Twisted Material Reveals Controllable Electrical Switching at the Nanoscale

Researchers are increasingly focused on harnessing the unique properties of twisted transition metal dichalcogenides, and a new study details the behaviour of ferroelectric domains within these materials. Wei Ren, Shiyu Guo, and Daochen Long, all from the School of Physics and Astronomy at the University of Minnesota, led the investigation, working with colleagues including Kenji Watanabe and Takashi Taniguchi from the National Institute for Materials Science, and Ke Wang. This collaborative effort has resulted in the fabrication and study of a twisted molybdenum disulfide point contact, allowing for the local probing of individual ferroelectric moiré domains. The team’s findings demonstrate an unusually long conductance plateau with significant electrical hysteresis, revealing the emergence of antiferroelectricity from alternating polarization and offering unprecedented insight into the microscopic dynamics of these domains, potentially enabling the development of advanced, tunable ferroelectric devices.

Scientists have achieved an advance in understanding and controlling ferroelectricity at the atomic scale within twisted two-dimensional materials, specifically twisted transition metal dichalcogenides (tTMDs). These materials exhibit alternating regions of ferroelectric polarization, termed moiré domains, created by atomic reconstruction. Researchers developed a method to locally probe the behaviour of a single moiré domain using a gate-defined quantum point contact (QPC) in twisted molybdenum disulfide (tMoS2). This innovative approach isolates a single domain’s properties, shielding them from broader material imperfections and variations in twist angle. Measurements reveal an unusually long conductance plateau extending over 150 nanometers, accompanied by substantial electrical hysteresis of approximately 120V between forward and backward scans of the gate voltage, indicating robust nanoscale ferroelectric behaviour. By comparing local and global measurements, the study confirms the emergence of antiferroelectricity arising from the alternating polarization of individual domains. The fabrication process involves creating a van der Waals heterostructure consisting of hexagonal boron-nitride (hBN), pre-doped few-layer graphite contacts, twisted MoS2 layers, and additional hBN, transferred onto atomically clean local bottom gates deposited on a silicon substrate, followed by etching to define the mesoscopic transport channel. Applying equal gate voltages electrostatically confines electrons, creating a one-dimensional conducting channel, the QPC, comparable in size to the moiré lattice constant, allowing for characterisation of local properties within a single or a few moiré domains. Measurements reveal unusually long conductance plateaus accompanied by substantial electrical hysteresis, a key signature of ferroelectric behaviour originating from a single moiré domain. The observed hysteresis loops exhibit device-to-device variation, attributed to spatial variations in moiré domain sizes resulting from twist-angle inhomogeneity. At low carrier density, each domain contains a single electron dipole, with equal and opposite polarizations between domains, mirroring conventional antiferroelectricity. Conversely, at higher carrier density, variations in domain size lead to differing charge carrier numbers and dipole moments, resulting in ferroelectric hysteresis loops characterised by distinct plateaus. Analysis of global conductance demonstrates the absence of single dipole switching behaviour, confirming that all moiré domains collectively contribute to the global transport signature and that the observed sharp slope originates from the anti-ferroelectric hysteresis loop. The research characterizes the time scale of ferroelectric domain evolution and single-dipole switching events, providing insight into the microscopic behaviour of a single moiré unit cell inaccessible through conventional Hall bar devices. A gate-defined twisted molybdenum disulfide (tMoS2) point contact (QPC) serves as the central tool for probing local ferroelectric behaviour within moiré domains. This mesoscopic device is fabricated by stacking hexagonal boron-nitride (hBN) layers, few-layer graphite contacts (FLGs), and two pieces of tMoS2 with a twist angle less than 1 degree, all transferred onto an atomically clean silicon dioxide/silicon substrate. Subsequent dry etching defines a mesoscopic conducting channel, ensuring that transport occurs solely via the gate-defined QPC. Electrical transport measurements are performed at 4 K, employing a configuration where equal gate voltages (VQPC) are applied to a pair of local bottom gates, electrostatically confining charge carriers into a one-dimensional channel, the QPC, between the gate separations. The QPC width is tuned from approximately 100nm down to pinch-off using VQPC, making it comparable to the moiré lattice constant. A silicon back gate voltage (Vg) modulates both the out-of-plane electric field (E0) influencing polarization and the carrier density, allowing for comprehensive control over the system. Four-probe resistance measurements, conducted as a function of Vg at a fixed VQPC of -7V, reveal large electrical hysteresis and multiple conductance plateaus, though not fully quantized due to limitations in carrier mobility within the tMoS2 material. For years, manipulating materials with atomic precision has promised revolutionary devices, but reliably creating and probing isolated pockets of controllable ferroelectricity has remained a substantial hurdle. This work bypasses the averaging effects of bulk measurements by focusing on individual ‘moiré’ domains, tiny regions within the twisted material exhibiting distinct electrical properties. The ability to not only observe but also dynamically influence these domains with an applied electric field is noteworthy. Previous studies demonstrated ferroelectric effects across an ensemble of domains, but this research reveals the signature of single atomic dipole flips, a level of control previously unseen, opening possibilities for designing devices where the local electrical state can be tuned with unprecedented accuracy. However, the technique relies on fabricating precise nanoscale point contacts, a process that may not scale easily to large-scale production, and the long-term stability of these isolated domains requires further investigation. Future work will likely focus on integrating these findings with other 2D materials to create more complex heterostructures and exploring the potential for room-temperature operation.