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ZnO Nanorod-Graphene Hybrid Architectures for Multifunctional Conductors Jung Min Lee, Yong Bum Pyun, Jaeseok Yi, Jae Woong Choung, and Won Il Park* DiVision of Materials Science Engineering, Hanyang UniVersity, Seoul 133-791, Korea ReceiVed: August 14, 2009; ReVised Manuscript ReceiVed: September 22, 2009
We report ZnO nanorod-graphene hybrid architectures (ZnO-G HAs) composed of regular arrays of ZnO nanorods formed on few-layer graphene films transferred to transparent and/or flexible substrates. The ZnO-G HAs exhibited a high current flow reaching ∼1.1 mA at an applied bias of 1 V and good optical transmittance in the range of 70-80%, comparable to those of a graphene layer. In addition, cathodoluminescence images and photoluminescence spectra of the ZnO-G HAs showed distinct light emission involving optical transitions in the ZnO nanorod array. Moreover, a bending test demonstrated that the ZnO-G HAs exhibit excellent mechanical flexibility and structural stability for the bending radius down to ∼4 mm. Our results suggest that the 1D-2D HAs provide unique and multiple functions as can be applicable for next-generation electronic and optoelectronic systems. Introduction
Experimental Section
As emerging electronic and optoelectronic systems require the development of new devices with multifunctionality, various research activity has been focused on the synthesis of novel materials and their integration into device platforms.1,2 One example is the use of one-dimensional (1D) forms of inorganic materials (including nanowires and carbon nanotubes) in flexible and transparent electronics.3-5 In this application, the 1D materials provide a significant enhancement in the mechanical flexibility and optical transparency, while their single crystalline structures hold promise for high device performance. In addition to 1D nanostructures, graphenes, two-dimensional (2D) layers of carbon atoms, have attracted much attention due to their excellent properties including good electrical and thermal conduction,6-8 mechanical strength and elasticity,9 and optical transparency.10,11 Recent achievement of large-area, free-standing graphenes by several routes (e.g., mechanical exploitation of highly oriented pyrolytic graphite,7a chemical routes,10,11 and chemical vapor deposition12-15) has led to the macroscopic use of flexible, conformable, and transparent electrodes. However, the applications of both materials are limited by their intrinsic properties. In this regard, the hybridization of different types of materials is crucial as it can enable versatile and tailor-made properties with performances far beyond those of the individual materials. Especially for 2D graphene, hybridization with 1D semiconductor nanostructures enables the construction of threedimensional architectures and the imposition of multifunctionalities.16 Here we demonstrate the synthesis of 1D-2D hybrid architectures (HAs) composed of regular arrays of ZnO nanorods formed on graphene layers. The 1D-2D HAs exhibited outstanding electrical conductivity, optical transparency, and mechanical flexibility, comparable to those of graphene. In addition, new optical functions inherited from the ZnO nanorods were introduced to the 1D-2D HAs, which, combined with the excellent electrical and mechanical properties, suggests a wide spectrum of applications ranging from transparent conducting electrodes to active components in wearable and flexible electronic, photonic, and photovoltaic systems.
Our approach to fabricating the ZnO nanorod-graphene HAs (ZnO-G HAs) is described in Figure 1. The key steps in the overall fabrication process include (i) large-area synthesis of graphene, (ii) transfer of graphene to an arbitrary substrate, and (iii) low-temperature selective growth of ZnO nanorods on the graphene. (i) Graphene synthesis (Figure 1a): A custom-built chemical vapor deposition (CVD) system was used to grow graphene films on Ni-coated SiO2/Si substrates. In our CVD system, the substrates were electrically connected to the metal electrodes and could be heated up to 1000 °C by current flow. Graphene growth began with the high-temperature thermal annealing of the Ni layer onto the SiO2/Si substrates in the CVD reactor under a H2 and Ar atmosphere at 850-1000 °C for 10 min, followed by introduction of 50 sccm methane (CH4) as a carbon source. After the system was maintained at this condition for 10 min, the samples were cooled to room temperature at a cooling rate of ∼10 °C/s by reducing the current through the sample continuously under ambient Ar. During cooling, the carbon atoms were segregated from the Ni films, resulting in the formation of a continuous form of few-layer graphene films (S1 and S2, Supporting Information).12 (ii) Graphene transfer (Figure 1b): As-grown graphene layers on the Ni-coated SiO2/Si substrates were coated with ∼200 nm thick poly(methyl methacrylate) (PMMA) layers to minimize the mechanical fracture of the graphene during the transfer.13,14 The samples were soaked in a HF solution to detach the PMMA/ graphene layeres from the substrates, followed by etching of the remaining Ni layers with an aqueous Ni etchant TFG. The freestanding PMMA/graphene films with hydrophobic surfaces were floated on the etching solution and then transferred to the various substrates, such as glass and poly(ethylene terephthalate) (PET), to grow the ZnO nanorods. (iii) Growth of the ZnO nanorod array on the graphene: After the transfer of PMMA/graphene film, the PMMA protecting layer was dissolved in acetone. Then, ZnO nanorods were grown on the graphene via a hydrothermal process in aqueous solution. Two approaches were used to fabricate an ordered array of ZnO nanorods on the graphene: (i) using holes in the graphene as a growth mask (Figure 1c) and (ii) using seed patterns on the
* To whom correspondence should be addressed. E-mail: wipark@ hanyang.ac.kr.
10.1021/jp9078713 CCC: $40.75 2009 American Chemical Society Published on Web 10/14/2009
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Figure 1. Schematic fabrication process to obtain ZnO nanorod-graphene HAs (ZnO-G HAs). (a) CVD synthesis of graphene. (b) Free standing PMMA/graphene by selective etching of the underlying SiO2 and Ni layers. (c) Growth of a ZnO nanorod array using holes in the graphene as a growth mask involves the following: (i) graphene transfer to glass substrate coated with textured ZnO seeding layer, (ii) patterning a regular array of holes in the graphene by photography and dry etching, and (iii) selective growth of ZnO nanorods via the holes in the photoresist/graphene growth mask. (d) Growth of a ZnO nanorod array using seed patterns on the graphene involves the following: (i) graphene transfer to a plastic substrate, (ii) defining an array of ZnO seed patterns, and (iii) selective growth of ZnO nanorods.
graphene (Figure 1d). In the first method, the floating graphene layer was placed on a glass or plastic substrate deposited with a textured ZnO seeding layer by metallorganic chemical vapor deposition (MOCVD) ((i) in Figure 1c). The seeding layer growth was performed at 170-180 °C using 50 sccm Ar, 20 sccm O2, and 3 sccm DEZn for 20 min, producing a ∼200 nm thick polycrystalline ZnO layer. The graphene transferred onto the ZnO layer coated glass substrate was then coated with a 1.2 µm thick photoresist (AZ1512), and a regular array of square, 5 µm holes was patterned by photolithography ((ii) in Figure 1c). By using the patterned photoresist as an etching mask, the array of holes was formed in graphene by exposure to the oxygen plasma:17 300 W (2.45 GHz, microwave) and 150 sccm O2 for 280 s. Finally, ZnO-G HA was formed by selective growth of ZnO nanorods via the holes in the photoresist/graphene growth mask in aqueous solutions containing zinc nitrate (Sigma-Aldrich) and hexamethylenetetramine (SigmaAldrich) at 80 °C for over 12 h, followed by removal of the photoresist with an acetone solution ((iii) in Figure 1c).18 In the second method, the graphene layer floating on the aqueous solution was transferred to bare substrate, such as plastic ((i) in Figure 1d). A photoresist (AZ1512) was then coated onto the sample, and a regular array of square, 5 µm holes was patterned by photolithography. A 100-200 nm layer of ZnO was deposited by MOCVD at 150 °C, followed by liftoff processes ((ii) in Figure 1d). ZnO nanorods were grown selectively with the ZnO seed patterns using the solution-based hydrothermal growth method ((iii) in Figure 1d).
Figure 2. Morphological and structural analyses of the ZnO-G HAs. (a) OM and (b) SEM images of a ZnO-G HA on glass coated with the textured ZnO layer. (c) SEM image of a ZnO-G HA grown using the ZnO epilayer as a seeding layer. (d) Representative Raman spectra of the graphene transferred to a bare glass substrate (G/glass) and the ZnO-G HA on a glass substrate coated with a ZnO layer recorded with the excitation wavelength of 514 nm.
Results and Discussion The optical microscope (OM) image of an as-fabricated ZnO-G HA on a glass substrate (Figure 2a) revealed that the regular array of ZnO crystals (dark spots) was formed over a large area (∼1 cm × 1 cm) as a continuous graphene film. The
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Figure 3. (a) I-V curves for the ZnO-G HA (red line) and ZnO layer (blue line). Inset: the change of the resistance as a function of the bending radii for the HA on PET substrate. (b) CL image of ZnO-G HA showing distinct light emission from the array of ZnO nanorods (inset). (c) PL spectrum of ZnO-G HA. (d) Optical transmittance spectra of the graphene transferred onto the ZnO coated glass substrate (G/ZnO/glass; black), the ZnO-G HA on the ZnO coated glass substrate (ZnO-G HA/ZnO/glass; blue), and the ZnO-G HA on PET substrate (ZnO-G HA/ PET; red). Strong absorptions occurred only in the ultraviolet region (λ < ∼380 nm) for the samples with ZnO layers due to the near-band-edge transition in ZnO. Inset: photograph of the transparent ZnO-G HAs on a glass substrate coated with ZnO layer. The dashed line indicates the HA area.
scanning electron microscope (SEM) image of the sample shows that the dark spots corresponded to a high density of ZnO nanorods with a mean diameter and length of ∼100 nm and ∼7 µm, respectively, grown via holes in the graphene layer (Figure 2b). Specifically, the ZnO nanorods were close-packed and oriented in the radial direction, which led to the formation of flowerlike ZnO nanorod bundles with a typical diameter of ∼15 µm.19 Since the morphologies and crystallographic orientations of ZnO nanorods are generally affected by the ZnO seeding layers, the nanorod growth was also investigated using an ZnO epilayer grown on a sapphire substrate. As shown in Figure 2c, hexagonal-faceted ZnO rods with diameters in the range of 5-10 µm grew vertically at the center of the graphene holes. Meanwhile, small-diameter ZnO nanorods grew along the edges of the ZnO rods and were radially oriented. This observation indicates that the seeding layers, as well as the sizes and shapes of the holes in the graphene, played an important role in determining the ZnO growth behaviors. Even after ZnO nanorod growth, the underlying graphene layer in the HAs was flat, and no major deformations, critical defects, or damages were formed. Only a slight contrast change was observed in the optical image (inset in Figure 2a), which was attributed to the thickness variation of the graphene layer. Raman spectroscopy was used to further investigate the structural characterizations and qualities of the graphene films in the ZnO-G HAs. Figure 2d compares the Raman spectra of the graphene transferred to the bare glass substrate (G/glass) and the ZnO-G HA fabricated on the glass substrate coated with a textured ZnO seeding layer. Both samples show distinct Raman peaks at 1580-1584 and 2711-2713 cm-1, corresponding to the G (∼1582 cm-1 in graphite) and 2D bands (∼2726 in graphite).20 The G to 2D band intensity ratio (∼1.5-2) together with the positions of the 2D band peak confirmed the formation of few-layer (3-5) graphene films,12-14,20 which
coincided with the transmission electron microscope (TEM) results (see Supporting Information, Figure S1). In addition, the relative intensity of the disorder-induced D band (∼1355 cm-1) was low and similar for both samples, indicating that no major degradation of the graphene films was induced, even after a series of fabrication processes. It was noted that the broad background signal, whose intensity gradually increased with increasing Raman shift, was observed for the HA, which may be from the textured ZnO seeding layer or the ZnO nanorods. Due to the high quality of the graphene layers in the ZnO-G HAs as confirmed by SEM and Raman measurements, it was expected that the HAs possessed the extraordinary characteristics inherent from the graphenes. First, the high current capability of the graphene in the ZnO-G HA was confirmed by twoterminal current-voltage (I-V) measurements. To measure the I-V characteristics, silver electrodes were deposited on both ends of the large-area (1 cm × 1 cm) graphene layer. As shown in Figure 3a, the I-V curve for the ZnO-G HA exhibited a high current flow reaching ∼1.1 mA at an applied bias of 1 V, which was more than 100 times greater than that through the ZnO layer even though the graphene thickness was roughly 100 times less than that of the ZnO layer. For comparison, bare graphene films transferred to insulating substrates were also measured and were revealed to have similar ranges of resistance (R ) ∼1-5 kΩ), indicating that no major degradation of the electrical property on the graphene occurred even after the solution-based growth of ZnO nanorods. Ignoring the contact resistance, the sheet resistance (Rs) of the sample was calculated from the resistance by Rs ) F/t ) RL/W, where F, t, L, and W are the resistivity, thickness, length, and width of the graphene film, respectively. The Rs converted from the R for ZnO-G HA was ∼1,000 Ω/square. Even though our two-terminal measurement did not exclude contact resistance, this value was significantly lower than those of the assembled graphenes10,11
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Figure 4. (a) Photograph of the experimental setup for the optical diffraction of a ZnO-G HA. Up to five orders of diffraction spots are clearly observed. (b-c) Optical diffraction patterns for ZnO-G HA on plastic substrates before bending (b) and after bending down to the radius curvature of 4 mm. (d) Evolution of optical diffraction from spotty to streaky patterns with regard to the bending radii (rB): (i) rB ) ∞ (ε ) 0%), (ii) rB ) 8.7 mm (ε ) 4%), (iii) rB ) 5.3 mm (ε ) 6.5%), (iv) rB ) 4 mm (ε ) 8.9%), and (v) rB ) ∞ (ε ) 0%).
and comparable to those from the continuous graphene layers, as measured by a four-probe method (300-1000 Ω/square).13,14 Second, variation of the resistance with respect to tensile strain induced by bending was evaluated because one of the attractive features of graphene is the excellent mechanical strength and elasticity.9,13 In case of the HA on the PET substrate, failure did not occur for bending radii as low as ∼0.38 cm (corresponding to a tensile strain of 9.2%), and a stable performance with a small change in the resistance below 25% was obtained (inset in Figure 3a). The reduced resistance was fully restored to the original value after recovery to the unbent state, and the response to the alternating bending-unbending was stably sustained up to several cycles. In addition to the excellent electrical properties, the array of ZnO nanorods offers new optical functions such as light emission and optical coupling to the HAs. The luminescent characteristics of the HAs were investigated using cathodoluminescence (CL) and photoluminescence (PL) measurements. The CL image confirmed the distinct light emission from the array of ZnO nanorods (Figure 3b). In addition, as shown in Figure 3c, the PL spectrum of the HA showed an ultraviolet (UV) emission peak centered at 380 nm and a visible emission band in the range of 450-800 nm. The UV emission peak corresponds to the near-band-edge emission related to the free exciton of ZnO, whereas the broad visible band reflects deeplevel defects in ZnO.21,22 The optical transparency of the samples fabricated on transparent substrates was investigated. Figure 3d shows the optical transmittance spectra of the graphene transferred onto the ZnO coated glass substrate (G/ZnO/glass), the ZnO-G HA on the ZnO coated glass substrate (ZnO-G HA/ZnO/glass), and the ZnO-G HA on PET substrate (ZnO-G HA/PET) with the substrate absorption removed. All of the samples showed good optical transparency in the visible spectral range, with a transmittance of ∼80% for the graphene on the ZnO coated glass substrate (black), 70% for the HA on the ZnO coated glass substrate (blue),
and 75% for the HA on the bare plastic substrate (red). Light scattering by the array of radially oriented ZnO nanorods led to the decrease in transmittance for the HAs. Meanwhile, light interaction via the nanostructured ZnO can increase the coupling light into and out of the active region of optoelectronic devices.23 Therefore, with the appropriate design, the ZnO-G HAs may be effectively integrated into various optoelectronic systems as transparent conducting layers with efficient light injection/extraction and/or antireflection characteristics.23,24 Scattering of the incident light by the regular arrays of ZnO nanorods on the transparent graphene also yields optical diffraction. In this experiment, the ZnO-G HAs, consisting of ZnO nanorods with the array spacing of 70 µm, were placed perpendicular to the incident beam of a diode laser with a wavelength of 660 nm (Figure 4a). The diffracted beams formed a square array of diffraction spots with a unit length of ∼4.9 cm on a screen placed 560 cm away from the sample (Figure 4b). The corresponding diffraction angle of 0.50° agreed well with the prediction based on Bragg’s diffraction rule. The contact areas between the individual ZnO nanorods and the graphene were very small, and thus the ZnO nanorods were less affected by the stress applied in the lateral direction. Due to this unique geometry, the ordering of ZnO nanorods can be maintained with regard to the axial (tension-compression) or flexural (bending) deformation. This characteristic was evaluated by a bending test for the HA on the PET substrate. Upon bending of the substrate, individual diffraction spots expanded upward and downward, and their widths increased gradually with a reducing bending radius (Figure 4c and d). The divergence of the spots resulted from the bending-induced deformation of the sample, as well as the light refraction on the curved surfaces. Notably, even after bending to a radius curvature of ∼4 mm (corresponding to a tensile strain of 8.9%), the diffraction patterns were fully recovered to the original feature upon release of the bending force (Figure 4d). In addition, repeated bending and unbending, up to tens of cycles,
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showed a stable response, demonstrating the excellent mechanical flexibility of the 1D-2D HAs as comparable to the intrinsic properties of the 2D graphene.
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References and Notes
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JP9078713
Conclusions We fabricated 1D-2D HAs by low temperature and selective growth of a regular array of ZnO nanorods on few-layer graphene films transferred to arbitrary substrates, such as glass or plastic. The ZnO-G HAs showed good electrical conductance and good optical transparency, comparable to that of a homogeneous graphene layer, as well as ultraviolet and visible light emissions inherited from the ZnO nanorods. In addition, light scattering by the regular array of ZnO nanorods on transparent graphene yielded distinct diffraction patterns for a laser beam, indicating the long-range ordering of the ZnO nanorods. A cyclic bending test for the ZnO-G HAs on plastic substrates showed that reiterations between spotty and streaky diffraction patterns were repeated up to tens of cycles, which reflects the excellent mechanical flexibility and structural stability of the HAs. The unique electrical, optical, and mechanical properties of the 1D-2D HAs demonstrate the potential application as multifunctional conducting layers in electronic and optoelectronic systems. Acknowledgment. This research was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund; KRF-2007-331D00194) and by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. KOSEF R01-2008-000-20778-0). Supporting Information Available: Figures showing the HR-TEM images (S1) and Raman spectra (S2) of CVD-grown few-layer graphene films. This material is available free of charge via the Internet at http://pubs.acs.org.
