  The success of isolating small flakes of atomically thin layers through
mechanical exfoliation has triggered enormous research interest in graphene and
other two-dimensional materials. For device applications, however, controlled
large-area synthesis of highly crystalline monolayers with a low density of
electronically active defects is imperative. Here, we demonstrate the
electrical imaging of dendritic ad-layers and grain boundaries in monolayer
molybdenum disulfide (MoS2) grown by vapor transport technique using microwave
impedance microscopy. The micrometer-sized precipitates in our films, which
appear as a second layer of MoS2 in conventional height and optical
measurements, show 2 orders of magnitude higher conductivity than that of the
single layer. The zigzag grain boundaries, on the other hand, are shown to be
more resistive than the crystalline grains, consistent with previous studies.
Our ability to map the local electrical properties in a rapid and
nondestructive manner is highly desirable for optimizing the growth process of
large-scale MoS2 atomic layers.
