Letter pubs.acs.org/JPCL
Direct Synthesis of Graphene Meshes and Semipermanent Electrical Doping Jaeseok Yi, Dong Hyun Lee, Won Woo Lee, and Won Il Park* Division of Materials Science & Engineering, Hanyang University, Seoul 133-791, Korea S Supporting Information *
ABSTRACT: Here we describe a new method for the direct patterned synthesis of graphene meshes on Cu foils that use self-assembled silica sphere arrays as growth masks. Structural analyses based on electron microscopy and Raman spectroscopy showed that the graphene meshes are mostly single- or double-layer necks with empty holes that have abrupt edges. On the basis of experimental observations, we proposed the model illustrating the dissociation of carbon atoms at the Cu/silica interface through catalytic hydrogenation of the graphene lattice. Moreover, our approach can minimize problems associated with the graphene etching process, including contamination and exposure to reactive plasma. This enables stable electronic doping through covalent C−N bonds at the edges of graphene meshes. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
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important challenge.20−22 Because bonding of sp2-hybridized carbon atoms in graphene is very strong, substitutional doping (i.e., replacing carbon atoms in a hexagonal lattice with atoms of another element) hardly occurs, except for chemical bonding between dopants and carbon atoms at imperfection sites, such as point defects or edges. To address the issues mentioned above, we present a new fabricating strategy that allows direct synthesis of graphene mesh structure while minimizing the problems associated with graphene patterning. The edges of graphene meshes synthesized by this approach are less exposed to contamination and reactive plasma than those produced by etching. After thermal reaction with NH3, stable N-doping was achieved through covalent C−N bonds at the edges. The electrical properties were confirmed to be more easily and permanently tuned than those of pristine graphene. The graphene meshes were synthesized by chemical vapor deposition (CVD) on a metal-catalyst layer covered with a selfassembled monolayer of silica spheres, as schematically illustrated in Figure 1. First, monodisperse silica spheres with average diameters of ∼600 nm were prepared by the Stöber method. Then, they were deposited on copper (Cu) foils in the form of hexagonally close packed colloidial monolayers by Langmuir−Blodgett assembly (Figure 1a). Samples were placed in a CVD reactor and thermally annealed at 900−1000 °C under H2 atmosphere to allow silica spheres to sink down within the Cu. After annealing for 30 min, a precursor of CH4
ince recent success in exfoliating a single-layer graphene from graphite,1 there has been tremendous research interest placed on the fundamental properties and potential applications of this emerging material and 2D layered materials.2−7 In particular, graphene, a zero-gap semiconductor with a linear energy dispersion relation near the Dirac points, provides an alternative platform to conventional heterostructure 2D electronic gas systems for studying mesoscopic electron transport. However, several challenges, associated with its one-atomthick nature as well as its intrinsic semimetallic, and zerobandgap property, still remain for its practical application in electronic devices. Graphene patterning is an essential issue because it is not only an indispensable step in device fabrication but also a feasible way to modulate the bandgap of graphene. Existing methods for patterning graphene are mainly based on lithography processes, which include photo- and electron beam lithography, block-copolymer lithography, and nanosphere lithography.8−13 These lithographical approaches enable the rational design of high-quality graphene patterns that then undergo etching. The etching process essentially involves contamination by residual polymer and a disordered arrangement of carbon (C) atoms at the etched edges, which significantly affects the transport properties of graphene device, such as lowered carrier mobility, direct point shift, and suppression of weak localization.14−17 Alternative graphene patterning methods are based on transfer printing18 or nanoscale-cutting,19 but it possesses practical limits on largescale fabrication due to lack of uniformity and reproducibility or difficulty in large-area assembly. The extrinsic chemical doping of graphene, which is additionally important in nanoelectronic processing to control electronic properties, also presents an © XXXX American Chemical Society
Received: May 4, 2013 Accepted: June 10, 2013
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Figure 1. Schematic illustrating CVD synthesis of graphene mesh. (a) Deposition of monodisperse silica spheres on Cu foil. (b) Silica sphere sinking within Cu foil. (c) CVD growth of graphene mesh consisting of hexagonal array of holes. (d) Transfer of graphene mesh to other substrate.
Figure 2. Graphene meshes with various hole diameters but with fixed interval of 600 nm. (a−c) SEM images of graphene meshes prepared under different temperatures: (a) 1000, (b) 950, and (c) 900 °C. (d) Temperature versus hole diameter for series of samples. Scale bars: 200 nm.
size of silica spheres. This allows one to increase or decrease the size of the hole and the interval between adjacent holes at the same time. The size of graphene mesh holes can also be tuned independently because the sinkage depth of spheres in Cu determines hole diameter. To address this issue, we tested the growth of graphene meshes by varying temperature in the range of 900−1000 °C. Because the degree of silica sinkage essentially depends on the softening of Cu metal, the size of each hole in the graphene meshes was strongly temperature-dependent. Figure 2d summarizes temperature versus hole diameter for a series of samples. Although hole intervals remained the same size as silica spheres (∼600 nm), average hole diameter decreased steadily from ∼290 to 260 nm and 170 nm when growth temperature decreased from 1000 to 950 °C and 900 °C, respectively. In addition, hole interval and hole sizes were reduced by introducing smaller size of silica sphere (Figure S2 in the Supporting Information). Accordingly, fine-tuning the width and shape of the neck between two neighboring holes can be achieved by choosing appropriate silica spheres and growth parameters. Accordingly, the width and shape of the
was introduced to initiate graphene growth. Because graphene growth underneath silica was suppressed, the CVD process produced graphene mesh structures consisting of a hexagonal array of holes (Figure 1b). After removing the silica spheres using a hydrogen fluoride (HF) solution, the graphene mesh was readily separated from Cu foils and transferred onto other substrates (see the Experimental Methods section for details) (Figure 1c,d). Representative SEM images of graphene meshes that were transferred to SiO2/Si substrate are shown in Figure 2a−c. Because silica spheres sank down to a certain depth into the Cu foil, the Cu surface can be divided into two domains: the hexagonal array of circular depressions underneath silica and the surrounding bare and flat Cu surface (Figure S1 in the Supporting Information). While a very thin graphene layer was formed on the bare Cu surface, no graphene or carbonaceous structures appeared on regions covered with sunken silica spheres, indicating that silica spheres suppressed graphene growth. This observation presents the possibility of controlling the size and interval of graphene mesh holes by changing the 2100
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of graphene mesh, which further confirms clean and empty holes with very abrupt edges. A fast Fourier transformation (FFT) image converted from the white dashed box in Figure 3c exhibits a single hexagonal spot pattern, while no distinct pattern was noticed from the hole regions (Figure S3 in the Supporting Information). Interestingly, FFT images from other graphene regions within Figure 3c also exhibit hexagonal patterns with identical sizes and orientation (Figure S4 in the Supporting Information). This is indicative of a single-domain crystal structure over a large area even with growth interruption caused by silica spheres. The Raman spectroscopy study was used to estimate the number of graphene layers and their atomic ordering. Two characteristic peaks, a G-peak centered at ∼1595 cm and a 2D peak centered at ∼2710 cm−1, are distinct. The intensity ratio between 2D and G peak (I2D/IG = ∼1.2) and a single Gaussian 2D peak confirm that graphene meshes consist of mostly single- or double-layered films.27,28 The D peak at ∼1350 cm−1, which is associated with a degree of disorder for the carbon film, is not distinctly recognized (ID/IG ≤ 0.10). This indicates that the graphene mesh has high structural quality as compared with normal graphene (Figure 3d), which is in agreement with TEM results. One can consider that silica spheres possibly block the supply of hydrocarbon source gas into silica sphere and Cu interfacial regions during graphene mesh formation. An insufficient supply of source gas consequently restricts the gas-phase decomposition and surface adsorption reaction for graphene formation. However, a previous study of graphene growth on Cu thin films and Si/SiO2 substrate showed that defects such as grain boundaries can provide a preferential diffusion path for carbon species to the interface between Cu and the underlying SiO2 layer.5 Accordingly, the above explanation illustrating pattern formation by a nonuniform supply of carbon species is not sufficient to apply to this phenomenon. Alternatively, we postulate that carbon atoms were dissociated from the graphene lattice at the Cu/silica interface. Thus, continuous graphene growth is hindered even with the presence of carbon species. Site-specific etching of graphene sheets in the presence of hydrogen and metal or SiOx nanoparticles has been reported in which nanoparticles catalyzed the dissociation of carbon atoms.29−32 To explore whether a similar mechanism could be applied in the current case, a thermal annealing test was performed for the graphene layer in the presence of silica spheres (Figure 4b). A silica sphere array was first deposited on the CVD-grown graphene layer that was transferred onto Si/SiO2 substrate and then thermally annealed under hydrogen atmosphere. The SEM image taken after removing the silica sphere (Figure S5 in the
neck between two neighboring holes can also be tuned by choosing appropriate silica spheres and growth parameters. The topographical AFM image in Figure 3a shows abrupt height differences at the hole edges of graphene mesh. From
Figure 3. (a) Typical noncontact-mode AFM image of graphene mesh deposited on Si/SiO2 substrate. Scale bar: 100 nm. (b) Height profile along line shown in inset is a closeup of the blue dashed square in panel a. (c) Typical TEM image of graphene mesh. Inset: FFT image converted from white dashed square. Scale bar: 200 nm. (d) Typical Raman spectrum of graphene (black) and graphene mesh (red).
the AFM line scan across hole edges (Figure 3b), graphene sheet thickness was measured to be in the range of ∼1 to 1.5 (∼1.33) nm. This measured value is relatively high considering the thickness of graphene (0.335 nm) and because the CVD grown graphene sheets were mostly single or double layers. The thicknesses of graphene sheets are generally overestimated by AFM measurement23−25 due to a layer of absorbed water or solvent on the graphene surface or between the graphene and substrate. Overestimations can also be caused by instrumental offsets. The normal offset of ∼0.5 nm, which is larger than the thickness of a single graphene layer, occurs due to different interaction forces.26 Further structural investigations of the graphene meshes were performed by transmission electron microscopy (TEM) and Raman spectroscopy. Figure 3c shows a typical TEM image
Figure 4. (a) Schematic of a possible mechanism for hole formation in graphene mesh. (b) SEM image of amorphous carbonaceous layers around silica spheres after thermally annealed under hydrogen atmosphere. Scale bar: 500 nm. (c) SEM image of striped patterned graphene, with the inset showing the enlarged SEM image. Scale bars: 200 (c) and 1 μm (inset). 2101
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Figure 5. (a) AES spectra of N-doped graphene (black) and N-doped graphene mesh (red). (b) AES spectra of N-doped graphene (black) and Ndoped graphene mesh (red) after vacuum annealing.
Figure 6. (a) Id−Vg curves of pristine (black) and N-doped graphene (red) at Vds = 0.1 V. (b) Id−Vg curves of pristine (black) and N-doped graphene mesh (red) at Vds = 0.1 V (c) Statistical analyses on variation of Dirac points of graphene and graphene mesh before and after N-doping.
Supporting Information) shows that the graphene underneath the silica spheres was etched away. Instead, amorphous carbonaceous layers formed around the silica spheres. Because the sp2-hybridized C−C bond in graphene is very strong, the selective etching only underneath silica demonstrates that silica acts as a catalyst for the dissociation of carbon atoms from graphene. Almost concurrently, these dissociated carbon species formed amorphous carbonaceous structures. The mechanism of graphene dissociation underneath silica spheres is attributed to the catalytic hydrogenation mechanism. The reaction, similar to methanation reactions and hydrocarbon production from coal, is given by31
patterns was probably suppressed by catalytic hydrogenation process as described above. Because of the very strong covalent carbon−carbon bonds in graphene, thermal annealing generally yields physisorption of gas molecules rather than substitutional doping.20,21 Although physisorbed species typically led to a chemical doping effect by shifting the Fermi level away from the Dirac point, they were easily desorbed from the graphene surface even under normal atmospheric ambient conditions, causing unstable doping. Conversely, the edges in graphene mesh should provide a more chemically reactive site for covalent functionalization with dopant species, thereby enabling more stable electrical doping in graphene. Doping of graphene meshes by high-temperature thermal annealing was then explored under NH3 atmosphere. After annealing, the existence of N-dopants in graphene was investigated by Auger electron spectroscopy (AES), a powerful technique for surface chemical analysis of nanostructured materials. Figure 5a compares the AES spectra of graphene mesh to that of normal graphene following thermal annealing in NH3 gas. While no N peak was detected in the spectrum of normal graphene after annealing in NH3, a N peak appeared at ∼400 eV in the spectrum of graphene mesh. The N peak remained even after vacuum annealing at 300 °C for 3 h. This result illustrates that N doping occurs mostly through covalent functionalization at the edges of graphene meshes and supports the possibility of semipermanent N doping in graphene (Figure 5b). To investigate the effects of N-doping on electrical properties, we fabricated field-effect transistors (FETs) on 300-nm SiO2/Si substrates using four types of graphene channels: pristine graphene, annealed graphene, pristine graphene mesh, and annealed graphene mesh. Representative drain current versus back-gate voltage (Id−Vg) curves of FETs
SiOx
C(s) + 2H 2(g) ⎯⎯⎯→ CH4(g)
The scalability and availability of the current approach was further investigated by introducing various patterns of SiO2 thin films as growth masks. First, a 30 nm thick SiO2 film was deposited on the Cu foil. This was followed by patterning with photolithography (e-beam evaporator) and chemical etching. After CVD synthesis of graphene on the patterned Cu foil, SiO2 patterns were removed using a buffered oxide etchant. The remaining graphene samples were transferred to SiO2/Si substrates for SEM inspection. SEM image of the patterned graphene sample achieved with striped SiO2 mask patterns is shown in Figure 4c. The sample is composed of gray and bright regions based on image brightness. The gray region corresponds to very thin (mostly mono- or bilayer) graphene formed on the bare Cu surface region. Meanwhile, the areas corresponding to the under region of SiO2 mask patterns were very bright in the SEM image, which occurred due to electron charging and an indicative of the absence of graphene or carbonaceous structure. The graphene growth under the SiO2 2102
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dipped into the water, placed below the close-packed silica sphere film, and then raised up to transfer the film. Graphene CVD Growth. The prepared Cu foil was placed in the center of a quart tube reactor that was pumped to vacuum and heated to 900−1000 °C. Pressure was maintained in the range of 500−800 mTorr with H2 gas flow. To recrystallize Cu and sink the silica sphere into Cu foil, samples were thermally annealed for 0−30 min. After the annealing process, CH4 gas was introduced as a precursor to initiate graphene growth. During the growth phase, pressure was maintained at 500 mTorr under a continuous flow of 35 SCCM CH4 and 5 SCCM H2, as described elsewhere.4,6,38 After removing the embedded silica spheres with diluted HF solution (H2O/HF 50:1), the graphene meshes were coated with a poly(methyl methacrylate) (PMMA) protecting layer. They were then detached from the copper foils and transferred to other target substrates following a method described in ref 6. Nitrogen Doping. As-grown graphene samples (normal graphene sheet and graphene meshes) were transferred to SiO2/Si substrates and then annealed at 1000 °C for 1 h with 500 SCCM NH3 gas flow at 1 Torr. Fabrication of Graphene- and Graphene Mesh-FETs. The graphene samples were transferred onto a heavily doped n+-Si wafer with a 300-nm-thick oxide layer covered with a hydrophobic self-assembled layer of hexamethyldisilazane (HMDS). The HMDS layer was used to prevent extrinsic dopant adsorption on the graphene surface.39 Channels of the graphene transistor with a 5 μm width and 15 μm length were defined by photolithography and oxygen plasma etching at 50 W for 1 min. A second photolithography, evaporation of Ti/Au (10 nm/50 nm), and lift-off process were then performed to create the source and drain electrodes. As-fabricated devices were thermally annealed at 300 °C for 30 min to improve electrical contact. Analysis of transistor characteristics was performed using a probe station with a semiconductor parameter analyzer (model HP4145A).
are shown in Figure 6a,b. Both pristine graphene and annealed graphene exhibited ambipolar characteristic Id−Vg curves, with only slight changes in their Dirac points (minimum conductance points) from −1 to −5 V (Figure 6a). A quite different behavior was discovered with graphene meshes. The FET based on pristine graphene mesh operated as a typical ptype FET without showing a Dirac point up to Vg of 80 V. Statistical analysis on the Dirac points achieved from other graphene- and graphene mesh-FETs are summarized in Figure 6c, which also illustrates similar behaviors. After annealing in NH3 atmosphere, the Dirac points of graphene mesh-FETs were shifted to negative direction in average from far above 80 V to below 10 V. Normal graphene FETs exhibited only small changes in Dirac points, from ∼0.2 to ∼−4.2 V. Although the neck width of graphene mesh is relatively large for a band-gap opening,33−36 the physical mechanism of p-type like behavior is attributed to dangling bonds or physisorbed molecules such as oxygen species in edge sites.20 During the annealing process, physisorbed molecules were desorbed and N elements were covalently functionalized at the edges of graphene meshes, which moved the Dirac point to the left-hand side. From the slope of the Id−Vg curve, hole carrier mobility of graphene mesh was calculated to be ∼107 cm2/v·s, which is slightly lower than that of pristine graphene (∼122 cm2/v·s), presumably due to carrier scattering in the edges of graphene meshes. After N doping, the motility of graphene mesh was further decreased to ∼14 cm2/v·s because the dopant atoms in graphene possibly restricted carrier transport. In conclusion, we introduced a new method for direct CVD synthesis of graphene mesh structures that uses the silica sphere array as a growth mask. As-grown graphene meshes exhibited good structural quality, even when growth was interrupted by silica spheres. Graphene meshes were confirmed to have clean and empty holes with very abrupt edge, and their size can be independently tuned by controlling the depth of sphere sinkage in Cu. Moreover, this approach minimizes the unintentional contamination and atomic disordering that generally occurs during conventional lithography and etching processes. Accordingly, more reliable N doping was achieved through covalent functionalization at the edges of graphene meshes. This work may provide a step toward the direct synthesis of graphene-based nanostructures with tunable geometry and electronic property for future electronics.
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of Cu foil after graphene growth with silica sphere. SEM images of graphene meshes with monolayers of silica spheres with various diameters. TEM image and FFT images of graphene mesh. SEM images of graphene after hydrogen annealing with silica sphere. This material is available free of charge via the Internet at http://pubs.acs.org.
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EXPERIMENTAL METHODS Self-Assembled Monolayer of Silica Spheres on Cu Foil. First, precut Cu foil (purchased from Alfa Aesar) was sequentially cleaned in ultrasonic baths of acetone, methanol, and DI water for 5 min each and blown dry in N2. Monodisperse silica spheres (diameter of ∼600 nm) in ethanol solution were prepared by the Stöber method and then self-assembled into hexagonally close-packed structures on the Cu foil surface by the Langmuir−Blodgett assembly method.37 Because silica spheres in ethanol solutions naturally agglomerate, the prepared solution was first sonicated and agitated. The silica sphere suspension was carefully introduced to the water surface inside a glass dish using a slide glass half-submerged in water at an incident angle of 15°. Silica spheres in the suspension slowly spread over the air−water interface and formed a hexagonally close-packed 2D particulate film on the water surface due to surface tension. By introducing surfactant, a well-ordered array of silica spheres with higher grain size and lower defect density was achieved. After stabilizing the suspension, Cu foil was
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (2012-001442) and by Future-based Technology Development Program (Nano Fields) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0029300). 2103
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