Direct TEM observations of growth mechanisms of two-dimensional MoS 2 flakes

A microscopic understanding of the growth mechanism of two-dimensional materials is of particular importance for controllable synthesis of functional nanostructures. Because of the lack of direct and insightful observations, how to control the orientation and the size of two-dimensional material grains is still under debate. Here we discern distinct formation stages for MoS 2 flakes from the thermolysis of ammonium thiomolybdates using in situ transmission electron microscopy. In the initial stage (400 °C), vertically aligned MoS 2 structures grow in a layer-by-layer mode. With the increasing temperature of up to 780 °C, the orientation of MoS 2 structures becomes horizontal. When the growth temperature reaches 850 °C, the crystalline size of MoS 2 increases by merging adjacent flakes. Our study shows direct observations of MoS 2 growth as the temperature evolves, and sheds light on the controllable orientation and grain size of two-dimensional materials.