Adsorbate-Dependent Conductivity of MoS2 FETs

Two-dimensional (2D) nanomaterials are heavily being investigated for future applications in physics, chemistry, and nanoelectronics. They appear to exhibit many novel phenomena, not found in their bulk counterparts, even when only a single atomic layer of the material is used in a device. Beyond the well known 2D material graphene, transition metal dichalcogenides (TMDs), such as MoS2 and WS2, appear to provide the ideal properties to fulfill the promise of eventually replacing silicon at the nanoscale. These films provide a sufficiently large band gap, holding a great promise in their use in future digital electronics and in low -power sensing applications. The reduced dimensions ensure a strong impact from surface effects, offering the potential for ultra-sensitive gas- and biosensors. However, this high sensitivity comes with a potential downside: If a change in the molecular make-up of the ambient conditions modifies the material behavior, then devices based upon these materials will be lim ited in their applicability. Therefore, in this project, we will study how small molecules which are typically found in the air around us affect 2D materials and the thereupon based electronic devices. We aim to provide a physical understanding of the adsorption and desorption mechanisms and the preferred adsorption sites for typical ambient molecules, such as H 2, O2, and H 2O, on pristine and defected TMD surfaces. Physical models will be developed which can be used to understand how the presence of various molecules impact the performance of devices and circuits based on these materials. This project requires an interdisciplinary approach, including ab-initio density functional theory (DFT) modeling and ultra-high vacuum x-ray photoelectron spectroscopy (XPS) / scanning tunneling microscopy (STM) characterization to understand the make-up and electrical properties of the 2D materials. A Monte Carlo framework will then be used to calculate the macroscopic conductivity based on the microscopic behavior of charge carriers. Finally, a drift- diffusion model will be devised to enable transient simulations to study the mixed -mode operation of TMD-based devices and circuits.