High Yield Fabrication of Monolayer WSe₂ Mechanical Resonators and a Study of Their Loss Mechanisms

As a result of the advances in the field of quantum computing, there is increasing demand for quantum transducers to enable coupling between different quantum systems over long distances. Monolayer tungsten diselenide (WSe₂) mechanical resonators have great potential as an answer to this demand: possessing large zero-point motion, being optically active due to its direct bandgap, and its ability to host quantum emitters. In order for monolayer WSe₂ resonators to reach their potential it is necessary to develop methods of improving their quality factors. The first step on this road is understanding the dominant mechanical loss mechanisms in monolayer WSe₂ mechanical resonators. In this thesis, I describe the method I have developed to fabricate fully suspended monolayer WSe₂ mechanical resonators using a high yield fabrication method based on gold exfoliation of WSe₂, polycarbonate (PC) transfer, and a delicate chloroform cleaning. With this I am able to produce thousands of resonators with low contamination and high room temperature quality factors, of up to 1050 for 5 µm diameter resonators and 500 for 2.5 µm diameter resonators. I demonstrate my use of a reflection based Michelson interferometer detection scheme to detect thermal motion of monolayer WSe₂ resonators at room temperature and how we are able to take direct measurements of resonator masses to determine cleanliness. I also discuss my use of UV laser thermomechanical driving to allow measurement of resonators at cryogenic temperatures of 4 K. Using these methods we are able to measure 42 resonators with 2.5 µm diameter and 19 resonators with 5 µm diameter at room temperature across a range of frequencies resulting from different levels of pre-tension induced during fabrication. I then fit a model of quality factor based on bending losses and find a good agreement with the data providing strong evidence that bending losses dominate mechanical losses in layered material (LM) resonators at room temperature, and that dissipation dilution will work as a method of improving quality factors. Finally I describe my investigation into mechanical loss mechanisms at cryogenic temperatures through the use of phononic shielding. I compare the results of measured quality factors from experiments done on 2.5 µm diameter and 5 µm resonators samples on TEM grid membranes to 4 µm diameter resonator samples utilising phononic shielding through the use of phononic crystal membranes. I find that implementing phononic shielding results in a fivefold enhancement of the maximum quality factor observed, increasing from Q₅ = 17,000 for 5 µm diameter resonators to Q₄ = 94,000 for the phononically shielded 4 µm diameter resonators. To the best of my knowledge, this represents the highest recorded quality factor for monolayer WSe₂ resonators in the literature today. Finally I discuss how this data indicates that phonon tunnelling losses appear to play a dominant role in mechanical losses in LM resonators at cryogenic temperatures and that phononic shielding may allow quality factors to be increased further. The development of a fabrication and detection method capable of producing and measuring large numbers of monolayer WSe₂ resonators opens up new possibilities for the systematic study of mechanical losses in LM resonators. Additionally, the ability to fabricate fully suspended resonators has created the opportunity to study new resonator geometries, such as ’membrane in the middle’ cavities and further research into phononic crystal supported resonators. Furthermore, the evidence indicating that bending losses dominate LM resonator mechanical losses at room temperature and phonon tunnelling losses play a role at cryogenic temperatures opens up the possibility of developing techniques to reduce mechanical losses of LM resonators, allowing further increases in quality factor in future.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2023

Author dc:creator

• Pitts, Michael

Advisors dc:contributor.advisor

• Kara, Dhiren
• Atature, Mete
• Ferrari, Andrea

• Layered materials
• Mechanical resonator
• Monolayer resonator
• Quality fatcor
• Suspended monolayer
• Tungsten Diselenide
• WSe2

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/b00e2da6-d96a-4833-bc02-67f5f2632ed1/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

Author Identifier

0000-0003-0907-9993

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/371537