Graphene, with its two-dimensional, hexagonal honeycomb lattice structure and semimetallic characteristics, has great potential for use in a diverse array of optoelectronic applications, especially now that synthetic routes for its large-scale synthesis have been demonstrated. One route to achieving this goal is through chemical functionalization, which can convert graphene, with its bandgap of zero, to a wide-bandgap semiconductor. In addition, patterned multifunctional regions could be used to form the superlattices required for devices such as chemical sensors and thermoelectrics. Toward these ends, J.M. Tour and colleagues at Rice University and Tianjin University have demonstrated a two-step process to first hydrogenate a pattern on the basal plane of graphene and then convert the hydrogens to a different functionality.
As reported in the November 29, 2011 issue of Nature Communications (DOI: 10.1038/ncomms1577), the researchers transferred graphene originally grown on Cu substrates to an insulating substrate (either quartz or SiO2/Si) and then used conventional lithography to expose defined regions of the graphene to atomic hydrogen. Fluorescence quenching microscopy (FQM) was used to image the regions of hydrogenated graphene, which are termed graphane; see (a) in the figure. Partial hydrogenation of the graphene was confirmed using Raman spectroscopy. Measurement of the electronic properties of this material using four-probe analysis demonstrated a gradual transformation from semimetallic graphene to a near insulating graphane-like material with increasing hydrogenation.
The researchers also further functionalized the graphene/graphane superlattice with 4-bromophenyldiazonium tetrafluoroborate. They proposed that spontaneous electron transfer occurs from the graphane to the diazonium salt, generating an aryl radical that attacks the sp3 C–H bonds to form new, covalent sp3 C–C bonds; see (b) in the figure. Transmission electron microscopy and electron diffraction patterns of the diazonium functionalized films confirmed that the graphene structure survives the functionalization reactions. The extent of diazonium functionality was investigated using x-ray photoelectron spectroscopy, which showed that functionalized films containing as much as one new sp3 C–C bond for every 21.5 C atoms in the graphane domains could be achieved using this methodology.
The researchers said that their “two-step controlled covalent functionalization process permits modulation of the electronic properties of graphene’s basal planes and could hold promise for specifically patterned optoelectronic and sensor devices based on this exciting new material.”