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Effect of Catalyst Morphology on the Quality of CVD Grown Graphene Hindawi Publishing Corporation Journal of Nanomaterials Volume 2013, Article ID 393724, 6 pages http://dx.doi.org/10.1155/2013/393724 Research Article Effect of Catalyst Morphology on the Quality of CVD Grown Graphene Ya-Ping Hsieh,1 Yi-Wen Wang,1 Chu-Chi Ting,1 Hsiang-Chen Wang,1 Kuang-Yao Chen,2 and Chang-Chung Yang2 1 Graduate Institute of Opto-Mechatronics, National Chung Cheng University, 168 University Road, Min-Hsiung, Chiayi 62102, Taiwan 2 Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan Correspondence should be addressed to Ya-Ping Hsieh; [email protected] Received 15 March 2013; Revised 9 July 2013; Accepted 17 July 2013 Academic Editor: Nadya Mason Copyright © 2013 Ya-Ping Hsieh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The strong interest in graphene has motivated large effort in the scalable production of high-quality material. The potential of chemical vapor deposition on Cu foil to produce such graphene is impeded by lacking understanding of the relation between catalyst properties and graphene performance. We here present a systematic analysis of the catalyst morphology and its effect on electrical properties of graphene. We find that nanometer edsiz particles increase the density of bilayer regions but have no significant effect on carrier transport. Long wavelength roughness (waviness), on the other hand, generates defective graphitic regions that deteriorate carrier mobility. These findings shed light on the graphene formation process on Cu substrates and open a route to improve graphene quality for electronics applications. 1. Introduction used Cu foil is usually covered with a layer of chromium oxide for anticorrosion protection [10] that can affect the properties Graphene, a single atomic layer of carbon atoms, has gen- of grown graphene. Consequently, the formation of smooth erated enormous attention due to its physical properties. Cu surfaces free of contaminants becomes a necessary step Proof-of-concept experiments demonstrated novel electronic for the synthesis of high-quality graphene. and optoelectronic devices in transistors [1], solar cells [2, 3], photodetectors [4], and so forth. The desire for large-scale Despite this importance of the catalyst pretreatment, application of this material has motivated the development little work has been done to correlate the condition of the of a number of methods to synthesize large-area graphene Cu catalyst with the properties of the obtained graphene. sheets. Amongst these approaches, the chemical vapor depo- We here present the first systematic study of the effect of sition (CVD) synthesis of graphene on Cu substrate [5, 6] catalyst morphology on the electrical and optical properties has shown great promise for producing high-quality single- of graphene. We find that a higher density of surface particles layer graphene. Despite significant efforts, the properties supports the formation of bilayer graphene regions but has of CVD graphene have yet to reach the requirements of little effect on the electrical properties of the graphene film. electronics applications for mobility and uniformity. Recent Low frequency roughness (waviness), on the other hand, reports emphasize the importance of the surface morphology deteriorates the quality of graphene significantly as studied of the catalytic Cu substrate in determining the homogeneity by Raman spectroscopy and electrical measurements. and electronic transport properties of the grown graphene These observations provide deeper understanding of film7 [ –9]. It was found that imperfections in the Cu substrate the graphene growth and have large significance for the interfere with graphene growth. Furthermore, commonly optimization of graphene quality for future applications. 2 Journal of Nanomaterials 15 15 15 10 10 10 5 5 5 15 15 15 10 10 10 0 5 0 5 0 5 0 (m) 0 (m) 0 (m) (a) (b) (c) 1 100 0.8 80 60 0.6 40 0.4 Waviness (nm) Waviness Particle density (%) density Particle 20 0.2 0 0 0 20 40 60 0 20 40 60 Electropolishing time (min) Electropolishing time (min) (d) (e) Figure 1: Change of Cu morphology upon electropolishing: (a)–(c) AFM images for 5 min (a), 10 mins (b), 40 mins (c) electropolishing, (d) waviness versus electropolishing time, and (e) particle density versus electropolishing time. 2. Experimental then rapidly cooled under 10 sccm hydrogen flow. Graphene samples were transferred by a PMMA assisted process in Electrochemical polishing was employed to control the mor- which Cu was etched away by 5% FeCl3, and graphene was phology of the Cu foil. First, copper foil (99.8%, Alfa-Aesar, transferred onto SiO2/Si wafers. Finally, the PMMA was no. 13382) was precleaned by sonication in acetic acid for dissolved by immersion in acetone. 5 min to remove the oxide layer. The dried Cu foil was Carrier mobility was obtained by Hall effect measure- then affixed on a glass slide to simplify handling. TheCu ments of macroscopic (10 × 10 mm) films thus transferred foil was placed into a breaker containing a solution of employing a Van der Pauw geometry. Upon optimization of 300 mL of H3PO4 (85%) and 100 mL of polyethylene glycol the transfer protocol, sample-to-sample variations were less (PEG, molecular weight 400, from Sigma-Aldrich Co.) as than 10%. electrolytic solution. This Cu foil was then used as working Gwyddion software was used to extract the waviness and electrode, and another Cu plate was used as counter electrode, particle density from AFM images. The waviness parameter and a DC power supply was used to apply a voltage of 1.5 V. was defined as the RMS roughness of a sample after high-pass After the electropolishing treatment, the sample was washed filtering removed features with wavevectors larger than k = −1 with copious amounts of deionized water and then rinsed 5 m . with isopropyl alcohol, followed by blow-drying with N2. A watershed algorithm was applied to identify particles In our acid etching pretreatment, nitric acid (TFB, and calculate their area densities. TRANSENE Company, Inc.) and 37% HCl (J.T baker com- pany) were used as obtained. ThepretreatedCufoilswerethenusedtocarryout 3. Results and Discussion graphene growth. For this, the reactor chamber was pumped down to 10 mTorr using a vacuum pump. Hydrogen gas was In a first set of experiments we analyzed the effect of ∘ introduced into the chamber during heating to 1000 C. The electropolishing duration on the morphology of the Cu foils were annealed for 70 min to initiate Cu grain growth foil. Atomic force microscope images reveal the change in and to remove organic residue and surface oxide. Graphene roughness as time progresses (Figures 1(a)–1(c)). The images ∘ growth was then performed at 1000 C for 100 min under a gas reveal, however, that the roughness does not simply decrease mixture of H2 (100 sccm) and CH4 (40 sccm). The sample was as assumed in previous electropolishing papers. Instead, two Journal of Nanomaterials 3 45 2000 ) 1800 −1 40 1600 /Vs) 2 1400 35 1200 1000 -band width (cm width -band 30 Mobility (cm Mobility G 800 25 600 0 0.5 1 1.5 2 20 40 60 80 100 Particle density (%) Waviness (nm) (a) (b) Figure 2: Graphene quality for different Cu morphology: (a) G peak width versus particle density; (b) carrier Hall mobility versus waviness. components of the roughness have to be distinguished: while trend of graphene defectiveness or carrier mobility with the long range roughness (or waviness) decreases as a func- particle density, however, does not support that conclusion. tion of electropolishing time (Figure 1(d)), small particles Instead, the observation confirms results by Tsen et al. start occurring (Figure 1(e)).Thisisthoughttobecausedby [13] tailoring electrical transport across grain boundaries in two different mechanisms taking place during electropolish- polycrystalline graphene that intergrain carrier transport is ing. The concentration of electric field around micrometer not the limiting factor for the film mobility for well connected high protrusions causes preferential electrochemical etching grains. of these features and consequently adecrease of the wavi- Surprisingly, however, we observe a strong correlation ness [7]. Nanometer sized particles, however, exhibit too of the substrate waviness with the carrier mobility of the small field enhancement and are not affected by the same grown graphene (Figure 2(b)).Itcanbeseenthatbeyond mechanism [11]. Consequently, protrusions will smooth out acriticalvalueof∼70 nm, the carrier mobility decreases until they reach a critical size but will remain unchanged rapidly with waviness. This behaviour can be explained afterwards. Therefore, increasing electropolishing time repre- when examining the graphene growth process during CVD. sents a tradeoff between decreasing waviness and increasing HydrocarbonsareadsorbedontheCusubstrateanddiffuse particledensityasshownwhencomparingFigures1(d) and on the surface until they attach to the edge of an existing 1(e). graphene grain or a step edge [14]. In the case of samples Based on the finding that waviness and particle density with larger waviness, more step edges occur. These features are both changed during electropolishing, we investigate the act as obstacles for high-quality graphene growth and instead effectofeachofthetwoparametersonthequalityofgraphene form disordered graphitic regions. The
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