Probing Anisotropy in Transition-Metal Dichalcogenides using Polarized Raman Spectroscopy

Highly-anisotropic, 2D, transition metal-dichalcogenides (TMD) have generated interest as a result of their polarization-dependent, Raman-active vibrational modes. Such polarization dependence offers a possible approach to practically characterize crystallographic axes, which are crucial for orientation-dependent, device applications. We systematically measure the polarized Raman spectra for the first-order Raman active modes in ReS$_2$, an anisotropic TMD. Mechanical exfoliation prepares few- and single-layer ReS$_2$ flakes on SiO$_2$/Si substrates. Control of sample orientation and incident/scattered polarization directions affords acquisition of Raman spectra as a function of the polarization angle. Additionally, we induce anisotropy in MoS$_2$, a normally isotropic TMD, through the application of strain. Monolayer MoS$_2$ is synthesized using CVD and transferred onto flexible PET substrates, to which mechanical strain is applied and polarized Raman spectra acquired.\footnote{A. McCreary \textit{et al.}, ACS Nano \textbf{10}, 3186 (2016).} We will discuss polarized Raman measurements with predictions from density function theory.$^1$