Energy Dissipation in WSe₂ Opto-Acoustic Resonators

The breathing vibrational modes of WSe₂ nanoflakes deposited onto a glass surface have been studied by transient absorption microscopy. A wide range of both vibrational frequencies and quality factors are observed in the measurements. The differences in the frequencies simply arise from differences in the thickness of the WSe₂ nanoflakes. However, the variation in quality factor has a more complicated origin. The measured quality factors range from less than 10 to over 200, with the lower value being close to that expected for WSe₂ nanoflakes deposited directly on glass. The higher values are attributed to the presence of an organic spacer layer between the WSe₂ nanoflakes and the glass that reduces mechanical contact and, thus, decreases the transfer efficiency of acoustic energy from the nanoflakes to the substrate. A simple one-dimensional continuum mechanics calculation is used to model the results. These calculations show that the data can be explained by the presence of a spacer layer with thicknesses between a few and tens of nanometers.