Electrochemical mechanism of ionic-liquid gating in antiferromagnetic Mott-insulating ${\mathrm{NiS}}_{2}$ single crystals

Electrochemical mechanism of ionic-liquid gating in antiferromagnetic Mott-insulating ${NiS}_{2}$ single crystals

Sajna Hameed, Bryan Voigt, John Dewey, William Moore, Damjan Pelc, Bhaskar Das, Sami El-Khatib, Javier Garcia-Barriocanal, Bing Luo, Nick Seaton, Guichuan Yu, Chris Leighton, and Martin Greven

Phys. Rev. Materials 6, 064601 – Published 7 June 2022

Abstract

We explore the effect of ionic-liquid gating in the antiferromagnetic Mott insulator ${NiS}_{2}$ . Through temperature- and gate-voltage-dependent electronic transport measurements, a gating-induced three-dimensional metallic state is observed at positive gate bias on single-crystal surfaces. Based on transport, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an $electrochemical$ gating mechanism involving a substantial decrease in the S:Ni ratio over hundreds of nanometers, which is both nonvolatile and irreversible. Such findings are in striking contrast to the reversible, volatile, two-dimensional $electrostatic$ gate effect previously seen in pyrite ${FeS}_{2}$ . We attribute this stark difference in electrochemical vs electrostatic gating response in ${NiS}_{2}$ and ${FeS}_{2}$ to the much larger S diffusion coefficient in ${NiS}_{2}$ . The gating irreversibility, on the other hand, is associated with the lack of atmospheric S, in contrast to the better understood oxide case, where electrolysis of atmospheric $H_{2} O$ provides an O reservoir. The present study of ${NiS}_{2}$ thus provides insight into electrolyte gating mechanisms in functional materials, in a relatively unexplored limit.

Received 28 December 2021
Accepted 18 May 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.064601

Physics Subject Headings (PhySH)

Electrical properties

Antiferromagnets Field-effect transistors Mott insulators

Atomic force microscopy Energy-dispersive x-ray spectroscopy Transport techniques X-ray photoelectron spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Sajna Hameed ^1,*,†, Bryan Voigt², John Dewey², William Moore², Damjan Pelc¹, Bhaskar Das ², Sami El-Khatib^3,2, Javier Garcia-Barriocanal⁴, Bing Luo⁴, Nick Seaton⁴, Guichuan Yu^4,5, Chris Leighton ^2,‡, and Martin Greven ^1,§

¹School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
²Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA
³Department of Physics, American University of Sharjah, Sharjah, United Arab Emirates
⁴Characterization Facility, University of Minnesota, Minneapolis, Minnesota 55455, USA
⁵Informatics Institute, University of Minnesota, Minneapolis, Minnesota 55455, USA

^*hamee007@umn.edu
^†Present address: Max-Planck-Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.
^‡leighton@umn.edu
^§greven@umn.edu

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Vol. 6, Iss. 6 — June 2022

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Figure 1
(a) Sheet resistance as a function of temperature for different applied gate voltages. In the legend, the listing of applied voltages is in chronological order. As a check of the volatility and reversibility of the gate effect, 0 and −3.0 V (dashed lines) were applied following the first positive gate voltage application of +0.5 V. The resistance of the sample after polishing all sides postgating is also shown (dotted line). The inset shows low-temperature $R_{s}$ vs $T$ obtained for the +2.0 V gated sample in a ${}^{3}{He}$ refrigerator. (b) Temperature derivative of the sheet resistances in (a), highlighting the anomaly at about 30 K due to the onset of weak ferromagnetism. The data are normalized to the value at a temperature slightly above 30 K (in the cases where an anomaly occurs). The anomaly disappears at an applied gate voltage of +1.3 V, but reappears after the gated surfaces are removed by mechanical polishing. Inset: schematic of the gating setup.
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Figure 2
(a) EDX analysis of a ${NiS}_{2}$ single crystal before and after gating (to +4.0 V). The spectra were obtained at an incident electron energy of 5 keV. The Ni and S peaks are marked by pairs of orange lines; common contaminant peaks are marked in red. The intensity is normalized to that of the most intense Ni peak. (b) S:Ni ratio obtained from EDX analysis of ungated and gated samples (+4.0 V) with different incident electron energies. The inset shows the depth below which 90% of the characteristic x rays are emitted from the sample for S and Ni, obtained from casino simulations (as discussed in the main text). The dashed lines in (b) are guides to the eye.
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Figure 3
XPS survey scans taken on the crystal surface (a) before gating (no IL applied) and (b) after gating (+4.0 V), but before argon-ion sputtering. The XPS peaks due to specific elements are marked in (a); in (b), only the peaks due to IL contamination are additionally marked. High-resolution XPS scans of the S $2 p$ and Ni $2 p_{3 / 2}$ peaks are shown in (c), (d), respectively. The bottom spectra in dark green and blue correspond to panels (a), (b), respectively. The rest of the spectra were obtained with argon-ion sputtering. The approximate thickness of material sputtered before the measurement of each spectrum is indicated. These sputter thicknesses are based on sputter rates that were calibrated on a $Si / Si O_{x}$ substrate and are thus approximate. Note that the spot at which the spectra were measured postsputtering (gray curve and all other curves above it) is different from the spot at which the spectrum was measured after gating, but before any sputtering (blue curve).
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Figure 4
Contact-mode AFM height images (15 $μ m \times 15 μ m$ ) of (a) ungated and (b) +2.0 V gated ${NiS}_{2}$ single-crystal surfaces. The common height scale for the images is displayed on the left; the lower limit for the height scale was set at 300 nm, for better contrast. (c) 1D power spectral density functions for gated (+2.0 V) and ungated samples, as obtained from the AFM height images in (a), (b), using Eqs. (1) and (2). The solid black lines are fits to the $k$ -correlation model (see Supplemental Material, Sec. E [57]) used to extract the correlation length. The latter is 4.8(3.6) and 0.45(0.04) $μ m$ from the ungated and gated fits shown. The inverse correlation lengths obtained for the gated and ungated samples are depicted by dashed lines with the respective colors.
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Electrochemical mechanism of ionic-liquid gating in antiferromagnetic Mott-insulating NiS2 single crystals

Sajna Hameed, Bryan Voigt, John Dewey, William Moore, Damjan Pelc, Bhaskar Das, Sami El-Khatib, Javier Garcia-Barriocanal, Bing Luo, Nick Seaton, Guichuan Yu, Chris Leighton, and Martin Greven

Phys. Rev. Materials 6, 064601 – Published 7 June 2022

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Electrochemical mechanism of ionic-liquid gating in antiferromagnetic Mott-insulating ${NiS}_{2}$ single crystals