Feb 4, 2010
Nanoengineering graphene with oxygen
Researchers from MIT and Beijing Institute of Technology have investigated graphene epoxide as an example of engineered graphene materials by functional groups. Their work focuses on the mechanical and electronic properties at various oxidation conditions. For regularly patterned epoxy structures, two phases are revealed to have considerable binding strength consistent with previous experimental observations: the clamped structure where the oxygen adsorbed on the sp2 bond and the unzipped structure where the epoxy binding breaks the sp2 bond. The clamped phase presents at high oxidation density and forms a regular lattice, while the unzipped phase is more stable and results in line defects in graphene.
The results lay the groundwork for the design and application of graphene-based functional materials through oxidation control methods; in other words, by altering the epoxidation density and arrangement patterns. Understanding the structural property relation is also critical for preparing graphene-related materials such as graphene oxide papers through chemical exfoliation methods. Furthermore, with the ability to control the oxidation and reduction process – for example, to modify the structure of graphene by oxidation in specific regions – one can expect to fabricate tunable graphene-related nanodevices, such as chemical sensors or patternable nanoelectric circuits.
To study the system, the researchers built an atomistic model of graphene epoxide where oxygen atoms bind on top of sp2 carbon bonds and form epoxy groups. The team performed first-principles calculations based on density functional theory and conducted a series of simulations for various epoxy symmetries and densities.
For graphene functionalized by very high-density epoxy groups (the ratio between carbon and oxygen atoms is 4:1), a clamped phase is observed that is stabilized by an energy barrier of 0.58 eV. In less-dense graphene oxide, the unzipped phase becomes the only possibility where the sp2 bond in the epoxy group is broken.
The effect of oxidation on the electronic and mechanical properties of graphene has been studied systematically. The bendable epoxy triangle leads to a half loss of Young's modulus of graphene, while leaving the tensile strength unaffected. It also changes the band structure remarkably, from semiconductor with a bandgap depending on oxidation density to flatband magnetism when the epoxy groups bind anti-symmetrically on both sides of graphene. These results have implications for applications such as graphene-based nanocomposites and tunable electronic devices.
Full details can be found in the journal Nanotechnology.
About the author
Dr Zhiping Xu is a postdoctoral associate in the Department of Civil and Environmental Engineering at the Massachusetts Institute of Technology, US. His work is focused on nanoscale and biological materials through multiphysics and multiscale simulation techniques, especially on mechanical, thermal and electronic properties. Dr Kun Xue is a lecturer in the State Key Laboratory of Explosion Science and Technology at Beijing Institute of Technology, China. Her recent research interest is the physics of amorphous solids.