Figure 1: Diagram and optical images of a flakes transfer process.
From: Transfer of monolayer TMD WS2 and Raman study of substrate effects
(a) WS2 flakes are grown on a SiO2 surface. (b) PMMA is spun onto the WS2 growth substrate. (c) The substrate with PMMA resist is placed into 1 M KOH heated to 70 °C which dissolves the SiO2 allowing the PMMA/flakes to be released to float on the surface of the KOH. (d) Using a glass slide the floating PMMA is removed from the KOH and transferred to a water bath for 30 minutes and then again, using a glass slide, is removed from the water and allowed to float on top of the glass slide. (e) A glass slide with double-sided Kapton tape is placed and pressed over the floating PMMA to secure it to the tape. (f) The glass slide with the PMMA is aligned over a target substrate secured to a separate glass slide and has two Kapton tape pillars that attach the two slides together upon contact. (g) The desired flake during alignment is visible under the optical microscope through the glass, tape, and PMMA and is outlined in red. A micromanipulator is used to move the flake to the desired position, with a Si3N4 window in the background. (h) The flake has been lowered onto the sample and pressure has been applied to secure the two slides together, providing good contact between the flake and the final substrate surface, as evidenced by the substrate surface and flake being in the same focal plane. (i) Si3N4 window and placed flake post overnight Acetone bath. All scale bars are 40 microns.
