An important part of PhD life is maintaining devices you are supervising. For example, besides other devices, I am responsible for our atomic force microscopy (AFM). This type of microscope uses a fine tip to characterize a surface with up to picometre precision. In comparison to scanning tunnelling microscopy, the feedback is generated from changes in cantilever properties rather from a tunnelling current. In my group, mainly use the so-called tapping mode, where the cantilever is resonantly driven and the change of the resulting amplitude due to the close by sample is used as feedback. This way of imaging a surface is advantageous since it reduces the force exerted on the sample.
Recently, our roughly ten-year-old machine was experiencing some troubles. Occasionally, the scanner was not detected any more when the scanning unit was moved. It is designed that way, that the scanner needs to be pulled out to exchange the cantilever with the tip. A semi-flexible multilayered PCB cable connects the scanning with the mainboard of the AFM. The dynamic-cyclic loading degraded this cable over time. To confirm our first assumption, I took the AFM apart, removed the cable and checked if the pins on both ends are all still connected. As expected, depending on the bending, several pins are shortcut and several lose their connection.
After reassembling the entire unit and making a few minor adjustments, the AFM will work fine until the replacement part arrives. Finding these temporary solutions is an important lesson during the doctoral studies, as it enables a variety of technical problems to be solved, whether just as a bridging solution or a complete repair. For my research, atomic force microscopes are a useful tool to measure the thicknesses of exfoliated crystals, to clean surfaces of flakes or flattening heterostacks as shown in the following images.
In the scope of ETH outreach week, my colleague Patrick and I showed high school students how thermal evaporation works. The idea of this week is to give high school students from Switzerland and abroad can get hands-on insights on different study programs, in our case materials science. We first introduced them to the general basics of thin films in general, absorption and different principles of film deposition.
In our case, a thermal evaporator was used to vaporise the source materials. Following, a sketch of the device is shown. It is an easy principle: in an evacuated volume, the material is heated inside a crucible until atoms and small cluster are evaporated. The vacuum is needed to prevent contamination of the film and to ensure the particles reach the substrate. At the substrate, the atoms and clusters condensate on the surface and form a film, just as any other surface inside the chamber.
The students were allowed to carry out all steps by themselves under supervision. Firstly, the lab safety glasses had to be prepared and cleaned well. Then, they were placed inside the chamber, which was evacuated next. Now, the source materials were melted by resistive heating, while slowly increasing the power the evaporation rate was observed via an oscillating quartz crystal. After the desired rate was reached, the shutters were opened and the Poly(methyl methacrylate) (PMMA) glasses were coated.
The students were able to decide from different materials, where aluminium and copper have been the most common choices. Some tried some more interesting ideas, like making a bronze alloy from copper and tin. As explained in our introduction, absorption rates vary among the used materials. For example, 100 nm thick aluminium films will completely reflect the light while copper of the same thickness is still quite transparent.
Especially for sunglasses, it is important to have UV filters to protect the eyes. We made sure that materials we evaporated block most of UV light (below 350 nm wavelength) to have safe sunglasses. Anyway, since we did not deposit a sealing layer, our films are not scratch resistant. Therefore, our sunglasses are more meant to be a nice souvenir from their week at ETH Zürich.
In the last two weeks, thanks to an early part secondment, I had the opportunity to get to know many different preparation and exfoliation possibilities at CIC nanoGUNE. Besides the many scientific experiences, I came into contact with many friendly and helpful scientists and was able to enjoy the time there, also because of the good weather. Besides Marco, who works with me in Zurich, I also got to know two other ESRs personally here, Mayank and Eoin. Fortunately, my time in San Sebastián/Donostia coincided exactly with a statistically significant accumulation of birthdays, so I got to enjoy freshly baked cake at work surprisingly often.
The view over one of the beaches of San Sebastián/Donostia towards the city center.
I spent most of my time at the glove box exfoliating various non-stable materials and building stacks with the stamping system. In the picture you can see the transfer process, where the flakes are transferred to a substrate using polydimethylsiloxane (PDMS). With this technique, precisely aligned complex hetero structures can be produced successively.
Stamping process mid stamping. The exfoliated flakes are on the PDMS which is on the bottom side of the glass slide.
Back in Zurich, I will continue the exfoliation of van-der-Waals crystals of different materials. The new knowledge I gained at CIC nanoGUNE and especially the personal tips on the exfoliation for specific materials will help me to fabricate different types of samples in the near future, either in a controlled environment or under ambient conditions.
Finally, I can only thank you for the wonderful time and send my greetings to Spain.