Piezoelectricity in NbOl₂ for piezotronics and nanogenerators

Piezoelectric materials offer fast response, high speed, and precise control, enabling sensors, actuators, and MEMS/NEMS. As devices miniaturize, 2D materials with piezoelectric properties have become attractive. Since the 2014 discovery of 2D piezoelectricity in monolayer MoS2, a variety of 2D systems (TMDCs, group IV monochalcogenides, MXenes, black phosphorus) have been explored for piezoelectric nanogenerators (PENGs). However, many reported 2D PENGs show weak current outputs due to limited piezoelectric constants (e.g., MoS2 ~0.6 pm/V), limiting integration for energy conversion. NbOI2, a newly identified anisotropic 2D semiconductor, has been predicted to possess one of the largest piezoelectric stress constants among thousands of candidate monolayers, suggesting strong electromechanical coupling and potentially higher output in PENGs under strain. This work aims to experimentally demonstrate the anisotropic in-plane piezoelectricity and piezotronic effect in multilayer NbOI2, quantify their impact on carrier transport via Schottky barrier modulation under strain, and realize wearable energy harvesting and motion sensing.

Early demonstrations of 2D piezoelectricity in monolayer MoS2 opened the field for 2D PENGs, including TMDCs (MoS2, WSe2), group IV monochalcogenides (e.g., a-In2Se3), MXenes, and black phosphorus. These materials can be grown or exfoliated into ultrathin layers and incorporated into flexible devices, but several challenges persist: limited piezoelectric constants leading to low current output, odd–even layer effects that suppress piezoelectricity in even layers (e.g., MoS2), and environmental instability in materials such as black phosphorus and some MXenes. Data-driven screening identified NbOI2 as having exceptionally large piezoelectric stress constants among 2940 candidate monolayers, and prior studies have shown its highly anisotropic electrical/optical properties and strong nonlinear optical responses. These findings motivate experimental verification of its piezoelectric/piezotronic properties and evaluation in nanogenerators.

Material synthesis: High-quality large NbOI2 single crystals were grown by chemical vapor transport (CVT) using iodine as the transport agent. Nb powder (99.99%), Nb2O5 (99.99%), and iodine (99.99%) were mixed in Nb:O:I = 1:1:2 with total mass 0.5 g, sealed under vacuum (~10^3 Pa) in quartz tubes (7 mm ID, 9 mm OD, 220 mm length). The horizontal two-zone furnace was heated to 600 °C in 10 h, held for 120 h, then cooled to 310 °C/240 °C over 240 h at hot/cold ends, followed by furnace cooling. Rectangular single crystals (~4 × 8 × 2 mm^3) were obtained at the cold zone. Structural characterization: Composition was verified by EDX with Nb:I ≈ 1:2 (13.19%:25.17%). XRD indicated layered structure with (1,0,0) surface. HRTEM on the ac-plane provided lattice spacings a ≈ 0.276 nm and c ≈ 0.285 nm. Raman spectroscopy (514 nm laser, <1 mW) on bulk NbOI2 showed peaks at 105, 209, 275, 520, and 617 cm⁻1, corresponding to Nb–I and Nb–O vibrational modes. PFM measurements: To probe in-plane piezoelectricity, the probe was oriented perpendicular to the bc-plane, assessing response associated with the a-axis. Out-of-plane PFM amplitude spectra under varying excitation voltages exhibited a clear resonance near 360 kHz with voltage-dependent amplitude. PFM amplitude images were acquired at progressively increasing drive voltages over the same 5 × 5 µm² region. Effective piezoelectric constants were extracted from linear fits of piezoresponse versus drive voltage: d11 (a-axis) ≈ 68–69.4 ± 8.58 pm/V; similarly, d22 (b-axis) ≈ 1.43 pm/V and d33 (c-axis) ≈ 4.04 pm/V (from supplementary analysis). Device fabrication: Multilayer NbOI2 flakes were mechanically exfoliated onto PDMS, then transferred to PET by heating at 80 °C for 5 min. Thicknesses were measured by AFM (tapping mode), with representative device thickness ~40.2 nm. Cr/Au electrodes (5 nm/45 nm) were patterned by standard lithography and deposited by e-beam evaporation. Silver paste connected wires to electrodes. The metal/NbOI2 contacts form Schottky junctions (Cr work function ~3.9 eV; NbOI2 electron affinity ~2.24 eV). Electrical measurement and strain application: I–V curves were measured with a Keithley 4200. Piezoelectric current signals were recorded using an SR-570 low-noise current preamplifier and a TBS 2000 series oscilloscope. For piezotronics, strain was applied by bending the PET substrate on a displacement stage. For nanogenerator output, cyclic tensile/release strain was applied using a linear motor (loading speed 500 mm/s; strain hold and release times 0.5 s; cycle period ~1.1 s). Measurements were conducted inside a Faraday cage. Strain calculations and control details are provided in supplementary notes. Control experiments with bare PET (no NbOI2 flake) confirmed no spurious signals.

• Anisotropic piezoelectricity: PFM shows strong a-axis response with resonance near 360 kHz and voltage-dependent amplitude. Effective piezoelectric constants extracted: d11 (a-axis) ≈ 68–69.4 ± 8.58 pm/V; d22 (b-axis) ≈ 1.43 pm/V; d33 (c-axis) ≈ 4.04 pm/V, evidencing pronounced in-plane anisotropy.
• Piezotronic effect: Under tensile strain, asymmetric Schottky barriers at Cr/NbOI2 contacts are modulated by strain-induced piezocharges, altering I–V characteristics. The Schottky barrier change ΔΦSB scales with applied strain; along the Nb–O (a) axis the modulation is strong, whereas along the Nb–I (c) axis it is about five times weaker under identical strain.
• Nanogenerator output: Under periodic tensile/release strain, the device produced stable, periodic current outputs. At 0.21% strain, the output along the Nb–O axis was 8× larger than along the Nb–I axis. Along Nb–O, current increased with strain and saturated at ~140 pA at 0.51% strain; approaching stability by ~0.59% strain. Durability tests at constant 0.21% strain over 5 minutes showed excellent stability.
• Wearable demonstration: Flexible PENGs attached to the human finger and wrist generated current signals synchronized with bending motions. For example, a finger bending angle of ~25° yielded ~30 pA, and increasing angles led to larger outputs, demonstrating applicability for motion sensing and biomechanical energy harvesting.
• Controls: No electrical signal was detected from bending bare PET substrates without NbOI2, confirming intrinsic origin of the measured outputs.

The experimental observations confirm that multilayer NbOI2 exhibits strong, anisotropic in-plane piezoelectricity, with a large d11 along the Nb–O direction and much smaller responses along orthogonal axes. This intrinsic anisotropy directly translates into strain-tunable carrier transport via the piezotronic effect: piezoelectric polarization charges modulate Schottky barrier heights at metal/semiconductor interfaces, enabling control of I–V characteristics with mechanical strain. The comparatively large piezoelectric response leads to higher current outputs at relatively small strains versus many reported 2D PENGs, addressing the common limitation of low current density in ultrathin devices. The observed saturation of output at higher strain is consistent with equilibrium charge distribution under sustained deformation. Demonstrations on human joints highlight both the sensitivity and temporal fidelity of the devices, underscoring potential in wearable sensing and energy harvesting. Compared with other 2D materials (e.g., TMDCs, black phosphorus, MXenes), NbOI2 combines strong piezoelectric coupling with environmental robustness sufficient for ambient operation, though encapsulation is advisable for long-term stability. These results substantiate NbOI2 as a promising 2D platform for piezotronics and self-powered wearable systems.

High-quality NbOI2 crystals were synthesized and integrated into flexible piezoelectric nanogenerators that exhibit pronounced anisotropic piezoelectricity and a strong piezotronic effect. The devices deliver stable current outputs up to ~140 pA at 0.51% strain and demonstrate reliable operation under repeated cycling. Wearable tests on finger and wrist bending validate their utility for biomechanical energy harvesting and motion sensing. The combination of large in-plane piezoelectric response and strain-tunable Schottky barriers positions NbOI2 as a compelling material for high-performance 2D PENGs and piezotronic devices. Future work should focus on scalable synthesis and transfer, optimization of contact engineering to maximize piezotronic modulation, device encapsulation to mitigate moisture-induced degradation, and system-level integration for self-powered sensing networks.

Performance depends on crystalline orientation and flake thickness, contributing to variability across devices. NbOI2 is susceptible to degradation under wet conditions, necessitating effective encapsulation for long-term operation. While large piezoelectric responses are demonstrated, output saturation at higher strains indicates limits set by internal charge equilibration. The use of Schottky contacts introduces asymmetry and contact-related variability; detailed contact engineering may be required to standardize device behavior.