Operando PhotoemissionImaging of the Energy Landscapefrom a 2D Material-Based Field-Effect Transistor

As the integration of transition metal dichalcogenides (TMDC) becomes more advanced for optoelectronics, it is increasingly relevant to develop tools that can correlate the structural properties of the materials with their electrical output. To do so, the determination of the electronic structure must go beyond the hypothesis that the properties of the pristine material remain unaffected after the device integration, which generates changes in the dielectric environment, including electric fields that are likely to renormalize the electronic spectrum. Here, we demonstrate that nanobeam photoemission spectroscopy is a well-suited tool to unveil the device energy landscape under operando conditions. Both the gate vertical field and the drain in-plane vectorial electric field can be determined with a sub-μm resolution. We provide a correlative description of a field-effect transistor to connect its bias-modified energy landscape with the transistor electrical output. The method appears highly suited to unveil how the actual geometry of the flake (thickness, edge effect, presence of structural defects, etc.) is driving the current flow within the device. Lastly, the method appears fully compatible with traditional device fabrication, therefore making it relevant for systematic rational optimization of TMDC-based electronic devices.