Simple approach for high-contrast optical imaging and characterization of graphene-based sheets

2 ABSTRACT A simple optical method is presented for identifying and estimating the optical properties of nanometer-thick graphene-based materials. The method is based on the use of substrates consisting of a thin dielectric layer, of controlled thickness, on silicon. The contrast between reflected light from the graphene-based material and the substrate is optimized through the choice of the optical properties and the thickness of the dielectric layer. Optimal substrate designs are calculated, and their properties are verified by confocal microscopy. By comparing the measured and predicted contrast of single layers of graphene oxide, the optical properties before and after a thermal treatment are obtained. Substrates optimized for high-contrast imaging should find further use for optically identifying and characterizing exceptionally thin platelets and also, perhaps, thin biological material that might otherwise not be discerned through " standard " optical microscopy. 3 Identifying and characterizing a single nanometer-scale layer, or a small number of layers, of materials such as graphite, any of a number of clays, or metal dichalcogenides such as WS 2 , is challenging, but is critical for the study of such materials. 1,2 Scanning probe microscopy (SPM) can both identify the presence of such thin platelets and determine their lateral and height dimensions. 3 SPM methods such as atomic force microscopy (AFM), though, are somewhat time-consuming. In order to obtain accurate values of height, thereby discriminating between a single layer of material and a bilayer, a relatively small area must be scanned. Scanning electron microscopy can also, in principle, be used for identification of individual layers versus multilayer platelets, but SEM imaging typically induces the formation of a layer of contaminant in the exposed region. 4 Optical methods, on the other hand, offer the potential for rapid, non-destructive characterization of large-area samples. Ellipsometry, for example, is widely used to determine the optical constants and thicknesses of thin films. Standard ellipsometers, though, require samples with lateral dimensions well over a millimeter. By contrast, imaging ellipsometry can have sub-micrometer resolution, and may be useful for probing optical constants and thicknesses. 5,6 This work is ongoing, and will be reported elsewhere. For the past two years, we have focused on simpler methods that allow the use of standard confocal microscopy for rapid identification and characterization of the optical response of thin platelets. Optical microscopy has previously been used for identification of single graphene sheets, by incorporating an intermediate dielectric layer between the …