Direct TEM observations of growth mechanisms of two-dimensional MoS2 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 MoS2 flakes from the thermolysis of ammonium thiomolybdates using in situ transmission electron microscopy. In the initial stage (400 °C), vertically aligned MoS2 structures 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.

The scale bar in the first frame also applies to their lower frames for each column. Note there was a short period of ca. 1-2 min for sample stabilization after heating or cooling at each time before imaging, which has not been reflected by the heating programs.  Figure 8A) which was similar to that in Figure 4D. Then, the E-chip was taken out from the TEM and re-dropcasted with (NH 4 ) 2 MoS 4 solution. After remounting the E-chip into TEM, the same area was located (Supplementary Figure 8B).
Consequently, the in-situ heating was re-conducted. At 500 o C, we can also identify  Figure 8F). Furthermore, the neighboring flakes were trended to adopt same orientation, as vividly evidenced by Supplementary Figure   8G. Although we have not obtained perfect flakes throughout this experiment, the current result showed significant possibility for growing large flake with our method.

Investigation of the effect of the electron beam irradiation on the growth dynamics
For TEM experiments, it is essential to assess the influences from the electron beam to the experimental phenomena. 4,5 In our situation, it is also important to confirm the current results to be thermally-assisted evolution or electron-induced process. Therefore, systematical investigations on the effect of the electron beam irradiation on the growth dynamics was carefully carried out. that the thermal heating processing plays the dominant role in the dynamics of layer-bylayer vertical growth as described in our work.
Secondly, the effect of the electron beam on the particle growth process at higher temperature was also quantitatively assessed. The precursor was directly heated to 840 o C (at a rate of 1 o C s -1 ) to form irregular nanoparticles, followed by slowly cooled down to 800 o C (at a rate of 2 o C s -1 ). The as-formed MoS 2 nanoparticles were then illuminated by constant electron beam (65 pA cm -2 ). Sequential images were taken every 10 minutes (column D in Supplementary Figure 9) during this process. No obvious coalescence event was observed in this 30 minutes process. We also randomly selected 20 nanoparticles in the frame and measured their size change during the assessment. As expected, the size change of the particles was within a narrow window of ± 3%, which is much less than the Subsequently, the precursor was heated to certain temperatures (at a rate of 1 o C s -1 ) while electron beam was turned off. After reaching the targeted temperature for a short period (ca. 2 minutes), the electron beam was turned on and the four positions were sequentially examined. In our situation, the experimental phenomena (the morphology 14 / 15 and density of vertical clusters, the size and shape of the horizontal particles, etc.) from these four positions were quite similar, which were also comparable with two previously unilluminated regions.
In summary, the above control experiments have systematically and quantitatively suggested that the as-revealed mechanisms (layer development, OA, and OR) are mostly resulted from the thermal heating process.