A scalable method to isolate graphene on insulating substrates is still a challenge. Recently it has been demonstrated that graphite can be exfoliated in certain solvents [1] and in water-surfactant solution [2] forming a stable colloidal suspension. Amongst several solvents investigated, exfoliation with 1-Methyl-2-pyrrolidone (NMP) has yielded the highest concentration of 1-5 graphene layers [1,3]. Here we present Langmuir-Blodgett (LB) films of pure graphene exfoliated in NMP and their optical and electronic characteristics [3]. For flakes with large lateral size, the films displayed sheet resistance of 5 KOhm/sq and transmittance of 75% at wavelength of 550 nm. Thin film transistor (TFTs) have been fabricated and tested at temperatures ranging from 77 K to 340 K. The field effect measurements displayed high degree of p-doping and this is possibly due to interaction between graphene and residual oxygen functional groups, as suggested by X-ray photoelectron spectroscopy (XPS). Below 150 K, the temperature dependence of the conductivity suggests transport by two-dimensional variable range hopping. Above 180 K, thermal activation of the charge carries appears to be the dominant mechanis. A clear dependence of the optoelectronic properties with the lateral size of graphene flakes has been observed. A critical analysis on the prospects of improving the electrical properties of graphene obtained via non covalent exfoliation will be discussed.

[1] Y. Hernandez et al. Nature Nanotech. Vol. 3, pp. 563-568, 2008.

[2] M. Lotya et al. J. Am. Chem Soc. Vol 131, pp.3611-3620, 2009.

[3] H. Kim et al. submitted.