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Optical modification of atomic-thickness graphene oxide

The absorption response and photoluminescence of a graphene oxide film are modified by using thermal annealing and oxygen plasma treatment to alter the ratio of sp2 to sp3 orbitals.
13 November 2012, SPIE Newsroom. DOI: 10.1117/2.1201210.004520

Graphene, a monolayer crystal of carbon atoms, exhibits excellent electronic, mechanical, and thermal behavior, making it of significant interest in physics.1–3 The extraordinary physical properties of this material include zero band gap, zero effective mass for electrons and holes, resistivity of 10−5Ω·cm (the lowest seen in any substance at room temperature), and electron mobility of 15,000cm2V−1s−1 in SiO2 substrate (200,000 in suspended structure).1 With the recent acceleration in graphene research, including an emphasis on large-scale production, some have begun envisioning new industrial possibilities in which carbon supplants silicon in transistors, sensors, resonators, and other electronic devices.3,4

Graphene oxide (GO), shown in Figure 1(a–d), is produced by exposing stacked graphene sheets to phenolic hydroxyl, epoxide, carbonyl, carboxyl, or other oxygen functional groups. Because of GO's polycrystalline structure and the presence of various chemical and physical defects, mostly attributable to these functional groups, oxidizing graphene degrades certain of its physical properties substantially.1, 5 Yet these same defects also give GO a measurable band gap, and as a result it exhibits such characteristics as optical absorption and photoluminescence (unlike graphene, which features neither a clear absorption edge nor a well-defined band gap). GO's optical absorbance is dominated by π–π* transitions between 225 and 275nm (4.5–5.5eV), and its photoluminescence under visible and UV light, shown in Figure 1(e) and the inset of Figure 1(f), is visible to the naked eye.6–8

Figure 1. (a) A colloidal solution of graphene oxide (GO) in deionized water. (b) Transmission electron micrograph of GO. (c-d) Atomic force micrograph of GO thin film. (e) Photoluminescent emission of oxygen plasma-treated GO film, showing pressure dependence. (f) Scanning electron micrograph of GO film, with inset showing photoluminescence.

Much research has been conducted on synthesizing GO-based composite materials by controlling the distribution of the attached functional groups.5,6,9–11 Pure graphene is sp2-bonded, meaning that in the valence shell of each carbon atom, the 2s orbital and two of the three 2p orbitals ‘mix’ together to produce three hybridized sp2orbitals. Under local oxidation, however, the 2s orbital mixes with all three 2p orbitals, producing four hybridized sp3orbitals, and thus the regions of GO where oxygen functional groups are attached are sp3-bonded. The ratio of sp2 to sp3can be measured and used to estimate a GO sample's electronic and optical properties. Alternatively, modifying the sp2/sp3ratio (and thus the band gap) allows GO to be treated as a tunable optoelectronic material.5, 12

We tailored the physical properties of a graphene oxide thin film by altering its sp2/sp3ratio via both reduction (to increase the sp2fraction) and oxidation (to increase the sp3fraction). We examined the properties using atomic force microscopy, scanning electron microscopy, UV-visible-near IR absorption spectra, Fourier transform IR spectroscopy, Raman spectroscopy, x-ray photoemission spectroscopy, transmission electron microscopy/electron energy loss spectroscopy, and photoluminescence spectroscopy.

We reduced GO to produce a higher sp2carbon fraction using two different techniques. The chemical method uses hydrazine (N2H4) or sodium borohydride (NaBH4), which is proper for surface—but not edge—functional groups, so thermal annealing (up to 850°C in vacuum conditions of about 10−4torr) should give better performance. A reduced monolayer GO film was fabricated by spin coating and thermal annealing. Its optical response confirmed a sharp peak at 4.55eV, matching that of graphene.13

We used oxygen plasma treatment to oxidize GO in order to produce a higher sp3 fraction. As oxygen atoms attach to random graphene sites, they cause carbonyl components to increase faster than epoxy components, due to the rearrangement of epoxy groups. Meanwhile, the isolation of sp2carbon clusters within an increasingly dense sp3matrix leads to electron-hole pair localization, resulting in intense excitation-dependent visible emission.13

Our work shows that altering graphene oxide's band gap by using thermal annealing and oxygen plasma treatment to manipulate the sp2/sp3 ratio is an effective and versatile way of tuning the optoelectronic properties of the material. The ability to adjust photoluminescence at the atomic level makes GO a unique candidate for future carbon-based optoelectronic devices, including LEDs, electroluminescent devices and displays, and photovoltaic devices. We are currently investigating the effects on fluorescence of metal grafting, substrate effect, and other factors. The results of this fundamental research may also be applicable to the production of solar cell converters.

Juhwan Lim, J. R. Rani
Nano Electromechanical Device Lab, Mechanical Engineering
Yonsei University
Seoul, South Korea

Juhwan Lim received his bachelor's from Yonsei University and is currently a master's candidate in mechanical engineering.

J. R. Rani received her PhD from the University of Kerala, India, in 2009. She is an assistant professor for the government of Kerala and is a postdoctoral research fellow in mechanical engineering at Yonsei University. Her research interests include fabrication of silicon and graphene-based optoelectronic devices.

Kyujin Choi, Jae Hoon Kim
Physics and Applied Physics
Yonsei University
Seoul, South Korea

Kyujin Choi received his master's from Yonsei University and is currently a doctoral candidate.

Jae Hoon Kim received his master's and PhD from Stanford University and held a postdoctoral position there before joining the University of Groningen, Netherlands. He is currently a professor.

Seong Chan Jun
Yonsei University
Seoul, South Korea

Seong Chan Jun received his master's from Cornell University and his PhD from Columbia University. He took his postdoctorate at Columbia before joining Samsung Advanced Institute of Technology. He is currently an associate professor of mechanical engineering.

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