Figure 1 a depicts the dip coating process for the production of certain classes of TMD structures. First, a piece of SiO 2 / Si or quartz wafer is immersed into an aqueous (NH 4 ) 2 MoS 4 (or (NH 4 ) 2 WS 4 ) solution (0.23% w/v (g mL −1 ) in DI water) and removed at different speeds to control the drying velocity. As the water evaporates, small nuclei are formed at the solution-substrate interface, and these nuclei initiate the growth of various solid (NH 4 ) 2 MoS 4 , or (NH 4 ) 2 WS 4 morphologies as intermediate states of MoS 2 and WS 2 . The self-assembled TMD morphologies are strongly dependent on the evaporation rate of the solvent and the diffusion rate of the TMD-precursors in solution. Figure 1 b,c presents optical images of the selfassembled (NH 4 ) 2 MoS 4 structures formed at different evaporation speeds and pH conditions. At very fast evaporation speeds (approximately 320 nl s −1 ) induced by high temperature (80 °C), a uniform thin fi lm of (NH 4 ) 2 MoS 4 was formed after the complete removal of the solvent (condition A, Figure 1 b) because there was insuffi cient time to induce the formation of nucleation seeds. In contrast, at evaporation speeds that were two orders of magnitude slower (0.67-1.72 nl s −1 ), dendritic structures were formed (condition B, Figure 1 c). The dendrite structures have many worm-like stems with a large number small side branches; self-assemblies of these seeds randomly formed on the substrate.In addition to these two nonoriented phases, the spontaneous formation of well-oriented wire patterns was observed when the acidity of solution increased from pH 6.41 to pH 5.02 at the same evaporation speed as condition B (condition C, Figure 1 d). Interestingly, these long wires, which grew from seeds formed in parallel arrays at the initial solution/substrate/air contact line, displayed uniform spacing between the wires without serious distortion of the arrays. The formation of aligned wires can be explained by the "fi ngering instability" phenomenon. [ 18,19 ] When the solvent begins evaporating at the solution/substrate contact line, fi ngering instability induced by the regulation of the evaporation speed, which affects the internal fl ow of the solution, leads to the periodic formation of nucleation-seed arrays. In turn, this results in the spontaneous growth of regular, unidirectional wire arrays from the seeds during the drying process (Figure 1 d). The critical dependence of the directional growth mechanism on the pH variation is not yet clear. However, we believe that the acidity of the reaction media can effectively suppress the gradation of the precursor diffusion, which is the driving force in the generation of the dendrite side-arms and the lateral expansion of the selfassembly, thereby preserving the main stream and, as a result, enhancing the formation of aligned wire arrays. [ 20 ] Thus, the evaporation speed, solution concentration and pH are the key factors that control the nucleation and growth processes, which lead to different self-assembled structures.A sub...
