Quantum confinement effect and exciton binding energy of layered perovskite

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1 Quantum confinement effect and exciton binding energy of layered perovskite nanoplatelets Qiang Wang, 1 Xiao-Dan Liu, 1 Yun-Hang Qiu, 1 Kai Chen, 1 Li Zhou 1,a) and Qu-Quan Wang 1,2,a) 1 Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan , China 2 The Institute for Advanced Studies, Wuhan University, Wuhan , China ABSTRACT: We report the preparation of monolayer (n = 1), few-layer (n = 2 5) and 3D (n = ) organic lead bromide perovskite nanoplatelets (NPLs) by tuning the molar ratio of methylammonium bromide (MABr) and hexadecammonium bromide (HABr). The absorption spectrum of the monolayer (HA) 2 PbBr 4 perovskite NPLs shows about 138 nm blue shift from that of 3D MAPbBr 3 perovskites, which is attributed to strong quantum confinement effect. We further investigate the two-photon photoluminescence (PL) of the NPLs and measure the exciton binding energy of monolayer perovskite NPLs using linear absorption and two-photon PL excitation spectroscopy. The exciton binding energy of monolayer perovskite NPLs is about 218 mev, which is far larger than tens of mev in 3D lead halide perovskites. I. INTRODUCTION Recent outstanding progresses in solar cells and light-emitting diode applications have exhibited promising optoelectronic properties of organic-inorganic lead halide perovskites. 1 9 These perovskites (such as MAPbX 3, FAPbX 3, CsPbX 3, X = Cl, Br, I) exhibit a large absorption coefficient, high charge carrier mobility, small exciton binding energy, as well as long charge diffusion length Their absorption and photoluminescence (PL) wavelength can be tuned from visible to near infrared through substituting cationic or anionic components The optical and excitonic properties of perovskites are dependent on the chemical component, and crystal structure Perovskite nanoplatelets (NPLs) have attracted extensive attention due to their unique optical properties Mechanical exfoliation, vapor transport chemical deposition method, ligand-assisted exfoliation, and dilution-induced approach have been reported to prepare perovskite NPLs The morphology and optical properties of perovskite NPLs are different from 3D counterparts due to the quantum confinement effect, which is 1

2 observed when the crystal size becomes comparable to or smaller than the exciton Bohr radius in the perovskite materials. 20,28 Sichert et al. report quantum confinement effect and thickness-dependent properties of organometal halide perovskite NPLs. 29 Tyagi et al. compare the optical characteristics of excitons in these two-dimensional (2D) systems to spherical perovskite nanocrystals (NCs) and the bulk counterparts. 30 Yuan et al. investigate the quasi-2d layered structure of perovskites and employ the quantum confinement effect to realize precise color tuning of emission. 31 Recently, the nonlinear properties of perovskites with interesting applications are widely presented. Amplified spontaneous emission from perovskite NCs and perovskite films as well as lasing from perovskite nanowires and nanodisks based on multi-photon absorption have been reported Walters et al. have reported two-photon absorption from MAPbBr 3 single crystals and demonstrated the prospect of perovskites as a two-photon absorber for the applications in nonlinear optics. 39 Zhang et al. have investigated the nonlinear optical absorption coefficient of the organic-inorganic perovskite nanocrystal films. 40 However, two-photon PL from monolayer (n = 1), few-layer (n = 2 3) and 3D (n = ) layered perovskites are seldom reported. In this paper, we prepare lead bromide perovskite NPLs at room temperature and manipulate the thickness of the NPLs by tuning the reactants molar ratio of methylammonium bromide (MABr) and hexadecammonium bromide (HABr). Compared with previous reported approaches 24 27, the synthesis method in this work is fast and facile. The novelty of this method is mixing the reactants in different molar ratio. The absorption spectra of perovskite NPLs show a large blue shift (138 nm) when the thickness of NPLs is decreased to monolayer scale. We further investigate the one-photon and two-photon PL from the monolayer (n = 1), few-layer (n = 2 3) perovskite NPLs and 3D (n = ) bulk perovskites. The exciton binding energy of n = 1 monolayer perovskite NPLs is measured by using linear absorption and two-photon PL excitation spectroscopy (TP-PLE). The exciton binding energy of monolayer perovskite NPLs is 218 mev, which is far larger than tens of mev in 3D counterparts. II. EXPERIMENT METHOD Preparation of Precursors. For MABr, 4 ml aqueous solution of methylamine (30%) and 4 ml aqueous solution of hydrobromic acid (45%) were mixed in a single-neck round bottom flask. Next, the flask was placed in an ice bath for 2 h. The volatiles were removed via a heating at 70 ºC with magnetic stirring and the orange solids were obtained. The solids were washed several times (at least three times) by anhydrous ether and dried in vacuum 2

3 at 70 ºC for 24 h. For HABr, hexadecylamine (2410 mg) was dissolved in ethanol (40 ml), and 2 ml hydrobromic acid was added. After 2 h ice bath, the volatiles were removed via a heating at 70 ºC with magnetic stirring. The obtained solids were washed several times through anhydrous ether and dried in vacuum at 70 ºC for 24 h. Preparation of Perovskite NPLs. MABr, HABr, and PbBr 2 were respectively dissolved in dimethylformamide (DMF) at concentration of 0.1 mol/l. The PbBr 2 powder was added into DMF then heated at 80 ºC until PbBr 2 was completely dissolved. The three precursor solutions were mixed in proper ratio (Table S1 in Supporting Information). Then, the mixed precursor solution was added dropwise into 10 ml of toluene in stirring at room temperature. The obtained perovskite NPLs were sonicated for 20 minutes and then centrifuged in 7000 rpm for 10 minutes. The products were washed three times through toluene for further usage. Characterization. Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) observations were performed with a JEOL 2010 FET transmission electron microscope operated at 200 kv and a Hitachi S The absorption spectra were recorded by a Varian Cary 5000 spectroscope and a TU-1810 UV-vis spectroscope (Purkinje General Instrument Co. Ltd. Beijing, China). The thickness of perovskite NPLs was measured by Smart SPM. Optical Measurement. The one-photon PL spectra were recorded using Hitachi F-4500 fluorescence spectrophotometer with a Xe lamp as the excitation source. For two-photon PL measurements, the excitation source was a mode locked Ti:sapphire laser (Mira 900, Coherent) with a pulse width of around 3 ps and a repetition rate of 76 MHz. The PL spectra were recorded by a spectrometer (Spectrapro 2500i, Acton) with liquid nitrogen cooled CCD (SPEC-10: 100B, Princeton). The absorption and PL measurement were performed on solution. The solvent is toluene and concentration of the sample is 0.2 mmol/l. III. RESULTS AND DISCUSSION A. Nanoplatelets Structure. Layered 2D perovskite NPLs are described by the general formula (R) 2 [ABX 3 ] n-1 BX 4, 24 where R, A, B and X are a long-chain alkylammonium ligand (HA), organic molecular cation (MA), divalent metal cation (Pb 2+ ), halide anion (chloride, bromide, iodide), respectively. The parameter n demonstrates the number of the metal cation layers between the two layers of the alkylammonium long chain. For example, n = 1, 2 and 3 correspond to the perovskite 3

4 NPLs with one, two and three layers of BX42- capped with long organic chains, respectively. With n =, it stands for the 3D counterparts, which are schematically illustrated in FIG 1a. We synthesize the perovskite NPLs through a facile and fast method (experiment section) and a mixture of perovskite NCs and NPLs is obtained (FIG S1). The perovskite NPLs are separated from the spherical NCs with several nanometer diameter through ultrasound treatment and centrifugation. The monolayer perovskite NPLs with the layer number n = 1 are depicted in Figure 1b. The lateral size of monolayer perovskite NPLs is nm, and the shape of the NPLs is rectangle with sharper corners (FIG S2a and S2b). The rounded corners are observed from TEM images (FIG S2c and S2d) of perovskite NPLs with MABr:HABr = 2:8. Figure 1c shows the bulk perovskite (3D structure with n = ) with lateral size nm. FIG 1. (a) Schematic representations of perovskite NPLs with different layers and the gradual change from monolyer NPLs to 3D bulk counterparts through regulating the molar ratio of MABr and HABr. (b) TEM image of monolayer perovskite NPLs (n = 1). The inset shows a single NPLs. (c) SEM image of 3D (n = ) perovskite counterparts. We measure the thickness of the NPLs by atomic force microscope (AFM). The AFM image (FIG S3a) demonstrates the thickness of the sample with MABr:HABr = 0:10. It is noted that several steps are shown in FIG 4

5 S3a and the smallest distance between adjacent steps is about 3 5 nm, which suggests these NPLs tend to stack together. We utilize centrifugation and sonic treatment to separate the NPLs from each other, however, these approaches don t work and further optimization of the synthesis procedure is needed to reduce the thickness dispersion in the sample. Therefore, we use the absorption and PL peaks from previous reports in Table 1 to determine the number of layers. 30, The thickness is estimated by the model reported by Tisdale et al. 42 For example, according to absorption peaks form Table 1, the monolayer perovskite NPLs have a sharp absorption peak at about 394 nm. Therefore, we determinate the NPLs with absorption peak in 390 nm in Figure 1b have just one layer. B. Optical Properties of Perovskite NPLs. The absorption spectra (FIG 2a) of perovskite NPLs can be tuned from 390 nm to 528 nm by manipulating the molar ratio of MABr and HABr. This large blue shift (138 nm) is attributed to quantum confinement effect, which will be discussed later. For the perovskite NPLs with MABr:HABr = 0:10, there is only a sharp absorption peak at 390 nm, indicating a pure monolayer perovskite NPLs. With the increasing ratio of MABr and HABr, the absorption spectrum shows a small but discernible peaks at 435 nm, 453 nm, 473 nm, corresponding to perovskite NPLs with n = 2, 3, 4 respectively. The absorption amplitude of n = 2, 3, 4 perovskite NPLs is about 8% of that in n = 1 NPLs, demonstrating these n = 2, 3, 4 NPLs constitute a small fraction of the sample. For MABr:HABr = 3:7, the absorption amplitude of n = 2 NPLs is strongest, illustrating n = 2 NPLs dominate in quantity in the product. Besides, it is noted that the absorption amplitude of n = 1 NPLs begins to reduce. At higher ratio, the absorption peaks of n = 3, 4 NPLs become obvious and the trend towards decreasing in absorption amplitude of n = 2 NPLs appears. With MABr:HABr = 4:6, n = 3 NPLs show a clear peak at 453 nm, in contrast, the signal originated from n = 1, 2 NPLs disappear. At MABr:HABr = 10:0, an absorption peak shows at 528 nm, which is attributed to the n = bulk perovskite counterparts. With the increasing of perovskite NPLs thickness, the absorption peaks change gradually from obvious sharp to less distinct, which is similar to that in the colloidal quantum dots (QDs) system including CdSe and CdTe QDs. 46 The perovskite bulk crystals usually exhibit the step like bandbap feature without excitonic transition at room temperature. The obvious extionic absorption peaks in the monolayer and few-layer perovskite NPLs indicate the strong quantum confinement effect and the large exciton binding energy. 47 5

6 Table 1. Summary of absorption and PL peaks for bromide perovskite NPLs with different n values. n (CH 3 NH 3 )(C 6 H 5 C 2 H 4 NH 3 ) 2 Pb 2 Br 7 in Ref [41] MAPbBr 3 in Ref [30] λ Abs (nm) L 2 PbBr 4 in Ref [42] CsPbBr 3 in Ref [43] NH 3 (CH 2 ) 12 NH 3 PbBr 4 in Ref [44] MAPbBr 3 in Ref [45] In present work Ref [41] λ PL (nm) Ref [30] Ref [42] Ref [43] Ref [44] 404 Ref [45] In present work λ Ex = 300 nm λ Ex = 750 nm The PL spectra shift from green (530 nm) to deep purple (396 nm) for the quantum confinement effect. A sharp PL peak at 397 nm is originated from n = 1 perovskite NPLs with MABr:HABr = 0:10. The full-width of halfmaximum (FWHM) of the emission is 14.8 nm, indicating the high purity of the product. At higher ratio, multiple PL peaks appear and they have a close agreement with that of the layered perovskites in Table 1. For example, at MABr:HABr = 6:4, the PL peaks located at 456 nm, 476 nm and 489 nm for n = 2, 3, 4 perovskite NPLs show in FIG 2b, suggesting a mixture of perovskite NPLs with different n values. It is noted that a shoulder at about 520 nm in cyan line is shown in FIG 2b. It might be originated from the perovskite NPLs with n > 6. We further observe the NPLs tend to stack together (FIG S2) and show a tendency to self-assemble into the pyramid structure and flat NPLs (FIG S3). 24,43 We further investigate two-photon PL (FIG 2c) from perovskite NPLs under the excitation wavelength of 750 nm. The peaks at 403 nm, 447 nm, 457 nm, 478 nm, 519 nm, 531 nm correspond to the NPLs with the molar ratio of MABr:HABr = 0:10, 2:8, 3:7, 4:6, 6:4, and 10:0, respectively In contrast to the PL spectra excited by one-photon, a slight red shift is observed for the two-photon PL excited by 750 nm, which is attributed to size inhomogeneity and 6

7 reabsorption effect To further confirm the two-photon PL in these perovskite NPLs, the excitation intensity dependent PL measurements are shown in FIG 2d and FIG S4. The slope k = logiemission/ logiexcitation is 1.82 confirms the two-photon PL (FIG 2d inset). FIG 2. (a) Absorption and (b) one-photon PL and (c) two-photon PL spectra of perovskite NPLs with different molar ratio of MABr and HABr. (d) Pump intensity dependent PL spectra from monolayer perovskite NPLs under excitation wavelength of 750 nm. The inset shows the slope. C. Quantum Confinement Effect. As shown in FIG 2a and 2b, the absorption and PL spectra of perovskite NPLs are blue-shifted compared to the 3D perovskite counterparts due to the quantum confinement effect. Thickness tuning in perovskite NPLs can be achieved by regulating the molar ratio of MABr and HABr, which leads to a one-dimensional quantum confinement effect in the thickness direction. 7

8 We have calculated the thickness-dependent bandgap of perovskite NPLs using the effective mass approximation (EMA). The estimated effective Bohr radius for MAPbBr 3 is 4.7 nm (Equation 1 in Supporting Information,) by using the values of reduced effective mass ( ) and effective dielectric constant ( eff ) given by Galkowski et al. 50 The dependence of the bandgap on the thickness is depicted in FIG 3. The calculated values (black line) follow the trend of experimental values (red dots). The reports from Kovalenko et al. and Brabec et al. can provide the support for our results. 18,51 FIG 3. Theory calculation of perovskite NPLs as a function of plate thickness (black line) and experimental results (red dot). The green line denotes the bandgap of 3D perovskite counterparts. D. Exciton Binding Energy. We use linear absorption and TP-PLE to measure the exciton binding energy of monolayer perovskite NPLs. The linear absorption (FIG 4a) of monolayer perovskite NPLs demonstrates the lowest energy 1s state, 28,52 53 which is identified as E 1s = ev. The p states and the bandgap energy can be probed by TP-PLE In FIG 4b, the measured TP-PLE experimental data are plotted as scattered black dots and the fitted line (solid green line). The TP- PLE spectra can be described by the sum of a Gaussian function (dotted red line) of p states, and a linear function 8

9 with a step (dotted blue line) at the bandgap energy (E g = ev). The exciton binding energy of monolayer perovskite NPLs is calculated by E g - E 1s = ev. FIG 4. (a) Linear absorption spectrum and (b) two-photon excitation spectrum measured on monolayer perovskite NPLs at room temperature. The experimental results (black dots) of two-photon PL excitation spectroscopy are fitted by green solid line, which can be described by the sum of a Gaussian function (dotted red line) and a linear function with a step (dotted blue line). In contrast to the 3D counterparts, the excitons are tightly confined in the 2D materials due to the reduced dielectric screening of Coulomb interactions. 24 A large exciton binding energy (0.5 ev 1 ev) in monolayer 2D materials (e.g. MoS 2 ) have been reported by theoretical works Zheng et al. measure the exciton binding energy of perovskite nanoparticles (0.32 ± 0.10 ev) via X-ray spectroscopies, which is 5 times higher than the value of bulk crystals (0.084 ± ev). 54 Blancon et al. report the the exciton binding energy of n = 1 layered 2D perovskites is 380 mev. 55 The exciton binding energy measured in this work is comparable with that reported in the highly sizeconfined nanostructures and is far larger than tens of mev in 3D lead halide perovskites. IV. CONCLUSIONS In summary, we report the preparation of monolayer (n = 1), few-layer (n = 2 5) perovskite NPLs and 3D (n = ) bulk perovskites by tuning the reactants molar ratio of methylammonium bromide (MABr) and hexadecammonium bromide (HABr). The absorption spectra can be tuned from 390 nm to 528 nm due to the quantum confinement effect. Two-photon PL from perovskite NPLs is investigated and we further measure the exciton binding energy of monolayer perovskite (n = 1) NPLs using linear absorption and TP-PLE. The exciton binding energy of monolayer NPLs is about 218 mev, which is far larger than tens of mev in 3D counterparts. 9

10 AUTHOR INFORMATION Corresponding Authors a) a) ORCID Qu-Quan Wang: Li Zhou: Notes The authors declare no competing financial interest. SUPPLEMENTARY MATERIAL See supplementary material for the TEM and AFM images, the pump-dependent PL spectra, the theory calculation details, and the table of ratio of precursors. ACKNOWLEDGMENT This study was supported by the National Key Research and Development Program of China (2017YFA ), NSFC ( and ), and the large-scale instrument and equipment sharing foundation of Wuhan University. REFERENCES 1 J. H. Im, I. H. Jang, N. Pellet, M. Grätzel and N. G. Park, Nature Nanotech. 9(11), 927 (2014). 2 X. Li, D. Bi, C. Yi, J. Décoppet, J. Luo, S. M. Zakeeruddin, A. Hagfeldt and M. Grätzel, Science 353(6249), 58 (2016) 10

11 3 L. M. Pazos-Outón, M. Szumilo, R. Lamboll, J. M. Richter, M. Crespo-Quesada, M. Abdi-Jalebi, H. J. Besson, M. Vrućinić, M. Alsar, H. J. Snaith, B. Ehrler, R. H. Friend and F. Deschler, Science. 351, 1430 (2016). 4 T. M. Kohn, K. Fu, Y. Fang, S. Chen, T. C. Sum, N. Mathews, S. G. Mhaisalkar, P. P. Boix and T. Baikie, J. Phys. Chem. C 118(30), (2013) 5 T. A. Berhe, W. Su, C. Chen, C. Pan, J. Cheng, H. Chen, M. Tsai, L. Chen, A. A. Dubale and B. Hwang, Energy Environ. Sci. 9, 323 (2016). 6 M. Manjappa, Y. K. Srivastava, A. Solanki, A. Kumar, T. Chien and R. Singh, Adv. Mater. 29, (2017). 7 Z. Tan, R. S. Moghaddam, M. L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A. Sadhanala, L. M. Pazos, D. Credgington, F. Hanusch, T. Bein, H. J. Snaith and R. H. Friend, Nature Nanotech. 9, 687 (2014). 8 Y. Ling, Z. Yuan, Y. Tian, X. Wang, J. C. Wang, Y. Xin, K. Hanson, B. Ma and H. Gao, Adv. Mater. 28, 305 (2016). 9 Y. Ling, T. Yu, X. Wang, J. C. Wang, J. M. Knox, F. Perez-Orive, Y. Du, L. Tan, K. Hanson, B. Ma and H. Gao, Adv. Mater. 28, 8983 (2016). 10 D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenbeger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent and O. M. Bakr, Science 347, 519 (2015). 11 A. A. Zhumekenov, M. I. Saidaminov, M. A. Haque, E. Alarousu, S. P. Sarmah, B. Murali, I. Dursun, X. Miao, A. L. Abdelhady, T. Wu, O. F. Mohammed and O. M. Bakr, ACS Energy Lett. 1, 32 (2016). 12 F. X. Xie, H. Su, J. Mao, K. S. Wong and W. C. H. Choy, J. Phys. Chem. C 120, (2016). 13 Q. A. Akkerman, V. D Innocenzo, S. Accornero, A. Scarpellini, A. Petrozza, M. Parto and L. Mana, J. Am. Chem. Soc. 137, (2015) 14 D. P. McMeekin, G. Sadoughi, W. Rehman, G. E. Eperon, M. Saliba, M. T. Hörantner,; A. Haghighirad, N. Sakai, L. Korte, B. Rech, M. B. Johnston, L. M. Herz and H. J. Snaith, Science 351, 151 (2016). 11

12 15 D. M. Jang,; K. Park, D. H. Kim, J. Park, F. Shojaei, H. S. Kang, J. Ahn, J. W. Lee and J. K. Song,. Nano Lett. 15, 5191 (2015). 16 A. Mancini, P. Quadrelli, C. Milanese, M. Patrini, G. Guizzetti and L. Malavasi, Inorg. Chem. 54, 8893 (2015). 17 G. Wang, J. Wei and Y. Peng, AIP ADV. 6, (2016). 18 L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. KRIEG, R. Caputo, C. H. Hendon, R. X. Yang, A. Walsh, and M. V. Kovalenko, Nano Lett. 15, 3692 (2015). 19 F. Zhu, L. Men, Y. Guo, Q. Zhu, U. Bhattacharjee, P. M. Goodwin, J. W. Petrich, E. A. Smith and V. Javier, ACS Nano 9, 2948 (2015). 20 X, Li, F, Cao, D, Yu, J, Chen, Z, Sun, Y, Shen, Y, Zhu, L, Wang, Y, Wei, Y, Wu, H, Zeng, Small 13, (2017). 21 Y. Hassan, Y. Song, R. D. Pensack, A. I. Abdelrahman, Y. Kobayashi, M. A. Winnik amd G. D. Scholes, Adv. Mater. 28, 566 (2016). 22 T. M. Koh, V. Shanmugam, J. Schlipf, L. Oesinghaus, P. Müller-Buschbaum, N. Ramakrishnan, V. Swamy, N. Mathews, P. P. Boix and S. G. Mhaisalkar, Adv. Mater. 28, 3653 (2016). 23 S. Sun, D. Yuan, Y. Xu, A. Wang and Z. Deng, ACS Nano 10, 3648 (2016). 24 L. Dou, A. B. Wong, Y. Yu, M. Lai, N. Kornienko, S. W. Eaton, A. Fu, C. G. Bischak, J. Ma, T. Ding, N. S. Ginsberg, L. Wang, A. P. Alivisatos and P. Yang, Science 349, 1518 (2015). 25 L. Niu, Q.Zeng, J. Shi, C. Gong, C. Wu, F. Liu, J. Zhou, W. Fu, Q. Fu, C. Jin, T. Yu, X. Liu and Z. Liu, Adv. Funct. Mater. 26, 5263 (2016). 26 V. A. Hintermayr, A. F. Richter, F. Ehrat, M. Döblinger, W. Vanderlinden, J. A. Sichert, Y. Tong, L. Polavarapu, J. Feldmann,; A. S. Urban, Adv. Mater. 28, 9478 (2016). 27 Y. Tong, F.Ehrat, W. Vanderlinden, C. Cardenas-Daw, J. K. Stolarczyk, L. Polavarapu, A. S. Urban, ACS Nano 10, (2016). 12

13 28 A. Chernikov, T. C. Berkelbach, H. M. Hill, A. Rigosi, Y. Li, O. B. Aslan, D. R. Reichman, M. S. Hybertsen and T. F. Heinz, Phys. Rev. Lett. 113, (2014). 29 J. A. Sichert,; Y. Tong,; N. Mutz, M. Vollmer, S. Fischer, K. Z. Milowska,; R. G. Cortadella,; B. Nickel,; C. Cardenas-Daw,; J. K. Stolarczyk, A. S. Urban and J. Feldmanm, Nano Lett. 15, 6521 (2015). 30 P. Tyagi, S. M. Arveson and W. A. Tisdale, J. Phys. Chem. Lett. 6, 1911 (2015). 31 Z. Yuan, Y. Shu, Y. Xin, B. Ma, Chem. Commun. 52, 3887 (2016). 32 Y. Wang, X. Li, V. Nalla, H. Zeng and H. Sun, Adv. Funct. Mater. 10, 1002 (2017). 33 M. Luisa De Giorgi, A. Perulli, N. Yantara, P. P. Boix and M. Anni, J. Phys. Chem. C 121(27), (2017). 34 E. Lafalce, C. Zhang, Y. Zhai, D. Sun and Z. V. Vardeny, J. APPL. PHYS. 120, (2016). 35 Y. Fu, H. Zhu, A. W. Schrader, D. Liang, Q. Ding, P. Joshi, L. Hwang, X. Zhu and S. Jin, Nano Lett. 16, 1000 (2016). 36 J. Xing, X. F. Liu, Q. Zhang, S. T. Ha, Y. W. Yuan, C. Shen, T. C. Sum and Q. Xiong, Nano Lett. 15, 4571(2015). 37 G. Xing, N. Mathews, S. S. Lim, N. Yantara, X. Liu, D. Sabba, M. Grätzel, S. Mhaisalkar and T. C. Sum, Nat. Mater. 13, 476 (2014) Q. Liao, K. Hu, H. Zhang, X. Wang, J. Yao, H. Fu, Adv. Mater. 27, 3405 (2015). G. Walters, B. R. Sutherland, S. Hoogland, D.Shi, R. Comin, D. P. Sellan, O. M. Bakr and E. H. Sargent, ACS Nano 9, 9340 (2015). 40 R. Zhang, J. Fan,; X. Zhang, H. Yu, H. Zhang, Y. Mai, T. Xu, J. Wang and H. J. Snaith, ACS Photonics 3, 371 (2016). 41 G. C. Papavassiliou and I. B. Koutselas, Synthetic Met. 71, 1713 (1995). 42 M. C. Weidman, M. Seitz, S. D. Stranks and W. A. Tisdale, ACS Nano 10, 7830 (2016). 43 Y. Bekenstein, B. A. Koscher, S. W. Eaton, P. Yang and A. P. Alivisatos, J. Am. Chem. Soc. 137, (2015). 44 Y. Takeoka, M. Fukasawa, T. Matsui, K. Kikuchi, M. Rikukawa and K. Sanui, Chem. Commun. 3, 378 (2005). 45 S. Bhaumik, S. A. Veldhuis, Y. F. Ng, M. Li, S. K. Muduli, T. C. Sum, B. Damodara, S. Mhaisalkar and N. Mathews, Chem. Commun. 52, 7118 (2016). 13

14 46 47 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc. 115, 8706 (1993). Y. H. Qiu, F. Nan, Q. Wang, X. D. Liu, S. J. Ding, Z. H. Hao, L. Zhou and Q. Q. Wang, J. Phys. Chem. C 121, 6916 (2017). 48 G. S. He, K. T. Yong, Q. D. Zheng, Y. Sahoo, A. Baev, A. Ryasnyanskiy, P. N. Prasas, Opt. Express 15 (20), (2007). 49 Y. Wang, X. Li, X. Zhao, L. Xiao, H. Zeng and H. Sun, Nano Lett. 16, 448 (2016). 50 K. Galkowski, A. Mitioglu, A. Miyata, P. Plochocka, O. Protugall, G. E. Eperon, J. T. Wang, T. Stergiopoulos, S. D. Stranks, H. J. Snaith and R. J. Nicholas, Energy Environ. Sci. 9, 962 (2016). 51 L. Levchuk, A. Osvet, X. Tang, M, Brandl, J. D. Perea, F. Hoegl, G. J. Matt, R. Hock, M. Batenschuk, C. J. Brabec, Nano Lett. 17, 2765 (2017). 52 K. He, N. Kumar, L. Zhao, Z. Wang, K. F. Mak, H. Zhao and J. Shan, Phys. Rev. Lett. 113, (2014). 53 F. Wang, G. Dukovic, L. E. Brus and T. F. Heinz, Science 308, 838 (2005). 54 K. Zheng, Q. Zhu, M. Abdellah, M. E. Messing, W. Zhang, A. Generalov, Y. Niu, L. Ribaud, S. E. Canton and T. Pullerts, J. Phys. Chem. Lett. 6, 2969 (2015). 55 J. C. Blancon, H. Tsai, W. Nie, C. C. Stoumpos, L. Pedesseau, C. Katan, M. Kepenekian, C. M. M. Soe, K. Appavoo, M. Y. Sfeir, S. Tretiak, P. M. Ajayan, M. G. Kanatzidis, J. Even, J. J. Crochet and A. D. Mohite, Science 10, 1126 (2017). 14

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