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.
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