  Reduced graphene oxide (rGO) is a bulk-processable quasi-amorphous 2D
material with broad spectral coverage and fast electronic response. rGO sheets
are suspended in a polymer matrix and sequentially photoreduced while measuring
the evolving optical spectra and ultrafast electron relaxation dynamics.
Photoreduced rGO yields optical absorption spectra that fit with the same Fano
lineshape parameters as monolayer graphene. With increasing photoreduction
time, rGO transient absorption kinetics accelerate monotonically, reaching an
optimal point that matches the hot electron cooling in graphene. All stages of
rGO ultrafast kinetics are simulated with a hot-electron cooling model mediated
by disorder-assisted supercollisions. While the rGO room temperature 0.31
ps$^{-1}$ electronic cooling rate matches monolayer graphene, subsequent
photoreduction can rapidly increase the rate by ~10-12$\times$. Such
accelerated supercollision rates imply a reduced mean-free scattering length
caused by photoionized point-defects on the rGO sp$^2$ sub-lattice. For visible
range excitations of rGO, photoreduction shows three increasing spectral peaks
that match graphene quantum dot (GQD) transitions, while a broad peak from
oxygenated defect edge states shrinks. These three confined GQD states donate
their hot carriers to the graphene sub-lattice with a 0.17 ps rise-time that
accelerates with photoreduction. Collectively, many desirable photophysical
properties of 2D graphene are replicated through selectively reducing rGO
scaffolded within a 3D bulk polymeric network.

Materials Science;Mesoscale and Nanoscale Physics;Chemical Physics