Intrinsic phonon bands in high quality monolayer $T'$ molybdenum ditelluride

Distorted octahedral ($T')$ transition metal dichalcogenide (TMDC) is a type of layered semimetal that has attracted significant recent attention because of its fascination physical, chemical and nontrivial topological properties.~ Unlike its hexagonal counterpart,~monolayer (1L) $T'$-TMDC is challenging to work with due to rapid sample degradation in air. In this talk, I will discuss well-protected 1L-$T'-$MoTe$_{\mathrm{2}}$~that exhibits sharp and robust intrinsic Raman bands, with intensities about one order of magnitude stronger than those from bulk~$T'$-MoTe$_{\mathrm{2}}$. The high quality samples enable us to reveal for the first time the set of all nine even-parity zone-center optical phonons. Crystal angle and light polarization resolved measurements further indicate that all the intrinsic Raman modes belong to either $z$-mode (vibrating along the zigzag Mo atomic chain) or $m$-modes (vibrating in the mirror plane). Moreover, with the knowledge of vibrational symmetry, we can effectively distinguish the intrinsic modes from Te-metalloid-like modes with energy around 122 and 141 cm$^{\mathrm{-1}}$~which are associated to the sample degradation.~ Our studies offer a powerful non-destructive method for assessing sample quality, providing the fingerprint as well as key insights in understanding the fundamental properties of 1L $T'$-TMDCs.