Liquid phase exfoliation of indium selenide: Achieving the optimum exfoliating parameters and unraveling the mechanism

The two-dimensional (2D) van der Waals indium selenide (InSe) exhibits the ultra-low intralayer Young's modulus yet the high interlayer cleavage and slipping energy, which could thus be obtained by the liquid phase exfoliation (LPE) method. Here, we designed 48 experiments to systematically achieve its optimum LPE parameters including the exfoliating solvent of N-Methyl-2-pyrrolidinone (NMP), ultrasonic power of 180 ?W, ultrasonic time of 12 ?h and exfoliating concentration of 6.25 ?mg/mL. The resulting optimal 2D InSe nanosheets show a uniform lateral size distribution of about 200 ?nm without impurities. We further analyzed the underlying exfoliating mechanism through the scanning electron microscopy (SEM) investigations. The results show that NMP has the more suitable Hansen solubility parameters to disperse and stabilize the 2D nanosheets. Increasing the ultrasonic power and time can provide plenty of energy to destroy the van der waals' force that exist between the interlayers of InSe and induce intralayer fragmentation. This work would provide a significant guideline for selecting exfoliating parameters to obtain the expected InSe nanosheets and thus extend its applications in optical sensors and nanocomposites.