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 MoS2 flakes from the thermolysis of ammonium thiomolybdates using in situ transmission electron microscopy. In the initial stage (400 °C), vertically aligned MoS2structures grow in a layer-by-layer mode. With the increasing temperature of up to 780 °C, the orientation of MoS2 structures becomes horizontal. When the growth temperature reaches 850 °C, the crystalline size of MoS2 increases by merging adjacent flakes. Our study shows direct observations of MoS2 growth as the temperature evolves, and sheds light on the controllable orientation and grain size of two-dimensional materials.
Distinct formation stages of MoS2 flakes from an amorphous precursor was observed by means of in situ heating. Researchers showed layer by layer formation of vertically aligned flakes at 400 C, vertical to horizontal transformation upon heating to 780 C and finally further growth in crystal size upon heating to 850 C. HRTEM imaging and diffraction patterns along with simulation studies provided an extended insight to the formation mechanism of the MoS2 flakes.