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FIG. 3. the deconvolved Fano resonance lineshape fit is plotted in dashed lines. Unlike pristine ml-graphene (black), the two rGO samples plotted also require two convolved Gaussians (dash-dot) suggesting molecular-like transition labeled π to π∗ and edge defect transitions, n to σ∗ (see inset). The resulting Fano-Gaussian convolved fits (dotted lines) show the graphene sub-lattice Fano-parameter, q increases with photoreduction consistent with more lattice disorder. lineshape with a renormalized peak resonance energy, Er that is red-shifted from the M-point by energy by ∼= 0.3 − 0.4 eeV.35,36 The asymmetric Fano lineshape accounts for the ratio of interference between the dis- crete (M-point) and continuum transition probabilities through the dimensionless Fano parameter, q.35 Thus, the tight binding model of the graphene absorption spec- trum in Fig. 3 is renormalized for effective electron-hole interaction effects by fitting to the below asymmetric Fano lineshape, TABLE I. Fano fitting parameters for data in Fig. 3 (dashed lines) show good agreement with our monolayer graphene data established literature values.35,36 The Fano parameter q of rGO5 best matches ml-graphene. Two convolved Gaus- sian for GQD π − π∗ and edge state defects are also required. ml-graphene. The molecular-like π − π∗ transitions are illustrated in Fig. 3 (inset), and show graphene quan- tum dot (GQD) states also contribute to the spectral weight and are centered near 4.6 eV.38 At 4.3 eV, rGO also contains sub-gap defect states between the π and π∗ states, which results from previously reported local oxygen-based disorder that creates edge defect state (n) to σ∗ transitions.2,39–42 Due to the heterogeneous oxygen coverage, these local disorder edge states have a much broader absorption FWHM. As rGO1 is further reduced, we observe in Fig. 3 that the peak area of the n−σ∗ Gaus- sian decreases as oxygen is removed, resulting in fewer edge states. Both our most oxidized samples (GOo and GOsolution) did not fit well to a Fano lineshape, suggest- ing only rGO samples have a graphene-like absorption lineshape in the IR and NIR regions. Table I contains a summary of the Fano fitting parame- ters, showing good agreement between the literature35,37 and our results for monolayer graphene and rGO5. rGO5 contains a large absorption from the linear dispersion near the K and K’ points, where excited carriers cou- ple strongly to the continuum, similar to monolayer graphene. For rGO1, the Fano parameter q decreases sig- nificantly from monolayer graphene, suggesting electron- hole interaction effects are increasingly screened for tran- sitions near the van Hove singularity. For GO and lightly reduced rGO, Table I shows the Fano parameter is many times larger than highly reduced samples and mono- layer graphene. This suggests the many edges states in more oxidized graphene couple strongly to continuum- like states. The inset of Fig. 3 shows a qualitative depiction of how the density of states changes from GO to rGO. As the samples are reduced, they contain larger area regions of non-interrupted sp2 carbon, leading to a more graphene- like distribution of continuum states, resulting in a better Fano lineshape fit. The two convolved Gaussians show the effect of reduction on the absorption spectra, with the amplitude of the n−σ∗ transition decreasing significantly, suggesting the removal of oxygen functional groups. We also see that the absorption peak in rGO1 shifts slightly to lower energy compared to rGO5. This shift has been theoretically predicted by Roy et al.22, who used DFT to calculate the band structure of GO at varying oxygen content, finding that the addition of oxygen decreases the band gap at the M-point. However, the underlying Under each linear absorption spectra (solid lines) A γ Fano 2 2 1+ γ(E−Er (E) = A (1) 2 (E − Er ) + q2 Sample ml-graphene [CVD] ml-graphene [exfoliated]35 rGO5 [highly reduced] rGO1 [barely reduced] Er (eV) 4.80 4.73 4.69 4.62 γ (eV) q 1.69 -3.2 1.30 -3.3 1.68 -3.2 2.16 -50 5 1.5 1.0 0.5 0.0 1.0 1.5 2.0 2.5 3.0 3.5 Energy (eV) 4.0 4.5 5.0 Absorbance E Fano-lineshape fits: rGO1, least reduction rGO5, highest reduction CVD ml-graphene (norm) edge defects GQDs: p-p* n-s* Fano p-p* n-s* where γ is the Lorentizian homogeneous linewidth and A is the amplitude scaling constant. Fig. 3 plots a hyperspectral measurement of CVD ml-graphene (black line) with its corresponding Fano lineshape fit (dashed line), given by equation 1 above. Table I gives the re- sulting Fano parameters and show excellent agreement of this work graphene values with the established liter- ature values.35,37 This provides an essential calibration base to quantitatively compare against the lineshape fit of rGO absorption spectra. Figure 3 shows good agreement between the absorp- tion spectra of rGO1 and rGO5, and the asymmetric Fano resonance after it is convolved with two Gaussians peaks at energies corresponding to the absorption of the n − σ∗ and π − π∗ transitions. This fitting analysis sug- gests that the absorption spectrum in rGO can be under- stood to contain a Fano resonance similar to that of CVDPDF Image | Graphene Oxide Photoreduction Recovers Graphene
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