Elsevier

Current Applied Physics

Volume 30, October 2021, Pages 8-13
Current Applied Physics

Electronic structure and charge-density wave transition in monolayer VS2

https://doi.org/10.1016/j.cap.2021.03.020Get rights and content

Highlights

  • We report an epitaxial growth of monolayer VS2 film on bilayer graphene by molecular beam epitaxy.

  • We demonstrate electronic band structure of monolayer VS2 with 1T phase by angle-resolved photoemission spectroscopy.

  • We observe temperature-dependent evolution of charge density wave gap with transition temperature at ~400 K.

  • The VS2 is an important building block for establishing next-generation electronics and renewable energy applications.

Abstract

Vanadium disulfide (VS2) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS2 monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transition above room temperature. Our observations provide important evidence to understand the strongly correlated electron physics and the related surface catalytic properties in two-dimensional transition-metal dichalcogenides (TMDCs).

Graphical abstract

Epitaxial growth and Fermi surface mapping of monolayer VS2 on graphene.

Image 1
  1. Download : Download high-res image (252KB)
  2. Download : Download full-size image

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.

References (66)

  • S. Manzeli et al.

    2D transition metal dichalcogenides

    Nat. Rev. Mater.

    (2017)
  • A. Majumdar et al.

    Interplay of charge density wave and multiband superconductivity in layered quasi-two-dimensional materials: the case of 2H−NbS2 and 2H −NbSe2

    Phys. Rev. Mater.

    (2020)
  • K. Rossnagel

    On the origin of charge-density waves in select layered transition-metal dichalcogenides

    J. Phys. Condens. Matter

    (2011)
  • M. Mulazzi et al.

    Absence of nesting in the charge-density-wave system 1T-VS2 as seen by photoelectron spectroscopy

    Phys. Rev. B

    (2010)
  • V.N. Strocov et al.

    Three-dimensional electron realm in VSe2 by soft-X-ray photoelectron spectroscopy: origin of charge-density waves

    Phys. Rev. Lett.

    (2012)
  • D. Won et al.

    Polymorphic spin, charge, and lattice waves in vanadium ditelluride

    Adv. Mater.

    (2020)
  • M. Yoshida et al.

    Memristive phase switching in two-dimensional 1T-TaS2 crystals

    Sci. Adv.

    (2015)
  • G. Liu et al.

    A charge-density-wave oscillator based on an integrated tantalum disulfide–boron nitride–graphene device operating at room temperature

    Nat. Nanotechnol.

    (2016)
  • Y.H. Huang et al.

    Electronic transport in NbSe2 two-dimensional nanostructures: semiconducting characteristics and photoconductivity

    Nanoscale

    (2015)
  • X. Chia et al.

    Electrocatalysis of layered Group 5 metallic transition metal dichalcogenides (MX2, M = V, Nb, and Ta; X = S, Se, and Te)

    J. Mater. Chem. A.

    (2016)
  • A. Gauzzi et al.

    Possible phase separation and weak localization in the absence of a charge-density wave in single-phase 1T-VS2

    Phys. Rev. B

    (2014)
  • J. Feng et al.

    Electronic structure and enhanced charge-density wave order of monolayer VSe2

    Nano Lett.

    (2018)
  • Y. Umemoto et al.

    Pseudogap, Fermi arc, and Peierls-insulating phase induced by 3D–2D crossover in monolayer VSe2

    Nano Res

    (2019)
  • M. Liu et al.

    Multimorphism and gap opening of charge-density-wave phases in monolayer VTe2

    Nano Res

    (2020)
  • K. Sugawara et al.

    Monolayer VTe 2 : incommensurate Fermi surface nesting and suppression of charge density waves

    Phys. Rev. B

    (2019)
  • Y. Ma et al.

    Evidence of the existence of magnetism in pristine VX2 monolayers (X = S, Se) and their strain-induced tunable magnetic properties

    ACS Nano

    (2012)
  • H.-R. Fuh et al.

    Newtype single-layer magnetic semiconductor in transition-metal dichalcogenides VX2 (X = S, Se and Te)

    Sci. Rep.

    (2016)
  • M. Bonilla et al.

    Strong room-temperature ferromagnetism in VSe2 monolayers on van der Waals substrates

    Nat. Nanotechnol.

    (2018)
  • G. Duvjir et al.

    Novel polymorphic phase of two-dimensional VSe2: the 1T′ structure and its lattice dynamics

    Nanoscale

    (2019)
  • P.K.J. Wong et al.

    Evidence of spin frustration in a vanadium diselenide monolayer magnet

    Adv. Mater.

    (2019)
  • J. Zhang et al.

    Synergistic interlayer and defect engineering in VS2 nanosheets toward efficient electrocatalytic hydrogen evolution reaction

    Small

    (2018)
  • W. Zhao et al.

    Colloidal synthesis of VSe2 single-layer nanosheets as novel electrocatalysts for the hydrogen evolution reaction

    Chem. Commun.

    (2016)
  • Y. Qu et al.

    Ultra-high electrocatalytic activity of VS2 nanoflowers for efficient hydrogen evolution reaction

    J. Mater. Chem. A.

    (2017)
  • Cited by (16)

    • On the oxidation of VS<inf>2</inf> 2D platelets using tip-enhanced Raman spectroscopy

      2023, Current Opinion in Solid State and Materials Science
      Citation Excerpt :

      Experiments conducted at a lower temperature from 5 K to 150 K revealed shifted Raman contributions and enhanced Raman intensity. Authors concluded that the enhancement and the splitting of some modes could be assigned to a charge density wave transition (CDW) that triggered a structural change in the VS2 phase. [23] Nevertheless, no additional results beyond the 330 K temperature were reported.

    • Progress and prospects of 2D VS<inf>2</inf> transition metal dichalcogenides

      2022, FlatChem
      Citation Excerpt :

      They have observed the temperature dependence of the gap size, which suggests the existence of charge-density-wave (CDW) phase transition above room temperature. Such type of study helps us to understand the strongly correlated electron physics and surface catalytic properties in 2D TMD materials [48]. Fig. 9 provides an overview of the discussed synthesis methods for VS2 nanomaterials.

    View all citing articles on Scopus
    View full text