Colloidal Organohalide Perovskite Nanoplatelets Exhibiting Quantum

May 4, 2015 - We find that excitonic features that had previously been attributed to quantum confinement in MAPbBr3 nanocrystals are in fact a propert...
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Colloidal Organohalide Perovskite Nanoplatelets Exhibiting Quantum Confinement Pooja Tyagi, Sarah M. Arveson, and William A. Tisdale* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: We prepare colloidal nanoplatelets of methylammonium lead bromide (MAPbBr3) perovskite and compare the optical signatures of excitons in these twodimensional systems to spherical perovskite nanocrystals and the corresponding bulk phase. We find that excitonic features that had previously been attributed to quantum confinement in MAPbBr3 nanocrystals are in fact a property of the bulk perovskite phase. Furthermore, we find that higher-energy absorption features originate from two-dimensional nanoplatelets, which are present in the nanocrystal reaction product. Upon further purification, we obtain colloidal nanoplatelets with predominantly single unit cell thickness and submicron lateral dimensions, which are stable in solution and exhibit a sharp excitonic absorption feature 0.5 eV blue-shifted from that of the three-dimensional bulk MAPbBr3 phase, representing a new addition to the growing family of colloidal two-dimensional nanostructures.

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However, the 2D perovskite materials investigated in each of these previous studies had out-of-plane dimensions too large for quantum size effects to be observed and were not stable in solution, thereby limiting their applicability.18,19,37 In this work, we prepare colloidal nanocrystals and 2D nanoplatelets of methylammonium lead bromide (MAPbBr3) perovskites and identify the optical signatures of their excitonic states using a combination of optical spectroscopy and electron microscopy. We find that the excitonic features that were previously assigned to quantum-confined excitons in MAPbBr338 are in fact characteristic of bulk 3D crystals. Moreover, we find that colloidal synthesis methods38 can yield a mixture of perovskite nanostructures with different morphologies and optical absorption and emission spectra, including nanoscale particles and 2D nanoplatelets (Figure 1). Upon further purification of the reaction product, we obtain single unit cell thick (n = 1) crystalline 2D perovskite nanoplatelets exhibiting clear evidence of quantum confinement with a sharp excitonic absorption 0.5 eV blue-shifted from that of the 3D bulk perovskite phase. The thickness-dependent absorption and emission spectra of these colloidal nanoplatelets closely resemble those of solid-phase layered perovskites.39 MAPbBr3 perovskite nanocrystals were synthesized using the method of Schmidt et al.,38 with octylammonium bromide as the long-chain ligand. The synthesis involved adding lead bromide, methylammonium bromide, and octylammonium bromide to a stirring solution of oleic acid and 1-octadecene at 80 °C. The reaction was quenched with acetone, and perovskite nanostructures were obtained upon centrifugation.

rganometal halide perovskites are a promising platform for future photovoltaic technologies.1−7 These materials have an ABX3 structure where A is an organic cation (e.g., CH3NH3+), B is a metal cation (e.g., Pb2+), and X is a halide anion (e.g., Cl−, Br−, I−). The electronic and optical properties of organometal halide perovskites are strongly dependent on the nature of the constituent ions. Doping with different metal (ABnB′1−nX3) and halide (ABXnX′3−n) units offers further control over their optoelectronic properties.8−16 Additionally, dimensionality may be used to tune the electronic and optical properties.17−20 Although the effects of reduced dimensionality and quantum confinement have been extensively investigated in conventional semiconductors,21−23 the extent of theoretical and experimental studies in low-dimensional perovskite systems is comparatively limited.17−20,24−29 A family of 2D layered perovskites can be described by (RNH3)2[CH3NH3PbX3]nPbX4, where R is a long-chain alkyl group (e.g., octyl, octadecyl) and n corresponds to the number of perovskite unit cells (Figure 1). For example, n = 1, 2, and 3 correspond to layered perovskites that are one, two, and three unit cells thick, respectively (Figure 1), and n = ∞ corresponds to the bulk perovskite phase. While most work on the photovoltaic applications of organohalide perovskites has focused on thin films of threedimensional (3D) bulk materials supported on planar and mesoporous substrates,8,12−15,30−36 2D hybrid layered perovskites have recently been shown to exhibit promise for lightemitting applications, with bromide-based white-light emitters reaching photoluminescence (PL) efficiencies of up to 9%. Iodide-based 2D perovskite nanoplatelets grown via chemical vapor deposition showed enhanced charge carrier diffusion relative to polycrystalline films obtained by spin-coating.30,37 © XXXX American Chemical Society

Received: March 31, 2015 Accepted: May 4, 2015

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DOI: 10.1021/acs.jpclett.5b00664 J. Phys. Chem. Lett. 2015, 6, 1911−1916

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(Figure 2b). Despite their identical PL maxima, the line widths of the bulk crystals and the unpurified solution were different. The PL spectrum of the unpurified nanocrystals had excess emission at higher energies, suggesting the presence of additional nanostructures emitting at wavelengths shorter than 531 nm. A scanning electron micrograph (SEM, Figure 2c) of the bulk MAPbBr3 sample revealed micron-scale crystals. Transmission electron micrographs (TEM, Figure 2d−f) of the unpurified nanocrystals revealed a mixture of nanostructures, including nominally spherical nanocrystals with an average diameter of ∼6 nm. It is intriguing that, despite a significant difference in size, both structural motifs (3D bulk crystals and unpurified nanocrystals) have identical positions of PL maxima (531 nm) and lowest excitonic absorption (525 nm). On the basis of previous experimental and theoretical work, the excitonic Bohr radius of MAPbBr3 is estimated to be 1.4−2.0 nm.40,41 The small Bohr radius suggests that nanocrystals 6 nm in diameter should not exhibit quantum confinement and should have bulklike optical properties. This hypothesis is consistent with our comparative absorption and PL measurements. We therefore conclude that the spectral features (525 nm excitonic absorption and 531 nm PL peak) that were previously assigned to quantum-confined perovskite nanocrystals38 are in fact characteristic of bulk MAPbBr3. In the unpurified colloidal reaction product, the bulk-like features arise from nanoscale crystals that are not quantum-confined. We note, however, that the absorption spectrum of the unpurified nanocrystal solution has additional high-energy features blue-shifted from that of the bulk band gap (Figure 2a). From the TEM images shown in Figure 2d,e, we can identify 2D sheets, or nanoplatelets, present in the reaction product in addition to spherical nanocrystals. These nanoplatelets are observable in previously reported TEM images of the same reaction product,38 but to our knowledge, they have not previously been identified or characterized. On the basis of the TEM contrast (Figure 2e), these nanoplatelets are estimated to be much thinner than the nanoparticles (thickness ≪ 6 nm). Thin 2D sheets of semiconductors such as CdSe and MoS2 are known to exhibit quantum confinement, resulting in blue shifting of excitonic absorption with respect to the bulk band gap.25,42,43 To establish whether the high-energy spectral features in the unpurified reaction product originate from quantum-confined 2D perovskite nanoplatelets, we attempted to isolate these structures using additional purification steps. To isolate the 2D perovskite nanoplatelets from the nanostructures solution, the “unpurified” colloidal solution was diluted by a factor of 10 and sonicated for 30 min to disaggregate any nanoparticles from nearby nanoplatelets. Immediately after sonication, the solution was passed through a 0.8 μm syringe filter. A small amount of acetone was added to the filtered solution to selectively precipitate the nanoplatelets, and the solution was centrifuged again, followed by sonication (see the Experimental Methods section for details). The purified product consisted of a colloidal solution of thin 2D nanoplatelets of submicron lateral dimensions and varying thickness (Figure 3a,b). Selected area electron diffraction (SAED) revealed highly crystalline nanoplatelets (Figure 3c). The nanoplatelets have a cubic crystal structure, with lattice spacing of approximately 6.0 Å, which is close to the bulk perovskite phase (5.937 Å) obtained using X-ray diffraction (see the Supporting Information). The inferred structure of

Figure 1. Schematic illustration of (a) a perovskite unit cell and (b) 2D perovskite nanoplatelets with n = 1 (one unit cell thick).

The precipitate was redispersed in toluene to obtain a colloidal solution of MAPbBr3 nanostructures, hereafter referred to as the “unpurified” product (see the Experimental Methods section for details). To compare the optical properties of perovskite nanostructures to those of the corresponding bulk material, we also synthesized bulk MAPbBr3 films. These films were prepared by drop-casting an equimolar solution of methylammonium bromide and lead bromide on a glass substrate and heating at 60 °C for 10 min in air (see the Experimental Methods section for details).36 The cubic perovskite phase of these films was confirmed by X-ray diffraction (Figure S1, Supporting Information). In Figure 2a, we compare the absorption spectrum of the unpurified colloidal MAPbBr3 nanostructures solution (red) to that of the 3D bulk MAPbBr3 crystals (black). The unpurified nanocrystal solution exhibited multiple peaks in the absorption spectrum, with the lowest-energy peak at 525 nm, whereas the bulk crystals had a single absorption peak at 525 nm characteristic of bulk semiconductor materials. On the other hand, the PL spectra of both the bulk and the unpurified nanocrystals exhibited only one peak, centered at 531 nm 1912

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Figure 2. Comparison of bulk MAPbBr3 microcrystals and unpurified colloidal nanocrystals. Normalized (a) absorption and (b) PL spectra of bulk crystals (black) and unpurified colloidal nanocrystals (red). (c) Scanning electron micrograph (SEM) of a drop-cast film of bulk MAPbBr3 showing micron-scale crystals. (d−f) Transmission electron micrographs (TEM) of the unpurified nanocrystals, showing a mixture of perovskite nanostructures with different morphologies.

with maxima at 475, 490, and 504 nm and a dominant peak at 530 nm (blue curve in Figure 4; Figure S3, Supporting Information). Because bulk MAPbBr3 crystals emit at 531 nm (Figure 2b), the lowest-energy peak at 530 nm is likely due to emission from bulk-like species in the purified product (large nanoparticles or thick nanoplatelets). The high-energy peaks, however, are specific to the nanoplatelets and are not observed in the bulk PL spectrum (Figure 2b). Given the narrow emission line widths of these peaks, it is unlikely that they are due to surface defects.19,44 We can also rule out the possibility that these are phonon replicas because a multipeak analysis showed that these emission peaks are not equally spaced in energy and therefore cannot be attributed to a phonon progression (Figure S3, Supporting Information). Because the positions of these PL peaks do not show any sample-to-sample variability, we conclude that they are an intrinsic property of the purified nanoplatelets. A close agreement between the emission peak positions of the purified colloidal product and solid-phase layered perovskites39 confirms our hypothesis that these PL peaks arise from nanoplatelets of different thickness (Table S2, Supporting Information). Specifically, the peaks at 475, 490, 504, and 530 nm can be assigned to nanoplatelets with n = 4, 5, 6, and ∞, respectively. These peak assignments are supported by TEM analysis, where the variety of contrast levels visible in the TEM images suggests the presence of nanoplatelets of different thickness in the purified product (Figure 3a,b). Because these nanoplatelets are quantum-confined, they have thicknessdependent optical and electronic properties, as revealed in the multipeak emission spectrum.25,45 A distribution of nanoplatelets with different thickness is also evident in the absorption spectrum of the unpurified product (Figure 2a). By comparing our absorption data to that of the

these nanoplatelets is a 2D sheet having the bulk MAPbBr3 crystal structure but with thickness as small as a single unit cell, with octylammonium bromide ligands bound to each face, as drawn schematically in Figure 1. This proposed structure is consistent with the dominance of diffraction peaks from (00l) planes in the X-ray diffraction pattern of the dried nanoplatelets solution (Figure S1, Supporting Information), as well as detailed analysis of the absorption and emission spectrum of the purified product, discussed below. The thickness of these nanoplatelets can be estimated from the MAPbBr3 lattice constant in the two-dimensional structure, as measured by SAED. An n = 1 nanoplatelet is t = (0.6 + 0.6) nm = 1.2 nm thick, including the two lead bromide octahedra (Figure 1), which is consistent with the relative TEM contrast between these nanoplatelets and the ∼6 nm thick nanocrystals. Absorption and PL spectra of the purified nanoplatelet solution are shown in Figure 4. The 2D nanoplatelets absorption spectrum is dominated by a single sharp excitonic absorption feature at 431 nm (red curve in Figure 4), which is 503 meV blue-shifted from the bulk excitonic absorption at 525 nm. This large blue shift is indicative of quantum confinement in one dimension.20,39 The absorption spectrum also has a small but discernible shoulder at 525 nm, likely originating from residual bulk-like nanostructures present in the purified product. The absorption amplitude of the 525 nm peak is 5% of that of the main absorption peak at 431 nm, confirming that these bulk-like nanostructures constitute a small fraction of the nanoplatelet population in the purified product. These bulk-like species could be nanocrystals remaining from the original unpurified solution or nanoplatelets that are thicker than the exciton Bohr radius. In contrast to the absorption spectrum, the PL spectrum of the purified nanoplatelets solution exhibits multipeak emission 1913

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Figure 4. Absorption (red) and PL (blue) spectra of the purified MAPbBr3 nanoplatelet solution. The inset shows an enlarged view of the blue shoulder of the emission spectrum, revealing discrete peaks corresponding to nanoplatelets with different thicknesses (n: number of layered unit cells). The green, brown, and orange curves are multipeak deconvolutions.

4). The dominance of the bulk-like peak (n = ∞) in the PL spectrum could be due to downhill energy transfer from the blue-emitting n = 1 nanoplatelets to the red-emitting nanocrystals and thicker nanoplatelets. Alternatively, the nanoplatelets may have intrinsically lower PL quantum yield due to efficient exciton dissociation into free charge carriers and subsequent quenching.46,47 Further optimization of the synthesis procedure is needed to reduce the thickness dispersion in the sample and enable further study of the thickness-dependent electronic and optical properties. We have presented a synthetic route to isolate crystalline, 2D nanoplatelets of MAPbBr3 perovskite in solution and identified the spectral signatures of quantum-confined excitons in these colloidal nanostructures. As shown by recent experiments, lowdimensional perovskites hold promise in a variety of applications including white-light emission and charge transport and may even outperform their 3D counterparts.18,19,37 Further work is needed in the purification of these materials to improve size selection and yield, so that their thickness-dependent properties can be characterized. In addition to size-dependent optical and electronic properties, these advances would enable study of the stacking properties of these perovskite nanoplatelets and possibly controllably build perovskite superstructures. Our preliminary data show evidence of self-assembly into stacked superstructures similar to CdSe nanoplatelets (Figure S4, Supporting Information).48 Ordered superstructures of organohalide perovskites would open up a range of possibilities to study nonlinear effects in these materials akin to those of layered semiconductors and van der Waals solids.49

Figure 3. (a,b) TEM of the purified MAPbBr3 nanoplatelet solution and (c) SAED pattern corresponding to the region shown in (b).

solid-phase layered perovskites,39 we can assign specific absorption features to nanoplatelets with a different number of unit cell layers, n (Table S2, Supporting Information). This comparison shows that our unpurified product primarily consists of colloidal perovskite nanoplatelets with n = 1, 3, 4, and ∞ with prominent absorption peaks at 431, 451, 472, and 525 nm, respectively. After purification, the colloidal product primarily consists of n = 1 nanoplatelets, in addition to a small amount of thicker nanoplatelets as confirmed by the PL spectrum (Figure 4). While quantized emission peaks are evident in the purified product PL spectrum (Figure 4), the same features are not as clearly defined in the unpurified product PL spectrum (Figure 2b), where an asymmetrically broadened bulk-like PL peak is observed instead. From the TEM images shown in Figure 2d,e, some size dispersion is evident in the nanocrystal sample. It is possible that the quantized nanoplatelet PL features are obscured by blue-shifted emission from the smallest nanocrystals in the distribution, which may be closer in size to the exciton Bohr radius. Although the purified product is predominantly composed of n = 1 nanoplatelets, the n = 1 emission peak (expected near 442 nm39) is conspicuously absent from the PL spectrum (Figure

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EXPERIMENTAL METHODS Preparation of Precursors. Methylammonium Bromide (CH3NH3Br). Methylamine (12 mL, 40 wt % solution in water, Sigma-Aldrich) was reacted with 5 mL of hydrobromic acid (48% in water, Sigma-Aldrich) in ethanol (50 mL) while stirring under a nitrogen atmosphere at room temperature for 2 h. Methylammonium bromide was obtained as a white, shiny powder upon crystallization using a rotary evaporator and washed three times with ethanol. 1914

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Octylammonium Bromide (CH3(CH2)7NH3Br). Octylamine (7 mL, Sigma-Aldrich) was reacted with 5 mL of hydrobromic acid (48% in water, Sigma-Aldrich) in ethanol (50 mL) while stirring under an nitrogen atmosphere at room temperature for 2 h. Octylammonium bromide was obtained as a white powder upon crystallization using a rotary evaporator and washed three times with ethanol. Synthesis of Bulk MAPbBr3. An equimolar solution (1 M) of methylammonium bromide and lead bromide (Sigma-Aldrich) in N,N-dimethylformamide (DMF, Sigma-Aldrich) was stirred at room temperature overnight. The solution was drop-casted on a glass substrate and heated at 60 °C for 10 min, giving a bright orange film of bulk MAPbBr3. Synthesis and Purif ication of MAPbBr3 Nanocrystals. MAPbBr3 nanocrystals were prepared using the method of Schmidt et al.38 A volume of 2 mL of 1-octadecene was added to 85 mg of oleic acid in a 20 mL glass vial. The solution was stirred and heated to 80 °C, followed by the addition of 12.6 mg of octylammonium bromide. Methylammonium bromide and lead bromide solutions were prepared in DMF (4.4 mg of methylammonium bromide in 100 μL of DMF and 36.7 mg of lead bromide in 200 μL of DMF) and added to the reaction vial. When the solution changed color to light yellow (1−2 min), acetone was added immediately to quench the reaction. The solution was centrifuged at 4000 rpm for 10 min. The clear supernatant was discarded, and the yellow precipitate was redispersed in toluene to obtain a colloidal solution of perovskite nanostructures. A small fraction of the colloidal perovskite nanostructures prepared above was further purified as follows: the unpurified colloidal solution was diluted by a factor of 10 and sonicated for 30 min. Immediately after sonication, the solution was passed through a 0.8 um syringe filter. A small amount of acetone was added to the filtered solution to selectively precipitate nanoplatelets, and the solution was centrifuged again (4000 rpm, 10 min). The precipitate thus obtained was redispersed in toluene and sonicated for 20 min. The final purified product consisted of perovskite nanoplatelets of submicron lateral dimensions (Figures 3 and S2, Supporting Information). Characterization. PL was collected in air using a CCD spectrograph (Princeton Instruments). Samples were photoexcited using a continuous-wave 405 nm diode laser. There was no difference between PL spectra acquired in air or in vacuum. TEM was performed on an FEI Tecnai (G2 Spirit TWIN) operating at 120 kV. SEM was performed on a Zeiss Merlin high-resolution SEM using an in-lens detector operating at 3 kV. XRD was performed on a Rigaku Smartlab with a Cu Kα source operating at 45 kV and 200 mA.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award Number DESC0010538. TEM, SEM, and XRD measurements made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under Award Number DMR-1419807.

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ASSOCIATED CONTENT

S Supporting Information *

Structural characterization, analysis of multipeak emission of 2D nanoplatelets, stacking properties of perovskite nanoplatelets, and supplementary figures including information on X-ray diffraction, emission spectrum deconvolution, additional TEM images, and peak positions and diffraction plane indices. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b00664.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 1915

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DOI: 10.1021/acs.jpclett.5b00664 J. Phys. Chem. Lett. 2015, 6, 1911−1916