Intercalation Dynamics, Charge Transfer and Defect Correlation in Transition Metal Dichalcogenides/Sapphire Interfaces

Due to their unique properties, offering prospects in multitudinous applications, two-dimensional (2D) materials have received significant interest in the research community. A pivotal factor to consider is their sensitivity to atmospheric gas species, as experimental observations have shown consistently that defective transition metal dichalcogenides (TMDs) layers interact with oxygen, water, and ambient gasses^1,2. Undoubtedly, as commonly used characterization techniques are often performed in an ambient or inert environment, they can contribute to unreliable quality checks. Although, a clear insight on the interaction mechanism with water and ambient has been established for graphene, the key role atmospheric adsorbates play in modifying the material properties of layered TMDs has not yet been fully understood. Therefore, in this work scanning probe microscopy techniques, such as Conductive - Atomic Force Microscopy (C-AFM), Kelvin Probe Force Microscopy (KPFM) and Scanning Tunneling Microscopy (STM) along with Time of Flight – Secondary Ion Mass Spectrometry (ToF-SIMS), are carried out in (ultra-)high vacuum to identify the intercalated adsorbates in the MoS₂/sapphire and WS₂/sapphire systems and to explore the complexity of the adsorption-desorption mechanism. Contact mode experiments reveal a conductivity increase with the removal of intercalated species. Re-exposure to atmospheric ambient proves intercalation to be a rapid process happening in the order of minutes. Our experimental data suggests that the intercalated species are water molecules at the interface, which is supported by ToF-SIMS findings. Moreover, the collected C-AFM data shows a local decrease in conductivity in the water intercalated regions, indicating that a charge transfer occurs to the adsorbates, depleting the <i>n</i>-type 2D layer. On the contrary, contact potential differences obtained by KPFM seemingly imply an improvement of <i>p</i>-doping over time. However, when considering the 2D screening of substrate interface charges³, this trend can rather be quantitatively interpreted as an enhancement in <i>n</i>-doping, which is in consonance with the C-AFM findings. Besides verifying that intercalation takes place at the interface, STM uncovers that the confined water layer (5.6 Å) is comprised of three phases: an ice, a quasi-liquid, and a triple water layer⁴. Lastly, time dependent STM has established a distinct interdependence between water intercalation and the presence of defects. Either defects form selectively and gradually in water intercalated areas or water intercalation is facilitated by pre-existing defects associated to local inhomogeneities in growth. Regardless, this discovery must be factored when evaluating the degradation of device performance with aging. Our work shows that TMD/sapphire samples are prone to water intercalation, which heavily affects the electronic properties, potentially leading to material degradation.<br/><br/>(1) Tongay, S.; Zhou, J.; Ataca, C.; Liu, J.; Kang, J. S.; Matthews, T. S.; You, L.; Li, J.; Grossman, J. C.; Wu, J. Broad-Range Modulation of Light Emission in Two-Dimensional Semiconductors by Molecular Physisorption Gating. <i>Nano Lett.</i> <b>2013</b>, <i>13</i> (6), 2831–2836.<br/>(2) Nan, H.; Wang, Z.; Wang, W.; Liang, Z.; Lu, Y.; Chen, Q.; He, D.; Tan, P.; Miao, F.; Wang, X.; Wang, J.; Ni, Z. <i>Strong Photoluminescence Enhancement of MoS 2 through Defect Engineering and Oxygen Bonding</i>.<br/>(3) Castellanos-gomez, A.; Cappelluti, E.; Roldán, R.; Agraït, N. Electric-Field Screening in Atomically Thin Layers of MoS 2 : The Role of Interlayer Coupling. <b>2013</b>, 899–903.<br/>(4) Sotthewes, K.; Bampoulis, P.; Zandvliet, H. J. W.; Lohse, D.; Poelsema, B. Pressure-Induced Melting of Confined Ice. <i>ACS Nano</i> <b>2017</b>, <i>11</i> (12), 12723–12731.