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second harmonic generation unit (OPE SHG) prior to modulation. Alternatively, a white-light supercontinuum is generated to provide a broadly tunable probe. Both beams are focused onto the sample by a single lens. The probe beam waist at the sample is approximately 80 mi- crons. The transmitted probe-beam is detected by pho- todiode lock-in amplification (Zurich Instruments, HFLI and MFLI) at 500 kHz modulation. To compare the rGO polymer physics to ml-graphene, similar measurements to the above were carried out using an ultrafast transient absorption (TA) microscopy setup with a 1 μm spot size. The ml-graphene was prepared by chemical vapor deposition (CVD) and wet-transferred to a thin silicon nitride grid. The above non-degenerate pump-probe scheme was used in a collinear geometry coupled to a 4f-confocal scanning microscope (Olym- pus BX51W). The absorption spectra of ml-graphene are taken on the same microscope by coupling in a tunable Xe-arc illumination source and detecting the full plane images on a camera (EMCCD, PI-ProEM) camera after background renormalization. III. RESULTS Spanning the UV to near-IR regions, Fig. 1c plots the absolute linear transmission of six graphene oxide (GO) samples in a polymer composite with increasing pho- tothermal reduction times labeled from rGO1 to rGO5. Additionally plotted on a renormalized scale, we overlay the linear absorption spectra of both pristine monolayer (ml) graphene (black line), and the starting as-grown commercial GO solution (gray line, GOsolution). The GO solution has a clear bandgap, peaking at the molec- ular π − π∗ transition. Conversely, ml-graphene gives an expected Fano resonance lineshape peaked at 265 nm, red-shifted from the M-saddle-point transition labeled in Fig. 1c (inset).31 The rGOo curve in Fig. 1c is the ‘as- grown’ GO after incorporation into a hybrid polyacrylic and PMMA polymer matrix described in the methods. The absolute absorbance increases monotonically with GO photothermal reduction time over the NIR and IR regions plotted (from 0.35 eV to 1.5 eV). Photoreduction of GO leads to a spectral lineshape that absorbs light more analogously to CVD monolayer graphene plotted in Fig. 1c. In the solution phase and most polymers, GO aggre- gates as it is reduced, resulting in colloidal mixtures that strongly scatter light. GO is incorporated in a polymer- sphere matrix scaffold that makes systematic photore- duction possible while maintaining pristine optical qual- ity films. Thus, we are able to compare the absorp- tion lineshapes, photoluminescence, and ultrafast hot electron cooling rates over a wide range of photoreduc- tion. Interestingly, the more heavily reduced graphene oxide samples in Fig. 1c have a transmittance lineshape and slope similar to ml-graphene throughout the near- infrared (NIR) regions. In the supplementary Fig. S2, this absorption spectrum is extended out past 3 μm to the IR-region where the strong similarity to graphene ab- sorption is maintained. Figure 1d plots the normalized transient transmission (∆T/T, semi-log scale) kinetics of sequentially photore- duced GO/rGO samples acquired with a 1.8 eV degener- ate pump and probe configuration. As the degree of re- duction increases, the kinetic relaxation rate accelerates. The data shown in both Figs. 1 and 2 fits (solid lines) to a least-squares algorithm requiring three-exponents (τ1, τ2 , and τ3 ) with pulse deconvolution for the 155 fs laser autocorrelation response. After GO is incorporated and stabilized in the polymer matrix, the relaxation dynamics accelerate monotonically with photoreduction time. In stark contrast, the as-grown solution of GO (gray line in Fig. 1d) has much longer TA relaxation dynamics at all timescales, bearing little resemblance to faster graphene. At a 1.8 eV visible probe energy, the GO polymer composite that received no reduction (highest oxygen content) has the longest TA relaxation kinetics with its τ3 component comprising 21% of total decay amplitude. The inset of Fig. 1d shows the τ2 lifetimes all decrease linearly from ∼1.2 to 0.9 ps with increasing lamp pho- toreduction time. All samples have a characteristic τ2 time similar to graphene’s characteristic ∼1 ps decay ex- pected for 1.8 eV probe, suggesting all five samples ex- hibit graphene-like hot-electron cooling dynamics. By analogy with monolayer graphene, the τ1 would be asso- ciated with relaxation by optical phonons, and τ2 with disorder-assisted hot electron cooling.29 The fitting pa- rameter for the fast and long decays are constant at τ1 = 0.15 ps and τ3 = 66 ps, and all parameters are shown in Fig. 2c-d. Figure 2 plots how the kinetic relaxation rates depend on the selected probe energy (Epr). Comparing Fig. 2a at Epr=1.3 eV to Fig. 1d at 1.8 eV, a similar pattern with photoreduction emerges. However, the longest com- ponent, τ3 is negligible for all five cases of photothermal reduction rGO1−5. In Fig. 2d the slower τ2 lifetime decreases linearly from 2.5 ps to 1 ps with increasing photoreduction time. τ1 varies the least with photore- duction. Interestingly, the most reduced samples relax even faster compared to monolayer CVD-grown graphene (black dashed line). Figures 2a show fits to a triexponen- tial decay curve showing lifetimes of ∼0.4 ps, 1-2.5 ps, and >30 ps for τ1, τ2 and τ3 respectively. Regardless of the incident TA probe energy (1.2 to 1.8 eV), rGO samples relaxed progressively faster as the pho- toreduction time increased. Figure 2b shows that TA dynamics of GO, rGO3 , and rGO5 are slower at Epr = 1.3 eV (closed circles, 2.6 eV pump) than the Epr= 1.2 eV (open circles, 2.2 eV pump) probe energy window. Interestingly, the most reduced sample, rGO5, always decays more quickly than ml-graphene. This faster de- cay relative to graphene suggests that the photothermal reduction is ultimately damaging the sp2 graphene-sub- lattice by causing increased disorder and defect sites. This symmetry-breaking results in low energy disorder 3PDF Image | Graphene Oxide Photoreduction Recovers Graphene
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