Photoluminescence and Band Gap Modulation in Graphene Oxide

We report broadband visible photoluminescence from solid graphene oxide, and modifications of the emission spectrum by progressive chemical reduction. The data suggest a gapping of the two-dimensional electronic system by removal of π-electrons. We discuss possible gapping mechanisms, and propose that a Kekule pattern of bond distortions may account for the observed behavior. Disciplines Physical Sciences and Mathematics | Physics Comments Suggested Citation: Luo, Z., P.M. Vora, E.J. Mele, A.T.C. Johnson and J.M. Kikkawa. (2009). "Photoluminescence and band gap modulation in graphene oxide." Applied Physics Letters. 94, 111909. Copyright 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters. and may be found at http://dx.doi.org/10.1063/1.3098358. This journal article is available at ScholarlyCommons: http://repository.upenn.edu/physics_papers/64 Photoluminescence and band gap modulation in graphene oxide Zhengtang Luo, Patrick M. Vora, Eugene J. Mele, A. T. Charlie Johnson, and James M. Kikkawa Department of Physics and Astronomy, The University of Pennsylvania, 209 South 33rd Street, Philadelphia, Pennsylvania 19104, USA Received 21 January 2009; accepted 21 February 2009; published online 19 March 2009 We report broadband visible photoluminescence from solid graphene oxide, and modifications of the emission spectrum by progressive chemical reduction. The data suggest a gapping of the two-dimensional electronic system by removal of -electrons. We discuss possible gapping mechanisms, and propose that a Kekule pattern of bond distortions may account for the observed behavior. © 2009 American Institute of Physics. DOI: 10.1063/1.3098358 Single layer and bilayer graphene systems can exhibit a remarkable diversity of phenomena, including observations of a room-temperature, unconventional quantum Hall effect, predictions of the quantum spin Hall effect, broken spin or pseudospin symmetries, and finite size effects that can be used to control band structure and magnetism. Taken together, these properties suggest that graphene is a promising platform for seamlessly exchanging information between different degrees of freedom. An outstanding challenge in this regard is photonic integration and band gap manipulation. Several theoretical works predict that a direct gap in the visible would occur for sufficiently small graphene nanoribbons, but no observations of this finite size effect have been reported. Additionally, modifications of the graphene sheet by oxidation can introduce direct gap behavior. Here we show that graphene, although intrinsically a zero-gap semimetal, may be oxidized in a manner that produces photoluminescence PL for solid, drop-cast samples. We find that despite the high surface area of graphene oxide GO and in marked contrast to carbon nanotubes, the strength of PL from GO flakes does not differ significantly between aqueous and drop-cast samples. The resilience of PL for solid GO samples is encouraging for technological applications, implying that GO may be a useful photonic material when incorporated in solid state devices. The large observed gap creates the possibility for spatially modulating the band structure within a single graphene flake by local control of the oxidation profile. Studies of progressive chemical reduction show quenching of PL for both drop-cast and aqueous samples, coordinated with changes in absorption. These studies also find signatures of band gap manipulation, albeit with different character for solid and liquid samples. Aqueous dispersions of single layer GO with an average area of 100 m2 were synthesized following a procedure described elsewhere. Solid samples s-GO were obtained by drop-casting the concentrated GO solution resulting from this procedure onto polished, low auto-fluorescence, Suprasil-2 substrates and then baking at 95 °C for 30 min. Liquid samples l-GO were held in quartz cuvettes, diluted a hundredfold or more as necessary to adjust optical density. PL for both l-GO and s-GO was collected at 90° degrees to the excitation, and the reflection transmission geometry for s-GO corresponded to collection on the same opposite side of the film. PL spectra were excited by Xe lamp passed through a monochrometer, and additional filters were employed on excitation and collection to reject excitation scatter, second order grating effects, and leakage of Xe lamp spikes. Spectra were spectrally corrected for detector efficiencies, and normalized by excitation power. All PL data shown here both maps and single spectra are further normalized to a maximum value of unity and taken at 300 K. Figure 1 a compares PL for both l-GO and s-GO samples. Both peaks in the visible with a long infrared emission tail. Differences in measurement geometry make quantitative comparisons of the quantum yield impossible, but generally little difference was seen in PL intensity. An interesting question is whether energy relaxation and spectral diffusion are qualitatively altered by aggregation. For isolated flakes in l-GO, diffusion of free carriers or bound excitons should be confined to the two dimensional GO plane. However, for s-GO, atomic force and optical microscopy, both indicate films of layered GO flakes, which could give rise to additional interflake relaxation pathways. If interlayer coupling is strong enough, the emission spectrum could redshift. s-GO indeed shows more PL spectral weight in the infrared, but the redshift in the PL peak position is not a robust feature of the experiment and was inconsistent from sample to sample perhaps due to variations in the oxidation density. In addition to exciton diffusion, several plausible changes could a Electronic mail: [email protected]. 600 80