Evidence for Chemical Vapor Induced 2H to 1T Phase Transition in Transition Metal Dichalcogenide Films

Transition metal dichalcogenides (TMDs) show remarkable potential for use in chemical vapor sensor devices. They are inexpensive, inherently flexible, low-power, can be grown in large areas, and have shown high sensitivity and selectivity to electron donor analyte molecules. We present both conductance and optical evidence that the phase transition can be induced in MoX₂ films by a saturating dose of strong electron donor vapor. We find that the conductance response to strong electron donors in both monolayer MoS₂ and MoSe₂ FET devices ceases after moderate exposure, with final value of the conductance being on order of that expected for the 1T phase. We also examine chemically exposed TMD films intermittently interrogated with Raman and photoluminescence spectroscopy. We observe the appearance of weak characteristic 1T phase Raman features for MoS₂ and we observed a quenching of the photoluminescence of both TMD films that is recoverable with annealing. We compare the electronic and optical data to what would be expected for doping mechanisms or oxidation alone, and we conclude that the data cannot by explained solely by doping mechanisms. Our results suggest a mechanism for a new type of optoelectronic chemical vapor sensor.

A.L. Friedman, et al. Sci. Rep. 7, 3836 (2017).