Thickness dependent transition from the 1T′ to Weyl semimetal phase in ultrathin MoTe2: electrical transport, noise and Raman studies

Abstract

Bulk 1T′-MoTe₂ shows a structural phase transition from the 1T′ to Weyl semimetallic (WSM) T_d 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 on ultra-thin 1T′-MoTe₂ (∼5 to 16 nm thick) field-effect transistor (FETs) devices as a function of temperature. The electrical resistivities for a thickness of 16 nm and 11 nm show maxima at temperatures of 208 K and 178 K, respectively, making a transition from the semiconducting to semi-metallic phase, hitherto not observed in bulk samples. Raman frequencies and linewidths for an 11 nm thick crystal show a change around 178 K, attributed to the additional contribution to the phonon self-energy due to the enhanced electron–phonon interaction in the WSM phase. Furthermore, 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 the WSM state in layered ultra-thin van der Waals materials.