Ionic-Liquid Gating of MoS2

Konstantin D. Schneider


University of Regensburg
Institute for Experimental and Applied Physics

March 19, 2024

Motivation

MoS2 excellent 2D semiconductor

  • Strong spin-orbit coupling
  • Spin split bands
  • Broken inversion symmetry

\(\Rightarrow\) Quantum Dots (Schock et al. 2023)

  • Intrinsic superconductivity (SC) in flakes

  • Also: Thickness dependend SC in WS2 nanotubes (NT) (Qin et al. 2018)

  • Objective:

    1. Find good fabrication and measurement methods using MoS2 flakes.
    2. Apply findings to MoS2 nanotubes.
    3. Achieve SC in MoS2 nanotubes.

MoS2 Nanotubes

  • Grown via chemical transport reaction by group of Prof. Dr. Maja Remškar (Ljubliana)
  • Nanotubes and -ribbons
  • Thickness from several hundred nanometers down to 20 nm
  • Length up to mm scale

Liquid-Ion Gating

(from Costanzo et al. 2016)

Liquid-Ion Gating

(from Costanzo et al. 2016)
  • ALiquid-Gate > ADevice
  • Apply voltage to liquid-gate

\(\rightarrow\) Ions deposit on device and create large capacitance

\(\rightarrow\) Increase of surface carrier density (n ~ 1014cm-2 (Zhang et al. 2012))

\(\Rightarrow\) Metallic transport in the MoS2 channel

\(\Rightarrow\) Transition to superconductivity for 1.5 K < T < 10 K

Device Preparation

  1. Transfer MoS2 via Blue Nitto Tape onto Si/SiO2
  2. Fabricate Bi/Au contacts with standard lithographic methods
  1. Apply protective resist layer with windows over MoS2 sample
  2. Add drop of DEME-TFSI

Design

Design

Finished Device

Interlude: Why Bismuth?

Liquid-Gate Sweep

Two Main Problems Arise

Device “Cracks” during Cooldown

Device Degradation

Two Main Problems Arise

Device “Cracks” during Cooldown

  • DEME freeze out leads to shear forces
  • Bi/Au contacts break

\(\rightarrow\) Cr/Au or Ti/Au contacts

Device Degradation

Two Main Problems Arise

Device “Cracks” during Cooldown

  • DEME freeze out leads to shear forces
  • Bi/Au contacts break

\(\rightarrow\) Cr/Au or Ti/Au contacts

Device Degradation

Two Main Problems Arise

Device “Cracks” during Cooldown

  • DEME freeze out leads to shear forces
  • Bi/Au contacts break

\(\rightarrow\) Cr/Au or Ti/Au contacts

Device Degradation

  • Leakage current indicates device degradation
  • Decreases with decreasing Temperature

\(\rightarrow\) Keep Ileak < 1nA, T \(\le\) 220K and VLG <= 3V

No more Bismuth!

  • Bismuth seems unsuited for liquid-ion gating
    • Cracks during cooldown
    • May react with DEME-TFSI and/or contaminate liquid
  • Alternatives:
    • Cr/Au or Ti/Au contacts for improved adhesion
    • Inert Au contacts to reduce device reactivity/interaction with DEME-TFSI

Suspend Nanotubes (1): Double Resist

  • MoS2 NT wedged between to layers of resist
  • Au contacts deposited around NT
  • Protective resist and a drop of DEME-TFSI

Suspend Nanotubes (2): Anthracene Transfer

  • Anthracene crystals are grown in our clean-room
  • MoS2 NT is picked up using an anthracene crystal stuck to PDMS and placed onto contacts
  • Anthracene is evaporated at 130 °C

\(\rightarrow\) Dry and very clean transfer

  • A drop of DEME-TFSI is applied

Summary & Outlook

  • Previous work on SC in MoS2 flakes and WS2 NT: SC in MoS2 NT expected
  • Unfortunately no functioning devices so far
  • Apparently, Bismuth is not suited for liquid-ion gating (especially at cryogenic temperatures)

Next Steps:

  • Improve sample cleanliness
    • Move away from optical resists and lithography.
    • Surface cleaning using AFM or HF/Ar annealing
  • Build suspended nanotube devices.

Acknowledgments

  • PD. Dr. Andreas K. Hüttel
  • Robin T. K. Schock
  • Prof. Dr. Maja Remškar

  • Prof. Alberto Morpurgo for his advice



Thank you very much for your attention!

References

Costanzo, Davide, Sanghyun Jo, Helmuth Berger, and Alberto F. Morpurgo. 2016. “Gate-Induced Superconductivity in Atomically Thin MoS2 Crystals.” Nature Nanotechnology 11 (4): 339–44. https://doi.org/10.1038/nnano.2015.314.
Qin, Feng, Toshiya Ideue, Wu Shi, Xiao-Xiao Zhang, Masaro Yoshida, Alla Zak, Reshef Tenne, et al. 2018. “Diameter-Dependent Superconductivity in Individual WS2 Nanotubes.” Nano Letters 18 (11): 6789–94. https://doi.org/10.1021/acs.nanolett.8b02647.
Schock, Robin T. K., Jonathan Neuwald, Wolfgang Möckel, Matthias Kronseder, Luka Pirker, Maja Remškar, and Andreas K. Hüttel. 2023. “Non‐destructive Low‐temperature Contacts to MoS2 Nanoribbon and Nanotube Quantum Dots.” Advanced Materials 35 (13). https://doi.org/10.1002/adma.202209333.
Shen, Pin-Chun, Cong Su, Yuxuan Lin, Ang-Sheng Chou, Chao-Ching Cheng, Ji-Hoon Park, Ming-Hui Chiu, et al. 2021. “Ultralow Contact Resistance Between Semimetal and Monolayer Semiconductors.” Nature 593 (7858): 211–17. https://doi.org/10.1038/s41586-021-03472-9.
Taniguchi, Kouji, Akiyo Matsumoto, Hidekazu Shimotani, and Hidenori Takagi. 2012. “Electric-Field-Induced Superconductivity at 9.4 k in a Layered Transition Metal Disulphide MoS2.” Applied Physics Letters 101 (4): 042603. https://doi.org/10.1063/1.4740268.
Zhang, Yijin, Jianting Ye, Yusuke Matsuhashi, and Yoshihiro Iwasa. 2012. “Ambipolar MoS2 Thin Flake Transistors.” Nano Letters 12 (3): 1136–40. https://doi.org/10.1021/nl2021575.

“DEME-TFSI is short for?”

Diethylmethyl(2-methoxyethyl)ammonium-bis(trifluormethylsulfonyl)imid

Konstantin D. Schneider | University of Regensburg