Optical modification of atomic-thickness graphene oxide

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.