"In order to chemically remove single sheets from a graphite crystal one has to overcome the huge attractive interactions present in the 3D crystal," Jan Englert, a PhD student at the Institute of Advanced Materials and Processes (ZMP) at the University Erlangen-Nuremberg, explains to Nanowerk. "First approaches to obtain graphene from graphite based on oxidation date back at least to 1899 ("Verfahren zur Darstellung der Graphitsäure"). During oxidation the introduction of carboxyl, carbonyl, epoxy and alcohol groups reduce attractive interaction of the carbon layers as coulomb repulsion is introduced. As early as 1961, Hanns-Peter Boehm – who also authored the IUPAC paper that initially defined the term 'graphene' – could show with graphitic acid that graphite does dissolve into single lamellae of carbon. These sheets are today called graphene oxide."
The general problem with this approach is however that is has been so far impossible to regenerate the undisturbed graphene lattice which was present prior to oxidation as decarboxylation inevitable takes place in the reduction step, which is effectively removing carbon atoms from the layers leaving back vacancies, holes or topological defects. In order to obtain defect-free graphene, researchers would need to find a way to suppress decarboxylation.
Atomic force microscopy image of large area chemically exfoliated graphene flakes. (Image: Institute of Advanced Materials and Processes)
In new work, Englert and a group of scientists from the University Erlangen-Nuremberg, led by Andreas Hirsch, have now demonstrated the first bulk wet chemical exfoliation of graphite in association with an in situ covalent functionalization of intermediately generated graphene. With this novel chemical method, it is now possible to achieve covalently bonded functionalities without mechanical or sonochemical treatment. The covalent functionalization also protects the single-layer graphene from reaggregation and substrate-induced doping.
As the team reports in the March 20, 2011, online edition of Nature Chemistry ("Covalent bulk functionalization of graphene"), they activated readily available graphite by reduction with a sodium/potassium alloy. The excessive negative charge eases the exfoliation of the sheets, and guarantees a facile reaction of in situ generated graphene with electrophiles.
"In contrast to oxidatively treated graphite, reduction by electrons does not cause any irreversible lattice damage to the hexagonal carbon plane" says Englert. "After efficient covalent functionalization of the entirely separated graphene sheets, completely intact layers can be fully restored by thermal treatment of the derivatized graphane material" (graphane is simply graphene modified by hydrogen atoms added to both sides of the matrix, which makes it an insulator).
Possible areas of application of such covalently altered graphene materials are generally to be seen in nanoelectronics and sensors. For instance, by altering its surface during attachment of organic addends, graphene's affinity towards specific substances could be generated and tuned.
One of the impediments towards its wider application in molecular electronics is the difficulty in opening a bandgap in graphene's gapless band-structure (i.e. it's metallic). In order to fabricate high-performance graphene-based transistors, the generation of a permanent intrinsic bandgap could be beneficial in achieving high on-off switching ratios.
"Due to alteration of the local geometry of single carbon atoms in the graphene sheets we are very optimistic that such a bandgap can be obtained as we proceed from graphitic structures to diamond related ones," says Englert.
While the separation of functionalized graphene material from unfunctionalized and unwanted graphitic debris still needs to be optimized, researchers also need to understand in detail if and how functionalization changes the electronic properties of the material. The results can then be exploited for future applications like smart windows, ultrasensitive photodetectors, or high-performance capacitors.