Friday, June 26, 2015By:
This week got off to a good start. I spent Monday as a nano-scale scullery maid! Translation: I suited up in a lab coat, gloves, and goggles and used a chemical process to clean off our silicon target substrate several times to be sure it was spic and span.
Tuesday morning we performed the chemical cleaning magic on a dummy substrate onto which will place MoS2 flakes. With our dummy substrate ready we pressed flake –laden tape onto its surface in a process called exfoliation. This left a lot of flakes far too small to be made into devices scattered over the substrate so to reduce the flake density our plan is to use a chemical transfer process to move the flakes from the dummy substrate (hereafter called the dummy) to another one called the target substrate on which the device will be built. This transfer process will leave many of the super small flakes behind so that our target substrate will have the desirable larger flakes and still be relatively free of clutter. The first step was to use a dropper to deposit a thin layer of a liquid polymer called PLLA onto the dummy and take it for a spin in the centrifuge so that the PLLA engulfs the MoS2 flakes. The first of our woes happened as we tried to peel the dummy off the double sided tape holding it to the carrier substrate on which it sat. Despite me asking it politely, the tape simply refused to let go of the dummy. In my attempt to pry the dummy free from the clutches of the tape I snapped it clean in half. Oops.
Ok, start over. We exfoliated onto another dummy and this time we made it one step further. After the spin coating comes the tricky part: use a razor blade to scrape off 1mm of the PLLA around the border of the dummy substrate whilst keeping all fingers intact. Then we placed a slab of a jello-like polymer, called PDMS, on top of the PLLA coating to form a sandwich. After a quick dunk into de-ionized water our sandwich was ready to be peeled apart. If all goes as planned (insert the universe’s mocking laugh here) the PDMS should act like a sticker that grabs the PLLA with the flakes in it so that they can be placed onto the target. No such luck. The PDMS and PLLA were evidently not on speaking terms. They refused to adhere to each other. We thought we could force them to if we just pressed them together more firmly...CRACK! Another substrate bites the dust. We thought the PDMS was to blame so we concocted a new batch and called it a day.
On Wednesday we were ready for a new round. We started back at square one with exfoliation. We made it to the point where we needed to scratch off the PLLA around the edge of the dummy. This time the PLLA was experiencing separation anxiety and did not want to be removed from the surface of the dummy. We tried to encourage it by scraping a little harder. Bad idea. Rather than relinquish the PLLA, the substrate shattered into a series of sharp shards. Sigh. Now a new batch of PLLA was in order. We concocted the PLLA potion and left it out to stew overnight. For the remainder of the day I resumed my nano-scale scullery duties to clean more dummy substrates because with track record we’ll need some extras. On Thursday we started from scratch with the exfoliation. Slowly but surely we proceeded through the same steps and got to the point where we tried out our fresh batch of PLLA. It had separation anxiety as well. After a much needed consultation/therapy session with our advisor we came up with a plan to systematically vary certain parameters to find a set of optimal conditions under which the dummy relinquishes the PLLA. Friday we carried out our plan which resulted in…drumroll please….widespread failure! Sigh. Back to the drawing board.