Infrared Study of the Pressure-Induced Isostructural Metallic Transition in Mo_0.5W_0.5S₂

Abstract

Ternary compounds of transition metal dichalcogenides are emerging as an interesting class of crystals with tunable electronic properties, which make them attractive for nanoelectronic and optoelectronic applications. Among them, Mo x W1–x S2 is one of the most studied alloys due to the well-known and remarkable features of its binary constituents, MoS2 and WS2. The band gap of this compound can be modeled varying Mo and W percentages in the sample, and its vibrational modes result from a combination of MoS2 and WS2 phonons. In this work, we report transmission measurements on a Mo0.5W0.5S2 single crystal in the far-infrared range. Absorbance spectra collected at ambient conditions enabled for the first time a classification of the infrared-active phonons, complementary to Raman studies. High-pressure measurements allowed the study of the evolution of both the lattice dynamics and the free carrier density up to 31 GPa, suggesting the occurrence of an isostructural semiconductor-to-metal transition above 18 GPa, in very good agreement with the theoretical calculation reported in the literature.

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

“…The SW trends also gave us a semiquantitative information on the different rate of increase of the DC conductivity in the two crystals, suggesting a slower transition in WS 2 compared with the MoS 2 case, in agreement with the results from resistivity measurements. It is worth noticing that a similar analysis conducted on Mo 0.5 W 0.5 S 2 gave a metallization threshold very close to the average P M FIR of MoS 2 and WS 2 , which is exactly what would be expected for an alloy compound. Moreover, a good correspondence between the metallization onset obtained from pressure-dependent FIR measurements and pressure-dependent transport measurements was found for MoTe 2 .…”

“…In the FIR range here explored, we see that the Drude band contributes to the spectrum as a wavenumber-independent background, suggesting a relatively large value of Γ compared to the ν̃ values considered and, thus, allowing us to work within the low-wavenumber approximation. Based on these considerations, we can say that the increase we observed in the FIR absorbance is directly related to an increase in σ 0 ,, and, thus, to the onset of the metallic behavior reported in the resistivity measurements of both MoS 2 and WS 2 . , …”

“…We found that the FIR spectrum can provide us with two effective descriptors that indicate both the onset of the metallic behavior and the origin of the free charge carriers formed in the process. The first one is the absorbance spectral weight, 17,19,24,25 second one is the line shape of the E 1u phonon, whose pressure evolution, interpreted in the framework of the Fano theory, 26 suggests a reliable explanation for the origin of the free charge carriers in the metallic state. The asymmetric profile shown by the E 1u phonon at ambient conditions is symptomatic of the coupling between the vibrational mode and a continuum of electronic transitions with comparable energy scales.…”

“…Several pressure-dependent studies have been carried out aiming at studying the TMD response to a strong enhancement of the interlayer interaction. − Most of the semiconducting crystals in the bulk form were found to undergo a pressure-induced transition toward a metallic state. − However, in many cases, a certain degree of ambiguity persists in the exact determination of the metallization pressure as well as in the explanation of the microscopic origin of the transition.…”

Broadband infrared study of pressure-tunable Fano resonance and metallization transition in 2H-$$\hbox {MoTe}_2$$

“… with and Drude relaxation time, the experimental absorbance is simply proportional to the square root of both the DC conductivity and the frequency , thus resulting . Therefore, the significant growth in the overall absorption at high pressure can be related to an increase of 37 and, thus, to the onset of a metallic behavior. To carry out a quantitative analysis, we define the absorption spectral weight at each pressure: .…”