Week 10 :

Last week in Germany! This week we finished aligning the second microscopy setup and all the mirrors and such should have the laser pointed at the right spot. We also had technicians from Hamamatsu come out and set up and streak camera for the second setup. Initially there was some strange oscillation that we would get from the streak camera, which we eventually fixed as a combination of the original signal and streak camera tweaking. The master's student Marius is visiting Dresden, as he is spending time in Berkeley Lab for part of his project. He gave a talk on TMDC heterostructures constructed with etched hBN grating. We did some more mappings for the perovskite heterostructures that we made, and I also finalized my presentation and everything is good to go.

 

Week 9 :

This Tuesday I gave the presentation during the group meeting on what I have been doing this summer at TU Dresden. This mostly focused on the technical aspects of the research, with two major focuses, one being the optical setup and two being the perovskite samples and heterostructures. There were some questions asked mostly by the PhD students in Regensburg, as the Dresden folks already know most of the things I've been doing. Dr. Chernikov gave some feedback on what I can improve in terms of the slides that I have prepared, including image contrast, highlighting more important points, include more introduction and explanation of the perovskites, and adding various schematics. I will also work on adding in some more experiences that I've had during the summer, in preparation for the presentation that I will give back in my own lab at UF. We also started to make the second laser path towards the new microscope, so now we can have two people working at the same time. We did have to figure out the optimal positioning for the spectrometer as it is shaped quite weirdly, with outputs going at 45 degree angles. We planned out the general path of the laser and prepared all of the necessary equipment (mirrors, periscopes, lenses, pinholes/shutters, etc.), and will align the laser to the desired precise pixel next week.

 

Week 8 :

This week the necessary component for the new Nikon microscope arrived, and the technician from Nikon installed the parts & gave us an introduction to the functionalities and features on this microscope. There's a motorized z-level control and has a lot of customization when it comes to the optical path and cameras to be installed. I also started working on the presentation which will summarize the research that I have done over the summer, and will present it at the group meeting next week. This Friday is the Dresden Night of Science, where many departments and organizations around Dresden set up activities showcasing various fields of science, aimed mainly towards the younger audience. Our lab is a part of the ct.qmat excellence cluster, which focuses a lot on topological materials and physics. Our station, since we work with optics and lasers, is to use a laser engraver to make custom name tags for the kids that complete the Kitty Q game developed by the cluster. 

 

Week 7 :

This week has mostly been solving problems with the current setup. We had a new can of helium delivered at the beginning of the week, and for some reason it just would not maintain proper pressure and we could not get the sample to stay at below 5K. It turns out that because of the low ceiling clearance, we bent the filter at the bottom tip of the transfer line that goes into the can, causing a crack. We carefully glued the part that had cracked and it seems to work well again. The pulsed laser was also out of focus, and we concluded that it was due to a mix of reasons, including the substrate material (in this case diamond) and the objective (the 60x objective has a zoom/adjustment where one could try and minimize interference through the glass on the cryostat). Started reading a paper about excitons in thin hybrid perovskites, which I will hopefully discuss next week with Dr. Chernikov.

 

Week 6 :

We were able to connect the balloon (which we got from the chemistry department) to the helium can so that we can properly cool the sample. We did have to disconnect some of the tubes that we had previously because the connector piece we had for the balloon restricted where we can connect to the helium clock. We also loaded some old samples (~a few months) of encapsulated perovskites to test out the setup and were able to replicate earlier results on the spectrometer and streak camera, so it's quite promising. In terms of our own samples we were eventually able to find a good flake of perovskite on a glass sample, and encapsulate it in hBN. The glass substrate is more difficult for transfer, and we had to use several hBN flakes as many just wouldn't stick, and would remain on the PDMS. This week I also discussed one of the papers I read about excitonic Rydberg series in TMDCs with Dr. Chernikov.

 

Week 5 :

For the perovskites, we sat up the ultrasonic bath for cleaning wafers in the lab. The wafers were cleaned using acetone and then IPA, before being brought to the plasma oven. We were able to exfoliate good samples of both perovskites and hBN, and was able to encapsulate one thin flake on silicon. More exfoliations will be needed to find a good flake on glass. After some communication we were also able to get our first helium can delivered, and we connected that to the installed helium clock, as well as to the cryostat, which contains strained MoSe2 and WSe2 samples, with the transfer line. The transfer line was just a bit too long when we needed to insert it, and so we had to remove an A/C filter cover on the ceiling for it to fit. We also sat up the vacuum and helium pumps for the cryostat for low temperature measurements. Seeing the frozen condensation on the outside of the tubes were pretty cool. We will need to install a balloon to the helium can to regulate pressure, which will in turn regulate temperature in the cryostat. 

 

Week 4 :

This week we had several visitors to TU Dresden. Two are PhD students who are still technically in our lab working under Dr. Chernikov, but because the lab had recently moved to Dresden and they're only writing their dissertations, they have opted to stay in Regensburg. Another visitor is a post-doc from a collaborator group in Columbia, who has collaborated with Dr. Chernikov's group previously, who came to gave a talk at TU Dresden. In the lab we did more detailed mappings on the MoSe2 where a previous rough mapping showed potential strain gradient. I started reading some papers about perovskites, which is a project that two of the PhD students in the group are working on. It will concern white light absorption and reflection of hBN encapsulated perovskite. We will try and exfoliate some perovskite samples with steps when it comes to different numbers of layers on the same sample. The perovskites will be exfoliated directly on silicon and glass wafers and hBN through the PDMS method.

 

Week 3 :

The beginning of this week was focused on the setup of the continuous wave laser, which appears green (forgot the exact wavelength). This follows the original beam path of the white light and will go into the pulse laser beam path before the periscope. I had a tour/instruction of the plasma oven in another lab, which we can use for cleaning wafers. I was also shown the exfoliation process in this lab for 2D materials, which is a bit different from the method that I have learned of directly exfoliating onto the silicon wafers. Instead the parent tapes are thinned on blue tapes, which is then pressed onto a PDMS square. On Friday we were able to connect the mapping software with the stage on the microscope, and began doing mapping on an hBN-encapsulated MoSe2 monolayer, which is deposited on a diamond substrate. The surface of the diamond substrate is rough enough to create many local strains, which we can measure through the shift in peak photo luminescence. Albert has been sadly decommissioned, as there's not too much of a difference having it on or off through careful observation. 

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Week 2 :

This week is cut short by the Ascension day on Thursday, which is a public holiday. As such most people take Friday off as well for a long weekend. For the laser setup we first tried to redo the whole setup with more 90 degree turns and making sure all the beam paths are horizontal. This did make the situation better but still not ideal. Then Dr. Chernikov suggested that we re-route the white light to come in at a different point, changing the whole setup but bypassing the periscope, which is now exclusively for the lasers. At the end of the week on Wednesday we were finally able to line up the pulse laser, the white light, and the SHG laser output to the points we wanted on the microscope. Albert is working pretty well, and so far we haven't seen humidity above 50% but as we go further in the summer it may get much more humid, so more observation and experimentation to follow.

 

Week 1 :

I first got settled into the office and lab at TU Dresden, and was introduced to the other members of the group. Technicians from Nikon came Monday morning to setup a newly purchased microscope, but it required a special adapter for the non-Nikon white light it will use. The goal was to align the white light for the existing spectroscopy setup, and to add in CW lasers and a beam path for the SHG. While we were able to line up the white light with the periscope for the most part, there would always be a shift compared to where we want it to be centered, which we suspect to come from the fact that the pulse laser uses a different (90/10) beam splitter before the objective, which is a thin piece of glass, whereas the white light uses the 50/50 beam splitter, which is a cube of glass and therefore much more material to travel through. By the end of the week we found that the pulse laser, which was calibrated previously, has somehow become misaligned as well, and in the end we were still not able to align everything simultaneously. We also had the purchase of "Albert" the dehumidifier, which was installed and setup in the lab. The humidity may cause problems with the streak camera, so we hope to monitor and control it effectively with Albert (perhaps multiple in the future if needed).