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Air-Stable Transparent Silver Iodide-Copper Iodide Heterojunction Diode Ji-Hyun Cha, and Duk-Young Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14378 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Air-Stable Transparent Silver Iodide-Copper Iodide Heterojunction Diode Ji-Hyun Cha and Duk-Young Jung* Department of Chemistry, Sunkyun Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon, Gyeonggido, 16419, Korea. Keywords: Silver Iodide, Copper Iodide, Vapor-Phase Iodization, Selective-Decomposition Patterning, Heterojunction pnDiode.

ABSTRACT: Transparent AgI-CuI heterojunctions with high rectifying diode behavior were prepared via vapor-phase iodization of metal thin films on transparent conducting oxide substrates. At room temperature, Ag and Cu metal thin films were quickly transformed into the transparent and well-crystallized β-phase of AgI and the γ-phase of CuI, respectively. The AgI and CuI films exhibited n-type and p-type semiconductor properties, respectively, with wide band gaps. The heterojunctions were obtained by applying the CuI film to the AgI film in a sequential iodization process. AgI compounds generally have poor air-stability under light, making them suboptimal for use in electronic applications. Here, we used a CuI top layer to inhibit the photodecomposition of the AgI bottom layer, resulting in an air-stable and smooth AgI-CuI film. We also propose a simple patterning method for the AgICuI layer using selective decomposition of the AgI without the need for lithography equipment or toxic chemicals. Although there is metal ion exchange between the two layers, each layer has a different chemical composition and crystal structure; therefore, the AgI-CuI heterojunction exhibits pn-diode behavior with a rectifying ratio of 9.4×104, which is comparable to other transparent pndiodes. These findings open a new path for electronic application of AgI materials.

INTRODUCTION Recently, halide compounds have emerged as valuable semiconducting materials as a result of their superior optoelectrical properties compared with those of traditional metal oxide and chalcogenide materials. Hybrid organic-inorganic halide perovskite materials, such as methylammonium lead iodide and formamidinium lead iodide, are widely investigated as lightabsorbing layers in high-efficiency photovoltaics (PV).1,2 Due to the tunable nature of their electrical properties, halide perovskite materials with complex chemical composition, including ternary and quaternary systems, are employed as semiconducting materials in electronics.3 Examples include the lightemitting layer of quantum-dot light-emitting diodes (QDLED),4 the resistance-changing layer of resistive random access memory (ReRAM),5 and the light-sensing layer of photodetectors.6 Additional study of the application and characterization of binary metal halide compounds is needed to understand their electrical and chemical properties for applications in electronics. Investigating the fundamental electrical and chemical behaviors of binary metal halide compounds will further the development of these halide material-enabled devices. AgI is conventionally applied to solid-state electrolytes, photographic films, photocatalysts, and cloud condensation and is valued for its photosensitivity, ionic conductivity, and crystal structure.7−9 AgI compounds have three main polymorphs with varying thermodynamic stability: the cubic α-phase (α-AgI), hexagonal β-phase (β-AgI), and cubic γ-phase (γ-AgI).10 The α-AgI polymorph is well-known as an ionic-conductive mate-

rial and can be stabilized by applying pressure to AgI nanoparticles with a diameter of 11 nm.11−13 At ambient pressure, βand γ-AgI exhibit greater stability than α-AgI below 147 ºC.14,15 The electrical properties of β- and γ-AgI were investigated using a Wagner-type polarization cell.16 The β-AgI compounds exhibit higher electrical conductivity as n-type semiconductors than as ionic conductors.17 The ionic conductivity of the AgI compound has been investigated in the solid electrolytes used to increase the safety and efficiency of batteries. However, AgI has not attracted significant attention as a semiconducting material in electronics devices because of its poor stability under light and ambient atmosphere.18 Although AgI-based ReRAM has been studied recently,19 the development of electronic devices incorporating AgI materials are challenging compared to those using other inorganic halide materials. CuI is a p-type semiconductor with high hole mobility originating from a copper vacancy.20 The CuI compounds have three main crystal phases: the γ-phase (γ-CuI) is a p-type semiconductor, and the α-phase (α-CuI) and β-phase (β-CuI) are ionic conductors.21 The γ-CuI phase has been investigated for use as a hole transport material of PV, p-type semiconductors of thermoelectric devices, and a photocatalyst of CO2 reduction.22−24 γ-CuI is a particularly promising candidate for perovskite-based PV because of its solution-processability and high hole mobility.22 In previous reports, CuI thin films were prepared via RF magnetron sputtering, thermal evaporation, electrochemical processing, solution coating, pulsed laser deposition, and Bädeker vapor route.20 In this paper, smooth and dense polycrystalline CuI thin films were obtained via chemi-

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cal reaction between Cu metal film and vapor-phase iodine under mild conditions, which is called as the Bädeker vapor route, in order to minimize the damage or influence to the bottom layer. Here, we present a novel metal iodide heterojunction structure consisting of transparent AgI and CuI thin films synthesized via vapor-phase iodization at room temperature. The simplest method to obtain highly crystallized metal halide thin films is the deposition of a pure metal layer and subsequent vapor phase iodization. AgI and CuI were utilized as an n-type and p-type semiconductor for the facile synthesis of stable metal halide heterojunction. The adhesion stability and chemical properties of the prepared metal halide layers were investigated in the sequential steps of film deposition. We developed a novel patterning method using the photodecomposition of the AgI layer and demonstrated the electrical performance of the AgI-CuI heterojunction.

RESULTS AND DISCUSSION Figure 1a shows the fabrication process of the AgI-CuI heterojunction diode on indium tin oxide (ITO) substrate. First, the Ag metal thin films were deposited via thermal evaporation with a thickness of 90 nm on a clean ITO substrate. The Ag thin films were transformed into AgI films through a vapor-phase iodization reaction, in which the Ag thin films were placed in a glass container with iodine powder. The Ag metal layer gradually changed to a yellowish transparent film after 10 minutes. (Figures S1)

EXPERIMENTAL METHODS Fabrication of AgI-CuI Heterojunction Thin Films Transparent AgI-CuI thin films were fabricated using the deposition of metal thin films and I2 vapor iodization of metal layers. A thermal evaporation system (SNTEK Company, REP-5004) was used for the deposition of Ag and Cu metal films on an ITO substrate. The highly pure Ag source (ORIGIN, shots, 99.99%) was thermally evaporated onto the cleaned ITO substrate at a deposition rate of 1.0 Å/sec under vacuum pressure, ~10−6 torr, without heating, to obtain an Ag layer 80 nm in thickness. The Ag-deposited substrate was fixed on the lid of the glass container, and 0.1g of iodine (JUNSEI, 99.8%) was added to the bottom of the container opposite the Ag film and iodine. The transformation from Ag into AgI was carried out for 10 minutes at room temperature (25°C). For the deposition of a Cu over-layer 150 nm in thickness, the highly pure Cu sources (ORIGIN, shots, 99.999%) were evaporated onto the as-synthesized AgI film at a deposition rate of 5.0 Å/sec under vacuum pressure, ~10−6 torr. The Cu thin films were patterned by attaching a metal shadow mask to the substrate and were transformed into CuI layers through iodization for 15 min. The exposed AgI layer, uncovered by the CuI layer, decomposed slowly under an ambient atmosphere. After 24 hours, the decomposed AgI area could be easily removed using a cotton swab soaked in hexane. Characterization of Devices In order to study the crystal structures and chemical composition of AgI-CuI, X-ray diffraction (XRD) patterns were obtained with a Rigaku X-ray diffractometer (Ultima IV) at 40 kV and 30 mA with Cu-Kα radiation (λ = 1.5405 Å). The surface morphologies and interface of the thin films were studied via scanning electron microscope (SEM, Philips, XL30) and a surface profiler (Bruker, DXT-A). The optical characteristics of the thin films were analyzed using a UV–Vis spectrophotometer (Scinco, S-3100). Twopoint electrical measurements of the patterned AgI-CuI heterojunction were conducted using an Au-coated tungsten tip and a semiconductor analyzer (Hewlett-Packard 4145B). Linear voltage sweeps were obtained in the range between −2 and +2 V without hold and delay times. The Hall effect of AgI and CuI film deposited on glass was measured using the van der Pauw technique (ECOPiA, HMS-3000).

Figure 1. (a) Schematic representation of the AgI-CuI heterojunction fabrication process using repetition of metal evaporation and iodization processes on the ITO substrate. (b) A photograph of the sample with a rectangular AgI-CuI diode prepared on a 2×2 cm ITO glass substrate. (c) Three-dimensional surface profile images and (d) cross-sectional images of the patterned multi-layer thin film. The vapor pressure of iodine (0.3 mmHg at 298 K) was sufficient to induce an oxidation reaction in the metal at room temperature, sufficient to prepare well-crystallized and smooth AgI from the Ag film up to a thickness of 200 nm. The Cu metal layer was selectively deposited via thermal evaporation onto the as-synthesized AgI substrate covered with a metal shadow mask. Patterned CuI thin films on AgI were obtained through an iodization process under the conditions described above. When the AgI-CuI sample was exposed to the ambient atmosphere, the AgI areas not covered by CuI became opaque due to the decomposition of AgI. These decomposed areas were easily lifted from the substrate using a cotton swab soaked in hexane. In comparison, the AgI layers covered with a CuI layer remained transparent after three days of exposure to air. We verified that the decomposed AgI was completely removed from the substrate on which the 20 AgI-CuI diodes remained. (Figure 1b) The dimensions of the AgI-CuI device shown in Figure 1c were 1.1 × 0.7 mm, with a thickness of 1.1(1) µm, and surface roughness of 35 ± 1 nm. Figure 1d shows a cross-sectional SEM image of the as-prepared AgICuI with clearly separated layers on the substrate. The total thickness of the metal iodide layer as determined via SEM was approximately 1.2 µm, with a 690 ± 30 nm top layer and 330 ± 40 nm bottom layer, in accordance with the surface profile data. The galvanic reaction at the interface of the AgI and CuI layers caused a compositional change to βʺ-(Ag0.6Cu0.4)I/γ′(Cu0.8Ag0.2)I (vide infra). Therefore, we demonstrated a repro-


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ACS Applied Materials & Interfaces ducible process to obtain patterned devices through the photodecomposition of AgI and the induced facile lift-off process without the need for complex lithography or chemical etchings, which represents a novel fabrication process for complex, narrow thin film devices. As a preliminary experiment, Ag, AgI, Cu, and CuI thin films were prepared separately on ITO substrates. (Figure S2) To compare the AgI-CuI multilayer films, we prepared the AgI and CuI layers on ITO. The grain size of the AgI polycrystalline was found to be in the range of 100−600 nm, and the thickness of the AgI layer (290 nm) was 3.9 times greater than that of the Ag layer (88 ± 3 nm) before iodization. (Figures S2 a−d) When the facecentered cubic (fcc) structure of Ag metal transformed into hexagonal AgI (wurtzite), (Figure S3) the unit cell volume increased theoretically from 67.9 Å3 to 276 Å3 or approximately 4 times. The direction of the crystal growth in the AgI polycrystalline film was observed to be exclusively perpendicular to the substrate. This was probably due to the prepared AgI layer being well adhered to the substrate. The thickness of the prepared CuI thin films was 4.7 times greater than that of the Cu metal layer, and the typical grain size within the CuI thin film was in the range of 50−450 nm. As shown in the SEM images in Figures S2 e−h, the volume expansion ratio of CuI to Cu film was calculated to be 4.8 based on the obtained experimental data. The measured value was close to the calculated value based on the lattice constants of CuI and Cu, which were obtained via XRD of the Cu and CuI films. (Figure S4) The polycrystalline CuI thin films were grown in the same direction as the AgI layer.

lifted off from the ITO substrate during the iodization process of the Ag on the γ-CuI layer. The successful process features layer-by-layer growth of AgI-CuI thin films, as depicted in Figure 2c. The two layers have different chemical compositions and crystal structures with a clear interface, as revealed via SEM and XRD measurements. Because the top layer of CuI is crucial for protecting the AgI bottom layer from decomposition, the micro-patterning of AgI-CuI devices was performed by lifting the decomposed β-AgI layer. XRD patterns were collected to analyze the crystal phase of thin films in each experimental step, as shown in Figure 3. The XRD pattern of Ag film was indexed to the fcc structure of Ag metal with (111) dominant growth. The lattice parameter was calculated using the least squares refinement as a = 4.084(1) Å, similar to the value of bulk crystal Ag (a = 4.08 Å, JCPDS 01-1167).

Figure 3. (a) The XRD patterns of Ag metal, AgI, AgI-Cu, and AgI-CuI in Figure 1a. (b) Magnified graph of XRD peaks in a range of 23° to 26°, which shows peaks shifting toward a high angle, following experimental steps. (c) Schematic illustration of the chemical composition of thin films in each deposition steps. Figure 2. Schematic illustration of various experimental processes in the film preparation as flowing sequence; (a) Ag deposition–Cu deposition–iodization, (b) Cu deposition–1st iodization–Ag deposition–2nd iodization, (c) Ag deposition–1st iodization–Cu deposition–2nd iodization. Figure 2 shows the summarized experimental results of the three methods of AgI-CuI film preparation to optimize the deposition procedure and formation of stable AgI-CuI heterojunctions. For all metal iodide thin films, the molar ratio between Cu and Ag metal thin films is fixed as Cu:Ag = 1.0 : 0.4. In the first preparation process, double-layer films consisting of Ag and Cu were iodized simultaneously, as shown in Figure 2a. The films were peeled from the substrate during the iodization process, as shown in Figure S5. In the case of Cu metal films deposited first onto an ITO substrate followed by iodization and thermal evaporation of the Ag films (Figure 2b), XRD results showed no diffusion of Cu and Ag ions between the Ag and the γ-CuI film. The film also spontaneously

The XRD pattern of the AgI film had a strong peak at 24.14° and three minor peaks. The three crystal structures of the AgI compounds are cubic (α-AgI), wurtzite (β-AgI), and zincblend (γ-AgI); β- and γ-AgI were observed at ambient pressure in the powder samples. The three XRD peaks of the asprepared sample observed at 2θ = 24°, 39°, and 46° are common to the β- and γ- AgI and can be indexed as (111), (220), and (311) for the γ-phase, respectively, or (002), (110), and (112) for the β-phase. (Figure S3) If the crystal phase of AgI is a zinc-blend structure (cubic), the isotropic crystal growth is thermodynamically stable, and the intensity ratio of the main peaks is similar to the simulated results of γ-AgI. AgI crystals grew anisotropically in the hexagonal unit cell along the [001] orientation, which decreased the intensity of the other peak. The XRD patterns of the as-prepared AgI thin films showed a highly oriented pattern with a strong intensity at 2θ = 24.14°, in contrast with the simulated XRD pattern of γ-AgI. Therefore, it is appropriate that the as-prepared AgI film is assigned to the wurtzite structure and the lattice parameters of a =



4.589(1) Å and c = 7.509(1) Å. Previous reports indexed the strongly oriented γ-AgI pattern as major products based on the XRD measurements;19 however, we believe that the (002) growth of the β-AgI films, formed as a result of iodization of the Ag layer, was affected by the crystallinity of the Ag metal layer.25 The crystal phase of the AgI thin films was independent of the crystal structure of the substrates and the reaction temperature. The XRD pattern of the AgI-Cu layer in Figure 3a shows diffraction peaks corresponding to Cu (JCPDS 011241), Ag (JCPDS 03-0921), and β′-phase AgI. The peaks of β′-phase AgI were shifted to a higher 2θ than the pristine βAgI phase due to a decrease in unit cell dimensions from the substitution of Cu with Ag atoms. The calculated lattice constants of β′-AgI (Cu-doped β-AgI) were a = 4.497(2) and c = 7.366(2) Å and suggest a composition of Ag0.7Cu0.3I based on Vegard’s rule.26 As the Cu concentration increased, the smaller ionic size of Cu+ (145 pm) compared to Ag+ (165 pm) likely resulted in a systematic decrease in the unit cell volume of AgI for the AgI-CuI solid solution. A metal ion exchange was observed at the interface between Cu and AgI, which we ascribed to the difference in the standard reduction potentials of AgI/Ag (Eo = − 0.15 V) and CuI/Cu (Eo = − 0.18 V) pairs. Cu was expected to be oxidized into Cu+ ions and dissolved in the β-AgI layers, as shown in the equation below. AgI(s) + e– ⇋ Ag(s) + I–

Eo = − 0.15 V

(1)

CuI(s) + e– ⇋ Cu(s) + I–

Eo = − 0.18 V

(2)

AgI(s) + Cu(s) ⇋ Ag(s) + CuI(s)

Eo =

(3)

0.03 V

film was indexed with β″-AgI and γ′-CuI, because the β″-AgI possesses higher Cu content than the previous β′-AgI, which has smaller lattice parameters of a = 4.42(1) Å and c = 7.27(1) Å. The chemical formula of βʺ-AgI calculated from the XRD data was Ag0.6Cu0.4I, showing that the Cu content increased compared to that in βʹ-AgI. Analysis of the γ′-CuI layer with a zinc-blend structure yielded an a = 6.148(1) Å. This is larger than the 6.051 Å found for the silver-free γ-CuI prepared on a glass substrate shown in Figure S4, with a chemical composition of Cu0.8Ag0.2I. In the solid solution of the AgI-CuI system, the zinc-blend structure dominates, resulting in an increase in Cu concentration due to thermodynamic stability.17 Although the chemical compositions and lattice parameters of both layers changed after the redox replacements, the two films were distinguished on the ITO substrates, as shown in the SEM cross-sectional image in Figure 1d. Figure 3c shows the compositional changes in each deposition step of the AgI-CuI heterojunction. The Ag metal film was deposited onto ITO and then transformed into β-AgI thin film through iodization. As the Cu metal films were deposited onto the β-AgI, β′-AgI films were obtained through a galvanic redox reaction. The second iodization produced stable β″-AgI on γ′-CuI thin films. Details related to the crystal structure and chemical composition of the thin films are presented in Table 1.

Table 1. The crystal structural and estimated compositional data of thin films prepared at each experimental step.

Phases

Unit cell

Lattice parameter (Å) a)

Estimated composition of film b)

Ag metal

Cubic

a=4.084(2)

Ag

β-AgI

Hexagonal

a=4.589(1) c=7.509(1)

AgI

βʹ-AgI

Hexagonal

a=4.497(2) c=7.366(2)

Ag0.7Cu0.3I

Cu metal

Cubic

a=3.607(4)

Cu

βʺ-AgI

Hexagonal

a=4.42(1) c=7.27(1)

Ag0.6Cu0.4I

γʹ-CuI

Cubic

a=6.148(1)

Ag0.2Cu0.8I

a)

The lattice parameters were calculated using the least square method; b) The chemical compositions were calculated using Vegard’s law from PXRD patterns. The deposited Cu layer oxidized to Cu+ ions and diffused into the AgI layer, and the reduced Ag cluster produced from the AgI layer was expected to exist primarily in the interface between the Cu and AgI layer. The XRD pattern of the AgI-CuI

Figure 4. Optical transmittance spectra of the as-prepared AgI and AgI-CuI exposed under air conditions and AgI and AgICuI thin films. The inset shows the absorbance spectra of asprepared AgI and AgI-CuI thin films, obtained from transmittance spectra from the Beer-Lambert equation. Figure 4 shows the optical properties of the as-prepared AgI and AgI-CuI films and exposed samples under an air atmosphere for 12 hours and AgI and AgI-CuI films. The fresh AgI thin film is yellowish color with an average transmittance of 60 % at the region of 450−700 nm wavelength. After exposure to air for 12 hours, the AgI films lost their transparency and adhesion properties, becoming opaque and easily scratched. This property results from ready decomposition of the hexagonal AgI even under weak light and ambient atmosphere. However, the transmittance of the AgI-CuI layer remained unchanged after 12 hours and remained steady for up to two weeks under ambient conditions. The photodecomposition of the AgI thin film under air is a significant obstacle to application of AgI film to electronics. While many researchers have investigated the electronic properties and application of AgI thin films, stable devices using AgI thin films have not yet


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ACS Applied Materials & Interfaces been reported. Our fabrication process provides a novel design for electronics based on AgI materials. The low transparency of AgI-CuI, which is approximately 30 % in the visible range, can be improved by reducing the film thickness and surface roughness. The spectra of the AgI-CuI film showed the absorption edges of the films shifting to a higher wavelength compared with the AgI film in the inset graph of Figure 4. According to the bonding orbital theory of band structure for covalent semiconductors, energy bands are derived from the extension of molecular orbitals in unit cells. The increased Cu concentration in the Ag-Cu-I compounds system leads to a decreased band gap and causes a red-shift in the absorption peak. The AgI-CuI film had approximately 2.9 eV of band gap energy between AgI (2.8 eV) and CuI (3.0 eV).20,27 (Figure S6) The band gaps of these compounds were obtained from thin films prepared separately on glass substrates, using the same experimental conditions as for metal deposition and iodization.

diodes such as p-CuI/n-ZnO, p-NiOx/n-TiOx, and p-NiO/nZnO.28−31 The ideality factor was estimated to be η = 1.00(9) via a linear fit in a ln (J) vs. V (V) plot in the regime +0.2V ≤ V ≤ +0.5V, indicating a range between η = 1(diffusion current) and η = 2 (recombination-generation). CONCLUSIONS In summary, transparent AgI-CuI heterojunction diodes were prepared via the vapor-phase iodization method of metal thin films and exhibited high rectifying behavior. The processing sequence of thin film deposition is the most important factor in obtaining stable and patterned AgI-CuI diodes on the substrate. Stable and patterned AgI-CuI heterojunction devices were obtained exclusively with AgI as a bottom layer; the other stacked thin film structures were not successful. The crystal structure and chemical composition of the final films were identified as βʺ-(Ag0.6Cu0.4)I and γʹ-(Cu0.8Ag0.2)I, respectively. Although modified chemical compositions were observed on both AgI and CuI layers, the heterojunction structure exhibited good diode behavior with a rectifying ratio of 9.4×104. The fabrication of n-type AgI thin films are possible for electronic device applications, as we found with CuI deposited as a passivation and p-type layer. The selective deposition of CuI overlay provides a synthesis strategy to prevent the photodecomposition of AgI layers in ambient conditions under visible light. This novel patterning process, utilizing the photodecomposition of AgI, will allow the design of more complicated device structures.

ASSOCIATED CONTENT Supporting Information Figure 5. Current density-voltage (J-V) curve of the AgI-CuI heterojunction using a linear scale. The insets show the J-V curve in a logarithmic and schematic diagram of the diode architecture. Figure 5 shows the current density-voltage (J-V) curve of the AgI-CuI diode with a linear scale, which was re-plotted using a logarithmic scale in the inset curve. When a bias voltage was applied between the CuI top contact and the ITO bottom electrode, rectifying behavior characteristic of a pn-diode was observed. The turn-on voltage of the device was estimated to be approximately 1 V, which is smaller than the bandgap of AgI or CuI films obtained via optical measurements. In previous reports, the major carrier of Ag-rich compositions was electrons, and the Cu-rich phase contained holes as major carriers in the I-VII mixed semiconductors. The major carrier types of AgI and CuI on glass were n- and p-types, measured via the Hall effect measurement shown in Table S1. We obtained a carrier concentration (N) for the AgI thin films of −1.7(6)×1013 cm−3, conductivity (ρ) of 7.00(4)×10−3 Ω−1m−1, and a mobility (µ) of 5(1)×101 cm2/Vs. For the CuI thin films, we obtained N = 2.43(1)×1018 cm−3, ρ = 3.201(9)×10−1 Ω−1m−1, and µ = 8.01(2) cm2/Vs, which are comparable to previous data in Table S1. Further effects of the AgI-CuI diode include the points of vanishing current flow, which deviate from zero bias by approximately 0.2 V. The on/off ratio was achieved using the current at V= +2V divided by the absolute value of the current at V= −2V. The on/off ratio of the n-AgI/p-CuI diodes was 9.4×104, comparable to transparent heterojunction

The Supporting Information is available free of charge on the ACS Publications website. Results of photographs, SEM images, XRD patterns, UV–Vis spectra, and electrical properties of metal and metal halide films.(PDF) This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *D.-Y.J.: e-mail: [email protected]

ORCID D.-Y.J.: 0000-0003-3152-6382.

Author Contributions J.-H.C. performed the experiment and acquired the data. J.-H.C. and D.-Y.J. wrote the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by Individual Basic Science & Engineering Research Program (NRF-2017R1D1A1B03032462) from National Research Foundation of Korea (NRF).

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