One-Step Vapor-Phase Synthesis and Quantum-Confined Exciton in

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

One-Step Vapor-Phase Synthesis and Quantum-Confined Exciton in Single-Crystal Platelets of Hybrid Halide Perovskites Zhixiong Liu, Yunhai Li, Xinwei Guan, Yang Mi, Abdulrahman Al-Hussain, Son Tung Ha, Ming-Hui Chiu, Chun Ma, Moh R. Amer, Lain-Jong Li, Jie Liu, Qihua Xiong, Jinlan Wang, Xinfeng Liu, and Tom Wu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00777 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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The Journal of Physical Chemistry Letters

One-Step Vapor-Phase Synthesis and QuantumConfined Exciton in Single-Crystal Platelets of Hybrid Halide Perovskites Zhixiong Liu,† Yunhai Li,¶ Xinwei Guan,† Yang Mi,§ Abdulrahman Al-Hussain,┴ Son Tung Ha,║ Ming-Hui Chiu,† Chun Ma,† Moh R. Amer,┴ Lain-Jong Li,† Jie Liu,┬ Qihua Xiong,║ Jinlan Wang,*¶ Xinfeng Liu*§ and Tom Wu*†‡ † Physical Science and Engineering Division, King Abdullah University of Science & Technology, Thuwal 23955-6900, Saudi Arabia. ‡ School of Materials Science and Engineering, University of New South Wales (UNSW), Sydney, NSW 2052, Australia. ¶ School of Physics, Southeast University, Nanjing 211189, P. R. China. § Division of Nanophotonics, CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China ┴ Center of Excellence for Green Nanotechnologies, Joint Centers of Excellence Program, King Abdulaziz City for Science and Technology, P.O Box 6086, Riyadh 11442, Saudi Arabia ║ Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ┬ Department of Chemistry, Duke University, Durham, NC 27708, USA Corresponding Author E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

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ABSTRACT To investigate the quantum confinement effect on excitons in hybrid perovskites, single-crystal platelets of CH3NH3PbBr3 are grown on mica substrates using one-step chemical vapor deposition. Photoluminescence measurements reveal a monotonous blue shift with decreasing platelet thickness, which is accompanied by a significant increase of exciton binding energy from approximately 70 meV to 150 meV. Those phenomena can be attributed to the 1D quantum confinement effect in the 2D platelets. Furthermore, we develop an analytical model to quantitatively elucidate the 1D confinement effect in such quantum wells with asymmetric barriers. Our analysis indicates that the exciton Bohr radius of single-crystal CH3NH3PbBr3 is compressed from 4.0 nm for the thick (26.2 nm) platelets to 2.2 nm for the thin (5.9 nm) ones. The critical understanding of the 1D quantum confinement effect and the development of a general model to elucidate the exciton properties of asymmetric semiconductor quantum wells pave the way towards developing high-performance optoelectronic heterostructures. TOC GRAPHICS

KEYWORDS CH3NH3PbBr3 platelet, quantum confinement, photoluminescence, exciton Bohr radius 2 ACS Paragon Plus Environment

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The past few years have witnessed intensive research efforts on metal halide perovskites (e.g., CH3NH3PbX3, X=Cl, Br, I) due to their attractive optical and electrical properties such as large bandgap tunability,1-3 high light absorption,4-6 outstanding fluorescence yield,1-3 long carrier lifetime5,6 and high ambipolar mobility.7,8 Together with low-temperature solution processing capability, these key features of hybrid perovskite led to their promising applications in optoelectronic devices like solar cells,4 photodetectors,9-11 phototransistors,7,12 and light-emitting diodes and light-emitting transistors.13-14 Notably, most of the progress has been based on polycrystalline perovskite films, but single crystals are important to reveal the intrinsic physical properties. Perovskite single crystals in the size of several centimeters were synthesized by antisolvent vapor-assisted or top-seeded solution growth methods.15,16 However, their large size of bulk perovskite single crystals prevents their direct integration into device applications. Therefore, there have been emerging efforts on synthesizing perovskite nanomaterials with well-defined morphology, high crystallinity and tunable physical properties. High-quality synthesis is the foundation for achieving optimal and reliable physical properties in perovskite materials. Several approaches have been developed to synthesize perovskite nanomaterials including zero-dimensional (0D) quantum dots,17 one-dimensional (1D) nanowires2,3,18-20 and two-dimensional (2D) thin platelets.20-26 In prior works, thin platelets were usually synthesized through two-step vapor conversion method,20-23 where lead halide platelets were first grown by vapor transport or solution casting, which was then converted to perovskite structures using methylammonium halide vapor. These 2D hybrid perovskite platelets opened alternative routes beyond conventional inorganic nanostructures in achieving near-infrared solidstate lasing.22 However, the introduction of grain boundaries and defects is inevitable during the phase conversion process,3,22,23 which might bring negative effects for device performance. 3 ACS Paragon Plus Environment


Alternatively, thin halide perovskite platelets were synthesized in solution with the help of organic ligands,25-26 and a clear quantum confinement effect was observed.25 However, such solutionprocessed perovskite platelets are coated with organic ligands, which might compromise their transport properties in optoelectronic devices. Moreover, these colloidal platelets usually have very small lateral dimensions on the scale of hundreds of nm, which complicates device fabrication based on individual platelets. In early works, MAPbCl3 and CsPbBr3 single crystal platelets with microscale lateral sizes were prepared via a one-step CVD method,27,28 however the physical properties of these have not been explored. Unlike 0D quantum dots and 1D nanowires,17,29,30 the quantum confinement effect in ultrathin perovskite platelets grown from vapor phase has not been quantitatively elucidated. Here we report the van der Waals growth of MAPbBr3 platelets (001) on muscovite mica (KAl2(Si3AlO10)(OH)2) (001) facet via a one-step chemical vapor deposition method. Our photoluminescence measurements revealed that the optical band gap is blue shifted with a decreasing thickness of the perovskite platelets, which is accompanied by an increase of the exciton binding energy. An exciton Bohr radius (aB) in the range of 2.2 – 4.0 nm was estimated by comparing the experimental data with a quantum well model. Overall, the direct observation of growth in such thin single-crystal platelets enriches the understanding of the low-temperature vapor-phase growth of perovskites, and these platelets offer exciting opportunities to explore quantum confinement effect and other phenomena in hybrid nanomaterials. The hybrid organic-inorganic perovskite platelets were prepared via a chemical vapor deposition (CVD) method on freshly cleaved muscovite mica, as schematically illustrated in Figure 1a. More synthesis details can be found in Figure S1 (Supporting information) and Experimental section. Figure 1b shows a typical optical image of the square-shaped perovskite platelets grown on a mica 4 ACS Paragon Plus Environment

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substrate at a growth temperature of 320 °C and a pressure of 140 torr for 20 min. The lateral dimension of the platelets can reach 10 µm, and they appeared uniform with smooth surfaces. The atomic force microscopy (AFM) image in Figure 1c further confirms the high structural quality of these perovskite platelets. The height profile in the inset of Figure 1c suggests a thickness of 7.8 nm. The root-mean-square (RMS) roughness of the particular platelet in Figure 1c is around 0.2 nm, much smaller than the lattice constant of MAPbBr3 (5.92 Å),31 indicating atomic level flatness and a single-crystalline nature.

Figure 1. Synthesis and characterization of MAPbBr3 single crystal platelets. (a) Crystal structure of MAPbBr3 and schematic of the CVD setup. (b) Optical and (c) AFM images of a MAPbBr3 platelet. (d) XRD patterns of the MAPbBr3 platelet single crystals. (001) and (002) peaks of MAPbBr3 are labeled, while the rest peaks belong to the mica substrate. For comparison, the simulated XRD pattern is shown at the bottom. (e) Raman spectra of both a perovskite sample and a mica substrate; The insets (i) and (ii) are optical image and Raman mapping at 320 cm-1, respectively, collected on a perovskite platelet with a lateral size of 8.5 μm. (f) Absorption and



photoluminescence of the MAPbBr3 platelets, the inset is PL mapping of a platelet with a lateral size of 7 μm at 530 nm. To further demonstrate the platelet crystal structure and quality on mica substrates, X-ray diffraction (XRD) measurement was carried out. Figure 1d shows the XRD patterns of the perovskite platelet sample on a mica substrate. MAPbBr3 possesses a cubic structure with a lattice constant of 5.92 Å at room temperature.31 Only peaks from MAPbBr3 crystals and mica substrates were observed. The two peaks locating at 14.90° and 30.10° are assigned to the (001) and (002) crystallographic planes of MAPbBr3, respectively.2 The dominant (00l) peaks indicate wellorientated perovskite platelets grown on the (001) mica substrate. The XRD result indicates that our products are MAPbBr3 and the platelets prepared by this CVD method possess very high crystalline quality. Raman spectroscopy is a powerful tool to characterize materials with valuable insights into chemical bonding and lattice dynamics. The Raman spectrum of the MAPbBr3 platelets grown on mica is presented in Figure 1e. Unlike tetragonal MAPbI3, the lattice of MAPbBr3 perovskites (Pm 3m) has a higher symmetry and only high-frequency Raman activity is expected for MA+ cation according to the symmetry-based selection rules.32 Indeed, as shown in Figure 1e, only one Raman peak was observed at 324 cm-1 for MAPbBr3, which can be assigned to the halide motion.33 In addition, there appears to be a strong broad Raman feature below 200 cm-1 although we cannot determine its exact position due to instrument limitation. It is reported that such peaks are typical for crystals with cubic lattice, which have no Raman-active phonon modes.32 In addition, the Raman mapping was collected at around 320 cm-1 (inset of Figure 1e), and the square shape of the MAPbBr3 platelet corresponds well with the optical image, indicating the pure phase of the platelet crystal. 6 ACS Paragon Plus Environment

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Absorbance and photoluminescence (PL) spectra collected on the perovskite platelet sample are presented in Figure 1f. The MAPbBr3 platelets on mica displayed an absorption cutoff at 540 nm and a sharp excitonic absorption peak at 530 nm. The PL peak is located at 529 nm. In addition, the PL mapping result recorded at around the peak wavelength (inset of Figure 1f) confirmed that only the perovskite platelets illuminate on the mica substrate. The stronger intensity at the platelet corners may be caused by the whispering-gallery-mode cavity effect as reported previously.22

Figure 2. Temperature and pressure effects on the morphology of perovskite crystals. In the growth of these typical samples, the temperature was increased from 320 °C (a, b, c, d), to 330 °C (e, f, g, h) and then 340 °C (i, j, k, l) while the pressure is 100 torr for (a, e, i), 125 torr for (b, f, j), 140 torr for (c, g, k) and 200 torr for (d, h, l). The growth time is fixed at 20 min. In fact, layered materials like muscovite mica are known to favor the van der Waals growth, and the growth of highly crystalline structures can be realized even with large lattice mismatch.27,34-37 7 ACS Paragon Plus Environment


The van der Waals growth on mica was reported for a wide range of materials including 2D metal chalcogenides35-37 and very recently another 3D perovskite MAPbCl3.27 Muscovite mica has a monoclinic structure (space group: C2/c) with lattice constants of a = 5.19 Å, b = 8.99 Å, c = 20.10 Å and β = 95.18°, while the MAPbBr3 lattice is cubic with a lattice constant of 5.92 Å above the structural transition temperature at 236 K.31 Therefore, the large lattice mismatch indicates the van der Waals growth of perovskite platelets. In addition, with finely adjusted growth conditions, the van der Waals interaction at the perovskite/mica interface may lead to other morphologies such as films, rods and wires.18,19 Because of inhomogeneous distribution of temperature and pressure in the growth chamber, sometimes multiple growth morphologies may exist even on the same piece of growth substrate. More in-depth investigations on the thermodynamic factors are required to achieve accurate control on the morphology and dimension of perovskite single crystals. We further investigated the effect of pressure and temperature on the morphology of MAPbBr3 platelets. We systematically adjusted the growth conditions and the results are presented in Figure 2. The growth time is fixed at 20 min. When the pressure and growth temperature are low, the square-shaped platelets cannot form. For example, at 100 torr and 320 °C, we observed the growth of 3D spheres with a uniform diameter of around 1.6 µm (Figure 2a). These spheres were confirmed to be MAPbBr3 using PL (Figure S2). As the pressure was increased, the coexistence of 2D platelets and 3D spheres was observed at an intermediate pressure of 125 torr (Figure 2b), and the 3D spheres finally disappeared at a pressure of 140 torr (Figure 2c), indicating a pressureinduced transition between 2D and 3D nucleation modes. In this case, we observed some platelets as thin as 5.9 nm (Figure S3). As shown in Figure 2d, larger and thicker platelets were obtained at a high pressure of 200 torr, which indicates that the growth rate is closely correlated with the pressure.38 The average thickness increases from 29 nm to 73 nm and the average lateral size 8 ACS Paragon Plus Environment

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increases from 6 um to 10 um with the pressure changing from 140 torr to 200 torr (Figure S4a – d). The growth of large-area platelets at high pressures (Figure 2c, d) indicates a strong wetting tendency of hybrid perovskite on the mica substrate surface. As a result, these perovskite single crystals adopt the 2D thin platelet morphology although MAPbBr3 has a 3D lattice structure. According to the thermodynamics of heterogeneous growth,38 the nucleation mode, either 2D or 3D, is determined by the surface energy difference (∆σ) between the material (σ), the interface (σi) and the substrate (σs) in a relationship of ∆σ = σ + σi - σs. Complete wetting, or ∆σ ≤ 0, naturally leads to 2D nucleation. In the case of ∆σ > 0, the 3D growth mode will prevail unless the precursor chemical potential ∆μ is sufficient to overcome the energy barrier ∆µcr. These two energy terms are shown below:38 ∆μ = kT ln(p/pe);

(1)

∆μcr = sc (σ + σi - σs),

(2)

where k, T, p pe, and sc denote Boltzmann constant, temperature, precursor pressure, pressure in balance and interface area, respectively. ∆μ increases with pressure. Therefore, a high growth pressure (>140 torr) can presumably provide a sufficient chemical potential to trigger the transition from 3D to 2D nucleation during the growth of MAPbBr3 platelets. Growth temperature also has a significant effect on the morphology of the perovskite structures. When the furnace temperature increases from 320 °C to 330 °C, very few platelets were observed at a pressure of 100 torr (Figure 2e), and some irregular structures were observed at 125 torr in addition to well-defined platelets (Figure 2f). While keeping the temperature at 330 °C and



increase the pressure to 140 torr (Figure 2g) and 200 torr (Figure 2h and Figure S5), we observed the formation of much larger perovskite structures composed of connected square-shaped platelets. The average thickness increased from 30 nm to 100 nm (Figure S4d, e), indicating much larger growth rates at high pressures. When the temperature further increased to 340 °C (Figure 2i - l), the pressure-induced morphology evolution of the perovskite structures is similar to the case of 330 °C, except that the thick platelets tend to connect together and form thick terrace structures (Figure S6). Finally, it should be noted that high growth temperatures and low pressures sometimes cause the platelets to decompose into amorphous structures (e.g., Figures 2e, 2f, 2i and 2j).

Figure 3. PL emission characteristics of MAPbBr3 platelets with various thicknesses. (a) Optical and (b) fluorescence images of MAPbBr3 platelets. (c) PL spectra of platelets with different thicknesses. (d) Dependence of optical band gap on the platelet thickness. The inset is the linear fit of band gap versus 1/d2.


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Because halide perovskites are an emerging class of luminescent materials with high tunability, we studied the optical properties of the MAPbBr3 platelets. We selected platelets with different thicknesses (Figure 3a), and the fluorescence images in Figure 3b indicates that the thicker platelets appear much brighter than the thinner ones. Also, all these platelets have smooth surfaces with RMS values less than 0.5 nm measured from AFM in Figure S7. More importantly, we observed that the photoluminescence emission spectra of MAPbBr3 platelets monotonously change with their thicknesses (5.9 nm to 26.2 nm). As shown in Figure 3c, the emission peaks are blue shifted from 526 nm at the platelet thickness of 26.2 nm to 510 nm at the thickness of 5.9 nm. The blue shift is 16 nm (78 meV), which is smaller than that reported for MAPbBr3 quantum dots (131 meV for a diameter down to 3.3 nm) with 3D quantum confinement.17 Recently, a very large blue shift of 255 meV was reported for CsPbBr3 nanowires with 2D confinement and diameter down to 3.4 nm.30 In platelets, the quantum confinement effect appears weaker, but it is essential to explain the thickness-dependent PL data. As shown in Figure 3d, the band gap of the platelets Eg varies with their thickness d according to a simple low: Eg ∝ d-2. This behavior is consistent with the particle-in-a-box model with 1D confinement.39 In quantum wells (QW), according to the effective-mass-approximation, the band gap has the following thickness dependence: Eg = Eg0 + ∆Eg = Eg0 + h2/(8d2)(1/(me*) + 1/(mh*)) = Eg0 + A⁄d2 ,

(3)

where, Eg, Eg0, ∆Eg, h, me* and mh* represent the bandgap for platelet, bandgap for the bulk, bandgap increase, the Planck constant, and the effective masses of electrons and holes, respectively. Because the reduced effective mass (µ) for MAPbBr3 was measured at only very low temperatures and it has a sizable temperature dependence, we decided to use the value of 0.13 m0 (m0: free 11 ACS Paragon Plus Environment


electron mass) for FAPbBr3 measured at high temperatures.40 MAPbBr3 and FAPbBr3 share the same crystal structure, and the same value of µ (0.117m0) was measured at 2 K for both materials.40 We estimated the value of A as 2.893 eV nm2, which is close to the slope of 2.903 eV∙nm2 extracted from the experimental data (Figure 3d). Also, Eg0 is extracted to be 2.35 eV, which matches well with previous reports.41,42 Hence, we can unambiguously attribute the blue shift observed in perovskite platelets to the 1D quantum confinement effect.

Figure 4. Thickness-dependent exciton binding energy and estimation of the exciton Bohr radius. (a) Temperature-dependent PL spectra for the MAPbBr3 platelets with different thicknesses from 5.9 nm to 26.2 nm. (b) Corresponding plots of the PL emission intensity as a function of temperature. (c) Plot of exciton binding energy (Eb) versus platelet thickness and the fitted curve according to our model. The inset schematic illustrates the 1D exciton confinement effect in a 2D perovskite platelet. (d) Dependence of the exciton radius on the platelet thicknesses. The inset schematic demonstrates the asymmetric quantum well model involved in the calculation.


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The good fitting in Figure 3d also excludes surface depletion as the mechanism behind the observed PL blue shift in the perovskite platelets; more discussions can be found in Figure S8. Recently, blue shift of PL peaks was discovered by Shi and coworkers, which was assigned to the strong van der Waals interaction between thin CsPbBr3 platelets and mica substrates.26 It was claimed that the distorted lattice near the mica surface may influence the electronic band structure of CsPbBr3 to give rise to shifted PL. The large lattice stress is evidenced by the clear well-aligned cracks presented in large CsPbBr3 platelets. However, in our case, no such crack was discovered (Figure 2), suggesting much weaker strain in the MAPbBr3 platelets. In addition, the exciton Bohr radius of MAPbBr3 has been reported in the range of a few nm,40,43,44 indicating that the quantum confinement effect should not be ignored in thin perovskite platelets. In semiconductors, the confinement of excitons is expected to increase the exciton binding energy.17,45 By measuring the temperature-dependent PL, the exciton binding energy can be calculated.17,46 Figure 4a shows the temperature-dependent PL data for MAPbBr3 platelets with a thickness from 5.9 nm to 26.2 nm. It is known that the peak PL intensity has the following temperature dependence: I = I0/(1 + A ∙ exp(-Eb/kBT)),

(4)

where, I0 is the intensity at 0 K, Eb is the exciton binding energy, and kB is the Boltzmann constant.17,46 The fitting result in Figure 4b indicates that the exciton binding energy for MAPbBr3 platelets with thicknesses of 5.9 nm, 7.8 nm, 10.1 nm, 14.3 nm and 26.2 nm is 147.6 meV, 123.5 meV, 92.1 meV, 71.4 meV and 69.5 meV, respectively. As shown in Figure 4c, the exciton binding energy of the thickest platelets is almost identical to the value reported for bulk MAPbBr3 single crystal (65 meV).17 It should be mentioned that the exciton binding energy of bulk MAPbBr3 has 13 ACS Paragon Plus Environment


been reported to be in the range of 25 meV to 65 meV, which may originate from different measurement techniques and sample preparation details.40,47 And the value of 147.6 eV for the 5.9 nm quasi-2D platelet is still much smaller than that of 2D hybrid perovskites (~250 meV).47 Although the confinement of exciton in the perovskite platelets is limited to the perpendicular direction, the two-fold increase of exciton binding energy in the thinnest platelets is quite significant. Unlike the cases of 0D quantum dots and 1D nanowires, a quantitative model to describe the quantum confinement effect in 2D platelets with asymmetric dielectric barriers is still lacking. In a QW where a semiconductor layer is sandwiched by barriers with a smaller dielectric constant and a larger energy gap, the electron-hole Coulomb attraction is enhanced because of the reduced screening effect.45 Consequently, the exciton binding energy of the well materials increases on reducing layer thickness d. Accordingly, the modulation of the exciton binding energy can be calculated as a function of the ratios of 2d/aB and ε2⁄ε1 (ε2 and ε1 are the dielectric constants for the well and the barrier materials, respectively).45 In the exciton model developed by Nagaosa and co-authors, it is assumed that the quantum well is sandwiched between two barrier layers with the same dielectric constant.45 However, in our case the perovskite layer is deposited on mica substrate (ε = 8.1),48 while the top barrier is vacuum (ε = 1). Thus, Nagaosa’s model must be modified to take into consideration of the different dielectric constant of the top barrier. As shown in the Supporting information, test calculations confirmed that our model is equivalent to Nagaosa’s result when the top barrier has the same dielectric constant with the bottom barrier. Using our modified model, we obtained the dielectric constant of well material (i.e., perovskite) ε2 as 3.52 by fitting to the experiment data (Figure 4c). As a note on the side, a larger effective dielectric constant of 7.5 was reported for perovskite films in an early study,40 but the 14 ACS Paragon Plus Environment

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preparation method is different from ours and the film thickness (350 nm) is at least an order of magnitude larger than our samples. Furthermore, in that study,40 the exciton model used to fit the dielectric constant is a 3D hydrogen atom model, not the 2D model as used in our work. For perovskite platelets, the lower dimensionality compared to their 3D counterparts will reduce the intensity of electronic screening and eventually the dielectric constant. Thus, we think that the effective dielectric constant of 3.52 obtained in our work is reasonable. Finally, the exciton radius was deduced for platelets with different thicknesses. As shown in Figure 4d, the exciton radius increases from 2.2 nm to 4.0 nm when the platelet thickness changes from 5.9 nm to 26.2 nm.

Figure 5. Simulated contour maps of exciton binding energy and exciton radius. Dependence of (a) exciton binding energy and (b) exciton radius on the barrier dielectric constant (ε1) and thickness. (c, d) Corresponding dependence on the well dielectric constant (ε2) and thickness. The experimental data measured on perovskite platelets with five thicknesses are indicated with white dots. 15 ACS Paragon Plus Environment


Figure 6. Thickness-dependent exciton lifetime. (a) TRPL decay profiles of the MAPbBr3 platelets with different thicknesses recorded at an excitation wavelength of 400 nm. The black lines are the fitting curves. (b) Thickness dependence of the deduced recombination rate (k1 and k2). To examine the potential of modulating the exciton properties of quantum-confined hybrid perovskites, we used the QW model to investigate the dependence of exciton energy and radius on key QW parameters, particularly barrier dielectric constant ε1, semiconductor thickness and dielectric constant ε2. A shown in the contour maps in Figure 5a, the exciton binding energy can be significantly enhanced when the barrier dielectric constant and the platelet thickness decreases. In contrast, the exciton Bohr radius has a much weaker dependence on the barrier dielectric constant (Figure 5b), but it is significantly reduced when the platelet becomes thinner, as evidenced in our experiment. It should be noted that in Nagaosa’s result the exciton radius has a much stronger dependence on the barrier dielectric constant as a result of the symmetric quantum well structure.45 In contrast, our QW structure is asymmetric with air on one side, and replacing mica substrates on the other side may lead to a limited tuning effect on the exciton radius. Moreover, the exciton binding energy is also sensitively dependent on the dielectric constant of the sandwiched material (Figure 5c), which is in coincidence with the previous work.45 Finally, the exciton radius is reduced when the dielectric constant of the sandwiched material and the platelet 16 ACS Paragon Plus Environment

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thickness decrease (Figure 5d). It should be noted that the quantum confinement effect in the 2D perovskite platelets is different from that in the 2D Ruddlesden-Popper halide perovskites where the perovskite layers are symmetrically sandwiched between organic layers.49 The simplistic approximation of Hydrogen exciton model has been often used to describe the strongly bound excitons in QWs. In contrast, the modified Nagaosa model developed here can be generalized to accurately describe the confinement effect in a wide range of QW structures, either symmetric or asymmetric, with different combinations of barrier and sandwiched materials. Finally, we carried out time-resolved photoluminescence (TRPL) experiments to explore the exciton and carrier decay in the perovskite platelets. The TRPL data shown in Figure 6a reveal valuable information on the radiative recombination in the perovskite platelets. In general, the radiative PL intensity (n) decay can be described as dn/dt = – k1∙n – k2∙n2, where k1 and k2 are recombination rates for exciton and free carriers, respectively.47 The extracted values of k1 and k2 for various platelets are shown in Figure 6b. The value of k1 increases with reducing thicknesses, which is in line with the enhancement of exciton binding energy as discussed above.40,47 The same trend occurs for k2, which could be a consequence of larger free carrier density in thinner platelets, as reported previously.50 In summary, single-crystal MAPbBr3 platelets was fabricated on muscovite mica substrates using a one-step chemical vapor deposition method. The transition between 2D and 3D growth modes was observed by tuning the temperature and pressure conditions, revealing the energetic nucleation process. Furthermore, the blue shift of the PL emission in the thin platelets provided a clear evidence of increased exciton binding energy, indicating the 1D quantum confinement effect. Using a modified Nagaosa model of a QW sandwiched by inequivalent dielectric barriers, we discovered an interesting phenomenon of “compressed exciton” in perovskite platelets with 17 ACS Paragon Plus Environment


quantum confinement: the exciton Bohr radius of single-crystal CH3NH3PbBr3 decreases from 4.0 nm for the thick (26.2 nm) platelets to 2.2 nm for the thin (5.9 nm) ones. Overall, our study enriches the understanding of vapor-phase growth of hybrid perovskite single crystals on 2D substrates, and these platelets offer an exciting platform to explore the physical properties of perovskites under nanoscale quantum confinement. EXPERIMENTAL SECTION The perovskite platelets were synthesized on freshly air-cleaved (001) muscovite mica substrates using vapor transport in a single-zone furnace (Lindberg Blue M TF55035C-1). MABr was prepared using a reported method.11 Their temperatures during growth are 320 ℃ - 340 ℃ and 218 ℃ - 230 ℃, respectively. Nitrogen gas was used as the carrier gas with pressure in the range of 100 torr - 200 torr. Mica (001) substrates were positioned downstream at a distance of 12 cm from the PbBr2 source. More synthesis details can be found in Supporting Information. Optical images of MAPbBr3 were obtained using a Leica DM1000 LED optical microscope. The fluorescence images were taken by an Olympus microscope, and the platelets were illuminated with an Olympus U-HGLGPS lamp after passing through a 355 nm bandpass filter. AFM (Bruker Dimension Icon) in the tapping mode was used to obtain the morphologies of MAPbBr3 platelets and to resolve the unit-cell steps on the surfaces. Scanning electron microscope (FEI Quantum 600FEG, FESEM) was used to observe the MAPbBr3 nanostructures. XRD was carried out on a Bruker D8-Advance diffractometer using Cu Kα radiation (λ = 1.5406 Å). Fast-scanning Raman microscopy was conducted using a confocal μ-Raman spectroscopy with automated stage (Renishaw). A step size of 100 nm was used to ensure stable mapping results. Raman mappings were produced using 100X objective lens using a 532 nm laser as the excitation wavelength with 18 ACS Paragon Plus Environment

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low laser power and short exposure time. Absorption spectra were recorded using an Agilent Cary 5000 UV–vis–NIR spectrometer. The PL mapping image was collected using a confocal Raman microscopy system (Witec Alpha 300) with the excitation wavelength of 532 nm, and temperaturedependent PL measurements from 77 to 300 K were carried out using a Horiba Aramis Raman system with a temperature controlling system. TRPL measurements were performed on a confocal microspectrometer using reflective symmetry (Horiba-JYT64000). A 400 nm pulsed laser was used as the excitation source, which was frequency doubled by a BBO crystal from a Coherent Mira 900 (center wavelength: 800 nm, 120 fs, 76 MHz). The backscattered signal was collected using time-correlated single photon counting (TCSPC, SPC-150), which has an ultimate temporal resolution of ∼40 ps. ASSOCIATED CONTENT Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the King Abdullah University of Science and Technology (KAUST), the Ministry of Science and Technology (No.2016YFA0200700, 2017YFA0205004, 2017YFA0204800), National Natural Science Foundation of China (No.21673054, 21525311). Q.X. acknowledges the support from and the Singapore Ministry of Education via AcRF Tier 2 grant (MOE2015-T2-1-047), and Tier 1 grants (2015-T1-001-175 and RG113/16). A. A and M.R.A would like to acknowledge King Abdulaziz City for Science and Technology (KACST) for their financial support through the center of excellence for green nanotechnologies (CEGN), part of joint centers of excellence program. 19 ACS Paragon Plus Environment


Supporting Information Brief descriptions in nonsentence format listing the contents of the files supplied as Supporting Information. Additional figures showing growth conditions for all samples, PL for platelets and spheres, AFM for ultrathin platelets, platelets size distributions, substrate photograph with platelets in large scale, AFM of the connected and terraced structures, perovskite platelets for the PL test, band gap fitting and analysis using other theories, summary of the methodology for the quantum well model. (PDF)


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One-Step Vapor-Phase Synthesis and Quantum-Confined Exciton in

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