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.