Dimensional-Hybrid Structures of 2D Materials with ZnO

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Dimensional-Hybrid Structures of 2D Materials with ZnO Nanostructures via pH-Mediated Hydrothermal Growth for Flexible UV Photodetectors Young Bum Lee, Seong Ku Kim, Yi Rang Lim, In Su Jeon, Wooseok Song,* Sung Myung, Sun Sook Lee, Jongsun Lim, and Ki-Seok An* Thin Film Materials Research Center, Korea Research Institute of Chemical Technology, Yuseong, P.O. Box 107, Daejeon 305-600, Republic of Korea S Supporting Information *

ABSTRACT: Complementary combination of heterostructures is a crucial factor for the development of 2D materials-based optoelectronic devices. Herein, an appropriate solution for fabricating complementary dimensional-hybrid nanostructures comprising structurally tailored ZnO nanostructures and 2D materials such as graphene and MoS2 is suggested. Structural features of ZnO nanostructures hydrothermally grown on graphene and MoS2 are deliberately manipulated by adjusting the pH value of the growing solution, which will result in the formation of ZnO nanowires, nanostars, and nanoflowers. The detailed growth mechanism is further explored for the structurally tailored ZnO nanostructures on the 2D materials. Furthermore, a UV photodetector based on the dimensional-hybrid nanostructures is fabricated, which demonstrates their excellent photocurrent and mechanical durability. This can be understood by the existence of oxygen vacancies and oxygen-vacancies-induced band narrowing in the ZnO nanostructures, which is a decisive factor for determining their photoelectrical properties in the hybrid system. KEYWORDS: dimensional hybrid, ZnO nanostructure, hydrothermal growth, graphene, MoS2, flexible photodetector

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structure.12−14 Even though graphene and MoS2 have an innate light-absorbing property over a very wide wavelength range and direct band gap for utilizing active materials for photodetectors, their absorption efficiency becomes low because of their atomically thin structure. Therefore, the hybridization of graphene or MoS2 with photosensitive materials such as ZnO is essential for their application in optoelectronic devices. Unfortunately, previous studies involving the synthesis of ZnO nanostructures have mainly focused on the synthesis mechanism or the structural properties of ZnO itself, and few studies have been devoted on the electrical, doping, or optical properties of ZnO hybridized with other 2D materials.15−20 The effect of hybridization of geometrically tailored ZnO nanostructures and 2D materials still remains elusive. In this article, ZnO nanostructures with various morphologies were synthesized on graphene and MoS2 layers by pH-

INTRODUCTION

Nanostructured solid mixtures based on 0D, 1D, and 2D materials have attracted enormous interest from two standpoints. Initially, many researchers have focused on lowdimensional nanostructures such as quantum dots, nanowires, nanotubes, and nanosheets because of their anisotropic electron transport properties that originated from their geometrical features.1−4 Importantly, a prerequisite for practical applications in next-generation electronics based on nanostructures is reliable control of the structural parameters because the diversity in electronic properties can be determined by the structural parameters of the nanostructures.5−7 Moreover, dimensional-hybrid nanostructures for complementary in terms of their properties or improved device performance have emerged as a promising candidate for multifaceted applications such as transparent electrodes, gas sensors, field effect transistors, and photodetectors.8−11 Two-dimensional materials, such as graphene and MoS2, exhibit exceptional electrical properties with excellent optical transparency and flexibility that originate from their atomically thin crystalline © 2017 American Chemical Society

Received: January 25, 2017 Accepted: April 13, 2017 Published: April 13, 2017 15031

DOI: 10.1021/acsami.7b01330 ACS Appl. Mater. Interfaces 2017, 9, 15031−15037

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ACS Applied Materials & Interfaces

Figure 1. (a) Correlation between the pH values and NH4 concentration in the solution for the hydrothermal growth of structurally tailored ZnO nanostructures. (b) Schematic diagram of the structural manipulation of ZnO nanostructures via tailoring the defect-dependent seeding site and the pH-mediated Zn2+ ion supplement. Representative SEM images of (c) graphene, (d) ZnO NWs, (e) NSs, (f) NFs on graphene, (g) MoS2, (h) ZnO NWs, (i) NSs, and (j) NFs on MoS2 onto SiO2 (300 nm)/Si(001) substrates. Graphene and the MoS2 layers transferred onto SiO2 and polyimide substrates were immersed into the solution at 80 °C for 3 h. Characterization of the Hybrid Materials. The structural characterization of the dimensional-hybrid nanostructure was performed using field-emission scanning electron microscopy (FESEM, S-4700, Hitachi). Resonant Raman spectroscopy (inVia Raman microscope, Renishaw) with an excitation wavelength of 514 nm was used for identifying the number of graphene and MoS2 layers. The chemical characterization of the dimensional-hybrid nanostructure was carried out using X-ray photoelectron spectroscopy (XPS, KAlpha, Thermo Scientific). The XPS spectra were recorded with a normal emission geometry using monochromatic Al Kα radiation (hν = 1486.6 eV) in an ultrahigh vacuum system (base pressure: ∼10−9 Torr). The pass energy was 20.0 eV, and the extracted spectra were deconvoluted by a standard nonlinear least-squares fitting procedure using Voigt functions.

mediated structural tailoring combined with a solvothermal reaction. We explored a detailed synthesis mechanism for structurally tailored ZnO nanostructures on graphene or MoS2 layers and how the process affects the amount of vacancies in ZnO nanostructures. Also, we applied these hybrid structures to a photodetector and explained the effects of ZnO nanostructures with different electronic structures on the performance of ZnO and 2D materials hybrid flexible photodetectors with improved performance.

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EXPERIMENTAL SECTION

Synthesis and Fabrication of 2D Materials. Graphene was synthesized on a 25 μm-thick Cu foil (Alfa Aesar, 99.8% purity) using conventional thermal chemical vapor deposition. In brief, a Cu foil was preannealed at 1050 °C while introducing H2 (10 sccm) for 30 min to form an atomically flat Cu surface. After the preannealing process, methane (CH4, 30 sccm) was injected as a carbon feedstock with H2 gas for 3 min to synthesize graphene. The chamber was then cooled to room temperature. The synthesized graphene on Cu was transferred onto SiO2 and PET substrates using a PMMA-assisted wet-transfer method.21 MoS2 was prepared using the thermal decomposition process of single-source precursor [ammonium tetrathiomolybdate, (NH4)2MoS4] devised by Lim et al.26 (NH4)2MoS4 (1.25 wt %) in ethylene glycol was spin-coated on SiO2 and polyimide substrates at 3000 rpm for 30 s. Afterward, the as-coated samples were annealed at 450 °C while introducing H2/N2 (100 sccm) at a pressure of 1.8 Torr for 30 min to the synthesized MoS2 layers. Synthesis of ZnO Nanostructures on 2D Materials. As-grown graphene and MoS2 layers were hybridized with structurally tailored ZnO nanostructures as follows: a solution containing 20 mM zinc nitrate and 5 mM hexamethyltetraamine (HMTA) was prepared for the formation of ZnO nanostructures by hydrothermal reaction. It should be noted that the structural manipulation of ZnO nanostructures was conducted by adding 0, 1, and 2 mM ammonia in the solution corresponding to NWs, NSs, and NFs, respectively.

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RESULTS AND DISCUSSION In a hydrothermal growth process for the synthesis of ZnO nanostructures, the pH value of the chemical bath solution is a crucial factor for manipulating the Zn ion in the solution. It should be noted that we carefully established the optimal conditions for controlling the morphologies of hydrothermally grown ZnO crystal structures on graphene or MoS2. Figure 1a shows the correlation between the pH values and the concentration of ammonia solution in the chemical bath solution for the hydrothermal growth of ZnO nanostructures, indicating a logarithmic relationship between the pH values and ammonia concentration. We precisely established the pH values required for the formation of structurally tailored ZnO nanostructures on graphene and MoS2 layers, as depicted in Figure 1b. The optimized pH values for the growth of ZnO nanowires (NWs), nanostars (NSs), and nanoflowers (NFs) correspond to 6.53, 8.18, and 9.18, respectively. The morphology manipulation of ZnO-based nanostructures can be understood by the pH-mediated reaction rate 15032


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Figure 2. Histogram showing (a,c) the size distribution and (b,d) the interdistance between adjacent nanostructures for ZnO NWs, NSs, and NFs on graphene and MoS2 layers. Raman spectra recorded at 514 nm excitation wavelength of (e) pristine graphene and (f) MoS2 layers onto SiO2/Si substrates. (g) XRD patterns for MoS2 layers, ZnO NWs, NSs, and NFs on MoS2 layers.

structural tailoring, respectively. Well-defined NWs, NSs, and NFs on both graphene and MoS2 layers are observed. The inevitably occurred defects in graphene and MoS2 layers during the growth or transfer process could act as the principal nucleation sites for the formation of ZnO-based nanostructures. On the basis of SEM observation, the size distribution and interdistance between adjacent nanostructures for ZnO NWs, NSs, and NFs on graphene and MoS2 layers are estimated. With increasing pH (NWs < NSs < NFs), the size of structurally tailored ZnO nanostructures on graphene and MoS2 layers becomes larger, as displayed in Figure 2a,c. The size distribution of the nanostructures on MoS2 layers is much narrower than that on graphene. Figure 2b,d exhibit the similar interdistance between adjacent nanostructures for ZnO NWs, NSs, and NFs on graphene or MoS2 layers, indicating that the effect of nanostructure density on the photoelectrical properties of the hybrid system was excluded. The density of nanostructures is sparser than that published in previous reports15 because the intrinsic defect sites present in the 2D materialsinstead of a coated seed layerwork as the nucleation sites. The structural characterization of dimensional-hybrid structures was performed by Raman spectroscopy and X-ray diffraction (XRD). Resonant Raman spectroscopy provides the crystallinity and the number of layers in 2D materials, such as graphene and MoS2. Raman spectra were recorded at an excitation wavelength of 514 nm. The incident laser power irradiated on the samples was adjusted to be 3 mW/cm2 to avoid any structural deformation induced by laserstimulated heating. Figure 2e displays the Raman spectrum for graphene transferred onto a SiO2/Si substrate, which exhibits the graphene fingerprints including the D-, G-, and 2D-bands unambiguously. The intensity ratios of D- to G-bands (ID/IG) and 2D- to G-bands (I2D/IG) correspond to 0.21 and 2.97,

modulation, which affects Thiele modulus (Φ), which indicates the ratio of the reaction rate on the surface to the diffusion rate.17 When the pH is close to the neutral condition, the ratedetermining step is the growing reaction (Φ < 1) that is governed by the reaction rate of surface consumption, resulting in the equal supplement of Zn ions and the formation of seeding sites for the growth of thin and high-density ZnO NWs. Positive redox potential under the neutral condition (Figure 1a) also indicates that the formation of ZnO from Zn ions is thermodynamically not preferred at room temperature and the reaction is relatively slower compared with the diffusion rate. In contrast, the reaction rate increases as pH is increased by adding ammonia solution; hence, the reaction of Zn ions is now governed by the diffusion of Zn ions to the surface (Φ > 1). Therefore, micro-sized and low-density ZnO nanostructures are formed on graphene or MoS2 because of the competition between seeding sites, which react with the diffused Zn ions. Redox potentials of the chemical bath after adding the ammonia solution show negative values at room temperature, which indicates a spontaneous and fast formation rate of ZnO. Figure 1c shows an SEM image of graphene transferred onto a SiO2 (300 nm)/Si(001) substrate using a poly(methyl methacrylate) (PMMA)-assisted wet-transfer method,21 indicating that the existence of graphene is confirmed by the formation of wrinkles of graphene. Solution-processed MoS2 layers directly formed on a SiO2/Si substrate possess an atomically flat surface without structural imperfection, as shown in Figure 1g. This was possible because of our previous study, where the optimized condition for the synthesis of MoS2 layers using the thermal decomposition process of single-source precursors was successfully established.26 Figure 1d−f,h−j exhibit SEM images of ZnO NWs, ZnO NSs, and ZnO NFs grown on graphene and MoS2 layers using pH-mediated 15033


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Figure 3. (a) C 1s and (b) Mo 3d core-level spectra for graphene and MoS2 layers. (c) Zn 2p and (d−f) O 1s core-level spectra of ZnO NWs, NSs, and NFs. Representative SEM images of the initial stage for (g) ZnO NWs, (h) NWs, and (i) NFs on SiO2/Si.

Figure 4. (a) Schematic illustration of a flexible UV photodetector based on the dimensional-hybrid nanostructures. Dark current (black), on current (red), and their photocurrents (blue) of the ZnO nanostructures hybridized with (b) graphene and (c) MoS2 exposed to UV light. (d) Spectral photoresponse of the ZnO NWs, NSs, and NFs exposed to light with a wavelength of 250 to 800 nm. Suggested band diagrams for (e) ZnO NWs, NSs, and NFs/graphene and (f) ZnO NWs, NSs, and NFs/MoS2 hybrid structures.

Further characterization of the dimensional-hybrid structure was conducted by XPS. The C 1s core-level spectrum obtained from graphene on SiO2/Si is exhibited in Figure 3a, in which predominant sp2 C−C, C−O, and CO bonding states were assigned. This result indicates that PMMA residue-free graphene with a small amount of oxygen functional group is found. The Mo 3d core-level spectrum acquired from MoS2 layers on SiO2/Si is shown in Figure 3b. The Mo 3d5/2, 3d3/2, and S 2s peaks corresponding to MoS2 without the formation of any Mo oxide bonding states are observed. The S 2p corelevel spectrum shows two clear peaks originating from the spin−orbit coupling of MoS2. The extracted atomic ratio of Mo and S is 1:2, revealing the growth of stoichiometric MoS2 layers. After hybridization with ZnO-based nanostructures using a pHmanipulated basic solution, XPS results of graphene and MoS2 layers still exhibit exact same signals, which means that highquality graphene and MoS2 layers are well-preserved without

respectively, indicating the successful synthesis of highcrystalline monolayer graphene. For MoS2, two typical Raman active modes including the A1g (out-of-plane vibration) and the E2g (in-plane vibration) modes are observed at 381.9 and 407.7 cm−1, as shown in Figure 2f. The difference between two Raman modes corresponds to 25.8 cm−1, which indicates the synthesis of few-layered MoS2.22 The crystal structure of ZnObased nanostructures was evaluated by XRD, as seen in Figure 2g. The diffraction pattern of ZnO NWs reveals three distinct peaks at 31.8°, 34.5°, and 36.3°, corresponding to (100), (002), and (101) crystal planes. The predominant peaks from ZnO NWs is the (002) crystal plane owing to the preferred growth induced by HMTA.23 By contrast, ZnO NSs and NFs reveal an equivalent intensity of (100), (002), and (101) crystal planes, which corroborates to the SEM results including the radial direction growth of ZnO-based nanostructures. 15034


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ACS Applied Materials & Interfaces Table 1. Comparison of Photodetector Parameters of Various ZnO-Based UV Photodetectors photodetector

λ (nm)

P0 (mW/cm2)

bias (V)

ZnO NWs/graphene foam ZnO NRs/graphene Co−ZnO NRs MoS2 ZnO NFs/graphene ZnO NFs/MoS2

365 365 365 254 350 350

1.3 1 1.3 12.5 1.2 1.2

5 1 5 5 1 1

Rs (A/W)

EQE (%)

reference

× × × × × ×

2.1 × 103 108 2.6 × 105 1.73 × 10−3 1.24 × 105 3.19 × 10−1

16 18 20 25 this work this work

6.1 3.0 7.7 2.4 3.50 8.99

100 105 102 10−6 102 10−4

(red), as shown in Figure 4b,c. The dark current of the hybrid materials was lower compared with that of pristine 2D materials because channel materials are now hybridized with n-type ZnO nanostructures with a lower work function. Interestingly, when UV light is illuminated on the hybrid materials, the current of ZnO/graphene hybrids decreases whereas that of ZnO/MoS2 hybrids increases. The detailed mechanism will be discussed later. Figure 4d shows the spectral photoresponse of ZnO NWs, NSs, and NFs under different illumination wavelengths (250− 800 nm). For ZnO NFs with a high density of oxygen vacancies, an abrupt increase in the photocurrent is observed at a wavelength below 500 nm compared with other hybrids. The photocurrent of ZnO NSs increases gradually below 400 nm and that of ZnO NWs exhibits relatively low photoresponses. These trends can be understood by the fact that the band gap narrowing of ZnO nanostructures is caused by an increase in the oxygen vacancy concentration.27−29 For an in-depth understanding of the photoexcited electron transport mechanism based on the results, the energy band diagrams for ZnO/graphene and ZnO/MoS2 systems are depicted in Figure 4e,f, respectively. As the density of oxygen vacancies in ZnO nanostructures was increased, the band gap of the structure became narrower because of the formation of vacancy defect states, which increases the amount of free electron concentration under UV light illumination.27−29 It is well known that graphene is a p-type conductor because of the existence of water molecules, whereas MoS2 is an n-type semiconductor because of the presence of sulfur vacancies. When the Fermi levels in graphene and MoS2 are upshifted by injecting photoexcited electrons, the carrier concentration of graphene decreases because it is initially p-type, whereas that of MoS2 increases significantly because of its n-type characteristic. Photoresponsivity (Rs), which is defined as the ratio of the photocurrent and the illuminating power, can be calculated with the equation

any structural deformation. After the formation of 2D materials hybridized with ZnO-based nanostructures, the Zn 2p and O 1s core-level spectra show two interesting features (Figure 3c). The Zn 2p bonding states of ZnO NSs and NFs are shifted to a lower binding energy compared with that of ZnO NWs. This discrepancy in terms of their binding energy could be understood by the alteration in their electronic structure. The O 1s core-level spectra (Figure 3d−f) after the formation of dimensional-hybrid nanostructures were deconvoluted into three components: loosely bound oxygen (CO3, O2, or H2O) on the ZnO surface (EB = 533.38 eV), O2− ions in the vicinity of oxygen vacancies (EB = 531.88 eV), and O2− ions surrounded by Zn atoms in the hexagonal wurtzite structure (EB = 530.18 eV).24 As the pH value is increased, oxygen vacancies-related peaks are also increased compared with that under neutral condition, which is presumably due to the fastgrowing rate of ZnO. Typically, ZnO exhibits n-type conductivity, which can be explained by the presence of oxygen vacancies, common native point defects, as a source of conductivity.25 Along these lines, we anticipate that various electrical properties would be induced by pH-mediated structural tailoring, which can differently interact with graphene or MoS2 layers. To elucidate the formation mechanism of ZnO NWs, NSs, and NFs, we aliquot the chemical bath solution to study the initial growing step. As soon as the chemical baths were heated to 80 °C, a few microliters of the solution were drop-cast on SiO2 substrates to examine the initial morphology and size of the nanostructure. Figure 3g displays an SEM image of the initial stage for ZnO NWs. As we mentioned earlier, HMTA allows the preferential growth of ZnO NWs along the (002) plane, resulting in the existence of honeycomb-shaped ZnO NWs. Conversely, aliquots of the ZnO NSs and NFs bath show the initial stage of ZnO-based nanostructures (Figure 3h,i), in which the density of the nanostructure is lower than that of ZnO NWs and similar shapes of ZnO NSs and NFs are found. However, the size of the initial nanostructure becomes enlarged when the pH value is increased, owing to the higher reaction time. The photoelectrical properties of the dimensional-hybrid materials were explored, as seen in Figure 4. We fabricated the dimensional-hybrid photodetectors consisting of ZnO NWs, NSs, and NFs on graphene or MoS2, as depicted in Figure 4a. We used 100 nm thick Au as electrodes for the devices, and the channel length and width were 50 and 2000 μm, respectively. Figure 4b,c reveal current responses of geometrically tailored ZnO-based nanostructures hybridized with graphene and MoS2 in dark and illuminated states and their photocurrent. The current−voltage analysis of the hybrid films in dark and illuminated states (350 nm UV light) is included in Figure S1. Summarized dark and bright current at 1 V bias is plotted for NWs, NSs, and NFs on graphene- or MoS2-based UV photodetectors. Their photocurrents (blue) are extracted by subtracting the dark current (black) from the bright current

Rs =

Iph P0A

where P0 is the illumination power density, Iph is the photocurrent, and A is the effective device area.16 The calculated values of photoresponsivity for ZnO NWs, NSs, and NFs with graphene were 1.45 × 102, 3.02 × 102, and 3.50 × 102 A W−1, respectively. For ZnO NWs, NSs, and NFs hybridized with MoS2 photodetectors, photoresponsivity values were 7.91 × 10−6, 1.02 × 10−4, and 8.99 × 10−4 A W−1, respectively. In addition, external quantum efficiency (EQE) for each structure was also extracted, which is defined as the number of photoinduced charge carriers per the number of photons incident to the device, calculated by the equation 15035


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Figure 5. (a) Photograph of ZnO NFs on MoS2 hybrid photodetectors fabricated onto a PI substrate. Representative time-dependent photocurrents for ZnO NWs, NSs, and NFs on (b) graphene and (c) MoS2 layers recorded using UV lamp at Vsd = 1 V. (d) Photocurrent variation of ZnO NFs on graphene (red) and MoS2 (blue) layers as a function of the bending cycles. Insets showing the bending process of UV photodetectors based on dimensional-hybrid structures (bending radius = 6 mm).

EQE =

Rs q/hν

by pH-mediated structural tailoring. The response time and recovery time are estimated by measuring the time taken to reach 90% from 10% and 10% from 90% of the maximum photocurrent, respectively. The estimated rise time and decay time are 10 and 67 s for ZnO NFs on graphene, 61 and 90 s for ZnO NFs on MoS2, respectively. This discrepancy in terms of their response time can be explained by the fact that the mobility of graphene is greater than that of MoS2. Additionally, the photocurrents of the dimensional-hybrid materials are proportional to the bias voltage because of the increase in the drift velocity of photoinduced electrons (not shown here). The photocurrents for ZnO NFs on graphene and MoS2 layers decreased to 74 and 76% from the initial photoresponses, respectively, after 104 cycles of 6 mm radius bending (Figure 5d).

where q is an elementary charge, ν is the frequency of incident photon, and h is Planck’s constant.16 EQE exhibited by each hybrid structures were 5.14 × 104, 1.07 × 105, and 1.24 × 105% for ZnO NWs, NSs, and NFs/graphene, and 2.81 × 10−3, 3.63 × 10−2, and 3.19 × 10−1% for ZnO NWs, NSs, and NFs/MoS2, respectively. These values are summarized and compared with previous results in Table 1. To ascertain the possibilities of flexible UV photodetectors based on the dimensional-hybrid structures, the photoelectrical properties of geometrically tailored ZnO-based nanostructures hybridized with 2D materials were explored. Notably, the ZnObased nanostructures with 2D materials reveal excellent transparency and abrupt UV-absorbing property. By contrast, graphene and MoS2 layers contribute as the basal/channel layer with superb flexibility and transparency. A photograph of ZnO NFs on MoS2 hybrid photodetectors fabricated onto a polyimide (PI) substrate is shown in Figure 5a. Remarkably, time-dependent photocurrents of the ZnO nanostructures on graphene or MoS2 reveal interesting phenomena, as seen in Figure 5b,c. First, under UV illumination with a wavelength of 256 nm, the current of graphene-based hybrid nanostructures decreases abruptly, whereas that of MoS2-based hybrid nanostructures increases significantly. These results can be explained by the difference between the electronic structure of graphene and MoS2 (Figure 4e,f). As shown in the band diagrams of the hybrid materials, photoexcited electrons generated from the ZnO nanostructures are transferred to the graphene or MoS2 layers under UV illumination. Significant photocurrents for graphene-based and MoS2-based hybrid nanostructures with enhanced switching behavior compared with a previous result26 under periodic UV illumination are observed unambiguously. With increasing pH (NWs < NSs < NFs), the photocurrent of the device is enhanced regardless of 2D materials, which can be understood by the increase in the density of the oxygen vacancies in ZnO nanostructures induced

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CONCLUSIONS We reported a facile methodology for the synthesis of dimensional-hybrid nanostructures including ZnO with various morphologies and graphene or MoS2 layers via pH-mediated structural tailoring. High-crystalline ZnO NWs, NSs, and NFs were formed on monolayer graphene and multilayer MoS2 by the pH modulation of hydrothermal reaction for determining the reaction rate. The amount of oxygen vacancies of ZnObased nanostructures on 2D materials can be modulated by their structure, which can affect the photoelectrical property of the dimensional-hybrid materials. We anticipate that complementary dimensional-hybrid structures with high mechanical durability and good photoelectrical performance will be in demand for flexible optoelectronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01330. Photoelectrical properties of current−voltage analysis for ZnO NWs, NSs, and NFs hybridized with graphene or MoS2 without and with UV light as Figure S1 (PDF) 15036


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.S.). *E-mail: [email protected] (K.-S.A.). ORCID

Wooseok Song: 0000-0002-0487-2055 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a grant (grant no. 20110031636) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, Information and Communication Technology and Future Planning and the Nano/Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2016M3A7B4900119), and by Korea Research Fellowship (KRF) program through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT & Future Planning (NRF2016H1D3A1938211).

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REFERENCES

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