Nanoscale Adhesion and Material Transfer at 2D MoS 2 -MoS 2 Interfaces Elucidated by In Situ Transmission Electron Microscopy and Atomistic Simulations.

Low-dimensional materials, such as MoS 2 , 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 MoS 2 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 MoS 2 were 604 ± 323 mJ/m 2 and 932 ± 647 mJ/m 2 , respectively, significantly higher than previously reported values for mechanically exfoliated MoS 2 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 MoS 2 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 MoS 2 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.