Vertical graphene by plasma-enhanced chemical vapor deposition: Correlation of plasma conditions and growth characterist
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Robert D. Cormia Foothill College and UCSC/NASA-ARC Advanced Studies Laboratories, NASA Ames Research Center, Moffett Field, California 94035
Shuhei Takahashi and Michael M. Oyea) Center for Nanotechnology and UCSC/NASA-ARC Advanced Studies Laboratories, NASA Ames Research Center, Moffett Field, California 94035; and Department of Electrical Engineering, University of California Santa Cruz, Santa Cruz, California 95064 (Received 30 June 2013; accepted 17 September 2013)
Vertically aligned graphene was grown by plasma-enhanced chemical vapor deposition using methane feedstock. Optical emission spectroscopy (OES) was used to monitor the plasma species, and Raman spectroscopy was used for characterizing the properties of as-grown vertically aligned graphene. OES-derived information on plasma species, such as C, C2, CH, and H, are correlated with the properties of the vertically aligned graphene. Graphene grown at 250 W and 15 sccm exhibited the lowest amount of defects. Although OES peak intensities occurred at the highest power and lowest flow conditions, the OES peak ratios of plasma species had a greater dependence on flow rate and exhibited a saddle point in the atomic C/H ratio corresponding to optimal growth involving the lowest amount of overall defects. Plasma diagnostics provides a valuable approach to optimize growth characteristics and material properties. I. INTRODUCTION
Graphene as a nanomaterial is considered for a variety of applications, including electronics, sensors, supercapacitors, composites, and others.1–3 Besides chemical synthesis, chemical vapor deposition (CVD) has emerged as a viable technique to prepare graphene on various substrates including silicon and copper.1–7 Successful demonstration of single, bi-, and multilayer graphene has been reported by many groups. Recently, plasma-enhanced CVD (PECVD) has also been used in the preparation of graphene.8–17 Growth has been reported on a wide range of substrates, including silicon, copper, nickel, tungsten, tantalum, Mo, Zr, SiO2, Al2O3, and stainless steel.10,11 There are several unique features about PECVD of graphene. First, it is possible to grow vertically aligned graphene, sometimes also referred to as carbon nanosheets or carbon nanowalls (CNWs).8–19 These structures may be useful in applications involving chemical sensors, biosensors, supercapacitors, batteries, and others. Second, growth is reported without the aid of a catalyst. Hori and coworkers8,9 showed that injection of hydrogen atoms to a hydrocarbon plasma enables growth of CNWs a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.293 J. Mater. Res., Vol. 29, No. 3, Feb 14, 2014
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without a catalyst. No growth was reported for their conditions in the absence of H radical injection. Wang et al.10–12 reported CNW growth on twelve different substrates using inductively coupled plasma with methane diluted in H2 at a concentration of 5–100%; CNWs ranging from one to three layers were
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