Multicolor Heterostructures of Two-Dimensional Layered Halide

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Multicolor Heterostructures of Two-Dimensional Layered Halide Perovskites that Show Interlayer Energy Transfer Yongping Fu, Weihao Zheng, Xiaoxia Wang, Matthew P. Hautzinger, Dongxu Pan, Lianna Dang, John C. Wright, Anlian Pan, and Song Jin J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07843 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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Journal of the American Chemical Society

Multicolor Heterostructures of Two-Dimensional Layered Halide Perovskites that Show Interlayer Energy Transfer Yongping Fu1 †, Weihao Zheng2 †, Xiaoxia Wang2, Matthew P. Hautzinger1, Dongxu Pan1, Lianna Dang1, John C. Wright1, Anlian Pan2*, Song Jin1*

1

Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison,

Wisconsin 53706, United States. 2

Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, College of

Materials Science and Engineering, Hunan University, Changsha 410082, P. R. China. * Corresponding author: E-mail: [email protected] (S. J.), [email protected] (A. P.) † These

authors contributed equally to this work.

ABSTRACT Fabrication of heterostructures using two-dimensional (2D) materials with different bandgaps creates opportunities for exploring new properties and device applications. Ruddlesden−Popper (RP) layered halide perovskites have recently emerged as a new class of solution processable 2D materials that demonstrate exotic optoelectronic properties. However, heterostructures using 2D halide perovskites have not been achieved. Here, we report a simple solution growth for making vertically stacked double heterostructures and complex multilayer heterostructures of 2D lead iodide perovskites [(PEA)2(MA)n-1PbnI3n+1, PEA = C6H5(CH2)2NH3+, MA = CH3NH3+] via van 1 ACS Paragon Plus Environment

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der Waals epitaxy. These heterostructures present atomically sharp interfaces and display distinct photoluminescence that allow fingerprinting the RP phases. Time-resolved photoluminescence measurements reveal internal energy transfer from higher energy bandgap (lower n-value) perovskite layers to lower energy bandgap (higher n-value) perovskite layers on the timescale of hundreds of picoseconds due to natural type I band alignments. These results offer new strategies to fabricate perovskite-perovskite heterojunctions by taking advantage of surface-bound ligands as spatial barriers to prevent ion migration across the junctions. These heterostructures capable of multicolor emission with high spectral purity are promising for light emitting applications. KEYWORDS: 2D heterostructures, layered lead halide perovskites, perovskite heterojunctions, energy transfer, multicolor emission, time-resolved spectroscopy

INTRODUCTION Semiconductor heterostructures are key components in solid-state device applications such as photovoltaics, photodetectors, transistors, and laser diodes.1,

2

They are also of fundamental

interest, as bandgap mismatch at the junction of heterostructures gives rise to possibilities of controlling and manipulating the generation, recombination, and transport of charge carriers. The families of group IV, III−V and II−VI semiconductors3-6 and transition metal dichalcogenides7-10 have been explored in heterostructures with intriguing physical properties and used in high performance devices. Recently, metal halide perovskites have emerged as a new class of high performance semiconductor materials for photovoltaics and optoelectronics with both technological and fundamental interests.11-15 The remarkable device performance and versatility of lead halide perovskites has been attributed to inherently low nonradiative recombination rates, long carrier lifetimes, widely tunable bandgap, and high photoluminescence (PL) quantum yield. 2 ACS Paragon Plus Environment

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In addition, the structural and compositional diversity of halide perovskites allows an extensive library of materials with distinct optical and electrical properties.16-18 Given the importance of semiconductor heterojunctions in optoelectronics, creating analogous heterojunctions between different halide perovskite materials would lead to new exciting physics and applications.19, 20 Even though stable heterostructures of halide perovskites with other more inert materials could be created and studied,21-24 the fabrication of halide perovskite heterostructures with abrupt and stable interface is challenging due to the soft and ionic crystal halide perovskite lattice. An exemplary system of what has been achieved this far is nanowire heterojunctions of three-dimensional (3D) CsPbX3 (X is a halide anion) perovskites with a spatial resolution of 500 nm created via local ion exchange.25 However, the facile solid-state ion interdiffusion in 3D perovskites imposes fundamental challenge in maintaining a stable heterojunction.26 To enable halide perovskite heterostructures with an atomically sharp junction, we consider the families of Ruddlesden−Popper (RP) layered halide perovskites, which are two-dimensional (2D) derivatives of the 3D perovskites formed by slicing the 3D frameworks into 2D slabs (Figure 1a).27, 28 RP layered perovskites have a general chemical formula of (RNH3)2(A)n−1MX3n+1, where R is a long-chain alkyl or aromatic group, A is a small monovalent cation such as methylammonium, M is a divalent metal cation such as Pb2+ or Sn2+, X is a halide, and n is an integer. In these intrinsic 2D crystal structures, each layer consists of an 2D network of cornersharing [MX6]4− octahedra and interceding bilayers of RNH3+ capping two sides to balance the charge. The unit layers are stacked together by van der Waals forces to form bulk crystals. Due to the presence of the insulating dielectric RNH3+ bilayers, 2D perovskites can be regarded as a class of natural quantum wells (QWs) with the degree of quantum confinement and the related optical properties tailored by varying the thickness of the inorganic layer (n value). Unlike the 3D 3 ACS Paragon Plus Environment


perovskites that behave as free-carrier semiconductors, strongly confined 2D perovskites (n ≤ 5) exhibit tightly bound excitons with binding energies up to several hundred meV.29 This gives rise to the observed intense PL with high color purity and strong exciton-photon coupling at room temperature.30 Lead halide RP perovskites also show increased chemical stability relative to 3D hybrid perovskites and have been demonstrated as an alternative to the 3D hybrid perovskites in photovoltaics.31,

32

Recently, 2D perovskites have been introduced as a new type of solution-

processed 2D materials with fascinating optical properties.33, 34 Notably, the chemical diversity of 2D perovskites and their quantum confinement effect allow strong PL emission spanning from ultraviolet to near-infrared spectral regions,35, 36 which are not available in previously studied 2D materials. Atomically thin nanosheets can be prepared by physical exfoliation or solution growth.34, 37

Nanostructures with suitable dimensions have also been grown with potential use as building

blocks in photonic devices and LEDs.38-41 Here, we report a facile one-step solution phase growth for the creation of vertically stacked double heterostructures and complex multi-heterostructures of 2D halide perovskites with different perovskite layer thickness (n values) via van der Waals epitaxy. The key for the success in making these heterojunction is that the self-assembled bilayer of RNH3+ cations can act as natural diffusion barrier and prevent ion migration across the adjacent perovskite layers, maintaining stable and atomically sharp perovskite junctions. The heterostructures display distinctive PL peaks from the individual layers exhibiting different degrees of quantum confinement, and their effective emission color can be readily tuned by excitation power density. Time-resolved PL (TRPL) studies of individual heterostructures reveal internal energy transfer with a timescale of hundreds of picoseconds from lower-n to higher-n layers (high bandgap to low bandgap) in each heterostructure. These heterostructures capable of emitting multiple colors with high spectral 4 ACS Paragon Plus Environment

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purity are attractive platforms to explore new properties and physics in 2D materials. They also show promise for building versatile photonic devices that are easily solution-processed.

RESULTS AND DISCUSSION We synthesized microplates of 2D lead iodide perovskite heterostructures by immersing a glass slide coated with a lead acetate (PbAc2) thin film into an isopropanol solution containing both methylammonium (MA) iodide and phenethylammonium (PEA) iodide (see details in Experimental Section). With a proper ratio (𝛼) of MAI to PEAI, the PEA cations can be incorporated into the crystal lattice to form 2D perovskites. Interestingly, as the difference in the thermodynamic stability of various RP phases with similar n members is very small, we found 2D perovskite layers with different n values could nucleate and grow on top of existing layers via van der Waals epitaxy. This facile mixed layer growth leads to the formation of vertically stacked heterostructures. We further achieved dimensionality (or composition) modulation of the heterostructures by simply adjusting the ratio of MAI to PEAI: a lower α yields heterostructures primarily comprised of perovskite layers with smaller n members and less complexity (i.e. less nvalue mixing). In the most extreme case of only using PEA (α = 0), the n=1 perovskite, i.e. (PEA)2PbI4, is formed. After reactions, UV-Vis absorption measurements and powder X-ray diffraction (PXRD) revealed the mixed dimensionalities in the samples. UV-vis absorption spectra (Figure 1b) show there are (at least) three pronounced excitonic absorption peaks centered at 520, 568, and 610 nm, which can be assigned to the 2D RP perovskites with n =1, 2, and 3, respectively. The additional broad absorption extending to ∼750 nm is ascribed to a group of 2D perovskites with very large n values or possible 3D-like (𝑛 = ∞) perovskite.

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Figure 1. Structural characterization of as-synthesized 2D perovskite heterostructures. (a) Crystal structures of RP perovskites with different n values. A 3D perovskite structure is equivalent to 𝑛 = ∞. (b) Absorption spectra of (PEA)2(MA)n−1PbnI3n+1 samples grown from a solution containing MAI and PEAI with molar ratios (𝛼) of 0.84 and 1.05. (c) PXRD of these samples, together with the standard PXRD patterns of 2D perovskites with n = 1, 2, and 3. (d,e) SEM images of the asgrown samples. (f) Top-view and (g) cross-sectional view SEM images show the microplates contain multiple stacking layers. (h) A representative AFM image of a rectangular microplate.

The majority of the PXRD diffraction peaks (Figure 1c) are associated with (PEA)2PbI4 (n=1) and (PEA)2(MA)Pb2I7 (n=2) for the samples grown with α of 0.84 and 1.05, respectively, whereas the peak intensities of higher n member structures are barely resolved. It should be noted that the 6 ACS Paragon Plus Environment

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lack of diffraction peaks for RP perovskites with n > 2 does not indicate the absence of these phases, but merely suggests that these phases do not have the long-range crystal periodicity that gives rise to diffract peaks sufficiently strong to be detected by PXRD. Therefore, the PXRD data do not have the resolution to quantify the average compositions of the heterostructure samples. Solutionphase 1H NMR measurements on re-dissolved samples were then performed to determine the average composition by integrating the 1H NMR peaks associated with the PEA and MA cations (Figure S1). The average n value is 2.26 for the sample with α = 1.05, and 1.75 for the sample with α = 0.84, which corresponds to (PEA)2(MA)1.26Pb2.26I7.78 and (PEA)2(MA)0.75Pb1.75I6.25, respectively. Scanning electron microscope (SEM) images (Figure 1d-e) show microplates were readily grown on the substrate in high density and some exhibit well-defined rectangular morphology. As each layer in 2D perovskites consists of self-assembled long-chain organic ligands (PEA) on the surface, the interactions between neighboring layers are characterized by van der Waals forces. This characteristic makes it feasible to epitaxially integrate different RP perovskite layers during the growth. The magnified SEM images (Figure 1f and 1g, and additional SEM images in Figure S2) and AFM image (Figure 1h) clearly reveal multiple thin sheets stacked in these microplates. To investigate their optical properties, the microplates were transferred to a clean Si/SiO2 substrate. Micro-PL spectra of a series of microplates excited by a 442-nm continuous-wave laser (Figure 2a) demonstrate that diverse heterostructures comprised of a wide range of RP perovskites can be readily formed. The simplest double heterostructure features dual color emissions (microplate II in Figure 2a): one peak centered at 527 nm, corresponding to (PEA)2PbI4 (n = 1), and the other peak centered at 575 nm, corresponding to (PEA)2(MA)Pb2I7 (n = 2). Note, microplates I and III display a single emission peak, corresponding to n = 1 and n = 2 RP 7 ACS Paragon Plus Environment


perovskites, respectively. The well-defined sharp PL peaks corresponding to specific RP perovskite phases and the clear steps on the surface of these plates (Figure S2) suggest sharp interface between different components of the heterostructures that should be separated by bilayers of PEA cations. Complex heterostructures with multicolor emission corresponding to different nvalues were also synthesized (microplate IV, V, VI, etc.). As an example, Figure 2b illustrates a vertical heterostructure containing 2D perovskites layers with n = 1, 2, and 3. Note that there could be multiple layers of the n = 1, 2 and 3 RP phases that could not be definitively determined just based on PL spectra alone. The most complex heterostructures exhibit multiple exciton emission peaks and a broad emission at ~750 nm (microplate X in Figure 2a). Representative optical images (Figure 2c) of various heterostructures under excitation demonstrate tunable emission and a strong waveguiding effect among the microplates, rendering them interesting building blocks for photonic devices. Deconvolution of the emission spectra yields a series of emission peaks located at 2.35, 2.14, 1.99, 1.88, and 1.84 eV, with a bandwidth of 76, 89, 92, 87 and 91 meV (Figure 2d), which can be assigned to the perovskite layers with n from 1 to 5, respectively. The extracted peak positions are consistent with those of previously reported single crystals of pure phase (C4H9NH3)2(MA)n−1PbnI3n+1 with the same n values.27,

28

These heterostructures with multiple

emission colors, controllable color tunability, and high PL quantum yield,42 are promising downconverters for wide color gamut displays (Figure 2e).


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Figure 2. Optical characterizations of a variety of microplates of (PEA)2(MA)n−1PbnI3n+1 heterostructures. (a) Confocal micro-PL spectra of various heterostructures excited by a 442-nm laser. (b) Schematic illustration of a vertical heterostructure formed within a microplate, which contains 2D perovskite layers with n = 1, 2, and 3. (c) Optical images of various heterostructures under 442 nm laser excitation, showing different emission colors. Scale bars are 5 µm. (d) PL peak position and bandwidth of 2D perovskites with n from 1 to 5, in comparison with those of 3D perovskite. (e) Color gamut of 2D perovskites with n = 1, 2, 3, and 4 plotted on the CIE 1931 color space chromaticity diagram. The solid and dashed white lines are color standards for LCD TV and NTSC TV, respectively.

These discrete 2D perovskite heterostructures with diverse optical properties are ideal model system to study energy/charge transfer. The general concepts of energy and/or charge transfer



through efficient coupling between higher bandgaps and lower bandgaps have been demonstrated in solution-processed films of 2D perovskites.43-47 However, these investigations were performed on poorly defined junctions consisting of multiple RP phases with a variety of n values. These film samples are not ideal systems to study the charge carrier dynamics due to significant mixing of nvalues (phases) and unknown stacking (and sometimes unknown orientations) in the continuous heterogeneous films. In contrast, the discrete heterostructures herein have more defined vertically stacked junctions and diverse, yet isolated scenarios of heterostructures that could be studied as a single object, thus represent simpler and clearer model systems to study the interlayer carrier dynamics. Based on the reported band positions of (PEA)2(MA)n−1PbnI3n+1 with different n values,43 we expect van der Waals heterojunctions constructed by these phases form type I band alignment. This alignment favors energetically allowed energy transfer from smaller-n to larger-n layer. Moreover, there are spectral overlaps between the excitonic absorption of Rydberg states and emission peaks between neighboring n RP perovskites, which increase as the n value increases.28,

48, 49

To elucidate the possible energy transfer, we investigated the charge carrier

dynamics using time-resolved PL spectroscopy and carried out power dependent PL measurements. We started with the simplest heterostructure comprised of (PEA)2PbI4 (n=1) and (PEA)2(MA)Pb2I7 (n=2) (see Figure 3a for a schematic illustration of this heterostructure), of which the PL spectrum shows two distinctive emission peaks at 532 nm and 576 nm under a low excitation density of ~44 nJ/cm2 (Figure 3b). The 532 nm emission peak (n = 1) is much stronger than the 576 nm (n = 2) emission, indicating the photo-excited carriers were primarily created in the n=1 layers. Figure 3c shows the 2D pseudo-color plot of emission spectra under different excitation power densities. The integrated PL intensities of the two peaks as a function of excitation power density, together with their corresponding fittings, are provided in Figure 3d. We fitted the 10 ACS Paragon Plus Environment

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dependencies using a relation of I∼Pβ, where I is the PL intensity, P is the excitation power density, and β is the exponent. The emission intensity of the n=1 peak grows linearly with the excitation power density (β=0.99), whereas that of the n=2 grows super linearly (β=1.45). As a result, the effective emission color of the heterostructures can be readily tuned with the excitation power density, which suggests electrically controlled color tunability may be achieved with these heterostructures if electrical injection can be implemented. Such phenomenon was also observed in the more complex heterostructures with more components (see Figure S3). Moreover, we found the β values for the same-n members varied in different heterostructures, the origin of which remains unclear.

Figure 3. Power dependent PL spectra and carrier dynamics of a (PEA)2PbI4/(PEA)2(MA)Pb2I7 heterostructure. (a) Schematic illustration of a heterostructure comprised of 2D perovskite layers



n = 1 and 2, respectively. (b) Confocal PL spectrum of a microplate shows two emission peaks at 532 nm and 576 nm, which can be assigned to 2D perovskites with n = 1 and 2. Inset schematic illustrates light emission from the heterostructure with energy transfer from n = 1 to n = 2 layer. GS: ground state, ES: excited state. (c) 2D pseudo-color plot of the emission spectra under different excitation power densities. (d) PL intensity of the 532 nm (n = 1) and 576 nm (n = 2) peaks as a function of excitation power density in double-logarithmic scale. The β indicates the slope of the relation. (e) TRPL curves of the heterostructure probed at both components’ emission wavelengths. The excitation power density is 0.88 µJ/cm2.

Convincing evidence for energy transfer comes from TRPL measurements. The individual heterostructures were excited with a femtosecond laser pulse, and the PL signals at different emission energies were then collected using a streak camera (see more details in Experimental Section). We found the PL kinetics could not be described with a typical mono-exponential decay used in a single material, particularly for the lower-bandgap emission. This is because that the carriers in each layer may experience a competitive dynamic between recombination within the layer and energy transfer from or to the neighboring heterojunction layer. We model such a dynamical process with a bi-exponential function:

( )

𝐷 (𝑡) ∝ 𝐼(𝑡) = 𝐴1exp ―

𝑡 𝑡 + 𝐴2exp ( ― ) 𝜏1 𝜏2

where D(t) is the exciton concentration which is assumed proportional to I(t), the time-resolved PL intensity, τ is the time constant and A is the amplitude of each component. The two components represent the change of exciton density due to energy transfer and carrier recombination, respectively. A negative amplitude indicates rise kinetics, whereas a positive amplitude indicates 12 ACS Paragon Plus Environment

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decay kinetics. Similar simulation models have been employed to study the dynamics of Förster resonance energy transfer between fluorescent donor and acceptor molecules.50-53 We can describe this simple heterostructure of (PEA)2PbI4 and (PEA)2(MA)Pb2I7 as a classical donor-acceptor system with two excited states, in which the n = 1 component is the energy donor and the n = 2 component is the energy acceptor. The TRPL curves of this heterostructure (Figure 3e) show the emission of the n = 1 peak decays significantly faster than that of the n = 2. We fitted each of the curves to the model equation convolved with a Gaussian instrument response function (IRF ~180 ps). The parameters from the curve fitting are tabulated in Table 1. For the peak at 532 nm (n=1), the lifetime of the faster decay component (𝜏1=185±2 ps, with 𝐴1 of 93%) was found to be about one order of magnitude shorter than that of slower decay component (𝜏2= 1658±55 ps, with 𝐴2 of 7%), indicating two characteristic channels of depopulation. Moreover, fitting for the peak at 576 nm (n = 2) yields a 𝜏1= 731±7 ps with 𝐴1 of -27% and a 𝜏2= 4057±127 ps with 𝐴2 of 127%. The negative value of 𝐴1 shows a rise kinetics of the PL signal, which is indicative of continuous population of the n = 2 component after photoexcitation. This increasing population at the n = 2 perovskite layers is matched with the initial fast decay of the n = 1 layers at a similar time scale, suggesting energy transfer from the n = 1 layers to n = 2 layers. The second exponential component with several nanoseconds of lifetime may be attributed to the slow carrier recombination within the layers, which reflects the carrier decay of individual perovskite phase without forming heterostructures. We note that if the carriers are primarily generated in the lowerbandgap phase by optical excitation, a rising kinetics will be hardly resolved, as the contribution from energy transfer becomes relatively small. These results suggest that the energy transfer in these vertical heterostructures occurs within ~102 ps time regime after excitation, which can effectively compete with the radiative recombination of the excitons that exhibit a longer lifetime 13 ACS Paragon Plus Environment


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(> ns). For comparison, the energy transfer time scale was found to be about 1-10 ns in heterostructures of quantum dots and transition metal dichalcogenides (TMDs),54, 55 50-100 ps in heterostructures of multilayer WS2 and tetracene thin films,56 and in the order of 1 ps in a MoSe2/WS2 heterostructure.57 It should be also noted that interlayer charge transfer instead of energy transfer dominates the dynamics in most 2D/2D heterostructures of TMDs due to the type II band alignment.58 Table 1. Fitting parameters of the heterostructure comprised of RP perovskite components of n = 1 and n = 2, as shown in Figure 3e. Peak (nm)

𝜏1 (ps)

𝐴1 (%)

𝜏2 (ps)

𝐴2 (%)

532

185 ± 2

93

1658 ± 55

7

576

731 ± 7

-27

4057 ± 41

127

We then turned to probe the PL kinetics in a more complex heterostructure comprised of three components. Figure 4a shows the PL spectrum of a specific heterostructure exhibiting three narrow emission peaks located at 534, 579, and 625 nm, which indicates the heterostructure is made of (PEA)2PbI4 (n = 1), (PEA)2(MA)Pb2I7 (n = 2), and (PEA)2(MA)3Pb3I10 (n = 3). The (PEA)2PbI4 component exhibits the most intense emission, implying that the n = 1 layers have the largest percentage of injected carriers in the heterostructure. In such a system, because the n = 1 and n = 3 layers have the highest and lowest bandgap, they act as the energy donor and acceptor, respectively. However, the n = 2 layers have an intermediate bandgap, therefore both energy funneling from n = 1 layers and energy transfer into n = 3 layers are possible. In such a case, the exponential component due to energy transfer reveals a “net” dynamical result of the two competitive processes. Figure 4b shows the PL decay curves of the three components and their corresponding fittings. The fitting parameters are tabulated in Table 2. For the n = 1 phase that can 14 ACS Paragon Plus Environment

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only act as energy donor, a fast exciton depopulation was clearly observed, showing a dominant fast decay with 𝜏1= 140±2 ps and 𝐴1 of 98%, and a slow decay with 𝜏2= 1198±171 ps and 𝐴2 of 2%. Following the fast decay, the kinetics of the n = 2 phase shows a rise process (indicated by a negative amplitude of -23%) with a lifetime 𝜏1 of 577±108 ps, and then a slow decay process with a lifetime of 5958±138 ps. For the n = 3 phase that can only act as energy acceptor, the rise of PL signal is even more pronounced after excitation, displaying a longer lifetime 𝜏1 of 1913±205 ps and a larger absolute ratio of the amplitudes |𝐴1 𝐴2|. We note the layer stacking sequence can be quite complex and multiple heterojunctions may exist in these heterostructures. Therefore, the rising kinetics for lower energy peaks may represent energy transfer processes across multiple heterostructure interfaces.



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Figure 4. Carrier dynamics of two examples of more complex heterostructures. (a) PL spectrum of a heterostructure shows three emission peaks at 534 nm, 579 nm, and 625 nm, indicating the heterostructure contains 2D perovskites with n = 1, 2, and 3. Inset is a schematic illustration of the heterostructure. (b) The corresponding TRPL curves of this heterostructure probed at selected wavelengths and their fittings. (c) PL spectrum of a representative complex heterostructure shows multiple emission peaks and a board emission at 760 nm. Inset scheme illustrates the energy transfer from layers with lower n to higher n. GS: ground state, ES: excited state. (d) The TRPL curves of this most complex heterostructure probed at selected wavelengths and their fittings. Table 2. Fitting parameters of the heterostructure comprised of RP perovskite components of n= 1, n = 2, and n = 3, as shown in Figure 4b. Peak (nm)

𝜏1 (ps)

𝐴1 (%)

𝜏2 (ps)

𝐴2 (%)

534

140 ± 2

98

1198 ± 171

2

579

577 ± 108

-23

5958 ± 138

123

625

1913 ± 205

-64

4647 ± 207

164

We further studied the PL kinetics of a much more complex representative multilayer heterostructure. The PL spectrum exhibits four narrow exciton peaks located at 528, 584, 624, and 655 nm, as well as a broad emission peak located around 760 nm (Figure 4c). The first four peaks correspond to 2D perovskites with n from 1 to 4, respectively, whereas the last broad peak is assigned to the overlapped exciton emissions of 2D perovskites with n > 5 and possibly confined 3D perovskite crystallites. Complex multilayer heterostructures such as this likely behave similarly to those previously studied solution-processed 2D perovskite films,45, 46, 59, 60 as they demonstrate similar optical features. Though the stacking order of these complex heterojunctions is not clear to us in this system, the directional energy transfer from higher bandgaps to lower bandgaps in 16 ACS Paragon Plus Environment

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general was also observed. Figure 4d shows PL kinetics of this heterostructure probed at different emission energies, and the fitting results are summarized in Table 3. For the n = 1 component, the carriers depopulated quickly with a 𝜏1 of 74 ps after excitation. For the n = 2-4 phases with midrange band gaps that are capable of both energy transfer in and energy transfer out processes, the fitted results reveal a net dynamic of the two processes. Interestingly, we found that the amount of energy-out was larger than the amount of energy-in for the n = 2 and n = 3 phases, as indicated by the fact that the initial kinetics contains a fast decay with a time constant of 251 and 312 ps, respectively. However, for the n = 4 component, the energy-in is larger than the energy-out, which shows a rising kinetics with 𝜏1 of 431 ps and 𝐴1 of -11.2%. The energy-in becomes more dominant for the phases with lower energy bandgaps. Evidence of this was shown by the kinetics for the emission peak of ~732 nm (corresponding to a collection of n > 5 2D perovskites), which shows a longer rising kinetics with a lifetime up to 1550 ps. Based on the above PL kinetics, we can conclude that the higher-bandgap 2D perovskites with n = 1-3 are energy donors, and their exciton transfer out relative to recombination decreases with increasing n value, which is indicated by the decrease of the absolute ratio of |𝐴1 𝐴2|. Moreover, a rising kinetics appears for the perovskites with n ≥ 4, which confirms energy transfer into the phases with lower-energy bandgaps. The rise times are found longer than those of previously reported quasi-2D perovskite films.46 For example, it is reported that the exciton localization between the adjacently stacked QWs is extremely fast (within sub-ps) and efficient (> 85% efficiency). However, the exciton localization between nonadjacently stacked QWs (that are located further away to each other) was proposed to be much slower with a time scale of ~200 ps,46 which may be the case for our heterostructures. Table 3. Fitting parameters of the most complex multi-heterostructure, as shown in Figure 4d.



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Peak (nm)

𝜏1 (ps)

𝐴1 (%)

𝜏2 (ps)

𝐴2 (%)

528

74 ± 4

97

510 ± 215

3

585

251 ± 5

64

2925 ± 24

36

625

312 ± 34

17

3938 ± 24

83

655

431 ± 131

-11

5143 ± 55

111

732

1552 ±158

-46

5164 ±128

146

Interestingly, we also found the PL kinetics measured at 760 nm (for phases close to 3D perovskite) did not show a rising kinetics, which alternatively could be fitted with a single exponential (Figure S4). There are some possible reasons. As the n value increases to ~20 and the exciton binding energy decreases, it is expected that the nature of photo-excited carriers will change from exciton to free carrier at room temperature.29 Therefore, other carrier recombination mechanisms (such as carrier trapping) may become more important, but they are not considered in the simple model above. Moreover, charge separation with electron transfer from smaller n to larger n phases and hole transfer in the opposite direction in the quasi-2D films has been reported,45, 59

which is different from the energy transfer mechanism. We think the discrepancy may come

from the differences in compositions and phase stackings of the ill-defined thin film samples. Particularly, the origins of the broad emission close to 3D perovskite may be very different from each other in the exact composition, and therefore the electronic states may differ, which are critical for determining charge carrier dynamics. A clear understanding requires further in-depth studies. However, based on the results of the simpler and more well-defined individual heterostructures reported herein, we confirm the occurrence of energy transfer in the vertically stacked heterostructures of 2D perovskites.


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CONCLUSION In summary, we have demonstrated a simple solution growth method to create vertically stacked heterostructures of 2D halide perovskites that have multi-color photoluminescence emission spectra. The heterojunctions are formed by monolithically integrating 2D RP lead iodide perovskites with different n value via relatively weak van der Waals interactions with atomically sharp interface. These new 2D heterostructures with type I band alignments provide an interesting platform to explore new properties and the dynamics of energy transfer across heterojunctions. We show the energy transfer from higher-bandgap perovskite layers to lower-bandgap perovskite layers occurs in the hundreds of picoseconds timescale after excitation. Future work on precisely controlling the compositions and the stacking sequence of each layer will be explored. The heterogeneous integration of diverse 2D halide perovskites with different properties at the atomic scale in a controllable way will lead to a novel class of solution-processed 2D heterostructures with unique functionality and a wide range of applications, such as solid-state lighting, highresolution displays, whit-light lasers, and light emitting diodes.

EXPERIMENTAL SECTION All chemicals and regents were purchased from Sigma-Aldrich and used as received unless noted otherwise. Growth of heterostructures of RP perovskites. The microscopic heterostructures were synthesized by immersing a piece of lead acetate (PbAc2) coated glass slide in a mixed solution of methylammonium iodide (MAI) and phenethylammonium iodide (PEAI) in isopropanol (IPA) at room temperature, with the PbAc2 coated side facing up (or down). The PbAc2 thin film was 19 ACS Paragon Plus Environment


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prepared by dropcasting 100 mg/mL PbAc2·3H2O aqueous solution on a glass slide. The coated substrate was then annealed at 50 °C in an oven before it was dipped into 1 mL precursor solution in a reaction vial. The precursor solution was made by mixing solutions of PEAI (40 mg/mL) and MAI (40 mg/mL) in IPA with a volume ratio of 0.65/0.35 or 0.60/0.40. Upon dipping the glass slide into the precursor solution, the surface of the film rapidly turned dark brown. After a reaction time of a few hours, the glass slide was taken out, and subsequently washed in isopropanol and dried under N2 flow. Structural characterizations. The 1H-NMR measurements were carried out using a Bruker 400 MHz NMR spectrometer, and by dissolving the samples in dimethyl sulfoxide-d6. The SEM images were collected on a LEO SUPRA 55 VP field-emission scanning electron microscope operated at 3 kV. The PXRD patterns were collected on as-grown samples on glass substrates using a Bruker D8 Advance Powder X-ray Diffractometer with Cu Kα radiation. The UV–Vis absorption was collected using a JASCO V-550 spectrometer. Atomic force microscopy (AFM) was performed using an Agilent 5500 AFM in the contact mode (sharp silicon tip on nitride cantilever with reflective gold back coating, SNL-10 from Bruker AFM Probes, k: 0.12 N/m). The samples for AFM measurement were directly grown on a Si substrate by a solution transport setup.39 Optical

characterization

and

time-resolved

photoluminescence

studies.

The

heterostructures were dry-transferred and dispersed onto a Si substrate for optical characterization. The photoluminescence (PL) spectra of individual microstructures (as shown in Figure 2a) was collected with an Aramis Confocal Raman Microscope using a 442 nm laser source. The PL spectra in Figure 3b, Figure 4a, and Figure 4c4 were collected using a confocal microscope (WITec, alpha300) with a excitation light of Ti:Sapphire laser pulses at 400 nm (repetition rate of 80 MHz, pulse 20 ACS Paragon Plus Environment

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width of 80 fs). The 400 nm excitation light was generated by the second harmonic of an 800 nm laser from a mode-locked oscillator (Tsunami 3941-X1BB, Spectra-Physics) after a BBO crystal. The laser beam was focused onto the sample surface by an objective lens (50×, Zeiss, 0.75 NA) with a spot diameter of 3 µm, and the PL emission was collected by the same objective lens. The laser power was calibrated with a power meter (PM100D from THORLABS). Time-resolved PL decay kinetics was collected using a streak camera (C10910, Hamamatsu) with the same optical setup, in which the emission signal was reflected onto the streak camera by a mirror. The excitation power densities for all the TRPL measurements are ~0.88 µJ/cm2. The instrument response function is ~180 ps. The TRPL decay curves were fitted by a deconvolution biexponential decay function with a IRF.

ASSOCIATED CONTENT Supporting Information Additional power-dependent PL of the heterostructures, and the fitting results of the TRPL decay kinetics of various heterostructures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A. P.); [email protected] (S. J.) Notes The authors declare no competing financial interest. 21 ACS Paragon Plus Environment


ACKNOWLEDGMENT This research is supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under award DE-FG02-09ER46664. S. J. also thanks the support from K. C. Wong Education Foundation. L.D. thanks NSF graduate fellowship for support. W. Z., X. W., and A. P. thank the support from National Natural Science Foundation of China (Nos. 51525202, 61574054).

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Multicolor Heterostructures of Two-Dimensional Layered Halide

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