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Vapor-Phase Epitaxial Growth of Aligned Nanowire Networks of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I) Jie Chen, Yongping Fu, Leith Samad, Lianna Dang, Yuzhou Zhao, Shaohua Shen, Liejin Guo, and Song Jin Nano Lett., Just Accepted Manuscript • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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Vapor-Phase Epitaxial Growth of Aligned Nanowire Networks of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I) Jie Chen1,2, Yongping Fu1, Leith Samad1, Lianna Dang1, Yuzhou Zhao1, Shaohua Shen2*, Liejin Guo2, Song Jin1* 1

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

Madison, Wisconsin 53706, United States. 2

International Research Center for Renewable Energy, State Key Laboratory of Multiphase

Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, P. R. China.

ABSTRACT: With the intense interest in inorganic cesium lead halide perovskites and their nanostructures for optoelectronic applications, high-quality crystalline nanomaterials with controllable morphologies and growth directions are desirable. Here, we report a vapor-phase epitaxial growth of horizontal single-crystal CsPbX3 (X = Cl, Br, I) nanowires (NWs) and microwires (MWs) with controlled crystallographic orientations on the (001) plane of phlogopite and muscovite mica. Moreover, single NWs, Y-shaped branches, interconnected NW or MW networks with six-fold symmetry, and eventually highly dense epitaxial network of CsPbBr3 with nearly continuous coverage were controllably obtained by varying the growth time. Detailed structural study revealed that the CsPbBr3 wires grow along the [001] directions and have the (100) facets exposed. The incommensurate heteroepitaxial lattice

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match between the CsPbBr3 and mica crystal structures and the growth mechanism of these horizontal wires due to asymmetric lattice mismatch were proposed. Furthermore, the photoluminescence waveguiding and good performance from the photodetector device fabricated with these CsPbBr3 networks demonstrated that these well-connected CsPbBr3 NWs could serve as straightforward platforms for fundamental studies and optoelectronic applications.

KEYWORDS: Cesium lead halide perovskites, Nanowire networks, Vapor-phase epitaxial growth, Optoelectronic devices, Waveguide


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Metal halide perovskites with a general formula of ABX3, where A is methylammonium (MA), formamidinium (FA) or Cs; B is Pb or Sn; X is Cl, Br, or I, have attracted much attention since the first report of a solar cell incorporating MAPbI3.1 Due to their remarkable photophysical properties and broad chemical tunability, these compounds have shown great promise in a variety of optoelectronic applications beyond photovoltaics,2-5 such as light emitting diodes,6 lasers,7-10 and photodetectors.11,

12

Recent efforts have shown that the

all-inorganic perovskites CsPbX3 possess competitive optoelectronic properties, even though they have not yet demonstrated excellent photovoltaic performance.9, 13-19 Importantly, they are more tolerant to moisture, heat, oxygen, light, or electric field as compared to the organic-inorganic hybrid perovskites.13, 14

Semiconductor nanowires (NWs) are useful building blocks for various electronic and optoelectronic applications.20-22 Particularly, NWs of lead halide perovskites have recently been used in both optoelectronic applications and fundamental studies because of their excellent intrinsic semiconductor properties and facile synthetic approaches.7-9, 12, 23-29 To date, several solution-phase growth methods, such as dissolution-recrystallization growth from lead precursors deposited on substrates reacting with high concentration of AX precursor solution,7-9,

16, 27

colloidal solution synthesis of very thin NWs,23,

28

template-directed

synthesis,24 and partial dissolution and recrystallization of perovskite films,25, 26 have been developed to synthesize perovskite NWs. However, the products from those solution methods have poorly controlled orientations and alignments and often produce other nanostructure morphologies, such as nanoplates and nanocrystals; hence, further transfer of the NWs is required for device fabrication and other physical studies. In this regard, large scale 3 ACS Paragon Plus Environment

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horizontally aligned NWs with controlled orientation on the substrate surface, either through direct epitaxial growth30-32 or through post-growth assembly,33 could facilitate the direct integration into practical devices.30-33

Most NWs of various lead halide perovskites were grown via solution methods so far, but in general vapor-phase grown nanostructures have better crystalline quality and lower defect density.20, 22 Especially, vapor-phase epitaxial growth has shown great success in creating high-quality nanostructures with controllable morphologies and orientations.30-32,

34-36

Important examples include both vertical epitaxial NW growth from crystalline substrates34 and guided growth of horizontal NWs of ZnSe,30 GaN,31 ZnO32 on miscut sapphire as well as WO3,35 Pb1-xSnxSe36 on mica substrate. Specifically, layered structured mica is a robust, inexpensive and universal substrate for vertical epitaxial growth of NWs37-39 and also van der Waals epitaxial growth of 2D materials.40, 41 Organic-inorganic hybrid perovskites, such as MAPbI3, are not very friendly for high temperature vapor-phase growth because of their poor thermal stability. However, vapor-phase synthesis can be a good choice to grow epitaxial nanostructures of inorganic perovskites (such as CsPbX3) with improved crystal quality, because they are thermally stable at moderately high temperatures.42 Here, we report a facile and general method for growing high-quality horizontally oriented NWs and microwires (MWs) of cesium lead halide perovskites CsPbX3 (X = Cl, Br, I) with high density on mica by vapor-phase epitaxy. Moreover, by controlling the deposition time, we can grow a variety of CsPbBr3 nanostructures, including single NWs, Y-shaped branches, interconnected NW or MW networks, and eventually highly dense epitaxial networks of CsPbBr3 with nearly continuous coverage. The heteroepitaxial lattice match and possible growth mechanism for 4 ACS Paragon Plus Environment

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these horizontally aligned CsPbBr3 wires were elucidated. Furthermore, we demonstrated that the high-quality interconnected NW networks grown on the transparent and insulating mica substrates can serve as straightforward platforms for optoelectronics device fabrication and applications.

The epitaxial growth of CsPbX3 NWs was carried out in a home-built chemical vapor deposition (CVD) reactor (see Supporting Information for experimental details and Figure S1 for the schematics of the CVD setup). Specifically, a mixture of CsX and PbX2 powders with a molar ratio of 1:1 were used as the precursors and heated in the middle of the heating zone of the furnace at about 300 oC to 350 oC (depending on the perovskite). The evaporated precursors were carried by flowing argon gas to the downstream region, where freshly cleaved phlogopite (p-mica) or muscovite mica (m-mica) was placed and used as an epitaxial substrate for vapor growth. Mica structures are composed of alternating sheets of positively charged potassium ions (K+) and negatively charged aluminosilicates, which consist of one layer of face-sharing AlO6 octahedra sandwiched with two layers of SiO6 octahedra.38, 39, 41 The adjacent aluminosilicate sheets in mica are bound by van der Waals attraction, thus the freshly cleaved mica surface is expected to be atomically smooth and free of dangling bonds. Such chemically inert substrates can relax stringent lattice matching conditions, facilitating the epitaxial growth of large-scale, high-quality crystals.

We first present the epitaxial growth of CsPbBr3 NWs on both p-mica and m-mica. As shown in Figure 1a and 1b, both samples after a CVD growth reaction at 325 oC for 2h exhibited similar product morphology: well-aligned surface-bound wires were horizontally 5 ACS Paragon Plus Environment

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orientated on the mica (001) surface and a network structure was formed by the elongated wires connecting with each other. At a growth time of 2 h, the wires typically have a width of ~1 µm and a length of tens of micrometers. Many wires are parallel to each other, and a six-fold symmetry of the growth directions can be clearly observed, as indicated by the 60° or 120° angle between almost every pair of interconnected wires. Note that there are a few wires with mismatched directions in both samples, possibly due to the defects or imperfect orientation of the atoms on the mica surface induced by stress or nucleation at the cleavage edge of mica.41 To quantitatively characterize the orientation of these wires, we further performed 2D Fast Fourier Transform (FFT) on the optical images with a large area of ~120 µm × 120 µm (insets in Figure 1a and b), which show six streaks evenly spread every 60° on a 360° turn. We noticed the growth directions of these NWs coincided with the six-fold symmetry structure present on the mica (001) surface, which is a result from the epitaxial growth and will be discussed later. It should be noted that, unlike the previous report of vapor-phase growth of CsPbBr3 plates,42 we only occasionally found a few nanoplates grown in a small region of the substrate. The reason might be that a much different growth condition of 325 ℃ at pressure of 80 mTorr was used here, which causes different partial pressures of the vapor precursors as compared to the 575 ℃ at 50 Torr in that report,42 kinetically leading to different growth morphology.


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Figure 1. Optical microscopy images of CsPbBr3 wire networks grown on (a) phlogopite mica and (b) muscovite mica (scale bar: 10 µm). Insets are 2D Fast Fourier Transform (FFT) of the corresponding images, showing these wires are oriented with a hexagonal symmetry. (c) EDX mapping of a CsPbBr3 network shows uniform spatial distribution of Cs, Pb, Br elements. (d) PXRD patterns of the CsPbBr3 grown on phlogopite mica and on muscovite mica together with those of freshly cleaved mica substrates and the standard PXRD for CsPbBr3 (PDF card: JCPDS # 54-0752). The peaks associated with CsPbBr3 in the products are marked with asterisks.

We then carried out powder X-ray diffraction (PXRD) and energy-dispersive X-ray (EDX) spectroscopy to confirm the phase and stoichiometry of the products. The EDX 7 ACS Paragon Plus Environment

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mapping (Figure 1c) shows that the Cs, Pb and Br elements are uniformly distributed and overlay perfectly with the corresponding SEM image of the network. Quantitative analysis of the spectrum collected on a single wire (Figure S3) further confirmed that the elemental composition of Cs: Pb: Br is ~1: 1: 3. Figure 1d shows the PXRD patterns of the as-grown samples, in comparison with the PXRD patterns of the mica substrates used and the standard CsPbBr3. The mica substrates display a set of diffraction peaks with nearly equal spacing that could be ascribed to the (00l) basal planes of the 1M polytype [KAl2(Si3Al)O10(OH)2, space group: C2/m, a = 5.387 Å, b = 9.324 Å, c = 10.054 Å, α = γ = 90°, β = 97°]. Both NW products show two additional diffraction peaks at 21.55° and 43.10°, which correspond well to the (110) and (220) planes of cubic phase CsPbBr3 (space group: Pm3തm, a = 5.830 Å). The absence of other diffraction peaks from CsPbBr3 strongly implies that these wires are well-oriented, which is a signature of epitaxial growth with the (110) lattice planes of CsPbBr3 parallel to the mica surface. One may notice that the CsPbBr3 wires can grow on both types of mica, despite the minor structural difference between p-mica and m-mica with approximately 0.2 Å and 0.4 Å difference in the a-axis and b-axis lattice constants, respectively. From now on, p-mica was used as the substrate for further study of the CsPbX3 wires in this work. In addition to CsPbBr3, epitaxial growth of CsPbCl3 and CsPbI3 wires can also be achieved (Figure S4 and S5) using very similar CVD conditions, confirming mica as an universal epitaxy substrate for horizontal 1D structures of cesium lead halide perovskites. We note that while the originally synthesized CsPbI3 wires show dark color at high temperature and are in cubic perovskite phase, they turned into a yellow non-perovskite structure (orthorhombic phase) at room temperature.43 Upon heating to 300 ℃, the 8 ACS Paragon Plus Environment

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orthorhombic phase can partially convert back to the perovskite phase as indicated by the color change of the sample (Figure S5).

In order to better understand the formation process of these NW networks, the morphological evolution was examined by monitoring the growth of CsPbBr3 NWs on p-mica as the reaction time increased from 5 min to 20 h. Even within a short growth time of 5 min, many sparsely distributed short nanorods with a length up to 1 µm and a width of a few hundred nanometers started to appear on the p-mica substrate (Figure 2a). As the growth time increased, CsPbBr3 preferably crystallized on those initially formed nanorods along the axial direction, leading to the formation of longer NWs with lengths up to 20 µm (Figure 2b). At the same time, nucleation sites on the clean substrate surface continued to emerge. Note that no wetting layer was deliberately deposited before growth, thus the growth should initially start with the heterogeneous nucleation of CsPbBr3 nanocrystals on the mica surface. Further prolonging the growth time yielded a higher density of NWs or MWs on the substrates and long wires gradually approached each other to eventually fuse together, forming interconnected branches and networks (Figure 2c and 2d). The surface coverage thus gradually increased as a consequence of the increasing width of these wires and emergence of more new wires to fill up the empty space in the network as growth time increased (Figure 2e). This implies the possibility of synthesizing epitaxial CsPbBr3 thin films with full coverage after a long growth time. As expected, a highly dense network of CsPbBr3 with nearly continuous coverage was formed when the growth time was intentionally increased to 20 h (Figure 2f). Although there are some small pinholes in it at this stage, increasing the growth time would further increase the coverage of the material to make it a continuous thin 9 ACS Paragon Plus Environment

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film. As highlighted by the inset in Figure 2f, the as-grown highly dense CsPbBr3 network covers the mica substrate with a scale of up to 10 mm2.

Figure 2. (a-f) SEM images of the CsPbBr3 NWs and MWs, networks, and continuous film grown on p-mica with different reaction time from 5 min to 20 h. The inset in (a) is a magnified SEM image. The inset in (f) highlights the highly dense CsPbBr3 network with a nearly continuous surface coverage over an area up 10 mm2. The scale bars for (a-f) are 10 µm. (g) The corresponding PXRD patterns of the CsPbBr3 samples, showing increasing intensity of the (110) and (220) peaks. The inset is the θ-rocking curve of the sample for the (110) peak after 4 h growth.

The growth time-dependent PXRD patterns of the samples (Figure 2g) further demonstrated this epitaxial growth progression. Note that the amount of products grown within 10 min was not enough to yield significant diffraction signal. When the growth time was 20 min or more, all PXRD patterns showed only two diffraction peaks corresponding to 10 ACS Paragon Plus Environment

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the (110) and (220) lattice planes of cubic CsPbBr3, after excluding the diffraction peaks from p-mica. The intensities of these two peaks increased with increasing growth time from 20 min to 20 h, which is consistent with the morphological evolution. The lack of out-of-plane diffraction peaks again confirms the existence of horizontally epitaxial growth up to 20 h. Furthermore, we performed θ-rocking curve on the 4 h sample at the (110) plane to evaluate the crystallinity of these NWs. The rocking curve (inset in Figure 2g) shows a very sharp and narrow peak with a small full-width-at-half-maximum (FWHM) of 0.075°. This value is close to those of previously reported single crystalline inorganic perovskites, such as CsPbBr3 (FWHM: ~0.074°, ref 44) and SrTiO3 (FWHM: 0.070°, ref 45), suggesting that these CsPbBr3 NWs and MWs are nearly single crystalline with the (110) planes parallel to the substrates.

To gain more detailed information on the morphology of the wires (i.e. facets and growth direction), we performed atomic force microscopy (AFM) and SEM cross sectional imaging on a sample grown for 2 hours (Figure 3). We chose a sample after long growth time because the facets and shapes of these wires are well-developed. A height profile generated from the AFM topographic image is provided in the inset of Figure 3a. The cross section profile of this wire shows a right triangle with a 90° apex angle and two 45° base angles, thus giving a height to width ratio of 1:2. SEM image (Figure 3b) also confirms the right triangular cross section. This is in clear contrast to the rectangular cross section typically observed in solution grown perovskite NWs.7-9, 16, 27


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Figure 3. (a) A representative AFM image of the CsPbBr3 wires synthesized with a growth time of 2 h. The inset shows the corresponding height profile along the line, yielding a height to width ratio of 1:2. (b) SEM image (45° tilted) of the CsPbBr3 wire highlights the triangular cross section.

From this morphology information and the PXRD patterns that only show diffraction peaks from the (110) lattice plane family, we can conclude that the CsPbBr3 wires are grown along the [001] direction and the exposed surfaces are (100) facets of the cubic perovskite phase, as illustrated in Figure 4a. The facets of the observed triangular wire are the same as one half of the cubic perovskite unit cell structure that is shaded with yellow color in Figure 4b, with the (110) plane interfacing with the mica (001) surface. It is also interesting to note that the exposed (100) facets have been thought to be the most stable surface facets with low surface defect density in lead halide perovskite structures.46 The mechanism of epitaxy for CsPbBr3 NWs grown on mica is nontrivial, especially considering the considerable lattice mismatch in the in-plane periodicities of the contacting surfaces of CsPbBr3 and mica. The lattice spacings are: dCsPbBr3 (001) = 5.830 Å, dCsPbBr3 (110) = 8.245 Å (JCPDS: #45-0752), dmica (100)

= 5.387 Å, and dmica (010) = 9.324 Å (JCPDS: #16-0344). The sheets of mica are bound by

a weak interlayer van der Waals force that originates from the electrostatic interactions between the alternately stacked K+ layer and the O2- from the negatively charged 12 ACS Paragon Plus Environment

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aluminosilicate layer.38, 39, 41 Upon cleavage, the interlayer of K+ cations would be divided onto the two face-to-face mica surfaces, leaving 50% K+ and 50% K+-vacancies on each surface.35 We hypothesize that the CsPbBr3 interacts with K+ and/or K+-vacancies resulting in heteroepitaxial match between the (110) plane of CsPbBr3 and (001) plane of mica at the interface. These interactions include: (1) the electrostatic attraction between Br- from CsPbBr3 and K+ from mica; (2) the Cs+ from CsPbBr3 might fill K+ vacancies on the surface, as also inferred from previous investigations on Cs+ absorption on mica minerals.47, 48 We propose two possible structural match models between CsPbBr3 and mica: (i) CsPbBr3 [001]//mica [100] and CsPbBr3 [110]//mica [010]; and (ii) CsPbBr3 [001]//mica [010] and CsPbBr3 [110]//mica [100] (see Figure S6 for detailed illustrations of the models). If we take the commensurate lattice match, the lattice mismatch factor f = (1 - doverlayer/dsubstrate) × 100%, where d is the lattice spacing, in model (i) is -8.22% for [001] direction and 11.6% for [110] direction of CsPbBr3 respectively. These f values in Model (ii) are 37.5% and -53.0%, respectively. Therefore, an incommensurate epitaxy could be more reasonable. In model (i), it can be calculated that 10 × dCsPbBr3 (110) ≈ 9 × dmica (010) with a f value of 1.7% and 8 × dCsPbBr3 (001) ≈

9 × dmica (100) with a f value of 3.8%. However, in model (ii), we found that 2 × dCsPbBr3

(110) ≈

3 × dmica (100) with a f value of -2.0% and 8 × dCsPbBr3 (001) ≈ 5 × dmica (010) with a f value of

0.04%. The smaller supercell with less lattice mismatch suggests the model (ii) is a more preferred incommensurate epitaxial match.


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Figure 4. (a) A schematic illustration of the epitaxial growth of CsPbBr3 NWs on mica. (b) A unit cell of CsPbBr3 crystal structure, the dotted lines outline the (110) plane. (c) Top view of the atomic arrangement of CsPbBr3 structure on (001) plane of p-mica and the proposed incommensurate heteroepitaxial relationship.

We then illustrate the top view atomic arrangement with the (110) plane of CsPbBr3 on (001) plane of mica using model (ii) (Figure 4c). As calculated above, the CsPbBr3 lattice matches very well to mica [010] direction along the [001] direction of CsPbBr3 but the CsPbBr3 [110] has a slight lattice mismatch along the mica [100] direction, which leads to unrestricted growth of the epitaxial crystal in the first [001] direction of CsPbBr3, but limits the width in the other [110] direction, resulting in the formation of a NW.49 The anisotropic growth of these surface-bound CsPbX3 NWs is different from the growth mechanisms for previously reported NWs of lead halide perovskites.7, 24, 26, 50 This kind of epitaxial NW growth mechanism is the closest to that observed for materials that have an appropriate but asymmetric lattice mismatch to the host substrate, such as metal silicide (e.g. ErSi2) on Si 14 ACS Paragon Plus Environment

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(001) surface.49 This is also unlike those well-known horizontal orientated GaN,31 ZnO32 and ZnSe30 NWs guided by the atomic steps or nanosteps on miscut sapphire. The growth of our CsPbBr3 NWs is initiated from the electrostatic attraction between CsPbBr3 and mica substrate then guided by the asymmetric lattice mismatch with substrate to enable the formation of long surface-bound NWs. Due to the approximately hexagonal symmetry of the mica (001) plane, the CsPbBr3 NWs grow in six directions with a 60° or 120° angle to each other. Similarly, CsPbCl3 and CsPbI3 NWs also prefer an epitaxial growth such that the [001] direction of the perovskite structures match along the [010] direction of mica with f = 3.8% and f = -1.2%, respectively. These slightly less favorable lattice matches perhaps explain the slightly less ordered NW alignments for CsPbCl3 and CsPbI3 NWs on phlogopite mica (Figure S4a and S5a).

Preliminary optical studies show that CsPbBr3 NWs exhibit a strong photoluminescence (PL) emission peak at 528 nm with a FWHM of ~16 nm, which is comparable to the solution grown CsPbBr3 NWs previously reported.9, 50 Interestingly, we found that green PL emission was not only observed at the excitation spot, but also on both ends of the wires (inset in Figure 5a). This is a typical feature of an optical waveguide inside a wire and it suggests that the CsPbBr3 wires are able to absorb the excitation light and propagate the PL emission towards both ends.51 More interestingly, the PL emission can also propagate from one branch to another (as marked with red arrows), suggesting that the wires are physically connected in the networks and a strong cavity coupling exists at the intersections.


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Figure 5. (a) PL spectra of the CsPbBr3 wire excited with a wavelength of 421 nm. The inset is an optical image showing strong waveguiding phenomenon in the CsPbBr3 NW network. The brightest spot is the position of laser excitation. (b) I-V characteristics of the CsPbBr3 NW network photodetector under dark and light conditions. The light is generated from a solar simulator with an intensity of 10 mW/cm2. The inset is the optical image of the device. (c) I-t curves of CsPbBr3 NW network photodetector with an applied bias of 2.5 V under different light intensities. From top to bottom, the light intensities are 20, 10, 4.7, 2.3, 0.7, and 0.5 mW/cm2. (d) Photocurrent rise and decay of the device measured at a bias of 2.5 V and a light intensity of 2.3 mW/cm2.

Taking advantage of the excellent connectivity of the CsPbBr3 NW networks, we further fabricated photodetector devices using the as-grown CsPbBr3 NW networks on insulating and transparent p-mica substrate to study the optoelectronic properties (see Supporting Information for detailed fabrication procedures and Figure S2). Two Au electrodes were thermally evaporated through a shadow mask onto the CsPbBr3 NW network, creating a light 16 ACS Paragon Plus Environment

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detecting area of 2 mm by 400 µm (inset in Figure 5b). The device was insulating in dark conditions, but was readily conductive under illumination (Figure 5b) due to the increased carrier concentration. These are simple photoconductive photodetectors. Under an applied voltage of 5 V, the current densities were 2.85 µA/cm2 and 0.002 µA/cm2 in the light and dark conditions, respectively. The on/off ratio is thus calculated to be >103, which is comparable to previous photodetectors based on CsPbBr3 nanosheets (see Table S1 in the Supporting Information for detailed comparison).19 To illustrate the time response of the CsPbBr3 NW network photodetectors, we measured the I-t curves as a function of incident light intensity at an applied voltage of 2.5 V. The on-and-off status of the illumination was controlled by a light shutter. As shown in Figure 5c, when the device was illuminated, the photocurrent rose sharply and a sharp fall in the photocurrent to dark current can be seen when the illumination was interrupted. Moreover, the photocurrent increased as the light intensity was increased from 0.5 mW/cm2 to 20 mW/cm2, because the number of photogenerated carriers is proportional to the absorbed photon flux. At an applied light intensity of 2.3 mW/cm2 (Figure 5d), it can be seen that both the rise time and decay time are limited by the time resolution of the potentiostat used (0.1 s), suggesting that the response time of the photodetector device could be actually smaller than 0.1 s. This response time is already faster than many previously reported perovskite film-based photodetectors (see Table S1).25, 52, 53 Moreover, unlike those photodetectors based on a single NW or nanosheet, our photodetector is simply fabricated on a large-area network with a working area of 0.8 mm2, therefore even though these photodetectors do not have the highest absolute performance (see Table S1 for detailed comparison), the respectable photodetector performance with a 103 on/off ratio and < 0.1 s 17 ACS Paragon Plus Environment

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response time clearly demonstrates that these NW networks are well-connected and the CsPbBr3 NWs have high crystalline quality.

In summary, single crystalline horizontal CsPbX3 NWs and MWs with controlled orientation were obtained via a facile vapor-phase epitaxial growth on mica substrates. Moreover, simply by varying the growth time, a variety of CsPbBr3 structures including single wires, Y-shaped branches, networks, and nearly continuous epitaxial CsPbBr3 films could be controllably grown. Detailed structural characterization revealed the heteroepitaxial lattice relationship between the CsPbBr3 and mica structures and suggested that the unusual growth mechanism of these surface-bound NWs is due to the asymmetric lattice match along different crystallographic directions. The PL waveguiding and the photodetectors fabricated with CsPbBr3 NW networks demonstrated the great potential of these structures toward direct integration into practical optoelectronic devices. Future work will focus on their photophysical and optoelectronic properties as well as the development of energy conversion applications. Furthermore, this epitaxial growth method could potentially be expanded to other inorganic or hybrid perovskite materials to realize well-ordered nanostructures.

ASSOCIATED CONTENT

Supporting Information Experimental methods, additional SEM images, PXRD patterns, the detailed illustrations of the lattice match models, and quantitative performance comparison of perovskite-based photodetectors. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION 18 ACS Paragon Plus Environment

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Corresponding Author *E-mail: [email protected] (S. J.); [email protected] (S. S.)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02-09ER46664. J. C. acknowledges support from the China Scholarship Council. L. S. and L. D. thank NSF Graduate Research Fellowship for support. REFERENCES 1.

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