Advised by
Exfoliation is the top-down approach of obtaining low dimensional materials from bulk. The two most common types of exfoliation is mechanical (aka Scotch-tape method) and liquid exfoliation. Mechanical exfoliation has existed since the famous Nobel prize discovery of the first 2D material, graphene by Novoselov et. al. in 2004. Since then, many ther layered materials have been exfoliated with this method. The advantages of the Scotch-tape method is its simplicity, low cost, and high quality result. A disadvantage is that often tape residues contaminate the sample and substrate and therefore transfer techniques need to be used to transfer the sample to a new substrate, leaving the tape residues behind.
There are many reported transfer techniques in literature, but usually they are not easy to reproduce due to the very sensitive nature of the materials used and skill required to transfer properly. Often, a thin film (i.e. PDMS, PMMA, and polystyrene are popular choices) which has a high interaction strength with the sample but not the substrate and tape residues is spin-coated on top of the sample and then peeled off. This film is then used to place the sample onto the clean substrate. Subsequently the film is dissolved, to ideally leave only the undamaged sample. In reality often, the film cannot be completely dissolved, leaving atomic thickness contamination on the sample. It is also possible for the very fragile low dimensional material to be damaged during the peeling and transfering of the film.
I worked on exfoliating transition metal dichalcogenide nanowires. During the mechanical exfoliation trials, I experienced many of the difficulties above and had to test many different approaches. I found that a water-mediated transfer using a sacrificial polystyrene film worked the best. The water helped peel off the hydrophobic polystyrene from the original sample. After transfer the polystyrene film could be almost completely dissolved with toulene. However, there remained some about 1 nm polystyrene residues on the surface. I was able to exfoliate the material down to about 20nm height nanowires and transfer them to SiO2 and Au surfaces.
I decided to try liquid exfoliation to see if the polystyrene residues issue could be avoided in this way. Liquid exfoliation usually involves ultrasonication of the material in a solvent. The energy due to sonication, breaks up the material. This technique has the advantage of having much higher yield than mechanical exfoliation (an exfoliated solution can be used to make dozens of samples by drop-casting) but disadvantage of poorer size control of the sample and possible chemical contamination or modification of the sample due to the solvents used. In order to avoid chemical modification, I used isopropyl alcohol. The general procedure, is to sonicate, then centrifuge, remove the supernatent, and sonicate it again, repeating the process until the solution looks clear. This process filters out the larger particles. I was able to exfoliate the nanowires to about 20nm, however as expected the control of sample size was poorer so larger nanowires of 100nm were also present.
.