Characterization of Graphene Grown on Bulk and Thin Film Nickel

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Characterization of Graphene Grown on Bulk and Thin Film Nickel Chun-Chieh Lu,† Chuanhong Jin,‡ Yung-Chang Lin,† Chi-Ruei Huang,† Kazu Suenaga,‡ and Po-Wen Chiu*,† † ‡

Department of Electrical Engineering, National TsingHua University, Hsinchu 30013, Taiwan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan ABSTRACT: We report on graphene films grown by atmospheric pressure chemical vapor deposition on bulk and thin film nickel. Carbon precipitation on the polycrystalline grains is controlled by the methane concentration and substrate cooling rate. It is found that graphene grows over multiple grains, with edges terminating along the grain boundaries and with dimensions directly correlated to the size of the underlying grains. This greatly restricts the resulting graphene size (50%). In addition, the number of layers can be better controlled in the bulk growth. Characterizations of the graphene sheets using transmission electron microscopy, Raman spectroscopy, and transport measurements in the field-effect configuration are also discussed.

1. INTRODUCTION Graphene, a single atomic layer of graphite comprising a planar hexagonal lattice of carbon atoms, has a host of characteristics that show great promise for the development of postsilicon electronics, 1 4 including a large room-temperature carrier mobility5 (20 000 cm2V 1 s 1 ) and long-range ballistic transport.6 The mechanical cleavage of highly oriented pyrolytic graphite (HOPG) is a method used to isolate graphene on insulating substrates.1 This method offers a simple means of obtaining high-quality graphene flakes and is the process used most frequently in experimental studies. The high-yield direct exfoliation of graphite is a more challenging approach. Lately, a number of attempts have been made to synthesize large quantities of graphene through exfoliation. Chemical exfoliation in the solution phase7 11 has shown potential in mass production, although the chemical treatments inevitably result in substantial defects that degrade the electronic properties of the resulting graphene. An alternative approach that has been actively pursued is the thermal decomposition of SiC, which is capable of wafersize epitaxial growth.12,13 However, this reaction requires an ultrahigh vacuum, high-temperature (1300 1600 °C) environment, incurring a considerable thermal budget that may act as the prime constraint against integration with current silicon technologies. Another focused synthesis of graphene makes use of the catalytic decomposition of hydrocarbons or carbon oxides on transition metals such as Ni(111),14,15Pt(111),16Ir(111),17and Ru(0001).18,19 The large-area synthesis of graphene on a polycrystalline Cu foil or a thin film by chemical vapor deposition (CVD) has also reported lately.20,21 Mono-, bi-, and few-layer (>3) graphene have been synthesized on nickel, which is of particular interest among the various metal templates because of its lower reaction temperature r 2011 American Chemical Society

(500 900 °C).21,22 Typical graphene growth on Ni is based on gas decomposition and the diffusion of atomic carbon into the bulk with subsequent carbon precipitation to the Ni surface under nonequilibrium thermodynamic conditions. As the Ni substrate slowly cools from high temperature, a large number of carbon atoms emerge from the bulk, forming a fairly thick graphitic overlayer. The surface precipitation of carbon can be greatly retarded by rapid cooling, allowing for the formation of mono-, bi-, and few-layer graphene.22 A recent study details a modified approach to graphene growth on a Ni surface in which a solid carbon source was supplied from the underlying SiC substrate.23 In the process, a 200 nm Ni thin film was deposited on a SiC substrate, followed by vacuum annealing at 750 °C during which both silicon and carbon dissolve in the Ni film. Upon cooling, carbon precipitates on the surface of the nickel silicide, forming mono- to few-layer graphene. Here, we compare the properties graphene grown by the gas-phase decomposition of methane on bulk and thin film Ni and provide a systematic characterization of graphene grown on bulk Ni, including Raman scattering, transmission electron microscopy (TEM) analysis, and transport measurements in a field-effect configuration.

2. EXPERIMENTAL SECTION 2.1. CVD Growth of Graphene. Figure 1 schematically illustrates the graphene synthesis process and property characterizations. Two different Ni substrates were used for growth: bulk pillars and thin films. The pillars are 1 cm in height and 0.5 cm in radius, and the thin films (1 μm) Received: June 11, 2011 Revised: September 9, 2011 Published: October 03, 2011 13748

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Figure 1. Schematic of graphene growth by CVD on bulk and thin film Ni. The reaction temperature versus time curve is shown for the CVD process. Following growth, graphene sheets are transferred onto silicon substrates with a 300-nm-thick oxide layer for Raman characterization and transport measurements. Gold markers are also made on top of graphene sheets, followed by Ni etching using hydrochloric acid. The graphene sheets become suspended after being transferred to TEM grids. were deposited using a thermal evaporator on Si substrates with 300 nm dry oxide. In the CVD process of bulk growth, the samples were first placed away from the center of the tube furnace, which was ramped up to 900 °C over 40 min under a steady 120 sccm flow of hydrogen and a 230 sccm flow of Ar. The samples were then moved to the center of the furnace. Graphene growth was carried out by the thermal decomposition of methane (99.99%) at 850 °C, with an 8 sccm constant flow of methane. After ∼8 min, the growth was stopped and the samples were moved outside the furnace and cooled at a rate of 8 °C/min in a temperature range of 850 400 °C under a flow of Ar. Graphene growth on Ni thin films was carried out by a similar CVD process with slightly reduced gas flows. 2.2. Material Characterization. Following growth, graphene was characterized by high-resolution TEM (JEOL-2010F) with a postspecimen image corrector operated at 80 kV to avoid the electron beam damaging the graphene during observations. Image recording was performed with a Gatan CCD 894. To produce suspended structures, graphene sheets on Ni pillars or films were coated with poly(bisphenol A carbonate) (PC) of ∼1.5 μm thickness,24 followed by etching in a 5% aqueous hydrochloric acid solution. The PC film, along with the attached graphene, was lifted off after etching. The whole structure was then transferred to a silicon substrate and rinsed with chloroform to remove the PC film. A 20 nm silicon(II) oxide film was deposited by thermal evaporation onto the graphene to serve as a protective layer prior to the spin-coating of poly(methyl methacrylate) (PMMA), allowing us to make gold markers on the graphene by e-beam lithography while preventing direct contact between the graphene and the PMMA. The gold pattern and the graphene were separated from the silicon substrate by etching in a buffered HF solution. Afterward, the gold pattern and graphene were transferred onto a copper grid for TEM observation. The drying process was conducted with care to avoid breaking the graphene. This technique provided a clean graphene surface for high-resolution TEM imaging and, importantly, is capable of correlating TEM with Raman or transport measurements. The acquired TEM images in the vicinity of the Raman spots provide a more accurate interpretation of the Raman spectra on the CVD-grown graphene. More details about this technique can be found elsewhere.24 Raman spectroscopy is an ingenious tool for fingerprinting sp2 carbon and providing an on-chip analysis of grown graphene in a manufacturing environment.25 27 In this study, the micro-Raman spectra were obtained with a commercial Raman microscope (HR800, HORIBA), with a laser excitation wavelength of 633 nm. A 100 objective was used to

Figure 2. (a) Optical and (b) SEM images of graphene flakes grown on Ni thin films. Optical images of graphene transferred to Si substrates for (c) thin film growth and (d) bulk growth. provide a diffraction-limited spot size of about 1 μm. A low power level (∼1 mW) was used to avoid the heating effect. To eliminate the graphene nickel interaction, the Raman spectra were taken on graphene sheets transferred to a silicon substrate.

3. RESULTS AND DISCUSSION Figure 2a,b depicts an optical photograph and a scanning electron microscopy (SEM) image of identical CVD graphene flakes on a Ni film, respectively. A clear color contrast between the graphene flakes and Ni can be seen in the optical photograph. The darkest pattern corresponds to a thick graphite slab (>10 layers), and the lighter patterns near the thick graphite are thin graphene flakes of one to three layers. These thin graphene flakes are distributed homogeneously over the surface of the Ni thin film, with the size strongly depending on the dimensions of the underlying grains. The typical size is about 3 5 μm. It was also found that graphene can grow over different Ni grains and terminates along the Ni grain boundaries, as can be seen in the comparison of the optical and SEM images in Figure 2a,b. To differentiate monolayer graphene better, the grown films were transferred to a Si substrate. Figure 2c shows such a film, in which graphene samples with different layer counts are mixed. Given an appropriate SiO2 thickness (∼300 nm), monolayer graphene can be easily identified under a conventional optical microscope, similar to the case of exfoliated graphene on a Si substrate.28,29 The linear dimension of monolayer graphene can be enlarged to hundreds of micrometers (Figure 2d) if the growth is carried out at elevated temperatures on Ni pillars that possess large crystalline grains. Compared to graphene grown on Ni thin films,21,22,30 multilayer graphene sheets are found less frequently under the bulk growth condition, presumably 13749

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Langmuir because of the reduced grain boundaries that facilitate carbon diffusion and precipitation. It should also be noted that, under our growth conditions, no wrinkles or other obvious corrugated structures were found in graphene grown on both Ni pillars and thin films. Following transfer to the Si substrate, the graphene sheets stay flat.

Figure 3. (a) Optical image of a large-area monolayer graphene sheet on a silicon substrate, covered with a Au pattern. (b) Optical image of the graphene/Au structure transferred to a TEM grid. High-resolution TEM images of (c) multilayer and (d) monolayer graphene grown on bulk pillars. The holes indicated by arrows were created by electron beam irradiation to assist in counting the stacking layers.

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Figure 3a shows large-area CVD graphene on the SiO2/Si substrate with Au patterns made by the aforementioned PMMAfree lithography process. To transfer the graphene to a TEM grid, buffered HF was used for SiO2 etching, with one side of the graphene protected with a thin layer of PC. Following etching, the PC/Au/graphene film was suspended on pure water and then transferred to a copper grid, as shown in Figure 3b. The PC film was dissolved in chloroform overnight to ensure the complete removal of PC. Figure 3c,d displays high-resolution TEM images of multilayer and monolayer graphene grown on bulk nickel, respectively . No remarkable defects were found inside the clean area of the graphene, consistent with the low intensity of the Raman D peak discussed below. The holes indicated by arrows were generated by electron irradiation and can be used to identify the number of layers with high accuracy. In the multilayer graphene, a domain boundary that marks off the upper and lower domains can be clearly seen. It should be noted that many small graphene derivatives (∼2 30 nm) appear on CVD-grown graphene, resembling amorphous carbon randomly decorating nanotube walls. These overgrown derivatives are presumably caused by the rough surface of the metal catalyst. The graphene derivatives, along with the grain boundaries, are responsible for the intensity of the Raman D peak. The most prominent features in the Raman spectra of graphene are the G, D, and 2D peaks, which lie at around 1580, 1330, and 2660 cm 1, respectively.25 The G peak position has been shown to be sensitive to charge density26,31,32 and the presence of strain in graphene.27,33 The D peak is generated by the breathing modes of sp2 carbon, and its relative intensity (compared to that of the G peak) is a measure of the amount of disorder. The second order of the D peak is called the 2D peak, which conveys information regarding the interlayer coupling as graphene is stacked layer by layer. Figure 4 compares the Raman spectra of the CVD graphene grown on Ni thin films and Ni bulk pillars with that of exfoliated HOPG graphene on SiO2/Si. The optical photographs in the insets show the graphene sheet on which the Raman spectra were recorded. For exfoliated HOPG graphene, three clear Raman features can be used to differentiate monolayer graphene from multilayer graphene:25 (a) the 2D peak is a single Lorentzian for monolayer graphene, but it splits into a multilayer because of the branching of the electronic bands

Figure 4. Raman spectra of mono- and bilayer graphene on SiO2/Si. (a) Exfoliated HOPG graphene. (b) Ni thin film CVD graphene. (c) Ni bulk pillar CVD graphene. (a c) The G peaks are scaled to have similar intensities. The corresponding optical images of the graphene flakes are shown in the insets. 13750

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Langmuir and the phonon dispersion around the Dirac point; (b) the 2D line width γ, which is the full width at half-maximum, broadens in the multilayer; (c) the intensity ratio I2D/IG is 20 V exhibit a higher electron mobility, and the hole mobility rises for the R Vtg curves at Vbg < 20 V.

4. CONCLUSIONS We have demonstrated that, given suitable control of the methane concentration and substrate cooling rate, monolayer graphene can be grown over multiple Ni grains. Compared to growth on thin films, large-area monolayer graphene (300 400 μm) can be readily formed on bulk Ni because of the large crystalline grain sizes at elevated temperatures. CVD synthesis of graphene on transition metals opens up a number of avenues for graphene application development and represents a vital step forward toward

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