Conductivity Enhancement in Transition Metal Dichalcogenides: A Complex Water Intercalation and Desorption Mechanism

The complexity of the water adsorption-desorption mechanism at the interface of transition metal dichalcogenides (TMDs) and its impact on their current transport are not yet fully understood. Here, our work investigates the swift intercalation of atmospheric adsorbates at the TMD and sapphire interface and between two TMD monolayers and probes its influence on their electrical properties. The adsorbates consist mainly of hydroxyl-based (OH) species in the subsurface region suggesting persistent water intercalation even under vacuum conditions, as determined by time-of-flight-secondary ion mass spectrometry (ToF-SIMS) and scanning tunneling microscopy (STM). Water intercalates there rapidly, within the order of a few minutes after being exposed to ambient atmosphere, this process tends to be partly reversible under (ultra)high vacuum, as observed by time-dependent scanning probe microscopy (SPM) based conductivity and ToF-SIMS measurements. A significant enhancement of the electronic properties is observed with the complete desorption of intercalated water clusters because of the pressure-induced melting effect under the tip of the SPM probe. Conversely, it also indicates that the characterization of TMD samples is substantially affected in air, in inert environments, and to some extent even in a vacuum if water intercalation is present. More importantly, STM analysis has uncovered a correlation between water intercalation and the presence of defects, showcasing their role in the gradual degradation of the material as it ages.