[show abstract][hide abstract] ABSTRACT: Engineering the electronic properties of graphene has triggered great
interest for potential applications in electronics and opto-electronics. Here
we demonstrate the possibility to tune the electronic transport properties of
graphene monolayers and multilayers by functionalisation with fluorine. We show
that by adjusting the fluorine content different electronic transport regimes
can be accessed. For monolayer samples, with increasing the fluorine content,
we observe a transition from electronic transport through Mott variable range
hopping in two dimensions to Efros - Shklovskii variable range hopping.
Multilayer fluorinated graphene with high concentration of fluorine show
two-dimensional Mott variable range hopping transport, whereas CF0.28
multilayer flakes have a band gap of 0.25eV and exhibit thermally activated
transport. Our experimental findings demonstrate that the ability to control
the degree of functionalisation of graphene is instrumental to engineer
different electronic properties in graphene materials.
[show abstract][hide abstract] ABSTRACT: The electrical properties of graphene are known to be modified by chemical species that interact with it. We investigate the effect of doping of graphene-based devices by toluene (C6H5CH3). We show that this effect has a complicated character. Toluene is seen to act as a donor, transferring electrons to the graphene. However, the degree of doping is seen to depend on the magnitude and polarity of an electric field applied between the graphene and a nearby electrode. This can be understood in terms of an electrochemical reaction mediated by the graphene crystal.
[show abstract][hide abstract] ABSTRACT: Graphene – a single layer of sp2 bonded carbon atoms arranged in a honeycomb pattern – is an indefinitely large aromatic molecule, of unique interest in the field of transparent organic electronics. This is a transparent material where charge carriers (relativistic Dirac fermions) exhibit mobilities (>10^6 cm^2/Vs) higher than Si at room temperature. However, the energy dispersion of graphene is gap-less, and this would limit its applications in electronic devices. For instance, in a graphene-based transistor the absence of the gap in the band-structure results in a relatively small resistance difference between the electro-neutrality (Dirac) region and a region with large carrier concentration (i.e., between the "on" and "off" states). Due to this significant limitation in the use of graphene in electronics, intensive research is currently underway aimed at the creation of a (tunable) gap in graphene's energy spectrum. The ability to chemically functionalize graphene, for instance with fluorine  and hydrogen  atoms, paved the way towards band-gap engineering. This type of functionalization transforms the graphene planar crystal structure, with sp2 bonds between the carbon atoms, into a three-dimensional structure with sp3 bonding between them. Theoretical predictions show that a band gap of 3.8 eV and 4.2 eV is expected for hydrogen and fluorine for 100% functionalization, respectively [3, 4]. Here I will review recent results on fluorinated graphene transistors  produced by mechanical exfoliation of natural graphite which is fluorinated to 24% and 100% (as measured by mass uptake). Transport measurements over a wide range of temperatures (from 4.2K to 300K) show a very large and strongly temperature dependent resistance in the electro-neutrality region. The strong temperature dependence of fluorinated graphene is due to the opening of a mobility in gap in the energy spectrum of graphene where electron transport takes place via localised electron states. Magneto-transport experiments through fluorinated graphene as a function of gate voltage, bias voltage, and temperature show that a magnetic field systematically leads to an increase of the conductance on a scale of a few tesla. This phenomenon is accompanied by a decrease in the energy scales associated to charging effects, and to hopping processes probed by temperature-dependent measurements. All these results demonstrate that disorder induced sub-gap states originate strong localization effects in the transport of charge carriers for energies below the energy-gap of fluorinated graphene.
Phys. Rev. B Science Phys. Rev. B J. Phys.: Condens. Matter. 01/2010; 82(21):73403-153401.