Formation mechanism of twin domain boundary in 2D materials: The case for WTe₂

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.