Tunable optical absorption and interactions in graphene via oxygen plasma

We report significant changes of optical conductivity (${\sigma }_1$) in single-layer graphene induced by mild oxygen plasma exposure and explore the interplay between carrier doping, disorder, and many-body interactions from their signatures in the absorption spectrum. The first distinctive effect is the reduction of the excitonic binding energy that can be extracted from the renormalized saddle point resonance at 4.64 eV. Secondly, ${\sigma }_1$ is nearly completely suppressed (${\sigma }_1\ll {\sigma }_0$) below an exposure-dependent threshold in the near-infrared range. The clear steplike suppression follows the Pauli blocking behavior expected for doped monolayer graphene. The nearly zero residual conductivity below ω ∼ 2$E_F$ can be interpreted as arising from the weakening of the electronic self-energy. Our data shows that mild oxygen exposure can be used to controllably dope graphene without introducing the strong physical and chemical changes that are common in other approaches to oxidized graphene, allowing a controllable manipulation of the optical properties of graphene.

Figure 1

Raman spectra (excitation wavelength of 514 nm) for pristine single layer graphene (SLG) on SiO${}_2$ (300 nm)/Si, and after consecutive intervals of oxygen plasma exposure: $t_{\mathrm{s}}$ = 2, 4, and 6 s.

Figure 2

SE data of single layer chemical vapor deposition (CVD) graphene on SiO${}_2$ (300 nm)/Si substrate. (a) Ψ and (b) Δ for substrate (CVD graphene) shown in red (blue). The inset in (a) shows the optical multilayer model used in extracting Ψ and Δ. Media 0, 1, 2, and 3 in the model denote air, a graphene layer with the thickness $d_1$ of 0.335 nm, a SiO${}_2$ layer with the thickness $d_2$ of 300 nm, and silicon bulk, respectively.

Figure 3

The experimental (a) Ψ and (b) Δ at an incident angle (${\theta }_0$) of 70° for pristine graphene on SiO${}_2$ (300 nm)/Si substrate and after cumulative oxygen plasma exposure ($t_s$) in 2-s steps. The experimental (c) Ψ and (d) Δ of SiO${}_2$ (300 nm)/Si substrate at an incident angle (${\theta }_0$) of 70°. The insets amplify Ψ and Δ at particular energy ranges.

Figure 4

Detail of the analysis of SE data for pristine graphene on the SiO${}_2$ (300 nm)/Si substrate. The experimental (a) Ψ and (b) Δ at different incident angles (${\theta }_0$) 70°, 65°, and 55° are shown in the solid blue, green, and red lines, respectively. The best match model extracted from the analysis is shown in dashed black lines. (c) Model dielectric constant used in the optical model for 300-nm SiO${}_2$ layer. (d) Model dielectric constant used in the optical model for silicon bulk. (e) Model dielectric constant extracted from the optical model for the 0.335-nm graphene layer. (f) Corresponding optical conductivity of the graphene extracted from (e).

Figure 5

Detail of the analysis of SE data for graphene on the SiO${}_2$ (300 nm)/Si substrate after 6s of oxygen plasma exposure time ($t_{\mathrm{s}}$). The experimental (a) Ψ and (b) Δ at different incident angles (${\theta }_0$) 70° and 60° are shown in the solid blue and green lines, respectively. The best match model extracted from the analysis is shown in dashed black lines. (c) Model dielectric constant used in the optical model for the 300-nm SiO${}_2$ layer. (d) Model dielectric constant used in the optical model for the silicon bulk. (e) Model dielectric constant ${\varepsilon }_1$ extracted from the optical model for 0.335-nm graphene layer. (f) Corresponding optical conductivity ${\sigma }_1$ of 0.335-nm graphene extracted from (e).

Figure 6

Corresponding real part of optical conductivity ${\sigma }_1$(ω) extracted from SE measurements. The dashed line below 1.4 eV is the best fit to the data for $t_s$ = 6 s, as discussed in the text. The inset shows this fit in more detail.

Figure 7

(a) Fit of the experimental conductivity using the phenomenological Fano line shape analysis discussed in the text for pristine single layer graphene on SiO${}_2$ (300 nm)/Si. Upper panel: the optical conductivity ${\sigma }_1$ extracted from SE is shown as red circles, and the best fit to Eq. (1) by the dashed line; the unperturbed band-to-band component ${\sigma }_{\mathrm{cont}}$ is shown in green/solid. Lower panel: Isolated Fano contribution ${\sigma }_{\mathrm{Res}.\mathrm{Fit}}$/${\sigma }_{\mathrm{cont}}$. (b) Comparison of the two individual fitting components (unperturbed band-to-band transitions, upper panel, and Fano resonance profile, lower panel) as the oxygen plasma exposure time increased. The grey area denotes the low-energy region below ∼2$E_F$ where ${\sigma }_1$ is Pauli-suppressed, which is not captured by the Fano approach.

Figure 8

Fano parameters extracted from the fit of ${\sigma }_1$ as function of oxygen plasma exposure time $t_s$. The corresponding disorder parameter $L_a$ (the in-plane crystalline grain size) derived from $t_s$ is depicted on top of the graphs. (a) The Fano line shape parameter $q$. (b) The exciton lifetime within Fano's model. (c) The peak position of ${\sigma }_{\mathrm{cont}}$ ($E_0$), and Fano resonance energy ($E_{\mathrm{res}}$). (d) The number of charge carriers ($n_e$) extracted directly from the Pauli blocking seen in the experimental traces of ${\sigma }_1$(ω).

Figure 9

(a) The integrated optical spectral weight ($W$) up to the photon energies in the horizontal axis. (b) Allowed optical transitions (vertical red arrow) in pristine graphene. Here, $E_d$ and $E_F$ denotes the Dirac point and Fermi energy, respectively. (c) Likewise, for hole-doped graphene. Optical transitions below $E_s$ are disallowed due to Pauli's exclusion principle. (d) In hole-doped and gapped graphene.