The Influence of Thermal Annealing to Remove Polymeric
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CARBON xxx (2013) xxx– xxx Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon The influence of thermal annealing to remove polymeric residue on the electronic doping and morphological characteristics of graphene Kitu Kumar, Youn-Su Kim, Eui-Hyeok Yang * Department of Mechanical Engineering, Stevens Institute of Technology, 1 Castle Point on Hudson, Hoboken, NJ 07030, USA ARTICLE INFO ABSTRACT Article history: The impact of polymer removal by forming gas and vacuum annealing on the doping, Received 15 April 2013 strain, and morphology of chemical vapor deposited (CVD) and mechanically exfoliated Accepted 26 July 2013 (ME) graphene is investigated using Raman spectroscopy and atomic force microscopy Available online xxxx (AFM). The behavior of graphene exposed and unexposed to polymer is compared. It is found that the well-known doping effect after forming gas annealing is induced in CVD– ME graphene by polymeric residue/hydrogen-functionalization. Further, forming gas annealing of ME graphene is shown to induce strain via pinning of the graphene layer to the substrate. It is found that vacuum annealing removes most polymeric residue, with minor doping and strain effects. Finally, a study of AFM step height and roughness mea- surements provides a comprehensive understanding of those annealing-based processes which create morphological changes and directly influence doping and strain in the graph- ene layer, such as removal of polymer, removal of the interfacial graphene–substrate water layer, environmental doping effects and deformation of the graphene layer. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction to desired substrates such as SiO2, glass or other plastic films. A common method for graphene transfer involves spin-coat- Graphene, an atomic layer of carbon atoms in a hexagonal lat- ing a polymer, typically poly(methyl methacrylate) (PMMA), to tice, has generated enormous interest in the research com- support the graphene during etching of the catalyst metal and munity because of its potential for use in various transfer to substrate [8]. Once the graphene–PMMA stack is applications such as electrodes in photovoltaic cells [1], tran- transferred, the polymer is removed and further, if the graph- sistors [1–3] and as the chemically active material in superca- ene is to be patterned, PMMA may be re-introduced as a mask pacitors [4], and gas [5] and chemical sensors [6]. Graphene for electron beam lithography. In both cases, PMMA is dis- may be fabricated by mechanical exfoliation from bulk graph- solved either through solvent rinses, thermal annealing or a ite, chemically reduced graphene oxide, Si sublimation from combination of the two. It is well known that solvent rinses bulk silicon carbide, and chemical vapor deposition (CVD) leave a layer of polymeric residue [9–11], and that these can on catalytic metal films. To be promising for commercial or be largely removed by thermal annealing in gaseous atmo- bulk production, graphene must be grown over large areas, spheres such as Ar [12–14],H2 [15],H2/Ar [5,16],N2,orinvac- for which CVD growth is most promising due to its high yield uum [7,17,18]. Current annealing is also explored as a route to and quality [7]. However, the large-area CVD graphene must remove polymeric contaminants [19,20], but can only be per- be transferred from the catalyst growth metal (usually Cu foil) formed after electrode fabrication on the graphene. Thermal * Corresponding author. E-mail addresses: [email protected], [email protected] (E.-H. Yang). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.07.088 Please cite this article in press as: Kumar K et al. The influence of thermal annealing to remove polymeric residue on the electronic doping and morphological characteristics of graphene. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.07.088 2 CARBON xxx (2013) xxx– xxx annealing, on the other hand, can be performed at any step in molecular weight PMMA (Sigma–Aldrich) dissolved in anisole the device fabrication process, either immediately after trans- at a spin speed of 4000 rpm for 1 min followed by 1000 rpm for fer or immediately after device fabrication. It has been shown 1 min (acceleration of 1000 rpm/s). The resulting PMMA thick- that PMMA residue on CVD graphene is substantially de- ness was approximately 50 nm. The sample was then dried at creased after ultra-high vacuum annealing steps, resulting 25 °C in laboratory ambient air for 12 h after which it was in twice as large carrier mobility [17]. Modifying the transfer placed in an etchant bath. For both PE and PN samples, the process by cleaning graphene in IPA to reduce the amount Cu foil was etched for approximately 12 h and then etched of interfacial water between graphene and substrate [7] and for an additional 24–30 h in a fresh citric acid bath to ensure annealing in ultra-high vacuum can also increase CVD graph- the complete removal of Cu. The samples were placed in ene carrier mobility to approach what is measured in two successive water baths of 12 h each and then transferred mechanically exfoliated graphene. Though ultra-high vac- to a 90 nm SiO2/Si substrate. After being allowed to dry in lab- uum annealing removes the bulk of PMMA residue, many oratory ambient air, the PE samples were placed in an acetone researchers opt to anneal graphene in forming gas at either bath at room temperature overnight followed by an acetone atmospheric pressure or low vacuum for ease. Recent TEM bath at 55 °C for 2 h to ensure an effective PMMA removal. studies [11,21] have shown that alternate annealing under The transferred CVD samples were then either subjected to hydrogen and oxygen atmospheres removes a substantial annealing in H2/Ar atmosphere at 350 °C for 2 h followed by majority of PMMA residue; however oxygen annealing intro- annealing in ambient atmosphere at 350 °C for 2 h or vacuum duces defects in the graphene lattice [21]. Despite significant annealing at 350 °C for 2 h. work in this area, the physical processes involved in the re- Mechanically exfoliated samples were subjected to similar moval of PMMA as well as the causes of the resulting doping conditions. Monolayer and bilayer graphene flakes were and strain in the graphene have not clearly been accounted placed in an acetone bath to remove adhesive residue and for. half of the samples (PN) were annealed as above. The other Here, we present a comprehensive analysis of thermal half were spin-coated with PMMA as above (PE), placed in annealing processes by comparing the effects of alternating an acetone bath overnight, followed by a 2 h acetone bath at hydrogen and oxygen annealing with the effects of vacuum 55 °C to remove the polymer and subsequently subjected to annealing on the Raman spectral peaks and atomic force annealing as above. microscope (AFM) topography of graphene exposed (PE) and unexposed (PN) to polymer. We also subject mechanically 2.2. Raman spectroscopy exfoliated (ME) graphene to this processing to further eluci- date the effect of thermal polymer removal on a pristine lat- Raman characterization of all samples was performed in tice. Utilizing Raman spectroscopy, which provides valuable ambient atmosphere at room temperature. The CVD graph- information on graphene doping levels, strain, and defects, ene was characterized after transfer to substrate (PN only) and AFM which provides information on graphene morphol- or after acetone removal of PMMA (PE only), and subsequently ogy, we show that annealing in purely H2/Ar atmosphere after each annealing step. The ME graphene was character- strongly dopes and strains graphene in addition to leaving be- ized after exfoliation, after acetone removal of PMMA (PE hind substantial polymeric residue, which may be removed in only) and after each annealing step. Typically, all were mea- an oxygen atmosphere, to the detriment of the graphene lat- surements taken from the same growth or flakes to avoid tice. We show that vacuum annealing largely removes this the effects from spatial heterogeneity. Raman maps were col- residue with relatively minor doping and strain of the graph- lected using a confocal WiTEC Raman spectrometer with a ene. We then illustrate competing annealing-induced physi- 532 nm laser excitation source and pinhole for additional res- cal processes such as interfacial water removal, increased olution. Both CVD–ME graphene were mapped from 5 · 5to corrugation of graphene, incomplete removal of polymer 20 · 20 lm2 areas with 0.5 lm step size. The laser power was and graphene deformation which generate doping and strain kept low to prevent sample burning and to avoid heating in- levels in graphene. These results are of importance for re- duced variation in the Raman spectra [22,23]. The D, G, and search on graphene devices where the electronic and G0 bands were extracted from the maps and fit to a Lorentzian mechanical performance of the graphene is critical. profile, where the peak parameters discussed below come from the Lorentzian fits. 2. Experimental section 2.3. Atomic force microscopy 2.1. Graphene growth/exfoliation and transfer Additional sets of CVD and mechanically exfoliated graphene The CVD graphene samples employed in this work were were prepared as above specifically for AFM scanning. Raman grown at atmospheric pressure on 25 lm thick Cu foils (Alfa point spectra were taken at strategic locations post-transfer Aesar, 99.999% purity) in a quartz tube furnace at 1000 °C with or post-PMMA removal (CVD only) to quickly ascertain mono- 1000 sccm of Argon, 30–50 sccm of H2 and 10 sccm of CH4 layer regions or flakes. AFM measurements were then taken flowing during growth.