Dielectric permittivity extraction of MoS₂ nanoribbons using THz nanoscopy

The nanoscale optical properties of high-quality MoS₂ nanoribbons areinvestigated using THz nanoscopy based on a scattering-type scanning probe. Thenanoribbons comprise a multi-layer core, surrounded by monolayer edges. Afeatureless complex permittivity spectrum covering the range 0.6-1.6 THz isextracted from experimental time-domain measurements through a minimizationprocedure, adopting an extended finite-dipole model of the probe-sampleinteraction. Real-space mapping of the nanoribbon reveals variations in thelocal permittivity down to the instrument-limited resolution, on the order of 30 nm. Clustering analysis statistically identifies regions of lower apparentpermittivity that we attribute to a high curvature at the edges of thenanoribbon causing an increase in local material strain or cross-talk in themeasured signal with topography-induced measurement artifacts. The core of thenanoribbon contains two regions that follow tightly distributed, but slightlyshifted Gaussian statistics in complex permittivity space, with the real partmean of both distributions lying around 5.4 and compatible with literaturevalues of the static permittivity of thin-film MoS₂ reported previously. Ourresults show that the nanoribbons exhibit a modest degree of dielectricvariation at the nanoscale that could be explained by heterogeneous doping orvariations in the local defect density. We believe that our approach could beuseful for the direct real-space measurement of dielectric disorder in otherlow-dimensional semiconducting material systems.