Transferring and cleaning suspended WS2 monolayers for electron ptychography characterization.

Despite their weak out-of-plane interactions, two-dimensional materials such as WS2 can retain appreciable contamination on their surfaces, complicating characterization in the electron microscope. Polymers and other heavy molecular weight compounds have been historically challenging to remove despite many conventional cleaning procedures attempted. We investigate the influence of sample exfoliation, sample transfer, and sample cleaning methods on contamination levels. While no one method was effective, a combination of solvent, vacuum-oven annealing at high temperatures, and indirect air-plasma cleaning removes pervasive WS2 sheet contaminants. At sufficiently clean levels, electron microscopy characterization such as contamination-sensitive ptychography is performed on these materials.