Charge Density Wave Phase Transitions in Large-Scale Few-Layer 1T-VTe₂ Grown by Molecular Beam Epitaxy

Abstract

Charge density wave (CDW) as a novel effect in two-dimensional transition metal dichalcogenides (TMDs) has obtained a rapid rise of interest for its physical nature and potential applications in oscillators and memory devices. Here, we report var der Waals epitaxial growth of centimeter-scale 1T-VTe2 thin films on mica by molecular beam epitaxy. The VTe2 thin films showed sudden resistance change at temperatures of 240 and 135 K, corresponding to two CDW phase transitions driven by temperature. Moreover, the phase transitions can be driven by an electric field due to local Joule heating, and the corresponding resistance states are nonvolatile and controllable, which could be applied to the memory device where the logic states can be switched by an electric field. The multistage CDW phase transitions in the VTe2 thin films could be contributed to electron–phonon coupling in the two-dimensional VTe2, which is supported by twice pronounced Raman blue shifts of the vibration modes associated with in-plane phonons at CDW phase transition temperature. The results open up a new platform for understanding the microscopic physical essence and electrical control of CDW phases of TMDs, expanding the functionalities of these materials for memory applications.

Polymorphic Spin, Charge, and Lattice Waves in Vanadium Ditelluride

“…The bond length difference between types 1 and 2 is limited to 0.1 Å, but the Laue diffraction patterns precisely reveal that the unit cell of the type 2 CDW is three times larger than that of type 1. Overall, the new phase, type 2 CDW, has an additional lattice distortion along the b ‐axis that has not been observed in previous reports on CDWs in VTe 2 ;15–19,21 the intervanadium distance of 3.61 Å along the b ‐axis is split into 3.52 and 3.65 Å, as shown in the right panel of Figure 2a.…”

“…Vanadium ditelluride (VTe 2 ), a group 5 TMD, provides a platform for investigating the unexplored correlation between CDW and other quantum states, particularly those with magnetism 15–19. The moderate CDW transition temperature of bulk VTe 2 ( T C ≈ 480 K) compared with other CDW materials, such as VSe 2 ,20 allows a thorough study of the magnetism in its CDW phase.…”

“…The moderate CDW transition temperature of bulk VTe 2 ( T C ≈ 480 K) compared with other CDW materials, such as VSe 2 ,20 allows a thorough study of the magnetism in its CDW phase. Since the late 1950s, both CDW and magnetic orderings have been separately reported in VTe 2 of various thicknesses;15–19,21 in the former studies, however, the powder and thin film‐based VTe 2 materials showed conflicting features regarding their lattice structure at room temperature 19,20,22. Recent studies of thin VTe 2 films show that the interlayer interaction plays a role for the structural changes 18,23,24.…”

Evidence of charge density wave with anisotropic gap in a monolayer VTe2 film

“…1(d)] are observed in the RHEED patterns before and after growth of 1T-VTe 2 film. Using the lattice constant of graphene as a reference, the lattice constant of VTe 2 is extracted to be a = 3.63Å, which is similar to that of the nanoplate 3.64Å [18] and previous thicker MBE films 3.67Å [19]. As shown in Fig.…”

“…Few-layer VTe 2 films, however, show 1T structure similar to VSe 2 . Ferromagnetism has been reported in nanoplates of 1T-VTe 2 [18], and two possible CDW transitions at 240 K and 135 K have been reported in films of approximately ten layers thick from resistivity measurements [19]. However, the corresponding periods of those CDW orders remain unknown.…”