Scanning Tunneling Microscopy of an Air Sensitive Dichalcogenide Through an Encapsulating Layer.

Many atomically thin exfoliated two-dimensional (2D) materials degrade when exposed to ambient conditions. They can be protected and investigated by means of transport and optical measurements if they are encapsulated between chemically inert single layers in the controlled atmosphere of a glovebox. Here, we demonstrate that the same encapsulation procedure is also compatible with scanning tunneling microscopy (STM) and spectroscopy (STS). To this end, we report a systematic STM/STS investigation of a model system consisting of an exfoliated 2H-NbSe crystal capped with a protective 2H-MoS monolayer. We observe different electronic coupling between MoS and NbSe from a strong coupling when their lattices are aligned within a few degrees to essentially no coupling for 30° misaligned layers. We show that STM always probes intrinsic NbSe properties such as the superconducting gap and charge density wave at low temperature when setting the tunneling bias inside the MoS band gap, irrespective of the relative angle between the NbSe and MoS lattices. This study demonstrates that encapsulation is fully compatible with STM/STS investigations of 2D materials.