High-throughput optical thickness and size characterization of 2D materials.

We describe a method using simple optical microscopy and image processing that simultaneously characterizes thousands of nanosheets in a sample area on the order of 1 mm2. Including data acquisition and processing, both the number of atomic layers and the lateral sizes of all sheets can be obtained within a few hours-approximately 100 times faster than with previous methods, such as atomic force microscopy. This is achieved by normalizing the optical image based on substrate brightness, which eliminates inhomogeneities usually limiting optical techniques. Ultimately, the method enables robust statistical analysis of populations of nanosheet materials. We demonstrate the utility of this method by examining fractions made from a sample of graphene oxide (GO) made using an emulsion-based method. Beyond providing the morphological composition of the samples, the reported method is sensitive enough to provide information about the oxidation level of a population of GO sheets and, correspondingly, optical constants of the material.

Altering and investigating the surfactant properties of graphene oxide.

Graphene oxide (GO) is a two-dimensional material with a hydrophobic carbon lattice functionalized with hydrophilic oxygen groups on the edges and basal plane. Its hydrophobic/hydrophilic nature allows it to behave as a surfactant, stabilizing emulsions of oil in water. In the investigation described here, we study GO's emulsifying ability by using it to template the polymerization of polystyrene while tuning the hydrophobicity and degree of oxidation of the GO sheets. SEM, TGA, optical microscopy, acoustic spectroscopy, and digital image analysis are used to characterize the system. The size of the GO templated PS spheres is found to be a function of the amount of GO in the system, with too little GO resulting in PS spheres with no GO shell. Increasing the hydrophobicity of the sheets is shown to lead to the formation of inverse emulsions, while increasing the graphitic character of the GO results in distorted styrene droplets as the GO sheets become more planar and less able to conform to a smoothly curving interface.