Fluorination of suspended graphene

Functionalization is a well-established method to manipulate the electronic properties of graphene. It consists in the substitution of carbon atoms in the hexagonal network by other elements such as heteroatoms (nitrogen or boron, the most common) or in the introduction of more complex functional groups. The customization of the graphene exceptional electronic properties by the functionalization opens different avenues for future applications including bio and chemical-sensors. Among various functionalization methods, plasma process and ion irradiation have been widely employed for the modification of surface chemical composition and properties. These techniques have attracted the attention of a vast scientific audience because they can be used to tailor the surface reactivity in different materials making them suitable for various applications ranging from chemical sensing to medical implants. In particular, the fluorination of graphene allows the tuning of the optical bandgap, introducing a progressive semiconducting behaviour for increasing fluorine content ending in insulating properties for fully fluorinated graphene.
In the present work, we study the fluorination of graphene, both suspended and supported on copper foil. By using the ion implantation functionalization strategy, we can probe two fundamental aspects: the effect of different ion kinetic energy and the influence of a substrate in the fluorination mechanism. A key point in this study is the use of spatially-resolved XPS performed with the scanning photoelectron microscope at the ESCAmicroscopy beamline at Elettra. With this technique, we are able to probe the effect of different fluorination parameters in individual flakes, thus avoiding contribution to the XPS spectra from atoms in undesired regions, e.g. supporting grid or substrate. Fig. 1 shows the experimental geometry and the F1s and C1s core levels recorded on a suspended flake. Performing the functionalization under the same ultra-high vacuum condition as for the measurements results in a contamination-free fluorination.
The precursor gas used for fluorination is the tetrafluoromethane (CF₄), which is fed in the plasma source and fragmented during the discharge. At low ion kinetic energy, CF_x functional groups are adsorbed on the graphene surface and in the C1s core level spectrum we can identify contributions from photoelectrons emitted from C-F₂ and C-F₃. When the fluorination is performed using accelerated ions, the C-F₃ component is not observed due to a higher fragmentation of the CF_x products prompted by the increased number of collisions. In addition, the accelerated ions create carbon vacancies in the graphitic network: these sites are more reactive, leading to fluorine-atom implantation, thus, the C-F₂ component is related to the creation of defective sites decorated with fluorine atoms that are covalently bonded to carbon. In contrast, fluorination with low kinetic energy ions reduces the probability of lattice ‘breaking’ leading to a decrease in the functionalization yield.

Figure 1. The changes induced on graphene flakes by fluorine ion implantation are evaluated by spatially resolved XPS measurements performed at the ESCAmicroscopy beamline. A zone plate (ZP) allows focusing the x-ray beam (hv) down to 120 nm diameter spot. The F1s (top) and C1s (bottom) experimental data (black symbols) are acquired from a selected area of suspended graphene after fluorine irradiation. The red solid lines represent the fitting curves for the components associated with different C–F and C–C bonds. A Shirley-type background is used (grey solid line).

By comparing the functionalization of suspended and supported graphene on copper foil, we can conclude that many ions traverse the suspended graphene without inelastic interaction. On the contrary, when fluorination is performed on supported graphene, the energy of the incident species is dissipated at the substrate surface generating backscattering, which increases the probability of fluorine grafting and defects’ creation as confirmed by the observed 12.5% increase in the sp³/sp² ratio.

In summary, spatially resolved chemical analysis allows to define the influence of the substrate on graphene fluorination. Suspended graphene provides a preferable model for investigating the intrinsic properties of irradiated carbon nano-systems, since the interference from a supporting substrate disturbing the graphene network is avoided. Varying the ion kinetic energy allows the formation of different C-F configurations, with an increased covalent bonding when 1 kV ion acceleration is used. 


This research was conducted by the following research team:

Claudia Struzzi¹, Mattia Scardamaglia¹, Nicolas Reckinger², Jean-François Colomer², Hikmet Sezen³, Matteo Amati³, Luca Gregoratti³, Rony Snyders^1,4, and Carla Bittencourt¹

¹ Chimie des Interactions Plasma-Surface, CIRMAP, University of Mons, 7000 Mons, Belgium
² Research Group on Carbon Nanostructures (CARBONNAGe), University of Namur, 5000 Namur, Belgium
³ Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
⁴ Materia Nova Research Center, 7000 Mons, Belgium