Electrical and structural mapping of friction induced defects in graphene layers

Developing low friction and high conduction layers for electrical contacts has been a challenge for many years. Graphene is known to display outstanding friction and electrical behaviors; lately much work has been conducted both in the applied and fundamental fields of friction in order to understand the mechanisms involved. In the past years we have done experimental work on the tribological behavior of galvanic gold surfaces coated with various types of graphene-like coatings deposited by spraying liquid phase exfoliated graphite. The results showed that the unevenly distributed graphene-like flakes on the surface could bring strong friction reduction. However such coatings are difficult to control and a more controllable deposition technique such as CVD is considered here. The aim of this work is to start investigating the effect of friction on the properties of graphene layers at a microscopic scale and at a macroscopic scale. The first aspect is presented here. In order to assess the effect of friction and wear on graphene films, an atomic force microscope (AFM) is used. The effects of friction of the cantilever tip on the graphene coated plane are investigated. Topographic and electrical properties of the wear scars are recorded as well as the structure of the coating, thanks to a new customized system consisting of a Conducting Probe-AFM (CP-AFM) fitted confocal Raman microscope. Simultaneous topographic maps, electrical maps and Raman maps can be recorded on the friction tracks produced on the graphene coated surfaces. Electrical maps are built from the local values of tip/surface resistance while structure maps are built from the intensity of chosen representative peaks of the Raman spectra. Preliminary results were acquired on CVD graphene transferred to a silicon substrate. This involves a special technique that can induce defects in the graphene film and leave some insulating contamination on the surface. Friction behavior is investigated for different numbers of cycles and for different normal loads. Both parameters seem to govern the behavior of the graphene film. The quantity of defects in the film structure and its wear off are correlated. From the understanding of the mechanisms involved in controlled samples such as CVD graphene on silicon, we aim at bringing insight in the behavior of graphene coatings deposited on gold for an electrical contact application. Such knowledge is necessary to assess the durability of the coatings in various situations such as wear and atmospheric ageing.