Strong charge-density-wave order of large-area 2D metallic VSe₂ nanosheets discovered by temperature-dependent Raman spectra

High-quality and large-area 1T-VSe₂ nanosheets with different thicknesses on flat mica substrates are grown by ambient-pressure chemical vapor deposition. Temperature-dependent Raman spectra between 90 and 350 K have been performed to investigate the phenomena of the charge density wave (CDW) order. It can be found that the frequency of the A_1g mode increases with the decreasing temperature. When the temperature reaches 110 K, the A_1g mode abruptly converts to a low wavenumber. This phenomenon proves the CDW formation of VSe₂ with the 5 nm thickness. It can be interpreted as arising from an enhanced electron-phonon coupling mechanism. Moreover, variable temperature-dependent Raman spectra for VSe₂ with different thicknesses showed that the phase transition temperature gradually increased with the thickness. In addition, the CDW phase transition has also been detected by the abrupt change of the resistance, and its slope is located at about 112 K. The present results can be helpful for the potential applications of VSe₂ crystals, such as spintronics devices and supercapacitors.