  Structural transformation between metallic (1T) and semiconducting (2H)
phases of single-layered MoS2 was systematically investigated by an in situ
STEM with atomic precision. The 1T/2H phase transition is comprised of S and/or
Mo atomic-plane glides, and requires an intermediate phase ({\alpha}-phase) as
an indispensable precursor. Migration of two kinds of boundaries ({\beta} and
{\gamma}-boundaries) is also found to be responsible for the growth of the
second phase. The 1T phase can be intentionally introduced in the 2H matrix by
using a high dose of incident electron beam during heating the MoS2
single-layers up to 400~700{\deg}C in high vacuum and indeed controllable in
size. This work may lead to the possible fabrication of composite nano-devices
made of local domains with distinct electronic properties.
