Electronic ground state evolution of two-dimensional transition metal dichalcogenide alloys throughout the metal-semiconductor transition

Transition metal dichalcogenide (TMD) alloys offer unprecedented versatility to engineer 2D materials with tailored properties for specific purposes. These include the unique opportunity to explore novel structural and electronic phase transitions in two dimensions (2D). In this work, we report on the atomic-scale evolution of the electronic ground state of a monolayer of Nb_1-δ Mo_δSe₂ with 0 < δ < 1 by means of low-temperature (300 mK) scanning tunneling microscopy/spectroscopy (STM/STS). In particular, we first investigate the atomic and low-energy electronic structure of this 2D alloy throughout the metal to semiconductor transition from the monolayer of NbSe₂ to the monolayer of MoSe₂. Our STS measurements enable to extract the effective doping of Mo impurities, the bandgap evolution as well as the band shifts.  Furthermore, we probe the existence and properties of collective electronic phases (charge density wave (CDW) and superconductivity) to demonstrate a remarkable robustness against impurities. Our results paint a clear and detailed picture of the evolution of the electronic structure in 2D TMD alloys, which is of upmost relevance for future 2D materials' design.