Spatially mapping work function changes and defect evolution in the fluorination of graphene

When graphene supported on SiO₂ is fluorinated, XPS reveals an increase in concentration of chemically-adsorbed fluorine (higher F/C ratio with C-CF, C-CF₂, C-F and C-F₂ but no C-F₃) on the graphene surface with time. Raman I_D/I_G ratio, i.e. a measure of non-sp²to Sp² bonding states, increases with time before showing a decrease suggesting a surface morphology change owing to C-F bonding followed by disordering of the π-electron system. AFM surface morphology scans reveal that defects (holes), which increases in size with time, are observed to form preferentially at the boundary of the graphene flakes. Synchronized Kelvin-Probe Force-Microscopy (KPFM) mapping of the graphene region surrounding these holes shows a higher work-function, φ, giving rise to a donut-shape contact potential difference (CPD) which increases from 4.9 ± 0.1 eV to 5.4 ± 0.1 eV with fluorination. Together with XPS and Raman results, the increase in φ can be attributed to the presence of a higher concentration of fluorine in the graphene region (C-F/C-F₂ bonds) surrounding these holes. The formation of the hole-defects on graphene and its subsequent increase in size with fluorination is thus a result of aggregation of adsorbed fluorine and removal of carbon likely in the form of CF₄ or C₂F₄.