Thickness dependent transition from 1T’ to Weyl semimetal phase in ultrathin MoTe2: Electrical transport, Noise and Raman studies
Bulk 1T'-MoTe2 shows a structural phase transition from 1T' to Weyl semimetallic (WSM) Td phase at ∽240 K. This phase transition and transport properties in the two phases have not been investigated on ultra-thin crystals. Here we report electrical transport, 1/f noise and Raman studies in ultra-thin 1T'-MoTe2 (∽ 5 to 16 nm thick) field-effect transistors (FETs) devices as a function of temperature. The electrical resistivities for thickness 16 nm and 11 nm show maxima at temperatures 208 K and 178 K, respectively, making a transition from semiconducting to semi-metallic phase, hitherto not observed in bulk samples. Raman frequencies and linewidths for 11nm thick crystal show change around 170 K, attributed to additional contribution to the phonon self-energy due to enhanced electron-phonon interaction in the WSM phase. Further, the resistivity at low-temperature shows an upturn below 20 K along with the maximum in the power spectral density of the low frequency 1/f noise. The latter rules out the metal-insulator transition (MIT) being responsible for the upturn of resistivity below 20 K. The low temperature resistivity follows ρ∝1/T, changing to ρ∝T with increasing temperature supports electron-electron interaction physics at electron-hole symmetric Weyl nodes below 20 K. These observations will pave the way to unravel the properties of WSM state in layered ultra-thin van der Waals materials.