Correlating structural, electronic, and magnetic properties of epitaxial VSe2 thin films

Abstract

The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono-and bilayer VSe 2 films grown using molecular beam epitaxy. VSe 2 has recently attracted much attention due to reports of emergent ferromagnetism in the two-dimensional (2D) limit. To understand this compound, high-quality 1T and distorted 1T films were grown at temperatures of 200°C and 450°C, respectively, and studied using 4 K scanning tunneling microscopy and spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T film shows a CDW pattern with 4a × 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wave vector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T VSe 2 film is weakly ferromagnetic, with ∼3.5 μ B per unit similar to previous reports.

Compositional Phase Change of Early Transition Metal Diselenide (VSe2 and TiSe2) Ultrathin Films by Postgrowth Annealing

“…The formation of such ordered intercalation compounds may also explain some controversial results regarding possible distorted lattices for ultrathin TMDC-films formed by direct MBE growth at elevated growth temperatures. 22,23,26,44 Transformation of layered chalcogenides into a different compositional phase by chalcogen-desorption induced by annealing or electron beam irradiation has been reported previously for other systems. 45,46,47,48,49 Thus, the transformations observed here fall into the same general category of phase engineering of layered materials and demonstrates that post-growth processing of ultrathin layered materials is a common approach for synthesizing new van der Waals materials.…”

“…films with a VSe 2 composition. 22 , 23 Compositional control in MBE growth of chalcogenides is, however, challenging because of the high vapor pressure of the chalcogens, which generally requires a high chalcogen overpressure. With varying growth temperatures and/or chalcogen flux, competing compositional phases may become favored.…”

Molecular Beam Epitaxy of Transition Metal (Ti-, V-, and Cr-) Tellurides: From Monolayer Ditellurides to Multilayer Self-Intercalation Compounds

“…In addition, VTe 2 undergoes a structural transition below ∼480 K to a distorted 1T structure, known as the 1T′′ or ribbon structure, which recently was considered to be associated with a CDW transition for these bulk materials . In variance of these known bulk structures, work of MBE grown multilayer structures have indicated structural variations in scanning tunneling microscopy (STM) studies of TiTe 2 , VSe 2 , , and VTe 2 . These have been interpreted as a transition into a distorted 1T-structure, known as the 1T′ structure − or the occurrence of periodic lattice distortions in a CDW .…”

“…In variance of these known bulk structures, work of MBE grown multilayer structures have indicated structural variations in scanning tunneling microscopy (STM) studies of TiTe 2 , VSe 2 , , and VTe 2 . These have been interpreted as a transition into a distorted 1T-structure, known as the 1T′ structure − or the occurrence of periodic lattice distortions in a CDW . Such interpretations are controversial, though, because these structures are not known for the bulk of these compounds and explanations for their formation in thin film form, such as interface-induced strain, is surprising because of the usually low strain in van der Waals epitaxy.…”

Topological Phase and Quantum Anomalous Hall Effect in Ferromagnetic Transition-Metal Dichalcogenides Monolayer 1T−VSe2

“…Particularly, presents ferromagnetic properties with the Curie temperature [ 14 ] above room temperature (RT). has also been shown to exhibit the charge density wave (CDW) phase below 130 K [ 14 , 15 , 19 ]. The wide temperature range for the ferromagnetic phase from 130 K up to RT achieves a high potential candidate for fulfilling the QAH effect in a single TMD layer.…”