Nanoscale Adhesion and Material Transfer at 2D MoS₂-MoS₂ Interfaces Elucidated by In Situ Transmission Electron...

Low-dimensional materials, such as MoS₂, hold promise for use in a host of emerging applications, including flexible, wearable sensors due to their unique electrical, thermal, optical, mechanical, and tribological properties. The implementation of such devices requires an understanding of adhesive phenomena at the interfaces between these materials. Here, we describe combined nanoscale in situ transmission electron microscopy (TEM)/atomic force microscopy (AFM) experiments and simulations measuring the work of adhesion (W_adh) between self-mated contacts of ultrathin nominally amorphous and nanocrystalline MoS₂ films deposited on Si scanning probe tips. A customized TEM/AFM nanoindenter permitted high-resolution imaging and force measurements in situ. The W_adh values for nanocrystalline and nominally amorphous MoS₂ were 604 ± 323 mJ/m² and 932 ± 647 mJ/m², respectively, significantly higher than previously reported values for mechanically exfoliated MoS₂ single crystals. Closely matched molecular dynamics (MD) simulations show that these high values can be explained by bonding between the opposing surfaces at defects such as grain boundaries. Simulations show that as grain size decreases, the number of bonds formed, the W_adh and its variability all increase, further supporting that interfacial covalent bond formation causes high adhesion. In some cases, sliding between delaminated MoS₂ flakes during separation is observed, which further increases the W_adh and the range of adhesive interaction. These results indicate that for low adhesion, the MoS₂ grains should be large relative to the contact area to limit the opportunity for bonding, whereas small grains may be beneficial, where high adhesion is needed to prevent device delamination in flexible systems.