described in terms of photonic bands and band gaps great promise in solar cells, photodetectors, LEDs and (forbidden intervals). PhCs, however, also exhibit many other lasers.[1b, 2] The optical properties of halide perovskite NCs interesting phenomena. 2D photonic crystals, very much as are readily tunable across the entire visible light spectrum, by other gratings, can diffract light very efficiently, which has means of their halide composition as well as their dimensio- been used e.g., to enhance the optical path in solar cells.[10] [*] D. Vila-Liarte,[+] Dr. J. L. Garcia-Pomar, Dr. A. Mihi Prof. Dr. L. M Liz-Marzn Institut de Cincia de Materials de Barcelona (ICMAB-CSIC) Centro de Investigacin en Red en Bioingeniera, Biomateriales Campus de la UAB y Nanomedicina (Ciber-BBN) 08193 Bellaterra, Catalonia (Spain) Paseo de Miramn 182, 20014 Donostia—San Sebastin (Spain) E-mail: amihi@icmab.es and M. W. Feil,[+] Dr. A. Manzi, Dr. H. Huang, Prof. Dr. J. Feldmann, Ikerbasque, Basque Foundation for Science Dr. L. Polavarapu 48013 Bilbao (Spain) Chair for Photonics and Optoelectronics, Nano-Institute Munich [+] These authors contributed equally to this work. Department of Physics, Ludwig-Maximilians-Universitt (LMU) Supporting information and the ORCID identification number(s) for Kçniginstrasse 10, 80539 Munich (Germany) the author(s) of this article can be found under: E-mail: l.polavarapu@lmu.de https://doi.org/10.1002/anie.202006152. Dr. M. Dçblinger  2020 The Authors. Published by Wiley-VCH GmbH. This is an Department of Chemistry, Ludwig-Maximilians-Universitt (LMU) open access article under the terms of the Creative Commons Butenandtstrasse 5–13 (E), 81377 Munich (Germany) Attribution License, which permits use, distribution and reproduc- D. Vila-Liarte,[+] Prof. Dr. L. M Liz-Marzn tion in any medium, provided the original work is properly cited. CIC biomaGUNE, Basque Research and Technology Alliance (BRTA) Paseo de Miramn 182, 20014 Donostia—San Sebastin (Spain) 17750  2020 The Authors. Published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2020, 59, 17750 – 17756 Angewandte Research Articles Chemie Many materials can be organized into photonic crystals by Results and Discussion periodically arranging them in the sub-micrometer range and thereby exhibit optical properties differing from those of the The colloidal CsPbBr3 nanocube ink used for the fabri- bulk material.[6b, 11] In this work, we propose a simple and cation of 2D photonic crystals was prepared by ligand-assisted scalable method to fabricate large area 2D photonic crystals ultrasonication (see experimental section in the Supporting using colloidal solutions of perovskite nanocubes. We used Information SI). The synthesized NCs are highly monodis- template-induced self-assembly to produce large-area arrays perse, with an average size of 10.7  0.9 nm (Figure S1). The from colloidal perovskite NC dispersions. The obtained colloidal dispersion was purified by centrifugation to obtain perovskite 2D PhCs have lattice parameters on the order of a perovskite NC ink in hexane at a 100 mg mL 1 concen- visible light wavelengths, with a strong iridescence indicating tration. The perovskite NCs exhibited green photolumines- the high quality of the structure. Furthermore, the photonic cence (PL) with a maximum at 513 nm (Figure S1). Figure 1 a architecture is engineered to couple the 800 nm pumping light illustrates the self-assembly process using poly(dimethylsil- into photonic modes propagating in the perovskite PhC layer. oxane) (PDMS) templates[12] with pre-patterned square This enhanced electric field intensity in the material, as arrays (see AFM image of the PDMS surface in Figure S2). revealed by FDTD simulations, is used in turn to produce The modified template-assisted self-assembly process is amplified spontaneous emission (ASE) in PhC films under illustrated in Figure 1. In the first step, a drop (10 mL) of fluence that is too low to obtain ASE from planar films CsPbBr3 colloidal dispersion is deposited on a glass substrate obtained under similar conditions. The combination of the and immediately covered with a PDMS template with holes NCs optical properties with the photonic environment facing the droplet (Figure 1 a-I). The resulting confinement provides an efficient use of optical pumping to boost the forces the colloidal dispersion into the holes of the PDMS light intensity-dependent mechanism of two-photon absorp- template, from where the solvent is left to evaporate for tion. 1 hour (Figure 1 a-II). Homogeneous distribution of the Figure 1. a) Schematic illustration of PDMS template-assisted fabrication of 2D photonic supercrystals made of perovskite NCs. b),c) Representa- tive SEM images of 2D photonic supercrystals made of CsPbBr3 arrays with lattice parameters of 600 nm and 1700 nm, respectively. The inset in (b) is a photograph of different CsPbBr3 SC arrays on glass substrates, displaying diffraction under white light illumination. Angew. Chem. Int. Ed. 2020, 59, 17750 – 17756  2020 The Authors. Published by Wiley-VCH GmbH www.angewandte.org 17751 Angewandte Research Articles Chemie dispersion and conformal contact between the substrate and the PDMS mold was ensured by placing a 700 g weight on top of the stamp during the entire drying step (see experimental section in SI). Finally, the PDMS mold is gently lifted-off from the substrate, yielding an array of NCs with the negative image of the template pattern on the substrate (Figure 1 a- III). The obtained PhC films exhibit strong green PL under UV light illumination (see left inset in Figure 4 b below) and appear iridescent under white light illumination as shown in the inset of Figure 1 b. The diffraction of white light into different colors originates from the subwavelength periodicity of the NC assemblies on the substrate. The 2D perovskite photonic crystals were further characterized by scanning electron microscopy (SEM). SEM images of the films fabricated using a PDMS template with a lattice parameter (L) of 600 nm clearly show large-area 2D arrays of NC assemblies (Figure 1 b). Interestingly, the CsPbBr3 NCs in the assemblies exhibit cubic close-packing into an ordered super- crystal, as revealed by high magnification SEM images (Figure 1 b). The lateral dimension of the supercrystal is Figure 2. SEM characterization of photonic crystals prepared by self-  200 nm. These supercrystals are formed during evaporation assembly of NCs from dispersions at concentrations of a)–c) 100 d)– of the solvent inside the holes of the PDMS template. f) 50 and g)–i) 10 mg mL 1. The concentration of NCs and ligands has However, parameters such as ligand concentration and a strong influence on the viscosity of the sample, altering the solvent composition should be investigated further to better distribution of NCs along the surface. Isolated pillars are formed at understand the mechanism of supercrystal formation under low concentrations, while a residual layer of NCs is formed all over the the template assembly.[6b] sample at 100 mg mL 1. Perovskite photonic crystals with different lattice param- eters were fabricated by different choice of PDMS molds. Optimization of the system allowed us to successfully residual nanocubes connecting the pillars. For the highest assemble the NCs into structures with L ranging from NC concentrations (80–100 mg mL 1) the final photonic 1700 nm down to 400 nm, thereby overcoming the increasing architecture was composed of an array of CsPbBr3 nanopillars difficulty involved in pattern miniaturization. SEM images of on top of a residual layer of nanocubes. The height of the PhCs with different lattice parameters are shown in Figure S3. pillars is in the range of 80–100 nm (Figure S6). This latter Bright field optical microscopy overview images of the configuration is particularly useful in the case of light samples confirm the formation of uniform and large-area emission studies (amplified spontaneous emission and lasing), 2D PhCs (Figure S4). Shown in Figure 1 c is a representative where thick layers of active material are required. We focus SEM image of a perovskite 2D PhC with a lattice parameter now on the optical properties of perovskite 2D photonic of 1700 nm, prepared on a glass substrate using the corre- crystals with lattice parameters of 400, 500 and 600 nm, sponding PDMS template. In this particular array, high produced from 100 mg mL 1 dispersions. These PhCs consist magnification SEM reveals that the NCs are closely packed of arrays of 100 nm high pillars, on top of a 250 nm flat film of within individual SCs (Figure 1 c). In order to better visualize NCs, as schematically illustrated in Figure 3 c. This film the individual CsPbBr3 NCs, we assembled the same NCs over thickness is required to observe a meaningful photolumines- carbon-coated grids and analyzed them by transmission cence signal from the sample. We first inspect the extinction electron microscopy (TEM, Figure S5). TEM images of these (1—transmittance) from the PhCs and compare it to the assemblies and their 2D fast Fourier transform (FFT) analysis extinction from a flat NC film (Figure 3 a). indeed confirm the formation of self-assembled perovskite The flat reference sample shows an excitonic peak at supercrystal arrays (Figure S5). In addition, the assemblies 512 nm, while PhCs with lattice parameters of 400, 500 and can be seen on the substrate as well after a thorough washing 600 nm exhibit excitonic peaks at 515, 519 and 527 nm, of the surface with methylacetate or acetone (Figure S6). respectively. However, a series of new peaks appear for the However, this process partially destroys the NCs and the patterned samples, which redshift with increasing lattice assemblies. spacing. These features are Rayleigh anomalies associated In addition to the dimensions of the template, we found to the offset of diffraction. These peaks indicate that light that both concentration and viscosity of the CsPbBr3 colloidal impinging normally to the sample is diffracted by the dispersion play a critical role on the final structure of the PhC. periodicity of the array.[6b] We carried out finite difference We fabricated a series of photonic crystals with fixed geo- time domain (FDTD) simulations from Lumerical Solutions metrical parameters and varying concentration of NCs (Fig- Inc., to theoretically obtain the corresponding extinction ure 2). We observed that isolated SC pillars were effectively curves (Figure 3 b) and to extract the spatial distribution of achieved with concentrations below 10 mg mL 1, whereas 800 nm incident light on a 600 nm lattice parameter perov- increasing the concentration of nanocrystals resulted in skite PhC, versus a flat film (see numerical details in SI). The 17752 www.angewandte.org  2020 The Authors. Published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2020, 59, 17750 – 17756 Angewandte Research Articles Chemie Figure 3. Lattice resonances and field enhancement: Experimental (a) and theoretical (b) extinction (1 T, where T is transmittance) spectra for a NC thin film (green curve) and for different PhCs with lattice parameters of 400 (blue), 500 (black) and 600 nm (red). The vertical dashed line in (a) corresponds to the 800 nm laser excitation wavelength used in the nonlinear experiments. c)–f) FDTD simulated spatial distribution of the electric field enhancement illustrating the effect of light diffraction, for the case of 600 nm lattice parameter versus a perovskite flat film (e,f). field profiles reveal that the electric field within the photonic crystal is 7.5 times higher than that in a flat film of the same thickness (Figure 3 c–f). Effectively, a perovskite PhC dif- fracts the incoming light and couples it to the perovskite residual layer under the pillars. This light trapping strategy is typically used in photovoltaics to couple light more efficiently to the active layer of a solar cell, by increasing the optical path of light in the material.[10] Similarly, this scheme has been exploited in the literature to enhance the two-photon absorption (TPA) of light emitters, a third order process whose absorption cross section scales non-linearly with the intensity of the incoming light (see numerical details in SI).[13] However, unlike previous reports in which quantum dot- decorated PhCs[14] were fabricated by costly lithographic techniques such as electron beam lithography (EBL), our approach readily produces a high quality PhC configuration within minutes. We thus applied this strategy to optimize the use of the 800 nm pumping light and look for an improved PL enhancement, following recent studies on bulk-perovskite metasurfaces that showed enhanced linear and nonlinear PL.[2a, 11] We selected the perovskite PhCs fabricated from 100 mg mL 1 dispersions (Figure 4 a) for PL characterization, since these resulted in the thicker architectures reproducing the light trapping scheme shown in Figure 3 c. We collected PL under 400 nm excitation from the PhC, as well as from a flat region outside of the patterned area, and compared it with the PL from the NC dispersion (Figure 4 b). The NCs in dispersion have a PL centered at 513 nm, with a Gaussian FWHM of 21 nm. Instead, both the NCs in the PhC pattern and in the flat reference region exhibited a spectrally red- shifted emission with respect to the PL of the NC dispersion, respectively at 528 nm (FWHM: 23 nm) and 524 nm (FWHM: 23 nm). Importantly, this redshift is not only observed in the emission but also in the extinction spectra Figure 4. a) SEM close view of SC arrangement in a perovskite PhC (Figure 3 a), as a redshift of the 1s excitonic transition. Such obtained from a 100 mg mL 1 NC dispersion, used for ASE studies. a redshift is known to stem from self-assembly of NCs into b) Photoluminescence spectra obtained from inside the patterned area colloidal superlattices.[8, 15] The physical origin of the red- (red), outside of the patterned area (blue) and from the initial colloidal shifted PL stems from the close vicinity of neighboring NCs, dispersion (green). Angew. Chem. Int. Ed. 2020, 59, 17750 – 17756  2020 The Authors. Published by Wiley-VCH GmbH www.angewandte.org 17753 Angewandte Research Articles Chemie which leads to overlap of their electronic wavefunctions. This coupling phenomenon between NCs leads to the formation of minibands, both in the conduction and in the valence bands. The split energy levels result in a lower energy for excitonic absorption and recombination (i.e., the observed redshifts).[8a] In our case, the redshifted absorption and emission from the PhCs underlines a higher degree of electronic coupling in such structures, with respect to the NCs in the reference flat area. Therefore, the SCs obtained inside the holes of the PDMS templates are of higher quality than those spontaneously formed in the unpatterned area, which evidences the potential versatility of this fabrication technique. Finally, to probe the effect of the field enhancement obtained by the presence of PhCs on the light emitting properties of perovskite NCs, we photoexcited the system with an 800 nm pulsed laser system, with a repetition rate of 1 kHz and a pulse width of 100 fs. This excitation wavelength corresponds to the electric field spatial distribution observed for the PhCs and is attributed to the increase in light diffraction, which is absent for unpatterned NCs (see Fig- ure 3 a). CsPbBr3 NC films have been shown to exhibit below band gap absorption and nonlinear amplified spontaneous emission (ASE).[16] We therefore explored the possibility to obtain ASE through nonlinear photon absorption in our PhC pattern, varying the intensity of the photoexcitation and comparing the observation to the ASE obtained from unpatterned NCs. ASE was reproducibly observed from our PhC pattern (see Figure 5 a), as evidenced by the emergence of an additional narrow emission peak on the low-energy side of the PL spectra. By integrating the PL intensity versus the pump fluence we obtained an ASE threshold intensity of 10.9 mJ cm 2 (Figure 5 b). Employing an unpatterned film from the same NC ink as a reference, we observed no ASE, even for the maximum excitation fluence of 13 mJ cm 2 (Figure S7). These results suggest that 2D perovskite PhCs make more efficient use of the excitation light by coupling to photonic modes with more intense field profiles at 800 nm, as predicted from FDTD simulations (Figure 3 e). This results in Figure 5. a) Fluence-dependent PL (evolution of ASE) of CsPbBr3 2D a decrease of the photoexcitation intensity necessary to photonic crystals film and b) the corresponding emission intensity achieve population inversion, hence ASE. This finding obtained by integration of the spectra. The inset in (a) is a photograph indicates how PhCs can be employed to tailor the field of the photonic crystal film showing waveguiding above excitation enhancement effects in optoelectronic devices based on threshold for ASE. The dotted lines in (b) are the best linear fits for perovskite NCs. We envision that the optimized use of the intensities below (green) and above (red) threshold. The spectra were optical pumping provided by the ordered nanostructuring of acquired with 800 nm femtosecond laser excitation. the NC ink not only broadens the range of applicability of this material beyond its visible band gap, but also will be particularly beneficial to light emitting applications under photonic crystal geometry. These resonances can be tuned NIR excitation. through the choice of lattice parameter (L) in the architec- ture. We reproduced the experimental spectra using FDTD simulations and investigated the field distribution in the Conclusion structure, revealing a strong field enhancement at below band-gap energies. Furthermore, we have taken advantage of In conclusion, we have demonstrated the fabrication of this light trapping scheme to enhance the two-photon large-area CsPbBr3 2D photonic crystals by template-assisted absorption of NIR light and obtained ASE. We anticipate self-assembly, using 10 nm nanocrystal inks. We produced that these results will have broader impact in the field of homogeneous photonic architectures with lattice parameters perovskites beyond ASE and open new opportunities to tailor ranging from 400 nm to 1700 nm. The PhCs obtained exhibit the optical properties of this novel material at the nanoscale. optical resonances in the extinction spectra under normal incidence produced by the onset of diffraction from the 17754 www.angewandte.org  2020 The Authors. Published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2020, 59, 17750 – 17756 Angewandte Research Articles Chemie Acknowledgements Bohn, E. Bladt, K. Wang, P. Mller-Buschbaum, S. Bals, A. S. Urban, L. Polavarapu, J. Feldmann, Angew. Chem. Int. Ed. 2017, 56, 13887 – 13892; Angew. Chem. 2017, 129, 14075 – 14080; c) Y. We acknowledge financial support by the Bavarian State Tong, E. Bladt, M. F. Aygler, A. Manzi, K. Z. Milowska, V. A. Ministry of Science, Research, and Arts through the grant Hintermayr, P. Docampo, S. Bals, A. S. Urban, L. Polavarapu, J. “Solar Technologies go Hybrid (SolTech)”. We thank local Feldmann, Angew. Chem. Int. Ed. 2016, 55, 13887 – 13892; research clusters and centers such as CeNS for providing Angew. Chem. 2016, 128, 14091 – 14096; d) Q. A. Akkerman, V. communicative networking structures. We also acknowledge D’Innocenzo, S. Accornero, A. Scarpellini, A. Petrozza, M. support from the Spanish Ministerio de Ciencia e Innovacin Prato, L. Manna, J. Am. Chem. Soc. 2015, 137, 10276 – 10281; through Grants No. SEV-2015-0496 and MDM-2017-0720, in e) A. De, N. Mondal, A. Samanta, Nanoscale 2017, 9, 16722 – the framework of the Spanish Severo Ochoa and Mara de 16727; f) Y. Tong, M. Fu, E. Bladt, H. Huang, A. F. Richter, K. Maeztu Centers of Excellence program. This project has Wang, P. Mller-Buschbaum, S. Bals, P. Tamarat, B. Lounis, J. Feldmann, L. Polavarapu, Angew. Chem. Int. Ed. 2018, 57, received funding from the European Research Council 16094 – 16098; Angew. Chem. 2018, 130, 16326 – 16330. (ERC) under the European Unions Horizon 2020 research [4] a) S. Paul, E. Bladt, A. F. Richter, M. Dçblinger, Y. Tong, H. and innovation program (Grant Agreement No. 637116, Huang, A. Dey, S. Bals, T. Debnath, L. Polavarapu, J. Feldmann, ENLIGHTMENT). H.H acknowledges the European Un- Angew. Chem. Int. Ed. 2020, 59, 6794 – 6799; Angew. Chem. ions Horizon 2020 research and innovation programme 2020, 132, 6860 – 6865; b) M. Imran, P. Ijaz, D. Baranov, L. under the Marie Sklodowska-Curie grant agreement No. Goldoni, U. Petralanda, Q. Akkerman, A. L. Abdelhady, M. 839042. Open access funding enabled and organized by Prato, P. Bianchini, I. Infante, L. Manna, Nano Lett. 2018, 18, Projekt DEAL. 7822 – 7831; c) D. Zhang, S. W. Eaton, Y. Yu, L. Dou, P. Yang, J. Am. Chem. Soc. 2015, 137, 9230 – 9233; d) S. Zou, Y. Liu, J. Li, C. Liu, R. Feng, F. Jiang, Y. Li, J. Song, H. 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