Generating intense electric fields in 2D materials by dual ionic gating

Article https://doi.org/10.1038/s41467-022-34158-z Generating intense electric fields in 2D materials by dual ionic gating Received: 10 February 2022 Accepted: 14 October 2022 1234567890():,; 1234567890():,; Check for updates Benjamin I. Weintrub 1, Yu-Ling Hsieh1,2, Sviatoslav Kovalchuk1, Jan N. Kirchhof 1, Kyrylo Greben1 & Kirill I. Bolotin 1 The application of an electric field through two-dimensional materials (2DMs) modifies their properties. For example, a bandgap opens in semimetallic bilayer graphene while the bandgap shrinks in few-layer 2D semiconductors. The maximum electric field strength achievable in conventional devices is limited to ≤0.3 V/nm by the dielectric breakdown of gate dielectrics. Here, we overcome this limit by suspending a 2DM between two volumes of ionic liquid (IL) with independently controlled potentials. The potential difference between the ILs falls across an ultrathin layer consisting of the 2DM and the electrical double layers above and below it, producing an intense electric field larger than 4 V/nm. This field is strong enough to close the bandgap of fewlayer WSe2, thereby driving a semiconductor-to-metal transition. The ability to apply fields an order of magnitude higher than what is possible in dielectricgated devices grants access to previously-inaccessible phenomena occurring in intense electric fields. Electric fields are widely used to control material properties and to explore diverse physical phenomena. The first group of phenomena appears due to changes of the carrier density induced in a material by the field at its surface, typically explored using field-effect transistors (FETs)1,2. Electric fields also cause a second, qualitatively different, group of effects when the field penetrates through the material’s bulk. In this case, the presence of a field inside the material breaks symmetries3–7, bends the band structure along the direction of the field3,5–7, and modifies the energetics of excitons with a dipole moment parallel to the field3–6,8,9. In conventional FETs, the induced carriers at the material’s surface almost completely screen the field in the bulk of the material. Therefore, a dual gate FET with a pair of gate electrodes above and below the material under study is used to explore the effects of an external electric field penetrating a material. In this configuration, the field strength is controlled by the potential difference between the bottom and top gates, while the Fermi level is determined by their sum5,7–9. However, an important limitation for studying large electric fields in conventional solid-state FETs is the breakdown of gate dielectrics happening at around 0.3 V/nm10–16 (somewhat larger dielectric strengths, ~1 V/nm, are measured using local probe techniques11,12). The limitation on the maximum achievable carrier density has been overcome during the last decade via ionic gating, which combines condensed matter physics with electrochemistry17,18. In that technique, an ionic compound such as ionic liquid (IL), a molten salt, is placed over a material under study17–20. A potential applied between the gate electrode inside the liquid and the sample falls predominantly over an atomically thick (≤1 nm) electric double layer (EDL) at the IL/sample interface, modeled as a capacitor with an exceptionally large geometric areal capacitance (~10 μF/cm2)17,19,21–27. The resulting electric field inside the EDL induces a carrier density inside the material27,28. Critically, the field generated here is not limited by the dielectric breakdown of gate dielectrics, which limit the performance of conventional solid-state FETs. Instead, the only significant limitation is electrochemical modification of the material or electrodes, which occurs when the potential drop across a particular interface is too large (outside the electrochemical window)27,29. Ionic gating enabled previously inaccessible carrier densities larger than 1014 cm−2 to be reached19,28,30,31. The interactions between electrons at these carrier densities result in structural phase transitions32 and electronic phases such as exotic superconductivity30,31 and gate-controlled ferromagnetism33,34. These effects are especially pronounced in 1 Department of Physics, Freie Universität Berlin, Berlin, Germany. 2Department of Mechanical Engineering, National Central University, Taoyuan City, Taiwan. e-mail: Nature Communications | (2022)13:6601 1 Article 2DMs such as graphene, transition metal dichalcogenides (TMDCs), or phosphorene, where the carriers are spatially confined to one or few atomic layers18. Despite the progress in using ionic gating to induce high carrier densities, dual ionic gating has not been used to generate intense external electric fields inside the bulk of materials. Although singlegated suspended 2DMs35,36 as well as hybrid dielectric/ion approaches to dual gating37–39 have been used, no ionic counterpart to dual gate FETs has been demonstrated. Because of that, a wide range of phenomena predicted to emerge at fields stronger than F⊥ ~ 1 V/nm remains inaccessible in solid-state devices. For example, for fields near F⊥ ~ 2–3 V/nm, the interlayer bandgap of bilayer (2L) TMDCs is expected to decrease to zero40,41. In this situation, interlayer excitons should start forming at zero energy costs and a transition into a different state of matter, an interlayer excitonic insulator42, may occur. Other predicted yet unobserved phenomena at intense fields include an insulator to topological insulator transition in phosphorene (F⊥ > 3 V/nm)43, topological insulator to semimetal to normal insulator transition in 1T’ TMDCs (F⊥ > 2 V/nm)44, structural change in chirality for monolayer Te (F⊥ > 7 V/nm)45, giant valley polarization ~65 meV in WSe2/CrSnSe3 heterostructures (F⊥ ~ 6 V/nm)46, field-dependent magnon dispersion in 1L and 2L Fe (F⊥ > 2 V/nm)47, and interlayer exciton condensates with high oscillator strength48. Here, we develop a double-sided ionic gating approach to generate ultrastrong electric fields. The approach can be viewed as a counterpart to conventional dual-gated FETs which is not limited by the breakdown of gate dielectrics. To generate the field inside a 2DM, we apply a potential difference between the two ILs above and below the 2DM, thereby generating a different type of EDL consisting of two ILs separated by an ultrathin membrane. We use a combination of electrochemical and electrical transport measurements to observe an electric field of more than 4 V/nm inside 2DMs, over 4 times larger than the biggest fields reported for conventional solid-state FET technologies and an order of magnitude larger than hBN-encapsulated devices. Results Device concept At the core of our approach to generate and measure large perpendicular electric fields is an electrically-contacted few-layer 2DM suspended between two volumes of IL (Fig. 1a). The potential difference between the top and bottom ILs, ΔVref, is controlled by separate top and bottom gate electrodes in (...truncated)