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Layered transition-metal dichalcogenides (TMDs) have recently attracted intense scientific and engineering interest because of their unique semiconducting and opto-electronic properties. We investigated the pressure-induced structural phase transition of layered semiconductor molybdenum disulfide (MoS2) using Raman spectroscopy and studied its metallization using infrared (IR) spectroscopy under both non-hydrostatic and quasi-hydrostatic conditions. Under quasi-hydrostatic and non-hydrostatic conditions, we found that the structural phase transition from 2 Hc stacking to 2 Ha stacking starts at approximately 16 and 21 GPa, respectively, and finishes at ∼35 and ∼41 GPa, respectively. Furthermore, the structural phase transition was followed by a semiconductor-to-metal (S-M) electronic transition. The pressure point of metallization under quasi-hydrostatic conditions is ∼5 GPa lower than that under non-hydrostatic conditions. [ABSTRACT FROM AUTHOR]

• The high pressure phase transitions of HfSe 2 were studied. • Pressure-induced semiconductor-metal transition appeared in the original P-3m1 phase. • The metallization was due to the overlap of valance and conduction bands with the contribution of Hf d and Se p orbitals. • A new metallic I 4 /mmm structure in HfSe 2 was found under high pressure. [Display omitted] In this study, we report the phase transitions and semiconductor-to-metal (S-M) transition of hafnium selenide (HfSe 2) under high pressure through experiments and theoretical calculations. The starting hexagonal structure transforms to an intermediate phase and I 4 /mmm structure above 10 GPa, and then converts into the I 4 /mmm structure completely above 47.2 GPa. Pressure-induced S-M transition at ~10 GPa was revealed by infrared (IR) reflectivity and resistance measurements. Theoretical calculations indicate that the pressure-induced metallization occurs in the initial hexagonal phase, which arises from the overlap of the valance and conduction bands with the contribution of the Hf d and Se p orbitals under high pressure. Upon pressure releases, the I 4 /mmm phase transforms into the initial hexagonal structure. This work provides new insights into the structures and electronic states of HfSe 2. [ABSTRACT FROM AUTHOR]

Cr 2 Ge 2 Te 6 is a rare layered ferromagnetic semiconductor that represents a promising material for novel thermoelectric and spintronic devices. High pressure is an effective way to change the electronic structure, and profoundly affects the physical and chemical properties of two-dimensional (2D) layered materials. Here we combined experimental results and theoretical calculations to investigate the structural evolution and properties of Cr 2 Ge 2 Te 6 under high pressure. It is found that Cr 2 Ge 2 Te 6 undergoes an isostructural phase transition from layered to non-layered structure at ~14 GPa, accompanied with a semiconductor to metal transition, which is distinct from the previous reports. The pressure-induced metallization is verified by the overlap of the valence and conduction bands, and the layered-to-non-layered isostructural transition is attributed to the enhanced interlayer Te–Te interaction by First-principles calculations. Our results provide a potential pathway to control the structure and property of layered materials and possibility for conceptually new devices via pressure engineering. • Cr 2 Ge 2 Te 6 undergoes an isostructural phase transition at ~14 GPa, and accompanied with a semiconductor to metal transition. • The semiconductor-to-metal transition was verified by IR and electrical experiments. • The metallization mechanism was interpreted by theoretical calculations. [ABSTRACT FROM AUTHOR]