To begin, clean the glass substrates in a UV Ozone Cleaner.
Cut the PDMS (product code: PF-3-X4, Gel-Pak) into small squares.
Remove the PDMS film with the protective polyethylene coversheet from the polyester substrate and carefully apply it to the glass.
Peel off the protective polyethylene coversheet.
Cut two pieces of blue tape and put the crystal on one of them. Ensure it is laid flat.
Put the second piece of tape on top and press gently with tweezers. Slowly peel the tape off.
Continue to distribute the bulk material over the surface of the tape by bringing the two pieces of tape together.
Once most of the surface is covered with thin layer of material, remove the bulk crystal from the tape.
Additional pieces of tape can be created by transferring some of the prepared material onto a fresh piece of tape.
Put one of the pieces of tape on top of the prepared PDMS film and apply gentle pressure with a cotton bud.
Peel the blue tape off. The rate of the peel defines the amount of material transferred onto PDMS; a faster peel rate will result in more material being transferred.
Inspect the PDMS substrate using an optical microscope to look for the most translucent flakes.
Optical contrast measurements can be used to identify the thickness of the flake...
by comparing the intensity of light reflected by the flake and the surrounding substrate.
The optical contrast of layered 2D materials changes linearly with increasing number of layers.
The value of the optical contrast for a monolayer flake will depend on the material, type of substrate, and optical system used.
But it should be close to the minimum difference between the adjacent thicknesses.
Some 2D materials, such as transition metal dichalcogenides, become fluorescent when their thickness is reduced to a single monolayer.
Therefore a fluorescence microscope can also be used to confirm the monolayer thickness.