Our scanning tunneling microscopy (STM) study observes, for the first time, twin domain boundary (TDB) formations on the surface of WTe₂ single crystal, which is glued by solidifying indium to Si substrate. In these TDB regions, a large inhomogeneous strain field, especially a critical shear strain of about 7%, is observed by geometric phase analysis. This observation does not obey the old believe that a small mechanical stress is sufficient to drive thermally-induced TDB formations in two-dimensional materials. To resolve the contradiction, we perform density functional theory calculations combined with elasticity theory analysis, which show that TDBs on WTe₂ are entirely displacement-induced, for which a critical strain is necessary to overcome the onset barrier.

Nanocrystal coalescence has attracted paramount attention in nanostructure fabrication in the past decades. Tremendous endeavor and progress have been made in understanding its mechanisms, benefiting from the development of transmission electron microscopy. However, many mechanisms still remain unclear, especially for nanocrystals that lack a permanent dipole moment standing on a solid substrate. Here, we report an in situ coalescence of Pt nanocrystals on an amorphous carbon substrate induced by electron-excitation-enhanced van der Waals interactions studied by transmission electron microscopy and first principles calculations. It is found that the electron-beam-induced excitation can significantly enhance the van der Waals interaction between Pt nanocrystals and reduce the binding energy between Pt nanocrystals and the carbon substrate, both of which promote the coalescence. This work extends our understanding of the nanocrystal coalescence observed in a transmission electron microscope and sheds light on a potential pathway toward practical electron-beam-controlled nanofabrication.