Staggered circular nanoporous graphene converts electromagnetic waves into electricity

Low-frequency EM waves (2–5 GHz) from modern telecommunications are largely unused, contributing to EM pollution, interference, and heat in electronics. Converting this ambient EM radiation into usable DC power could mitigate pollution and provide energy. However, single-component materials have struggled to perform both strong EM absorption (which typically correlates with high permittivity and high thermal conductivity) and efficient thermoelectric conversion (which requires low thermal conductivity and high Seebeck coefficient). Graphene is an excellent EM dissipator due to high permittivity, but its intrinsically high thermal conductivity, low Seebeck coefficient, and zero bandgap hinder thermoelectric conversion. Prior approaches (doping, nanostructuring) tune intralayer bonding; interlayer interaction engineering (e.g., small-angle twisted graphene) also yields emergent properties. This study targets a single-component solution by simultaneously engineering intralayer and interlayer characteristics via ordered nanopores and a staggered pore alignment across layers to enable EM-to-heat-to-DC conversion.

Previous work shows graphene’s strong EM dissipation but poor thermoelectric properties due to high thermal conductivity and zero bandgap. Intralayer modification strategies (elemental doping, nanoribbons) adjust electronic/phononic behavior, while interlayer manipulation (e.g., magic-angle twisted graphene) reveals rich phenomena. Existing pore fabrication methods often yield micrometer-scale, fully overlapped pores with low edge density and weak intralayer effects. Conventional EM absorbers exhibit dipole relaxation at higher frequencies (>8 GHz), limiting low-frequency harvesting. There remains a need for monodisperse, nanometer-scale ordered pores with controlled interlayer staggering to introduce strong edge dipoles, tune phonon transport, and open bandgaps for enhanced Seebeck response.

Synthesis: Monodisperse Fe3O4 nanoparticles were grown in situ on CVD graphene via thermal decomposition (oleic acid/oleyl amine, staged heating to 300 °C). Subsequent annealing oxidized underlying graphene to form nanopores; residual Fe3O4 was removed in HCl (pH 1.5–2), yielding monodisperse, nanometer-sized pores. Pore shapes (circular, square, hexagonal) were controlled by nanoparticle geometry; pore size <3 nm achieved via H2O2 oxidation (30 wt%, 80 °C, 1.2 h). Multilayer graphene (bi-, tri-, multilayer) exhibited partially overlapped (staggered) pores due to successive etching and interlayer slipping. N- and S-doped graphene were synthesized as controls. Characterization: TEM imaged pore morphology and layer structures; XPS and XRD tracked phase evolution and interlayer spacing; Raman and FT-IR probed bonding and edge chemistries. EM dissipation characterized by temperature-dependent permittivity using a vector network analyzer on silicone/graphene toroids (30 vol%) with effective medium back-calculation. Electrical properties (four-probe conductivity), Hall carrier density/mobility, Seebeck coefficient (Ulvac-Riko ZEM-3), and thermal conductivity via laser flash (LFA-467) with κT = ρ Cp λ were measured (with stated uncertainties). Molecular dynamics simulated phonon density of states and coupling/scattering near pore edges; density functional theory computed band structures and projected DOS for staggered porous bilayer graphene with defined overlap ratios. Device fabrication and testing: A ~10 ± 0.8 µm porous-graphene film was cut into 25 mm × 5 mm strips; one end (5 mm) coated with ~690 nm parylene-C (adiabatic layer) via CVD. Six strips were assembled in series on a 20 µm PDMS substrate with 5 mm spacing. Devices were exposed to 2.45 GHz EM radiation (100 W magnetron) in a custom cavity; temperature profiles and gradients were monitored via IR thermometer. Open-circuit voltage, I–V curves under varying load (0–150 Ω), power output, power density, cycle durability, and temperature dependence (10–85 °C) were recorded. Finite-element analysis modeled temperature gradients under EM exposure.

• Synthesis and structure: Achieved monodisperse, nanometer-sized circular pores (~6 ± 1 nm) and other shapes; created staggered overlap of pores across layers (bi-, tri-, multilayer) verified by TEM. Edge regions host non-graphitized carbon and polar groups (C–O, C=O, C–OH).
• EM dissipation: Bilayer graphene with staggered circular pores exhibits η up to 160.8 (2–5 GHz), exceeding nonporous graphene (116.9) and most conventional EM dissipators. Circular pores outperform pristine graphene; monolayer/bilayer η > trilayer/multilayer in 2–5 GHz. Cole–Cole analyses indicate strong low-frequency dipole polarization relaxation due to edge dipoles; relaxation intensity increases with temperature.
• Electrical transport: Ordered nanoporous graphene shows reduced mobility and Hall carrier density versus nonporous; electrical conductivity 1000–3000 S/cm from 300–500 K (about 20–30% of nonporous bilayer graphene).
• Thermal transport: Bilayer porous graphene κT = 3.5–2.1 W m−1 K−1 (300–500 K), two orders lower than nonporous bilayer; κT decreases with more layers. κl dominates κT reduction; MD reveals weakened phonon coupling (50–65 THz) and strong edge-induced backscattering (<50 THz). Covered vs exposed atoms in staggered pores and reduced interlayer spacing further weaken phonon coupling.
• Temperature gradients under EM: For a single strip, ΔT/L reached 1.86 K mm−1 at 180 s under 2.45 GHz irradiation, ~4× nonporous (0.52 K mm−1); returns to zero after EM off, consistent with FEA.
• Seebeck and thermoelectrics: Bilayer porous graphene |S| = 69–83 µV K−1 (300–500 K), ≥6× nonporous (<10 µV K−1); monolayer and trilayer |S| are ~50% and ~15% lower than bilayer, respectively. ZT ≈ 0.33 (300–500 K); thermoelectric coefficient of devices −14.4%. DFT shows Dirac point splitting, open bandgap ~0.13 eV (overlap ratio 1/3), van Hove singularities and Dirac trap effect, enhancing |S| via stronger electron–phonon interactions (phonon drag).
• Power generation: Six-strip device achieved open-circuit voltage Uoc ≈ −17.7 mV at 180 s; nonporous device ~0.8% of this. Maximum output power −1.5 µW at RL = 60 Ω after 180 s; power density −204 W m−3. Energy per 360 s cycle (180 s on/180 s off): −0.23 mJ; stable for ≥500 cycles. Overall EM-to-electricity conversion ≈ −5.6%. Performance robust from 10–85 °C and after 500 bending cycles. Power increases with strip thickness/number.

Engineering monodisperse, staggered nanopores in graphene concurrently optimizes properties needed for EM-to-DC conversion within a single material: (i) abundant edge dipoles shift polarization relaxation to 2–5 GHz, boosting low-frequency EM dissipation; (ii) pore-edge scattering and staggered interlayer interactions strongly suppress lattice thermal conductivity, sustaining temperature gradients; and (iii) staggered pore-induced electronic reconstruction (Dirac point splitting, bandgap opening, Fermi surface fragmentation) elevates the Seebeck coefficient. The combined effects create sizable temperature gradients under EM irradiation and enable thermoelectric conversion to DC power. The resulting device outperforms nonporous graphene by orders of magnitude, with EM dissipation factor and power density surpassing many conventional materials, validating the feasibility of single-component EM-heat-DC conversion. These insights link pore geometry, overlap ratio, and layer number to tunable electronic/phononic behavior with practical implications for energy harvesting and EM management.

This work demonstrates a nanoparticle-templating method to fabricate ordered, monodisperse, staggered nanoporous graphene that simultaneously offers high permittivity, ultralow thermal conductivity, and enhanced Seebeck response. Mechanistically, edge dipoles enable low-frequency EM absorption, pore-edge phonon scattering reduces κ, and staggered pores reshape the band structure to boost |S|. Devices based on this material convert ambient 2.45 GHz EM waves into measurable DC electricity with high power density, stable cycling, and mechanical robustness. The approach advances understanding of structure–property relationships in porous graphene and offers a route to mitigate EM pollution while harvesting energy. Future research will experimentally control pore overlap ratio and further optimize device architectures for self-powered and self-charging wearable electronics and broader applications in thermal management, EM shielding/absorption, thermoelectrics, and photocatalysis.

• Measurement uncertainties: Permittivity measurements carry ~10–12% uncertainty due to sample preparation; electrical output has ~15–25% uncertainty; temperature error ~1 K.
• Effective medium extraction: Permittivity of graphene derived from silicone/graphene composites may introduce additional error.
• Structural control: Precise experimental control of interlayer pore overlap ratio is identified as future work, indicating current limitations in tuning this parameter.
• Device specifics: Reported performance is at 2.45 GHz under specific device geometry and conditions; generalization to other frequencies/geometries was not detailed.