Determining the Thickness of Weyl Semimetal Nanoflakes Using Silicon Raman Scattering

It is known that for very thin layers a gap opens in van der Waal layered Weyl semimetals enabling spin Hall insulators. There is a great need to find easy means for determining the thickness of exfoliated nanoflakes. Here we use the Raman scattering intensity from the underlying silicon substrate to provide a direct measure of the thickness of a nanoflake. Nanoflakes were exfoliated from WTe₂ and NbIrTe₄ single crystals and transferred to a Si substrate which has a 500 nm thick SiO2 layer on the top surface. Raman phonon spectra were obtained both directly on the bare substrate and also on the Weyl semimetal nanoflake. Full Raman spectra were obtained from both the Silicon phonon from the substrate and also WTe2 or NbIrTe4 nanoflakes of varying thicknesses. Because the incident laser and also the inelastically scattered photons from the Silicon substrate which go through the nanoflake have a reduced intensity because of absorption, it is possible to use the Silicon Raman intensity as a metric for determining the flake thickness. We calibrate these measurements using a number of other techniques which enables us to accurately and rapidly determine the thickness of nanoflakes which are less than 100 nm thick. This is particularly important in these materials since the WTe2 or NbIrTe4 phonons are known to shift only weakly for thicknesses below 4 monolayers. We compare these measurements with Red-Green-Blue contrast imaging which enable us to rapidly identify flakes of a variety of thicknesses from 100 nm down to the monolayer limit.

We acknowledge the financial support of NSF through grants DMR 1507844, 1531373, 1505549 and 1906325 and ECCS 1509706.