Davisson-Germer Prize in Atomic or Surface Physics Talk: Studies of Two-dimensional Materials using Tunneling Electrons*

Over the past decade, much research world-wide has focused on two-dimensional (2D) materials, in which the electrons are localized within a single atomic plane. Obtaining μm-size flakes of 2D material by “exfoliating” (peeling apart) layers using adhesive tape has been a standard practice for decades, but only recently has this method been applied to produce small, microfabricated electronic devices on the flakes (Geim and Novoselov, Nobel Prize 2010). However, for practical electronics of the future, such devices must be produced on grown (deposited), large-area 2D layers, rather than on flakes. In this talk, studies of the structure of grown 2D layers will be described, focusing on heterobilayers of MoS₂ on WSe₂. The method of scanning tunneling microscopy is used to obtain detailed, atomic-scale views of the structure of the layers. Additionally, through spectroscopic measurements with the tunneling microscope, band gaps of the materials and band offsets between neighboring layers are determined. We find, in particular, the occurrence of localized electron states associated with the moiré pattern that forms when one layer of a 2D material (MoS₂) is placed on another layer (WSe₂) with different lattice constant.