Nanochannel-Assisted Perovskite Nanowires: From Growth

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Nanochannel-Assisted Perovskite Nanowires: From Growth Mechanisms to Photodetector Applications Qitao Zhou, Jun Gyu Park, Riming Nie, Ashish Kumar Thokchom, Dogyeong Ha, Jing Pan, Sang Il Seok, and Taesung Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03826 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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Nanochannel-Assisted Perovskite Nanowires: From Growth Mechanisms to Photodetector Applications Qitao Zhou,† Jun Gyu Park,† Riming Nie,‡ Ashish Kumar Thokchom,† Dogyeong Ha,† Jing Pan,§ Sang Il Seok,*,‡ and Taesung Kim*, † †

Department of Mechanical Engineering, Ulsan National Institute of Science and

Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea ‡

School of Energy and Chemical Engineering, Ulsan National Institute of Science and

Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea §

School of Chemical Engineering Sungkyunkwan University (SKKU), 2066, Seobu-ro,

Jangan-gu, Suwon 440-746, Republic of Korea

ACS Paragon Plus Environment 1

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ABSTRACT Growing interest in hybrid organic–inorganic lead halide perovskites has led to the development of various perovskite nanowires (NWs), which have potential use in a wide range of applications, including lasers, photodetectors, and light emitting diodes (LEDs). However, existing nanofabrication approaches lack the ability to control the number, location, orientation, and properties of perovskite NWs. Their growth mechanism also remains elusive. Here, we demonstrate a micro-/nanofluidic fabrication technique (MNFFT) enabling both precise control and in situ monitoring of the growth of perovskite NWs. The initial nucleation point and subsequent growth path of a methylammonium lead iodide-dimethylformamide (MAPbI3·DMF) NW array can be guided by a nanochannel. In situ UV-vis absorption spectra are measured in real-time, permitting the study of the growth mechanism of the DMF-mediated crystallization of MAPbI3. As an example of an application of the MNFFT, we demonstrate a highly sensitive MAPbI3-NW-based photodetector on both solid and flexible substrates, showing the potential of the MNFFT for low-cost, large-scale, highly efficient, and flexible optoelectronic applications.

KEYWORDS: micro-/nanofluidic fabrication technique (MNFFT), crack-photolithography, perovskite nanowire, growth mechanism, flexible photodetector


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Extraction of salt from water is one of the oldest natural evaporation-based processes developed by humans. Such solution-mediated crystallization is also one of the most common chemical phenomena in nature.1 Despite the fact that solution-mediated, wet-chemical approaches are widely used for nanocrystal growth, harnessing these processes to finely control crystal structures and monitor their growth in situ is still challenging.2 Recently, inorganic and hybrid organic–inorganic lead halide perovskites have attracted attention as a result of their outstanding optoelectronic properties, leading to their adoption in a wide range of applications. Examples include their use as absorbing layers in photovoltaic cells (with certified power conversion efficiencies of up to 22.1 %),3 photon detectors,4 light-emitting devices,5 and lasers.6 These applications are based on high-quality perovskite films or structures, mostly fabricated by wet-chemical methods7, 8 or solution-mediated processes9 because of the relative simplicity and economical potential of these approaches.10-12 Nanoscale perovskite structures, such as nanowires (NWs), are particularly useful for lasers8 and photodetectors13-16 because of their high sensitivity, rapid response time, and low power consumption.17 However, the ability to manipulate the number, location, orientation, and properties of perovskite NWs in a controllable manner using the aforementioned wet-chemical or solution-mediated methods remains a significant challenge. A promising solution comes from microfluidic and/or nanofluidic technologies, which have played innovative roles in facilitating biological,18 biomedical,19 and biochemical research20 over the past two decades. Micro-/nanofluidic technologies can precisely manipulate not only fluid flows (e.g., solvents) but also mass transport of small molecules (e.g., solutes) at micro- and nanoscales. Previous work suggests that micro-/nanofluidic technologies can potentially provide controllable and high-throughput parametric screening in wet-chemical or solution-mediated


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fabrication processes, leading to the precise growth of perovskite colloidal semiconductors, perovskite nanocrystals, or quantum dots.21,

22

In addition, micro-/nanofluidic fabrication

techniques (MNFFTs) that use soft-lithography for rapid and repeated replication of the same micro-/nanofluidic devices23 have been used to obtain well-defined perovskite NW arrays, although the micro-/nanofluidic devices were mainly utilized as intermediate fabrication templates or nanoimprint molds.24, 25 Such fabrication methods do not appear to fully utilize the advantages of micro-/nanofluidic technologies. Furthermore, for nanoscale perovskite structures, the methods relied on conventional microfabrication and/or nanofabrication technologies (e.g., nanoimprinting), which are inefficient with respect to cost, time, and throughput. A few unconventional fabrication methods have been reported, including slip-coating13 and blade-coating14 methods. These methods are useful in aligning the orientation of NWs on various substrates but lack the ability to control the growth, number, and location of NWs. Here, we describe a cracking-assisted MNFFT that makes it possible to manipulate the initial nucleation point and subsequent growth path of perovskite NWs using methylammonium lead iodide (MAPbI3). Although we use only standard photolithography with a micron feature size (>2 µm), we are able to produce a hybrid-scale micro-/nanofluidic channel network. We subsequently utilize the micro-/nanofluidic platform to grow perovskite NWs by controlling the spatiotemporal concentrations of perovskite solutes via evaporation. Additionally, the MNFFT makes it possible to explore the underlying growth mechanisms of such MAPbI3 NWs in real-time. A confined micro-/nanochannel network enables precise control of the growth of MAPbI3 NWs, and thus, we are able to control the number, location, and orientation of the NW arrays. Furthermore, we develop a MAPbI3-NW-array-based photodetector with high quality and performance, demonstrating the potential of the MNFFT in solution-mediated NW applications.


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RESULTS AND DISCUSSION A solution-mediated crystallization process often observed near saline lakes is shown in Figure 1a. Typically, salt crystals are formed around a lake as the lake water evaporates. In a similar manner, we observed a droplet of MAI (CH3NH3I)/PbI2/DMF solution on a glass surface (Figure 1b and 1c) and found micro-/nanoscale MAPbI3 (CH3NH3PbI3) structures crystallized in an agglomerated morphology, which grew at the interface between the solution and the glass surface as a result of the evaporation of DMF. Since rapid evaporation occurs at the liquid-air-solid interface of the droplet,26 the crystallization of MAPbI3 NWs begins at the same interface, and this interface shrinks over time. The generated NWs appeared to be radially oriented, and they were not well organized. Consequently, while their location can be somewhat adjusted, manipulating their number, density, and orientation remains challenging. Figure 1d describes a micro-/nanofluidic fabrication method for the production of MAPbI3 NWs in a controllable manner. We used a micro-/nanofluidic device in which a crack-assisted nanochannel connected the two microchannels. We used the “crack-photolithography” established in our previous work (see photograph in Figure S1) to facilitate the production of PDMS-based

micro-/nanofluidic

channel

networks

using

only

microfabrication

and

soft-lithography technologies.27 Figure 1e shows the scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of a typical micro-/nanofluidic device with a single central nanochannel that is about 2.5 µm wide and 350 nm deep. Figure 1f shows the fabrication process for the MAPbI3 NW using the micro-/nanofluidic device (see the Experimental Section for details). We simply loaded a droplet of MAI/PbI2/DMF solution into the device so that the solution flowed along the two yellowish microchannels.


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Quantifying the molar ratio of CH3NH3I to PbI2 with high accuracy is difficult using current weighing methods. In addition, it is difficult to confirm that CH3NH3I completely reacts with PbI2. Hence, we prepared a MAI/PbI2/DMF solution through a solvent-assisted purification process to aid in quantification (details are found in the Experimental Section).28 The interconnecting nanochannel was also filled with the solution; the trapped air in the nanochannel between the neighboring microchannels was dissolved by the solution and diffused out to the atmosphere through the gas-permeable PDMS. Next, the device was heated to 75 °C for 2 h to evaporate the DMF in a N2 environment and then annealed at 100 °C for 1 h, as illustrated in Figure 1f. After peeling off the PDMS device from the Si/SiO2 substrate, we obtained a MAPbI3 NW from the nanochannel in a single-crystal format and bulk MAPbI3 microwires (MWs) from the microchannels. It appeared that although NWs were generated in the microchannels, they merged together over time, turning into MWs. This indicates that micro-/nanofluidic channels can restrict the domain size of MAPbI3 NWs and MWs. The enlarged images in Figure 1g show an as-prepared MAPbI3 NW with a very smooth surface. Further, the AFM image shows that the as-prepared MAPbI3 NW is arch-shaped with a width and height of 2.0 µm and 300 nm, respectively, which is consistent with the nanochannel dimensions (Figure 1h). Figure 1i shows a typical TEM image of the MAPbI3 NW; the single-crystal characteristic is demonstrated by the selected-area electron diffraction patterns of the same NW taken from different positions (see Figure S2).29 The XRD pattern displays peaks typical for the perovskite structure of MAPbI3, without any impurity peaks (Figure 1j). The XRD measurements reveal identical patterns, indicating that no phase change has occurred after 100 days of storage and confirming the high stability of the MAPbI3 NW. This high stability can be mainly attributed to

the solvent-assisted fabrication method relying on both the


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micro-/nanofluidic device and the MAI/PbI2/DMF solution. In addition, the micro-/nanofluidic channel network made it possible to control the number, location, and orientation of the NWs, which remains elusive with conventional solution-mediated NW crystallization methods. We further monitored the crystal growth process of the NWs in real-time by replacing the SiO2/Si wafer substrate with a glass slide (see Figure S3). We found that when both the upper and lower microchannels are filled with a MAI/PbI2/DMF solution, NW crystallization occurs in two stages: (1) a long NW generation stage and (2) an aggregation/retention stage. As the monitoring experiments were carried out at room temperature (298 K), MAPbI3·DMF NWs were generated first as the precursors for MAPbI3 (see Supplementary Movie S1).30 In fact, recent research has shown that the growth of perovskite precursors (MAPbI3·DMF) is driven by the 1D Lewis adduct of PbI2·DMF, which coordinates partially with MAI (PbI2·DMF-related crystals) and its formation can be attributed to the low solubility of PbI2 in DMF.31 Since the mixture of CH3NH3I, PbI2, and DMF was injected into the right side of the microchannels, their local concentrations increased on the left side as a result of the fast and vigorous evaporation of DMF. Consequently, the critical concentration at which PbI2·DMF-related nanocrystals appear was reached earlier on the left side than on the right side of the microchannels. In other words, since high supersaturation occurs on the left side, nucleation dominates, leading to the initial formation of a large number of nuclei and the rapid consumption of reagents. This explains why a number of NWs are created simultaneously32 and why the MAPbI3·DMF NWs form from the left side to right side. Interestingly, it is hard to distinguish the generation of PbI2·DMF-related nanocrystals from the growth of MAPbI3·DMF NWs, since they appear to occur simultaneously. As mentioned earlier, the solvent, i.e., DMF, plays an important role during the generation of MAPbI3·DMF NWs, but the mechanism and evolution process is not yet fully understood. In


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particular, no technique is currently available for the in situ analysis of the growth of a single MAPbI3·DMF NW. Thus far, most in situ XRD or UV-visible spectroscopy analyses were based on films or NW bunches. Figure 2 shows another fabrication method using a similar micro-/nanofluidic device, which makes it possible to manipulate the nucleation point and subsequent growth path of a single MAPbI3·DMF NW. The number of MAPbI3·DMF NWs can be controlled by choosing the initial points and paths of PbI2·DMF nanocrystal precursors when designing the micro-/nanofluidic channel network; this has been well demonstrated in our previous work.27, 33 As illustrated in Figure 2a (see Supplementary Movie S2), a MAI/PbI2/DMF solution was injected only into the upper microchannel while the lower microchannel was kept empty. The solution completely filled the upper microchannel and then quickly filled the nanochannel as well (t = 0 min). As soon as the solution reached the other end of the nanochannel, where the lower microchannel and the nanochannel meet, the solvent underwent vigorous evaporation because the solution at the end of the nanochannel (350 nm in depth) encountered a microchannel (10 µm in depth and 100 µm in width) with an order of 104–105 magnitude increase in volume. As previously mentioned, the low solubility of PbI2 in DMF forces PbI2·DMF-related crystals to crystallize first in the nanochannel. The crystallization of PbI2·DMF-related species subsequently causes a MAPbI3·DMF NW to grow within the nanochannel. Interestingly, we observed that the MAPbI3·DMF NW extended out of the nanochannel and continued to grow in the microchannel (t = 23 to t = 35 min in Figure 2a). The DMF in the microchannels continues to evaporate, leading to a relatively high concentration of the mixture solution within the microchannels. As a result, crystallization can continue to take place, resulting in bundles of NWs (t = 45 min). In the next step, we confirmed that the proposed growth mechanism of the MAPbI3·DMF


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NW is correct—specifically, we confirmed that DMF evaporation and the crystallization of PbI2·DMF-related crystals play key roles in the initial nucleation in the nanochannel and continuous extension in the microchannel. Firstly, we investigated the dependence of crystallization starting points on the nanochannel size using three micro-/nanofluidic devices with different sizes of nanochannels, as shown in Figure 2b (see the AFM topographic images in Figure S4). Initially, the relatively deep and wide nanochannel was empty and clear at t =0 min (Figure 2b (i) solution penetration in channel, 500 nm deep and 4 µm wide). However, after the nanochannel was filled with the MAI/PbI2/DMF solution, it became less clear because the refractive index changed (t = 119 min). On the other hand, when the nanochannel was relatively shallow (Figure 2b (iii) nanochannel collapse, 200 nm deep and 1 µm wide), the MAI/PbI2/DMF solution could not flow because the nanochannel collapsed and blocked the fluid flow. As a result, MAPbI3·DMF NWs grew from the top side of the upper microchannel (t = 34 min), resulting in failure to control the nucleation point and subsequent path of the NWs. Only when the nanochannel was fabricated with dimensions of ~350 nm deep and 2 µm wide were MAPbI3·DMF NWs successfully grown from the nanochannel up towards the upper microchannel (Figure 2b (ii) NW growth). These results indicate that the starting point of crystallization can be controlled by the size of the nanochannel. Crystallization requires a critical concentration, and the concentration distribution can be controlled by DMF evaporation—a process that is highly dependent on the size of the nanochannel. At the same time, the growth path can also be controlled very well. It can be seen in Figure S5 that the MAPbI3·DMF NWs can grow into the microchannels with a width of 20 µm. We further confirmed that the MAPbI3·DMF NW growth is guided by both DMF evaporation from the nanochannel and the crystallization of PbI2·DMF-related crystals by


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loading PbI2/DMF solution into the upper microchannel, which showed that a PbI2·DMF NW grew out from the nanochannel (Figure 2c and Supplementary Movie S3). Unlike the MAPbI3·DMF NW, however, it seemed that the PbI2·DMF NW grew rapidly not only in the vertical direction, parallel to the nanochannel, but also in other directions (i.e., radially). This tendency was similar to the PbI2·DMF NW growth behavior observed in the solution droplet (Figure S6). It has been thought that the association between DMF and MAPbI3 or PbI2 may potentially create an energetic or entropic barrier which optimizes crystallization kinetics and suppresses exaggerated (or encourages directed) growth.34 It appeared that the PbI2·DMF NWs drive the growth of MAPbI3·DMF NWs. In addition, the transformation from PbI2·DMF NWs to MAPbI3·DMF NWs changed the growth behavior, inhibiting growth along the radius. Thus, we hypothesized that the growth mechanism of MAPbI3·DMF NWs can be explained in two steps. First, as the PbI2·DMF-related crystal NW grows, PbI2 and DMF are consumed, which results in the local enrichment of MAI near the PbI2·DMF-related crystal precursors, allowing the nucleation of MAPbI3·DMF NW. Second, since DMF is consumed, the decrease in PbI2 concentration is compensated for so that the growth of PbI2·DMF can be carried out continuously. Lastly, we further corroborated the proposed growth mechanism of MAPbI3·DMF NW formation by in situ monitoring with a UV-Vis microspectrometer, as shown in Figure 2d. The growth of a MAPbI3·DMF NW can be divided into two main steps. Before the MAPbI3·DMF NW grows into the microchannel, the absorption band edge shows no obvious changes, and we refer to this period as the mass enrichment step. The absorption is mainly caused by the enrichment of PbI2·DMF-related crystal nanoparticles at the junction. During the growth and


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transformation of PbI2·DMF-related crystal NWs into MAPbI3·DMF NWs, the absorption edge extended from 432 nm (t = 22 min) to around 602 nm (t = 35 min) while the absorption spectra integral kept moving upward. In addition to the red shift of the absorption edge, the decrease of the band gap energy repeatedly confirmed the continuous transformation and maturity of the MAPbI3·DMF NWs. The details describing the estimation of band gaps and absorption edges are given in Figure S7. The changes in the UV-visible absorption spectra are similar to those observed for perovskite films formed by drying of a CH3NH3I solution on PbI2 films using a spin-coating method. UV-vis absorption spectra were also obtained during the growth of the PbI2·DMF NW (Figure S8). While the changes in absorption during the PbI2·DMF NW growth were not obvious, the variation tendency was similar to the initial stage of MAPbI3·DMF NW growth. These results suggest that the properties of the PbI2·DMF-related crystals generated during the MAPbI3·DMF NW growth might be closer to those of PbI2·DMF NWs rather than MAPbI3·DMF NWs. As time progresses, additional MAI molecules are coordinated with PbI2·DMF-related crystal NWs, thereby changing them into MAPbI3·DMF NWs. It appeared that the growth of MAPbI3·DMF NWs followed the growth of the PbI2·DMF-related crystal NWs. Thus, the results further confirm that the generation of PbI2·DMF-related crystals drives the growth of MAPbI3·DMF NWs, and the generation of PbI2·DMF-related crystals is guided by the nanochannel (Figure 2e). The crystallization caused by solvent evaporation can be studied in another way. As shown in Figure 3a (see also Movie S4), when the upper microchannel was filled with MAI/PbI2/DMF solution at half of the standard solution concentration (0.0625 M), crystallization took longer to start (t = 39 min compared with t = 23 min in Figure 2a). Most importantly, the first NW growth did not persist because the concentration increased only near the nanochannel entrance in the


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upper microchannel. Thus, the NW dissolved as soon as it exited the nanochannel and entered the upper microchannel (t = 41 min). As time progressed, the concentration increased again due to the evaporation of DMF. Thus, NWs grew again; after t = 104.5 min, the growth behavior was similar to that observed when the upper microchannel was filled with a solution at the standard concentration at the beginning of the experiment (Figure 2a). In a previous report regarding MAPbI3·DMF,35 the formula was suggested to be PbI2·3MAI·DMF or PbI2·MAI·2MA·DMF, which means that generating MAPbI3·DMF NWs requires a critical concentration of MAI that is much higher than that of PbI2. This explains why the first NW growth could not be maintained when a low concentration MAI/PbI2/DMF solution (0.0625 M) was used. Although we believe that this work provides a better understanding regarding the MAPbI3·DMF NW precursor, understanding how the MAPbI3·DMF precursor converts into MAPbI3 continues to remain ambiguous. Our results show an additional advantage of the MNFFT in controlling the growth of NWs. We fabricated an arched channel as shown in Figure 3b (see Supplementary Movie S5). Since the precursor NWs showed high flexibility, the NWs grew along the arched channel for more than 130° from t = 21.3 min to t = 26.3 min. After turning around the arched channel, the NW growth continued until it reached the side wall of the upper microchannel (t = 26.3 min). After impacting the wall of the microchannel, the MW broke into several MWs (t = 29.7 min). As time progressed (t = 33 min), the concentration increased in the microchannel, leading to a MW bundle, as observed previously. We quantified the growth rate of MAPbI3·DMF NWs under various concentrations of MAI/PbI2/DMF solution and temperatures. Figure 3c shows the growth rate, which is nearly linear with a slope of 1.477 µm s–1 M−1 as the concentration of MAI/PbI2/DMF solution increased from 0.0625 to 0.5 M at constant room temperature (T = 298


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K). The growth rate was also found to depend on temperature when the temperature was increased from 298 to 318 K. The log of the growth rate is linear with respect to the reciprocal of the temperature with a slope of 0.024 µm s−1 K−1 at the concentration of MAI/PbI2/DMF = 0.125 M. We demonstrated a practical application of our MNFFT with a MAPbI3 NW based photodetector, as shown in Figure 4. A schematic diagram of the working mechanism of the photodetector that we developed using a single MAPbI3 NW is illustrated in Figure 4a. The photosensitivity of the photodetector is shown in Figure 4b, which reached ~7,400 % under 532-nm laser illumination at 1 mW cm−2 when compared with the dark current. Similarly, the photodetector showed a linear response from 50 µW cm−2 to 50 mW cm−2 (see Figure S9). The response time and recovery time of the photodetector were ~0.22 ms and 0.79 ms, respectively (Figure 4c), and are comparable to those of other photodetectors utilizing perovskite NWs (see Supplementary Table S1).13, 14, 36 The spectral responsivity (R) also has been measured, which can be expressed as R = Jph/Llight, where Jph is the photocurrent (PC) density and Llight is the incident light intensity. The PC density is given by Jph = Iph/S, where Iph and S are the PC and effective device area, respectively.14 The R value for a single MAPbI3 NW is estimated to be 410 AW−1 (photocurrent Iph ≈ 0.82 µA and S ≈ 40 µm2, 20 µm in length and 2 µm in width) under 532-nm light irradiation (e.g., 5 mW cm-2) and bias voltage of −1 V, which is very high.13, 14, 37 From the calculated R value, the specific detectivity (D*) can be estimated, which is given by D* = (AB)1/2/NEP, where A is the area of the device, B is the given bandwidth, and NEP is the noise equivalent power.14 Assuming that the shot noise from the dark current is the major contribution, the specific detectivity can be expressed as D* = S1/2 R/(2eId)1/2, where e is the elementary charge (1.6 × 10-19 C) and Id is the dark current (Id ≈ 2.56 × 10-9 A).38 Thus, the D* value for the same


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single MAPbI3 NW can be estimated to be 9.1 × 1012 Jones, which is not only higher than that of the commercial Si-based photodetectors (ca. D* ≈ 4 × 1012 Jones) but also comparable to other literature values of perovskite-based photodetectors.14, 39 We demonstrated another application of the MNFFT by using it to fabricate a flexible photodetector with a NW array (Figure 4d). A MAPbI3 NW array was fabricated on a polyethylene terephthalate (PET) substrate, which has excellent flexibility and durability, showing the potential to create flexible optical sensors and wearable electronic devices. This was easily demonstrated by simply placing the PDMS device with multiple crack nanochannels onto a PET substrate and repeating the fabrication process described above. We investigated the PC and dark-current stabilities of the devices with 4 NWs (Figures 4e–g) at various bending radii under illumination at 650 nm to evaluate the potential of the aligned MAPbI3 NW array for application in flexible optoelectronics. As shown in Figure 4g, no obvious PC or dark current change was observed under four different bending states, indicating that the performance of the flexible device remains stable under external stresses. Even after 2000 bending cycles the device performance did not exhibit an obvious decrease (Figure 4h). We note that the devices were bent in the perpendicular direction to the NW length because the flexibility of the MAPbI3 NWs deteriorated significantly after annealing. However, the strong bonding between the NWs and the substrate remained durable and robust during the cyclic experiment. CONCLUSIONS In summary, we demonstrated a MNFFT that can precisely control and monitor the growth of MAPbI3 NWs in situ and in real-time. The initial nucleation point and growth path were controlled simultaneously under the guidance of nanochannels. We used an in situ analysis by


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UV-vis absorption spectroscopy during NW growth to understand the mechanism of the DMF-mediated crystallization of CH3NH3PbI3. First, a PbI2·DMF-related crystal grew along the nanochannel and then continuously extended into the microchannel, due to both DMF evaporation at the point where the nanochannel met the empty microchannel and the lower solubility of PbI2 in DMF. Second, the over consumption of PbI2 resulted in the enrichment of MAI near the PbI2·DMF-related crystal precursors, allowing the MAPbI3·DMF NW to nucleate alongside it. The consumption of PbI2 was compensated for by the consumption and continued evaporation of DMF, allowing the continued growth of the PbI2·DMF-related crystal. Next, the MAPbI3·DMF NW continuously grew under the guidance of the nanochannel. Both solid and flexible substrate materials were used successfully with the PDMS micro-/nanofluidic platform for solution-mediated perovskite NW growth. As a practical application, we demonstrated a MAPbI3-NW-based photodetector with high sensitivity. The present work explores only some of the potential capabilities of our proposed technique, and, consequently, we anticipate that the proposed fabrication method and micro-/nanofluidic platform could be used widely for low-cost, large-scale, highly efficient, and flexible optoelectronic applications. Methods Reagents and materials: A Sylgard 184 silicone elastomer kit (Dow Corning, Midland, USA) was used for PDMS-based soft lithography. A polyurethane acrylate (PUA) solution, MINS-311RM, and a polyethylene terephthalate (PET) film (Minuta Tech., Osan, Korea) were used for replication of the SU-8 (SU-8 2010, MicroChem, Westborough, USA) master mold. Lead(II) iodide (PbI2), methylammonium iodide (MAI, CH3NH2·HI), isopropanol (IPA) and N,N-dimethylformamide (DMF) were purchased from Sigma Aldrich (St. Louis, USA). All reagents were of analytical grade and were used without further purification.


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Fabrication of various NWs using the micro-/nanofluidic platform: MAI/PbI2/DMF solution was prepared by the solvent-assisted method: a 400 µL mixture of CH3NH3I in DMF and PbI2 in DMF with a molar ratio of 1:1 (0.25 M:0.25 M) was heated to 75 °C in a N2 gas container. MAPbI3 was generated after complete evaporation of DMF. It was difficult to confirm that the CH3NH3I and PbI2 have completely reacted with each other, and the product was therefore washed with IPA several times, which dissolves the CH3NH3I and PbI2 residues, but not the MAPbI3 product. MAPbI3 with high purity was obtained by evaporating IPA at 75 °C in the same N2 gas container. The product was again dissolved in different volumes of DMF to obtain MAI/PbI2/DMF solutions with different concentrations. A standard mixture concentration of 0.125 M was used unless otherwise noted. Following dissolution, 5 µL of MAI/PbI2/DMF solution was injected into each of the inlets of two parallel PDMS microchannels. Subsequently, the micro-/nanofluidic device was relocated into a glass container (Lock & Lock, USA) into which nitrogen gas flowed continuously for protection. The glass container was put on a hot plate and heated at 75 °C for 2 h. Lastly, the NW samples were annealed at 100 °C for an additional 1 h when required. In order to monitor the stability, the NWs were placed in a glass container with a lid and then stored in a dark and ambient environment at a temperature of 25 °C and a relative humidity of around 40 %, which is similar to other experimental conditions used in the literature.40 Characterization and measurements of the photoresponse of NWs: The real-time growth of MAPbI3 NW was monitored and corresponding images were taken from the beginning of the injection of MAI/PbI2/DMF solution with different recording rates, depending on the growth rates (i.e., 1 frame per 5 min at 298 K, 1 frame per 30 s from 303 K to 313 K, and 1 frame per 6 s at 318 K). To evaluate the structural properties of fabricated NWs, AFM (DI-3100, Veeco, New


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York, USA), high-resolution transmission electron microscopy (HR-TEM, JEM2100F, JEOL, Japan), X-ray diffraction (XRD, D8 Advance, Bruker AXS), field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan) were used. The in situ absorption spectra of MAPbI3·DMF NW or PbI2·DMF NW were collected using a UV-visible-NIR microspectrophotometer

(20/20

PV™

UV-visible-NIR

microspectrophotometer,

Craic

Technologies, CA, USA). Electrical measurements of the photodetector nanosystems were conducted with a semiconductor characterization system (Keithley, Model 4200); two Au electrodes were sputtered on both sides of the MAPbI3 NW with a 20 µm spacing using a shadow mask, as described in our previous work.33 Monochromatic light in the UV–vis range was obtained by filtering the white light from a high power xenon lamp with optical filters, and a light intensitometer was used to determine the light intensity. A home-built measurement system that combined a laser diode (532 nm or 650 nm) and a pulse generator was used to quantify the photoresponse time of the photodetector nanosystems under differently pulsed light illumination conditions. ACKNOWLEDGMENT This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2017R1A4A1015564 and 2017R1A2A1A17069723). All microfabrication processes and analyses were performed at the UNIST Central Research Facility Center. ASSOCIATED CONTENT


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Supporting Information. Figure S1, photographic image of the micro-/nanofluidic platform; Figure S2, the electron diffraction corresponding to the entire TEM image of the MAPbI3 NW; Figure S3, time-lapse microscopic images showing the temporal MAPbI3 NW fabrication process over 900 min; Figure S4, The three cross-sectional profiles of the AFM topographic images shown in Figure 2b. Figure S5, the growth of MAPbI3·DMF NW in a micro-/nanofluidic PDMS platform with different channel size; Figure S6, optical images of the needle-like PbI2·DMF precursors growing from a droplet of PbI2/DMF at 298 K; Figure S7, the switched Tauc plots of the (αhυ)2 versus hυ curves of the MAPbI3·DMF NW; Figure S8, the in situ UV-vis absorption spectra of the PbI2·DMF NW; the UV-vis absorption spectra of the components and the UV-vis absorption spectrum of a MAPbI3 NW after annealing; Figure S9, photocurrents of a MAPbI3 NW under 532-nm-laser illumination conditions; Table S1, comparison of the responsivity and response time of the photodetectors; Movie S1 to S5 are in situ monitoring of the growth of NWs under different conditions. This material is available free of charge via the Internet at http://pubs.acs.org.


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REFERENCES 1.

Haferburg, G.; Gröning, J. A. D.; Schmidt, N.; Kummer, N.-A.; Erquicia, J. C.; Schlömann, M. Microbial Diversity of the Hypersaline and Lithium-Rich Salar de Uyuni, Bolivia. Microbiol. Res. 2017, 199, 19-28. 2. Mojibola, A.; Dongmo-Momo, G.; Mohammed, M.; Aslan, K. Crystal Engineering of l-Alanine with l-Leucine Additive Using Metal-Assisted and Microwave-Accelerated Evaporative Crystallization. Cryst. Growth. Des. 2014, 14, 2494-2501. 3. Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H. Iodide Management in Formamidinium-Lead-Halide-Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376-1379. 4. Liu, J.; Xue, Y.; Wang, Z.; Xu, Z.-Q.; Zheng, C.; Weber, B.; Song, J.; Wang, Y.; Lu, Y.; Zhang, Y. Two-Dimensional CH3NH3PbI3 Perovskite: Synthesis and Optoelectronic Application. ACS Nano 2016, 10, 3536-3542. 5. Wang, N.; Cheng, L.; Ge, R.; Zhang, S.; Miao, Y.; Zou, W.; Yi, C.; Sun, Y.; Cao, Y.; Yang, R. Perovskite Light-Emitting Diodes Based on Solution-Processed Self-Organized Multiple Quantum Wells. Nat. Photonics 2016, 10, 699-704. 6. Li, Y. J.; Lv, Y.; Zou, C.-L.; Zhang, W.; Yao, J.; Zhao, Y. S. Output Coupling of Perovskite Lasers from Embedded Nanoscale Plasmonic Waveguides. J. Am. Chem. Soc. 2016, 138, 2122-2125. 7. Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jesper Jacobsson, T.; Grätzel, M.; Hagfeldt, A. The Rapid Evolution of Highly Efficient Perovskite Solar Cells. Energy Environ. Sci. 2017, 10, 710-727. 8. Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636-642. 9. Gholipour, B.; Adamo, G.; Cortecchia, D.; Krishnamoorthy, H. N. S.; Birowosuto, M. D.; Zheludev, N. I.; Soci, C. Organometallic Perovskite Metasurfaces. Adv. Mater. 2017, 29, 1604268. 10. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering for High-Performance Inorganic–Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897-903. 11. Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.-H.; Liu, Y.; Li, G.; Yang, Y. Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process. J. Am. Chem. Soc. 2014, 136, 622-625. 12. Dou, L.; Wong, A. B.; Yu, Y.; Lai, M.; Kornienko, N.; Eaton, S. W.; Fu, A.; Bischak, C. G.; Ma, J.; Ding, T. Atomically Thin Two-Dimensional Organic–Inorganic Hybrid Perovskites. Science 2015, 349, 1518-1521. 13. Horváth, E.; Spina, M.; Szekrényes, Z.; Kamarás, K.; Gaal, R.; Gachet, D.; Forró, L. Nanowires of Methylammonium Lead Iodide (CH3NH3PbI3) Prepared by Low Temperature Solution-Mediated Crystallization. Nano Lett. 2014, 14, 6761-6766.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

14. Deng, W.; Zhang, X.; Huang, L.; Xu, X.; Wang, L.; Wang, J.; Shang, Q.; Lee, S.-T.; Jie, J. Aligned Single-Crystalline Perovskite Microwire Arrays for High-Performance Flexible Image Sensors with Long-Term Stability. Adv. Mater. 2016, 28, 2201-2208. 15. Zhang, Y.; Wang, Y.; Xu, Z.-Q.; Liu, J.; Song, J.; Xue, Y.; Wang, Z.; Zheng, J.; Jiang, L.; Zheng, C.; Huang, F.; Sun, B.; Cheng, Y.-B.; Bao, Q. Reversible Structural Swell–Shrink and Recoverable Optical Properties in Hybrid Inorganic–Organic Perovskite. ACS Nano 2016, 10, 7031-7038. 16. Xu, X.; Zhang, X.; Deng, W.; Huang, L.; Wang, W.; Jie, J.; Zhang, X. Saturated Vapor-Assisted Growth of Single-Crystalline Organic–Inorganic Hybrid Perovskite Nanowires for High-Performance Photodetectors with Robust Stability. ACS Appl. Mater. Inter. 2018, 10, 10287-10295. 17. Chen, X.; Wong, C. K.; Yuan, C. A.; Zhang, G. Nanowire-Based Gas Sensors. Sensors and Actuat. B-Chem. 2013, 177, 178-195. 18. Lecault, V.; VanInsberghe, M.; Sekulovic, S.; Knapp, D. J. H. F.; Wohrer, S.; Bowden, W.; Viel, F.; McLaughlin, T.; Jarandehei, A.; Miller, M.; Falconnet, D.; White, A. K.; Kent, D. G.; Copley, M. R.; Taghipour, F.; Eaves, C. J.; Humphries, R. K.; Piret, J. M.; Hansen, C. L. High-Throughput Analysis of Single Hematopoietic Stem Cell Proliferation in Microfluidic Cell Culture Arrays. Nat. Meth. 2011, 8, 581-586. 19. Sackmann, E. K.; Fulton, A. L.; Beebe, D. J. The Present and Future Role of Microfluidics in Biomedical Research. Nature 2014, 507, 181-189. 20. Sun, J.; Xianyu, Y.; Jiang, X. Point-of-Care Biochemical Assays Using Gold Nanoparticle-Implemented Microfluidics. Chem. Soc. Rev. 2014, 43, 6239-6253. 21. Maceiczyk, R. M.; Dümbgen, K.; Lignos, I.; Protesescu, L.; Kovalenko, M. V.; deMello, A. J. Microfluidic Reactors Provide Preparative and Mechanistic Insights into the Synthesis of Formamidinium Lead Halide Perovskite Nanocrystals. Chem. Mater. 2017, 19, 8433-8439. 22. Lignos, I.; Maceiczyk, R.; deMello, A. J. Microfluidic Technology: Uncovering the Mechanisms of Nanocrystal Nucleation and Growth. Acc. Chem. Res. 2017, 50, 1248-1257. 23. Whitesides, G. M.; Ostuni, E.; Takayama, S.; Jiang, X.; Ingber, D. E. Soft Lithography in Biology and Biochemistry. Annu. Rev. Biomed. Eng. 2001, 3, 335-373. 24. Deng, W.; Huang, L.; Xu, X.; Zhang, X.; Jin, X.; Lee, S.-T.; Jie, J. Ultrahigh-Responsivity Photodetectors from Perovskite Nanowire Arrays for Sequentially Tunable Spectral Measurement. Nano Lett. 2017, 17, 2482-2489. 25. Mao, J.; Sha, W. E. I.; Zhang, H.; Ren, X.; Zhuang, J.; Roy, V. A. L.; Wong, K. S.; Choy, W. C. H. Novel Direct Nanopatterning Approach to Fabricate Periodically Nanostructured Perovskite for Optoelectronic Applications. Adv. Fun. Mater. 2017, 27, 1606525. 26. Thokchom, A. K.; Zhou, Q.; Kim, D.-J.; Ha, D.; Kim, T. Characterizing Self-Assembly and Deposition Behavior of Nanoparticles in Inkjet-Printed Evaporating Droplets. Sensors Actua. B- Chem. 2017, 252, 1063-1070. 27. Kim, M.; Ha, D.; Kim, T. Cracking-Assisted Photolithography for Mixed-Scale Patterning and Nanofluidic Applications. Nat. Commun. 2015, 6, 6247.


Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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28. Yang, S.; Zheng, Y. C.; Hou, Y.; Chen, X.; Chen, Y.; Wang, Y.; Zhao, H.; Yang, H. G. Formation Mechanism of Freestanding CH3NH3PbI3 Functional Crystals: In Situ Transformation vs Dissolution-Crystallization. Chem. Mater. 2014, 26, 6705-6710. 29. Cho, N.; Li, F.; Turedi, B.; Sinatra, L.; Sarmah, S. P.; Parida, M. R.; Saidaminov, M. I.; Murali, B.; Burlakov, V. M.; Goriely, A.; Mohammed, O. F.; Wu, T.; Bakr, O. M. Pure Crystal Orientation and Anisotropic Charge Transport in Large-Area Hybrid Perovskite Films. Nat. Commun. 2016, 7, 13407. 30. Petrov, A. A.; Pellet, N.; Seo, J.-Y.; Belich, N. A.; Kovalev, D. Y.; Shevelkov, A. V.; Goodilin, E. A.; Zakeeruddin, S. M.; Tarasov, A. B.; Graetzel, M. New Insight into the Formation of Hybrid Perovskite Nanowires via Structure Directing Adducts. Chem. Mater. 2017, 29, 587-594. 31. Li, Y.; Zhao, Z.; Lin, F.; Cao, X.; Cui, X.; Wei, J. In Situ Observation of Crystallization of Methylammonium Lead Iodide Perovskite from Microdroplets. Small 2017, 13, 1604125. 32. Ji, Z.; Wang, X.; Zhang, H.; Lin, S.; Meng, H.; Sun, B.; George, S.; Xia, T.; Nel, A. E.; Zink, J. I. Designed Synthesis of CeO2 Nanorods and Nanowires for Studying Toxicological Effects of High Aspect Ratio Nanomaterials. ACS Nano 2012, 6, 5366-5380. 33. Kim, D.-J.; Ha, D.; Zhou, Q.; Thokchom, A. K.; Lim, J. W.; Lee, J.; Park, J. G.; Kim, T. A Cracking-Assisted Micro-/Nanofluidic Fabrication Platform for Silver Nanobelt Arrays and Nanosensors. Nanoscale 2017, 9, 9622-9630. 34. Wu, X.; Wang, J.; Yeow, E. K. L. Ultralong Perovskite Microrods: One- versus Two-Step Synthesis and Enhancement of Hole-Transfer During Light Soaking. J. Phys. Chem. C 2016, 120, 12273-12283. 35. Guo, X.; McCleese, C.; Kolodziej, C.; Samia, A. C. S.; Zhao, Y.; Burda, C. Identification and Characterization of the Intermediate Phase in Hybrid Organic–Inorganic MAPbI3 Perovskite. Dalton Trans. 2016, 45, 3806-3813. 36. 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. 37. Dong, R.; Fang, Y.; Chae, J.; Dai, J.; Xiao, Z.; Dong, Q.; Yuan, Y.; Centrone, A.; Zeng, X. C.; Huang, J. High-Gain and Low-Driving-Voltage Photodetectors Based on Organolead Triiodide Perovskites. Adv. Mater. 2015, 27, 1912-1918. 38. Gong, X.; Tong, M.; Xia, Y.; Cai, W.; Moon, J. S.; Cao, Y.; Yu, G.; Shieh, C.-L.; Nilsson, B.; Heeger, A. J. High-Detectivity Polymer Photodetectors with Spectral Response from 300 nm to 1450 nm. Science 2009, 325, 1665-1667. 39. Dou, L.; Yang, Y. M.; You, J.; Hong, Z.; Chang, W.-H.; Li, G.; Yang, Y. Solution-Processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Commun. 2014, 5, 5404. 40. Wang, B.; Chen, T. Exceptionally Stable CH3NH3PbI3 Films in Moderate Humid Environmental Condition. Adv. Sci. 2015, 3, 1500262.


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Figures

Figure 1. A typical example of solution-mediated crystallization in nature and in a microcosm. (a) Optical image of a saline lake. (b) and (c) Optical microscope images of a microdroplet containing MAI/PbI2/DMF solution at 298 K. After evaporation of DMF near the edge of the droplet, MAPbI3 micro- and nanoneedles are randomly but radially grown. (d) Schematic of the cracking-assisted micro-/nanofluidic device attached on a SiO2/Si substrate. (e) SEM image of a micro-/nanofluidic channel network in which a nanowire can be fabricated along a nanochannel. Enlarged image of the crack nanochannel. An AFM image of the crack nanochannel was obtained and the cross-cut along A–A′ is shown. (f) Schematic of the solution-mediated fabrication method for a MAPbI3 NW. Filling the nanochannel with the MAI/PbI2/DMF solution and then annealing it at high temperatures in a N2 environment makes it possible to produce a single-crystal MAPbI3 NW. (g) SEM images of the perovskite NW, which was grown with its crystallization restricted in the nanochannel domain. (h) AFM image of the single-crystal MAPbI3 NW. (i) TEM image of the single-crystal MAPbI3 NW. (j) XRD patterns of the MAPbI3


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NW and the same NW after storage in air for 100 days under darkness.


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Figure 2. Growth mechanism of MAPbI3·DMF NWs. (a) A micro-/nanofluidic platform shows a nanochannel between microchannels. The growth of a MAPbI3·DMF NW was monitored in real-time. The upper microchannel was filled with a MAI/PbI2/DMF solution, while the lower microchannel was empty during the NW fabrication process. (b) Three different nanochannels were tested and their influence on the growth of a MAPbI3·DMF NW were compared. (c) Real-time growth monitoring of a PbI2·DMF NW with a PbI2/DMF solution. (d) The in situ UV-vis absorption spectra of the MAPbI3·DMF NW. Optical microscopic images were taken at three different stages during the MAPbI3·DMF NW growth. (e) Illustration depicts the growth mechanism of a MAPbI3·DMF NW. Scale bars are 100 µm.


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Figure 3. Growth characterization of MAPbI3·DMF NWs. (a) Real-time growth monitoring of a MAPbI3·DMF NW from a nanochannel to the microchannel network at the upper side filled with a low concentration of MAI/PbI2/DMF solution (0.0625 M). (b) Real-time growth monitoring of two MAPbI3·DMF NWs from two separate but parallel nanochannels toward an arched microchannel on the upper side. (c) Dependence of the NW growth rate on the concentration of a MAI/PbI2/DMF solution at 298 K. (d) Dependence of the NW growth rate on temperature. Scale bars are 100 µm.


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Figure 4. Photoelectric property characterization of MAPbI3 NWs. (a) Schematic representation of the working mechanism of the photodetector based on a single MAPbI3 NW. (b) Dark and laser-illuminated I−V curves obtained from the MAPbI3 NW under different illumination intensities of 532-nm wavelength. (c) Temporal response with –4 V bias under 532-nm LED illumination with light intensities of 100 µW cm−2 and a modulation frequency of 250 Hz. (d) Photograph of a micro-/nanofluidic PDMS device with four nanochannels on a PET substrate. (e) Microscopic image of MAPbI3 NWs on the PET substrate. (f) Photographic image of the as-prepared flexible photodetector with Au electrodes. (g) Current of the flexible photodetector bent to various radii at a bias voltage of –5 V. Middle insets show corresponding photographs of the device under the four bending conditions. (h) The PC and dark current after different bending cycles.


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For Table of Contents Only

90×30mm (300×300 DPI) We not only developed a micro-/nanofluidic fabrication technique that enables precise control and in situ monitoring of the growth of perovskite nanowires but also demonstrated a MAPbI3-NW-based photodetector with high sensitivity.


Nanochannel-Assisted Perovskite Nanowires: From Growth

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