Scanning Auger Micro-spectroscopy for thickness evaluation of Graphene and Graphene Oxide layers

Graphene is the archetype 2D material, and its unique properties widely appeal to applications, from electronics, to energy storage, photonics, composite materials and sensor technology. Graphene oxide (GO) can be easily dispersed in solvents and deposited with controlled thickness onto different substrates. Because of the variety of its oxygen-functional groups, GO can interact with wide range of organic and inorganic materials. Since it is tunable from insulator to semi-metal, it is promising for applications such as plastic electronics, solar cells and biosensors. As is in general for 2D materials, the physical properties of graphene and GO crucially depend on the number of stacked atomic layers composing the film. It is therefore of the utmost importance to quantify the number of layers and the overall thickness of the films obtained from different growth processes. Nanoscale thickness metrology techniques are possible, like atomic force microscopy (AFM), optical microscopy, contrast spectroscopy, confocal Rayleigh scattering microscopy and micro-Raman (μ-Raman) spectroscopy. Among these, AFM and μ -Raman spectroscopy directly identify the number of layers. However, depending on the substrates, Raman technique may not be applicable and AFM is mainly limited by the small field of view and the need for a step-shaped sample to refer to substrate for levelling. Recently, Scanning Auger Microspectroscopy (SAM) has been demonstrated to be sensitive for thickness measurements of graphene layers on silica substrates. We extend SAM characterization to generic substrates and to GO, by evaluating reference layer number by AFM and μ-Raman, where applicable, and by overcoming the limitations of AFM and μ-Raman themselves. Firstly, updated values for the electron effective attenuation lengths in graphene and GO were experimentally determined with unprecedented accuracy, for each film/substrate combination. Then, the absolute thickness of graphene and GO flakes, up to 4 monolayers thick, was evaluated by SAM; results are consistent with AFM and Raman measurements, with a standard deviation well below a single monolayer. SAM, associated with SEM imaging, represents a method for quantitative thickness mapping of large area graphene and GO films and related devices with sub-monolayer resolution. This method is capable of detecting chemical impurities, on a very wide variety of substrates, from conductors to insulators and from heavy to light atomic weight compounds.