The Influence of Thermal Annealing to Remove Polymeric

CARBON xxx (2013) xxx– xxx

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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 measurements provides a comprehensive understanding of those annealing-based processes which create morphological changes and directly inﬂuence doping and strain in the graphene layer, such as removal of polymer, removal of the interfacial graphene–substrate water layer, environmental doping effects and deformation of the graphene layer. 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 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 elucidate 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 reduced 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. The graphene transfer process was in ambient conditions and subsequently after each annealing as follows. Each sample was cut in half, where the first half step. AFM was carried out in non-contact mode and care was (PN) was placed directly in citric acid etchant (Transene), taken to avoid tip–sample interactions caused by the free and the second half (PE) was spin-coated with a 495,000 amplitude of the tip [24,25]. Scans were performed in a low humidity environment to mitigate tip response dampening

caused by a tip–substrate interfacial water layer. Step heights respectively, subjected to either vacuum annealing (Table 1 and root-mean-square roughness, (r), were carefully ex- and 2 top) or H2/Ar annealing and subsequent oxygen anneal- tracted from regions free of wrinkles, impurities or residues. ing (Table 1 and 2 bottom). The indicated mean values (and

standard deviations) of the frequencies (xG0, xG, xD), line- 3. Results and discussion widths (CG0, CG, CD), and intensity ratios (IG0/IG, ID/IG) were ob- tained from Raman spatial maps. A rapid qualitative assessment of the effects of thermal We ﬁrst focus on a comparison of the as-transferred values annealing can be accomplished by inspecting the Raman of all graphene to establish baseline doping and strain charac- 0 maps of the G band in Fig. 1. CVD-PN graphene (Fig. 1a) sub- teristics in the samples. The CVD graphene has broadened CG0 0 jected to gas annealing shows a decrease in G band intensity and CG, upshifted xG0 and xG, and presence of a D band (and 0 after the H2/Ar anneal and an increase in areas of G intensity therefore an ID/IG) compared to its ME graphene counterparts. suppression after a subsequent oxygen anneal indicating the It is well known that broadened linewidths in CVD graphene formation of defects in the graphene lattice (see Fig. S1 for de- are attributed to increased disorder [42] caused by grain tailed D, G, and G0 Raman maps). On the contrary, vacuum boundaries and other transfer-induced contaminations that annealing on separate CVD-PE graphene shows no such in- single crystal ME graphene, transferred under dry conditions, 0 crease in defects, evidenced by the largely unaltered G maps does not possess. Of note is that the upshift in xG0 is greater in imaged before and after the annealing step (Fig. 1b). the more disordered CVD samples (gas annealed ID/IG = 0.42 The Raman sensitive phonons corresponding to the D, G, and 0.13) than in the less disordered samples (vacuum an- 0 and G bands are sensitive to defects [26–29], electronic dop- nealed ID/IG = 0.04 and 0.06). Additionally, the broadened line- ing [30–34] and strain [34–39], amongst other effects. The G widths, in conjunction with the upshifts of xG0 and xG, (1580 cm1) and G0 (2400–2800 cm1) bands are the strongest indicate high levels of hole doping [33], which is attributed Raman peaks in crystalline graphene. The G band is due to a to a combination of increased water contact on CVD graphene

ﬁrst-order scattering process involving the E2g phonon at the stemming from the interfacial graphene–substrate water C point of the Brillioun zone. The G0 band is a second-order layer [43–45] following the wet transfer method and greater double-resonance process near the K point. The D (1200– high aspect-ratio ripples which may enhance the adsorption 1400 cm1) and D0 (1600–1630 cm1) bands arise from double of atmospheric hole dopants [43,46]. However, in all cases, resonance processes involving phonons near the K and C DxG0 (CVD xG0 –ME xG0 ) is around twice as great as DxG point, respectively, and require defects for activation [27,40]. (CVD xG ME xG). Typically, xG is more sensitive to doping

Defects, which include structural disorder such as grain than xG0 [30,43], where the DxG0 /DxG with respect to increas- boundaries or vacancies, edges, and functionalized groups, ing charge carrier density is around 0.5 [31,33].ADxG0/DxG 2 0 have the effect of increasing D and D peak intensity (ID, ID0), behavior is seen in monolayer ME graphene subjected to 0 0 decreasing G band intensity (IG0 ) and increasing (broadening) increasing strain [34], where the phonons involved in the G the G [27,29,32,41] and the G0 [27] full width at half-maximum, double resonance process are highly sensitive to strain [36] or linewidths (CG0, CG). Tables 1 and 2 present a summary of since their equivalent wave vectors are designated by the dis- Raman data for the G0, G and D modes for CVD–ME graphene, tance between the K points in the Brillioun zone (for this rea-

Fig. 1 – Optical images of CVD graphene transferred to SiO2 (a) without and (b) with PMMA. The graphene in (a) is wrinkled and torn without a robust polymer backing layer, but is predominantly monolayer. The sample in (b) is large-area monolayer graphene exceeding 1 · 1cm2 area. Images to the right are G0 band Raman maps taken before and after each annealing step. The third Raman map in (a) details the effects of visible lattice damage (white arrows) due to the oxygen anneal. (c) Repre- sentative monolayer graphene signal taken from CVD graphene transferred without polymer exposure. The scale bar in each image is 4 lm.

Table 1 – Mean values of linewidths (C), frequencies (x) and intensity ratios of the G0, G, and D bands in CVD graphene subjected to vacuum annealing (top) and gas annealing (bottom). Mean values were taken from 50 points on each sample. Corresponding errors are one standard deviation from the mean.

1 1 1 1 1 1 Process step Type CG0 (cm ) xG0 (cm ) CG (cm ) xG (cm ) CD (cm ) xD (cm ) IG0/IG ID/IG As transferred PE 30.3 ± 1.9 2679.2 ± 1.8 16.9 ± 1.0 1585.6 ± 1.0 70.5 ± 10.7 1351.3 ± 2.3 1.62 ± 16 0.06 ± 0.05 Vacuum annealed 31.6 ± 2.2 2679.4 ± 1.8 18.6 ± 1.6 1585.7 ± 1.4 72.6 ± 5.5 1345.3 ± 1.0 1.71 ± 0.15 0.09 ± 0.02 As transferred PN 29.2 ± 1.2 2676.9 ± 1.8 15.0 ± 1.0 1584.9 ± 0.8 58.1 ± 13.1 1343.3 ± 4.6 1.53 ± 0.04 0.04 ± 0.01 Vacuum annealed 32.5 ± 1.6 2681.9 ± 1.5 13.3 ± 1.1 1587.0 ± 1.0 56.2 ± 5.1 1351.2 ± 3.0 1.48 ± 0.17 0.10 ± 0.03 As transferred PE 30.6 ± 2.1 2680.3 ± 1.9 18.6 ± 1.1 1585.5 ± 1.2 53.1 ± 8.0 1345.0 ± 4.8 1.38 ± 0.26 0.419 ± 0.31 H2/Ar annealed 39.3 ± 1.8 2683.0 ± 1.1 27.8 ± 2.6 1590.4 ± 3.0 85.0 ± 6.5 1356.9 ± 5.5 0.9 ± 0.05 0.801 ± 0.03 Oxygen annealed 42.8 ± 1.5 2683.7 ± 1.1 19.9 ± 2.9 1598.5 ± 3.0 35.3 ± 4.8 1344.6 ± 2.7 1.17 ± 0.21 0.425 ± 0.35 As transferred PN 29.7 ± 1.8 2678.8 ± 1.0 17.3 ± 1.6 1584.5 ± 0.4 63.0 ± 8.6 1343.9 ± 3.5 1.35 ± 0.11 0.13 ± 0.03 H2/Ar annealed 44.3 ± 3.0 2685.4 ± 2.3 33.3 ± 3.2 1589.0 ± 0.8 78.4 ± 7.7 1359.8 ± 5.4 0.73 ± 0.05 0.83 ± 0.05 Oxygen annealed 39.6 ± 3.4 2680.8 ± 1.5 22.7 ± 1.9 1592.7 ± 2.0 34.5 ± 9.1 1346.2 ± 2.0 1.24 ± 0.17 0.16 ± 0.06

Table 2 – Mean values of linewidths (C), frequencies (x) and intensity ratios of the G0, G, and D bands in ME graphene subjected to vacuum annealing (top) and gas annealing (bottom). The D band was not present in most samples, even after thermal annealing. Mean values were taken from 25 monolayer to 25 bilayer samples. Corresponding errors are one standard deviation from the mean.

1 1 1 1 1 1 Process step Type CG0 (cm ) xG0 (cm ) CG (cm ) xG (cm ) CD (cm ) xD (cm ) IG0/IG ID/IG As transferred PE 24.6 ± 0.9 2671.2 ± 0.9 14.2 ± 1.4 1579.7 ± 0.8 22.61 ± 1.2 1351.3 ± 1.1 1.6 ± 0.14 0.53 ± 0.27 Vacuum annealed 24.1 ± 1.0 2671.8 ± 1.5 12.9 ± 2.4 1580.7 ± 1.0 22.41 ± 2.3 1345.3 ± 1.0 1.6 ± 0.15 0.52 ± 0.27 As transferred PN 24.3 ± 0.3 2665.9 ± 1.6 13.9 ± 0.4 1578.8 ± 0.8 N/A N/A 1.6 ± 0.03 N/A Vacuum annealed 29.0 ± 1.0 2669.5 ± 1.2 17.0 ± 0.8 1581.1 ± 0.4 N/A N/A 1.7 ± 0.03 N/A As transferred PE 24.6 ± 1.4 2668.1 ± 2.4 12.7 ± 2.4 1580.3 ± 1.4 N/A N/A 2.7 ± 0.39 N/A H2/Ar annealed 31.1 ± 2.9 2678.6 ± 4.1 11.3 ± 2.0 1589.4 ± 2.8 N/A N/A 0.9 ± 0.15 N/A Oxygen annealed 32.7 ± 3.0 2678.2 ± 4.5 11.6 ± 2.8 1591.2 ± 1.7 N/A N/A 1.0 ± 0.15 N/A As transferred PN 24.8 ± 1.4 2666.3 ± 3.4 10.0 ± 1.0 1580.8 ± 2.0 N/A N/A 2.3 ± 0.83 N/A H2/Ar annealed 33.1 ± 2.9 2676.3 ± 3.8 12.6 ± 1.3 1589.0 ± 2.7 N/A N/A 0.7 ± 0.17 N/A Oxygen annealed 30.1 ± 2.9 2671.9 ± 3.3 11.5 ± 1.1 1591.3 ± 1.7 N/A N/A 0.9 ± 0.21 N/A

son, xD is also highly sensitive to strain). In this case, how- defective than that subjected to vacuum annealing, illus- ever, the comparison is between CVD–ME graphene with dif- trated by the greater mean ID/IG ratio (0.04 vs 0.13 for PN fering levels of relative strain. A recent work by Bissett et al. and 0.06 vs 0.42 for PE) and lesser mean IG0/IG ratio (1.53 vs [38], compared the effects of strain on CVD–ME graphene 1.35 for PN and 1.62 vs 1.38 for PE). This greater defectiveness, and showed that, for increasing compressive strain until due to variation in graphene domain size, was not found to 0.21%, the G peak of CVD graphene is upshifted with respect significantly affect the annealing-induced shifts in peak to that of ME graphene, with the G0 band showing a similar 2· parameters. We find that the as-transferred values of CVD- increase with increasing compressive strain for both CVD–ME PE xG0 and, xG are slightly upshifted, just outside one stan- graphene [38]. This difference in strain between CVD–ME dard deviation, indicating a relative increase in hole doping, unexposed to PMMA may arise from the wrinkle-inducing with respect to the CVD-PN xG0 andxG values. This is consis- post-transfer water evaporation in CVD graphene. Table 1 fur- tent with a previous study by Pirkle et al., where FET measure- ther demonstrates the similarities of the as-transferred val- ments showed that exposure to PMMA induced hole doping in ues of xG0, xG, CG0, and CG to within one standard deviation, CVD graphene [17]. We also find this behavior occurring in the between the CVD graphene subjected to gas annealing and ME graphene in Table 2, where the ME-PE G and G0 bands are the CVD graphene subjected to vacuum annealing. These re- upshifted with respect to the ME-PN values, outside one stan- sults are a measure of material homogeneity between various dard deviation, further confirming that PMMA is a source of locations of the pristine monolayer graphene. As mentioned hole doping in graphene transferred to a SiO2 substrate. above, all defects such as vacancies, grain boundaries and We now examine Tables 1 (top) and 2 (top), respectively, transfer contaminations would activate the D band, ensuring which detail Raman peak parameter changes in CVD–ME a baseline D band value. Given this baseline and despite the graphene due to vacuum annealing. It is immediately appar- 0 homogeneity of the G and G peaks between samples, the ent that the mean CG0, CG, xG0 and xG post-anneal shifts from as-transferred graphene subjected to gas annealing is more the CVD-PE and ME-PE graphene are negligible, being within

one standard deviation, whereas the mean post-anneal shifts as-transferred values and largely within variation of spectral from the CVD-PN and ME-PN are more signiﬁcant. To further values. We discuss several processes that could account for investigate this, we study the Raman spectra from represen- this effect. Initially, the removal of PMMA would reduce hole tative CVD-PE, CVD-PN, ME-PE and ME-PN graphene in doping, but uncover sites where atmospheric H2OorO2 could Fig. 2. Post-anneal G0 band alteration of both CVD–ME poly- adsorb and then increase hole doping [43,47]. Next, any ther- mer exposed graphene is negligible while that of the PN mal annealing process (including gas annealing) removes the graphene is considerably broadened and stiffened (CVD interfacial water layer between the graphene and substrate, 1 1 1 DCG0 = 5.0 cm , DxG0 = 6.7 cm and ME DCG0 = 3.9 cm , decreasing H2O-assisted hole doping [44,45] and thus brings 1 DxG0 = 3.4 cm ) The G band of the CVD-PE graphene broadens the graphene layer closer to the substrate, increasing n-type (+1.3 cm1) and stiffens (+1.8 cm1) slightly, while that of the charge injection from the substrate [48]. As the graphene 1 ME-P graphene sharpens (2.1 cm ) and stiffens insigniﬁ- layer conformably contacts the corrugated SiO2, the distortion cantly (+0.5 cm1). The G band slightly sharpens for the of the lattice would relax short range C–C bonds and allow in- CVD-PN graphene (0.6 cm1) and broadens for the ME-PN creased chemical reactivity [49], especially in the grain graphene (+2.4 cm1) with stiffening in both cases (CVD boundaries [50], to adsorbates such as atmospheric hole do- 1 1 DxG = 3.9 cm and ME DxG = 1.6 cm ). pants. Overall, the net effect would be to maintain charge Vacuum annealing of PE graphene is expected to desorb doping in the graphene layer as we discern above. Any minor

PMMA residues [17,18] and other atmospheric contaminants. strain effects from the conformal SiO2 contact in CVD-PE 0 This would result in a softening of xG0 and xG as hole dopants graphene (stiffened G and softened G with compressive are removed, which would be observed if the Raman spectra strain) [38] would then be overwhelmed by the shifts from of the samples were measured in vacuum [17,47].However, doping. upon exposure to atmospheric conditions after the vacuum AFM measurements conﬁrm many of these processes annealing, the G and G0 bands (i.e., post-annealing values) (Fig. 3). PMMA is clearly burned away as evidenced by the de- would stiffen compared to pre-annealing values [47]. This creased step heights and r values of the CVD-PE graphene change is not clearly observed in our experiments from the post-anneal (Fig. 3a). We also conﬁrm the thermal annealing post-anneal values; the G band positions, especially being removal of the interfacial water layer from all CVD graphene more sensitive to charge doping, are almost the same as the by the AFM step height reduction in a CVD-PN graphene sam-

Fig. 2 – Raman spectra of (a) pristine CVD graphene and (b) ME monolayer graphene taken (1) before and (2) after vacuum annealing. Graphs to right are magniﬁcations of the G and G0 peaks clearly showing peak position and intensity shifts. Here the solid lines correspond to (1) and dotted lines to (2).

Fig. 3 – Non-contact mode AFM topographic images and line scans of (a) CVD-PE graphene, (b) CVD-PN graphene, and (c) ME- PN graphene taken (1) before and (2) after vacuum annealing. The boxes in the left-hand side images show regions where roughness (r) measurements were taken. In (a), averaged line scans show a decrease in step height from 2.87 to 0.53 nm as polymeric residue is removed. This effect is noticeable in the 3-D image on the right as rCVD-PE decreases from 0.50 to 0.29 nm. The annealing effect on the step height is seen in the line scans of (b) as CVD-PN graphene height decreases from 0.77 to 0.34 nm. The 3-D images on the right detail the effect of graphene conforming to the substrate. In the location marked r r by the arrow, the graphene is largely indistinguishable from the substrate. The SiO2 and CVD-PN is 0.64 and 1.12 nm, respectively in (1) and 0.16 and 0.24 nm, respectively in (2). Line scans and images in (c) illustrate the annealing effect of conformal contact with the substrate in ME-PN graphene. The AFM detector is unable to clearly distinguish graphene and r r substrate, and post-annealing SiO2 and ME-PN are 0.25 and 0.27 nm, respectively. These scans corroborate that as-transferred step heights are larger for CVD-PN graphene than for ME-PN graphene due to the larger interfacial graphene–substrate layer in CVD samples from water exposure during the transfer process. Because ME graphene does not have an interfacial layer to this extent, it is able to conform dramatically to the substrate upon annealing. The dotted lines in the 2-D images in (b) and (c) are a guide showing the edge of the graphene layer post vacuum anneal. The scale bars in the 2-D images in (a), (b), and (c) are 1 lm, 1 lm, and 625 nm, respectively. The 3-D scans are all 2.5 · 2.5 lm2. ple (Fig. 3b), which has no polymer residue to mask the true after the thermal cycling, its effects would be neutralized in height values. AFM scans of ME-PN graphene further corrob- the Raman spectra from the inﬂuence of charge doping as orate exceptional conformal contact with the corrugated mentioned above or from smaller localized deformation since

SiO2 substrate (Fig. 3c); here the graphene is indistinguishable the PE samples never quite reach the conformal contact levels from the substrate except at the sample edge and surface of PN graphene (Fig. 3a vs b). We also note that IG0/IG increased roughness values match that of the underlying SiO2. This con- and ID/IG decreased for all polymer exposed graphene consis- tact was more pronounced in ME-PN graphene due to the tent with the removal of polymeric functional groups [27,51], smaller interfacial water layer and less transfer-induced com- however, the increase in CD of the CVD-PE graphene indicates pressive strain (see previous section) than CVD-PN graphene, a general increase in lattice disorder upon annealing. A more although it can be seen in some regions of the CVD-PN AFM detailed study of the effect of thermal annealing on sheet dis- scan (Fig. 3b). order needs to be performed in the future to elucidate the The important processes affecting the Raman spectra in cause of this width increase. CVD-PN and ME-PN graphene are thus removal and re- We now examine Tables 1 (bottom) 2 (bottom), respec- adsorption of surface contaminants, removal of the interfa- tively, which detail Raman peak parameter changes in CVD– cial water layer, increased reactivity of graphene due to closer ME graphene due to H2/Ar annealing and subsequent oxygen conformal contact to, and minor charge injection from the annealing. Of immediate note is the drastic broadening and substrate. The stiffening of the G peak and decrease of IG0/IG substantial stiffening of all CVD Raman peaks after H2/Ar in the CVD-PN graphene are consistent with an increase in annealing. Similar effects are seen in ME graphene, with dras- hole doping [33], although the greater increase of G0 peak po- tic peak stiffening and significant peak broadenings (except sition signifies the effect of approximately 0.1% compressive the mean ME-PE G band sharpening by 1.4 cm1). The repre- strain [38]. Here, the G peak position stiffening from hole dop- sentative spectra (Fig. 4a) of CVD graphene show a broad fluo- ing would be in competition from the softening which would rescent signal from 1200 to 1650 cm1 superimposed upon the 0 occur under this compressive strain. The DxG0 /DxG in the ME- greatly broadened D and G peaks and the suppressed G peak

PN graphene is also indicative of compressive strain from after the H2/Ar anneal for both CVD-PE and CVD-PN graph- thermal cycling (0.05%) [34,38]. However, if this compressive ene. This signal may evidently be attributed purely to the strain also exists in the polymer exposed CVD or ME graphene existence of amorphous carbon (mixture of sp2- and sp3-

(a)CVD - PE (b) ME - PE CVD - PN ME - PN

2 Edge Δ = 0.91 contamination 3 Δ = 1.46 CVD-PE

Fig. 4 – Raman spectra of (a) pristine CVD graphene and (b) ME monolayer graphene taken (1) before H2/Ar annealing, (2) after

H2/Ar annealing and (3) after oxygen annealing. AFM images and line scans (c) of CVD-PE graphene showing increase in step height to 4.51 nm caused by scission of PMMA after (2). Residue removal occurs during (3), bringing the step height down to

0.91 nm. The scale bar is 1.5 lm. AFM images (d) of ME-PE and CVD-PE graphene (1) before and (2) after H2/Ar annealing showing edge contamination only on the ME graphene, but a layer of residue atop the CVD graphene. The images are · l 2 r r r 2.5 2.5 m .The measurements were taken from the boxed areas. SiO2 and ME-PE is 0.18 and 0.22 nm, respectively in (1) r r and could not be measured post anneal due to scanning artifacts from edge contamination. SiO2 and CVD-PE is 0.32 and 0.48 nm, respectively in (1) and 0.36 and 2.56 nm, respectively in (2). bonded carbon) from the scission of PMMA [21,52–54] which is removal, environmental exposure, and/or conformal contact not largely removed as in vacuum annealing. Indeed, Lin et al. with the substrate. AFM scans show residue attached pre- [11] report that a similar signal arises when PMMA-trans- dominantly to the more reactive edges of the ME graphene ferred graphene is suspended, and is dampened after several ﬁlm (Fig. 4d) (not seen in scans of graphene subjected to vac-

H2/Ar and oxygen anneals, as the PMMA is removed. We agree uum annealing in Fig. 3c), which we believe is a partial cause that the signal is partially due to the existence of amorphous of strain fields (discussed below) within the graphene. This is carbon as evidenced by the drastic broadening of the D peak unlike the residue spread over the polycrystalline CVD graph- and increase of the CVD ID/IG signal to 0.80 [42,55–57], fur- ene (Fig. 4d) which contains more reactive sites such as ther noting the presence of this residue in AFM scans strained grain boundaries [50]. Thus, there is supporting evi- (Fig. 4c) of CVD-PE graphene. However, here we see this signal dence here that pre-existing defects allow or enhance sp3 arise in graphene transferred both with and without PMMA functionalization; however, research on this defect-enhanced post-H2/Ar anneal. Therefore, we attribute the fluorescence functionalization and the cause of the fluorescence in the ME and increased disorder to the functionalization of graphene graphene is still ongoing. 3 with H2 and other contaminants forming sp sites during Subsequently, upon oxygen annealing, we find that the the annealing process [42,58]. This sp3 functionalization is fluorescent signal largely disappeared in CVD graphene known to superimpose a strong fluorescent peak in carbon (Fig. 4a) and IG0/IG returned to its original values. However, with high hydrogen content [57,59]. Interestingly, there is a the samples are now highly hole-doped as evidenced by the weaker fluorescent signal and suppressed G0 peak in ME G0 and G band stiffening compared to the as-transferred posi- graphene subjected to H2/Ar anneal (Fig. 4b), but no D band tions. Interestingly, the CVD-PE CD and xD largely returned to activation as would occur if hydrogen were creating disor- the as transferred values of 24 cm1 and 1345 cm1 with 3 0 dered sp sites. However, from Fig. 4, the strong G peak stiff- ID/IG also returning to very close to as transferred values 1 1 ening (ME-PE DxG0 = 12.3 cm and ME-PN DxG0 = 13.0 cm ) (0.14 vs pre-annealed 0.16 for CVD-PE and 0.32 vs pre-an- 0 1 and G peak broadening (ME-PE DCG0 = 8.0 cm , ME-PN nealed 0.16 for CVD-PN), pointing to removal of both poly- 1 3 DCG0 = 6.5 cm ) coupled with the visible IG0/IG reduction indi- meric residue and hydrogen-functionalized sp sites cate that the H2/Ar anneal strained [34,38] the graphene, over- [12,43,58]. The ME graphene did not display this reversibility whelming any doping effects from PMMA or contamination (Fig. 4b). Although the fluorescent signal was slightly damp-

(b) P12 P22

(a) P11 P21

Fig. 5 – Raman spectra and AFM scans of bilayer ME graphene subjected to gas annealing. (a) Raman spectra taken (1) before 0 H2/Ar annealing, (2) after H2/Ar annealing, and (3) after oxygen annealing. (b) Zoom-in of G band in (a-1) showing 4 possible 0 double resonance phonons. Inset is of broad, single peaked G band post H2/Ar anneal. (c) AFM scans of multilayer graphene

taken (1) before H2/Ar annealing, (2) after H2/Ar annealing, and (3) after oxygen annealing. The arrows correspond to where r was measured over a 250 nm2 box (not shown). Black arrow is on monolayer graphene, blue arrow is on bilayer graphene, r r r yellow arrow on SiO2 substrate. The SiO2, monolayer, and bilayer is 0.31, 0.14, and 0.23 nm, respectively in (1) and 0.32 nm, 0.32, and 0.27 nm, respectively in (3), showing close conformal contact of the monolayer to the corrugated substrate. As in Fig. 3c, the monolayer region cannot be distinguished from the surrounding substrate. The r measurements were not taken in (2) because of the scanning artifacts from edge contamination. The 3-D scans are 2.5 · 2.5 lm2. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

ened, the IG0/IG values remained near the post H2/Ar anneal from the contamination. After the oxygen anneal, the poly- value of 0.9 (compared to the higher pre-anneal values of meric contamination is removed, but the strain signal re- 2.8 for CVD-PE and 1.6 for CVD-PN). mains. At this point, AFM measurements show that the We now show that the spectral changes in ME graphene monolayer region was highly conformed to the substrate upon H2/Ar anneal are a strain induced effect, which remain as to be indistinguishable (Fig. 5c, highlighted) from it. In- r r after the oxygen anneal. In bilayer ME graphene (Fig. 5), the deed, both SiO2 and monolayer is 0.32 nm, whereas the bi- G0 peak may be ﬁtted using four Lorentzian peaks, corre- layer is smoother, with r of 0.27 nm. The weaker attraction sponding to four double resonance processes, or phonons [64] between the bottom and top graphene layers in bilayer

(P11,P22,P12,P21 in Fig. 5b). However, after H2/Ar annealing, in conjunction with the interaction caused by the strong the asymmetrical, multi-peaked G0 line shape typical of AB conformal contact from bottom layer pinning to the sub- stacked bilayer disappears to be replaced by a slightly asym- strate is a strong indicator of unequal strain fields between metric, single peaked band (Fig. 5b, inset), more typical of a the layers. Thus, in bilayer graphene, the G0 band would dis- monolayer or rotated bilayer [60,61]. It has to be noted that play a misoriented peak shape, especially if the underlying there is no accompanying presence of a D band, indicating graphene layer conformally contacted the substrate during 3 that there are no defects or adsorbed sp sites, which may the H2/Ar anneal. The second thermal anneal would have contribute to the signal. We attribute the appearance of the effect of further pinning the underlying graphene layer asymmetric band after H2/Ar annealing to unequal strain (or single monolayer) to the substrate, thus the strain signal fields between the two graphene layers [37], which would remains subsequent to oxygen annealing. break the inversion symmetry of the lattice [62,63] and mis- orient the stacking between the layers. In Fig. 5c, H2/Ar an- 4. Conclusions neal-induced edge contamination is visible in an AFM scan of bilayer graphene with the bottom monolayer region We have systematically investigated the effect of both gas and extending outward from beneath it. The strain signal first vacuum annealing on the removal of polymer from CVD–ME arises after this annealing, presumably due to the edge con- graphene and the subsequent electronic and morphological tamination, however, we were unable to take accurate step changes induced in the graphene. Data from Raman spectros- height or roughness measurements due to scan artifacts copy and AFM, combined with previous electrical character-

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