Terahertz Electromagnetic Shielding and Absorbing of MXenes and Their Composites

doi:10.15541/jim20230453

Abstract

Abstract:

Developing novel low-dimensional materials for terahertz electromagnetic shielding and absorbing applications represents a critical research frontier. Their unique electrical, mechanical, and electromagnetic responses hold great potential in enabling more efficient solutions for electromagnetic shielding and absorbing. Two-dimensional transition metal carbides, nitrides, and carbonitride MXenes have already demonstrated excellent electromagnetic shielding and absorbing performance in the low-frequency spectrum. MXenes possess high conductivity, low density, and high flexibility, which are advantageous for future portability and integration of terahertz devices and systems. However, practical implementation of MXene-based terahertz electromagnetic shielding and absorption materials faces challenges in adhesion stability, environmental resilience, and high-temperature tolerance, hindering their suitability for aerospace and future next generation communication applications. Moreover, in terahertz frequency band, lacking more comprehensive and reliable electromagnetic scattering and absorbing measurement methods limits the development of THz shielding and absorbing materials. Extensive research efforts have targeted on these limitations, exploring fundamental architectural and theoretical aspects of prevalent electromagnetic materials. This review specifically highlights the terahertz electromagnetic shielding and absorption characteristics inherent in various MXenes and their compositions, such as Ti₃C₂T_x, Mo₂Ti₂C₃T_x, Mo₂TiC₂T_x, Nb₄C₃T_x, and Nb₂CT_x. Additionally, this review envisages the forthcoming challenges and prospects of MXenes as a pivotal electromagnetic shielding and absorbing material within the terahertz frequency band.

Key words: two-dimensional material, MXenes, terahertz, electromagnetic shielding and absorbing material, perspective

CLC Number:

TN927

WAN Hujie, XIAO Xu. Terahertz Electromagnetic Shielding and Absorbing of MXenes and Their Composites[J]. Journal of Inorganic Materials, 2024, 39(2): 129-144.

Figures/Tables 13

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)

Table 1 Terahertz electromagnetic shielding and absorption mechanism[37-38]

Classification		Absorbing mechanism	Remark
Salisbury screen		A resistive sheet is placed ${{\lambda }_{0}}/4$ in front of a metal ground plane, usually separated by some lossless dielectrics. The reflectivity drops to zero due to destructive interference between the scattering paths in the device.	(a) Narrow band (b) Destructive interference (c) Easy to process
Jaumann absorber		The Jaumann absorber, an extension of the Salisbury screen, primarily comprises multiple thin impedance layers, lossless dielectric layers, and a metal layer. The electromagnetic characteristics of each impedance layer and the thickness of the dielectric layer are designed to operate at distinct frequencies, enabling destructive interference absorption across multiple frequency points and achieving broadband absorption.	(a) Broadband frequency (b) Destructive interference (c) Substantial thickness and intricate processing methods
Dällenbach absorber		The impedance matching layer is designed for specific high imaginary part of dielectric constant and permeability, while matching the free-space impedance.	(a) High imaginary part of dielectric constant and permeability ${\varepsilon }''$,${\mu }''$ (b) Free-space impedance matching (c) The absorption bandwidth is related to the conduction-frequency characteristics (d) Easy to process
Classification	Absorbing mechanism		Remark
Impedance gradient multilayer absorber	Similar to the Dällenbach absorber, the layer-by-layer impedance matching design minimizes the reflected component of the interface when waves are incident.		(a) Gradient interface impedance matching (b) The absorption bandwidth is related to the conduction- frequency characteristics (c) Complicated process
Pyramidal type absorber	The pyramidal-type absorber, an evolution from the Dällenbach absorber framework, adopts a specific angle design to maximize incident wave capture, minimizing reflection. Electromagnetic waves undergo multiple reflections along the cone structure. Both pyramidal and tetrahedral pyramidal configurations exhibit reduced demands concerning the polarization direction of electromagnetic waves.		(a) The macrostructure captures the incident wave and reduces scattering (b) The absorption bandwidth is related to the conduction- frequency characteristics (c) Easy to process
Metamaterial absorber	The resonant response exhibited by distinctive metamaterial structure serves the purpose of dissipating incident electromagnetic waves. Owing to the notably high degree of freedom within this characteristic structure, metamaterials can be engineered for either single-frequency or wide-band resonant absorption. To achieve broadband absorption, multiple distinct single-frequency resonant structures are employed and combined. However, due to practical limitations in the manufacturing process, there remains a gap in absorption between the multiple resonant frequencies, leading to partial absorption within specific frequency bands.		(a) Electromagnetic resonance characteristic structure (b) Adjustable electromagnetic parameters ($\varepsilon,\mu $) (c) The processing difficulty is related to the wavelength
Coherent perfect absorber	When two normal incidence plane waves with an odd times π phase difference enter the impedance layer from both ends, interference cancelation occurs, and all electromagnetic waves will be absorbed.		(a) Coherent wave (b) Phase difference:$(2n+1)\text{ }\!\!\pi\!\!\text{ }$ (c) The absorption bandwidth is related to the conduction- frequency characteristics
Artificial blackbody	In the domain of artificial black holes, spheres and cylinders stand as prevalent design frameworks. Multi-layer spheres and cylinders are designed to increase the dielectric constant of the medium layer by layer from the outside to the inside. The center is full of material with high imaginary part of dielectric constant, and the incident wave is deflected to the center layer by layer and converges and loses.		(a) Gradient dielectric constant $\varepsilon (r)=\left\{ \begin{align} & {{\varepsilon }_{0}},\ r>R \\ & {{\varepsilon }_{0}}{{\left( \frac{R}{r} \right)}^{2}},\ {{R}_{\text{c}}}<r<R \\ & {{\varepsilon }_{\text{c}}}+\text{i}\gamma,\ r<{{R}_{\text{c}}} \\ \end{align} \right. $ (b) The overall geometry is larger than the wavelength diffraction limit

Table 1 Terahertz electromagnetic shielding and absorption mechanism[37-38]

Classification		Absorbing mechanism	Remark
Salisbury screen		A resistive sheet is placed ${{\lambda }_{0}}/4$ in front of a metal ground plane, usually separated by some lossless dielectrics. The reflectivity drops to zero due to destructive interference between the scattering paths in the device.	(a) Narrow band (b) Destructive interference (c) Easy to process
Jaumann absorber		The Jaumann absorber, an extension of the Salisbury screen, primarily comprises multiple thin impedance layers, lossless dielectric layers, and a metal layer. The electromagnetic characteristics of each impedance layer and the thickness of the dielectric layer are designed to operate at distinct frequencies, enabling destructive interference absorption across multiple frequency points and achieving broadband absorption.	(a) Broadband frequency (b) Destructive interference (c) Substantial thickness and intricate processing methods
Dällenbach absorber		The impedance matching layer is designed for specific high imaginary part of dielectric constant and permeability, while matching the free-space impedance.	(a) High imaginary part of dielectric constant and permeability ${\varepsilon }''$,${\mu }''$ (b) Free-space impedance matching (c) The absorption bandwidth is related to the conduction-frequency characteristics (d) Easy to process
Classification	Absorbing mechanism		Remark
Impedance gradient multilayer absorber	Similar to the Dällenbach absorber, the layer-by-layer impedance matching design minimizes the reflected component of the interface when waves are incident.		(a) Gradient interface impedance matching (b) The absorption bandwidth is related to the conduction- frequency characteristics (c) Complicated process
Pyramidal type absorber	The pyramidal-type absorber, an evolution from the Dällenbach absorber framework, adopts a specific angle design to maximize incident wave capture, minimizing reflection. Electromagnetic waves undergo multiple reflections along the cone structure. Both pyramidal and tetrahedral pyramidal configurations exhibit reduced demands concerning the polarization direction of electromagnetic waves.		(a) The macrostructure captures the incident wave and reduces scattering (b) The absorption bandwidth is related to the conduction- frequency characteristics (c) Easy to process
Metamaterial absorber	The resonant response exhibited by distinctive metamaterial structure serves the purpose of dissipating incident electromagnetic waves. Owing to the notably high degree of freedom within this characteristic structure, metamaterials can be engineered for either single-frequency or wide-band resonant absorption. To achieve broadband absorption, multiple distinct single-frequency resonant structures are employed and combined. However, due to practical limitations in the manufacturing process, there remains a gap in absorption between the multiple resonant frequencies, leading to partial absorption within specific frequency bands.		(a) Electromagnetic resonance characteristic structure (b) Adjustable electromagnetic parameters ($\varepsilon,\mu $) (c) The processing difficulty is related to the wavelength
Coherent perfect absorber	When two normal incidence plane waves with an odd times π phase difference enter the impedance layer from both ends, interference cancelation occurs, and all electromagnetic waves will be absorbed.		(a) Coherent wave (b) Phase difference:$(2n+1)\text{ }\!\!\pi\!\!\text{ }$ (c) The absorption bandwidth is related to the conduction- frequency characteristics
Artificial blackbody	In the domain of artificial black holes, spheres and cylinders stand as prevalent design frameworks. Multi-layer spheres and cylinders are designed to increase the dielectric constant of the medium layer by layer from the outside to the inside. The center is full of material with high imaginary part of dielectric constant, and the incident wave is deflected to the center layer by layer and converges and loses.		(a) Gradient dielectric constant $\varepsilon (r)=\left\{ \begin{align} & {{\varepsilon }_{0}},\ r>R \\ & {{\varepsilon }_{0}}{{\left( \frac{R}{r} \right)}^{2}},\ {{R}_{\text{c}}}<r<R \\ & {{\varepsilon }_{\text{c}}}+\text{i}\gamma,\ r<{{R}_{\text{c}}} \\ \end{align} \right. $ (b) The overall geometry is larger than the wavelength diffraction limit

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]

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)

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)

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)

Table 2 MXenes fitting parameters for terahertz electron relaxation time and backscattering based on the Drude-Smith model [60⇓-62,64-65]

MXenes	Film fabrication method	Thickness/nm	$\tau $/fs	c	Ref.
Ti₃C₂T_x (MILD)	Interfacial thin film technology	16	6±1	-0.97±0.03	[60]
Ti₃C₂T_x (MILD)	Interfacial thin film technology	25±5	19±1	-0.68	[62]
Ti₃C₂T_x	/	/	67±3	~ -0.65	[64]
Al-Ti₃C₂T_x	Self-assemble technology	3.0±0.2	9.62±0.1	-0.82±0.0015	[65]
		6.4±0.3	10.61±0.2	-0.79±0.0016
		8.0±0.7	10.73±0.2	-0.76±0.0019
		10.2±0.1	10.98±0.2	-0.72±0.0016
		11.8±0.5	12.60±0.2	-0.68±0.0013
		13.3±0.5	12.59±0.1	-0.66±0.0012
Mo₂Ti₂C₃T_x (HF TBAOH)	Spin-coating technology	~80	16±3	-0.941±0.007	[61]
Annealing-Mo₂Ti₂C₃T_x (HF TBAOH)	Spin-coating technology	~80	20±4	-0.875±0.013	[61]
Mo₂TiC₂T_x (HF TBAOH)	Drop-cast technology	~1300	36±4	-0.864±0.007	[61]
Annealing-Mo₂TiC₂T_x (HF TBAOH)	Drop-cast technology	~1300	31±3	-0.895±0.005	[61]
Nb₄C₃T_x-few layer (HF TMAOH)	/	/	52±4	~ -0.7	[64]

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)

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)

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)

Table 3 Terahertz electromagnetic shielding and absorption properties of MXenes and their composites [57,59,62,76,78⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓ -89]

	Composition	Density/(g·cm^-3)	Thickness/μm	SE/dB	SSE/t/(dB·cm²·g^-1)	Absorption /(RL·dB^-1)	Frequency band/THz	Ref.
Compact & laminated structure	Ti₃C₂T_x	/	0.15	20	/	/	1.0	[59]
	Ti₃C₂T_x	ca. 2.39	0.025	~2.5	~7×10⁵	/	0.25-2.25	[62]
	Ti₃C₂T_x	/	25	55~70	/	/	0.3-0.7	[78]
	Ti₂CT_x/PDMS	/	/	~6	/	/	0.2-3	[79]
	PAN/Ti₃C₂T_x/AgNPs	/	3.85	9.11	/	/	0.2-1.2	[80]
	Ti₃C₂T_x/copolymer-polyacrylic	/	38.3	64.9	/	/	0.2-1.6	[57]
	PVA/Ti₃C₂T_x/MWCNT	/	42	23~36	/	/	0.2-2.0	[81]
	Ti₃C₂T_x/extracted bentonite	/	11	47	/	/	0.2-1.3	[76]
	Ti₃C₂T_x/ polyaramids	/	20	52.7	/	/	0.2-1.6	[82]
Porous structure	Ti₃C₂T_x/GO	/	4000	/	/	37 dB	0.2-2.0	[83]
	Zn²⁺/Ti₃C₂T_x/GO	0.11	85	51	451.0	/	0.2-2.0	[84]
	Ti₃C₂T_x/polyurethane	/	2000	/	/	99.99%	0.3-1.65	[85]
	Ti₃C₂T_x/PAA/ACC nanoparticle	/	130	45.3	/	23.2	0.2-2.0	[86]
	Ti₃C₂T_x/polyurethane/SCA/silica	/	2000	/	/	99.6%	0.3-1.2	[87]
	Ti₃C₂T_x/rGO	/	148	~30	/	99.999%	0.37-2.0	[88]
	Ti₃C₂T_x/polysiloxane	/	2500	/	/	27.3	0.2-1.4	[89]

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)

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.)

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