Self-Catalyzed Vapor–Liquid–Solid Growth of Lead Halide Nanowires

Nov 7, 2017 - Lead halide perovskites (LHPs) have shown remarkable promise for use in photovoltaics, photodetectors, light-emitting diodes, and lasers...
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Self-Catalyzed Vapor-Liquid-Solid Growth of Lead Halide Nanowires and Conversion to Hybrid Perovskites Jonathan K. Meyers, Seokhyoung Kim, David J. Hill, Emma E. M. Cating, Lenzi Jessmaen Williams, Amar Kumbhar, James R. McBride, John M. Papanikolas, and James F. Cahoon Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03514 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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Self-Catalyzed Vapor-Liquid-Solid Growth of Lead Halide Nanowires and Conversion to Hybrid Perovskites Jonathan K. Meyers,† Seokhyoung Kim,† David J. Hill,† Emma E. M. Cating,† Lenzi J. Williams,† Amar S. Kumbhar,‡ James R. McBride,δ John M. Papanikolas,† and James F. Cahoon†,* †

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina

27599-3290, United States, ‡Chapel Hill Analytical & Nanofabrication Laboratory, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States, δVanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States KEYWORDS: lead iodide, self-catalyzed vapor-liquid-solid growth, lead halide perovskite, lightemitting diode, solar cell ABSTRACT: Lead halide perovskites (LHPs) have shown remarkable promise for use in photovoltaics, photodetectors, light-emitting diodes, and lasers. Although solution-processed polycrystalline films are the most widely studied morphology, LHP nanowires (NWs) grown by vapor-phase processes offer the potential for precise control over crystallinity, phase, composition, and morphology. Here, we report the first demonstration of self-catalyzed vapor-liquid-solid (VLS) growth of lead halide (PbX2; X = Cl, Br, I) NWs and conversion to LHP. We present a kinetic model of the PbX2 NW growth process, in which a liquid Pb catalyst is supersaturated with halogen X through vapor-phase incorporation of both Pb and X, inducing growth of a NW. For PbI2, we show that the NWs are single-crystalline, oriented in the 〈1 21 0〉 direction, and composed of a stoichiometric PbI2 shaft with a spherical Pb tip. Low-temperature vaporphase intercalation of methylammonium iodide converts the NWs to methylammonium lead iodide (MAPbI3) perovskite while maintaining the NW morphology. Single-NW experiments comparing measured extinction spectra with optical simulations show that the NWs exhibit a strong optical antenna 1 ACS Paragon Plus Environment

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effect, leading to substantially enhanced scattering efficiencies and to absorption efficiencies that can be more than twice that of thin films of the same thickness. Further development of the self-catalyzed VLS mechanism for lead halide and perovskite NWs should enable the rational design of nanostructures for various optoelectronic technologies, including potentially unique applications such as hot-carrier solar cells.

Lead halide perovskites (LHPs) of the form APbX3 (A = organic or inorganic cation; X = Cl, Br, or I), including methylammonium lead iodide (MAPbI3), have garnered substantial attention because of the high-performance photovoltaic (PV) devices that can be fabricated with solution-processed films.1-4 Since the initial reports1,2 of high-efficiency LHP solar cells, there has been a growing interest in developing these materials for PV devices,3,4 photodetector applications such as X-ray detectors,5 and photonic technologies6 such as light-emitting diodes7 and lasers.8,9 The high performance of these materials can be attributed to the high absorption coefficients and extraordinarily long diffusion lengths of both electrons and holes10,11 for a solution-processed material. Furthermore, altering the halide ratio in mixed halide perovskites, MAPbX3-xYx (X; Y = Cl, Br, or I), allows for a tunable bandgap and lasing emission,12 situating LHPs as an adaptable and efficient platform for optoelectronic applications. A range of methods has been developed to synthesize LHPs in morphologies that include thin polycrystalline films,1-4 macroscopic single crystals,13,14 ultrathin conformal coatings,15,16 quantum dots,17,18 and nanowires12,19-29 (NWs). Although thin films are the most widely explored morphology, there is building interest in the development of lower dimensional forms of LHP as a means to study fundamental properties on well-defined crystalline materials as well as invoke new properties found in nanoscale materials. One-dimensional NWs offer the potential to rationally design structures with desired electronic and photonic properties. Building on the widely explored thin-film syntheses, LHP NWs have been grown through solution-phase processes. For instance, disordered NW networks have been synthesized, primarily by 2 ACS Paragon Plus Environment

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utilizing PbX2 precursors in a dissolution-recrystallization mechanism20,29 with a variety of modifications to affect crystallization.20 Similarly, LHP NWs with mixed cations and/or halogens have been grown, and these techniques have yielded single-crystalline NWs capable of low-threshold lasing.12,21,22 In order to better control solution-phase syntheses, templates28,29 such as anodic aluminum oxide19 have been used but typically produce disordered polycrystalline materials. Although there has been significant progress, solution-phase LHP NW syntheses have not yet yielded precise nanoscale control of morphology, crystallinity, and composition – characteristics that have been demonstrated in other semiconductor NW systems and that are needed for many technological applications. Vapor-phase LHP NW syntheses reported to date generally offer the same growth handles as solution-phase processes. For example, single-crystalline PbX2 NWs were grown by vapor-phase transport of PbX2 and then converted through the vapor phase to perovskite NWs of MAPbI3, MAPbBr3, and MAPbI3-xClx.23 A similar method was used to directly grow single-crystal NWs of CsPbX3 (X = Cl, Br, I).24 These growth processes have gained an additional level of control through the use of epitaxial25,26 and templated26 growth. Despite these prior reports on the vapor-phase syntheses of LHP NWs, the synthetic control over this material is still limited compared to other semiconductor NWs, especially those grown by the vapor-liquid-solid (VLS) mechanism. The VLS growth mechanism, first reported for Au-catalyzed Si NWs more than fifty years ago,30 remains a topic of intensive research. In this mechanism, a metallic liquid droplet serves as a catalyst particle that is supersaturated through the vapor-phase addition of material, inducing the growth of a single-crystalline semiconductor NW at the liquid-solid interface between the catalyst and wire. The VLS mechanism has been reported for a wide-range of metal catalysts and semiconductor materials, including group IV, III-V, and II-VI semiconductors,31,32 and there are several reports of PbS VLS growth.33-36 Selfcatalyzed growth, in which one element of the semiconductor also serves as the catalyst, has been developed for some materials37-40 as a way to preclude contamination of the semiconductor with impurity metal atoms. The long-standing interest in the VLS process stems both from the high-quality, singlecrystalline nature of the material produced and the precise control over morphology, growth density, 3 ACS Paragon Plus Environment

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position, and composition afforded by the VLS mechanism, which also permits modulation of composition along the axis of the NWs. Moreover, unique and unexpected phases of semiconductor materials have been formed, such as InGaN NWs that support higher In fractions than would be possible in a bulk material,41 GaAs NWs that can crystallize in the wurtzite phase,42 and GaP/Si NWs that allow the growth of the hexagonal phase of Si.43 These numerous considerations motivate the development of a VLS growth process for lead halide and perovskite materials. In this letter, we report the first observation of self-catalyzed VLS growth of single-crystalline PbX2 NWs using liquid Pb catalysts, and we demonstrate the vapor-phase conversion of single-crystalline PbI2 NWs to MAPbI3 NWs. The mechanism of VLS growth in this system can be understood from the Pb-I binary equilibrium phase diagram44 shown in Figure 1A. As indicated in the diagram, Pb melts at 328 °C, and above this temperature it can form an equilibrium liquid phase with a wide range of atomic percent I. Above this equilibrium percentage, indicated by the shaded region in Figure 1A, the liquid Pb alloy will be supersaturated with I and produce a thermodynamic driving force for the precipitation of solid PbI2. Thus, as illustrated in Figure 1B, VLS growth will occur if a nanoscale liquid Pb droplet can be supersaturated with I and be continuously supplied with both Pb and I in the vapor-phase, enabling the growth of a single-crystalline PbI2 NW. The phase diagrams for Pb-Cl and Pb-Br are shown in Figure S1 and are qualitatively similar to the Pb-I phase diagram, indicating that the growth of PbCl2, PbBr2, and PbI2 should all be possible by the VLS mechanism. Pb catalyst particles were prepared by thermal evaporation of Pb on SiO2/Si growth wafers, and VLS NW growth was performed in a low-pressure chemical vapor deposition (CVD) system illustrated in Figure 1C (see Methods in Supporting Information for details). Alternatively, we found that we could use commercially-available Sn particles as catalysts because they rapidly alloy with Pb and fully convert to Pb catalysts under the growth conditions used herein (see Figure S2). At a pressure of 200 Torr and carrier gas flow of 120 sccm Ar, PbI2 powder in the first zone of the furnace served as the source of vapor-phase Pb and I (furnace set point of 415 °C) while the growth wafer in the second zone was held at a fixed furnace set point in the range of 275-325 °C. Because of the large temperature gradient between 4 ACS Paragon Plus Environment

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the two zones, we estimate the local catalyst and NW growth temperature to be between these two set points. Holding these conditions for 60 min led to a good yield of one-dimensional NWs growing out of the plane of the SiO2/Si substrate, as shown by the scanning electron microscopy (SEM) image in Figure 1D. As shown by the gallery of SEM images in Figure 1E, every NW exhibited a pronounced spherical tip, indicative of catalyzed one-dimensional growth through a nanoparticle. Moreover, control experiments on substrates without pre-deposited Pb show little to no growth of spherically-capped NWs under these conditions, confirming the necessity of the pre-deposited Pb to induce a high yield of NW growth. A representative single NW harvested from the growth substrate is shown in Figure 1F. As exemplified by the images in Figures 1D-F, the NWs are tens of microns in length and have a rectangular cross section with side lengths ranging from 100 nm to 2 µm. Similar CVD growth procedures were performed with PbCl2 and PbBr2 precursors, and wires with distinct spherical tips were observed (see Figure S1), indicating that the same NW growth process occurs with PbCl2, PbBr2, and PbI2. High-resolution images of PbI2 NWs were obtained on a 200 kV scanning transmission electron microscope (STEM) equipped for energy-dispersive X-ray spectroscopy (EDS). The elemental maps collected by EDS (Figure 2A) show that the spherical tip of the NW is composed almost entirely of Pb, and the NW is composed, within the detection limits of EDS, of stoichiometric PbI2. The spherical Pb tip also contains a thin shell of I that is 10-15 nm in thickness (see Figure S3 for EDS line scans across the catalyst and NW). Integrating the EDS signal over the center of the tip yields a net I atomic percent of 2.3%. This value is consistent with the expected equilibrium solubility of I in Pb if the tip was liquid during NW growth at ~365 °C and solidified at the end of the growth. The O EDS elemental map in Figure 2A also reveals the presence of an oxide shell that is 10–30 nm in thickness across the entire length of the NW (see O EDS line scans in Figure S3). This oxide layer was presumably formed by exposure of the NW to ambient conditions prior to STEM imaging. Considering the low percentage of I in the catalyst, the spherical morphology of the Pb particle, the presence of a Pb particle at the tip of all NWs, and the Pb-I phase diagram in Figure 1A, we conclude that the NWs were grown by the VLS 5 ACS Paragon Plus Environment

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mechanism through supersaturation of I in a liquid Pb catalyst. Note that the diameters of many of the Pb catalyst particles are larger than the initial size of the particles deposited on the substrate surface, suggesting that the Pb particles may increase in size during NW nucleation either through Ostwald ripening or vapor-phase incorporation of Pb. However, additional investigation of the nucleation process will be needed to understand the correlation between the initial diameter of the as-deposited Pb particles and the final diameter of the NWs. The crystalline quality of the PbI2 NWs was evaluated through transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) as shown in Figures 2B and 2C, respectively. From the SAED pattern, the NW growth direction was determined to be 〈1 21 0〉 (equivalent to 〈010〉). Diffraction patterns collected from different points along the same NW and from different NWs all confirm the same growth direction, indicating that this growth direction is preferred and suggesting that the NWs are single crystalline. The SEM image of a PbI2 NW cleaved perpendicular to its growth axis in Figure 2D clearly shows stacked (0001) planes parallel to the growth axis, further confirming the crystallographic orientation of the NW. Although the NW growth axis is oriented in the 〈1 21 0〉 direction, the STEM-EDS elemental maps in Figure 2E show that more than one orientation of the catalyst interface relative to the wire axis is possible, showing examples at 0° and 30°. Based on these angles, these images suggest that both the 1 100 and 1 21 0 planes can act as interfacial growth planes; however, further investigation of the liquid-solid interface during VLS growth is needed. Although the PbI2 NWs are relatively uniform, they exhibit a small degree of tapering from the base of the NW to the tip, as exemplified by the SEM images and diameter profile in Figure 3A. Because of the variability in catalyst size and NW cross-sectional ratio, we assume a cylindrical geometry to simplify our analysis of the NW morphology. We define the radial tapering parameter, σ, as σ = dr/dL, where r is the NW radius and L is the axial length of the NW.45-47 For the NW shown in Figure 3A, a linear fit of the diameter profile yields σ ≈ −2 nm/µm. Across multiple NWs, we found the degree of tapering to range from approximately 0 to −13 nm/µm. In addition to tapering, we also found that some

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fraction of NWs exhibited a platelet morphology (see SEM images in Figure S4), which we attribute to epitaxial growth extending the basal plane of the NW as a result of either vapor-solid deposition on the NW sidewall48 or secondary Pb catalyst induced growth on the sidewall, an effect that has been observed for Au-catalyzed InP NWs.49 To understand the morphology and VLS growth mechanism of lead halide NWs, we propose a simple model of the growth kinetics. As illustrated in Figure 3B, our model includes the microscopic kinetic processes of vapor-phase Pb and diatomic halogen (X2) incorporation into the liquid catalyst and X

2 crystallization of PbX2 into the solid NW. We define the rate of Pb and X2 incorporation as RPb i and Ri ,

respectively, and the rate of PbX2 crystallization as Rc. For incorporation, each rate can be expressed with M M M M a first-order rate expression, RM i = ki P (M = Pb; X2), where ki and P refer to the first-order rate

constant and partial pressure, respectively. Although in the experiments reported herein we use PbX2 as the precursor material, we still include separate rates of Pb and X2 incorporation because of the possibility for dissociation and differential sublimation of PbX2 into Pb0 and X2 vapor, permitting a different stoichiometry to be present in the vapor phase compared to the precursor solid phase.50 Similarly, the rate of crystallization can be expressed as Rc = kc[X2], where [X2] is the concentration of halide in the liquid catalyst, and kc is the pseudo first-order effective rate constant for the crystallization of PbX2, where Pb is considered to be in excess. For simplicity we do not consider the process of dissolution of the solid NW back into the catalyst; thus, kc in this analysis depends on the concentration of halogen, and kc should approach zero as [X2] approaches the maximum equilibrium concentration (i.e. liquidus line). Note also that for simplicity we write all halogen concentrations in terms of diatomic species X2, and we do not consider the process of evaporation from the liquid to the vapor phase. In this model, the NW morphology (i.e. the tapering parameter, σ) is dictated by the ratio of the rates of incorporation for X2 and Pb, which is defined by the parameter α such that α = Ri 2 /RPb i . If we X

assume the atomic percentage of X2 in the liquid catalyst (i.e. the degree of supersaturation) is constant during VLS growth, then for a hemispherical catalyst particle we find that

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1−χ

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(1)

where χ is the atomic ratio of X2 to Pb in the liquid catalyst, i.e. χ = [X2]/[Pb]. The full derivation of eq 1 can be found in the Supporting Information. Using this expression, values of σ as a function of α are displayed in Figure 3C for several distinct atomic percentages of X in the liquid Pb catalyst. Based on this analysis, we can identify several growth regimes which are illustrated schematically in Figure 3D. If  ≤ , the catalyst cannot reach the supersaturation required to nucleate a NW and VLS growth will not occur; instead, the liquid Pb droplet will accumulate Pb and increase in diameter without forming a NW. On the other hand, if α ≫ 1, the liquid Pb catalyst will quickly saturate with halogen and crystallize as PbX2, resulting in little or no NW growth. However, if the incorporation rates are similar, α ≈ 1, the NW can nucleate and grow with minimal tapering (σ ≈ 0). Even if α is not precisely unity, the atomic percentage of X2 in the catalyst can still be maintained and VLS growth should be stable. However, to maintain a constant χ, the catalyst will either be slowly ripened (α < 1) or consumed (α > 1), causing an increase (σ > 0) or decrease (σ < 0), respectively, in the diameter of the NW as it grows. For the experimental value of σ measured from the NW in Figure 3A, we estimate the value of α to be 1.011 ± 0.001, assuming an I concentration in the liquid catalyst of < 10 at.% (i.e. χ < 0.06). This result suggests that the CVD growth conditions used here provide a slight excess of I2 from the vapor phase, but the incorporation rates of Pb and I2 are sufficiently similar to enable stable VLS growth with relatively minimal tapering of the NW. Note that it is difficult to ascertain the role of vapor-solid overcoating for the NWs with minimal tapering, and the vapor-solid process is a topic to be addressed in future growth studies. After obtaining reproducible and high-quality VLS-grown PbI2 NWs, we converted them to MAPbI3 perovskite NWs through a low-temperature vapor-phase procedure.23 Conversion by vapor-phase methylammonium iodide (MAI) was performed in the quartz-tube reactor illustrated in Figure 1C by placing the MAI precursor and PbI2 NW growth substrate in the first and second zones, respectively, and

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holding both furnace set points at 100 °C for 60 min under a total pressure of 50 Torr and carrier gas flow of 100 sccm Ar. After conversion, the morphology of the NWs was substantially altered, exhibiting a rougher surface and a linear expansion up to ~80% (see Figure S5). Moreover, some NWs become slightly bowed, as observed in Figure 4A, due to strain experienced during expansion.51 The TEM images collected on MAPbI3 NWs, shown in Figure 4B, indicate that the NWs are polycrystalline. SAED patterns (Figure 4B, right) taken at multiple points along the axis of the NW show that the conversion to LHP forms randomly-oriented polycrystalline domains, which is consistent with prior reports on the vaporphase conversion of PbI2 single crystals.51 However, the quality and mechanism of conversion in single NWs as a function of process conditions will be a topic of future investigations. Photoluminescence (PL) spectra were collected from PbI2 and MAPbI3 NWs and are shown in Figure 4C. Emission maxima are observed at 516 nm and 768 nm for PbI2 and MAPbI3 NWs, respectively. Similar to other MAPbI3 perovskites,52 the NW has an emission full width at half maximum of 110 meV centered at 1.6 eV, and the relatively blue emission wavelength suggests a strong contribution either from surface effects53 or a small crystallite size in the material.54 PL maps of individual NWs were generated by raster scanning a 425 nm laser beam focused to a diffraction-limited spot size across the samples and collecting the resulting PL emission. The PL maps of PbI2 and MAPbI3 NWs (Figure 4D) show that both materials emit across the entire NW. Variations in the PL intensity most likely result from changes in the coupling of the pump laser light into the structure, which can strongly depend on the local NW geometry.55 The absence of PL near the bandgap of PbI2 in the MAPbI3 NW PL spectrum, combined with the expected emission wavelength throughout the NW structure, suggests that the conversion of PbI2 to MAPbI3 was complete. Semiconductor NWs exhibit a range of unique photonic characteristics related to their high refractive index and sub-micron diameters, which cause the NWs to behave as sub-wavelength optical cavities that support leaky-mode optical resonances. As a result, NWs can exhibit an optical antenna effect that causes enhanced light absorption and scattering because the absorption and scattering cross sections exceed the physical cross section of the wires.56 This effect has been observed in Si NW 9 ACS Paragon Plus Environment

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photovoltaic devices, which exhibit improved short-circuit current densities relative to planar films.57-59 Because of the relatively high refractive indices of both PbI2 and MAPbI3,60,61 we expected that in a NW geometry, both materials would exhibit the optical antenna effect. Optical measurements and simulations (Figure 5) confirm this expectation. Figure 5A and 5B display simulated absorption, scattering, and extinction efficiency spectra along with experimental measurements of extinction for PbI2 and MAPbI3 NWs, respectively. We consider NWs that are oriented horizontally and illuminated perpendicular to their growth axis (see Figure S6 for details on the angle-dependence of illumination).62 Optical simulations were performed using the finite-element method in the wave-optics regime, and experimental spectra were collected by measuring the differential optical signal from single NWs using a home-built microscope with a supercontinuum laser source. The simulated spectra in Figure 5A were calculated for a rectangular PbI2 NW 225 nm wide and 150 nm thick, reflecting the approximate dimensions measured from SEM images, and the simulated extinction spectrum is in good qualitative agreement with the experimental measurement. Snapshots of the electric field profile inside the NW at four wavelengths (denoted I-IV) are also shown in Figure 5A and highlight the complex optical mode profiles57-59 that arise because the NW behaves as a sub-wavelength optical cavity. For wavelengths longer than 520 nm, the extinction spectrum is dominated by two peaks: a broad and a narrow scattering resonance (labeled I and II) centered at 700800 nm and 550-600 nm respectively. The scattering efficiency of each peak exceeds 400%, indicative of a strong optical antenna effect in which the scattering cross section exceeds the physical cross section. For wavelengths below ~520 nm, absorption is significant, and a comparison of the absorption efficiency from the top 150 nm of a bulk PbI2 film (green dashed-dotted line) and the NW (blue dashed-dotted line) shows an absorption enhancement in the NW of up to 500%. Figure 5B displays optical spectra from a single MAPbI3 NW, and simulated spectra were calculated for a rectangular geometry 550 nm wide and 350 nm thick, in accord with measurements from SEM images. Similar to the PbI2 NW in Figure 5A, the simulated extinction spectrum is in good qualitative agreement with the experiment. However, because of the higher absorption coefficient of 10 ACS Paragon Plus Environment

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MAPbI3 and the larger size of the NW compared to PbI2 NW, the absorption efficiency (red dasheddotted line) makes a large contribution to the overall extinction efficiency across the entire spectrum. Several peaks, labeled I-V, are apparent in the absorption spectrum, and absorption mode profiles at the wavelengths corresponding to each peak are shown on the right-hand side of Figure 5B. For comparison, the absorption efficiency of the upper 350 nm of a bulk perovskite film is shown as the green dasheddotted line. The absorption efficiency of the NW is larger than the film across the entire visible spectrum, and because of the lower absorption coefficient of the material toward the band edge (~770 nm), the absorption in the NW is more than double that of the film. These results confirm the presence of the optical antenna effect in both PbI2 and MAPbI3 NWs, causing substantially enhanced scattering and absorption that can be used for various photonic and optoelectronic technologies. In summary, we have described the self-catalyzed VLS growth of single-crystalline PbX2 (X = Cl, Br, I) NWs and have shown that the VLS-grown PbI2 NWs are easily converted to organic-inorganic hybrid perovskite materials such as MAPbI3. A simple kinetic model of the growth process, requiring supersaturation of a liquid Pb catalyst with X, provides guidelines for further development of stable VLS growth of uniform NWs. Optical experiments and simulations on single NWs show that the structures exhibit leaky-mode resonances and the optical antenna effect, which substantially enhances absorption in the NWs compared to planar films. Further development of this VLS process should lead to direct control of NW diameter and the in-situ synthesis of heterojunctions for various optoelectronic and solar energy applications. The growth of high-quality and customizable hybrid perovskite NWs could lead to the rational design of new devices, such as nanoscale electrically-injected lasers63 and hot-carrier solar cells.64,65

Author Information. Corresponding Author *Email: [email protected]

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Associated Content. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Detailed CVD reactor parameters, VLS growth of PbCl2 and PbBr2 NWs, Snassisted self-catalyzed PbI2 NWs, EDS line scans of PbI2 NW, existence of platelet morphology, linear expansion through conversion to perovskite, optical efficiencies and profiles of PbI2 and MAPbI3, and derivation of eq 1.

Acknowledgements. We thank Kyoung-Ho Kim and Joseph D. Christesen for fruitful discussions on optical simulations and NW growth, respectively. This research was primarily supported by a Packard Fellowship for Science and Engineering. J.F.C. acknowledges a Sloan Research Fellowship, J.K.M. and D.J.H. acknowledge National Science Foundation Graduate Research Fellowships, and S.K. acknowledges a Kwanjeong Scholarship. This work made use of instrumentation at the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the NSF (ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI).

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Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P.; Lu, Z.; Kim, D. H.; Sargent, E. H. Perovskite Energy Funnels for Efficient Light-Emitting Diodes. Nat. Nanotechnol. 2016, 11, 872-877. Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-Temperature Solution-Processed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13, 476-480. Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D.-D.; Higler, R.; Hüttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; Atatüre, M.; Phillips, R. T.; Friend, R. H. High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. J. Phys. Chem. Lett. 2014, 5, 1421-1426. Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344-347. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science. 2013, 342, 341-344. Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X.-Y. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636-642. Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-Hole Diffusion Lengths >175 µm in Solution Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967-970. Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O. M. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519-522. Sutherland, B. R.; Hoogland, S.; Adachi, M. M.; Wong, C. T. O.; Sargent, E. H. Conformal Organohalide Perovskites Enable Lasing on Spherical Resonators. ACS Nano 2014, 8, 10947-10952. Sutherland, B. R.; Hoogland, S.; Adachi, M. M.; Kanjanaboos, P.; Wong, C. T. O.; McDowell, J. J.; Xu, J.; Voznyy, O.; Ning, Z.; Houtepen, A. J.; Sargent, E. H. Perovskite Thin Films via Atomic Layer Deposition. Adv. Mater. 2015, 27, 53-58. Schmidt, L. C.; Pertegás, A.; González-Carrero, S.; Malinkiewicz, O.; Agouram, S.; Mínguez Espallargas, G.; Bolink, H. J.; Galian, R. E.; Pérez-Prieto, J. Nontemplate Synthesis of CH3NH3PbBr3 Perovskite Nanoparticles. J. Am. Chem. Soc. 2014, 136, 850-853. Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M. Quantum Dot–Induced Phase Stabilization of α-CsPbI3 Perovskite for High-Efficiency Photovoltaics. Science 2016, 354, 92-95. Ashley, M. J.; O’Brien, M. N.; Hedderick, K. R.; Mason, J. A.; Ross, M. B.; Mirkin, C. A. Templated Synthesis of Uniform Perovskite Nanowire Arrays. J. Am. Chem. Soc. 2016, 138, 10096-10099. Im, J.-H.; Luo, J.; Franckevičius, M.; Pellet, N.; Gao, P.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M.; Park, N.-G. Nanowire Perovskite Solar Cell. Nano Lett. 2015, 15, 2120-2126. Fu, Y.; Zhu, H.; Schrader, A. W.; Liang, D.; Ding, Q.; Joshi, P.; Hwang, L.; Zhu, X.-Y.; Jin, S. Nanowire Lasers of Formamidinium Lead Halide Perovskites and Their Stabilized Alloys with Improved Stability. Nano Lett. 2016, 16, 1000-1008.

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(22) Eaton, S. W.; Lai, M.; Gibson, N. A.; Wong, A. B.; Dou, L.; Ma, J.; Wang, L.-W.; Leone, S. R.; Yang, P. Lasing in Robust Cesium Lead Halide Perovskite Nanowires. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 1993-1998. (23) Xing, J.; Liu, X. F.; Zhang, Q.; Ha, S. T.; Yuan, Y. W.; Shen, C.; Sum, T. C.; Xiong, Q. Vapor Phase Synthesis of Organometal Halide Perovskite Nanowires for Tunable Room-Temperature Nanolasers. Nano Lett. 2015, 15, 4571-4577. (24) Park, K.; Lee, J. W.; Kim, J. D.; Han, N. S.; Jang, D. M.; Jeong, S.; Park, J.; Song, J. K. Light– Matter Interactions in Cesium Lead Halide Perovskite Nanowire Lasers. J. Phys. Chem. Lett. 2016, 7, 3703-3710. (25) Chen, J.; Fu, Y.; Samad, L.; Dang, L.; Zhao, Y.; Shen, S.; Guo, L.; Jin, S. Vapor-Phase Epitaxial Growth of Aligned Nanowire Networks of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I). Nano Lett. 2017, 17, 460-466. (26) Wang, Y.; Sun, X.; Shivanna, R.; Yang, Y.; Chen, Z.; Guo, Y.; Wang, G.-C.; Wertz, E.; Deschler, F.; Cai, Z.; Zhou, H.; Lu, T.-M.; Shi, J. Photon Transport in One-Dimensional Incommensurately Epitaxial CsPbX3 Arrays. Nano Lett. 2016, 16, 7974-7981. (27) Spina, M.; Bonvin, E.; Sienkiewicz, A.; Náfrádi, B.; Forró, L.; Horváth, E. Controlled Growth of CH3NH3PbI3 Nanowires in Arrays of Open Nanofluidic Channels. Scientific reports 2016, 6, 19834. (28) Zhuo, S.; Zhang, J.; Shi, Y.; Huang, Y.; Zhang, B. Self-Template-Directed Synthesis of Porous Perovskite Nanowires at Room Temperature for High-Performance Visible-Light Photodetectors. Angew Chem Int Ed Engl 2015, 54, 5693-5696. (29) Zhu, P.; Gu, S.; Shen, X.; Xu, N.; Tan, Y.; Zhuang, S.; Deng, Y.; Lu, Z.; Wang, Z.; Zhu, J. Direct Conversion of Perovskite Thin Films into Nanowires with Kinetic Control for Flexible Optoelectronic Devices. Nano Lett. 2016, 16, 871-876. (30) Wagner, R. S.; Ellis, W. C. Vapor-Liquid-Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4, 89-90. (31) Schmidt, V.; Wittemann, J. V.; Gösele, U. Growth, Thermodynamics, and Electrical Properties of Silicon Nanowires. Chem. Rev. 2010, 110, 361-388. (32) Thelander, C.; Agarwal, P.; Brongersma, S.; Eymery, J.; Feiner, L. F.; Forchel, A.; Scheffler, M.; Riess, W.; Ohlsson, B. J.; Gösele, U.; Samuelson, L. Nanowire-Based One-Dimensional Electronics. Mater. Today 2006, 9, 28-35. (33) Afzaal, M.; Ahmad, K.; O'Brien, P. Probing the Growth Mechanism of Self-Catalytic Lead Selenide Wires. J. Mater. Chem. 2012, 22, 12731-12735. (34) Bierman, M. J.; Lau, Y. K. A.; Jin, S. Hyperbranched PbS and PbSe Nanowires and the Effect of Hydrogen Gas on Their Synthesis. Nano Lett. 2007, 7, 2907-2912. (35) Nichols, P. L.; Liu, Z.; Yin, L.; Turkdogan, S.; Fan, F.; Ning, C. Z. CdxPb1–xS Alloy Nanowires and Heterostructures with Simultaneous Emission in Mid-Infrared and Visible Wavelengths. Nano Lett. 2015, 15, 909-916. (36) Nichols, P. L.; Sun, M.; Ning, C.-Z. Influence of Supersaturation and Spontaneous Catalyst Formation on the Growth of PbS Wires: Toward a Unified Understanding of Growth Modes. ACS Nano 2011, 5, 8730-8738. (37) Gomes, U. P.; Ercolani, D.; Zannier, V.; David, J.; Gemmi, M.; Beltram, F.; Sorba, L. Nucleation and Growth Mechanism of Self-Catalyzed InAs Nanowires on Silicon. Nanotechnology 2016, 27, 255601. (38) Mandl, B.; Stangl, J.; Hilner, E.; Zakharov, A. A.; Hillerich, K.; Dey, A. W.; Samuelson, L.; Bauer, G.; Deppert, K.; Mikkelsen, A. Growth Mechanism of Self-Catalyzed Group III−V Nanowires. Nano Lett. 2010, 10, 4443-4449.

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(39) Priante, G.; Patriarche, G.; Oehler, F.; Glas, F.; Harmand, J.-C. Abrupt GaP/GaAs Interfaces in Self-Catalyzed Nanowires. Nano Lett. 2015, 15, 6036-6041. (40) Tersoff, J. Stable Self-Catalyzed Growth of III–V Nanowires. Nano Lett. 2015, 15, 6609-6613. (41) Kuykendall, T.; Ulrich, P.; Aloni, S.; Yang, P. Complete Composition Tunability of InGaN Nanowires Using a Combinatorial Approach. Nat. Mater. 2007, 6, 951-956. (42) Jacobsson, D.; Panciera, F.; Tersoff, J.; Reuter, M. C.; Lehmann, S.; Hofmann, S.; Dick, K. A.; Ross, F. M. Interface Dynamics and Crystal Phase Switching in GaAs Nanowires. Nature 2016, 531, 317-322. (43) Hauge, H. I. T.; Verheijen, M. A.; Conesa-Boj, S.; Etzelstorfer, T.; Watzinger, M.; Kriegner, D.; Zardo, I.; Fasolato, C.; Capitani, F.; Postorino, P.; Kölling, S.; Li, A.; Assali, S.; Stangl, J.; Bakkers, E. P. A. M. Hexagonal Silicon Realized. Nano Lett. 2015, 15, 5855-5860. (44) Okamoto, H. I-Pb (Iodine-Lead). J. Phase Equilib. Diffus. 2010, 31, 320-321. (45) Krylyuk, S.; Davydov, A. V.; Levin, I. Tapering Control of Si Nanowires Grown from SiCl4 at Reduced Pressure. ACS Nano 2011, 5, 656-664. (46) Sharma, S.; Kamins, T. I.; Williams, R. S. Diameter Control of Ti-Catalyzed Silicon Nanowires. J. Cryst. Growth 2004, 267, 613-618. (47) Wang, Y.; Schmidt, V.; Senz, S.; Gösele, U. Epitaxial Growth of Silicon Nanowires Using an Aluminium Catalyst. Nat. Nanotechnol. 2006, 1, 186-189. (48) Perea, D. E.; Hemesath, E. R.; Schwalbach, E. J.; Lensch-Falk, J. L.; Voorhees, P. W.; Lauhon, L. J. Direct Measurement of Dopant Distribution in an Individual Vapour-Liquid-Solid Nanowire. Nat. Nanotechnol. 2009, 4, 315-319. (49) Kelrich, A.; Sorias, O.; Calahorra, Y.; Kauffmann, Y.; Gladstone, R.; Cohen, S.; Orenstein, M.; Ritter, D. InP Nanoflag Growth from a Nanowire Template by in Situ Catalyst Manipulation. Nano Lett. 2016, 16, 2837-2844. (50) Fornaro, L.; Saucedo, E.; Mussio, L.; Yerman, L.; Ma, X.; Burger, A. Lead Iodide Film Deposition and Characterization. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 458, 406-412. (51) Brenner, T. M.; Rakita, Y.; Orr, Y.; Klein, E.; Feldman, I.; Elbaum, M.; Cahen, D.; Hodes, G. Conversion of Single Crystalline PbI2 to CH3NH3PbI3: Structural Relations and Transformation Dynamics. Chem. Mater. 2016, 28, 6501-6510. (52) Wong, A. B.; Lai, M.; Eaton, S. W.; Yu, Y.; Lin, E.; Dou, L.; Fu, A.; Yang, P. Growth and Anion Exchange Conversion of CH3NH3PbX3 Nanorod Arrays for Light-Emitting Diodes. Nano Lett. 2015, 15, 5519-5524. (53) Wu, B.; Nguyen, H. T.; Ku, Z.; Han, G.; Giovanni, D.; Mathews, N.; Fan, H. J.; Sum, T. C. Discerning the Surface and Bulk Recombination Kinetics of Organic-Inorganic Halide Perovskite Single Crystals. Adv. Energy Mater. 2016, 6, 1600551. (54) D'Innocenzo, V.; Srimath Kandada, A. R.; De Bastiani, M.; Gandini, M.; Petrozza, A. Tuning the Light Emission Properties by Band Gap Engineering in Hybrid Lead Halide Perovskite. J. Am. Chem. Soc. 2014, 136, 17730-17733. (55) Mehl, B. P.; Kirschbrown, J. R.; Gabriel, M. M.; House, R. L.; Papanikolas, J. M. Pump–Probe Microscopy: Spatially Resolved Carrier Dynamics in ZnO Rods and the Influence of Optical Cavity Resonator Modes. J. Phys. Chem. B 2013, 117, 4390-4398. (56) Cao, L.; Fan, P.; Vasudev, A. P.; White, J. S.; Yu, Z.; Cai, W.; Schuller, J. A.; Fan, S.; Brongersma, M. L. Semiconductor Nanowire Optical Antenna Solar Absorbers. Nano Lett. 2010, 10, 439-445. (57) Kempa, T. J.; Cahoon, J. F.; Kim, S.-K.; Day, R. W.; Bell, D. C.; Park, H.-G.; Lieber, C. M. Coaxial Multishell Nanowires with High-Quality Electronic Interfaces and Tunable Optical Cavities for Ultrathin Photovoltaics. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 1407-1412. 15 ACS Paragon Plus Environment

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(58) Kim, S.-K.; Day, R. W.; Cahoon, J. F.; Kempa, T. J.; Song, K.-D.; Park, H.-G.; Lieber, C. M. Tuning Light Absorption in Core/Shell Silicon Nanowire Photovoltaic Devices through Morphological Design. Nano Lett. 2012, 12, 4971-4976. (59) Kim, S.-K.; Zhang, X.; Hill, D. J.; Song, K.-D.; Park, J.-S.; Park, H.-G.; Cahoon, J. F. Doubling Absorption in Nanowire Solar Cells with Dielectric Shell Optical Antennas. Nano Lett. 2015, 15, 753-758. (60) Ahuja, R.; Arwin, H.; Ferreira da Silva, A.; Persson, C.; Osorio-Guillén, J. M.; Souza de Almeida, J.; Moyses Araujo, C.; Veje, E.; Veissid, N.; An, C. Y.; Pepe, I.; Johansson, B. Electronic and Optical Properties of Lead Iodide. J. Appl. Phys. 2002, 92, 7219-7224. (61) Leguy, A. M. A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J.; Docampo, P.; Barnes, P. R. F. Reversible Hydration of CH3NH3PbI3 in Films, Single Crystals, and Solar Cells. Chem. Mater. 2015, 27, 33973407. (62) Zhang, X.; Pinion, C. W.; Christesen, J. D.; Flynn, C. J.; Celano, T. A.; Cahoon, J. F. Horizontal Silicon Nanowires with Radial p–n Junctions: A Platform for Unconventional Solar Cells. J. Phys. Chem. Lett. 2013, 4, 2002-2009. (63) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Single-Nanowire Electrically Driven Lasers. Nature 2003, 421, 241-245. (64) Zhu, H.; Miyata, K.; Fu, Y.; Wang, J.; Joshi, P. P.; Niesner, D.; Williams, K. W.; Jin, S.; Zhu, X.-Y. Screening in Crystalline Liquids Protects Energetic Carriers in Hybrid Perovskites. Science 2016, 353, 1409-1413. (65) Guo, Z.; Wan, Y.; Yang, M.; Snaider, J.; Zhu, K.; Huang, L. Long-Range Hot-Carrier Transport in Hybrid Perovskites Visualized by Ultrafast Microscopy. Science 2017, 356, 59-62.

Figure captions

Figure 1. Synthesis of PbI2 NWs. (A) The Pb-I binary phase diagram.44 Shaded area denotes the region of the phase diagram corresponding to where a liquid Pb droplet could be supersaturated with I and be used as a catalyst for NW growth. (B) Schematic of VLS growth of PbI2 NWs through vapor-phase addition of precursor at the vapor-liquid interface and crystallization at the liquid-solid interface. (C) Schematic of the hot-walled CVD reactor for NW growth with two independently-controlled temperature zones for the source powder (on left) and growth substrate (on right). (D) Tilt-view (52°) SEM image of the growth substrate; scale bar, 5 µm. (E) Gallery of spherical Pb tips on PbI2 NWs grown at similar conditions; scale bar, 1 µm across all images. (F) Representative VLS-grown PbI2 NW with Pb catalyst tip; scale bar, 1 µm.

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Figure 2. Characterization of PbI2 NWs. (A) High-angle annual dark field STEM image (far left) and EDS elemental maps (right) showing Pb (red), I (yellow), and O (blue) of a PbI2 NW; scale bar, 400 nm. (B) TEM image of a PbI2 wire showing the crystallographic growth direction along the wire axis as obtained from the SAED pattern in panel C; scale bar, 2 µm. (C) Representative SAED pattern of the PbI2 NW show in panel B. The zone axis is [0001]; scale bar, 2 nm-1. (D) SEM image showing the stacking of the (0001) planes in a cross section of a PbI2 NW made by cleaving the NW upon transfer from the growth substrate; scale bar, 100 nm. (E) EDS maps of two NWs with different apparent growth planes. White line denotes the presumed interfacial plane between the liquid catalyst and solid NW. The PbI2 crystal structure is overlaid on the images; scale bars, 200 nm.

Figure 3. Kinetic model of PbX2 NW growth. (A) SEM image of a tapered NW (upper) and corresponding diameter profile (lower) extracted from the image. Solid red line denotes a linear fit to the data with the RMS deviation from the fit shaded in grey, yielding the tapering parameter, σ, from the slope of the fit. Inset: schematic of the tapering parameter. Black circles represent diameter measurements from magnified SEM images (right), showing the base (I) and tip (II) of the NW; scale bars, 200 nm. (B) Microscopic kinetic processes considered in the model for VLS growth of PbX2 NWs. (C) Plot of the tapering parameter, σ, as a function of the ratio of precursor incorporation rates, α. Orange, green, blue, and purple curves represent atomic percentages of 0, 35, 55, and 66%, respectively, for halogen X in the liquid Pb catalyst, corresponding to χ of 0, 0.27, 0.61, and 0.97, respectively. Inset: magnified view of the approximate region of interest for stable PbI2 NW growth. (D) Illustration of five growth regimes depending on the ratio of incorporation rates, α, showing no NW growth (far left and right), tapered NW growth (center left and right), and uniform NW growth (center).

Figure 4. Conversion of PbI2 NWs to MAPbI3 NWs. (A) SEM image of a MAPbI3 NW; scale bar, 1 µm. (B) Left: TEM image of a MAPbI3 NW; scale bar, 500 nm. Right: SAED patterns collected from the four spatial positions, labeled I-IV, denoted in the TEM image; scale bars, 2 nm-1. (C) Normalized PL spectra 17 ACS Paragon Plus Environment

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of a PbI2 NW (blue curve) and MAPbI3 NW (red curve). (D) PL maps of a PbI2 NW and MAPbI3 NW imaged at wavelengths of 517 nm and 770 nm, respectively. Spectra in panel C were collected from the same NWs shown in panel D.

Figure 5. Optical properties of PbI2 and MAPbI3 NWs. (A) Optical spectra (left) for a single PbI2 NW, showing simulated spectra (blue lines) for absorption (dashed-dotted), scattering (dashed), and extinction (solid) efficiencies of a rectangular NW 225 nm wide and 150 nm thick under plane-wave illumination incident perpendicular to the NW growth axis at 30° from the substrate normal with transverse magnetic (TM) polarization. Solid black line represents an experimental measurement of extinction with the intensity scaled to match the simulation. Green dashed-dotted line represents the absorption efficiency of the top 150 nm of a bulk PbI2 film. Peaks and dips in the extinction spectrum are labeled I-IV, and the images to the right show the normalized electric field spatial profile of each feature upon plane-wave illumination at normal incidence; scale bars, 50 nm. (B) Optical spectra (left) for a single MAPbI3 NW, showing simulated spectra (red lines) for absorption (dashed-dotted), scattering (dashed), and extinction (solid) efficiencies for a rectangular NW 550 nm wide and 350 nm thick under plane-wave illumination incident perpendicular to the NW growth axis at 30° from substrate normal with transverse magnetic (TM) polarization. Solid black line represents an experimental measurement of the extinction with the intensity scaled to match the simulation. Green dashed-dotted line represents the absorption efficiency of the top 350 nm of a bulk MAPbI3 film. Peaks in the absorption spectrum are labeled I-V, and the images to the right show the normalized absorption mode spatial profiles of each peak upon plane-wave illumination at normal incidence; scale bars, 50 nm.

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