Chiral Ion Gating in Materials & Devices

• Chiral ion gating is a form of ionic gating where chiral ionic liquids impart molecular handedness to interfaces, modulating both magnetic and ionic phenomena.
• Experiments using electric-double-layer transistors reveal that chiral ionic liquids can bias magnetic domains and alter key observables like coercivity and Hall conductivity.
• This approach paves the way for innovative spintronic and ion-transport devices by harnessing interfacial symmetry breaking for controlled material responses.

Searching arXiv for the cited papers on chiral ion gating and closely related chirality-controlled transport. arXiv search query: "(Matsuoka et al., 28 Jul 2025) chiral ionic gating FeSi(111)" Chiral ion gating denotes gating phenomena in which the ionic medium that establishes electrostatic or electrochemical control is itself chiral, so the gated interface acquires molecular handedness in addition to charge accumulation. In current arXiv usage, the clearest realization is electric-double-layer transistor control of FeSi(111) surface ferromagnetism by chiral ionic liquids, where handedness-dependent symmetry breaking biases magnetic domains without an external magnetic field (Matsuoka et al., 28 Jul 2025). A broader, materials-driven usage appears in chiral spin-crossover polymers whose electrical response is dominated by ionic migration and interfacial polarization; in that setting, the relevant functionality is not straightforward electron-dominated spin transport, but switchable chiral ionic transport or ion-gating behavior (Zaz et al., 11 Mar 2026).

1. Definition and scope

In the strict interfacial sense, chiral ion gating is a form of ionic-liquid gating in which the ions in the electric double layer are not merely electrostatically active but also chiral. The essential distinction is therefore not the existence of an electric-double-layer transistor as such, but the symmetry of the adsorbed ionic layer: an achiral ionic liquid can tune interfacial carrier density and electric field, whereas a chiral ionic liquid can additionally break interfacial symmetry in a handedness-dependent way (Matsuoka et al., 28 Jul 2025).

A broader use of the term emerges when a chiral solid exhibits transport dominated by mobile ions and interfacial charge redistribution. In that case, the material can be viewed as a candidate platform for chiral ionic transport or chiral ion gating because chirality, dielectric response, and ionic motion are coupled, even if the measured conduction is not dominated by mobile electrons or holes (Zaz et al., 11 Mar 2026). This suggests that the term spans both a narrow EDLT meaning and a wider class of chirality-conditioned ionic transport phenomena.

System	Chiral element	Principal gating or transport outcome
FeSi(111) EDLT	Chiral ionic-liquid cation $[(S)\text{- or }(R)\text{-MBMIm}]^+$ with TFSI$^{-}$	Handedness-dependent zero-field magnetic-domain bias plus reversible modulation of $H_c$ and anomalous Hall conductivity
$[Fe(NH_2trz)_3](X\text{-CSA})$	Chiral camphorsulphonate counterion $X=L$ or $D$	Thermally switchable chirality and ionic-migration-dominated electrical response

2. Interfacial chiral ionic gating in FeSi(111)

The most direct demonstration of chiral ion gating is the electric-field control of two-dimensional ferromagnetism in epitaxial FeSi(111) thin films patterned into EDLT devices (Matsuoka et al., 28 Jul 2025). FeSi is described there as a narrow-gap, nonmagnetic insulator in the bulk whose surface hosts a metallic, spin-orbit-coupled ferromagnetic state derived from a nontrivial Zak phase. Because the ferromagnetism is surface-confined and chemically stable while the bulk carries no magnetic moment, the interfacial magnetic state is unusually sensitive to adsorption and hybridization at the liquid-solid boundary.

The device architecture uses an ionic liquid in place of a solid gate dielectric. Three gating modes are explicitly compared. Electrochemical doping is performed at elevated temperature, where redox reactions are active. Achiral EDLT uses a conventional ionic liquid such as $[\mathrm{DEME}][\mathrm{TFSI}]$, where the action is electrostatic. Chiral EDLT uses a liquid containing a chiral cation, specifically $[(S)\text{- or }(R)\text{-MBMIm}]^+$ paired with TFSI$^{-}$. In both EDLT cases, positive gate voltage accumulates charge and modifies the surface electric field; only in the chiral case does the interfacial layer itself have handedness (Matsuoka et al., 28 Jul 2025).

This distinction is central because the ordinary gate response and the chirality-specific response separate experimentally. Both achiral and chiral EDLTs modify coercivity and anomalous Hall conductivity, but only chiral ionic gating biases the relative population of up- and down-magnetized domains in a handedness-dependent manner. The paper therefore treats chiral ion gating not as a synonym for ionic gating with chiral ions, but as a symmetry-selective interfacial control mode (Matsuoka et al., 28 Jul 2025).

3. Experimental signatures and quantitative observables

The FeSi study quantifies chiral ion gating through sheet resistance, sheet anomalous Hall conductivity $\sigma_{xy}^{\mathrm{sheet}}$, coercive field $^{-}$0, and a zero-field domain polarization ratio expressed as a normalized quantity such as $^{-}$1, or equivalently the zero-field anomalous Hall signal divided by its saturated value (Matsuoka et al., 28 Jul 2025). These observables distinguish electrochemical, achiral-electrostatic, and chirality-specific effects.

Under electrochemical gating, the sheet resistance decreases strongly, the anomalous Hall conductivity increases by about $^{-}$2 times, and the coercive field decreases. The paper attributes this mainly to redox-driven changes including oxygen removal from the native surface oxide and proton intercalation into the bulk. Under achiral EDLT, the sheet resistance barely changes, but the coercive field decreases by about $^{-}$3 and the anomalous Hall conductivity drops by about $^{-}$4 at $^{-}$5 V. Chiral EDLT produces a similar reversible electrostatic response, with the coercive field reduced by about $^{-}$6 and the Hall conductivity by about $^{-}$7 (Matsuoka et al., 28 Jul 2025).

The chiral-specific observable is the zero-field magnetic-domain bias. After zero-field cooling, the initial anomalous Hall signal is normally near zero in the as-grown device and in achiral or racemic liquid cases, consistent with nearly equal populations of up- and down-magnetized domains. With a chiral ionic liquid on the surface, a clear offset appears even at zero gate voltage. The handedness reverses the preferred domain orientation: the $^{-}$8-enantiomer favors down-magnetized domains, while the $^{-}$9-enantiomer favors up-magnetized domains, and a racemic mixture suppresses the effect. The domain polarization ratio can reach as high as about $H_c$0 in the best cycles for the chiral device, whereas achiral and racemic cases remain near zero (Matsuoka et al., 28 Jul 2025).

A common misconception is that all observed magnetic changes in such devices are intrinsically chirality driven. The comparative data show otherwise: changes in $H_c$1 and $H_c$2 occur in both achiral and chiral EDLTs, so they are assigned primarily to ordinary interfacial electric-field and carrier-accumulation effects. The handedness-dependent domain selection is the distinctive signature of chiral ion gating (Matsuoka et al., 28 Jul 2025).

4. Mechanistic interpretation and unresolved questions

The mechanism is interpreted as chirality-induced symmetry breaking at the interface between the chiral ionic liquid and the FeSi surface ferromagnet (Matsuoka et al., 28 Jul 2025). From spatial symmetry alone, a chiral adsorbate on a surface lacking inversion symmetry can in principle distinguish up and down magnetization directions. The more difficult issue is how a stable domain bias persists without an applied magnetic field.

The paper argues that the effect is unlikely to come simply from the applied gate voltage, because the domain polarization persists even at $H_c$3 after cooling and does not depend strongly on gate bias. Since the ionic liquid is frozen below its glass transition and ionic motion is suppressed, the authors suggest that long-range excitations such as chiral phonons, or interfacial coupling between vibrational modes of the chiral molecules and the surface spins, may mediate the symmetry breaking (Matsuoka et al., 28 Jul 2025). This is an interpretation rather than a completed microscopic derivation.

Conceptually, the work places chiral ion gating adjacent to, but distinct from, the broader CISS literature. The FeSi paper explicitly connects the interfacial effect to the CISS framework, in which molecular chirality couples to electron spin and can produce spin-selective phenomena (Matsuoka et al., 28 Jul 2025). A related theoretical line argues that in screw-symmetric chiral conductors, propagating states carry pseudo-angular momentum and that at chiral-achiral interfaces wave-function matching converts pseudo-angular-momentum polarization into spin polarization (Wang et al., 2023). That interface-based picture is not presented as the direct microscopic mechanism in FeSi, but it clarifies why handed interfacial symmetry is treated as a genuine transport control variable rather than a chemically incidental detail.

5. Spin-crossover polymers and ionic-migration-dominated chiral gating

A second, broader realization of chiral ion gating arises in the chiral Fe(II) spin-crossover coordination polymer $H_c$4, with $H_c$5 or $H_c$6, where $H_c$7 is 4-amino-1,2,4-triazole and CSA is camphorsulphonate (Zaz et al., 11 Mar 2026). The two samples are enantiomeric because the counterion is chiral. The chiral camphorsulphonate counterion imposes a handed asymmetric environment on the polymeric Fe-triazole coordination framework, yielding enantiomers of comparable structure but opposite handedness.

The material is a classic Fe(II) triazole spin-crossover system, with Fe(II) centers that thermally switch between a diamagnetic low-spin state and a paramagnetic high-spin state. Its chiroptical response is strongly spin-state dependent. UV-vis CD spectra from $H_c$8–$H_c$9 nm show substantial, well-defined circular dichroism in the low-spin state and strongly quenched CD in the high-spin state for both enantiomers, summarized as $[Fe(NH_2trz)_3](X\text{-CSA})$0 (Zaz et al., 11 Mar 2026). Magnetometry shows a cooperative thermally driven spin crossover near room temperature, specifically in the $[Fe(NH_2trz)_3](X\text{-CSA})$1–$[Fe(NH_2trz)_3](X\text{-CSA})$2 K range, with clear thermal hysteresis; the $[Fe(NH_2trz)_3](X\text{-CSA})$3-enantiomer transitions at slightly lower temperature than the $[Fe(NH_2trz)_3](X\text{-CSA})$4-enantiomer (Zaz et al., 11 Mar 2026).

Fe $[Fe(NH_2trz)_3](X\text{-CSA})$5-edge X-ray absorption spectroscopy on $[Fe(NH_2trz)_3](X\text{-CSA})$6 provides a direct spectroscopic marker of the spin-state transition. Temperature-dependent line-shape changes across the $[Fe(NH_2trz)_3](X\text{-CSA})$7 and $[Fe(NH_2trz)_3](X\text{-CSA})$8 edges are consistent with changes in 3d occupancy, ligand-field strength, and multiplet structure, and are interpreted as evidence for Fe(II) electronic reconfiguration across spin crossover. The paper explicitly links the quenching of chirality in the high-spin state to this Fe-centered electronic reorganization and altered hybridization with ligand states (Zaz et al., 11 Mar 2026).

The electrical measurements, however, shift the interpretation away from straightforward electron transport. Thin films on Au interdigitated electrodes show pronounced $[Fe(NH_2trz)_3](X\text{-CSA})$9 hysteresis in both low-spin and high-spin states, and small-signal $X=L$0 measurements at $X=L$1 kHz show cycling-dependent capacitance. These behaviors are interpreted as characteristic of field-driven redistribution of mobile ionic species, interfacial polarization, slow rearrangement of internal charge distributions, slow dipolar polarization, and formation and relaxation of interfacial charge layers. The explicit conclusion is that transport is dominated by ionic migration and interfacial polarization rather than by purely electronic carriers (Zaz et al., 11 Mar 2026).

The important corrective point is therefore that a chiral, switchable material need not be a robust CISS-based electronic conductor. The paper argues that this system is not suitable for a robust electronic spintronic device under the present conditions, but may instead be better understood as a chiral ionic-transport or ion-gating platform (Zaz et al., 11 Mar 2026). A plausible implication is that chiral ion gating can designate not only ionic-liquid interfaces controlling a separate conductor, but also intrinsically chiral solids whose dielectric and transport response is governed by mobile ions.

6. Conceptual boundaries and related gating usages

The term should be distinguished from several neighboring concepts. First, ion gating need not be chiral. In the bacterial flagellar motor literature, a contact-dependent gating mechanism is proposed in which the MotA-FliG interface modulates the ion-release rate of the proximal MotB channel, with stronger gating in CCW than in CW rotation; this explains directional asymmetry in the torque-speed relation, but the asymmetry is directional rather than a handed molecular gating medium of the type meant by chiral ion gating (Zhu et al., 1 Apr 2026).

Second, chirality-selective gating need not be ionic. In adjacent literatures, “gating” refers to symmetry-selective control by other means: pseudo-angular-momentum filtering at chiral-achiral interfaces (Wang et al., 2023), electrostatic control of chiral Majorana interference in magnetic topological insulators (Osca et al., 23 Aug 2025), holonomic discrimination of enantiomers in a trapped-ion qudit (Liu et al., 2022), digital gate-based chiral discrimination on IBM hardware (Akbar et al., 25 Aug 2025), and programmable trapped-ion simulation of spin-dependent transfer through a synthetic chiral lattice (Li et al., 11 Jun 2026). These studies are related through chirality-conditioned control of transport or state evolution, but they are not examples of chiral ion gating in the strict ionic-liquid or ionic-transport sense.

Taken together, the current literature supports two defensible usages. The narrow usage denotes handed ionic-layer control of an interfacial electronic or magnetic state, as in FeSi(111) EDLTs (Matsuoka et al., 28 Jul 2025). The broader usage denotes switchable, chirality-coupled ionic transport or dielectric response in intrinsically chiral materials, as in the chiral spin-crossover polymer $X=L$2 (Zaz et al., 11 Mar 2026). The boundary between them is not merely semantic: it determines whether the key observable is interfacial symmetry breaking in an electronically conducting channel, or ionic migration and interfacial polarization within a highly dielectric chiral medium.