Layer-selective hydrogenation and proton transport in twisted bilayer graphene

Abstract: Recent work investigated graphene's hydrogenation with independent control of the electric field, E, and charge density, n, in the crystal and showed that the process is controlled by n. Here, we demonstrate layer-selective conductor-insulator transitions in twisted bilayer graphene, driven by hydrogenation at fixed n under strong E. This process is accompanied by proton transport through the bilayer, enabling several parallel and configurable logic gates in the devices. Selectivity arises because the large twist angle decouples the two layers' electronic systems, enabling independent control of their charge densities. Polarisation by the field then induces a charge imbalance at fixed total n, triggering hydrogenation when one of the layers' charge densities reaches the threshold for monolayer hydrogenation. Our results introduce a new type of electrode-electrolyte interface in which electrochemical processes are controlled with two decoupled 2D electron gases, opening new design opportunities for energy and information processing devices.

• The paper demonstrates that dual-gated tBLG enables reversible layer-selective hydrogenation via field-driven conductor-insulator transitions with ON/OFF ratios over 10^4.
• The paper reveals that high transverse electric fields activate out-of-plane proton transport in tBLG, yielding currents an order of magnitude lower than in monolayer graphene.
• The paper establishes a novel platform for configurable parallel logic operations by merging electronic and protonic information processing in a single nanoscale device.

Layer-Selective Hydrogenation and Proton Transport in Twisted Bilayer Graphene

Introduction

This work explores the electrochemical and transport phenomena in twisted bilayer graphene (tBLG) under dual electrostatic gating, focusing on the emergent layer-selective hydrogenation and out-of-plane proton transport mechanisms. By leveraging the twist-angle-induced electronic decoupling between graphene layers, the authors demonstrate unprecedented control over chemical and ionic processes, revealing new prospects for energy storage and ion-based information processing devices.

Experimental Framework and Electronic Structure

Twisted bilayer graphene devices were fabricated by mechanically stacking monolayer graphene sheets at large twist angles (predominantly $>10^\circ$) to induce electronic decoupling, separated and supported over micron-scale apertures. Double gating was realized via independent top and bottom PdHx electrodes with an ion gel electrolyte, allowing independent modulation of the transverse electric field ($E$) and net carrier density ($n$). The double-gated configuration introduces two decoupled two-dimensional electron gases (2DEGs), which can be polarized independently, in contrast to conventional bilayers or monolayers.

The electronic decoupling is evidenced by the splitting of Dirac neutrality lines into two distinct curves in $E$-$n$ maps, consistent with theoretical predictions for tBLG under strong perpendicular electric fields and substantial electrolyte capacitance. The observed X-shaped splitting directly reflects the ability to independently control charge in each graphene layer, in marked contrast with AB-stacked bilayers where only a single neutrality line is observed.

Proton Transport and Layer-Selective Hydrogenation

Proton transport across tBLG was activated under high transverse electric fields, with measured currents an order of magnitude smaller than monolayer graphene, attributed both to the limited areal fraction of AA stacking ($\approx 30\%$) and higher permeation barriers ($\sim 1.4-1.7$ eV, DFT-calculated). Notably, AB-stacked bilayers show complete suppression of proton permeation, confirming the necessity of twist-induced decoupling for ionic transparency.

At fixed sub-threshold carrier density ($n < 10^{14}$ cm$^{-2}$), sufficiently high $E$ induced conductor-insulator transitions exclusively in the layer facing the incoming proton flux. Raman spectroscopy (D band enhancement) and electrical transport (conductance drop $E$0) confirmed selective and reversible hydrogenation of single layers. This layer-selective hydrogenation was repeatedly cycled over 1000 times with minimal degradation (cycle-to-cycle variation $E$1). The hydrogenation polarity reliably follows the direction of $E$2, demonstrating control over which graphene layer undergoes the phase transition.

Hydrogenation thresholds mapped across the $E$3-$E$4 parameter space further revealed that the minimum $E$5 required to induce hydrogenation increases as $E$6 decreases. For $E$7 cm$E$8, hydrogenation occurs even at $E$9. The boundaries of the hydrogenated regions track the neutrality curves, indicating that hydrogenation is triggered when the local $n$0 in one layer exceeds the monolayer threshold ($n$1 cm$n$2) due to strong polarization, even if the total $n$3 is below this value.

Theoretical Analysis

The analytical electrostatic model, corroborated by experiment, describes each graphene layer as a coupled plate capacitor system, subject to differential gating and quantum capacitance effects. This model predicts the observed neutrality line splitting and explains threshold conditions for hydrogenation in terms of the charge accumulation in individual layers. DFT calculations identify the energetic barriers for proton permeation and hydrogenation, showing that the stacking configuration critically impacts both processes: AB stacking favors lower hydrogenation barriers but remains impermeable to protons, while AA regions have higher barriers similar to monolayer graphene.

Configurable Parallel Logic Gates

Exploiting the selective hydrogenation and proton conduction, the device architecture enables parallel logic gate operation:

• In-plane electronic transport from individual layers implements NOT operations, each with robust ON/OFF ratios ($n$4).
• Interlayer tunneling, diminished upon hydrogenation of either layer, realizes NOR logic, and total in-plane current switching by dual-layer hydrogenation implements a NAND gate.
• Out-of-plane proton current concurrently enables XOR operations, with ON/OFF ratios $n$5.

All logic functions are configurable via input waveform modulations of $n$6 and $n$7, allowing for multiple parallel logic gates in a single tBLG platform. Remarkably, this dual-mode electroionic operation merges charge-based and ionic (protonic) information processing, with potential for non-volatile memory states derived from metastable hydrogenated phases.

Implications and Future Directions

The demonstration of field-driven, layer-selective chemical modifications in tBLG establishes a new class of electrode-electrolyte interfaces where surface electrochemistry is governed by independently tunable 2DEGs. This lays foundational groundwork for:

• New energy technologies based on electrostatically actuated ionic gating (hydrogen evolution, ion storage, and redox catalysis).
• Multi-modal, multi-bit logic and memory architectures exploiting independent, reversible insulator switching and ionic transport in a single nanoscale device.
• Broader application to other van der Waals heterostructures and 2D systems, where twist angle and gating offer new degrees of freedom for controlling electronic, ionic, and chemical processes.

Theoretical investigations could further elucidate the interplay between stacking, polarization, and electronic correlation effects in reactive processes. Experimentally, integrating additional 2D phases (e.g., TMDs, ferroelectric or magnetic layers) could provide expanded control and device functionalities.

Conclusion

This study presents a robust strategy to achieve layer-selective hydrogenation and controlled proton transport in twisted bilayer graphene by exploiting twist-induced electronic decoupling and independent dual gating. The resultant interface allows for reconfigurable electronic and ionic logic operations, setting the stage for electrochemically reconfigurable devices and hybrid charge-protonic information systems, and highlighting the broader promise of engineered 2D interfaces for functional nanoelectronics and energy systems (2603.29342).