2, University of Minnesota, Minneapolis, Minnesota, United States
The family of two-dimensional (2D) materials and their applications continue to expand as novel layered materials are synthesized and used in various devices. One common application is in field effect transistors (FETS), where the 2D material serves as a channel between the metallic source and drain contacts. The interface between the metal and the 2D material is critical to the device performance, but its local atomic and electronic structure remains poorly understood. Theoretical studies using atomistic simulations have shown that the embedded 2D material interacts with the metal contacts and alters its atomic structure. However, few, if any, experimental studies have supported or expanded on these theoretical claims, likely because the device-embedded 2D material is difficult to access and investigate experimentally at the necessary atomic scale. At this scale, transmission electron microscopy (TEM) may be the only viable option, but TEM investigations of devices embedded with 2D materials still remains scarce due to the susceptibility of 2D materials to electron irradiation damage.
Here, we present an experimental study that shows direct observation of bonding interactions between the metallic contact and the 2D material channel within a device using scanning TEM (STEM) in conjunction with electron energy loss spectroscopy (EELS). Using FETs with Ti contacts and MoS2 channels serving as the representative system, atomic-resolution STEM images of the Ti-MoS2 interface show distinct differences in the crystal structure between interacting and non-interacting areas within the FET. Furthermore, EELS measurements collected layer-by-layer shows a clear and systematic change in the local density of electronic states (DOS) at each layer of MoS2 as a function of distance from the Ti contact. These experimental results together with theoretical predictions show quantitatively not only how the metallic contact affects the innate structure of 2D materials but also how many layers the 2D material must be to preserve its expected properties, both of which has significant implications for device performance.