Substrate dependent photochemical oxidation of monolayer graphene

Abstract

In this report we show that the photochemical oxidation of monolayer graphene is strongly dependent on its underlying substrate. It was found that chemical vapor deposition-grown monolayer graphene transferred onto a silicon wafer is more easily oxidized compared to graphene that was left on the copper substrate or transferred onto a H₂-annealed copper foil. The differences in the degree of oxidation were tentatively attributed to the varying energies of adhesion between the graphene and the underlying substrate. Our result has significant implications for the outdoor use of graphene as a transparent conducting material.

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Introduction

Graphene, a two-dimensional nanomaterial consisting of carbon atoms arranged in a hexagonal pattern, has attracted significant attention as a result of its outstanding electronic and mechanical properties.¹ These properties have given graphene much potential for applications in photovoltaics,² field-effect transistors,³ energy storage,⁴ and sensing.⁵ Graphene's transparency and conductivity have made it an especially interesting material for photovoltaic applications, including as a replacement for the expensive and inflexible indium tin oxide (ITO).^6–8 Many of these applications, however, require that graphene be supported by an underlying substrate such as glass, silicon or mica. As a result of its atomic thinness, graphene is heavily influenced by its supporting substrate. For example, Lui et al. demonstrated that the formation of ripples in graphene sheets may be determined by the supporting substrate.⁹ Wang et al. showed that graphene's reactivity in electron-transfer reactions can be controlled by changing its underlying material.¹⁰ With techniques such as chemical-vapor-deposition (CVD), large-area graphene can now easily be transferred onto any suitable surface.¹¹ Herein we show that the degree to which chemical-vapor-deposition synthesized graphene oxidizes in an ultraviolet-ozone (UVO) environment is dependent on the substrate that supports it. These results could have practical implications in situations where graphene is to be exposed to ultraviolet radiation, such as in photovoltaics or outdoor displays. The selection of a suitable substrate may have significant consequences in determining the lifetime of a photovoltaic graphene device. Furthermore, understanding how the substrate effects graphene's reactivity is crucial to improving the understanding of graphene's interfacial chemistry.

Results and discussion

Graphene was synthesized on copper foil using a well-established low pressure chemical vapor deposition method using methane as the carbon precursor.¹² The synthesized graphene covered copper foil (Cu_CVD–G) was then allowed to cool to room temperature and subsequently cut into smaller pieces and transferred onto various substrates including: silicon wafer with 300 nm thermal oxide layer (300 nm-SiO₂–G), silicon wafer with a ca. 2 nm of native oxide layer (native-SiO₂–G), annealed copper foil (Cu_Trans–G), and mica (mica–G). The annealed copper foil was prepared following the same procedure used to synthesize graphene except without methane gas as a carbon source (i.e., annealed in a H₂ atmosphere at 1000 °C). Transferring of graphene from the original copper foil onto desired substrates was accomplished by coating the Cu_CVD–G with poly-methyl methylacrylate (PMMA) and subsequently etching the copper in FeCl₃ solution and transferring the resulting PMMA–graphene film onto the desired substrate. After the PMMA covered graphene was transferred onto the desired substrate, the PMMA layer was dissolved in acetone to yield graphene on the target substrate.¹¹ All experiments were carried out within one week of the initial CVD synthesis of graphene in order to minimize oxidative damage from extended air exposure.

The CVD procedure used here is known to produce monolayer graphene and upon inspection of the 300 nm-SiO₂–G sample with optical microscopy, it was noted that there were only two color contrasts present, one representing areas of graphene coverage and the other representing uncovered areas. Multiple layer graphene is expected to produce more than two color contrasts near holes and edges, corresponding to the varying number of layers present. Additionally, thickness of our graphene sample measured by AFM (Fig. S1†) was within the range of reported values of transferred CVD single layer graphene.¹³

Raman spectra were obtained on the 300 nm-SiO₂–G, native-SiO₂–G, and mica–G samples before any photochemical treatment. Along with an un-transferred pristine sample of Cu_CVD–G, the transferred samples were simultaneously placed in a UV-Ozone (UVO) chamber and treated with UVO for 30 minutes. After the treatment, the samples that were transferred onto copper foils were again transferred using the same transfer process onto silicon wafers. All samples were then characterized with Raman spectroscopy. The 300 nm-SiO₂–G, Cu_CVD–G, and Cu_Trans–G samples were also characterized with atomic force microscopy (AFM) and optical microscopy to observe the graphene film structure post-treatment. Additional experimental details are available in the ESI.†

After treatment for 30 minutes in UVO, Raman spectra were obtained for each sample. Fig. 1 shows the Raman spectra of 300 nm-SiO₂–G, Cu_CVD–G, and Cu_Trans–G samples. Fig. 1a shows the Raman spectrum of 300 nm-SiO₂–G before any treatment. The peak at ca. 1350 cm⁻¹ corresponds to the D peak, which is related to the defect density on the graphene surface. The G peak at ∼1580 cm⁻¹ corresponds to the in-plane sp² C=C stretching of the carbon atoms in the graphene lattice and its intensity can also be used to gauge the degree of oxidation present as the attachment of oxygen moieties results in sp³ hybridization of the carbon atoms in the graphene lattice. Lastly, the 2D peak at ∼2700 cm⁻¹ is an overtone of the D peak but does not require nearby defects to become Raman active as does the D peak.^14,15 We noticed a very weak D peak in the untreated 300 nm-SiO₂–G sample, which corresponds to a low defect density that is commonly observed in chemical vapor deposition synthesized graphene samples. Note that as the same batch of graphene was transferred onto the various substrates, the spectrum of the untreated 300 nm-SiO₂–G sample is representative of the graphene on all samples prior to treatment with UVO.

Fig. 1 The Raman spectrum of (a) 300 nm-SiO₂–G before any UV-ozone treatment, (b) 300 nm-SiO₂–G after 30 minutes of UV-ozone treatment, (c) Cu_CVD–G after treatment, (d) Cu_Trans–G after treatment. All samples were treated with UVO simultaneously and all Raman measurements were obtained under identical conditions. No scaling has been performed on the spectra. For (c) and (d), the samples were transferred onto Si wafers with 300 nm of thermal oxide after UVO oxidation and before Raman characterization.

Fig. 1b shows the 300 nm-SiO₂–G sample after 30 min of UVO oxidation. All three Raman peaks (D, G, and 2D) have significantly decreased in intensity with the 2D peak almost completely vanished (see Table 1 for 2D peak areas). This observation is consistent with significant oxidation resulting in the loss of sp² atomic structure of the graphene sheet. Fig. 1c shows the Raman spectrum of graphene that was left on its growth copper substrate (Cu_CVD–G) when placed in the UVO chamber for 30 minutes. Note that this graphene sample was transferred onto a silicon wafer with 300 nm of thermal oxide before Raman characterization and the same condition was used in collecting the spectra; as such, the Raman signal intensity can be directly compared with the 300 nm-SiO₂–G sample. From Fig. 1c it is clear that compared to the case of 300 nm-SiO₂–G sample, the Cu_CVD–G sample gave higher intensity in all peaks; a significant 2D peak was also observed, which was almost vanished in the treated 300 nm-SiO₂–G sample. Both facts indicate that the graphene that was left on its native copper foil suffered much less oxidation than the graphene that was transferred onto 300 nm thermal oxide silicon wafer prior to UVO treatment. Similarly, for graphene that was transferred onto a mica substrate prior to UVO treatment, we also observed severe oxidation similar to the case of 300 nm-SiO₂–G (Fig. S2†). The ratio of the area under the D peak to the area under the G peak (A_D/A_G) may also be used as a quantitative measure of the degree of oxidation.¹⁶ The calculated A_D/A_G ratios (Table 1) are consistent with a greater degree of oxidation on the graphene that was transferred onto either silicon wafer or mica prior to UVO treatment than the graphene that was left on its growth copper foil.

Table 1 The ratio of the area of the D peak to that of the G peak (A_D/A_G) and the area of the 2D peak (A_2D) for graphene samples before and after UVO oxidation. Note that the absolute intensity of the 2D peak depends on the substrate; therefore, the A_2D values for native-SiO₂–G and mica-G samples were not compared. Values shown are averages acquired from three Raman spectra. Also note that A_D/A_G alone cannot determine the degree of oxidation of these samples. See text for details

Sample	AD/AG	A2D
300 nm-SiO2–G before treatment	0.10 ± 0.04	(1.48 ± 0.15) × 10^5
300 nm-SiO2–G after treatment	1.40 ± 0.08	(4.7 ± 0.5) × 10^3
CuCVD–G after treatment	1.06 ± 0.12	(1.20 ± 0.08) × 10^5
CuTrans–G after treatment	1.51 ± 0.08	(2.7 ± 1.2) × 10^4
Native-SiO2–G after treatment	1.45 ± 0.32
Mica–G after treatment	1.50 ± 0.31

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Lastly, Fig. 1d is the Raman spectrum of a sample of graphene that was transferred onto an annealed copper foil prior to UVO treatment; after the UVO oxidation, it was transferred again onto a silicon wafer for Raman characterization. From this spectrum it is seen that the D peak is significant and the G and 2D peaks have decreased in intensity. This observation indicates a degree of oxidation that is greater than that observed in the Cu_CVD–G sample but less than that observed in the 300 nm-SiO₂–G sample. Importantly, this experiment demonstrates that the majority of the defects were formed during the UVO oxidation and not the transferring process, as the Cu_Trans–G has been transferred twice – once onto the copper foil prior to UVO treatment and once again onto silicon wafer post treatment – yet has a lesser degree of damage than the 300 nm-SiO₂–G sample which was only transferred once. It was noted that despite the peaks showing very little intensity in the case of 300 nm-SiO₂–G sample, the A_D/A_G ratio for 300 nm-SiO₂–G was lower than that of Cu_Trans–G. Although on the surface this observation may suggest a lesser degree of oxidation for the 300 nm-SiO₂–G sample compared with Cu_Trans–G, the severely decreased intensity of the G peak and the absence of the 2D peak clearly indicate that the 300 nm-SiO₂–G sample actually suffered greater structural damage. In fact, it is known that in the progression of graphene damage, the A_D/A_G ratio initially increases and subsequently decreases upon severe atomic-scale damage.^16,17

We also conducted a similar experiment in which X-ray photoelectron spectroscopy (XPS) was used to probe the oxygen and carbon content of graphene after it had undergone UVO treatment while supported on different substrates. Just as described earlier, graphene samples were prepared in which graphene was either left on its growth copper substrate (Cu_CVD–G), transferred onto annealed copper foil (Cu_Trans–G), or transferred onto silicon wafer with 300 nm thermal oxide layer (300 nm-SiO₂–G). The samples were put through identical UVO treatments as described before and the graphene that was supported by copper foils was transferred onto identically cleaned silicon wafers (with 300 nm of oxide layer) prior to XPS measurements. The C1s and O1s peaks of the treated samples are shown in Fig. 2. These XPS measurements are consistent with the results obtained from Raman spectroscopy. The graphene supported by silicon wafer undergoes the greatest amount of oxidation, as indicated by a large O1s peak and a small C1s peak, and the graphene that was left on its growth copper foil showed the least amount of oxidation. The graphene that had been transferred to an annealed copper foil undergoes an intermediate amount of oxidation.

Fig. 2 C1s and O1s peaks of graphene samples that had undergone UVO treatment while supported by either silicon wafer with 300 nm thermal oxide layer (300 nm-SiO₂–G), annealed copper foil (Cu_Trans–G), or growth copper substrate (Cu_CVD–G). Samples that were supported on metal foils were transferred to identically cleaned silicon wafers (with 300 nm of thermal oxide) prior to XPS measurements. Note that the O1s XPS peaks include contribution from the (same type of) underlying substrate.

As UVO oxidation is sensitive to the amount of ultraviolet light illuminating the samples, it is important to ensure that these observed results are not simply due to an optical phenomenon. In order to verify this, we performed an experiment in which we transferred CVD synthesized graphene onto both silicon wafer with native oxide and silicon wafer with 300 nm of thermal oxide. Both samples were then treated with UVO under identical conditions. It is known that the oxide layer of the silicon wafers could result in strong interference effects.¹⁸ Raman spectra were subsequently obtained on the treated samples as shown in Fig. 3. From these spectra, it is clear that in both cases of graphene supported by silicon with native oxide or thermal oxide, the D and G peaks significantly decreased and the 2D peak nearly vanished. These results suggest that the degree of oxidation on graphene supported by silicon wafers with varying oxide thicknesses was approximately the same, eliminating the possibility that the observed substrate dependent oxidation was due to optical phenomena. In addition, the fact that Cu_CVD–G and Cu_Trans–G samples suffered very different degrees of oxidation despite both being supported by copper foil rules out the possibility that this substrate dependence is strictly an optical phenomenon.

Fig. 3 Raman spectrum of 300 nm-SiO₂–G sample (a) before and (b) after 30 min of UVO oxidation, and Raman spectrum of native-SiO₂–G (c) before and (d) after 30 minutes UVO oxidation. Note: the peaks near 1554 cm⁻¹ and 2330 cm⁻¹ in (c) and (d) are due to atmospheric O₂ and N₂, respectively. These peaks are not visible in (a) and (b) because the Raman response of graphene is much stronger due to the interference enhancement effect of the substrate (300 nm thermal oxide Si wafer).

Whereas Raman spectroscopy and XPS probe the atomic-scale structure of graphene, we used AFM to examine how the substrate dependent oxidation altered the nano- to micron-scale structure of graphene. It is known that in mechanically exfoliated graphene, thermal oxidation preferentially occurs at defect sites resulting in resolvable pits and linear cracks.¹⁹ In the case of CVD synthesized monolayer graphene, however, it has been reported that photochemical oxidation is non-selective and results in uniform oxidation of the graphene sheet.¹⁴ Consistent with these previously reported results, we did not observe the formation of any pits or line cracks on the graphene sheet after 30 minutes of UVO oxidation. Fig. 4 shows AFM and optical images of Cu_CVD–G, 300 nm-SiO₂–G, and Cu_Trans–G samples after treatment in UVO for 30 minutes. Note that for Cu_CVD–G and Cu_Trans–G samples, the graphene was transferred to a silicon wafer after UVO oxidation and prior to the AFM analysis. The AFM images were taken near the graphene film edges to show contrast between the underlying silicon wafer and the graphene film.

Fig. 4 AFM (left) and optical (right) images of the graphene samples after UVO oxidation. (a) and (b) Cu_CVD–G sample, (c) and (d) Cu_Trans–G sample, and (e) and (f) 300 nm-SiO₂–G sample. Scale bars in the optical images are 100 μm.

In the cases of Cu_CVD–G and Cu_Trans–G we did not notice any significant microscopic defects other than occasional holes which may have formed during the transfer process. It should be noted that the Cu_Trans–G sample has been transferred twice – once onto copper foil prior to UVO treatment and again onto silicon wafer post-treatment – yet still showed little microscopic damage demonstrating that the transfer process does not significantly damage the film structure. For the case of 300 nm-SiO₂–G, although a faint film structure was visible under optical microscopy, it was noted that there was very little contrast between the film and underlying silicon. Upon characterization of the 300 nm-SiO₂–G sample with atomic force microscopy, it was found that the graphene film structure had actually degraded into patches or islands that were unresolvable using optical microscopy (Fig. 4e). Given the lack of 2D peak in the Raman spectrum of the 300 nm-SiO₂–G sample and the patchy morphology of the carbon as seen under AFM, we believed that the UVO treatment has oxidized the 300 nm-SiO₂–G sample into closely-spaced patches of amorphous carbon-like structure. This draws stark contrast with the intact film structure observed in the graphene samples that were either left on the growth copper foil or transferred onto annealed copper foil prior to UVO treatment, demonstrating the significant influence of the underlying substrate on the reactivity of graphene.

We hypothesize that this substrate dependent oxidation is due to varying amounts of attraction between the graphene film and underlying substrate. As oxygen functionalities are added onto the graphene lattice, the planar sp² structure of graphene is destroyed as sp³ hybridized carbon atoms form out-of-plane protrusions. Energetically, this increase in distance between the carbon atom and substrate surface requires an additional energy cost. Thus, we believe that the adhesion energy of the graphene and underlying substrate could be a significant factor that influences the susceptibility of graphene to undergo addition of oxygen functionalities during UVO oxidation.

In the case of CVD monolayer graphene on its growth copper foil, Yoon et al. found the adhesion energy between graphene and the copper foil, measured by double cantilever beam fracture mechanics,²⁰ to be 0.72 ± 0.07 J m⁻². Na et al., through the use of coupling interferometry,²¹ found the adhesion energy of wet transferred CVD graphene and silicon to be 0.357 ± 0.016 J m⁻². These adhesion energy values are qualitatively consistent with our hypothesis that the adhesion energy between graphene and the substrate relates to the amount of oxidation incurred in UVO treatments. The graphene that was transferred onto silicon wafer has a lower adhesion energy and so the formation of out-of-plane sp³ carbon centers is not as energetically expensive as in the cases of copper foil-supported samples. Similarly, it is interesting to note that graphene that was transferred onto annealed copper foil, Cu_Trans–G, underwent greater oxidation than the un-transferred pristine Cu_CVD–G sample. This may be attributed to a greater average distance between the graphene and copper surface brought about by molecular and nanoscale impurities from the transferring process in the case of Cu_Trans–G. These impurities weaken the adhesion between the graphene and copper surface, resulting in greater reactivity for the Cu_Trans–G sample.

In addition to varying adhesion energies between the graphene and underlying substrates, we note that other factors may also be at play. For example, owing to the difference in the wettability of the underlying substrates, the amount of water or O₃ adsorbed or trapped in the samples may vary significantly.^22–24 Such difference in the adsorbed/trapped water or O₃ may also contribute to the observed substrate-dependence in the photochemical oxidation. Last but not least, the presence of substrate may alter the type and amount of photo-generated radical intermediates.

Conclusion

In conclusion, we have shown that the substrate supporting graphene strongly influences the degree to which graphene oxidizes during UVO treatment. It was found that CVD-grown graphene left on its growth copper foil or transferred onto annealed copper foil suffered less oxidation than graphene that was transferred onto silicon wafer. The variation in the amount of oxidation incurred with different substrates was tentatively attributed to the varying adhesion energies of the graphene–substrate interface although other substrate-dependent factors such as water or O₃ adsorption cannot be ruled out. Nevertheless, the results presented here show that it is necessary to thoroughly consider the substrate supporting graphene in the context of its reactivity. In applications where graphene will experience UV radiation, such as in photovoltaics, selection of the substrate may have direct impact on the lifetime of the graphene device.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, Fig. S1 and S2. See DOI: 10.1039/c5ra20713d

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