Infrared study of the 2H-MoTe2 electronic structure under pressure

Transition Metal Dichalcogenides (TMDs) are a well-known class of layered materials with stoichiometry MX2, where M is a transition metal atom and X is a chalcogen element [1]. Among TMDs, molybdenum-based compounds with a 2H hexagonal lattice and semiconducting nature with bandgaps in the infrared region are of particular interest for a wide range of electronic and optoelectronic applications [2,3]. This stimulated several investigations pointing out the strong dependence of the electronic structure on the number of layers and on their interaction, as witnessed by an indirect to direct bandgap transition when the crystal sample is reduced down to mono-layer [4,5]. The application of pressure, which mainly acts on the inter-layer distance, thus represent a powerful and clean tool to tune and study the electronic properties of these systems.
Here, we report the infrared spectroscopic study of the electronic properties of bulk 2H-MoTe2 under pressure up to about 20 GPa. Infrared transmission measurements have been performed from the far to the near infrared range at the synchrotron SOLEIL on the SMIS and AILES beamlines. The pressure dependence of the bandgap and the pressure-induced increase of the spectral weight in the far infrared range indicate that a semiconductor to insulator transition takes place at 13±1 GPa. This result is in well agreement with theoretical predictions based on the density functional theory [6]. We also propose a simple method based on the Drude model, which in the metallic phase associates the integral of the far-infrared absorption to the free electron density. This allows us to provide a semi-quantitative description of the slow pressure-induced increase of the electron density that characterizes the metallization process in bulk 2H-MoTe2.
Finally, this study shows the importance of performing broadband infrared spectroscopy at high pressure at synchrotron facilities. For this reason, an upgrade of the high-pressure optical setup available the SMIS beamline is currently under commissioning. The main features of this upgraded setup are also presented.

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