In situ generation and manipulation of mirror twin boundaries in transition metal dichalcogenides

The mirror twin boundary (MTB) within semiconducting transition metal dichalcogenides (TMDs) not only provides a versatile platform to explore exotic phenomena of one-dimensional (1D) systems, but also offers an effective building block to introduce novel electronic states in the TMDs. In spite of extensive efforts on introducing the MTBs in the TMDs, creating and manipulating the MTBs with custom-designed sites remains an outstanding challenge in experiment. Here we present a facile method for in situ generation of MTBs in three different TMDs at predetermined positions by scanning tunneling microscope (STM) tip pulses. In $\mathrm{MoT}{\mathrm{e}}_{2}$, the length and density of the MTBs can be further manipulated using a STM tip. Moreover, we demonstrate the ability to introduce uniform-sized and orderly arranged MTB triangular loops, which generate low-energy flat electronic bands in the $\mathrm{MoT}{\mathrm{e}}_{2}$. A periodic modulation in the density of states with a $3a$ period ($a$ is the lattice constant) is observed in the MTBs. Unexpectedly, our experiment indicates that the MTBs exhibit length preference with $3\mathit{Na}$ ($N$ is an integer), implying that the mechanism for the periodic modulation in the density of states also plays a role in affecting the formation of the MTBs.