Pressure-Induced Phase Transition in Multilayered Vanadium Diselenide Nanosheets

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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 (VSe 2 ) 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 ( P 3̅ m 1) 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 VSe 2 nanosheets recover the original ambient structure upon decompression. Raman spectroscopy studies at high pressures identify an A 1g soft phonon (∼236 cm –1 ) and an E g phonon (208 cm –1 ) that show normal hardening and consequently phase instability at ∼7 GPa. Using our experimental Raman mode Grüneisen parameters γ i, the thermal expansion coefficient α v of the Vse 2 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 Vse 2 nanosheets under an extreme stress environment.