Unveiling the sp2─sp3 C─C Polar Bond Induced Electromagnetic Responding Behaviors by a 2D N‐doped Carbon Nanosheet Absorber

Abstract The infertile electromagnetic (EM) attenuating behavior of carbon material makes the improvement of its performance remain a significant challenge. Herein, a facile and low‐cost strategy radically distinct from the prevalent approaches by constructing polar covalent bonds between sp2‐hybridized and sp3‐hybridized carbon atoms to introduce strong dipolar polarization is proposed. Through customizing and selectively engineering the N moieties conjugated with carbon rings, the microstructure of the as‐synthesized 2D nanosheet is gradually converted with the partial transition from sp3 carbons to sp2 carbons, where the electric dipoles between them are also tuned. Supported by the DFT calculations, a progressively enhanced sp2─sp3 C─C dipolar polarization is caused by this controllable structure evolution, which is demonstrated to contribute dominantly to the total dielectric loss. By virtue of this unduplicated loss behavior, a remarkable effective absorption bandwidth (EAB) beyond ‐10 dB of 8.28 GHz (2.33 mm) and an ultrawide EAB beyond ‐5 dB of 13.72 GHz (4.93 mm) are delivered, which upgrade the EM performance of carbon material to a higher level. This study not only demonstrates the huge perspective of sp2─sp3‐hybridized carbon in EM elimination but also gives pioneering insights into the carbon–carbon polarization mechanism for guiding the development of advanced EM absorption materials.

solely treated in the same route at a pyrolysis temperature of 750 ℃, and the resultant bare carbon product was named AC750.
Material Characterizations.The X-ray diffraction (XRD) patterns were collected on a PANalytical X'Pert PRO diffractometer with a Cu Kα radiation source.Fourier transform infrared (FTIR) spectrum was carried out with a Thermo Scientific NICOLET iS10 FTIR spectrophotometer.The X-ray photoelectron spectroscopy (XPS) and the X-ray Auger-electron spectroscopy (XAES) were performed in a ThermoFischer ESCALAB 250Xi spectroscopy using an Al-kα monochromatic excitation source.The visible resonant Raman spectra (vis-Raman) and photoluminescence (PL) spectra were recorded on a Renishaw in-Via spectrometer with a 532 nm laser, and the Ultraviolet Raman spectra (uv-Raman) were conducted on an Andor SR-500i Raman spectrometer with a 325 nm laser.The transmission electron microscope (TEM) study, including the related characterization such as scanning TEM-electron energy loss spectroscopy (STEM-EELS) was conducted using an aberration-corrected Titan G2 microscope operated at 300 kV.The content of sp 2 and sp 3 carbons was calculated by the integrated area based on the two-window method, as follows: Electromagnetic Measurements.The electromagnetic parameters were measured using an Agilent N5230A vector network analyzer (VNA) in the 2-18 GHz frequency range.To prepare the test specimen, the samples were uniformly mixed with paraffin wax at a mass ratio of 30% and then compacted into a coaxial ring with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm.The reflection loss (RL) was calculated based on the transmission line theory as follows: RL=20log in which Zin and Z0 successively represent the input impedance of the absorber and the impedance of free space, d is the thickness of absorber, and c is the velocity of light in vacuum.

theoretical calculations
Radar cross-section (RCS) simulation based on Frequency domain.The RCS performance of AC-CN nanosheets was studied by a frequency domain-based simulation on CST Studio Suite 2019 to investigate their promising practical applications.The numerical simulation was accomplished by calculating the following formula: where S and λ represent the area of the absorber and the wavelength of electromagnetic wave,

Figure S3
. FTIR spectra of AC-CN nanosheets, where the C-N stretching region originates from the triazine rings, [1] and the diminished C-N stretching with the elevated pyrolysis temperature indicates the gradual degradation of triazine units.Co3O4@WSe2-MWCNTs -56.9 6.56 1.41 Is represent the integrated intensities of the 1s→π* transition peak and the 1s→σ* transition peak, respectively.The reference sample of glass carbon is denoted as g, while a refers to the as-prepared AC-CN nanosheets.The atomic force microscopy (AFM) images were obtained by a Bruker Dimension Icon microscopy.Electrochemical impedance spectroscopy (EIS) was recorded using a conventional three-electrode system in 3.5 wt% NaCl solution on a CHI 660E electrochemical workstation (scanning speed 0.001 V/s, frequency range of 10 5 Hz-0.1 Hz).In a typical method, the working electrode is fabricated with the carbon cloth (4.6 mm*4.4 mm) serving as supporter, and the auxiliary electrode and reference electrode are platinum foil and saturated calomel, respectively.The electron paramagnetic resonance (EPR) was performed on a Bruker EMX plus under the temperature of 77K, with the frequency of 9.84 GHz and the center field of 3410 G.
respectively, and Es and Ei successively are the electric field intensity of transmitting waves and the electric field intensity of the receiving wave.In the simulation model, the electromagnetic absorption layer (180mm*180mm*2.5mm)sticks to a perfect electric conductor (PEC, 180mm*180mm*0.5mm)layer with open boundary conditions, and the incident electromagnetic wave (9.00 GHz) penetrates the upper absorption layer along the z-axis.The setting of the length and width is accordance with the standard size of the template plates for the arch method to measure electromagnetic absorption performance, the thickness of absorption layer is dependent on the matching thickness of the as-prepared AC-CN nanosheets.DFT computational Details.The geometry optimizations were performed with ωB97X-D/def2SVP level of theory.Harmonic vibration frequency calculations were performed for all stationary points to confirm them as a local minimum at the same theoretical level.The singlepoint energy (SP) calculations were performed on the optimized geometries at ωB97X-D3(0)/def2-TZVPPD theoretical level.The geometric optimization and subsequent frequency analysis were carried out by Gaussian 16 programs and the SP calculations were carried out by the ORCA 5.0.3.The analyses of dipole moment and electrostatic potential (ESP) on molecular vdW surface were finished by Multiwf.The ESP map was rendered by VMD program based on the outputs of Multiwfn.The component of the dipole moment of the whole molecule can be computed as   = ∑ [    − ∫   ()() d] ∈ (Equation S5)where R is the position of the nucleus, Z is the nuclear charge, ρ(r) is the electron density, and w(r) is the atomic space partition function.

Figure S6 .
Figure S6.High-resolution XPS spectra of a) C 1s and O 1s of AC750, which demonstrates the

Figure S7 .
Figure S7.Uv-Raman spectra of AC750, where no T band is exhibited but an obvious D band

Figure S8 .
Figure S8.Core-loss spectrum images and the EELS mappings of C-K edge and N-K edge of

Figure S10 .
Figure S10.a) Frequency dependence of permeability and the b) magnetic loss factor (tan δM)

Figure S11 .
Figure S11.Frequency dependence of a-e) complex permittivity and f) dielectric loss factor (tan

Figure S12 .
Figure S12.PL spectra of AC-CN nanosheets, where AC750 shows no PL peak but AC-CN

Figure S13 .
Figure S13.Frequency dependence of permittivity of g-C3N4, which indicates negligible

4 Figure
Figure S14.a) Mulliken charge of N, sp3 C and sp2 C atoms of the bond between them in the

Figure S16 .
Figure S16.a) Frequency dependence of electromagnetic parameters and the b) effective

Figure
Figure S17.a-b) The 2D contour of the impedance matching characteristic Z=|Zin/Z0| for AC-CN nanosheets, where AC-CN750 exhibits largest impedance matching region with a Z located within 0.8-1.2.

Figure S18 .
Figure S18.The model employed in the RCS simulation, in which the absorption layer with a

Table S1 Parameter
D of XAES spectra, ID/IG of vis-Raman spectra, and sp 3 /sp 2 carbon fraction derived from EELS

Table S2 N
/C fractions derived from XPS and EELS spectra TableS3EM performance comparison between the AC-CN nanosheets prepared in this study with the recently reported state-of-the-art carbon-based absorbers