Conductive-probe AFM characterization of graphene sheets bonded to gold surfaces
Highlights
► Graphene sheets are bonded to polyaminophenylene films grafted on gold substrates. ► CP-AFM is a powerful tool to discern different numbers of stacked graphene sheets. ► Friction is reduced on graphene sheets in comparison with the organic coating. ► Current–voltage curves display characteristics of metal–molecule–metal junctions.
Introduction
Electronic transport properties of graphene [1] films have recently attracted extensive attention. Graphene is considered by academic and industrial researchers as a potential candidate for next generation applications in nanoelectronics and miniaturised devices. Graphene is a two dimensional (2D) hexagonal lattice of carbon atoms, which is the building block for the formation of 3D graphite. Graphene can thus be expected to have the good friction properties of graphite which is a well known solid lubricant [2]. In order to use graphene in nanoelectronic devices, it is thus of major interest to study the electrical and mechanical properties of graphene layers at a nanoscopic scale. The atomic force microscope is one of the devices which allow investigating atomic scale phenomena. Local electronic transport phenomena have been explored by STM techniques [3], [4], [5] and more recently by conducting probe atomic force microscopy (CP-AFM) [6], [7]. Lateral force microscopy has proven to be useful when materials with different surface energies or cohesion need to be characterized [8], [9] in order to study their friction properties. In spite of numerous theoretical [10], [11], [12], [13] studies and more and more experimental [14], [15] works on graphene, there is no study combining topographical, electrical and frictional measurements of graphene layers grafted on gold coated substrates. Banerjee et al. [16], [17] and Wielgoszewki et al.[18] reported electrical studies of graphene ribbons on highly oriented pyrolytic graphite using AFM. Few friction studies on graphene have been reported [19], [20], [21]. Friction properties of single and bilayer graphene films grown epitaxially on SiC have been studied by Filleter et al. [20], who showed that graphene reduced the friction with respect to SiC, especially for bilayer films. More recently, Kellar et al. [21] characterized epitaxial graphene domains on partially graphitized SiC (0 0 0 1) using a suite of scanning probe techniques, including CP-AFM and LFM.
Graphene can be prepared using mechanical exfoliation with “scotch tape” [13], [14], [15], chemical exfoliation by oxidation [22], electrostatic deposition [23] or epitaxial growth [20]. The “scotch tape” method, which removes thin sheets from graphite, is convenient for trace amounts of material but requires a thick layer of adhesive as thin layers do not resist to mechanical stress. Thick layers of insulating adhesive are not very suitable for CP-AFM. On the other hand, oxidation of graphite to graphene oxide (GO) disturbs the graphitic nature of the resulting nanosheets and leads to a loss of conductivity [24], even after reduction by chemical, thermal or electrochemical treatments, due to partial recovery of the graphitic character of the chemically converted graphene. Viel et al. [25] have successfully immobilized graphene sheets on gold substrates coated with self-adhesive surfaces of polyaminophenylene (PAP) films prepared from aryldiazonium salts.
In the present paper, we report the first results of the investigations at a microscopic scale of the mechanical, electrical and frictional properties of graphene layers chemically bonded on PAP films grafted on Au-coated wafers.
Section snippets
Experimental
Imaging, current–voltage (I–V) and deflection–resistance (DRz) curves were performed under ambient conditions with an atomic force microscope (D.I. Nanoscope III), to which a home-made current amplification and conversion device, called “Resiscope” was added to investigate the local electrical properties. The first version of the device was presented in a paper published in 1996 [26]. Since then, a larger range of resistance values can be measured, from 102 to 1012 Ω, the protective resistance RO
Results and discussion
Fig. 1 gives a typical example of the three AFM images (topographical, local electrical resistance and friction images) acquired on a graphene layer immobilized on the PAP modified gold substrate as described above. Representative cross-sectional profiles are also displayed for each image. The figure shows the AFM images of the edge of a very thin graphene film. On the topography image, shown in Fig. 1(a), we can see the graphene layer on the right hand side, slightly brighter than the PAP
Conclusion
In summary, we have shown that CP-AFM is very useful to characterize graphene layers and is a powerful tool to discriminate between very thin and thicker graphene flakes. A few sheets of graphene bonded to a grafted PAP film on gold are hardly seen on topographical AFM images, but lateral force microscopy proved efficient to distinguish these graphene layers from the underlying organic film. We were thus able to locate flakes of a very small number of graphene sheets. However, the most
Acknowledgement
This project has been supported by the RTRA “Triangle de la Physique” through the GRAMINE project under contract number 2008-055T.
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