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

Abstract

High pressure is a proven effective tool for modulating inter-layer interactions in semiconducting transition metal dichalcogenides, which leads to significant band structure changes. Here, we present an extended infrared study of the pressure-induced semiconductor-to-metal transition in 2H-$$\hbox {MoTe}_2$$ MoTe 2 , which reveals that the metallization process at 13–15 GPa is not associated with the indirect band-gap closure, occurring at 24 GPa. A coherent picture is drawn where n-type doping levels just below the conduction band minimum play a crucial role in the early metallization transition. Doping levels are also responsible for the asymmetric Fano line-shape of the $$\hbox {E}_{1u}$$ E 1 u infrared-active mode, which has been here detected and analyzed for the first time in a transition metal dichalcogenide compound. The pressure evolution of the phonon profile under pressure shows a symmetrization in the 13–15 GPa pressure range, which occurs simultaneously with the metallization and confirms the scenario proposed for the high pressure behaviour of 2H-$$\hbox {MoTe}_2$$ MoTe 2 .

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

“…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 . These results obtained on three different TMD semiconductors and a TMD alloy indicate the SW analysis of the FIR spectrum as a reliable approach for the detection of the metallic behavior in this class of materials.…”

“…In bulk MoTe 2 , resistivity measurements found evidence of the onset of a metallic behavior at P M res ∼ 10−12 GPa, 16,18 while the effective closure of the band gap was observed at ∼24 GPa through absorption measurements in the near-infrared (NIR). 19 In a similar vein, resistivity 20 and impedance 21 measurements carried out on bulk MoS 2 suggested the crystal becomes metallic at P M res ∼ 20 GPa, while absorption measurements in the NIR/visible range showed that the band gap closes at pressures higher than 26 GPa. 22 As for the other semiconductors belonging to the class of TMDs, like WS 2 , in the majority of cases, the definition of the metallization threshold only relies on transport measurements, 23 while there lacks a direct estimate of the pressure-dependent trend of the band gap.…”

“…The pressure evolution of the MoS 2 and WS 2 absorbance spectra closely resembles that of MoTe 2 , previously studied and explained within the aforementioned scheme. 19 The As shown in Figure 1, in both samples, all the measured absorbance A( ) curves (see Methods) display quite intense oscillating fringes due to interference effects between the internal diamond faces through the hydrostatic medium in I ( ) 0 , the backround intensity (see Methods). In this case, a Fourier transform smoothing procedure is not effective to remove the fringes owing to the reduced number of oscillations b), (f) MoS 2 , WS 2 absorbance spectra at selected pressures after the subtraction of the polynomial background and the sinusoidal fringes.…”

“…In bulk MoTe 2 , resistivity measurements found evidence of the onset of a metallic behavior at P M res ∼ 10–12 GPa, , while the effective closure of the band gap was observed at ∼24 GPa through absorption measurements in the near-infrared (NIR) . In a similar vein, resistivity and impedance measurements carried out on bulk MoS 2 suggested the crystal becomes metallic at P M res ∼ 20 GPa, while absorption measurements in the NIR/visible range showed that the band gap closes at pressures higher than 26 GPa .…”

“…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.…”