This work fits into the broader research on twistronics and layered materials, where neighboring studies explore twisted graphene and heterostructure interfaces, magnetic patterns, and charge flow switching. Unlike graphite, whose sixfold symmetry cycles through equivalent states, hexagonal boron nitride, with its threefold symmetry and chemically distinct boron and nitrogen atoms, offers several nonequivalent commensurate stackings separated by moiré states. This allows dynamic reconfiguration of the interface between states with different electrical polarization, which is important for controlling functionality within a single material.
Experiments were performed on hexagonal boron nitride flakes about 250 nm thick, cut into pillars with circular contacts ranging from 200 to 500 nm in radius. Rotation of the top pillar relative to the bottom was achieved using an atomic force microscope cantilever in a nitrogen atmosphere with water and oxygen content below 1 ppm. Force and angular displacement were recorded with an oscilloscope, and torque versus rotation angle curves were constructed. To elucidate microscopic mechanisms, quasistatic atomistic simulations were performed using the LAMMPS package with Tersoff and ILP potentials, where a rigid ring drove a flexible circular layer into rotation.
Experimental torque curves showed pronounced peaks at angles that are multiples of 60°, with the peak at 120° (AA′ state) systematically higher than the peak at 60° (AB state). The ratio of peak torques was approximately 0.7. Peak values increased linearly with contact area, contradicting the rigid-body model that predicts a dependence on area to the power of 1.5, and indicating the decisive role of edge elasticity. Simulations reproduced the asymmetry and the linear scaling, and analysis of the local registry index revealed two distinct yielding mechanisms: ductile for AA′, with gradual propagation of the metastable AB′ phase from the edge, and brittle for AB, with abrupt sliding.
This work differs from neighboring studies by demonstrating reversible mechanical switching between two nonequivalent commensurate states in a homogeneous hexagonal boron nitride contact, using torque asymmetry as a selective mechanism. It provides the first experimental evidence that different stackings can have different switching thresholds, opening the possibility to control polarization by simple rotation. Compared to graphite, where all commensurate states are equivalent, the threefold symmetry here yields two distinguishable states, expanding the twistronics toolkit.
The topic could evolve toward device miniaturization and integration with electrical contacts to read out the polarization state. The influence of rotation speed, temperature, and defects on switching dynamics could be studied. It is promising to extend the approach to other heteropolar layered materials, such as transition metal dichalcogenides, where stacking asymmetry is also expected.
The results will impact the development of reconfigurable nanoelectromechanical systems, information storage devices, and elements with tunable polarization. They are also important for fundamental understanding of friction and adhesion at atomically thin interfaces.
The next step could be to measure the electrical polarization directly during rotation to link the mechanical state to the electrical response. Long-term stability of switching over repeated cycles should also be investigated.
This work addresses the unresolved problem of controlled manipulation of interlayer stackings in layered materials, where no reliable method existed to reversibly switch between states with different polarization. It also touches on the role of edge effects in the mechanics of nanoscale contacts, which remains open for many two-dimensional systems.
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What the work does
Experimental demonstration of reversible switching between two stable stackings (AA′ and AB) in a homogeneous hexagonal boron nitride contact under an applied external torque. Combining atomic force microscopy and atomistic simulations revealed an asymmetry in the maximum torques (ratio ~0.7) and a linear increase of peak values with contact area, indicating the dominant role of edge elasticity.
Where its strength is
This work demonstrates controlled mechanical switching between nonequivalent commensurate states with different polarization, using torque asymmetry as a selective mechanism. It paves the way for reconfigurable nano- and microelectromechanical devices based on hexagonal boron nitride.
Where it could go next
Worth trying
Why this may matter
Such switching could underpin nanoscale memory elements and logic devices, where the state is defined by layer stacking and controlled mechanically, expanding the functionality of layered materials in twistronics.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.52, against a usual 0.64. It looks like few have drilled nearby.
The torque required to rotate a disk of 500 nm radius is only about 10⁻¹⁸ N·m, comparable to the moment of force exerted by the weight of a single bacterium on a lever arm of one micrometer.