Electron-beam-induced Contactless Manipulation of Interlayer Twist in van der Waals Heterostructures

The ability to dynamically control the relative orientation of layers in two dimensional (2D) van der Waals (vdW) heterostructures represents a critical step toward the realization of reconfigurable nanoscale devices. Existing actuation methods often rely on mechanical contact, complex architectures, or extreme operating conditions, which limit their applicability and scalability. In this work, we present a proof-of-concept demonstration of contactless electrostatic actuation based on electron-beam-induced charge injection. By locally charging an insulating hexagonal boron nitride (hBN) flake on an electrically grounded graphene layer, we create an interfacial electric field that generates in-plane electrostatic torque and induces angular displacement. We validate the induced rotation through in-situ scanning electron microscopy (SEM) and twist-dependent Raman spectroscopy.

💡 Research Summary

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The paper introduces a novel, contact‑free method for dynamically controlling the interlayer twist angle in van der Waals (vdW) heterostructures composed of graphene and hexagonal boron nitride (hBN). Traditional approaches to twist‑angle engineering rely on mechanical contact, complex micro‑electromechanical systems (MEMS), or extreme conditions such as high temperature or high voltage, which limit scalability and device integration. Here, the authors exploit electron‑beam‑induced charge injection to generate an electrostatic torque that rotates the hBN layer relative to a grounded graphene “stator”.

In the experimental configuration, a monolayer graphene sheet is transferred onto a Si/SiO₂ substrate and patterned into square pads that are electrically grounded. An hBN flake (~30 nm thick) is placed on top of the graphene, acting as a mechanically decoupled “rotor”. Gold electrodes are fabricated on the hBN to provide a uniform charge‑collection surface. During scanning electron microscopy (SEM) exposure, a focused 5 keV electron beam (≤ 100 pA, 20 µm aperture, 1 s per frame) scans a 50 µm × 50 µm area, delivering a dose of 2–4 µC cm⁻². This dose is low enough to avoid lattice damage, as confirmed by Raman spectroscopy before and after exposure.

Electron injection builds up a negative surface charge on the insulating hBN, creating a potential difference with the grounded graphene. The resulting electric field has both a vertical component (pulling the layers together) and a lateral component that exerts an in‑plane torque on the hBN rotor. Because the interlayer shear strength is extremely low for incommensurate graphene/hBN stacks, the torque can overcome static friction and rotate the hBN until electrostatic and mechanical torques balance.

The authors monitor rotation in real time with SEM imaging and independently verify twist‑angle changes via twist‑dependent Raman signatures (shifts and splittings of the graphene 2D band and moiré‑induced features). Image registration based on a rigid‑rotation minimization of mean‑squared error yields quantitative rotation angles; for example, sample S1 shows a counter‑clockwise rotation of ≈ 3.2°, while sample S2 exhibits a clockwise rotation of ≈ 2.5°. Raman maps of the graphene 2D‑band full width at half maximum (FWHM) before and after actuation corroborate the SEM‑derived angles, confirming that the observed changes stem from genuine interlayer twist modification rather than artefacts.

Key advantages of this technique include: (1) complete avoidance of physical contact, eliminating strain, contamination, or damage associated with tip‑based or MEMS actuation; (2) precise control of torque magnitude and direction through beam parameters (energy, current, scan pattern) and optional biasing of graphene pads; (3) simplicity of device architecture—only a grounded graphene layer and a charge‑collecting hBN flake are required. The method also leverages the large bandgap (~6 eV) and low out‑of‑plane conductivity of hBN, which promote localized charge accumulation and strong interfacial fields.

Limitations are acknowledged. The approach depends on access to an SEM or equivalent electron‑beam system, raising questions about scalability to wafer‑level production. Long‑term stability of the injected charge and repeatability of multiple actuation cycles were not fully explored. Moreover, a quantitative model linking charge distribution, electric field geometry, and the resulting torque to the observed angular displacement remains to be refined.

In conclusion, the study demonstrates that electron‑beam‑induced electrostatic torque can reliably and reversibly adjust the twist angle in graphene/hBN heterostructures. This contactless actuation opens a pathway toward reconfigurable 2D nano‑devices, twist‑engineered quantum materials, and dynamic control of moiré superlattices without the drawbacks of conventional mechanical or thermal methods. Future work will likely focus on optimizing charge‑injection strategies, integrating parallelized beam sources, and extending the concept to other vdW material combinations and device platforms.