Radiation induced changes in chemical and electronic properties of few-layer MoS2 and MoTe2 films

We investigate the changes in chemical and electronic properties of partially oxidized MoS2 and MoTe2 films of a few atomic layers during X-ray photoelectron spectroscopy (XPS) measurements at different power densities and exposure times. Our results show that MoTe2 films undergo severe photon-induced chemical changes from surface TeO2 to MoTe2 by Te-O dissociation, while MoS2 undergoes much less change. We find that the reduction of p-dopant TeO2 in MoTe2 results in an upward shift of the Fermi level due to electron injection. These electronic changes in the materials can lead to misleading interpretations of device performance and operating mechanisms during operando analysis. To minimize the damage, we propose two protocols, namely "unscanned mode" and "raster mode", with high X-ray power density and short acquisition time. These protocols enable reliable operando-XPS analysis of 2D FETs using high photon flux and allow intermediate states to be observed under device operation within seconds to minutes.