A Few layer Graphene synthesized by A Catalytic ...

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M. G. Herb Brody, Tony Scully, Nick Haines, Nature OUT LOOK,483, S29 (2012). 4. ... E. L. Fred Schedin, Antonio Lombardo,§ Vasyl G. Kravets, Andre K. Geim,.
A Few layer Graphene synthesized by A Catalytic Thermal Chemical Vapor Deposition Ravi. K. Biroju * and P. K. Giri *, # * Centre for Nanotechnology, # Department of Physics Indian Institute of Technology, Guwahati, Assam, India- 781039. Abstract: The discovery of uniform, large area and high quality continuous layers of graphene deposited on copper by chemical vapor deposition (CVD) has created significant interest. CVD graphene grown on copper is less defective over large area in nanometre scale and excellent optoelectronic properties can be achieved. In this work, thermal chemical vapor deposition (CVD) growth of graphene is demonstrated over a copper foil of thickness 25μm. A typical CVD reaction is done in a quartz tube placed inside a horizontal muffle furnace with a heating zone of 15 cm. The reaction is carried out by flowing CH₄ gas (18 sccm), H₂ gas (200 sccm) for 30min at a temperature of 1000⁰C and a pressure of 4.0 mbar. As deposited Graphene on Cu foil is transferred to Si/SiO2 substrate by wet transfer technique using PMMA and Fe(No₃)₃ etchant solution. The quality of graphene grains is characterized by Raman spectroscopy, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) imaging. From the line shape analysis of 2D peak in the Raman Spectra, HRTEM and AFM images it is found that the number of layers of graphene grown are only a few layers (2-4). The average grain size of graphene is found that few to tens of microns area. The structural quality of the graphene layer is found to be superior to that grown by chemical methods. Key words: Graphene, Chemical Vapor Deposition, Raman Spectroscopy

1. Introduction: The remarkable properties of graphene like large carrier mobilities up to 20000 cm2V-1S-1 (1), optical transparency ~97.7% (2), mechanical strength, robustness and environmental stability makes it as a new star of the material world (3). Low absorption (in visible-NIR region) and low sheet resistance of the graphene are major advantages for various optoelectronic devices such as photodetectors, transperant electrodes (4), LEDs and solar cells (5). Fabrication of the device requires wafer scale deposition that can be processed using CMOS fabrication techniques. Graphene was first produced by mechanical exfoliation by simply pealing of scotch tape on graphite flake(1), later several methods are developed in order to produce the graphene in large scale, followed as chemical methods (6), unzipping of carbon nanotubes (7, 8), plasma enhanced chemical vapour deposition (PECVD), metallorganic chemical vapour deposition (MOCVD) (9) etc. However, majority of these methods are not much successful to achieve high quality, large area, defect free graphene. The most promising, less expensive and convenient approach to produce the high quality graphene is the chemical vapor deposition (CVD) on to metal catalysts (Ni, Ru, Ir, Pd, stainless steel etc.) (10-14). Out of all these catalysts, large area, defect free, monolayer

of graphene growth is usually achieved by copper. Most of the depositions were performed on copper foil (12) or on thermally evaporated film or e-beam evaporated films. CVD growth of large area wafer scale graphene using copper catalysts have been reported for various electronic device fabrication (15), most recently milli-meter area defect free monolayer CVD graphene (16) has been reported. In this article, we provide a detailed study on the synthesis of graphene by a home built thermal CVD setup and characterize its grain size, surface coverage and number of layers as determined by Raman Spectroscopy, SEM, FESEM, TEM and AFM.

2. Materials and Methods: 2.1 Sample Preparation: To synthesize graphene we have designed and developed a catalytic CVD system in our laboratory, as shown in the schematic diagram of Fig.1. It consists of a horizontal muffle furnace, a closed cylindrical quartz tube with vacuum pumping and gas flow option through a mass flow controller (MFC) unit. Inlet of the quartz tube is connected to the MFC and outlet is connected to the vacuum pump. The quartz chamber is placed Figure 1: Schematic diagram of the in-house developed thermal CVD setup. inside the horizontal muffle furnace. During the growth process, chamber pressure is controlled by the gate valve connected at the mouth of rotary pump. Copper foil of thickness 25 μm (Alfa-Aesar) cut in to 1x1 sq.inch is used as a substrate to grow graphene. Cu foil was inserted into the quartz chamber and flushed with Ar gas for 5 min. Chamber is pumped to base vacuum 4x10-4 mbar and temperature was increased to growth temperature with ramp 25 ⁰C min-1.Substrate was pre annealed in reduced environment by flowing 200 SCCM H₂ gas for 30 min. The reaction is carried out by controlled flow of CH₄ (18 sccm) and H₂ (200 sccm) for 30 mins at a temperature of 1000⁰C and a pressure 4.0 mbar. Poly (methyl methacrylate) (PMMA) is dissolved in dimethyl formamide (DMF) and is drop casted on the graphene on Cu and etching of the underlying Cu is done in Fe(NO3)3. PMMA/Graphene floating on the Fe(NO3)3 solution is rinsed in the DI water and then transferred to Si/SiO2 substrate. This is a well known wet transfer technique to transfer of graphene on an arbitrary substrate.

2.2 Characterization Tools: Micro Raman measurements were performed with a commercial Raman setup (Horiba, Lab Ram HR), equipped with He-Ne laser excitation source at 633 nm. Excitation source was focused with 100x lens, spot size 2μm and the signal was collected by a CCD in a back scattering geometry sent through a multimode fibre grating. Electron microscopy has been performed by SEM (LEO 1430VP), FESEM (Carl Zeiss, Sigma, Germany, Operating Voltage 0.5-30 KV), HRTEM (JEOL-2100, 200kv) and Atomic Force Microscopy (Agilent Technologies 5500 SPM).

3. Results and Discussion: Raman spectroscopy is a versatile tool to characterize graphene related sp2 hybridized carbon nano structures (17), number of layers, disorder due to defects (quality), especially for less than 10 graphene layers. Fig 2(a-d) represents the (a)

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Figure 2: SEM images of multilayer graphene flowers and circular shaped grains up to 10 μm area.

various graphene films of different shapes (flower and circular grains) after transferring to Si/SiO2 substrate from the copper foil. The sample was grown at temperature 1000oC for duration 30min under low pressure 4.0 mbar. Typical Raman spectrum of as-grown graphene sample is shown in Fig 3. It shows the prominent features: D band at 1324cm-1, G band at 1585 cm-1 and 2D band at 2642 cm-1 that are significant Raman finger prints of Figure 3: Typical Raman Spectra represents graphene. The G-peak is from the first order the signatures D(1324cm-1), G(1585cm-1) and sharp 2D(2642cm-1) and some second order Raman scattering process and is attributed vibrational (2460, 2910 cm-1) modes few layer graphene

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of the

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Figure 4: Lorentzian fitting of the Raman signatures of graphene (a) 2D-band- 2642 cm-1, (b) G-band1585cm-1, (c) D-band- 1324cm-1

to the double degenerate in-plane longitudinal optical (iLO) mode and in-plane transverse optic (iTO) phonon modes (E2g symmetry) of sp2 hybrid carbons at the Γpoint. The D and 2D bands originate from second order Raman process involving one iTO phonon. The D-band is a signature of disorder or defects in graphene and the 2D band is the overtone of the D-band involving two iTO phonons with opposite momenta (18). However, D-band is weak for the infinite number of graphene layers but Raman active for few layers with considerable defects. So, the presence of D-peak (1324cm-1) suggests that the formed graphene is disordered and has inherent defects which include vacancies, wrinkles leading to non uniformity, hexagonal/non-hexagonal(heptagons, octagons) distortions, corrugation and twisting of the layers as shown in the TEM, HRTEM and SAED images of Fig 4. The weak Raman feature~ 2460cm-1 is significant for intervalley double resonance Raman processes similar to 2D band but involving one LO and one iTO phonon (19).Another weak feature(D+D’) at 2910 cm-1 is attributed to combinational mode (18). Number of layers and disorder due to defects in the graphene film are estimated by simply fitting Lorentzian line shape to D, G and 2D-bands, as shown in Fig 4(ac) and by monitoring the shape evolution, peak position and peak intensity ratios (I2D/IG, IG/ID). Raman spectra of Fig 5(a) are recorded at different circled positions on graphene film as shown in the optical image and we calculated that I2D/IG~ 0.5-1.5, IG/ID~ 1.5-1.65 and (c)

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Figure 5: (a) Raman Spectra of graphene grains recorded at different positions marked as circular rings in the optical microscope image (b). (c) Raman spectra of monolayer graphene.

FWHM~ 42-45 cm-1 characteristic of the bi- or tri-layer graphene. Fig 5(c) is a typical Raman spectrum of a single layer graphene with I2D/IG~ 1.88 and FWHM~ 35 cm-1 (12). We acquired Raman spectra at several locations and found that 80% of the graphene layers have a few (2-3) layer structure. D band shown in Fig. 6(a) is possibly due to the disorder on the graphene surface arising while transferring the film from copper foil to (a)

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Figure 6: (a)TEM image of the twisted graphene layers, arrows indicates the residual Cu, PMMA nano particles (b) Hexagonal lattice fringes of few layer graphene (c) corresponding SAED pattern.

Si/SiO2 substrate. Fig. 6(a) shows the TEM image of the twisted graphene layer, with Residual metal nanoparticles, Fig. 6(b) show the lattice fringe of the graphene layer (bilayer graphene) and Fig. 6(c) shows the corresponding SAED pattern. The secondary diffraction patterns are observed due to residual Cu nano particles and PMMA adsorbed on the surface of the graphene. The possible disorder due to edge defects, wrinkles and twisting of graphene is due to the residual Cu nano particles or PMMA left in the transfer process. It is clearly observed in the HRTEM and AFM topography images shown in the Fig 7 (a)-(c), which is consistent with the Raman Dpeak at ~1353 cm-1 and some second-order combinational modes at 1831 cm-1, 2460 cm-1 and 2910 cm-1. (b)

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Figure 7: (a) A few layer graphene on the top of Cu nanoparticle, (b) AFM line profile of the residual Cu nano particles and graphene.

4. Conclusions: In the present study, well organized few layer graphene was synthesized by a laboratory assembled thermal CVD setup. The presence of single, bi, try and multi layer graphene over micrometer area is established by Raman spectroscopy, electron microscopy and AFM measurements. Raman data in combination with FESEM and TEM suggest graphene growth as a few layer (2-3) and size of few to 10 μm2 area. We also find the decoration of Cu nanoparticles on the graphene layers as observed in HRTEM. A possible surface enhanced Raman scattering (SERS) is possible with Cu nano particle coating on graphene which may probe new features related to structural properties of the graphene. Note that SERS of graphene with gold thin film and gold nano particles(20) is well reported.

5. Acknowledgements: We thank to Department of Science and Technology(DST), Govt.of.India and Central Instrument Facility(CIF), Indian Institute of Technology Guwahati for providing the Instruments TEM, Raman ,FESEM for characterization. Thanks to Indrajit Talukdar for his assistance in TEM measurement.

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