Graphene/h-BN heterostructures under pressure: From van der Waals to covalent

Electromechanical Modulations in Transition Metal Dichalcogenide Nanosheets: Implications for Environmental Sensors

“…In the experiment itself, a biased tip applies a desired force onto the investigated flake and, at the same time, inject charges into it. The extent of transferred charges is subsequently determined via a conventional EFM imaging procedure − (see the Supporting Information). Therefore, the result of this experiment is a plot of injected charges as a function of applied force for all TMDs investigated in this work and the reference MLG flake.…”

“…Therefore, the origins of each modification must be identified and any eventual interplay between them investigated. Our ab initio calculations indicate that the first transition is associated with pressure-induced hydrogenation of the chalcogen atom, which results in available conducting states. − In the environment of our experiment, water, from the ubiquitous contamination layer atop the samples originated from any previous exposure to ambient atmosphere, is believed to be the current H source. − The second modification, observed at high pressures, is associated with both layer–layer approximation (for FL flakes) and bond-angle modification (for both ML and FL flakes). − These distinct processes, which are now concomitantly observed in a single experiment, evidence the unique environment-related electromechanical behavior of TMD nanosheets.…”

“…In a pressure-dependent experiment, we observe an initial electrical response modification at low pressure values (which requires previous exposure to an ambient atmosphere) that is followed by a second modification at higher pressure values (which occurs even without previous exposure to an ambient atmosphere). Experimentally, such electromechanical modulations are characterized by increasing plateaus in the charge injection efficiency, which normally are a signature for gap closure and/or conducting states availability. − Even though some previous studies have reported on a single environment-related modification of TMDs (either pressure-related − or adsorbent-related − ), to our knowledge, the concomitant observation of two distinct environment-related modifications has never been reported. Therefore, the origins of each modification must be identified and any eventual interplay between them investigated.…”

“…Within this methodology, eventual electromechanical effects are investigated via a biased electrostatic force microscopy (EFM) tip that is used to simultaneously apply forces and inject charges on all TMD flakes. − Subsequent EFM imaging monitors transferred charges, evidencing any pressure-induced effects on the electrical properties of the material under investigation. Additionally, the sample atmosphere and temperature can be controlled via a homemade environmental chamber and an in situ heater, enabling the investigation of additional environment-related parameters. − …”

“…Figure a, on the left, shows two views of the atomistic models of MoSe 2 monolayer, both pristine (top) and with an upper surface completely functionalized with H atoms (bottom). We assume that this functionalization originates in reactions which occur under pressure involving water molecules found in the TMD surface in ambient conditions. − However, we emphasize that the surface reaction may have other outcomes (functionalization with −OH, for instance), including partial, rather than complete, coverages, which are also consistent with our model (see the Supporting Information for details and examples). In our discussion, we will focus on the hydrogenation since it provides a systematic scheme to investigate pressure effects.…”