Annealing Temperature and Substrate Effects on the Raman Spectra of Transferred CVD Graphene
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Annealing Temperature and Substrate Effects on the Raman Spectra of Transferred CVD Graphene Barbara M. Nichols, Yasmine R. Doleyres, and Gregory P. Meissner U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, U.S.A. ABSTRACT This work explores the influence of the post-transfer anneal temperature and the substrate on transferred graphene and its Raman properties. Graphene grown by low pressure chemical vapor deposition on copper foils was transferred to SiO2/Si, fused silica, and silicon substrates via a process that involves coating the graphene with PMMA as a protective handling layer during the wet chemical etching of the copper and then placing the PMMA/graphene onto the substrate. The PMMA layer was then removed by heating in a hydrogen/argon atmosphere at temperatures ranging from 350 to 550 °C or by exposing the PMMA to heated acetone vapor/liquid. Raman spectroscopy measurements, taken before and after PMMA removal, reveal differences in the prominent Raman features, the G and G' peaks, upon annealing. These changes include (1) a shift in the average G and G' peak positions when comparing Raman spectra before and after PMMA removal and (2) a decrease in the G':G peak intensity ratio (IG'/IG), which is typically used as a measure of the number of graphene layers. For both the astransferred graphene and graphene removed by the heated acetone, the IG'/IG peak ratio was approximately 2, indicating single layer graphene. However, when the graphene was annealed at temperatures above 350 °C, the IG'/IG intensity ratio varied from 0.5 to 1.5. These changes in the Raman spectra are similar to those observed in exfoliated single layer graphene supported on SiO2/Si substrates and are indicative of graphene-substrate interaction effects that lead to hole doping of the graphene [1,2]. These trends were observed for graphene transferred to all three substrates, regardless of the substrate surface roughness and/or composition. INTRODUCTION Graphene is a monolayer thick material comprised entirely of sp2-bonded carbon atoms that form a honeycomb-like lattice structure. Because it is only one atom thick, it is the prototypical two-dimensional material and as such exhibits unique physical properties. These interesting properties include a high intrinsic mobility (200,000 cm2/Vs) [3], a high breakdown current density exceeding 108 A/cm2 [4], and a constant optical absorption of 2.3% per layer over a wide spectral range [5]. These properties make graphene an attractive material for many electronic applications, including transparent conductors, field-effect transistors, and frequency multipliers. Thus far, the electrical properties of graphene have been primarily explored using graphene produced by mechanical exfoliation. Interactions between the exfoliated graphene and the device substrate, in particular thermally grown silicon dioxide, have been shown to influence the transport properties of the material [1,2,6]. In order for graphene-based electronic devices to be fully realized, large area, high qualit
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