🔬 mesoscale physics

Electrostatic control of Li+ density and transport rate in double-gated van der Waals devices

This study demonstrates that double-gated van der Waals devices enable independent two-dimensional control of Li+ ion density and transport rate, creating hybrid ionic-electronic transistors with memory and logic capabilities that overcome the limitations of traditional one-dimensional ion transport control.

Original authors: E. Hoenig, X. Zhang, C. Li, J. Tong, G. Chen, L. Chen, D. Domaretskiy, D. R. da Costa, F. M. Peeters, M. Lozada-Hidalgo

Published 2026-08-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: E. Hoenig, X. Zhang, C. Li, J. Tong, G. Chen, L. Chen, D. Domaretskiy, D. R. da Costa, F. M. Peeters, M. Lozada-Hidalgo

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of electronics, the transistor is the fundamental switch that powers our digital lives. It works by controlling the flow of electrons, tiny charged particles, through a material. By adjusting a voltage, engineers can decide exactly how many electrons are present and how fast they move, allowing for complex calculations and memory storage. Scientists have long wondered if ions—charged atoms rather than electrons—could be controlled with similar precision. Ions are the workhorses of batteries and biological systems, moving through materials to store energy or transmit signals. However, controlling ions has been difficult because the force that pushes them forward usually also determines how many of them are present. This creates a bottleneck: to get more ions to move, you often have to change the entire environment, making it hard to manage their density and speed independently. This limitation has held back the development of new types of computers and energy storage devices that rely on ion movement rather than electron flow.

A team of researchers at the University of Manchester has now found a way to break this bottleneck. They built a tiny device using layers of atom-thin materials, specifically stacking graphene or molybdenum disulfide on top of a sheet of hexagonal boron nitride. These layers are so thin that they are essentially two-dimensional, and they were placed over a microscopic hole in a silicon support. The researchers coated both sides of this stack with a liquid electrolyte containing lithium ions. Crucially, they placed two separate electrical gates, one on the top and one on the bottom, allowing them to apply two different voltages simultaneously. This setup, known as double gating, gave them two independent knobs to turn. One control adjusted the total number of lithium ions sitting at the interface between the layers, while the other control adjusted the electrical push that made those ions move along the channel.

When the researchers tested their device, they discovered that they could switch the flow of lithium ions on and off with remarkable stability. By adjusting the first voltage, they could trap the ions in specific, stable layers, creating distinct states where the number of ions remained constant. Once the ions were in place, the second voltage could be used to speed them up or slow them down without changing how many were there. This is a significant departure from previous methods, where changing the speed of the ions would inevitably change their density. The device behaved like a transistor for ions, capable of holding a state of high flow or low flow. The researchers observed that the ions moved in a hysteresis pattern, meaning the device remembered its previous state. Once the ions were loaded into the channel, they stayed there even after the initial loading voltage was removed, only leaving when a specific reverse voltage was applied.

The stability of this new system was impressive. While similar devices using a single voltage control often failed after just a few dozen cycles, these double-gated devices survived more than one thousand switching cycles without any signs of wear. The researchers also measured how the ions moved at different temperatures. They found that the movement required a specific amount of thermal energy to get started, confirming that the ions were hopping into the space between the layers rather than just leaking through cracks. By analyzing the electronic signals in the material alongside the ion flow, they confirmed that the number of ions in each stable state was perfectly balanced by the electrical charge of the material itself. This balance allowed them to calculate the density of the ions, finding that they could pack a significant number of them into the tiny interface.

Perhaps the most exciting aspect of this discovery is the ability to control the speed of the ions independently of their density. In traditional setups, making ions move faster often requires increasing the voltage so high that it damages the device or causes unwanted chemical reactions. In this new design, the researchers could keep the ion density fixed and simply increase the push along the channel to accelerate the flow. This means the device can operate faster without subjecting the materials to damaging high voltages. The researchers demonstrated this by using the device to perform basic logic operations. They could write information into the device by setting the ion density to a high or low state, and then read that information by measuring the current flow, which could be tuned continuously. The device retained this information even when power was removed, acting as a form of memory.

This work suggests a new way to think about controlling matter at the atomic scale. By decoupling the number of particles from the force that moves them, scientists can now explore operating regimes that were previously impossible. The ability to store and move ions with such precision opens the door to more efficient energy storage systems and new types of computing that mimic the way biological brains process information. The researchers showed that by using two independent controls, they could create a hybrid device that manages both electronic and ionic currents with high reliability. This breakthrough does not just improve existing technology; it establishes a new framework for designing devices where the flow of matter can be manipulated with the same finesse as the flow of electricity.

Technical Summary: Electrostatic Control of Li+ Density and Transport Rate in Double-Gated van der Waals Devices

Problem Statement
In conventional ion transport systems, particularly within crystalline hosts and intercalation materials, the applied potential serves a dual function: it simultaneously dictates the ionic distribution (density) and the transport rate (kinetics). This coupling restricts ion transport to a one-dimensional control space, where increasing the ion flux inherently requires increasing the electrochemical potential, which can lead to parasitic side reactions or device degradation. While nanofluidic devices have achieved some decoupling by exploiting surface charge dominance in narrow channels, intercalation systems in van der Waals materials remain constrained by this one-dimensional operating regime. The challenge is to achieve independent control over ionic occupancy and transport velocity in layered materials to enable advanced functionalities such as logic operations and memory retention.

Methodology
The authors developed double-gated van der Waals devices to decouple charge density and electric field control. The device architecture consists of mechanically exfoliated few-layer graphene (FLG) or MoS₂ stacked on a hexagonal boron nitride (hBN) flake containing a circular hole (3–5 µm diameter). This stack is supported on a silicon nitride substrate with a matching aperture. The entire assembly is coated with a non-aqueous lithium-conducting electrolyte (LiTFSI in PEG) and contacted by two independent gate electrodes (top and bottom).

The 2D crystals are patterned such that the van der Waals interface between the hBN and the 2D material forms a radially symmetric channel connecting the electrolyte reservoirs. Since the basal planes of pristine graphene and MoS₂ are impermeable to Li⁺, ionic transport is confined strictly to this interface. The system is operated in an inert argon atmosphere at 50 °C.

Control is achieved via two independent gate voltages, $V_t$ (top) and $V_b$ (bottom), which are transformed into two control variables:

$\Sigma V = V_b + V_t$ : Controls the total charge density in the device (ionic occupancy).
$\Delta V = V_b - V_t$ : Controls the electrochemical potential drop along the channel (driving force for transport).

The devices are monitored via ionic current measurements along the interface and simultaneous in-plane electronic conductivity measurements using a drain-source bias ( $V_{ds}$ ).

Key Results

Independent Control of Density and Rate: The study demonstrates that $\Sigma V$ switches the device between an "OFF" state (low ionic density) and an "ON" state (high ionic density) characterized by discrete plateaus. Within these plateaus, the ionic density ( $n_{Li}$ ) remains fixed and balanced by electronic charge. Conversely, $\Delta V$ independently modulates the ionic transport rate. The ionic current ( $i$ ) exhibits a linear dependence on $\Delta V$ only when the system is in the ON state ( $i \propto n_{Li} \Delta V$ ), confirming that the transport rate can be accelerated without altering the ionic density.
Hysteresis and Discrete States: The ionic current response displays pronounced hysteresis with well-defined plateaus during charge and discharge cycles. The intercalation (charging) process shows a sharp onset, while deintercalation (discharging) occurs in three distinct steps. These steps correspond to specific electronic charge-discharge events, indicating that the system occupies discrete ionic-density states balanced by electronic charge.
Thermally Activated Transport: Temperature dependence measurements reveal that the ionic current in the ON state increases exponentially with temperature, yielding activation energies of $E_a \approx 0.6 \pm 0.2$ eV for FLG and $0.7 \pm 0.1$ eV for MoS₂. This confirms that the transport is thermally activated and intrinsic to Li⁺ entry at the 2D crystal/hBN interface.
Enhanced Stability: Double-gated devices sustain over 1,000 ON-OFF switching cycles without degradation. In contrast, single-gated configurations of the same materials exhibit irreversible leakage and failure after approximately 10 cycles, highlighting the critical role of the double-gate geometry in stabilizing the interface.
Device Performance: The devices function as hybrid ionic-electronic transistors with ON/OFF ratios exceeding two orders of magnitude ( $\approx 10^2$ ). The effective mobility of Li⁺ is estimated at $\approx 1 \times 10^{-4}$ cm² V⁻¹ s⁻¹.

Significance and Claims
The paper claims to have expanded the control parameter space for ion transport in layered materials from one-dimensional to two-dimensional. By decoupling ionic density and transport rate, the authors demonstrate a new operating regime where transport can be accelerated without necessarily increasing the bias at the ion-supplying reservoir. This capability offers a pathway to mitigate high-potential parasitic reactions often associated with fast charging in battery interfaces.

Furthermore, the work establishes these devices as functional components for iontronic applications. Specifically, the authors demonstrate:

Logic Operations: The ability to use $\Sigma V$ as a digital "WRITE" signal (switching the state) and $\Delta V$ as an analogue "READ" signal (tuning the current magnitude).
Memory Retention: The hysteresis in the charge-discharge cycle enables non-volatile memory, where the device retains its ionic state (ON or OFF) after the removal of the write pulse, while simultaneously providing a volatile electronic response.

The authors conclude that this approach opens new possibilities for energy storage architectures and ion-based computing by providing a robust method to control interfacial transport kinetics independently of ionic concentration.

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