Electronic structure and charge-density wave transition in monolayer VS2
Graphical abstract
Epitaxial growth and Fermi surface mapping of monolayer VS2 on graphene.
Introduction
Complex phases in two-dimensional (2D) layered transition-metal dichalcogenides (TMDCs) attract great interest for fundamental phenomena, such as band-gap transition, charge-density wave (CDW), and superconductivity, as well as their potential energy applications, such as solar energy harvesting and efficient catalytic electrode alternatives [[1], [2], [3], [4]]. Due to inherent 2D layered geometry, TMDCs have shown diverse CDW phases in metallic TMDCs, such as Ta(S,Se)2, Nb(S,Se)2, V(S,Se,Te)2 [[5], [6], [7], [8], [9]]. Those metallic TMDCs also recently show potential performances as phase switching device for nonvolatile memory [10], CDW-based oscillator [11], and photodetector [12], and catalytic electrode for hydrogen evolution reaction [13].
Vanadium dichalcogenides, especially, attract much attention due to their intriguing properties, such as CDW, ferromagnetism, and surface catalytic behavior. Bulk VS2 shows CDW phase transition at 304 K as detected by previous electron diffraction pattern, temperature-dependent resistivity and magnetic susceptibility, and nuclear magnetic resonance measurements [14,15]. However, angle-resolved photoemission spectroscopy (ARPES) study of bulk VS2 concluded the absence of Fermi surface (FS) nesting during the CDW transition [7]. On the other hand, bulk VSe2 has a CDW phase (105 K) with 4 × 4 × 3 periodicity which is attributed to FS nesting [8], while bulk VTe2 has strong CDW phase (482 K) with 4 × 4 × 3 periodicity [9]. Recent extensive studies have revealed that VSe2 and VTe2 exhibits transition from 3D nesting vector to 2D one, depending on the thickness, while the detailed role of thickness, interface, or stoichiometry remained controversial [[16], [17], [18], [19]] In fact, huge attention was focused on vanadium dichalcogenides due to its theoretically calculated ferromagnetism in their monolayer (ML) form [20,21]. While extensive theoretical results predicted both 1H and 1T phase for ML VX2 (X = S, Se, Te) with spin-polarized band structures, most experiments show the 1T phase ML with non-magnetic band structures in the case of VSe2 and VTe2. Detection of room temperature ferromagnetic signal in ML VSe2 was understood as a result of either non-stoichiometric defects or interface-driven effect [[22], [23], [24]]. Another interest is surface catalytic property found at the surface of ultrathin VS2 and VSe2, whose reactivity due to metallic conductivity and atomic distortion shows quite promising results [[25], [26], [27]]. Therefore, it is important to study fundamental electronic structure properties of the single-crystalline ultrathin films with combination of molecular-beam epitaxy (MBE) and ARPES.
However, the MBE growth of vanadium chalcogenides have been confined with Se and Te, because high vapor pressure of sulfur is difficult to be incorporated with the ultra-high vacuum (UHV) instruments. As an alternative of elemental sulfur, metal sulfides, such FeS and FeS2, have been recently attempted to obtain high quality epitaxial sulfide thin films for limited number of sulfide compounds, such as MoS2, WS2, NbS2, and TaS2 [28,29]. Still, there is only limited number of reports on the epitaxially grown VS2 ML [30], in which temperature dependent electronic structure have not been understood yet.
Here, we successfully prepared ML VS2 on graphene substrates by using MBE system. ML VS2 shows similar in-plane lattice parameter to its bulk value and is epitaxially aligned to the graphene with small tensile strain. ML VS2 film shows a FS map with six-fold symmetry, well aligned to the band structure of graphene. The electronic band structure is compared with the theoretical band structure of freestanding ML VS2 with 1T phase. Temperature dependence of gap sizes are analyzed for understanding the CDW in this ML VS2.
Section snippets
Experiments
ML VS2 was grown on bilayer graphene (BLG) on 4H–SiC (0001) using a home-built UHV MBE with a base pressure of 1.0 × 10−9 torr [31]. After outgassing at 650 °C for a few hours, the substrates were annealed up to 1300 °C for 6 min for preparation of BLG on SiC substrates while monitoring reflection high-energy electron diffraction (RHEED) images. High-purity V (99.8%) rod and FeS (99.98%) powder were used as vanadium and sulfur sources, and simultaneously evaporated by an electron-beam
Results and discussion
Fig. 1(a) display a schematic model of ML VS2 stacked on a BLG/SiC substrate. The crystal structure consists of a flat hexagonal plane of vanadium atoms sandwiched between two sulfur layers, forming a 1T structure with octahedral symmetry, similar to the other ML vanadium dichalcogenides, VSe2 and VTe2 [33,34]. Fig. 1(b and c) shows RHEED images of BLG and ML VS2 with same measurement parameters, respectively. The BLG shows several main streaks with additional diffraction spots due to different
Conclusion
In summary, we performed ARPES study of electronic structure in an epitaxial ML VS2 grown on bilayer graphene by MBE. ML VS2 is aligned to the graphene with the in-plane lattice parameter similar to its bulk value. We find that ML VS2 has Fermi surface with six elliptic bands centered at the M point, confirming the theoretical calculation of 1T phase. Interestingly, the elliptic bands present rather round shape, different from the rather straight sides observed in both ML VSe2 and ML VTe2.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Research Foundation (NRF) grants funded by the Korean government (No. NRF-2019K1A3A7A09033389 and NRF-2020R1A2C200373211). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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