Ultrafast Spectral Migration of Photoluminescence in Graphene Oxide

Abstract

We use subpicosecond time-resolved photoluminescence measurements to study the nature of photoluminescence in graphene oxide and reduced graphene oxide. Our data indicate that, in contrast to prior suggestions, the photoluminescence spectra of graphene oxide and reduced graphene oxide are inhomogeneously broadened. We observe substantial energy redistribution and relaxation among the emitting states within the first few picoseconds, leading to a progressive red shift of the emission spectrum. Blue shifts that arise in time-integrated spectra upon photothermal reduction are easily understood within this dynamical context without invoking a modified distribution of dipole-coupled states. Rather, reduction increases the nonradiative electron-hole recombination rate and curtails the red-shifting process, which is consistent with an increase in quenching through the introduction of larger and/or more numerous sp(2) clusters. Polarization memory measurements show energetic signatures of electron-hole correlations, established on a subpicosecond time scale and developing little thereafter.

Spectroscopy and Microscopy of Graphene Oxide and Reduced Graphene Oxide

“…Two characteristic peaks are observed, located in the blue (~490 nm) and red (~690 nm) portions of the visible spectrum. This agrees with other reported ensemble GO spectra [27][28][29]. As described earlier, GO's broad emission is generally attributed to various causes including the presence of intrasheet sp 2 domain size distributions [24,27], quasi-molecular ligand/sp 2 states [29], and heterogeneous carrier relaxation kinetics [28].…”

“…In the latter case, optical Kerr-gate photoluminescence measurements have revealed clear spectral dynamics following excitation. Specifically, GO's emission redshifts from 460 to 690 nm over the course of~10 ps following excitation and clearly indicates the existence of heterogeneous carrier relaxation kinetics [28].…”

“…An additional difference from graphene is that, upon absorbing UV or visible light, GO fluoresces [23,24,[27][28][29][30]. Two broad (200-300 nm full width at half maximum, fwhm) peaks are often observed, centered in the blue (400-500 nm) and red (600-700 nm) regions of the visible spectrum [27,31].…”

“…Conventional wisdom suggests that it arises from carrier recombination in isolated/confined sp 2 clusters where varying domain sizes lead to a distribution of energy gaps that results in a spectrally broad emission [31]. However, quasimolecular transitions and even heterogeneous carrier relaxation kinetics have been invoked to explain its origin and appearance [28,29]. In the former case, intriguing evidence for quasi-molecular photoluminescence, arising from basal plane oxygenfunctionalized aromatic clusters, has been observed in GO excitation spectra [29].…”

Direct Observation of Quantum‐Confined Graphene‐Like States and Novel Hybrid States in Graphene Oxide by Transient Spectroscopy

“…If this universal feature is observed and demonstrated, it will give us a greater understanding of the electronic nature of GO. At the same time, it will also be remarkably different from previous reports in the fi eld of ultrafast spectroscopy for GO, which would usually involve only featureless excited-state absorption signals [36][37][38] or just emission [ 39 ] . According to our transient spectral evolution of GO/rGO, the experimental results about confi ned graphenelike states are highly consistent with previously expected universal features.…”