Pressure-Induced Phase Transition in Multilayered Vanadium Diselenide Nanosheets

Layered transition-metal dichalcogenides have recently attracted considerable attention due to their unique mechanical and opto-electronic properties. Here, we report the investigation of structural, vibrational, and electronic properties of vanadium diselenide (VSe2) nanosheets up to a pressure of 33 GPa by diamond anvil cell-based pressure-induced studies. The experimental results indicate a structural transition from the metallic trigonal (P3̅m1) to a metallic monoclinic (C2/m) phase at ∼7 GPa, consistent with our ab initio calculations. A decrease in the metallic nature of the trigonal phase is evident from the reduction in the width of the Fermi level band crossing in the high-pressure monoclinic phase. Transmission electron microscopy analyses reveal that VSe2 nanosheets recover the original ambient structure upon decompression. Raman spectroscopy studies at high pressures identify an A1g soft phonon (∼236 cm–1) and an Eg phonon (208 cm–1) that show normal hardening and consequently phase instability at ∼7 GPa. Using our experimental Raman mode Grüneisen parameters γi, the thermal expansion coefficient αv of the Vse2 nanosheets at ambient temperature is obtained as −0.96 × 10–6 K–1. This pressure-tuned behavior of these layered nanomaterials can be beneficial in the calibration and development of novel nanodevices using Vse2 nanosheets under an extreme stress environment.