Rapid fabrication of complex nanostructures using room-temperature ultrasonic nanoimprinting

Nanostructures underpin many applications in sensing, catalysis, and thermoelectrics. Complex nanostructures with designed features such as nanogaps or heterojunctions can dramatically enhance performance, e.g., boosting SERS sensitivity or catalytic activity. Existing scalable synthesis methods (solution-phase growth, electroplating, lithography) suffer from drawbacks such as poor uniformity, limited material choices, high cost, and energy-intensive processes. Conventional nanoimprinting is largely limited to polymers or to high-temperature techniques for metals, which risk alloying, oxidation, and recrystallization and are unsuitable for crystalline metals at room temperature. The research question is whether one can achieve rapid, room-temperature, scalable nanoimprinting to fabricate complex, multi-material nanostructures directly from bulk or multilayer films, including hard and brittle crystalline metals, without high-temperature processing. The study introduces an ultrasonic nanoimprinting method (nanojackhammer) that leverages high-frequency vibrations and nanoscale energy concentration to deform materials at room temperature, and investigates its mechanisms and capabilities, including a biosensing demonstration.

The paper reviews scalable nanostructure synthesis routes: solution-phase methods (chemical reduction, seed-mediated, microwave-assisted, microemulsion) yield heterogeneous products requiring purification and struggle with precise complex architectures. Electroplating in templates can produce nanowires and some heterojunctions but with limited metals. Advanced lithography with lift-off provides precision but at high cost and low throughput. Conventional nanoimprinting suits soft polymers; hard material imprinting typically needs high-energy lasers or elevated temperatures (e.g., bulk metallic glasses above Tg), which are energy-inefficient, costly, and limited to amorphous metals. Direct superplastic nanoimprinting at around half the melting temperature has been used for polycrystalline metals, but still requires heating and makes complex heterojunctions difficult due to alloying. Prior understanding in metal forming suggests difficulty in deforming crystalline metals into nanoscale features due to grain size constraints. Ultrasonic assistance and acoustoplasticity concepts exist in macro/micro forming, but not demonstrated for room-temperature fabrication of complex multi-material nanostructures at the nanoscale.

Ultrasonic nanoimprinting (nanojackhammer): An anodic aluminum oxide (AAO) template serves as the mold with nanoscale pore walls acting as energy directors. The AAO mold is placed on a substrate (e.g., 100-µm metal foil) and pressed by a 20 kHz ultrasonic horn (Branson 2000X). High-frequency vibration concentrates energy through sub-100-nm AAO walls, generating dislocations in the surface layer and driving material flow into nanopores, forming nanowires/nanorods. Process parameters (holding force 200–1600 N and horn vibration amplitude 10–30%) control imprint depth and nanowire aspect ratio. Demolding is achieved either mechanically (with solid lubricant such as buckminsterfullerene and mild sonication for short rods <5 µm) or by chemical etching of AAO (typically 1 M NaOH up to 2 h; for Al substrates, 1.5 wt% chromic acid + 6 wt% phosphoric acid at 70 °C up to 3 h). Mold selection determines diameter; customized molds can yield varied shapes. Multilayer thin films deposited on substrates (E-beam evaporation, ~1 Å s⁻¹ for metals; ALD Al2O3 at 250 °C, ~1 Å per cycle) enable multi-segment heterojunction nanorods and built-in nanogaps via sacrificial layers followed by selective etching. Materials and characterization: AAO molds (commercial), metal foils (Au, Ag, Cu, Sn, Al, Bi, Ni, 99.999%), silicon wafers, and chemicals were standard. SEM (JEOL 7600F), TEM/HRTEM/EDS (JEOL 2100F, 200 kV), FIB cross-sections (Zeiss Crossbeam 540) with protective Pt deposition; XRD for single-crystal Cu. Heterojunction and nanogap fabrication: Multilayer metal stacks (e.g., Au/Bi, Au/Sn) deposited on substrates and imprinted in AAO with ~100–130 nm pores. For plasmonic sensing substrates, Ag/Al2O3/Ag trilayers on Ag foil were imprinted; partial etching of alumina in NaOH formed ~13 nm mid-rod nanogaps in ~130 nm diameter, ~360 nm tall rods. Atomistic simulations: Large-scale MD (LAMMPS) modeled nanoimprinting on single-crystal Ag using EAM potentials for Ag; LJ interactions between mold and substrate. Samples: 10×10×10 nm³ (55,296 Ag atoms), equilibrated at 300 K (NPT, 100 ps). A rigid mold mimicking AAO pore edge was constructed using a graphene/CNT shape template; an alternative obtuse-corner mold (102°) tested geometry effects. A virtual planar indenter imposed displacement under loading patterns defined by a dimensionless parameter δ = (d_adv − d_ret)/(d_adv + d_ret), with δ=1 for direct loading and 0<δ<1 for cyclic loading. Imprinting speed: 0.01 nm per 1000 steps (10 m s⁻¹); total displacement 4 nm; cyclic frequency ~4.76×10⁹ s⁻¹ (period ~210 ps), scalar displacement per cycle 2.1 nm. Temperatures maintained at 300 K (NVT). Additional nanoindentation simulations at 300 K and 0.1 K validated generality. Au–Ag bilayer heterojunction imprinting simulated with appropriate EAM potentials. Electromagnetic simulations: COMSOL RF module simulated electric field enhancement for hexagonally packed nanorod arrays (130 nm diameter, 360 nm height) on Ag substrate with mid-rod alumina layer (13 nm) and a 20 nm notch representing etched gap; material permittivities: alumina 3.10, silver −29.96−0.38i. Periodic boundary conditions on unit cell; plane wave at 784.5 nm; solved with MUMPS. Biofilm growth and Raman detection: Pseudomonas aeruginosa PAO1 (10⁸ CFU in 20 µl LB) cultured on plasmonic substrates at 37 °C, humid, 18 h. Erythromycin (100 µg ml⁻¹) added to assess QS inhibition. Raman spectra acquired at 784.5 nm (120 mW, 10 s exposure).

• Room-temperature ultrasonic nanoimprinting (nanojackhammer) rapidly fabricates nanowires/nanorods from diverse materials (Au, Ag, Cu, Sn, Al, Bi, Ni) on various substrates with preserved crystallinity. Vertically aligned Au nanorods (~100 nm diameter, ~2 µm length) were imprinted within 1 min; Ag nanowires up to >10 µm length formed within minutes.
• Aspect ratio control: With constant 20% vibration amplitude, superplastic-like deformation dominated at room temperature even below ~700 N; aspect ratio increased sharply with force from ~700 to ~1200 N before saturating. At fixed forces (1200, 1500 N), aspect ratio scaled linearly with vibration amplitude in the 10–25% range.
• Diameter tunability via AAO pore sizes achieved Au nanowires of ~20–300 nm diameters.
• Multi-material heterojunctions fabricated with abrupt, flat interfaces at room temperature, including Bi–Au (brittle–malleable), Sn–Au (low–high melting points), Cu–Au (hard–soft), Au–Ag (inert–reactive), and Au–polycarbonate (metal–plastic), confirmed by STEM/EDS.
• Atomistic simulations showed cyclic ultrasonic loading (0<δ<1) significantly reduces imprinting load compared with direct loading (δ=1). The mechanism is alternating dislocation generation during loading and partial recovery during retreating half-cycles, localized near the nanowire root corner, leading to softening and more homogeneous deformation. Steady-state mean imprinting force increased monotonically with δ; δ=1 yielded the largest forces often exceeding machine limits in experiments.
• Plasmonic sensing platform: Ag–alumina–Ag nanorods (~360 nm tall, 130 nm diameter) with built-in ~13 nm mid-rod nanogaps exhibited calculated electric field enhancement up to ~1180 in the gap, versus ~15 between pure Ag nanorods without gaps. Experiments with 4-MPy confirmed nanogap-dominated SERS.
• Quorum sensing detection: P. aeruginosa biofilms formed on the gapped Ag nanorod arrays produced strong Raman signals of pyocyanin with <10% spatial variation across the substrate. Addition of 100 µg ml⁻¹ erythromycin suppressed the pyocyanin Raman signature despite similar biofilm morphology, confirming QS origin of the signal.
• Process advantages: Room-temperature, atmospheric-pressure operation reduces oxidation/alloying, is fast (seconds to minutes), scalable, energy-efficient, cost-effective, and compatible with thin-film microfabrication to realize complex designs with nanoscale precision.

The nanojackhammer approach addresses key barriers in nanoimprinting of crystalline metals and complex multi-material nanostructures at room temperature. Concentrating ultrasonic energy through nanoscale AAO walls enables plastic flow into nanopores without heating, thus avoiding alloying and oxidation that plague high-temperature processes and enabling abrupt heterointerfaces. Parameter control (force and vibration amplitude) tunes aspect ratios, while choice of mold sets diameter and geometry, providing design flexibility. MD simulations elucidate a cyclic-loading softening mechanism—alternating dislocation generation and partial recovery—explaining the markedly reduced imprinting loads and improved homogeneity under ultrasonic conditions. The trade-off between imprinting force and speed, encapsulated by the loading parameter δ, guides practical optimization for throughput versus mechanical limits. The plasmonic demonstration highlights how complex metal–oxide–metal nanorods with engineered nanogaps can be rapidly fabricated with wafer-scale uniformity, achieving strong and homogeneous SERS suitable for biosensing. Successful detection and QS inhibition validation showcase application potential in microbiology and drug screening. Overall, the findings indicate broad relevance for optoelectronics, sensing, and catalysis, where precise, multi-material nanostructures are required at scale.

The study introduces a room-temperature ultrasonic nanoimprinting technique (nanojackhammer) that concentrates ultrasonic energy via nanoscale mold features to shape diverse solid materials into nanostructures rapidly and cost-effectively. It enables controllable fabrication of single- and multi-component nanowires/nanorods, including abrupt metal–metal and metal–oxide heterojunctions and built-in nanogaps. Atomistic simulations reveal that cyclic ultrasonic loading promotes alternating dislocation generation and recovery, lowering imprinting forces and improving uniformity relative to direct loading. A plasmonic Ag–alumina–Ag nanorod array with engineered nanogaps demonstrates robust, uniform SERS for quorum sensing detection in bacterial biofilms and for inhibitor screening. Future work could explore broader mold geometries for complex 3D shapes, integration with roll-to-roll or large-area tools for industrial-scale manufacturing, extension to additional material systems (semiconductors, ceramics), optimization of ultrasonic waveforms for throughput and fidelity, and application-specific device integration in optoelectronics, catalysis, and biomedical diagnostics.

• MD simulations employ high strain rates relative to experiments and simplified rigid mold representations; while mechanisms are qualitatively consistent, quantitative forces and kinetics may differ.
• Accurate interatomic potentials were not available for all materials; heterojunction simulations were limited to Au–Ag, excluding systems like Bi–Au and Sn–Au.
• The loading parameter δ involves a trade-off between reduced force and slower imprinting speed; practical optimization is required for throughput.
• Demolding for longer nanorods (>5 µm) requires wet etching, which may limit reusability of molds and introduces chemical handling constraints.
• While a variety of metals and substrates were demonstrated, broader validation on brittle ceramics and complex semiconductors would further generalize the approach.