Graphene and Graphene Oxide

Graphene and graphene oxide layers are localized and characterized on different substrate materials by means of the spectroscopic imaging ellipsometer nanofilm_ep3se. The thickness and the dispersion functions of the refractive index n and of the extinction k of a few μm-wide layers are obtained. The results of imaging ellipsometry agree with the results obtained by the combination of AFM and confocal microscopy within the error margins. In contrast to the latter methods, the measurement time is much shorter with imaging ellipsometry. Introduction Graphene is one two-dimensional layer of the three-dimensionl graphite. The interest in graphene is supported by the remarkable electronic properties of this material and potential electronic applications. It exhibits high crystal quality, ballistic transport of electrons on a submicron scale (even under ambient conditions) and its charge carriers accurately mimic massless Dirac fermions. Graphene samples currently used in experiments are usually fabricated by micromechanical cleavage of graphite, i.e. slicing this strongly layered material by gently rubbing it against another surface. Although graphene is probably produced every time one uses a pencil, it is extremely difficult to find small graphene crystallites in the ‘haystack’ of millions of thicker graphitic flakes which appear during the cleavage. In fact, modern visualization techniques (including atomic-force, scanning tunneling and electron microscopies) have difficulties in finding graphene because of their extremely low throughput at the required atomic resolution or the absence of clear signatures distinguishing atomic monolayers from thicker flakes. Even Raman microscopy, which recently proved itself as a powerful tool for distinguishing graphene monolayers, has not yet been automated to allow search for graphene crystallites. Moreover, the differences between two layers and a few layers of graphene sheets are not obvious in Raman spectra. Although atomic force microscopy (AFM) measurement is the most direct way to identify the thickness of graphene, the method has a very slow throughput and may also cause damage to the crystal lattice during measurement. Furthermore, the interpretation of the obtained thickness values is hard to interpret, due to adsorbents on the sheet under ambient conditions[1]. Until now, the only way to isolate graphene is to cleave graphite on top of an oxidized Si wafer and then carefully scan its surface in an optical microscope[2]. Thin flakes are sufficiently transparent to add to an optical path, which changes their interference color with respect to an empty wafer. Contrast spectra and contrast images on SiO2 or Si3N4 or PMMA films can be used to determine the number and optical properties of graphene layers[2, 3]. The same inspection method applies to layers of graphene oxide, which is a material that can be exfoliated to form stable colloidal suspensions in water. Accurion GmbH Stresemannstrasse 30 37079 Goettingen, Germany Phone: +49-[0]551-999 60-0 Fax: +49-[0]551-999 60-10 E-Mail: info@accurion.com Web: www.accurion.com APPLICATION NOTE Graphene and Graphene Oxide MATTHIAS VAUPEL1), ULRICH STÖBERL2) Fig. 1: Thickness map of graphene layer n GaAs substrate, calculated in a few seconds from the Delta map in fig. 5.