Article pubs.acs.org/cm
Direct Observation of Poly(Methyl Methacrylate) Removal from a Graphene Surface Xiaohan Wang,¶ Andrei Dolocan,¶ Harry Chou,¶ Li Tao,‡ Andrew Dick,§ Deji Akinwande,‡ and C. Grant Willson*,¶,§ ¶
Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, United States Department of Electrical and Computer Engineering, University of Texas at Austin, Austin, Texas 78758, United States § McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, United States ‡
S Supporting Information *
ABSTRACT: Poly(methyl methacrylate) (PMMA) is commonly used as a temporary support layer for chemical vapor deposition (CVD) graphene transfer; it is then removed by a chemical or thermal treatment. Regardless of the method used for PMMA removal, polymer residues are left on the graphene surface, which alter its intrinsic properties. A method based on isotope labeling of PMMA and time-of-flight secondary ion mass spectrometry (ToF-SIMS) has now been developed to identify, locate, and quantify these residues. It is shown that vacuum annealing does not completely remove the PMMA residues but, instead, transforms them into amorphous carbon. In contrast, air annealing under optimized conditions generates a PMMA-free surface with limited damage to the graphene structure. This cleaned graphene surface demonstrates low friction which is comparable with that of pristine graphene film.
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INTRODUCTION Graphene, a two-dimensional honeycomb lattice of carbon, holds great promise for applications in electronics, filtration, and surface coatings. Unlike three-dimensional materials, all of the atoms in graphene are at the surface, rendering its intrinsic properties susceptible to the environment. For example, the transport and wetting behavior of graphene can be dramatically altered by the airborne molecules.1,2 This offers a design for sensors, but it poses a challenge to the study of intrinsic electrical and surface properties and impedes the applications of graphene in some fields.3 To exploit the intriguing features of graphene, a detailed understanding of the surface contaminants and their influence on graphene is required. This work reports a systematic study of poly(methyl methacrylate) (PMMA) residues on a graphene surface. As a temporary support layer, PMMA is used to protect graphene from tearing or cracking during film transfer.4 It is then removed by dissolution in a solvent such as acetone, chloroform, or acetic acid.5,6 Regardless of the solvent used, a thin (1−2 nm) layer of PMMA residue is believed to remain on the graphene surface, isolating the film from its environment7 and degrading the properties of the transferred graphene.8 Although new materials and processes have been reported to produce clean graphene transfer,9−11 PMMA is still the most commonly used because of its ready availability, desirable material properties, and ease of processing.12 Thus, we choose to explore achievement of clean transfer from this very popular process. © 2017 American Chemical Society
Much effort has been spent on removing the residues which are presumed to be PMMA. Thermal annealing at 300−500 °C under a high vacuum or reactive (H2 + Ar or O2) environment was found to be the most efficient method for eliminating them.5,8,13,14 The improvement in surface cleanness is usually demonstrated by TEM, AFM, XPS, Raman spectroscopy, and electrical measurements. However, none of these techniques possess the chemical sensitivity and selectivity required to unambiguously identify PMMA. In a recent study based on sum-frequency vibrational spectroscopy, the surface contaminants were found to be mostly alkane-based, rather than the previously indicated residues of PMMA.15 Such discrepancies demand a revisit to the surface cleaning strategies in a more chemically sensitive way. In this work, deuterium was used to chemically label PMMA (deuterated PMMA (D-PMMA)). By following the unique mass signature of deuterium, it was possible to identify, locate, and quantify the PMMA film and residues throughout the transfer process. After dissolution in acetone, a submonolayer of PMMA residue was observed on the resulting graphene surface. Those residues were transformed into amorphous carbon upon vacuum annealing, while air annealing was able to completely remove them. By optimizing the temperature and duration of the air annealing, a PMMA-free surface was Received: September 19, 2016 Revised: January 17, 2017 Published: January 26, 2017 2033
DOI: 10.1021/acs.chemmater.6b03875 Chem. Mater. 2017, 29, 2033−2039
Article
Chemistry of Materials
Figure 1. Schematic illustration of D-PMMA-assisted transfer of graphene. The inset shows the molecular structure of D-PMMA.
Figure 2. PMMA removal from graphene surface in an acetone bath. (a) Raman spectrum before and after acetone treatment. In addition to the graphene bands (G and 2D, top panel), D-PMMA bands were observed between 2000 and 2300 cm−1 before the treatment (bottom panel). (b) Positions of the G band and the 2D band, and the 2D/G height ratio as a function of acetone bath duration. (c) ToF-SIMS depth profiles of 2H−, C3−, and SiO2−, representing D-PMMA residues, full-monolayer graphene, and SiO2 substrate, respectively (normalized to the maximum signal of each species). (d, e) ToF-SIMS secondary ion maps (acquired in high lateral resolution) of C3− and 2H−, which correspond to a submonolayer graphene film and surface D-PMMA residues, respectively. changing the gas flow rate. CH4, 5 sccm (10 min) or 0.5 sccm (1 min), was introduced to obtain full-monolayer (Figure S1a) or submonolayer (Figure S1b) graphene film, respectively. The system was then cooled to room temperature. Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS). For chemical analysis and depth profiling, we employed a TOF.SIMS 5 (ION-TOF GmbH, Germany, 2010) instrument that provided a pulsed, Bi3+ cluster, analysis ion beam (30 keV ion energy, 20 ns pulse duration, 0.8 pA measured sample current) and a Cs+ sputtering ion beam (500 eV ion energy, ∼40 nA measured sample current). During depth profiling, the sputtering beam raster scanned over a 300 × 300 μm2 area while the analysis beam raster scanned over a 100 × 100 μm2 area centered within the regressing sputtered area.
achieved with minimized damage to the underlying graphene structure. This resulted in a low-friction graphene surface which is comparable with that of pristine chemical vapor deposition (CVD) graphene on copper substrate.
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EXPERIMENTAL SECTION
LPCVD Growth of Graphene. Monolayer graphene was produced by low-pressure chemical vapor deposition (LPCVD) as reported by Li et al.16 A 25 μm thick Cu foil (Alfa Aesar, item no. 13382) was loaded into a tube furnace and heated to 1030 °C in H2 with a flow rate of 5 sccm and pressure (Pfurnace) of 4.9 × 10−2 mbar. After reaching 1030 °C, the sample was annealed for 30 min without 2034
DOI: 10.1021/acs.chemmater.6b03875 Chem. Mater. 2017, 29, 2033−2039
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Figure 3. (a) Depth profiles of 2H− in a transferred graphene film before and after different thermal treatments. These profiles are divided into three regions based on corresponding major component as shown at the top of the figure. (b) Integral of 2H− intensity within each depth region before and after different thermal treatments. The Bi3+ cluster analysis beam was chosen to enhance the secondary ion signal of the organic fragments. The data was acquired at a 10−9 Torr base pressure while all collected secondary ions had negative polarity. Due to the insulating properties of the SiO2 substrate, a constant current electron beam (21 eV electron energy) was directed onto the sample surface during the analysis. The mass resolution was >3000 (m/δm) for all fragments of interest. Friction Measurement. The AFM used for lateral force measurement was a Park Scientific model, CP Research, with a Bruker (part number MSCT) microlever A cantilever and a sharpened SiN tip. Scan speed was 1 Hz, with normal force of 1 nN. To eliminate the effect of tip contamination/damaging, aluminum oxide was used as a reference before and after each scan.
acetone, the doping level of graphene dropped dramatically. If PMMA was the major dopant of graphene, then the majority of PMMA was removed from the graphene surface during this 3 h period. The doping level continued to decrease with longer acetone treatment time but leveled off after about 5 h. Exchanging the acetone (every 24 h) had a negligible effect. After acetone treatment, the graphene film was analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS), a technique that offers subnanometer surface sensitivity and ultrahigh chemical sensitivity.22 A Bi3+ cluster analysis beam was chosen to enhance the secondary ion signal of the organic fragments.22 To determine the graphene and D-PMMA distributions perpendicular to (that is, in depth) and within the graphene plane, we acquired depth profiles and high lateral resolution maps of C3− (representing graphene) and 2H− (representing D-PMMA; see the Supporting Information for details) signals.23 The higher order carbon cluster C3− was used as a marker for graphene in order to limit the residual signal from oxides and other adventitious organic fragments. Figure 2c shows the normalized intensity of different chemical species as a function of depth, where the sputtering time was converted into the depth by applying a sputtering-rate model that assumes the instantaneous sputtering rate at the graphene/SiO 2 interface to be a linear combination of the sputtering rates for the individual species (see the Supporting Information for details).22 The 2H− (red curve) is concentrated on the film surface, and its intensity rapidly drops when penetrating into the graphene structure (green curve). The surface D-PMMA appears thinner than a single atomic layer of graphene, consistent with a submonolayer of polymer residues randomly adsorbed on the graphene surface. In addition to the depth profiling, high lateral resolution (∼200 nm) mapping was employed to determine the graphene and D-PMMA in-plane distributions (Figure 2d,e). To better visualize the correlation of their distributions, a submonolayer graphene film (Figure S1b) was prepared and transferred onto the SiO2 substrate with D-PMMA. In this case, isolated graphene domains (orange color, Figure 2d) were observed in the C3− map. The domain shape and size is consistent with that documented before transfer (Figure S1b). The 2H− map of the same region (yellow color, Figure 2e) shows significantly higher deuterium intensity (roughly by a factor of 2) atop the graphene domains compared to the surrounding SiO2 substrate. This indicates a preferential adsorption of D-PMMA residues on the graphene surface
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RESULTS AND DISCUSSION A chemical vapor deposition (CVD) method was used to grow monolayer graphene on copper foils (Figures 1 and S1a).16 Graphene quality was evaluated by SEM and Raman spectroscopy. In order to protect graphene from tearing or cracking in the following transfer process, fully deuterated PMMA (inset of Figure 1, Polymer Source Inc., P2674-dPMMA by anionic polymerization) was dissolved in chlorobenzene and spincoated on top of the CVD graphene. Graphene was separated from the copper surface by electrochemical delamination.17 This produced isolated PMMA/graphene films, which were then rinsed with deionized water and transferred onto SiO2/Si substrates for analysis. The Raman spectrum of the delaminated D-PMMA/ graphene film (Figure 2a, black curve) shows clear graphene bands (G and 2D) and no detectable D band (∼1350 cm−1). This indicates that there are very few structural defects in graphene.18 A series of nongraphene bands were observed between 2000 and 2300 cm−1. They are assigned to the carbondeuterium stretching vibrations in D-PMMA,19,20 and their positions match well with corresponding infrared (IR) absorption frequencies of the pure polymer (Figure S2). Upon acetone treatment, these D-PMMA bands gradually weakened and disappeared (Figure 2a, red curve), suggesting that the polymer was removed. The graphene bands (G and 2D), on the other hand, remained but shifted to lower wavenumbers after the acetone treatment. This is primarily due to a decrease in surface charges contributed by the D-PMMA, which manifests as a suppression of p-type doping in graphene.8 By monitoring the band shifts and changes in the 2D/G height ratio (I2D/IG),21 the doping evolution was studied as a function of time (Figure 2b). It was found that, within the first 3 h in 2035
DOI: 10.1021/acs.chemmater.6b03875 Chem. Mater. 2017, 29, 2033−2039
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Figure 4. (a) Raman spectra on a transferred graphene before and after different thermal treatments. (b) Thermally induced shift of G (ωG) and 2D (ω2D) band positions. The data points correspond to Raman spectra obtained by raster-scanning the laser spot within a 20 × 20 μm2 graphene film before and after thermal annealing.
after a 4 min air exposure at 450 °C (red curve). Its intensity is nearly constant at any depth and is very close to the 2H− intensity in SiO2 bulk. This suggests production of a graphene film free of deuterium-related fragments and therefore DPMMA residues. The samples were further analyzed by Raman spectroscopy in order to better understand the behavior of graphene and DPMMA residues in thermal annealing. Figure 4a shows a series of Raman spectra collected at the same spot on a graphene film before and after different thermal treatments. Broad D and G bands appeared after vacuum annealing at 350 °C or above. These features are consistent with sp2-rich amorphous carbon (a-C)26 that is believed to be derived from decomposition of PMMA residues.14,27 Specifically, PMMA could undergo sidegroup (methoxycarbonyl) elimination upon vacuum annealing and form a series of conjugated CC double bonds.28 The PMMA-derived a-C was quantified on the basis of normalized D band area (relative to that of the silicon band, Figure S4), and its amount was found to increase as a function of annealing temperature. Longer annealing in the UHV system could not completely remove the amorphous carbon, but if air was introduced, the D band decreased significantly (red curve in Figure 4a). Air annealing at high temperatures also oxidizes the underlying graphene.29 To minimize this effect, a series of annealing temperatures and durations were tested, and 450 °C for 4 min was found to be optimal. This condition removes the amorphous carbon but limits damage to the underlying graphene film. This is attributed to the fact that graphene has a higher resistance to oxygen etching compared to the organic residues.30 A similar trend was observed by directly annealing transferred graphene in air (Figure S5). In this case, a less intense D band appeared at 350 °C, and it then disappeared upon heating to 450 °C. An additional ToF-SIMS experiment confirmed that the air-annealed-only graphene film was also free of D-PMMA residues. In addition to degrading the D-PMMA residues, thermal annealing also affects the doping level of graphene.5,8,27,29,31,32 To study this effect, we have collected hundreds of Raman spectra and monitored the positions of G (ωG) and 2D (ω2D) band along with the thermal treatment. The results are shown in Figure 4b, where each data point corresponds to a spectral average over 0.25 μm2. Before the thermal annealing, (ωG and ω2D) (black points) are averaged at 1588.8 ± 1.8 and 2680.0 ± 2.0. Both values are higher than those of suspended pristine
compared with that on silicon dioxide, which is likely a result of differences in substrate hydrophobicity, i.e., hydrophilic (SiO2) vs hydrophobic (graphene). As shown above, ToF-SIMS is able to identify and locate trace amounts of D-PMMA residues. Although it is not able to provide an absolute quantification of the residual D-PMMA, a relative quantification is possible between samples that contain the same supporting matrix (here, graphene/SiO2). Therefore, ToF-SIMS was further employed to study the removal of these residues from graphene surface. A full-monolayer graphene was first transferred with D-PMMA onto a SiO2/Si substrate. After acetone treatment, the sample was loaded into an ultrahigh vacuum (UHV, base pressure = 10−9 Torr) chamber and heated to temperatures sequentially from 350 to 600 °C. ToF-SIMS was used in situ after each step of the treatment to analyze for deuterium. The corresponding depth profiles of 2H− are shown in Figure 3a. Since the graphene and SiO2 profiles remain relatively constant throughout the thermal treatment (Figure S3), their data are not included but instead their positions are indicated at the top of the figure. Before vacuum annealing (black curve), the 2H− intensity is the highest on the surface, indicating adsorption of D-PMMA residues on top of the graphene. However, a deuterium signal was also detected at the depth of graphene or the graphene/SiO2 interface. This is likely due to the presence of graphene wrinkles or defects, where the D-PMMA can be trapped or even penetrate the graphene film. After 1 h of annealing at 350 °C (purple curve), the 2H− profile decreased slightly. This is not consistent with previous reports, where annealing at 300 °C or even lower is able to significantly remove PMMA.8,13 This discrepancy may be due to the different PMMA polymerization mechanism, i.e., free radical polymerization (previous work) vs anionic polymerization (this work). Reportedly, anionic PMMA has a much higher thermal stability and begins to degrade around 350 °C compared with the onset in free-radical PMMA which is 160 °C.24,25 After 1 h of annealing at 600 °C (green curve), the 2H− intensity above the graphene layer dropped by more than 1 order of magnitude, but at the depth of graphene or the graphene/SiO2 interface, the deuterium signal did not diminish significantly. This is evident in Figure 3b. It was found that vacuum annealing efficiently removes the residues above graphene but has limited influence on residues trapped in or below the graphene film. To solve this problem, air annealing was applied to the vacuumannealed sample. The 2H− signal was found to further decline 2036
DOI: 10.1021/acs.chemmater.6b03875 Chem. Mater. 2017, 29, 2033−2039
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Chemistry of Materials graphene (1581.6 ± 0.2, 2676.9 ± 0.7),32 and this is attributed to the p-type doping by D-PMMA residues.5,8 In addition to the charge doping, tensile strain is another factor that determines the (ωG, ω2D) of graphene.33−35 To separate these two factors, we have considered their distinct contribution to the shift of ωG and ω2D (ΔωG, Δω2D): (Δω2D/ΔωG)tensile strain = 2.2 vs (Δω2D/ΔωG)p‑type doping = 0.7.32 In the ωG−ω2D space, this means that the vector, (ΔωG, Δω2D), can be decomposed by unit vectors representing tensile strain and p-type doping, respectively.32 In this work, we only focus on the component of (ΔωG, Δω2D) along the direction of unit vector for doping. When vacuum annealing starts at 170 °C in UHV, the position of (ωG, ω2D) (purple points) is mostly unchanged, indicating that the doping level is unaffected. However, the data cluster shifted in the opposite direction of the doping vector after vacuum annealing at 350 °C (blue points). This suggests the suppression of p-type doping in graphene.32 Considering the onset of D-PMMA degradation at this temperature, we speculate that the resulting a-C has less doping effect compared with PMMA residues. This trend is consistent with the electrical measurement on thermally annealed graphene samples,5 where the Dirac point shifted toward 0 V (i.e., less doping) after vacuum annealing at 300 °C. Further increasing the annealing temperature (600 °C) caused a significant shift of (ωG, ω2D) (green points) along the direction of doping vector, which manifests as heavier p-type doping. Such doping is also observed on graphene devices annealed above 400 °C.5 This is likely due to a closer coupling between graphene and the SiO2 substrate; thus, enhanced charge transfer between graphene and ambient oxygen molecules is expected.31 Subsequent air annealing could remove the residual a-C as discussed before, but it continued to enhance the doping level of graphene indicated by the shift of (ωG, ω2D) point cluster (red points). Similar p-type doping was also observed after one-time air annealing at 450 °C (Figure S6a), the optimal condition for DPMMA removal. To eliminate the adverse effect of doping on carrier mobility,5,36 the graphene can be exposed to inert gases. As a result, the adsorbed dopants could detach,31 which restores the electrical properties of graphene.8,37 A low contact resistance was also obtained between the annealed graphene and metal (Figure S6b), which is promising for graphene-based nanoelectronics such as sensors, light emitting diodes, and solar cells. The frictional properties of graphene were studied after the thermal decomposition of D-PMMA residues. Figure 5 shows the difference in lateral tip deflection between a trace and retrace AFM scan on a transferred graphene film (see the Experimental Section for details), which can be loosely interpreted as a measure of the friction coefficient.38 Despite the acetone treatment, the transferred graphene exhibited surface friction close to that of a spin-coated D-PMMA reference. This suggests that the submonolayer of polymer residues played a dominant role in determining the frictional properties. When the graphene was annealed at 350 °C under vacuum, the surface friction was found to decrease by 50%. This can be attributed to better adhesion of graphene to the substrate upon annealing,5,31 which suppresses local film puckering around the sliding tip,39 and the transformation of PMMA to sp2-rich a-C, which is expected to have a lower friction coefficient than sp3 carbon.40 Upon vacuum annealing at higher temperatures, the friction was not reduced even at 600 °C. This is not consistent with the observation of continuing
Figure 5. Difference of lateral tip deflection between a trace and retrace AFM scan (which represents the surface friction) on a transferred graphene film. The film underwent a series of post-transfer treatments. The corresponding friction change is shown together with the friction of D-PMMA and as-grown CVD graphene as references.
loss of deuterium signal in ToF-SIMS (Figure 3) and calls for future work. When air annealing (450 °C, 4 min) was applied, the friction decreased again and approached that of clean CVD graphene on a copper substrate. This is likely due to the chemical changes rather than the suppression of the puckering effect. At this temperature, amorphous carbon will burn, but graphene is stable up to at least 500 °C.29
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CONCLUSIONS We have directly studied PMMA residues on a graphene surface by using deuterium isotope labeling and ToF-SIMS. This method provides spatially resolved chemical information at extremely high sensitivity. We found that UHV annealing does not completely remove the deuterium signals (presumed to be PMMA residues); instead, it transforms the residues into more stable amorphous carbon. Annealing in air further decomposes this structure and, under optimized conditions, a PMMA-free surface with minimized structural damage of underlying graphene film was achieved. This cleaned surface exhibits low friction, suitable for advanced coating and device applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03875. SEM of CVD graphene, IR spectra of D-PMMA before and after acetone treatment, ToF-SIMS data processing, ToF-SIMS depth profiles of transferred graphene upon thermal annealing, thermally induced D band evolution, air-annealed-only graphene, and one-time annealed graphene (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiaohan Wang: 0000-0003-2608-0718 Andrew Dick: 0000-0001-9116-0082 2037
DOI: 10.1021/acs.chemmater.6b03875 Chem. Mater. 2017, 29, 2033−2039
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Chemistry of Materials
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C. Grant Willson: 0000-0002-2072-3981 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Richard D. Piner, Prof. Yufeng Hao, Prof. Ji Won Suk, Prof. Shanshan Chen, and Prof. John Ekerdt for valuable discussions and manuscript revision. We thank Yuxuan Chen for providing the UHV annealing chamber. We appreciate funding support from NSF-NASCENT Engineering Research Center (Cooperative Agreement No. EEC-1160494). We also acknowledge the NSF grant DMR-0923096 used to purchase the ToF-SIMS instrument at Texas Materials Institute, UTA.
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