Amino Acid-Mediated Synthesis of CsPbBr3 Perovskite Nanoplatelets


Sep 13, 2018 - ... of Suzhou, College of Chemistry, Chemical Engineering and Materials Science, Soochow University , Suzhou 215123 , P. R. China. Chem...
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Amino Acid-Mediated Synthesis of CsPbBr3 Perovskite Nanoplatelets with Tunable Thickness and Optical Properties Jinyuan Zhao, Sunan Cao, Zhi Li, and Nan Ma Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02396 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Amino Acid-Mediated Synthesis of CsPbBr3 Perovskite Nanoplatelets with Tunable Thickness and Optical Properties Jinyuan Zhao, Sunan Cao, Zhi Li, Nan Ma* The Key Lab of Health Chemistry and Molecular Diagnosis of Suzhou, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, P. R. China

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ABSTRACT Metal halide perovskites (MHPs) have recently attracted considerable attentions due to their remarkable optoelectronic properties. Colloidal synthesis of MHPs provides a versatile route to precisely control their dimensionality and the related optoelectronic properties. Herein, we report on amino acid (Trp)-mediated synthesis of quasi-two-dimensional CsPbBr3 perovskite nanoplatelets (NPLs) with well-controlled and tunable thickness. The number of layers (n) could be finely tuned from 4 to 12 by simply adjusting Trp concentration, leading to gradual red shift of their photoluminescence (PL). This allows us to directly correlate their thickness with the band gap energy in the quantum confinement regime, which is in good agreement with theoretical model. We found that Trp is the only hydrophobic amino acid to promote the formation of 2D CsPbBr3 NPLs. The use of Trp derivatives for CsPbBr3 nanocrystal synthesis indicates that the carboxyl group, amine group, and indole ring of Trp act synergistically on CsPbBr3 NPLs formation.

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INTRODUCTION MHPs have exhibited great potential for solar cells, light-emitting diodes, photodetectors, and lasers because of their superior photovoltaic and optoelectronic properties.1-9 The all-inorganic CsPbX3 (X = Cl, Br, I) nanocrystals represents an important type of MHPs with improved air stability, high photoluminescence efficiency, tunable optical properties, and narrow emission line widths.10 The optoelectronic properties of these nanocrystals are dictated by the quantum confinement effect, and precise dimensional control provides an inherent route to investigate the relationship between the nanocrystal sizes and their optical properties.11 In this context, hotinjection and reprecipitation approaches have been developed for the synthesis of high-quality perovskite nanocrystals with controlled morphology and tunable emission properties.10-13 However, facile preparation of quasi-two-dimensional (2D) CsPbX3 nanocrystals with fine thickness control remains challenging and less accomplished. 2D CsPbX3 NPLs have been previously synthesized via conventional hot-injection routes followed by size-selective precipitation.14 Acetone-induced reprecipitation produces CsPbX3 NPLs with relatively limited thickness tunability between 3 and 5 monolayers.15 Biomolecules are naturally occurring molecules with sophisticated chemical structures and wellpreserved biological functions. Intriguingly, many biomolecules could serve as excellent ligands for the synthesis of a variety of inorganic nanomaterials.16-19 For example, DNA and peptide molecules have been harnessed to mediate the growth of metal chalcogenide semiconductor nanocrystals and metallic nanoparticles with tunable morphology and properties.20-22 The synthesis is usually carried out under mild conditions in aqueous solution. Crucial nucleotidesand amino acids-derived functional groups have been identified to control nanoparticles growth and modulate their properties.18,23,24 Despite these successes, biomolecule-directed synthesis of

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CsPbX3 perovskite nanocrystals has not been previously demonstrated. Because perovskite materials are unstable in aqueous solution, the synthesis needs to be conducted in non-polar solvents. This excludes hydrophilic biomolecules as ligands for CsPbX3 nanocrystal synthesis. The use of hydrophobic biomolecules to mediate nanomaterial synthesis in non-polar solvents still remains an unexplored area.

RESULTS AND DISCUSSION Herein, we report tryptophan (Trp)-mediated synthesis of CsPbBr3 NPLs with well-controlled thickness that could be finely tuned by adjusting Trp concentration. We screened five amino acids (Trp, Cys, His, Leu, Phe) that are soluble in non-polar solvents for the synthesis of CsPbBr3 nanocrystals via a modified reprecipitation protocol. Briefly, PbBr2 and amino acids were dissolved in DMF containing oleic acid (OA) and oleylamine (OLA). This precursor solution was mixed with Cs-oleate in ODE and then quickly added into toluene under vigorous stirring. The rapid formation of CsPbBr3 nanocrystals was accompanied with appearance of strong photoluminescence (PL). In the absence of amino acids, CsPbBr3 nanocubes (mean size = 11.3 nm) were produced with an emission maximum of 514 nm (Figure 1a, 1b). Cys, His, and Leu exhibited minimal effects on the PL wavelength of the products (Figure 1c-1e). Phe promotes the formation of smaller CsPbBr3 nanoparticles (5.6 nm) along with a blue shift of the PL peak from 514 nm to 480 nm (Figure 1f and SI Figure S1). Interestingly, Trp induced pronounced shift of the PL peak in a Trp concentration-dependent manner. As shown in Figure 2, the lowest Trp/Pb molar ratio (0.05) induced the largest blue shift of PL peak from 514 nm to 467 nm. A shoulder at 495 nm indicates the presence of a second product in different

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morphology. A gradual red shift of PL was observed with increasing Trp/Pb molar ratio from 0.05 to 0.6 (Pb2+ concentration was kept constant) (Figure 2a-2c). The red shift terminated at 520 nm when Trp/Pb molar ratio reached 0.7 (Figure 2c). Meanwhile, the exciton peak in the absorption spectra also exhibited gradual red shift with increasing Trp/Pb molar ratio (Figure 2d). This indicates that higher concentration of Trp leads to products with smaller band gap energy. The quantum yield (QY) of the prepared materials gradually increases from 33% to 94% as the Trp/Pb molar ratio increases from 0.1 to 0.6 (SI Table S1). The QY values are comparable to other studies (SI Table S2).13 The full width at half maximum (FWHM) of the ensemble spectra of prepared materials is typically 4-5 nm broader than that of the single particle spectra25 (SI Table S3), which could be attributed to small inhomogeneity of the product thickness. When stored in dark, the PL of prepared CsPbBr3 nanocrystals was retained up to 3 days and then gradually decreased after 7 days (SI Figure S3a). When continuously illuminated with UV light (405 nm), more than 80% PL intensity was retained after 24 hours illumination (SI Figure S3b). The products were subsequently characterized by energy-dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD), and transmission electron microscopy (TEM). EDX spectrum confirms the elemental composition of Cs, Pb, and Br (SI Figure S3a). XRD pattern of the products synthesized under various concentrations of Trp were in agreement with cubic phase (Pm3m space group) of CsPbBr3 (SI Figure S3b-S3d). The single peak at ~30° indicates that the nanocrystal is not orthorhombic phase.26 TEM images show gradual evolution of CsPbBr3 nanocrystals from thin NPLs to thick NPLs (Trp/Pb molar ratio from 0.05 to 0.5) and finally to nanocubes (Trp/Pb molar ratio of 0.6) with increasing Trp/Pb molar ratio (Figure 3a). The prepared CsPbBr3 NPLs tend to stack on their sides and oriented perpendicularly with respect to the carbon film, which allows direct measurement of their thickness for relatively thick NPLs

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(Trp/Pb ≥ 0.27). Similar self-assembly of perovskite NPLs was previously observed in other studies.14,15 The average thickness of NPLs synthesized with Trp/Pb molar ratio of 0.27, 0.4, 0.5 were measured to be 2.58 nm, 3.71 nm, and 4.71 nm according to TEM images, which are ascribed to 4, 6, and 8 layers respectively (the thickness of one unit cell is ~0.6 nm) (Figure 3b). Representative high-resolution TEM images of CsPbBr3 NPL and nanocube were shown in Figure 3c. Lattice fringes confirm the formation of crystalline materials. d spacing of 0.29 nm, 0.41 nm, and 0.58 nm correspond to the (200), (110), and (001) facets of the cubic phase CsPbBr3 respectively. The surface of prepared CsPbBr3 nanocrystals was investigated with X-ray photoelectron spectroscopy (XPS). The overview spectra of CsPbBr3 NPLs reveal the presence of Pb, Br, O, N, C on the surface of the crystal (SI Figure S4). The core level spectra of N 1s for CsPbBr3 NPLs contain two peaks at binding energy 400.4 eV and 402.4 eV, respectively. The peak at 400.4 eV is assigned to the nitrogen in the indole ring and the other peak at 402.4 eV is assigned to the nitrogen in the amine group (Figure 4). These results indicate the presence of Trp on the surface of the crystal. The fitted core level spectra of O 1s contain two peaks at 532.3 eV and 533.3 eV, respectively (SI Figure S5). The peak at 532.3 eV is assigned to two nonequivalent O atoms of carboxylic acid; the peak at 533.3 eV is assigned to two chemically equivalent O atoms from carboxylate species of deprotonated oleic acid. These results indicate the presence of OA on the surface of the crystal. A theoretical model was used to predict the band gap energy (Etheo) of CsPbBr3 NPLs with different number of layers.27 The experimental band gap energy (Eexp) is derived from the emission wavelength (λem) and the number of layers of each product is determined as described above. As shown in Figure 5 and Table 1, there is a good agreement between Etheo and Eexp for the products with n between 4 and 12. The Bohr radius of CsPbBr3 nanocrystal is 3.6 nm, and it is

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expected that CsPbBr3 NPLs exhibit strong quantum confinement within thickness of 7.2 nm (12 layers). Next, we proceeded to investigate the role of Trp in CsPbBr3 NPL formation. In the absence of Trp, varying the precursor concentrations of PbBr2 and Cs-oleate has little effect on the λem of the product (SI Figure S6). However, pronounced blue shift of λem was observed in the presence of Trp (Trp/Pb = 0.2 and 0.4). Moreover, varying the OA and OLA concentrations does not affect λem in the absence of Trp (SI Figure S7). These results indicate that Trp play a determinative role in mediating CsPbBr3 NPL formation. XPS measurements indicate that the percentage of bound Trp increases with higher Trp/Pb molar ratio (SI Figure S8). Interestingly, we observed that Trp could significantly slower the growth kinetics of CsPbBr3 nanocrystals. In contrast to instantaneous formation of CsPbBr3 nanocrystals in the absence of Trp, Trp-mediated formation of CsPbBr3 NPLs and nanocubes usually takes minutes to complete. This allows us to monitor the intermediate during synthesis. For example, we observed the formation of small nuclei and subsequent 2D growth of CsPbBr3 NPLs at Trp/Pb molar ratio of 0.4 (SI Figure S9). This indicates that Trp play a crucial role in breaking the inherent cubic symmetry of CsPbBr3 nanocrystal and dictate its 2D growth. We observed that Trp could be dissolved in DMF only in the presence of PbBr2, which is indicative of complex formation between Trp and Pb2+.28 It has been previously reported that pyridine could direct the growth of 2D MAPbBr3 nanostructures.29 The nitrogen in the aromatic ring of pyridine forms a bond with Pb through the pyridine lone pair to the under-coordinated Pb cation, which slows down the vertical growth rate and leads to the formation of 2D nanostructures.29 Likewise, in our study the nitrogen in the aromatic ring of indole could coordinate with Pb ion on the surface of the crystal, which slows down the growth rate and promotes 2D growth of CsPbBr3 nanocrystals.

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The amine group, carboxyl group, and nitrogen atom in the indole ring of Trp are three possible sites to interact with the surface of the nanocrystal and affect the growth of CsPbBr3 nanocrystal. To further deconvolute the role of each functional group in nanocrystal synthesis, four Trp derivatives – 1-methyl-tryptophan (A), N-acetyl-tryptophan (B), tryptophan ethyl ester (C), and N-acetyl-tryptophan ethyl ester (D) were used in place of Trp for the synthesis (Figure 6a). Only compound A induced the formation of CsPbBr3 NPLs in a concentration-dependent manner similar to natural Trp (Figure 6b). The relatively lower solubility of A limits the PL tunability of the products. Conversely, B induced blue shift of PL with increasing B concentration (Figure 4b). TEM image shows the formation of small nanoparticles rather than NPLs (Figure 6c). C induced formation of nanocubes along with blue shift of PL with increasing C concentration. We found that the nanocubes synthesized at various C concentrations possess similar size (SI Figure S10), which rules out the size effect on the PL shift. The blue shift of PL could be attributed to the anion exchange of Br- with Cl- of compound C in hydrochloride form. D did not induce PL shift at various D concentrations. Taken together, we conclude that the amine and carboxyl group of Trp are also required for the growth of NPLs. Methylation of nitrogen atom in the indole ring does not directly affect the formation of NPLs, suggesting that the methylation would not perturb the coordination of indole ring with Pb2+ ion which is crucial for 2D growth of CsPbBr3 nanocrystal. It is likely that the amine and carboxyl group play a cooperative role in stabilizing the binding of Trp with Pb2+ ion on crystal surface. On the basis of the above results, we propose a model for Trp-directed growth of CsPbBr3 NPLs (Figure 7). The nitrogen in the indole ring of Trp coordinate with Pb of [PbBr6]4- during nucleation. The binding of Trp with the growing nanocrystal surface inhibits its vertical growth, and the monomers selectively attach to the crystal along (001) plane to form 2D platelet.29-31 The

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binding of OA and OLA onto the surface of nanocrystal finally terminates the growth and stabilizes the final structures. CONCLUSIONS In summary, we demonstrated a new facile synthetic route of CsPbBr3 NPLs with wellcontrolled thickness using Trp as ligands. In particular, the thickness of CsPbBr3 NPLs could be finely tuned by simply adjusting Trp concentration, which allows us to directly correlate their dimensionality with the optical properties in quantum confinement regime. It opens up new possibilities to modulate the morphology and optoelectronic properties of perovskite nanocrystals using biomolecules as templates or scaffolds. Also, the reported method provides an efficient, environment-friendly, and scalable synthetic route of low-dimensional perovskite nanocrystals.

EXPERIMENTAL SECTION Materials. Cs2CO3 (Aladdin, 99.9%), PbBr2 (Aladdin, 99.0%), octadecene (ODE, Sigma-Aldrich, 90%), oleic acid (OA, Sigma-Aldrich, 90%), oleylamine (OLA, Acros, 80%–90%), N,Ndimethylformamide (DMF, Aladdin, 99.9%), cysteine (Cys, TCI, 98%), histidine (His, Aladdin, 99%), leucine (Leu, Aladdin, 99%), phenylalanine (Phe, Aladdin, 99%), tryptophan (Aladdin, 99%), 1-methyl-tryptophan (Sigma-Aldrich, 95%), N-acetyl-tryptophan (Aladdin, 98%), tryptophan ethyl ester hydrochloride (TCI, 95%), N-acetyl-tryptophan ethyl ester (Aladdin,

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98%), and toluene (Shanghai Lingfeng Chemical Reagent Co. Ltd., ≥ 99.5%) were purchased and used without further purification. Preparation of Cs−Oleate. Cs2CO3 (0.407 g) was loaded into a 50 mL, 3-neck round-bottom flask along with octadecene (10 mL) and oleic acid (1.25 mL), the mixture was degassed under argon flow at 120 °C for 1 hour. Then the reaction was kept at 150 °C under an argon atmosphere for another 30 min until all the Cs2CO3 reacted with OA. After cooling to room temperature, the Cs-Oleate was transferred to a glovebox. Cs-oleate needs to be preheated before use because it will precipitate out of ODE at room temperature. Synthesis of CsPbBr3 perovskite nanocrystals with Trp, Cys, His, Leu, Phe, and Trp derivatives. In a typical experiment, 2 mL DMF and 0.1 mmol PbBr2 were loaded into a 5 mL vial, followed by the addition of different amount of Trp (0.005 mmol to 0.08 mmol). Then, 200 µL OA and 100 µL OLA were directly injected into the precursor solution. Subsequently, 200 µL Cs-Oleate (0.22 mol/L in ODE) was added after complete dissolution of PbBr2 and Trp. Finally, 50 µL of the mixed precursor solution was rapidly added into a new vial containing 2 mL toluene under vigorous stirring. Bright emitting NCs were formed after different reaction time. The reaction proceeded slower as the amount of Trp increased, varying from a few seconds to several minutes. The products were centrifuged at 8000 rpm for 5 min, and the supernatant was collected and stored in a glass bottle. Other samples were synthesized by replacing Trp with other amino acids while keeping other conditions unchanged. Optical characterization.

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Photoluminescence spectra were recorded using a fiber fluorescence spectrophotometer (AvaSpecULS2048-USB2) equipped with a 405 nm laser (110 mW) as excitation light source. The integration time was set to 1 ms. Absorption spectra were recorded using a UV-Vis spectrophotometer (Agilent 8453). Quantum yield measurements. The quantum yields (QYs) of the NCs at room temperature were determined by comparing the integrated emission of the CsPbBr3 NC samples in solution with that of a fluorescence dye (fluorescein

sodium,

QY

=

92% Φ! = (

in

water)

according

to

the

equation

below:

𝑆! 𝜂! ! 𝐴!" )×( ! )×( )Φ!"   𝑆!" 𝜂!" 𝐴!

where Φ is the quantum yield, S is the measured integrated emission intensity, η is the refractive index of the solvent, and A is the optical density. The subscript “st” refers to standard with known quantum yield and “x” refers to the CsPbBr3 NCs. Fluorescence spectra were measured under 405 nm excitation. TEM Characterization. The sample was dropped onto a copper grid, and placed in an oven at 60 °C to quickly evaporate the toluene. TEM characterization was performed on a Tecnai G20 (FEI, United States) transmission electron microscope operated at 185 kV. HRTEM characterization and related elemental analysis were performed on a Tecnai F20 (FEI, United States) transmission electron microscope operated at 200 kV. XRD characterization.

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The sample was dropped onto clean glass plate and dried in an oven at 60 °C to quickly evaporate the solvent. This process was repeated several times to form a thin film on the glass plate. X-ray diffraction characterization of CsPbBr3 nanocrystalline films was performed under ambient condition using a Philips X’Pert-Pro MRD X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å). XPS characterization. The sample was drop casted onto clean silicon substrate and dried to form a film. XPS was recorded using a Thermo Scientific ESCALAB 250Xi instrument equipped with Al Ka radiation (hν = 1486.6 eV).

ASSOCIATED CONTENT Supporting Information. Additional TEM images; PLQYs; FWHM data; optical stability; EDX spectrum; XRD spectra; XPS spectra; Abs and PL spectra; theoretical model. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENT

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This work was supported in part by the NSFC (21475093, 21522506), the National High-Tech R&D Program (2014AA020518), 1000-Young Talents Plan, PAPD, and startup funds from Soochow University. REFERENCES (1) Stranks, S. D.; Snaith, H. J. Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices. Nat. Nanotechnol. 2015, 10, 391-402. (2) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O. M. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519-522. (3) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as VisibleLight Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (4) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643647. (5) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. (6) Tan, H.; Jain, A.; Voznyy, O.; Lan, X.; García de Arquer, F. P.; Fan, J. Z.; QuinteroBermudez, R.; Yuan, M.; Zhang, B.; Zhao, Y.; Fan, F.; Li, P.; Quan, L. N.; Zhao, Y.; Lu, Z. H.; Yang, Z.; Hoogland, S.; Sargent, E. H. Efficient and Stable Solution-Processed Planar Perovskite Solar Cells via Contact Passivation. Science 2017, 355, 722-726. (7) Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao,

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(23) Hinds, S.; Taft, B. J.; Levina, L.; Sukhovatkin, V.; Dooley, C. J.; Roy, M. D.; MacNeil, D. D.; Sargent, E. H.; Kelley, S. O. Nucleotide-Directed Growth of Semiconductor Nanocrystals. J. Am. Chem. Soc. 2006, 128, 64-65. (24) He, X.; Zhao, Z.; Xiong, L. H.; Gao, P. F.; Peng, C.; Li, R. S.; Xiong, Y.; Li, Z.; Sung, H. H.; Williams, I. D.; Kwok, R. T. K.; Lam, J. W. Y.; Huang, C. Z.; Ma, N.; Tang, B. Z. Redox-Active AIEgen-Derived Plasmonic and Fluorescent [email protected] Nanoparticles for Multimodality Bioimaging. J. Am. Chem. Soc. 2018, 140, 6904-6911. (25) Cho, J.; Jin, H.; Sellers, D. G.; Watson, D. F.; Son, D. H.; Banerjee, S. Influence of Ligand Shell Ordering on Dimensional Confinement of Cesium Lead Bromide (CsPbBr3) Perovskite Nanoplatelets. J. Mater. Chem. C. 2017, 5, 8810-8818. (26) Zhang, D.; Eaton, S. W.; Yu, Y.; Dou, L.; Yang, P. Solution-Phase Synthesis of Cesium Lead Halide Perovskite Nanowires. J. Am. Chem. Soc. 2015, 137, 9230−9233. (27) Tong, Y.; Bladt, E.; Aygüler, M. F.; Manzi, A.; Milowska, K. Z.; Hintermayr, V. A.; Docampo, P.; Bals, S.; Urban, A. S.; Polavarapu, L.; Feldmann, J. Highly Luminescent Cesium Lead Halide Perovskite Nanocrystals with Tunable Composition and Thickness by Ultrasonication. Angew. Chem. Int. Ed. 2016, 55, 13887-13892. (28) Ma, L J.; Liu, Y. F.; Wu, Y. A Tryptophan-Containing Fluoroionophore Sensor with High Sensitivity to and Selectivity for Lead Ion in Water. Chem. Commun. 2006, 2702-2704. (29) Ahmed, G. H.; Yin, J.; Bose, R.; Sinatra, L.; Alarousu, E.; Yengel, E.; AlYami, N. M.; Saidaminov, M. I.; Zhang, Y.; Hedhili, M. N.; Bakr, O. M.; Bred́ as, J.-L.; Mohammed, O. F. Pyridine-Induced Dimensionality Change in Hybrid Perovskite Nanocrystals. Chem. Mater. 2017, 29, 4393-4400. (30) Teunis, M. B.; Johnson, M. A.; Muhoberac, B. B.; Seifert, S.; Sardar, R. Programmable

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Colloidal Approach to Hierarchical Structures of Methylammonium Lead Bromide Perovskite Nanocrystals with Bright Photoluminescent Properties. Chem. Mater. 2017, 29, 3526-3537. (31) Aharon S.; Etgar, L. Two Dimensional Organometal Halide Perovskite Nanorods with Tunable Optical Properties. Nano Lett. 2016, 16, 3230-3235.

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Figure 1. Characterization of CsPbBr3 nanocrystals synthesized under different conditions. (a) PL spectrum of CsPbBr3 nanocrystals synthesized in the absence of amino acids; (b) TEM image of CsPbBr3 nanocrystals synthesized in the absence of amino acids; (c-f) PL spectra of CsPbBr3 nanocrystals synthesized with Cys, His, Leu, and Phe respectively with various amino acid/Pb molar ratios.

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Figure 2. Optical characterization of CsPbBr3 nanocrystals prepared under different Trp/Pb molar ratios from 0 to 0.8. (a) Normalized PL spectra of as-prepared CsPbBr3 nanocrystals; (b) Photographs of CsPbBr3 nanocrystals solutions excited under UV light; (c) Relationship between the emission maxima and Trp/Pb molar ratio; (d) Absorption spectra of as-prepared CsPbBr3 nanocrystals.

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Figure 3. Nanocharacterization of CsPbBr3 nanocrystals prepared under different Trp/Pb molar ratios. (a) TEM images of as-prepared CsPbBr3 nanocrystals; (b) Statistics of the thickness and length of as-prepared CsPbBr3 nanocrystals; (c) HRTEM images of as-prepared CsPbBr3 nanocrystals.

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Figure 4. Fitted XPS core level spectra of N 1s for CsPbBr3 nanocrystals synthesized in the absence and presence of Trp (Trp/Pb = 0 and 0.4).

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Figure 5. Experimental and theoretical band gap energy as a function of the number of layers of as-prepared CsPbBr3 NPLs.

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Figure 6. Characterization of CsPbBr3 nanocrystals synthesized with four Trp derivatives (compound A – D). (a) Chemical structures of the four Trp derivatives; (b) Normalized PL spectra of CsPbBr3 nanocrystals synthesized with four Trp derivatives respectively under different concentrations; (c) TEM images of the as-prepared CsPbBr3 nanocrystals.

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Figure 7. Schematic illustration of the proposed growth mechanism for CsPbBr3 NPLs.

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Number of Layers (n)

λem (nm)

Eexp (ev)

Etheo (ev)

4

481

2.590

2.626

5

484

2.563

2.569

6

492

2.530

2.530

8

501

2.481

2.483

12

510

2.437

2.435

Table 1. Experimental and theoretical band gap energy of CsPbBr3 NPLs with different number of layers.

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TOC

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