Graphene has emerged as a promising material for a variety of applications including transistors, photovoltaics, batteries and sensors. In particular graphene holds promise to replace the expensive ITO in photovoltaics and serve as scaffold for engineering electrochemically stable electrodes for batteries. To this end the work function of graphene needs to be tuned, while maintaining its remarkable conductivity and transparency. We recently developed an electrochemical tool  which allows the controlled covalent modification of the graphene using a variety of compounds (i.e. donors or acceptors). The graphene functionalization is performed using diaryliodonium salts which are preferred to the well-studied diazonium salts in order to avoid spontaneous functionalization. We show that the grafting density of nitrophenyl groups (-NO2Ph) for instance can be precisely tuned between 4. 1013 up to 3.1014 molecules/cm2. Local electronic structure and nature of chemical bonding is studied via a combination of Low Temperature Scanning Tunneling Microscopy (LT-STM), Scanning Tunneling Spectroscopy (STS) and Non-Contact Atomic Force Microscopy (NC-AFM) using an Omicron LT-STM/SPM system. We worked with a variety of epitaxial single layer graphene on both insulating and conductive (0001) SiC with and without buffer layer. We show that removal of the buffer layer allows clear observation of the nitrophenyl modifications and are an ideal platform for work function and bandgap engineering studies in graphene. Contrary to suggestions in the literature we do not observe an increased reactivity at defect sites or SiC step edges. Large increase in the density of states is observed at the modification site. STS and XPS data show that the -NO2Ph grafts induce n-type doping in graphene consistent with charge transfer from the grafts to the graphene.
1. C. K. Chan, T. E. Beechem, T. Ohta, M. T. Brumbach, D. R. Wheeler, K. J. Stevenson, J. Phys. Chem. C 2013, 117, 12038.