Two-dimensional vanadium diselenide : phase transition and magnetic properties

Abstract

Two-dimensional (2D) van der Waals (vdW) ferromagnets carry the promise of ultimately miniature spintronics and information storage devices. Although monolayer 1T-phase vanadium diselenide (VSe2) with ferromagnetism was theoretically predicted, the controversial results with both theoretical and experimental evidences raised ambiguity. This thesis reports our experimental investigation on the magnetic properties of 2D VSe2 with optical spectroscopic techniques. The first part of this thesis mainly focuses on the structural phase transition in 2D vanadium diselenide. VSe2 usually exists in a crystallographic form of 1T-phase with metallic behavior. We first materialized and observed a structural phase transition from 1T- to 2H-phase in multilayer VSe2, through annealing treatment, accompanied by the metal-insulator transition. With the help of Raman spectroscopy and in-situ selected area electron diffraction (SAED), we revealed the structural phase transition by determining the variation of Raman vibrational modes and SAED patterns. When the dimensionality decreases, 2D VSe2 thermodynamically favors ambient-stable 2H-phase instead of readily oxidizable 1T-phase, which is consistent with the thermal dynamic calculation. Additionally, the electrical measurements further provided the evidence of the metal-insulator transition, where 1T-VSe2 is metallic while 2H-VSe2 shows a semiconducting behavior. In the second part, we turn the spotlight on the noteworthy magnetic properties in 2D Vanadium diselenide. VSe2 is among the family of transition metal dichalcogenides (TMDs) which are typically believed to be paramagnetic. In this thesis, we studied the magnetic properties of the 2D VSe2 single crystal samples with the polar magnetic circular dichroism (MCD) microscopy and the second harmonic generation (SHG) technique. Strong ferromagnetism was observed in the 2H-phase of VSe2 with a Curie-Weiss temperature up to 425 K, and this ferromagnetism softens with the increased sample thickness. We attribute the change of the magnetic properties to the magnetic anisotropy that fundamentally arises from the structural inversion symmetry breaking and spin-orbit coupling in the 2D limit. According to the ferromagnetism of 2H-VSe2, we may further explore its potential of magnetic proximity effect in the heterostructure with another TMDs material.