Direct observation of ordered-disordered structural transition of MoS2-confined ionic liquids

Ionic liquids (ILs) are an emerging class of media of fundamental importance for chemical engineering, especially due to their interaction with solid surfaces. Here, we explore the growth phenomenon of surface-confined ILs and reveal a peculiar structural transition behavior from order to disorder above a threshold thickness. This behavior can be explained by the variation of interfacial forces with increasing distance from the solid surface. Direct structural observation of different ILs highlights the influence of the ionic structure on the growth process. Notably, the length of the alkyl chain in the cation is found to be a determining factor for the ordering trend. Also, the thermal stability of surface-confined ILs is investigated in depth by controlling annealing treatments. It is found that the ordered monolayer ILs exhibit high robustness against high temperatures. Our findings provide new perspectives on the properties of surface-confined ILs and open up potential avenues for manipulating the structures of nanometer-thick IL films for various applications.