Room temperature 3D carbon microprinting

Carbon-based materials are in high demand for flexible electronics, health monitoring, neuroscience, and related applications, but current additive manufacturing approaches often require multiple steps and offer limited spatial resolution. Traditional carbon growth methods rely on high-temperature processes that restrict substrate compatibility, especially for polymers and fabrics, and make out-of-plane and complex freeform structures difficult. Achieving high-aspect-ratio carbon features with micron-scale diameters and sub-150 µm pitch using scalable, eco-friendly methods remains a key challenge. This study aims to develop a low-energy, room-temperature 3D patterning approach for carbon microstructures with precise control over geometry and placement, broad substrate compatibility, and functional optical/electrical properties.

Conventional routes to carbon micro/nanostructures include pyrolysis of organic precursors, which requires multi-step processing (e.g., spinning, stabilization, carbonization) at temperatures typically above 1200 °C. Catalytic conversion of hydrocarbons using transition metals (Ni, Fe, Co) generally operates between 400 and 1500 °C or other high-energy sources. Laser-assisted chemical vapor deposition (L-CVD) can directly crack hydrocarbons with UV/visible lasers at powers typically exceeding 1 W, but growth demands high temperatures and precise maintenance of the growth front temperature synchronized with laser focus motion. Laser-induced graphene on polymers enables planar electrodes but struggles with forming on-demand out-of-plane, high-aspect-ratio features. As a result, scalable, energy-efficient, and substrate-friendly methods for complex 3D carbon architectures are lacking.

Catalyst engineering: Pristine h-BN powder was dried in vacuo at 400 °C for 12 h, transferred into an Ar glovebox, and ball-milled under inert conditions (2 g h-BN, 45 mL zirconia vial with one 19.05 mm zirconia ball, SPEX 8000M, 120 min) to create defect-laden h-BN (dh-BN). The powder was then prepared on selected substrates inside an Ar glovebox. Carbon growth: dh-BN was loaded in a custom reactor (sapphire window) and pressurized with propene (reactant gas) at 40 psi (276 kPa) at 25 °C. A 532 nm visible laser (10–100 mW) was focused using objectives (typically 10×, NA 0.2; also 20× and 50× tested) at or above the catalyst surface to initiate photocatalytic hydrocarbon dehydrogenation. Initial activation was achieved by extended illumination (minutes to hours). Arrays were patterned by rastering with controlled pitch and dwell times via a confocal Raman microscope’s stages (fine X–Y–Z control; notch filter used for visualization). Growth on diverse substrates (loose dh-BN powder bed, dh-BN-coated Kevlar textile fibers, PDMS, glass, quartz) was demonstrated. Process–structure studies included varying exposure time, laser focus height relative to the catalyst, laser magnification/NA, and reactant gas pressure. Tip-following strategies extended rod lengths by (i) segmenting: refocusing on the newly formed tip after each 30 s growth segment, and (ii) continuous tip tracking during growth to form millimeter-long rods and branched 3D architectures. Characterization: Morphology was examined by SEM. Raman and PL spectroscopy/mapping (excitation: 532, 473, 633, 785, 1064 nm; low power <10 mW for Raman acquisition) monitored reaction progress and assessed graphitization (D, G, G′, D+G bands). XRD (Cu Kα) of dense arrays quantified graphitic (002) peak and layer spacing. XPS (Thermo Escalab Xi+, Al Kα) surveyed chemical states (C 1s, B 1s, N 1s, O 1s), assessed bonding environments and hybridization (including C KLL Auger), and evaluated composition at tips, shells, and sputtered interiors. PL lifetime imaging/decays used a 466 nm pulsed diode laser (5 MHz) with TCSPC to extract biexponential decay constants. Electrical I–V and impedance (Nyquist) measured ohmic behavior and failure thresholds. Single-microrod strain and temperature sensors were assembled on flexible substrates (Wheatstone bridge with 4 V DC excitation) and benchmarked against a commercial strain gauge.

• Demonstration of one-step, room-temperature, visible-light-driven photocatalytic growth of carbon microstructures using a metal-free, defect-engineered h-BN (dh-BN) catalyst.
• Rapid formation (seconds to minutes) of rod-shaped microstructures with diameters 2–10 µm and tunable lengths; initial linear growth rate ~40 µm/s up to ~1 s, slowing to ~0.1 µm/s after ~30 s with a fixed focal plane.
• Achieved lengths up to 2.99 mm and aspect ratios up to ~500 by segmented or continuous tip-following refocus, enabling complex 3D and multi-branched structures.
• Arrays with pitches down to ~10 µm (comparable to rod diameters) and centimeter-scale patterned areas; growth on diverse substrates including Kevlar textiles and PDMS, indicating excellent substrate compatibility at room temperature.
• Growth requires photon energies >2 eV (visible absorption of dh-BN); focusing ≥4× magnification needed for out-of-plane growth; optimal focus positioning above the catalyst (e.g., up to ~60 µm) can increase length by ~1.5×; no growth beyond ~80 µm above or >25 µm below the surface.
• Raman shows graphitic G (~1600 cm⁻1), D (~1335 cm⁻1), G′ and D+G bands; rods exhibit core–shell structure with differing graphitization: estimated graphitic domain size ~3.1 nm (shell) vs ~5.4 nm (core). XRD shows a graphite-like (002) peak at 26.1° indicating turbostratic character with interlayer spacing ~0.342 nm (vs graphite ~0.337 nm); h-BN peak at 26.6° present.
• XPS confirms predominantly sp² carbon with some sp³, oxidized carbon features, and evidence of interactions near tips; shells are boron- and nitrogen-free graphitic carbon with defect sites.
• Strong visible photoluminescence from shell and localized clusters; PL peaks at ~540–566 nm; PL lifetimes are biexponential with τ1 ~2 ns (ZPL-related emission) and τ2 ~6–8 ns (phonon-assisted), consistent with carbon/dh-BN defect interactions.
• Electrical measurements show ohmic behavior; long rods (>1 mm) behave as simple resistors. Breakdown in air at ~40 mW for a 2.54 µm diameter, 0.36 mm length rod corresponds to a current density of 7.89 × 10^5 A/cm² (better than copper interconnects, comparable to CVD graphene and below CNTs).
• Sensing: Single microrod strain sensor shows ~30× higher sensitivity than a conventional gauge (reported gauge factor −59.9 µV/με at 4 V vs ~2 µV/με); flexible deformation up to ~2% strain without breakage (loop radius ~243 µm for 5 µm diameter rod). Temperature sensing shows −35% fractional resistance change between 20 and 100 °C.
• Proposed tip-growth mechanism at room temperature via photocatalytic dehydrogenation of hydrocarbons on reactive tips/shells; VLS mechanism is unlikely without metals/high temperatures.

The study addresses the need for an eco-friendly, scalable method to fabricate complex, high-aspect-ratio carbon microstructures on diverse, including polymeric and textile, substrates. By leveraging defect-engineered h-BN as a visible-light photocatalyst, the work demonstrates controlled room-temperature growth of graphitic carbon rods and 3D architectures without metal catalysts or high temperatures. Process–structure relationships—such as the dependence on photon energy, light focusing conditions, and tip refocusing—enable deterministic control over length, branching, and array pitch while maintaining constant widths set by the optical focal profile and gas conditions. The resulting materials exhibit conductive (ohmic) behavior and strong visible photoluminescence with characteristic lifetimes, indicating a hybrid of graphitic carbon with localized defect interactions that can be harnessed for optoelectronics. Electrical robustness (current density approaching that of leading carbon conductors) suggests utility as 3D interconnects for multi-level integration, particularly on flexible substrates. The superior strain and temperature sensing performance of single microrods highlights immediate application potential in wearable/biocompatible devices, aided by the absence of metallic contaminants.

This work introduces a low-power, room-temperature, metal-free photocatalytic method for 3D carbon microprinting, achieving micron-diameter rods with millimeter-scale lengths, ultrahigh aspect ratios (~500), dense arrays (pitch ~10 µm), and intricate multi-branched freeform structures on diverse substrates. Structural and spectroscopic analyses confirm predominantly graphitic carbon with a core–shell organization and turbostratic signatures, while optical and electrical measurements show strong visible PL and ohmic conduction with promising breakdown current densities. Demonstrations of high-sensitivity strain and temperature sensors underscore the technology’s potential for flexible electronics and biosensing. Future directions include beam shaping and dynamic focus strategies for finer control of geometry, environmental control for tuning morphology and composition, and engineered defect patterning to define reactive sites at the micro/nanoscale. These advances can unlock eco-friendly manufacturing routes for next-generation carbon devices spanning flexible electronics, quantum technologies, sensing, catalysis, and biointerfaces.

Growth relies on defect-engineered h-BN as a photocatalyst and requires visible photon energies >2 eV. Out-of-plane growth needs sufficient focusing (≥4× objectives) and appropriate focal plane positioning; no rods form when the focus is placed too far above (~>80 µm) or below (>25 µm) the catalyst surface. With a fixed focal plane, growth rates decrease markedly after ~1 s, yielding identical lengths for long exposures unless the focus is advanced with the tip. Width control is limited by optical focal volume and reactant pressure, rather than exposure time. Electrical robustness depends on geometry and environment; in air, catastrophic failure occurs above ~40 mW for certain dimensions, and optical/electrical characterization required care to avoid sample degradation (e.g., low-power spectroscopy, inert sandwiching for PL lifetime). The study focuses on propene at a set pressure (40 psi) and specific optical parameters; broader generalization across gases/pressures/optics and long-term stability were not explored here.