Correlated Optical and Atomic Force Microscopy Characterization of Two-dimensional Atomic-layered Materials

Single layers of two-dimensional atomic-layered materials, such as graphene–a single layer of graphite–exhibit unique physical properties as compared to the corresponding bulk materials. These properties offer possibilities of engineering atomically thin devices. Single layers can be synthesized by chemically and/or mechanically processing bulk materials, but the yield of single-layer material produced relative to less-desirable multi-layer material depends strongly on the process parameters. These materials can be characterized quickly with optical microscopy (OM), but it is non-trivial to determine the layer thickness with OM, so it is difficult to know which structural features are single layers versus multiple layers. Atomic force microscopy (AFM) precisely measures layer thickness, but AFM is relatively slow. Here, we describe our efforts on correlating AFM images with OM images in order to establish a high-speed OM technique for single-layer material characterization and identification. Two-dimensional atomic-layered materials processed by mechanical exfoliation were transferred to oxidized silicon wafers and imaged by both AFM and OM. Single-layer and few-layer features identified in AFM images were compared to corresponding features in OM images. By processing OM images using Image J software, we established a quantitative correlation between OM image contrast and the layer thickness of the material thereby enabling rapid, easy identification of single-layer materials with OM. These results indicate a viable route for high-speed characterization of atomic-layered materials synthesis processes.