Ultrafast Exfoliation of 2D Materials by Solvent Activation and One-Step Fabrication of All-2D-Material Photodetectors by Electrohydrodynamic Printing

Large-scale liquid exfoliation of two-dimensional materials such as molybdenum disulfide, tungsten disulfide, and graphene for the synthesis of printable inks is still inefficient due to many hours of exfoliation time needed to achieve a highly concentrated dispersion that is useful for printing. Here, we report that soaking the bulk 2D material powders in a variety of solvents (water, ethanol, isopropanol, acetone, methanol, dimethylformamide, N -methyl pyrrolidone, and hexane) briefly as short as 5 min “activates” them to be much more easily exfoliated afterward. The unsoaked powder yielded a negligible concentration of dispersed nanosheets (less than 0.01 mg/mL) even after long hours of sonication, while the powders soaked in water resulted in dispersed nanosheets of 1.21 mg/mL for MoS 2 and 1.28 mg/mL for WS 2 after 6 and 4 h of sonication, respectively, a more than 100 time increase. For graphene, soaking in methanol for 5 min prior to sonication for 6 h yielded an increase in the dispersed nanosheet concentration to 0.13 mg/mL, a more than 10 time increase in concentration. The enhanced exfoliation is originated not from the intercalated solvent molecules but from the slightly increased d -spacing of the bulk powders during soaking due to the different dielectric environments in the solvents, which assists in the exfoliation afterward. We further fabricated MoS 2 and WS 2 photodetectors with graphene as electrodes by one-step electrohydrodynamic (EHD) printing using highly concentrated inks (>2 mg/mL) obtained by ultrafast liquid exfoliation, which have light sensitivity down to 0.05 sun. We believe that this ultrafast exfoliation technique combined with the one-step device printing technique enables a big step toward the mass production of functional devices fabricated from solution-processed 2D material inks.