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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01330 • Publication Date (Web): 13 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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ACS Applied Materials & Interfaces

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 Post Office Box 107, Daejeon 305-600, Republic of Korea

*

Corresponding authors E-mail: [email protected] and [email protected]

Keywords - dimensional hybrid, ZnO nanostructure, hydrothermal growth, graphene, MoS2, flexible photodetector

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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 growing solution, which resulted in the formation of ZnO nanowires, nanostars, and nanoflowers. 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 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.

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Introduction Nanostructured solid mixtures based on 0D, 1D, and 2D materials have attracted enormous interest from two standpoints. Initially, many researchers focus on low-dimensional nanostructures such as quantum dots, nanowires, nanotubes, and nanosheets due to their anisotropic electron transport properties 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 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 2D materials, such as graphene and MoS2, exhibit exceptional electrical properties with excellent optical transparency and flexibility originated from their atomically thin crystalline structure.12-14 Even though graphene and MoS2 have innate light absorbing property over a very wide wavelength range and direct bandgap for utilizing active materials for photodetectors, however, their absorption efficiency becomes low due to their atomicallythin structure. Therefore, hybridization of graphene or MoS2 with photosensitive materials such as ZnO is essential for the application in optoelectronic devices. Unfortunately, previous studies involving synthesis of ZnO nanostructures have mainly focused on the synthesis mechanism or the structural properties of ZnO itself, and few researches have been devoted on 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 remain elusive. In this article, ZnO nanostructures with various morphologies were synthesized on graphene 3



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and MoS2 layers by pH-mediated structural tailoring combined with 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 photodetector and explained the effects of ZnO nanostructures with different electronic structures to performance of ZnO and 2D materials hybrid flexible photodetectors with improved performance.

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 (TCVD). In brief, a Cu foil was pre-annealed at 1050 °C with introducing H2 (10 sccm) for 30 min in order to form atomically-flat surface of the Cu. After the pre-annealing process, methane (CH4, 30 sccm) was then injected as a carbon feedstock with H2 gas for 3 min to synthesize graphene. The chamber was then cooled to room temperature. Synthesized graphene on Cu was transferred onto SiO2 and PET substrates using a PMMA assisted wettransfer method21. MoS2 was prepared using the thermal decomposition process of single source precursor (ammonium tetrathiomolybdate, (NH4)2MoS4) devised by Lim et. Al26. 1.25 wt% (NH4)2MoS4 in ethylene glycol was spin-coated on SiO2 and polyimide substrates at 3000 rpm for 30 sec. Afterward, the as-coated samples were annealed at 450oC with introducing H2/N2 (100 sccm) at a pressure of 1.8 Torr for 30 min to 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 4


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mM zinc nitrate and 5 mM hexamethyltetraamine (HMTA) was prepared for the formation of ZnO nanostructures by hydrothermal reaction. It is 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. Graphene and MoS2 layers transferred onto SiO2 and polyimide substrates were immersed into the solution at 80oC for 3 hours.

Characterization of the hybrid materials: The structural characterization of dimensionalhybrid nanostructure was performed by field-emission scanning electron microscopy (FESEM, S-4700, Hitachi). Resonant Raman spectroscopy (inVia Raman microscope, Renishaw) with an excitation wavelength of 514 nm was employed for identifying the number of graphene and MoS2 layer. The chemical characterization of dimensional-hybrid nanostructure was carried out using X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Scientific) The XPS spectra were recorded with a normal emission geometry using monochromatic Al Kα radiation (hυ = 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.

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Results and discussion

Figure 1. (a) The correlation between the pH value and NH4 concentration in the solution for the hydrothermal growth of structurally-tailored ZnO nanostructure. (b) A schematic diagram of structural manipulation of ZnO nanostructures via tailoring the defect-dependent seeding site and 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.

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 concentration of ammonia solution in the 6


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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 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 modulation, which affects Thiele modulus (Φ) that indicates the ratio of reaction rate on surface to diffusion rate17. When the pH is close to the neutral condition, rate-determining 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 in neutral condition (Figure 1a) also indicates the formation of ZnO from Zn ions is thermodynamically not preferred in room temperature and the reaction is relatively slower compared to the diffusion rate. In contrast, the reaction rate increases as pH increased by adding ammonia solution, hence, the reaction of Zn ions is now governed by diffusion of Zn ions to the surface (Φ > 1). Thereby, micro-size and low-density ZnO nanostructures are formed on graphene or MoS2 due to the competition between seeding sites which react with the diffused Zn ions. Redox potentials of the chemical bath after adding ammonia solution show negative values in room temperature which indicates spontaneous and fast formation rate of ZnO. Figure 1c shows a SEM image of graphene transferred onto a SiO2 (300 nm)/Si(001) substrate using a poly(methyl methacrylate) (PMMA)-assisted wet transfer method21, indicating the existence of graphene is confirmed by the formation of wrinkles of graphene. 7



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Solution-processed MoS2 layers directly formed on a SiO2/Si substrate possess atomicallyflat surface without structural imperfection, as shown in Figure 1g. This was possible because of our previous study, where the optimized condition for synthesis of MoS2 layers using the thermal decomposition process of single source precursors were successfully established26. Figure 1d-f and h-j exhibit SEM images of ZnO NWs, ZnO NSs, and ZnO NFs grown on graphene and MoS2 layers using pH-mediated structural tailoring, respectively. Well-defined NWs, NSs, and NFs on graphene and MoS2 layers are both observed. The inevitably-occurred defects in graphene and MoS2 layers during the growth or transfer process could act as principal nucleation sites for the formation of ZnO-based nanostructures.

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Figure 2. Histogram showing (a, c) the size distribution and (b, d) inter-distance 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.

Based on SEM observation, the size distribution and inter-distance 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 inter-distance between adjacent nanostructures for ZnO NWs, 9



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NSs, and NFs on graphene or MoS2 layers, indicating that the effect of nanostructure density on the photoelectrical properties of hybrid system was excluded. The density of nanostructures is much sparse than previous reports15 because intrinsic defect sites present in the 2D materials—instead of a coated seed layer—works 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 in order to avoid structural deformation induced by laser-stimulated 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 ratio of D- to G-bands (ID/IG) and 2D- to G-bands (I2D/IG) correspond to 0.21 and 2.97, respectively, indicating successful synthesis of high-crystalline 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, respectively. The difference of two Raman modes corresponds to 25.8 cm-1, which indicates the synthesis of few-layered MoS222. The crystal structure of ZnO-based nanostructures was evaluated by XRD, as seen in Figure 2g. The diffraction pattern of ZnO NWs reveals three distinct peaks at 31.8o, 34.5o, and 36.3o, corresponding to (100), (002), (101) crystal planes. The predominant peaks from ZnO NWs is (002) crystal planes owing the preferred growth induced by hexamethyltetraamine (HMTA)23. In contrast, ZnO NSs and NFs reveal equivalent intensity of (100), (002), and (101) crystal planes, which corroborates to the SEM results including the radial direction growth of ZnO-based nanostructures. 10


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Figure 3. (a)The C 1s and (b) Mo 3d core level spectra for graphene and MoS2 layers. (c) The 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.

Further characterization of the dimensional-hybrid structure was conducted by X-ray photoelectron spectroscopy (XPS). The C 1s core level spectrum obtained from graphene on SiO2/Si is exhibited in Figure 3a, in which a 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 founded. 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 formation of any Mo oxide bonding states are observed. The S 2p core level spectrum shows clear two peaks originated from the spin-orbit coupling of MoS2. The extracted atomic ratio of Mo and S is 1:2, revealing the growth of stoichiometric 11



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MoS2 layers. After hybridization with ZnO-based nanostructures using pH-manipulated basic solution, XPS results of graphene and MoS2 layers still exhibit exact same signals, which mean that high-quality graphene and MoS2 layers is well-preserved without 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 lower binding energy compared to 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 wurzite structure (EB = 530.18 eV)24. As the pH value is increased, oxygen vacancies-related peaks are increased compared to that of the neutral condition, which presumably due to the fast growing 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 conductivity25. 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 micro-liters of the solution were dropcasted on SiO2 substrates to examine the initial morphology and size of the nanostructure. Figure 3g displays a 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 12


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bath show the initial stage of ZnO-based nanostructures (Figure 3h,i), in which the density of nanostructure is lower than that of ZnO NWs and the similar shapes of ZnO NSs and NFs is founded. However, the size of the initial nanostructure becomes enlarged when the pH value is increased, owing to higher reaction time.

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Figure 4. (a) Schematic illustration of flexible UV photodetector based on the dimensionalhybrid 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 of wavelength from 250 nm to 800 nm. Suggested band diagrams for (e) ZnO NWs, NSs, and NFs/graphene and (f) ZnO NWs, NSs, and NFs/MoS2 hybrid structures.

Table 1. Comparison of photodetector parameters of various ZnO based UV photodetectors

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The photoelectrical properties of the dimensional-hybrid materials were explored, as seen in Figure 4. We fabricated the dimensional-hybrid photodetectors consisting ZnO NWs, NSs, and NFs on graphene or MoS2, as depicted in Figure 4a. 100-nm-thick Au was employed as electrodes for the devices and the channel length and width were 50 and 2000 µm, respectively. Figure 4b and 4c reveal current responses of geometrically-tailored ZnO-based nanostructures hybridized with graphene and MoS2 in dark and illuminated states and their photocurrent. Current-voltage analysis of the hybrid films in dark and illuminated states (350nm UV light) is included in supporting information (Figure S1). Summarized dark- and bright-current at 1V bias is plotted for NWs, NSs, and NFs on graphene or MoS2-based UV photodetectors. Their photocurrents (blue) are extracted by subtracting dark-current (black) from bright-current (red) as shown in Figure 4b, c. Dark-current of the hybrid materials was lower compared to that of pristine 2D materials because channel materials are now hybridized with n-type ZnO nanostructures with 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 spectral photoresponse of ZnO NWs, NSs, and NFs under different illumination wavelength (250–800 nm). For ZnO NFs with high density of oxygen vacancies, an abrupt increase in the photocurrent is observed at wavelength below 500 nm compared to those of 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 bandgap narrowing of ZnO nanostructures is caused by increase of oxygen vacancy concentration27-29. 15



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For an in-depth understanding of the photo-excited electrons transport mechanism based on the results, the energy band diagrams for ZnO/graphene and ZnO/MoS2 system are depicted in Figure 4e, f, respectively. As density of oxygen vacancies in ZnO nanostructures was increased, the band gap of the structure becomes narrower due to formation of vacancy defect states which increases amount of free electron concentration under UV light illumination.27-29 It is well-known that graphene is p-type conductor due to existence of water molecules, while MoS2 is a n-type semiconductor because of presence of sulfur vacancies. When the Fermi levels in graphene and MoS2 are upshifted by injecting photo-excited electrons, the carrier concentration of graphene decreases because it is initially p-type, whereas that of MoS2 increases significantly due to its n-type characteristic. Photoresposivity ( ) which is defined as the ratio of the photocurrent with illuminating power can be calculated with the equation below;

=

where is illumination power density, is 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 A W-1, 3.02 × 102 A W-1, 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 A W-1, 1.02 × 10-4 A W-1, 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 photo-induced charge carriers per the number of photons incident to the device, by the equation below:

EQE =

/ℎ

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where q is an elementary charge, is the frequency if 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.

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Figure 5. (a) 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 bending process of UV photodetectors based on dimensional-hybrid structures (bending radius = 6 mm).

In order to ascertain the possibilities of flexible UV photodetectors based on the dimensional-hybrid structures, the photoelectrical properties of geometrically-tailored ZnObased nanostructures hybridized with 2D materials were explored. Notably, the ZnO-based nanostructures with 2D materials reveal excellent transparency and abrupt UV absorbing property. In contrast, graphene and MoS2 layers contribute as the basal/channel layer with superb flexibility and transparency. A photograph of ZnO NFs on MoS2 hybrid 18


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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 wavelength of 256nm, the photocurrent of graphene-based hybrid nanostructures decreases abruptly, whereas that of MoS2-based hybrid nanostructures increases significantly. These results can be explained by the difference of electronic structure of graphene and MoS2 (Figure 4e, f). As shown in the band diagrams of the hybrid materials, photo-excited 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 to 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 increase in the density of the oxygen vacancies in ZnO nanostructures induced 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. Estimated rise time and decay time are 10 s, and 67 s for ZnO NFs on graphene, 61 s, 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 due to the increase in drift velocity of photo-induced electrons (not shown here). The photocurrents for ZnO NFs on graphene and MoS2 layers decreased to 74 and 76% from initial photoresponses, respectively, after 104 cycles of 6 mm radius bending (Figure 5d).

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Conclusion In summary, 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 ZnO-based 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 dimensionalhybrid structures will be demand for flexible optoelectronics with high mechanical durability as well as good photoelectrical performance.

Acknowledgement This research was supported by a grant (Grant No. 2011-0031636) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, Information & Communication Technology (ICT) and Future Planning and Nano/Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF2016M3A7B4900119).

Supporting Information 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

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Dimensional-Hybrid Structures of 2D Materials with ZnO

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