Far-Infrared Signatures for a Two-Steps Pressure-Driven Metallization in Transition Metal Dichalcogenides

TL;DR

This study uses far-infrared spectroscopy under high pressure to clarify the pressure-induced transition from semiconductor to metal in MoS2 and WS2, revealing a two-step metallization process involving doping and band-gap closure.

Contribution

It provides new spectral evidence and a mechanistic model for the pressure-driven metallization in transition metal dichalcogenides, reconciling previous conflicting reports.

Findings

Metallization threshold identified by spectral weight increase

Asymmetric E1u peak suggests n-type doping origin of free carriers

Proposes a two-step pressure-induced metallization mechanism

Abstract

We present a high-pressure investigation of the semiconductor-to-metal transition in MoS2 and WS2 carried out by synchrotron-based far-infrared spectroscopy, to reconcile the controversial estimates of the metallization pressure found in the literature and gain new insight into the mechanisms ruling this electronic transition. Two spectral descriptors are found indicative of the onset of metallicity and of the origin of the free carriers in the metallic state: the absorbance spectral weight, whose abrupt increase defines the metallization pressure threshold, and the asymmetric lineshape of the E1u peak, whose pressure evolution, interpreted within the Fano model, suggests the electrons in the metallic state originate from n-type doping levels. Combining our results with those reported in the literature, we hypothesize a two-step mechanism is at work in the metallization process, in which the pressure-induced hybridization between doping and conduction band states drives an early metallic behaviour, while the band-gap closes at higher pressures.