M. Burghard, C. G´omez-Navarro, R.S. Sundaram, K. Kern, J.C. Meyer1, A. Chuvilin1, S. Kurasch1and U. Kaiser1
The extraordinary properties of graphene have triggered an enormous amount of research into the development of novel synthesis procedures that yield large quantities of this fascinat-ing two-dimensional material [1]. One promis-ing, low-cost, and easily up-scalable approach involves the chemical reduction of graphene oxide (GO), obtainable by oxidation of graphite and subsequent exfoliation in water. As a par-ticular advantage of GO, it can be deposited in controllable density onto a large variety of sub-strates. Upon reduction, the electrical conduc-tivity of GO is enhanced by several orders of magnitude (Fig. 10). Further improvement has been achieved through a chemical vapor depo-sition (CVD) process performed after the
reduc-tion step [2]. The electrical conductivity of thus obtained chemically derived graphene (denoted as CVDGO) lacks behind that of mechanically exfoliated graphene by only one order of mag-nitude. Interestingly, the electrical conduction in such sheets is still governed by thermally ac-tivated hopping.
Despite this advancement, further optimiza-tion of the chemical route to graphene requires detailed information about the atomic struc-ture of the sheets. We have thoroughly ex-plored the nature and distribution of the de-fects in reduced graphene oxide (RGO) us-ing aberration-corrected, low-energy (80 keV) transmission electron microscopy (TEM) [3].
Figure 10: Room temperature electrical conductiv-ities measured on monolayers of graphene oxide (GO), chemically reduced GO (RGO), as well as GO subjected to a chemical vapor deposition-based healing process (CVDGO). The conductivity of pris-tine (i.e., mechanically exfoliated) graphene is in-cluded for comparison.
To this end, free-standing GO membranes were prepared by dipping a TEM grid with 1 µm-sized holes into an aqueous dispersion of GO.
The attached GO was then reduced via heating the grid under high vacuum, which represents an alternative means to eliminate most of the oxygen-containing functional groups. Figure 11 depicts a representative TEM image of a RGO monolayer prepared in this manner. It reveals a wealth of structural features (marked in dif-ferent colors), which could not be detected by previous spectroscopic and microscopic inves-tigations. The light gray regions are clean, well-crystallized areas, where the hexagonal lattice of graphene is clearly visible. Their average size ranges from 3 nm to 6 nm, and they cover approximately 60% of the surface. The sheet furthermore comprises regions where carbona-ceous adsorbates and also heavier atoms are trapped (marked in dark gray). As similar fea-tures are known from TEM examination of me-chanically exfoliated graphene, they most likely represent contamination from the environment, rather than debris originating from the strong
oxidative treatment of the graphite in the ini-tial fabrication step. Another similarity to ex-foliated graphene is the formation of larger holes (marked in yellow) under electron irra-diation, albeit these emerge somewhat faster in the present samples. A specific feature of the RGO is the presence of a significant amount of topological defects, which can be classi-fied into extended (clustered) topological de-fects appearing as quasi-amorphous single layer carbon structures (marked in blue), and isolated topological defects (pentagon-heptagon pairs, marked in green). The extended topological de-fects cover more than 5% of the surface. Re-markably, despite the presence of such a sig-nificant defect density, the long-range orienta-tion of the hexagonal lattice is maintained, as has been concluded from both direct images and diffraction patterns. In general, prolonged irra-diation by the e-beam resulted in the relaxation of the topological defects, rather than the cre-ation of additional defects.
Figure 11: Atomic resolution transmission electron microscopy (TEM) image of a free-standing, re-duced graphene oxide (RGO) monolayer. Color is used to mark the different features. The defect-free crystalline graphene areas appear light gray while contaminated regions are shaded in dark gray. Blue regions correspond to extended topological defects, and red areas to individual ad-atoms or substitu-tional atoms. The green areas highlight isolated topological defects, whereas holes and their edge re-constructions are colored in yellow. The scale bar represents 1 nm.
Figure 12: Close-up TEM image of clustered topo-logical defects within a RGO monolayer. Carbon pentagons, hexagons, and heptagons are highlighted in magenta, blue, and green, respectively. The blurred central region proved unstable under the e-beam irradiation. The red dashed line at the bot-tom marks a strong lattice deformation. Scale bar corresponds to 1 nm.
In Fig. 12, a more detailed view of clustered topological defects is presented. From the ab-sence of such clusters in exfoliated graphene, it follows that they must have evolved from the originally strongly oxidized areas in the GO.
It is noteworthy that all carbon atoms within the defective regions are bonded to three neigh-bors in a planarsp2-configuration, correspond-ing to a quasi-amorphous bondcorrespond-ing configuration composed of pentagons, hexagons, heptagons, and occasionally also octagons. By comparison,
many of the isolated topological defects incor-porate two separate pentagon-heptagon pairs, which often act like dislocation cores within the sheets. Their formation can be ascribed to the impact of pronounced plastic deformation dur-ing fabrication of the sheets.
The above observations suggest the following formation mechanism of RGO: Upon oxidation of graphite, isolated highly oxidized areas with a size of a few nm are created while at least 60% of the surface remains undisturbed. Upon reduction, the oxidized areas are restored tosp2 -bonded carbon networks, which however lack the perfect crystalline order of intact graphene.
The reduced disordered areas, which are best described as clustered topological defects, are seamlessly connected to the crystalline areas, and may induce strain as well as in-plane and out-of-plane deformations in the surrounding RGO. The gained knowledge about the defects is an important prerequisite for devising strate-gies to remove them, or alternatively to exploit them for further attachment of functional moi-eties.
References:
[1] Park, S. and R.S. Ruoff.Nature Nanotechnology4, 217–224 (2009).
[2] L´opez, V., R.S. Sundaram, C. G´omez-Navarro, D. Olea, M. Burghard, J. G´omez-Herrero, F. Zamora and K. Kern.Advanced Materials21, 4683 (2009).
[3] G´omez-Navarro, C., J.C. Meyer, R.S. Sundaram, A. Chuvilin, S. Kurasch, M. Burghard, K. Kern and U. Kaiser.Nano Letters10, 1144–1148 (2010).
1Universit¨at Ulm