Terahertz response of monolayer and few-layer WTe₂ at the nanoscale

Tungsten ditelluride (WTe₂) is an atomically layered transition metal dichalcogenide whose physical properties change systematically from monolayer to bilayer and few-layer versions. In this presentation, we use apertureless scattering-type near-field optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to study the distinct THz range electromagnetic responses of mono-, bi- and trilayer WTe₂ in the same multi-terraced micro-crystal. THz nano-images of monolayer terraces uncovered weakly insulating behavior that is consistent with transport measurements. The near-field signal on bilayer regions shows moderate metallicity with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations involving thermally activated carriers favor the semimetal scenario with  over the semiconductor scenario for bilayer WTe₂. Also, we observed clear metallic behavior of the near-field signal on trilayer regions. Our data are consistent with the existence of surface plasmon polaritons (SPPs) in the THz range confined to trilayer terraces in our specimens. In addition, we imaged the THz SPPs of few-layer WTe₂ in the phase chanel. Finally, data for microcrystals up to 12 layers thick reveal how the response of a few-layer WTe₂ asymptotically approaches the bulk limit.