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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 .

We report on Raman experiments performed on a single crystal MoTe 2 sample. The system belongs to the wide family of Transition Metal Dichalcogenides which includes several of the most interesting two dimensional materials for both basic and applied physics. Measurements were performed in the standard basal plane configuration, by placing the ab plane of the crystal perpendicular to the wave vector k i of the incident beam to explore the in plane vibrational modes, and in the edge plane configuration with k i perpendicular to the crystal c axis, thus mainly exciting out-of-plane modes. For both configurations we performed a polarization-dependent Raman study and we were able to provide a complete assignment of the observed first-and second-order Raman peaks fully exploiting the polarization selection rules. Present findings are in complete agreement with previous first-order Raman data whereas a thorough analysis of the second-order Raman bands, either in basal-or edge-plane configurations, provides new information and a precise assignment of these spectral structures. In particular, we have observed Raman active modes of the M point of the Brillouin zone previously predicted by ab-initio calculations and ascribed to either combination or overtone but never previously measured.

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.

The exceptional photovoltaic properties of hybrid organic–inorganic perovskites have attracted increasing interest in the past decades. Among these materials, FAPbI3 shows two structural phases: the high temperature perovskite α-phase, with direct bandgap close to the Shockley–Queisser limit, and the much less photoactive non-perovskite δ-phase, stable at ambient conditions. Although the presence of the δ-phase has been usually regarded as a limitation for FAPbI3 optoelectronic applications, recent studies have found that devices with increased stability and efficiency can be designed by mixing α- and δ-phases. This has brought out the need for a deeper understanding of the physical properties of δ-FAPbI3. In this paper, we present an original high-pressure Raman and photoluminescence study to address the effects of compression on the lattice and optoelectronic response of the sample. Also, based on the previous findings on different hybrid perovskites, our results for δ-FAPbI3 show that the cation configuration goes from a dynamically disordered regime at ambient conditions to a statically ordered phase at ∼1.5 GPa. On further increasing pressure, above 7 GPa, a statically disordered regime takes place, where the cations are locked at random orientations in the inorganic framework, giving rise to an amorphous-like state. Compared with α- FAPbI3, we found that the hexagonal δ-phase is less affected by external compression, as both the first detectable structural transition and the amorphous-like behavior occur at higher pressures.