1 Department of Micro- and Nanotechnology, Technical University of Denmark2 Nanointegration, Department of Micro- and Nanotechnology, Technical University of Denmark3 University of Copenhagen4 Chinese Academy of Sciences5 Center for Nanostructured Graphene, Center, Technical University of Denmark
Chemically converted graphene is highly relevant for transparent conducting film applications such as display and photovoltaic uses. So far, the major obstacle for realizing the potential has been to fully reduce/deoxygenate the graphene oxide (GO), which is challenging in part due to the pronounced aggregation that accompanies deoxygenation of GO in solution. Surface immobilization of monolayered graphene oxide (mGO) in Langmuir-Blodgett (LB) films was investigated as a method to circumvent this problem. Two types of LB films with different density of mGO flakes were prepared, i.e., diluted and coherent, and efficiently deoxygenated in a three-step reduction procedure involving subsequent treatment with hydrazine in dimethylformamide (DMF), sulfuric acid, and high temperature annealing. The stepwise reduction process was evaluated with optical microscopy, Raman microscopy, X-ray photoelectron spectroscopy (XPS) along with electrical characterization. XPS measurements confirmed a full conversion into virtually oxygen-free chemically converted graphene. The electrical characterization revealed large variations in the conductivity for single sheets in the diluted LB films, with an average conductivity of 100 S/cm. A similar conductivity was found for macroscopic devices made from the coherent LB films with overlapping mGO sheets. The large variation in single sheets conductance is assigned to overoxidation of the GO leading to formation of holes, which cannot be recovered in the chemical reduction procedure. The study shows that the applied three-step reduction procedure is chemically complete and that the conductivity of this chemically converted graphene is limited by structural defects/holes rather than remaining oxygen functionalities.
Chemistry of Materials, 2013, Vol 25, Issue 24, p. 4839-4848