I finally got to try my hand at sample preparation these past 2 weeks. Materials can exhibit striking electronic and mechanical properties when stripped to few layers / monolayer. Graphene (single layer graphite) is one popular example, first to be discovered among the family of 2D materials in 2004. But the search for conducting 2D materials has now extended beyond graphene.
I tried to exfoliate a single layer of molybdenum disulfide (MoS2), one type of transition metal dichalcogenides (TMD), under the guidance of my lab colleague. The properties of these TMD in the form of 2D sheets have application in more efficient electronics, semiconductors, solar cells, and touch screen display panels, just to name a few.
Exfoliation in this context is actually applying the material of interest onto a scotch tape, then folding and opening the tape several times until the flakes on it are as thin as possible. Judging with my naked eyes, this usually just means for the flakes to not appear as shiny (for metal compounds) anymore. Then, I will have to transfer the flakes on the Scotch tape to a tiny piece of transparent gel (PDMS gel). The procedure is pretty simple, but the flakes produced are not guaranteed to be thin enough. Through my many trials, I find random thickness of flakes distributed all over the gel under optical microscope. And among them, we need to locate the thinnest of all that could potentially be a monolayer.
Below is the flake that I suspect to be a single layer of MoS2 at different magnification level. Can you find it in the last picture?
Here is the relative size of the PDMS gel on which all these flakes lie.
This also happened when I managed to transfer plenty of flakes onto the PDMS gel, while most of the time I do not get enough to choose from when observed under the microscope. After identifying this flake, I had to transfer it onto another substrate (such as a silicon nitride grid), before I can put it into an electron microscope that offer higher resolution through electrons scattering method. The transfer patience takes some patience, as pressing or lifting the gel too quickly may break the hard-found flake. So it is just a lot of going back and forth between the sample preparation table and microscope to get one nice specimen. But it is important to get through this process for us to get to the characterization part, which is where all the fun stuff about science comes in!
