Metal to Insulator Quantum-Phase Transition in Few-Layered ReS₂

In ReS₂, a layer-independent direct band gap of 1.5 eV implies a potential for its use in optoelectronic applications. ReS₂ crystallizes in the 1T′-structure, which leads to anisotropic physical properties and whose concomitant electronic structure might host a nontrivial topology. Here, we report an overall evaluation of the anisotropic Raman response and the transport properties of few-layered ReS₂ field-effect transistors. We find that ReS₂ exfoliated on SiO₂ behaves as an n-type semiconductor with an intrinsic carrier mobility surpassing μ_i ∼ 30 cm²/(V s) at T = 300 K, which increases up to ∼350 cm²/(V s) at 2 K. Semiconducting behavior is observed at low electron densities n, but at high values of n the resistivity decreases by a factor of >7 upon cooling to 2 K and displays a metallic T²-dependence. This suggests that the band structure of 1T′-ReS₂ is quite susceptible to an electric field applied perpendicularly to the layers. The electric-field induced metallic state observed in transition metal dichalcogenides was recently claimed to result from a percolation type of transition. Instead, through a scaling analysis of the conductivity as a function of T and n, we find that the metallic state of ReS₂ results from a second-order metal-to-insulator transition driven by electronic correlations. This gate-induced metallic state offers an alternative to phase engineering for producing ohmic contacts and metallic interconnects in devices based on transition metal dichalcogenides.