Sub-Micron Imaging of Encapsulated 2D layers of Graphene and Transition Metal Dichalcogenides by Conductive Scanning Probe Microscopy

Encapsulation of 2D materials protects them from environmental disturbances and significantly improves their quality. However, these benefits are lost if impurities or structural defects become trapped within the encapsulating layers. It is therefore crucial to detect these prior to embarking on time-consuming device processing. While encapsulated flakes can be detected via post-processing of optical images or by confocal Raman microscopy, these techniques lack the sub-micrometer resolution to identify structural defects and charged impurities within the encapsulated layer. We demonstrate a facile technique to visualize charged contaminants within the heterostructure by measuring surface potential fluctuations using Kelvin probe force microscopy (KPFM). By applying a fixed tip bias larger than the surface potential fluctuations, the encapsulated flakes and their sub-micron structural defects, cracks, and bubbles can be detected through electrostatic force microscopy (EFM). We show that these methods, which are standard extensions of atomic force microscopy (AFM), are perfectly suited for imaging encapsulated conductors and their local charge environments.