Correlating structural, electronic, and magnetic properties of epitaxial $\mathrm{V}{\mathrm{Se}}_{2}$ thin films

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Correlating structural, electronic, and magnetic properties of epitaxial $V {Se}_{2}$ thin films

Guannan Chen, Sean T. Howard, Aniceto B. Maghirang, III, Kien Nguyen Cong, Rovi Angelo B. Villaos, Liang-Ying Feng, Kehan Cai, Somesh C. Ganguli, Waclaw Swiech, Emilia Morosan, Ivan I. Oleynik, Feng-Chuan Chuang, Hsin Lin, and Vidya Madhavan

Phys. Rev. B 102, 115149 – Published 23 September 2020

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 $V {Se}_{2}$ films grown using molecular beam epitaxy. $V {Se}_{2}$ has recently attracted much attention due to reports of emergent ferromagnetism in the two-dimensional (2D) limit. To understand this compound, high-quality $1 T$ and distorted $1 T$ 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 $1 T$ phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the $1 T$ film shows a CDW pattern with $4 a \times 4 a$ 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 $1 T$ 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 $1 T VS e_{2}$ film is weakly ferromagnetic, with ∼3.5 $μ_{B}$ per unit similar to previous reports.

Received 15 December 2019
Revised 8 July 2020
Accepted 14 August 2020

DOI:https://doi.org/10.1103/PhysRevB.102.115149

Physics Subject Headings (PhySH)

Density of states Magnetic order Phonons

Thin films Transition metal dichalcogenides

Molecular beam epitaxy Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Guannan Chen^1,2, Sean T. Howard^1,2, Aniceto B. Maghirang, III ³, Kien Nguyen Cong ⁴, Rovi Angelo B. Villaos³, Liang-Ying Feng³, Kehan Cai⁵, Somesh C. Ganguli ⁶, Waclaw Swiech², Emilia Morosan⁷, Ivan I. Oleynik ⁴, Feng-Chuan Chuang ³, Hsin Lin ⁸, and Vidya Madhavan^1,2

¹Department of Physics, University of Illinois Urbana-Champaign, Urbana, Illinois 61801, USA
²Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, Illinois 61801, USA
³Department of Physics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
⁴Department of Physics, University of South Florida, Tampa, Florida 33620, USA
⁵Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA
⁶Department of Applied Physics, Aalto University School of Science, P.O. Box 11100, 00076 Aalto, Finland
⁷Department of Physics and Astronomy and Rice Center for Quantum Materials, Rice University, Houston, Texas 77005, USA
⁸Institute of Physics, Academia Sinica, Taipei 11529, Taiwan

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Vol. 102, Iss. 11 — 15 September 2020

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Figure 1
Schematic structure of $1 T$ and distorted $1 T VS e_{2}$ . The blue circles represent V atoms and the orange circles represent Se atoms. (a) Side view and (b) top view of $1 T - VS e_{2}$ . (c) Side view and (d) top view of distorted $1 T - VS e_{2}$ . The green and pink ribbons highlight the lower and higher top Se atoms, correspondingly.
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Figure 2
Characterization of epitaxially grown $V {Se}_{2}$ films by RHEED and STM. (a)–(d) RHEED pattern of BLG grown on 6H-SiC (0001) substrate, LGT and HGT films after 0.5-layer growth, LGT film after 1.5-layer deposition, and HGT film after 1.5-layer deposition. In (d) the extra streaks are marked by the orange arrows, which indicate the formation of the distorted $1 T$ phase. (e), (g) Large STM topographies of 1.5-layer LGT and 1.5-layer HGT samples $(150 nm \times 150 nm)$ . (f), (h) Height profiles of the orange lines in (e), (g), respectively.
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Figure 3
STM data on first- and second-layer LGT $V {Se}_{2}$ films. (a) Atomic resolution STM topography of the first layer in LGT sample (10 nm × 10 nm). The 2D CDW vectors are shown as the orange arrows and green lines indicate the 1D stripe order with periodicity d. Inset: 2D fast Fourier transform (FFT). The orange and green circles represent the 2D CDW order and 1D stripe order, respectively. (b) Calculated phonon dispersion relations for monolayer 1T ${VSe}_{2}$ . (c) The typical dI/dV spectrum of the first layer LGT. The vertical arrows correspond to peaks at −32 and +18 meV. (d) The atomic resolution STM topography of the second layer in LGT sample (10 nm × 10 nm) with 2D FFT as inset. The pink circles in the FFT indicate the peaks associated with $4 a \times 4 a$ CDW. (e) Calculated phonon dispersion relations for bilayer $V {Se}_{2}$ . (f) Comparison of $dI / dV$ spectra of the second layer of LGT sample (blue line) and density of state curve from first-principles calculation (red line). The arrows indicate the peaks in the calculated density of states at −25 and +62 meV, very similar to the experimental data.
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Figure 4
STM data on first layer and second layer of the HGT $V {Se}_{2}$ film. (a), (c) STM topographic images of the first layer and second layer of HGT samples (10 nm × 10 nm). The insets are the corresponding 2D FFT images. In (c) the blue and pink circles highlight the peaks related with the top Se lattice (blue) and stripes caused by the structural distortion (pink). (b), (d) Typical $dI / dV$ curves measured on the first layer and second layer of HGT $V {Se}_{2}$ films, respectively. The arrows in (b) indicate peaks at −56 and +36 meV. (e)–(g) The height profiles of the orange and green line in (a) and red line in (c), respectively. In (f), (g), the black dashed lines show the positions of atoms.
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Physical Review B

covering condensed matter and materials physics

Correlating structural, electronic, and magnetic properties of epitaxial $V {Se}_{2}$ thin films

Guannan Chen, Sean T. Howard, Aniceto B. Maghirang, III, Kien Nguyen Cong, Rovi Angelo B. Villaos, Liang-Ying Feng, Kehan Cai, Somesh C. Ganguli, Waclaw Swiech, Emilia Morosan, Ivan I. Oleynik, Feng-Chuan Chuang, Hsin Lin, and Vidya Madhavan

Phys. Rev. B 102, 115149 – Published 23 September 2020

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Physical Review B

covering condensed matter and materials physics

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

Guannan Chen, Sean T. Howard, Aniceto B. Maghirang, III, Kien Nguyen Cong, Rovi Angelo B. Villaos, Liang-Ying Feng, Kehan Cai, Somesh C. Ganguli, Waclaw Swiech, Emilia Morosan, Ivan I. Oleynik, Feng-Chuan Chuang, Hsin Lin, and Vidya Madhavan

Phys. Rev. B 102, 115149 – Published 23 September 2020

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Correlating structural, electronic, and magnetic properties of epitaxial $V {Se}_{2}$ thin films