Millimeters long super flexible Mn5Si3@SiO2 electrical nanocables applicable in harsh environments

Nanoscopic electrical interconnects are critical components in micro/nano-electronics, optoelectronics, bioelectronics, and integrated nanosystems. Metallic nanowires such as Ag and Cu are widely used due to high conductivity and current-carrying capacity, but they suffer under extreme conditions: high temperatures in air, oxidizing and acidic environments, and mechanical deformation can cause oxidation, structural damage, and irreversible plastic deformation (local necking, dislocations), leading to electrical failure or degradation. There is a need for nano-interconnects that maintain electrical performance and mechanical integrity in harsh environments. Alloy silicides offer metallic-like transport and compatibility with silicon technology, alongside ceramic-like physicochemical stability and mechanical robustness. Mn5Si3, historically studied as an antiferromagnet, is explored here as a core material protected by an amorphous SiO2 sheath, aiming to deliver stable, flexible, and chemically resistant nano-interconnects. This work investigates the synthesis, structure, electrical properties, and environmental robustness (thermal, acidic, oxidizing, and complex electrical fields) of millimeter-long Mn5Si3@SiO2 nanocables.

• Metallic nanowires (Ag, Cu) have enabled transparent, flexible electrodes for devices like solar cells, touch panels, and sensors, but their performance degrades below ~200 °C and they oxidize readily in aqueous or organic environments, evolving into degraded morphologies.
• Mechanical issues in metallic nanowires include size-dependent fracture, plasticity leading to necking, and dislocation-driven deformation under strain, causing electrical degradation.
• Metal silicides (e.g., Ni2Si, TaSi2, TiSi2) show metallic-like transport and are compatible with silicon device processes, with ceramic-like stability (high modulus and hardness).
• Mn5Si3 has been studied primarily for magnetic properties (antiferromagnetism, magnetocaloric effects), but its electrical transport and stability in nanowire form as interconnects are less explored.
• Prior studies highlight the need for interconnects that are stable against oxidation and corrosion while maintaining high current density; this motivates core–shell architectures where an inert shell protects a conductive core.

Synthesis:

• Growth was conducted in a home-made horizontal tube furnace via CVD in a molten Si–Mn–O glassy matrix enabling eutectic transport and sustained Mn5Si3 precipitation for ultra-long core–shell nanowires.
• Experiment A (matrix preparation): Mn and SiO powders (molar ratio 6:1; 900 mg total) were mixed, placed on a ceramic sheet, and processed at 1250 °C for 3 h under H2 flow (100 sccm) at 20 kPa after forevacuum. The system cooled under vacuum with water cooling.
• Experiment B (nanocable growth): 90 mg of Mn:Si powder mixture (1:3 molar ratio) and 100 mg SiO2 were placed on a ceramic sheet; similar thermal and atmospheric conditions as Experiment A were used to synthesize Mn5Si3@SiO2 nanocables.

Structural/Compositional Characterization:

• SEM, TEM, STEM-EDS, SAED, and HRTEM confirmed core–shell morphology with a crystalline Mn5Si3 core (growth along c-axis) and amorphous SiO2 shell; XRD matched Mn5Si3 (PDF#42-1285). Diameter uniformity along ~300 μm showed <4% fluctuation (largely from SiO2 shell).

Mechanical Testing:

• Bendability was probed using a tungsten probe; nanocables sustained up to 16.7% bending strain without fracture. Additional tests on Si3N4 membranes with apertures showed no core fracture at 11.52% and 13.05% strain.

Device Fabrication and Electrical Measurements:

• Double-electrode devices were made in a dual-beam FIB-SEM workstation. SiO2 shell was removed locally using Ga+ FIB (current density −5 pA·μm^-2, 10 s) to expose the Mn5Si3 core, followed by FIB-induced Pt/C deposition (− pA·μm^-2, 120 s) for contacts. Two movable tungsten probes connected to a Keithley 2634B sourced I–V.
• Multiple devices along single nanocables were fabricated to assess longitudinal uniformity. Resistivity and maximum current density were extracted up to breakdown.
• For lithographically defined devices, Au/Ti (90/10 nm) electrodes were patterned; channels for Figs. 4–5 used lithography and lift-off.

High-Temperature Electrical Testing:

• A device with ~250 μm channel, core ~150 nm (shell ~206 nm) was measured at room temperature and on a hot plate. Substrate surface temperature was calibrated by thermal imaging (Fluke TiS55) and used as nanowire temperature. Resistance was recorded from 26 °C to 317 °C (heating/cooling cycles).
• Long-term stability: a ~75 μm channel device (core ~81 nm) drove a red LED at ~1 mA with 8.5 V, then was heated to ~300 °C; current over ~24 h was recorded.

Acidic and Oxidizing Environment Tests:

• HCl (pH=1) test: A ~1 mm channel device was epoxy-sealed leaving only the channel exposed; auxiliary electrodes (HCl+ and HCl−) applied a periodic rectangular pulse (peak 0.1 V) across the electrolyte. I–V and I–t were measured with and without HCl.
• H2O2 (10%) test: Side-by-side nanocables were immersed; I–V was measured after 1, 5, 10, and 20 h. Parallel Ag nanowires were subjected to the same oxidizing environment and thermal tests for comparison.

Comparison Benchmark:

• Ag nanowires were measured for resistivity and maximum current density using the same methodology for context.

• Electrical performance of Mn5Si3 core (30 nanocables, core diameters 62–102 nm): resistivity 1.28–3.84 ×10^-6 Ω·m; maximum current density 1.22–3.54 ×10^7 A·cm^-2. A representative device (97±3 nm core, 58.5 μm channel) showed ρ = 2.28 ×10^-6 Ω·m and j_max = 1.61 ×10^7 A·cm^-2.
• Uniformity: Diameter fluctuation along ~300 μm length <4% (mostly from shell); electrical properties were homogeneous along individual nanocables.
• Mechanical flexibility: Single nanocables withstood up to 16.7% bending strain without fracture; additional tests showed no core fracture at ~11.5–13.0% strain.
• High-temperature behavior in air: Resistance increased by ~14.45% from 84.42 kΩ (26 °C) to 92.62 kΩ (317 °C); heating/cooling R–T curves overlapped, indicating metal-like conduction. Devices operated stably at ~300 °C for ~24 h at ~1 mA driving an LED, with minimal current fluctuation.
• Thermal robustness limit: Morphology unchanged after 2 h anneal at 500 °C in air; core begins to degenerate at 600 °C. Ag NWs broke above ~160 °C in air in comparison.
• Chemical/oxidation resistance: In HCl (pH=1), I–V and I–t were essentially unchanged; stable operation under a superimposed pulsed field (0.1 V) across the electrolyte with negligible fluctuation. In H2O2 (10%), Mn5Si3@SiO2 devices retained electrical performance after up to 20 h immersion; Ag NWs suffered serious damage after 2 h.
• Comparison to Ag nanowires: Ag NWs measured resistivity ~1.7 ×10^-7 Ω·m and j_max ~6.5 ×10^7 A·cm^-2, but with poor stability in oxidizing and high-temperature environments.
• Structural characteristics: Single-crystal Mn5Si3 cores grow along c-axis with amorphous SiO2 shells, forming millimeter-length nanocables.

The study addresses the need for nano-interconnects that are electrically capable and robust under harsh mechanical, thermal, and chemical conditions. By combining a conductive Mn5Si3 core with an inert, amorphous SiO2 sheath, the nanocables achieve metal-like electrical transport with respectable resistivity and high current densities while the shell protects against oxidation and corrosion in air, strong acid (HCl, pH=1), and strong oxidizer (10% H2O2). The SiO2 sheath also dissipates strain energy, enabling large bending strains (up to 16.7%) without fracture of the brittle ceramic core, preventing plastic deformation and associated electrical degradation that often occur in metallic nanowires. The devices maintained stable resistance across temperature cycling and over prolonged operation at ~300 °C, and remained electrically functional after annealing at 500 °C, demonstrating suitability for high-temperature electronics where Ag/Cu nanowires fail. The nanocables’ compatibility with silicon processing (FIB, lithography, standard metallization) and their mechanical flexibility make them promising for flexible electronics, bioelectronics, and complex 3D wiring where exposure to ionic, acidic, and oxidizing environments is expected.

The work demonstrates the CVD growth of millimeter-long Mn5Si3@SiO2 nanocables with ultra-high aspect ratio, uniformity, and a single-crystal Mn5Si3 core encapsulated by a conformal amorphous SiO2 shell. These nanocables deliver resistivity in the 10^-6 Ω·m range and maximum current densities exceeding 10^7 A·cm^-2, while exhibiting exceptional stability at elevated temperatures (stable operation ≥317 °C; morphological integrity to 500 °C), in strong acid (HCl, pH=1), and in a strong oxidizer (10% H2O2). They withstand up to 16.7% bending strain without failure. Compared to common metallic nanowires, they provide superior environmental robustness with competitive electrical performance. The nanocables are compatible with standard micro/nanofabrication, enabling selective shell removal and contact formation. Future directions include integration into functional flexible and high-temperature devices, complex interconnect architectures, systematic fatigue testing under cyclic bending, and scaling/assembly strategies for large-area or multi-wire interconnect networks.

• The maximum in-operando test temperature was limited by Au/Ti electrodes (~317 °C); separate annealing suggests core degeneration occurs around 600 °C, indicating an upper bound to thermal robustness.
• Electrical property variability increased for thinner nanocables (62–70 nm cores), attributed to higher relative measurement uncertainty in dimension determination.
• Mechanical tests demonstrated high single-bend strain tolerance, but long-term cyclic bending fatigue data were not reported.
• While chemical robustness was shown in HCl (pH=1) and H2O2 (10%), broader chemical compatibility (e.g., bases, other oxidants/reductants, biological media) was not explored within this study.