Lattice Transparency of Graphene - Nano Letters (ACS Publications)

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Lattice Transparency of Graphene Sieun Chae, Seunghun Jang, Won Jin Choi, Youn Sang Kim, Hyunju Chang, Tae Il Lee, and Jeong-O Lee Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04989 • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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Lattice Transparency of Graphene Sieun Chaea,b,‡, Seunghun Jangc,‡, Won Jin Choia,, Youn Sang Kimb,e, Hyunju Changc, Tae Il Leed,* and Jeong-O Leea,* a

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, South Korea b

Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul 08826, South Korea c

Center for Molecular Modeling and Simulation, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, South Korea

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Department of BioNano Technology, Gachon University, Seongnam, Gyeonggi-Do 13120, South Korea

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Advanced Institutes of Convergence Technology, 145 Gwanggyo-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16229, South Korea

*Corresponding Author: [email protected], [email protected]

ABSTRACT: Here, we demonstrated the transparency of graphene to the atomic arrangement of a substrate surface, i.e., the “lattice transparency” of graphene, by using hydrothermally grown

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ZnO nanorods as a model system. The growth behaviors of ZnO nanocrystals on graphenecoated and uncoated substrates with various crystal structures were investigated. The atomic arrangements of the nucleating ZnO nanocrystals exhibited a close match with those of the respective substrates despite the substrates being bound to the other side of the graphene. By using first-principles calculations based on density functional theory, we confirmed the energetic favorability of the nucleating phase following the atomic arrangement of the substrate even with the graphene layer present in between. In addition to transmitting information about the atomic lattice of the substrate, graphene also protected its surface. This dual role enabled the hydrothermal growth of ZnO nanorods on a Cu substrate, which otherwise dissolved in the reaction conditions when graphene was absent.

KEYWORDS: graphene, transparency, ZnO nanorod, hydrothermal growth, lattice

Ever since its discovery, graphene has received great attention due to its distinctive features that arise from its essentially two-dimensional (2D) structure.1–4 Graphene has shown, for example, potential as a transparent and flexible electrode due to its transparency to visible light as well as its high flexibility and excellent electrical conductivity.5–7 However, unlike conventional transparent materials, the transparency of graphene does not originate from its energy band structure, but from its extreme thinness.8–9 Due to its single-atom thickness, graphene has shown more unique physical properties than have other so-called 2D materials such as silicene, phosphorene and MoS2, which are actually significantly thicker than graphene.10-12 Besides visible light, the transmission of other forms of physical quantities through atomically thin graphene has recently been studied. Rafiee et al. demonstrated graphene to be transparent to


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wetting to a significant degree, and attributed this phenomenon to the extreme thinness of graphene allowing van der Waals (vdW) interactions to take place between substrate atoms on one side of a graphene sheet and water molecules on the other side.13 The “electron transfertransparency” of graphene has also been reported, and this property accounts for redox reactions that occur at the graphene surface even though graphene itself is neither an oxidizing nor reducing agent.14 Other examples of the transparency of graphene have been found in interface engineering studies of graphene, including the engineering of its chemical reactivity carried out by deploying substrates that bind to its surface,15–17 and the delicate tailoring of the electrical properties of its devices by engineering the substrate.18–29 It is generally accepted that such substrate sensitivity of graphene devices is originated from the amount of charge impurities in the substrate that induce electron-hole puddles in the graphene.15,30,31 Basically, all of the features that occur via the transmission of the surface properties of the substrate are based upon graphene’s one atom thickness; thus these phenomena can be thought of as “tunneling of a given atomic scale electrostatic field” through graphene. Although there is a consensus that graphene is transparent to a variety of physical as well as chemical interactions, transparency of graphene has not yet been fully understood or observed on the atomic scale up to now. Here, we present another interesting transparency feature of graphene–lattice transparency–and show graphene to be able to transmit the periodic atomic potential of a substrate surface. As a model system to show the lattice transparency of graphene, we used a hydrothermal process to grow of ZnO nanocrystals on substrate-supported graphene surfaces.32 The model system introduced in this study was suitable for observing the lattice transparency on the atomic scale because the result of thermodynamically spontaneous nucleation and growth on a given substrate can indicate how strongly the monomers to be


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precipitated sense the ordered atomic potential of the substrate surface. Note that the mild temperature and pH of the ZnO growth conditions in the hydrothermal process exerted negligible chemical and mechanical effects on the graphene. The substrates on which graphene was supported were selected from evaporated metal films and metal oxides films with varying crystallinity. The nucleation and growth behaviors of the ZnO nanocrystals on the various substrates, each coated with a monolayer of graphene, were investigated using scanning electron microscopy and high-resolution transmission electron microscopy. Furthermore, in order to elucidate the effects of the underlying substrates on the nucleation and growth, we performed density functional theory (DFT) calculations. By comparing the crystal orientation of the underlying substrate with that of the nanocrystals grown on the substrate with monolayer graphene in between, it was possible to draw a conclusion that the crystal lattice of the substrates transmitted atomic arrangement through graphene.


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Figure 1. (a) Schematic of the experimental process used for the hydrothermal growth of ZnO nanorods on a substrate with and without a monolayer of graphene in between. (b,g,i,k) Schematic illustrations of various substrates with and without graphene after the ZnO nanorod


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growth: (b) polycrystalline substrates including quartz, evaporated Au film, and a sol-gelprocessed ZnO thin film, (g) thermally oxidized Si, (i) a Cu foil, and (k) an atomic layer deposition (ALD) ZnO thin film. (c,d,e,f,h,j,l) Scanning electron microscope (SEM) images of various substrates with and without graphene after the nanorod growth: (c) quartz, (d) Au annealed at 300°C, (e) as-deposited Au, (f) ZnO thin film prepared using the sol-gel method, (h) thermally oxidized Si, (j) a Cu foil, and (l) a ZnO thin film prepared by using the ALD method. The scale bars indicate 2 µm.

The graphene studied in the present work was synthesized on a copper foil by using the chemical vapor deposition method33 and was then transferred onto target substrates using the wet-transfer method. Details regarding the preparation of each target substrate are illustrated in METHODS. Raman spectroscopy was used to confirm that the deposited graphene was a monolayer and maintained a good condition without significant defects (Supporting Information Fig. S1). Then, to synthesize ZnO nanocrystals on the substrates, the fabricated substrate and the same substrate deposited with a monolayer of graphene were each submerged in an aqueous solution of 50mM Zn(NO3)6H2O and 50mM hexamethylenetetramine (HMTA) and heated up to 85°C for two hours in an oven. A simple schematic description of this process is shown in Figure 1 (a). Figures 1 (c)-(f), (h), (j), (l) show cross-sectional 45°-tilted scanning electron microscope (SEM) images of seven different kinds of underlying substrates, each with and without a monolayer graphene coating, after the ZnO nanorod growth for two hours in this condition. For crystalline SiO2 (quartz) and Au substrates, which are chemically stable in the growth solution,34,35 ZnO nanorods grew on both the bare and graphene-coated substrates with similar


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shapes and similarly high densities and shapes. Yet, it is interesting to note that regardless of whether graphene was deposited, only a few nanorods grew on each of the silicon wafers with 500 nm-thick thermal oxide, even though the substrate material is SiO2, as did the quartz substrate. The growth of the ZnO nanorods on an as-deposited Au film and an Au film annealed at 300°C exhibited distinctive behaviors as well: the nanorods grown on the as-deposited Au substrate showed random orientations and were intertwined together; while those grown on thermally annealed Au were oriented mostly perpendicular to the substrate and displayed a higher density. A bare Cu substrate dissolved as the reaction proceeded, and thus the growth of nanorods on this substrate could not be observed. A graphene-coated Cu substrate, however, did support the growth of many nanorods on its entire surface, as shown in Figure 1 (j). Graphene has been previously shown to act as an excellent protection layer for Cu and for a Cu/Ni alloy,36 where a monolayer graphene coating provided efficient chemical passivation and prevented the Cu from dissolving. The growth behaviors on two different ZnO thin films, one prepared by using atomic layer deposition (ALD) and the other using the sol-gel method, were also investigated. The Zn precursor molecules in the growth solution appeared to strongly bind to the bare ALD-prepared ZnO thin film due to the precursor and film having the same chemical functionality, but the reaction product on this surface did not appear to have the nanorod shapes observed on the other substrates. No precipitate was found when a single layer of graphene was coated on this substrate. In stark contrast to ALD-grown ZnO thin films, nanorods grew in diverse directions both on bare and graphene-coated ZnO thin films fabricated when using the sol-gel process. In the case of the bare sol-gel-prepared ZnO substrate, the nanorods exhibited heavily branched shapes, presumably because each nucleus particle contained many complex facets (Supporting Information Fig. S2h). When the reaction occurred on the bare ZnO substrate,


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the reaction was more complex; the substrate locally dissolved and new molecules attached on preferred planar sites. The growth features of ZnO nanorods were observed to strongly depend on the type of surface that was exposed to the growth solution. However, for each graphene-coated substrate, despite its outermost layer, i.e., the graphene, having the honeycomb lattice of carbons, the growth behavior seemed to be largely affected by the nature of the underlying substrate.


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Figure 2. Nucleation on various graphene surfaces. Schematics of the various graphene surfaces in the growth solution and the corresponding scanning electron microscope (SEM) images for (a) freestanding graphene, (b) graphene on an amorphous substrate, and (c) graphene on a polycrystalline substrate, after hydrothermal growth of ZnO nanocrystals for 30 minutes. The scale bars indicate 1 µm.


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To determine the causes of the different growth features observed in Figure 1, the initial growth stage, i.e., when a very small nucleating phase started to form, was analyzed. Here we categorized the system into three distinctive systems: intrinsic graphene, graphene on amorphous substrates, and graphene on polycrystalline substrates. Figure 2 shows schematics of these three systems and results of nanorod nucleation. Since it is difficult to perform a wet process such as hydrothermal growth on one-atom thick freestanding graphene and to find a substrate that does not interfere with graphene, we prepared a graphene substrate where a large portion of the area of the graphene was essentially suspended from the substrate.37 As shown in Figure 2 (a), after the substrate (a model system for freestanding graphene) was subjected to the growth condition for 30 min, nucleus particles were barely observed; i.e., nucleation of ZnO essentially did not occur on pristine graphene surfaces. However, when the graphene layer rested conformally on the supporting substrate, there was a clear tendency to form ZnO nuclei that were closely correlated with the crystal structure of the supporting substrate rather than of the graphene itself. Thermally oxidized Si and the asdeposited ALD-prepared ZnO substrate are composed of amorphous phase SiO2 and ZnO, respectively.38 When graphene was coated on these amorphous substrates, no nuclei formed. However, the nucleation reaction efficiently occurred on the graphene surface lying on substrates with some amount of crystalline phase. Many ZnO nuclei particles formed on the graphenecoated polycrystalline Au, ZnO and SiO2 substrates (quartz). X-ray diffraction (XRD) was used to acquire crystallographic information for each the substrates, as shown in Supporting Information Fig. S3. Nuclei tended to grow into more ordered, higher-density nanorods on the thermally annealed Au films than on the as-deposited Au films (Figure 1 (e), (f) and Figure 2


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(c)). As the size of the substrate crystalline surface increased, so apparently did the density of the nuclei, and the nuclei tended to grow vertically as the reaction proceeded. Nucleation theory in thermodynamics predicts that transformations from solutions to solids via nucleation lower the energy of the system.39 Nucleation preferentially occurs on pre-existing surfaces, especially those whose atomic structures closely match a particular plane of the nucleating phase so that the interfacial free energy can be minimized.39 Since the growth behavior of the ZnO nanocrystal in the experiment was largely affected by the crystal structure of the substrate plane underneath the graphene layer, the nucleations at the various graphene surfaces can be understood in light of the ‘transparency’ of the graphene to information about the crystal structure at the substrate plane. In order to correlate ‘transparency’ with graphene, one has to consider the effect of graphene layers, and possible transparency of other 2D materials rather than graphene. In Supporting Information Figure S4, we investigated lattice transparency of graphene by varying number of graphene layers. Since it is hard to achieve intrinsically bi- or trilayer graphene, we took the route of sequential transfer of graphene. As shown in the figure, number of ZnO nanorods dramatically diminishes with bilayer and trilayer graphene. This confirms that ‘transparency’ of graphene is indeed originated from atomic thinness of graphene, although we could not completely rule out the effects of organic residues derived from multiple transfer processes and lattice mismatches between each layers.


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Figure 3. Lattice transparency of graphene. (a) 45°-tilted SEM image of a ZnO nucleus precipitated on a monolayer-graphene-covered polycrystalline ZnO thin film/SiO2 substrate. (b) Bright-field transmission electron microscope (TEM) image of the FIB cross-section of the sample. (c)-(e) HRTEM images and the corresponding FFT images obtained at the each region labeled in (b). The scale bars indicate 2 µm. (f) The element line mapping of a region at the graphene interface and the element (C, O, Si, Zn) intensity line scan profiles.

In order to confirm the “lattice transparency” of graphene and to obtain further insights regarding the atomic interfaces in the ZnO nucleus/graphene/substrate composites, highresolution transmission electron microscopy (HRTEM) was performed. For this measurement,


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we chose a composite consisting of ZnO nucleus on graphene-covered sol-gel-processed ZnO thin films as the model system as shown in Figure 3. These films were observed to consist of polycrystalline domains with clear lattice fringes, and with each domain having dimensions of ~ 10 nm (Supporting Information Fig. S5). Figure 3(a) shows an electron microscope image of a ZnO nucleus formed on such a film. The surface of this nucleus was observed to be uneven, implying that the nucleus consisted of multiple domains. Figure 3(b) shows a cross-sectional image of the composite shown in Figure 3(a), and includes the interface between the ZnO nucleus and the graphene-covered sol-gel-processed ZnO thin film. We analyzed three different regions of the ZnO nucleus/graphene/sol-gel-processed ZnO film composite, as shown in Figs. 3c-e. As clearly shown in these figures, although the lattice orientations of the substrate ZnO domains from these three regions differed from one another, the lattice orientation of the nucleus phase perfectly matched that of the domain right below it, despite the presence of the graphene membrane in between. This result was also evident in the corresponding fast Fourier transform (FFT) patterns. The chemical compositions of the different layers were also characterized by carrying out element line mapping using electron dispersive X-ray spectroscopy (EDS) as shown in Figure 3(f). These analyses confirmed the presence of graphene at the interfaces. Taken together, matching of the lattices of two crystals across a graphene film has confirmed, and hence supported the transparency of graphene to information about the crystal structure of the substrate.


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Figure 4. (a) A comparison of the binding energies (BEs) calculated using DFT for five model substrates. Charge density difference plots and optimized structures for (b) c-otZnO/freestanding Gr, (c) c-ot-ZnO/Gr/c-znt-ZnO, (d) c-ot-ZnO/Gr/c-znt-ZnO, (e) Gr/c-znt ZnO, and (f) Gr/do-znt ZnO. The yellow and blue isosurfaces (±5.4×10-3 e/Å3) correspond to electron charge accumulation and depletion zones, respectively. In (b)-(d), to focus the contact interfaces, atomic structures were enlarged around upper layer (c-ot-ZnO). The red, white, and black balls represent O, Zn, and C atoms, respectively. Next, transparency of graphene and MoS2 has investigated using computational method. We calculated binding energies (BEs) of five types of model substrates for crystalline oxygenterminated ZnO layers (c-ot-ZnO) by using first-principles calculations based on density


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functional theory (DFT). The substrates tested were freestanding graphene (Gr), disordered zincterminated ZnO (do-znt-ZnO), crystalline znt-ZnO (c-znt-ZnO), Gr/do-znt-ZnO, and Gr/c-zntZnO, models of which are displayed in Fig. S6. The computational methods used to generate the do-znt-ZnO geometry, as a simulated amorphous ZnO structure, are provided in the Supporting Information (Fig. S7). In order to minimize the strain between ZnO and Gr, the polar ZnO (001) and (001ത) surfaces were adopted as model substrates in our calculation. We plotted the BEs (per unit area) for each model structure in Figure 4(a). The binding energy was defined as BE = Ec-otZnO/substrate

– ( Ec-ot-ZnO + Esubstrate ), where Ec-ot-ZnO/substrate is the total energy of the c-ot-ZnO

adsorbed on the particular model substrate, and Ec-ot-ZnO and Esubstrate are the total energies of the c-ot-ZnO layer and substrate, respectively. The calculated BE of c-ot-ZnO for the c-znt-ZnO substrate was greater than that for the do-znt-ZnO in both with and without the intermediate graphene layer, and that for freestanding Gr was the lowest. This BE trend for the c-znt-ZnO and do-znt-ZnO is maintained regardless of existence of graphene, which presents the lattice transparency of graphene and matched the experimentally determined number of ZnO nucleus particles formed on the graphene-coated polycrystalline substrates, as shown in Fig. 3. Yet, it is worth note that binding energies between the top ZnO and bottom ZnO are still larger without graphene intermediate layer; graphene is not 100% transparent for atomic lattice orientations. In addition, in order to more clearly understand the mechanism for the lattice transparency, we calculated the charge density differences, ∆ρ = ρc-ot-ZnO/substrate – ( ρc-ot-ZnO + ρsubstrate ), for the contacts between c-ot-ZnO and three model substrates of freestanding Gr, Gr/c-znt-ZnO, and cznt-ZnO, where ρc-ot-ZnO/substrate, ρc-ot-ZnO, and ρsubstrate are the electron charge densities of the c-otZnO/substrate, c-ot-ZnO, and substrate, respectively. In Fig. 4(b), weak charge redistribution and large equilibrium distance of the contact between c-ot-ZnO and freestanding Gr indicates weak


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vdW interaction character.40 On the other hand, the contact between c-ot-ZnO and Gr/c-znt-ZnO in Fig. 4(c) represents relatively stronger charge redistribution and shorter equilibrium distance. This implies that bottom ZnO substrate “dresses” graphene up with charges so that nucleating ZnO binds more strongly with graphene. In other words, graphene gains “ZnO-mimic” charge distribution by supported on ZnO substrate, and the interaction between nucleating ZnO and graphene would be vdW interaction reinforced with Coulomb interaction. With respect to the lattice transparency of graphene, to investigate why the calculated BE of the c-ot-ZnO layer for Gr/c-znt-ZnO was larger than that for Gr/do-znt-ZnO, we calculated the charge density differences, ∆ρ = ρGr/znt-ZnO – ( ρGr + ρznt-ZnO ), for the Gr/c-znt-ZnO and Gr/doznt-ZnO structures, where ρGr/znt-ZnO, ρGr, and ρznt-ZnO are the electron charge densities of the Gr/znt-ZnO (c-znt-ZnO or do-znt-ZnO) substrate, isolated Gr, and bare znt-ZnO layer, respectively. As shown in Figs. 4(e) and 4(f), the electrons (negative charges) were calculated to accumulate near Gr at the Gr/ZnO interface, but to be depleted at the uppermost layer of znt-ZnO in both structures. This result was consistent with the results of a previous theoretical study for a similar system.40 Meanwhile, the two charge density differences resulted in the considerably different charge distribution configurations as seen in Figs. 4(e) and 4(f). While Gr/c-znt-ZnO was designed to represent a periodic charge difference along ZnO, Gr/do-znt-ZnO with its disordered ZnO surface was modeled to show an irregular charge redistribution. Thus, an irregular electrostatic potential could be induced on the surface of Gr/do-znt-ZnO, resulting in the lower BE of c-ot-ZnO for the Gr/do-znt-ZnO substrate. Finally, to find out whether observed transparency is a unique character of graphene, we calculated binding energies of ZnO on freestanding monolayer MoS2 and on MoS2 supported on Zn-terminated ZnO. Growth (or exfoliation) of relatively large monolayer MoS2 (which is rather


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challenging at the current stage) must be preceded before investigating transparency of MoS2 by experimental techniques. As in the case of graphene, binding energy of ZnO on MoS2 supported by ZnO is larger than freestanding MoS2, one may expect some degree of transparency with MoS2 as well (Supporting Information Figure S8). However, growth of ZnO is not an ideal system to confirm transparency of MoS2 since ZnO grows well on MoS2 due to the excellent lattice matches between them.

In this work, we explored the transparency of graphene to an ordered arrangement of atoms (lattice) by using ZnO nanocrystals nucleated on various graphene-covered substrates as model systems. To conclude, for a ZnO crystal hydrothermally nucleated and grown on a graphenecoated substrate, the atomic arrangement of the nucleating phase energetically prefers to match the crystal structure at the surface of a supporting substrate on the other side of the graphene; graphene has some degree of lattice transparency. As a result, the growth feature of ZnO nanorods differed for different crystal structure at the substrate surface. This lattice transparency of graphene was determined to be due to its extreme thinness, which allowed for the transmission of the geometric pattern of atomic potentials on the surface of the underlying substrate. This unique feature of graphene may have huge potential in creating new epitaxial heterojunctions that have not been realized previously, since a graphene membrane can transmit features of the crystal growth template of a substrate while protecting the substrate surface from otherwise damaging chemical reactions.

METHODS


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Preparation of various supporting substrates. The target substrates were fabricated as follows. First, to produce the SiO2 substrates, bare 550-µm–thick Si wafers each with a 300-nmthick thermal oxide layer and 500-µm–thick x-cut polished quartz substrate were used, after being cut and washed. A 100-nm-thick Au film was deposited on the wafer by carrying out thermal evaporation. To produce the Au film annealed at 300°C, the as-deposited Au film was annealed at 300°C for one hour under 1000 sccm N2. Two types of ZnO thin films were prepared: one type by using a sol-gel solution processing method, where the wafer was spincoated with a solution of 0.001 mole of zinc oxide (Sigma Aldrich 99.9%) dissolved into 12 mL ammonium hydroxide (Sigma Aldrich 28% NH3 in H2O), followed by an annealing step at 300°C for one hour; and the other type of ZnO thin film was made by carrying out 50 cycles of atomic layer deposition (ALD) at fixed deposition temperature of 150°C using diethylzinc and H2O as the reactant and oxidant, respectively. To produce the Cu substrate that we used, asgrown graphene on a Cu substrate was treated with O2 and N2 plasma to remove graphene. Synthesis of ZnO nanorods. A mixed solution consisting of 50mM zinc nitrate hexahydrate (Sigma Aldrich, 99.0%) and 50mM hexamethylenetetramine (Sigma Aldrich, 98.0%) was prepared. Each target substrate was immersed in 20mL of this mixed solution in a glass vial. The glass vial including these contents was kept in an oven at 85 °C for two hours. The target substrate on which the ZnO nanorods were synthesized was then rinsed with deionized water and dried. Preparation of cross-sectional TEM samples and high-resolution STEM characterization. TEM cross-sectional samples were prepared by performing focused ion beam (FIB) milling (JIB4601F, JEOL). To reduce the amount of damage caused by the ion beam, a carbon film was pre-


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deposited onto the ZnO/graphene/ZnO film sample. The sample material was then Ga-FIBmilled into thin slices (80 ~ 90 nm thick) at an ion energy of 30 kV. High-resolution STEM images and were taken with a Cs-corrected STEM operated at 200 kV (JEM-ARM 200F, JEOL). Elemental analysis was carried out by using an STEM-EDS (INCA Energy TEM for JEM-ARM200F) Computational details. The atomic geometry, binding energy, and charge transfer properties of all of the model structures were calculated using the Vienna ab initio simulation package (VASP).41,42 The exchange–correlation functional was approximated using the Perdew–Burke– Ernzerhof (PBE) expression.43 In particular, the optB86b-vdW functional, implemented in VASP by Klimeš et al. to account for weak van der Waals (vdW) interactions, was used for all of the calculations.44 Electron–ion interactions were modelled using the projector-augmented wave (PAW) method.45 The electronic wave functions were expanded in a basis set of plane waves with a kinetic energy cutoff of 500 eV. Geometry relaxation step was repeated until the ionic forces were reduced to below 0.01 eV/Å. The k-space integration step was performed with finite sampling of the k-points on a 3 × 3 × 1 mesh in the Brillouin zone in order to optimize the geometry of each modelled structure. To minimize strain between the ZnO surface and the Gr, we chose polar ZnO (001) and (001 ̅) surfaces. The c-ot-ZnO and c-znt-ZnO surfaces were built by cleaving the bulk ZnO with alternating distances of R1 = 0.642 Å and R2 =2.011 Å (see Fig. S6 (a)) along the axis perpendicular to the (001) and (001 ̅) planes.46 The c-ot-ZnO and c-znt-ZnO surfaces were each atomically modelled with four ZnO layers. For both ZnO surfaces, the two bottom ZnO layers were fixed during structure optimization. In all models studied using this calculation, at least 17 Å-thick vacuum regions along the


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perpendicular direction (z) to the two-dimensional slab were included to minimize interactions between neighboring image cells. To study the influence of amorphous ZnO/Gr substrate on ZnO nanocrystal growth, we built a disorder ZnO structure for mimicking the amorous ZnO. First, we devised a simple method to generate disordered ZnO with optimized c-znt-ZnO, expeditiously. Firstly, O and Zn atoms were mutually exchanged in second ZnO layer from bottom of optimized c-znt-ZnO. And then, two ZnO upper layers including exchanged Zn and O atoms were relaxed. As a result, as shown in Fig. S7(c) we could get disordered (do)-znt ZnO surface. To check the validation of do-znt ZnO surface we calculated the radial distribution function (RDF) of Zn−O pair in the c-znt-ZnO and do-znt-ZnO. In Fig. S9, we can see that the small multiple peaks near first (~2 Å) and second (~ 3.8 Å) nearest-neighbor distance arose after disordering process. This indicates that the disordered phase of ZnO is effectively simulated the amorphous structure in local region through our proposed method.

ASSOCIATED CONTENT Supporting Information. Raman spectra of graphene on different substrates studied, SEM images of ZnO nucleus grown on different substrates without a graphene monolayer, XRD of different substrates, transparency of graphene with respect to the number of graphene layers, TEM image of the FIB cross-section of a ZnO nucleus/graphene/sol-gel ZnO/SiO2 interface, model structures of graphene interfaced with various substrates, calculation details for generating do-znt ZnO surface, a comparison of the binding energies calculated from DFT for MoS2 monolayer and MoS2 monolayer/c-znt-ZnO, comparison of radial distribution functions of Zn-O pair between c-znt-ZnO and a-znt-ZnO (PDF).


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions Sieun Chae conducted most of the experimental works and wrote the first draft of the paper, Seunghun Jang performed computational simulations under the supervision of Hyunju Chang and wrote the computational simulation part of the paper. Won Jin Choi initiated the idea and achieved early part of the data. Youn Sang Kim supervised experimental analysis. Funding acquisition has done by Jeong-O Lee, and Tae Il Lee and Jeong-O Lee supervised the experimental design, analysis and manuscript preparation. All contributors proof-read the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT This research was supported by the Creative Allied Program (CAP-12-1) through the Korea Research Council of Fundamental Science and Technology funded by the Ministry of Science, ICT and Future Planning and by the Focused Research Program funded by KRICT. Tae Il Lee acknowledges the support from the Next-generation Medical Device Development Program for the Newly Created Market of the National Research Foundation (NRF) funded by the Korean government, MSIP (NRF-2015M3D5A1065907).


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TOC Graphic

Hydrothermal growth of ZnO nanorods were performed on graphene-coated substrates with various crystal structures. The atomic arrangements of the nucleating ZnO nanocrystals prefers to match with those of the underlying substrate despite the presence of monolayer graphene in between. The “lattice transparency” is graphene’s unique feature due to its extreme thinness.


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Lattice Transparency of Graphene - Nano Letters (ACS Publications)

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