图1 MXenes的结构示意图

Fig.1 Chemical structures of MXenes 2D MXenes have a general formula of Mn+1XnTx, where M is an early transition metal, X is carbon and/or nitrogen, and Tx represents surface terminations of the outer metal layers. The n value in the formula can vary from 1 to 4, depending on the number of transition metal layers (and carbon and/or nitrogen layers) present in the structure of MXenes, for example, Ti2CTx (n = 1), Ti3C2Tx (n = 2), Nb4C3Tx (n = 3), and (Mo,V)5C4Tx (n = 4)

图2 典型电磁吸收体的原理示意图

Fig. 2 Schematic illustration of electromagnetic absorber (a) Salisbury screen absorber. The resistive film placed at 1/4 wavelength in front of the metal plate[39]; (b) Dällenbach absorber. The impedance of resistive film is equal to free space impedance to reduce reflection[38]; (c) Impedance gradient multilayer absorber. The impedance decreases layer by layer from large to small, reducing reflections while increasing conductive loss absorption[38]; (d) Jaumann absorber. All the layers are designed to operate at different wavelengths, so that the distance between each sheet is approximately$\lambda /4$, producing multiple reflection minima around some center frequency ${\lambda }_0$[38]; (e) Pyramidal type absorber. Multiple reflection losses along the cone, so the absorber has a wide band of electromagnetic wave absorption characteristics for both polarization direction electromagnetic waves[44-45] ; (f) Metamaterial absorber. Electromagnetic resonance loss depends on subwavelength metal-structure[37]; (g) Coherent perfect absorber in free space[49]; (h) Artificial black holes consisting of multiple layers of asymptotically varying dielectric constant spheres and/or cylinders, with the dielectric constant increasing from outside to inside, from small to large[52]

图3 太赫兹电磁屏蔽效能(a)和反射损耗(b)测试原理图[57]

Fig. 3 Schematic illustration of a THz-TDS system for electromagnetic interference shielding efficiency (a) and reflection loss measurements (b)[57] (Reprinted from Ref. [57] with permission, Copyright 2021, American Chemical Society)

图4 基于金纳米狭缝天线的MXene电磁屏蔽增强现象[59]

Fig. 4 Enhanced THz electromagnetic shielding phenomenon of MXene based on gold nano-slit antenna[59] (a) Scheme of MXene-coated nano-metamaterial antennas; (b, c) Thin film of MXene made by drop-casting of MXene flakes followed by drying in air for many cycles to achieve the desirable thickness; (d) SEM image of the as-prepared MXene film with 150 nm width antenna; (e) Normalized transmittance spectra of the bare silicon and various thicknesses of MXene films on the silicon substrate; (f) Normalized transmittance spectra of the antenna and various thicknesses of MXene on the 500 nm width antenna array; Electric field intensity distributions of transmitted THz wave near the 500 nm width antenna, obtained by analytical calculation of antenna (g) without MXene, with (h) 100 nm thick MXene on the antenna and (i) 300 nm thick MXene on the antenna. The yellow and gray colors indicate the gold nano-slot antenna and MXene, respectively. In (h, i), the MXene filled the gap of the nano-antenna (Reprinted from Ref. [59] with permission, Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

图5 MXenes的太赫兹电导响应特性

Fig. 5 Terahertz conductivity characteristics of MXenes (a) Complex THz conductivity of the 16 nm Ti3C2Tx film, and lines-global fitting of the real and imaginary conductivity to the Drude-Smith model (solid red line, fit to σ1, dashed blue line, fit to σ2)[60]; (b) Schematic representation of a Ti3C2Tx film: five atomic layers thick Ti3C2 cores of individual flakes are terminated by -OH, -F, or =O groups; (c) 2 μm by 2 μm AFM micrograph of a 25 nm thick Ti3C2Tx film; (d) THz spectroscopy experiment; (e) THz probe pulse transmitted through the substrate with a Ti3C2Tx film; (f) Corresponding THz electric field amplitude; (g) THz complex conductivity (solid symbols represent real and open symbols represent imaginary conductivity components; lines show a global fit of both components to the Drude-Smith model); (h) EMI SE calculated from data in (e)[62]; (i) Terahertz conductivity of Nb4C3Tx and Ti3C2Tx MXene thin films; (j) Real component of THz conductivity in three MXenes films (Ti3C2Tx, Mo2Ti2C3Tx and Nb2CTx) measured by the THz time-domain spectroscopy. Symbols are experiment data, and lines are fit to the Drude-Smith model. Extrapolation 0 THz yield static, DC conductivity[63-64]; TDS of (k, l) Mo2Ti2C3Tz and (m, n) Mo2TiC2Tx (k, m) as deposited and (l, n) after a mild 200 ℃ vacuum annealingz. Solid symbols represent real and open symbols represent imaginary conductivity, with lines showing global fits of both the real and imaginary conductivity to the Drude-Smith model with parameters σDC, τDS and c indicated on individual panels[61] (Reprinted from Ref. [60] with permission, Copyright 2022, IOP Publishing; Reprinted from Refs. [61-62] with permission, Copyright 2020, American Chemical Society; Reprinted from Ref. [63] with permission, Copyright 2022, Wiley-VCH.; Reprinted from Ref. [64] with permission, Copyright 2022, Springer Nature)

图6 自组装Ti3C2Tx薄膜的层数依赖太赫兹响应特性[65]

Fig. 6 Layer-dependent terahertz (THz) conductivity observed in self-assembled Ti3C2Tx films[65] (a) Ultrabroadband terahertz absorption and conductivity of a 17.2 nm Ti3C2Tx film measured by an air plasma-based THz-TDS system; (b) Terahertz sheet conductivity and resistance of Ti3C2Tx assemblies with different thicknesses for 0.5-4.5 THz; (c) Theoretical transmittance, reflectance and absorption simulated by impedance theory; (d) Conductivity as a function of frequency for a Drude-Smith model with different relaxation times. The shaded bars show the regions where the average absorption is 49% and fluctuation is 1%; (e) Relaxation time τ and sheet carrier concentration N for Ti3C2Tx assemblies with different thicknesses. The blue and red colours indicate the higher and lower impedance, respectively (Reprinted from Ref. [65] with permission, Copyright 2023, Springer Nature)

图7 基底普适性Ti3C2Tx复合薄膜涂料(MWP)的制备、黏度及太赫兹电磁屏蔽性能[57]

Fig. 7 Synthesis of MWP, and viscosity and EMI SE measurements of different MXene filler content MWP[57] (a) Schematic of the fabrication of Ti3C2Tx water dispersion and Ti3C2Tx MXene waterborne paint; (b) Photos of 30% MWP coating on different substrates: printing paper, aluminum foil, copper foil, sponge foam, glass slide, Kapton tape, silicon wafer, and quartz; (c) Viscosity of 40 mg·mL−1 MXene water dispersion and MWP with different filler contents; (d) EMI SE versus thickness of 30% MWP on quartz; (e) EMI SE at 1 THz of 30% MWP coated on different types of substrates: flexible, rigid, porous (Reprinted from Ref. [57] with permission, Copyright 2021, American Chemical Society)

图8 利用水氧吸附竞争机制的MXene与层状蒙脱土耐高温太赫兹电磁屏蔽复合薄膜[76]

Fig. 8 High-temperature terahertz electromagnetic shielding composite film of MXene and layered montmorillonite utilizing a water-oxygen adsorption competition mechanism[76] (a) Schematic showing the oxidation process of pristine Ti3C2Tx (starting from the edge) and suppressed oxidation of MEB under high-temperature annealing with the presence of oxygen; (b) Charge density difference plots for the stable configurations of one O2 adsorbed on Ti3C2O2, EB, and Ti3C2O2/EB heterostructure. In the heterostructure, EB is subject to saturated oxygen adsorption. The isosurface level is set to be 0.0002 e/Å3 except for O2 adsorbed on EB with a value of 0.0006 e/Å3. The yellow area indicates charge accumulation, and the green region represents charge depletion; (c) Thermal gravimetric (TG) curves in the air with mass spectrometry analysis (MS) for the atomic mass unit (amu) of 18/H2O and 44/CO2 for Ti3C2Tx and EB (MEB); (d) EMI SE in 0.2-1.3 THz of Ti3C2Tx-Atmos-500C-2, 600C-2, and MEB-Atmos-500C-2, 600C-2; (e) Average THz SE of the samples showed in (d) (Reprinted from Ref. [76] with permission, Copyright 2022, Springer Nature)

图9 Ti3C2Tx及其复合多孔吸波体的太赫兹吸收特性

Fig. 9 Terahertz absorption characteristics of Ti3C2Tx and its composite porous absorber (a) Schematic diagram of the MXene/GO foam (MGOF) preparation process; (b) RL curves of 4 mm MGOF with different contents of MXene added to the GO foam[83]; (c) Schematic illustrating the ion-diffusion-induced gelation process; (d) THz EMI shielding effectiveness of the Zn2+ MXene-based foams[84]; (e) Schematic illustration of the fabrication of MSF; (f) THz absorption mechanism of MXene filled 3D porous structure with above typical fill state; Two typical filling states: continuous MXene film on pores (g) and/or discontinuous MXene film on skeletons (h); (i) THz absorption by MSF with different pore sizes but fixed 2 mm thickness and (2.8±0.5) mg Ti3C2Tx loading[85] (Reprinted from Ref. [83] and Ref. [84] with permission, Copyright 2019 and 2020, American Chemical Society; Reprinted from Ref. [85] with permission, Copyright 2020, Wiley-VCH GmbH)

图10 Ti3C2Tx及其复合多孔水凝胶与定向冷冻干燥气凝胶的太赫兹吸收特性

Fig. 10 Terahertz absorption properties of Ti3C2Tx and its composite porous hydrogels and directional freeze-dried aerogels (a) Schematic illustrating the formation of MXene composite hydrogel; (b) Proposed absorption-dominated EMI shielding mechanism of the MXene composite hydrogel; (c) RL curves of MXene composite hydrogels; (d) Comparison of terahertz EMI shielding and absorption performances between MXene composite hydrogel and reference samples; (e) SEM image of MXene composite hydrogel[86]; (f) Schematic illustration of the fabrication of CMXene; (g) RL curves in THz band of all MXene-based absorbers[89] (Reprinted from Ref. [86] with permission, Copyright 2021, American Chemical Society; Reprinted from Ref. [89] with permission, Copyright 2023, Elsevier B.V.)