Interface Engineering to Drive High-Performance MXene/PbS Quantum Dot NIR Photodiode.

The realization of a controllable transparent conducting system with selective light transparency is crucial for exploring many of the most intriguing effects in top-illuminated optoelectronic devices. However, the performance is limited by insufficient electrical conductivity, low work function, and vulnerable interface of traditional transparent conducting materials, such as tin-doped indium oxide. Here, it is reported that two-dimensional (2D) titanium carbide (Ti3 C2 Tx ) MXene film acts as an efficient transparent conducting electrode for the lead sulfide (PbS) colloidal quantum dots (CQDs) photodiode with controllable near infrared transmittance. The solution-processed interface engineering of MXene and PbS layers remarkably reduces the interface defects of MXene/PbS CQDs and the carrier concentration in the PbS layer. The stable Ti3 C2 Tx /PbS CQDs photodiodes give rise to a high specific detectivity of 5.51 × 1012  cm W-1  Hz1/2 , a large dynamic response range of 140 dB, and a large bandwidth of 0.76 MHz at 940 nm in the self-powered state, ranking among the most exceptional in terms of comprehensive performance among reported PbS CQDs photodiodes. In contrast with the traditional photodiode technologies, this efficient and stable approach opens a new horizon to construct widely used infrared photodiodes with CQDs and MXenes.

Electrochemically modulated interaction of MXenes with microwaves.

Dynamic control of electromagnetic wave jamming is a notable technological challenge for protecting electronic devices working at gigahertz frequencies. Foam materials can adjust the reflection and absorption of microwaves, enabling a tunable electromagnetic interference shielding capability, but their thickness of several millimetres hinders their application in integrated electronics. Here we show a method for modulating the reflection and absorption of incident electromagnetic waves using various submicrometre-thick MXene thin films. The reversible tunability of electromagnetic interference shielding effectiveness is realized by electrochemically driven ion intercalation and de-intercalation; this results in charge transfer efficiency with different electrolytes, accompanied by expansion and shrinkage of the MXene layer spacing. We finally demonstrate an irreversible electromagnetic interference shielding alertor through electrochemical oxidation of MXene films. In contrast with static electromagnetic interference shielding, our method offers opportunities to achieve active modulation that can adapt to demanding environments.

Solution-Processed Ti3 C2 Tx MXene Antennas for Radio-Frequency Communication.

Highly integrated, flexible, and ultrathin wireless communication components are in significant demand due to the explosive growth of portable and wearable electronic devices in the fifth-generation (5G) network era, but only conventional metals meet the requirements for emerging radio-frequency (RF) devices so far. Here, it is reported on Ti3 C2 Tx MXene microstrip transmission lines with low-energy attenuation and patch antennas with high-power radiation at frequencies from 5.6 to 16.4 GHz. The radiation efficiency of a 5.5 µm thick MXene patch antenna manufactured by spray-coating from aqueous solution reaches 99% at 16.4 GHz, which is about the same as that of a standard 35 µm thick copper patch antenna at about 15% of its thickness and 7% of the copper weight. MXene outperforms all other materials evaluated for patch antennas to date. Moreover, it is demonstrated that an MXene patch antenna array with integrated feeding circuits on a conformal surface has comparable performance with that of a copper antenna array at 28 GHz, which is a target frequency in practical 5G applications. The versatility of MXene antennas in wide frequency ranges coupled with the flexibility, scalability, and ease of solution processing makes MXene promising for integrated RF components in various flexible electronic devices.

Tailoring Electronic and Optical Properties of MXenes through Forming Solid Solutions.

Alloying is a long-established strategy to tailor properties of metals for specific applications, thus retaining or enhancing the principal elemental characteristics while offering additional functionality from the added elements. We propose a similar approach to the control of properties of two-dimensional transition metal carbides known as MXenes. MXenes (Mn+1Xn) have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites presents promising routes for tailoring the chemical, optical, electronic, or mechanical properties of MXenes. Herein, we systematically investigated three interrelated binary solid-solution MXene systems based on Ti, Nb, and/or V at the M-site in a M2XTx structure (Ti2-yNbyCTx, Ti2-yVyCTx, and V2-yNbyCTx, where Tx stands for surface terminations) showing the evolution of electronic and optical properties as a function of composition. All three MXene systems show unlimited solubility and random distribution of metal elements in the metal sublattice. Optically, the MXene systems are tailorable in a nonlinear fashion, with absorption peaks from ultraviolet to near-infrared wavelength. The macroscopic electrical conductivity of solid solution MXenes can be controllably varied over 3 orders of magnitude at room temperature and 6 orders of magnitude from 10 to 300 K. This work greatly increases the number of nonstoichiometric MXenes reported to date and opens avenues for controlling physical properties of different MXenes with a limitless number of compositions possible through M-site solid solutions.

Tunable electrochromic behavior of titanium-based MXenes.

Two-dimensional transition metal carbides, nitrides and carbonitrides, popular by the name MXenes, are a promising class of materials as they exhibit intriguing optical, optoelectronic and electrochemical properties. Taking advantage of their metallic conductivity and hydrophilicity, titanium carbide MXenes (Ti3C2Tx and others) are used to fabricate solution processable transparent conducting electrodes (TCEs) for the design of three-electrode electrochromic cells. However, the tunable electrochromic behavior of various titanium-based MXene compositions across the entire visible spectrum has not yet been demonstrated. Here, we investigate the intrinsic electrochromic properties of titanium-based MXenes, Ti3C2Tx, Ti3CNTx, Ti2CTx, and Ti1.6Nb0.4CTx, where individual MXenes serve as a transparent conducting, electrochromic, and plasmonic material layer. Plasmonic extinction bands for Ti3C2Tx, Ti2CTx and Ti1.6Nb0.4CTx are centered at 800, 550 and 480 nm, which are electrochemically tunable to 630, 470 and 410 nm, respectively, whereas Ti3CNTx shows a reversible change in transmittance in the wide visible range. Additionally, the switching rates of MXene electrodes with no additional transparent conductor electrodes are estimated and correlated with the respective electrical figure of merit values. This study demonstrates that MXene-based electrochromic cells are tunable in the entire visible spectrum and suggests the potential of the MXene family of materials in optoelectronic, plasmonic, and photonic applications, such as tunable visible optical filters and modulators, to name a few.

Scalable Manufacturing of Free-Standing, Strong Ti3 C2 Tx MXene Films with Outstanding Conductivity.

Free-standing films that display high strength and high electrical conductivity are critical for flexible electronics, such as electromagnetic interference (EMI) shielding coatings and current collectors for batteries and supercapacitors. 2D Ti3 C2 Tx flakes are ideal candidates for making conductive films due to their high strength and metallic conductivity. It is, however, challenging to transfer those outstanding properties of single MXene flakes to macroscale films as a result of the small flake size and relatively poor flake alignment that occurs during solution-based processing. Here, a scalable method is shown for the fabrication of strong and highly conducting pure MXene films containing highly aligned large MXene flakes. These films demonstrate record tensile strength up to ≈570 MPa for a 940 nm thick film and electrical conductivity of ≈15 100 S cm-1 for a 214 nm thick film, which are both the highest values compared to previously reported pure Ti3 C2 Tx films. These films also exhibit outstanding EMI shielding performance (≈50 dB for a 940 nm thick film) that exceeds other synthetic materials with comparable thickness. MXene films with aligned flakes provide an effective route for producing large-area, high-strength, and high-electrical-conductivity MXene-based films for future electronic applications.

Beyond Ti3C2Tx: MXenes for Electromagnetic Interference Shielding.

New ultrathin and multifunctional electromagnetic interference (EMI) shielding materials are required for protecting electronics against electromagnetic pollution in the fifth-generation networks and Internet of Things era. Micrometer-thin Ti3C2Tx MXene films have shown the best EMI shielding performance among synthetic materials so far. Yet, the effects of elemental composition, layer structure, and transition-metal arrangement on EMI shielding properties of MXenes have not been explored, despite the fact that more than 30 different MXenes have been reported, and many more are possible. Here, we report on a systematic study of EMI shielding properties of 16 different MXenes, which cover single-metal MXenes, ordered double-metal carbide MXenes, and random solid solution MXenes of M and X elements. This is the largest set of MXene compositions ever reported in a comparative study. Films with thicknesses ranging from nanometers to micrometers were produced by spin-casting, spray-coating, and vacuum-assisted filtration. All MXenes achieved effective EMI shielding (>20 dB) in micrometer-thick films. The EMI shielding effectiveness of sprayed Ti3C2Tx film with a thickness of only ∼40 nm reaches 21 dB. Adjustable EMI shielding properties were achieved in solid solution MXenes with different ratios of elements. A transfer matrix model was shown to fit EMI shielding data for highly conductive MXenes but could not describe the behavior of materials with low conductivity. This work shows that many members of the large MXene family can be used for EMI shielding, contributing to designing ultrathin, flexible, and multifunctional EMI shielding films benefiting from specific characteristics of individual MXenes.

Ultralight and Mechanically Robust Ti3C2Tx Hybrid Aerogel Reinforced by Carbon Nanotubes for Electromagnetic Interference Shielding.

Lightweight materials with high electrical conductivity and robust mechanical properties are highly desirable for electromagnetic interference (EMI) shielding in modern portable and highly integrated electronics. Herein, a three-dimensional (3D) porous Ti3C2Tx/carbon nanotube (CNT) hybrid aerogel was fabricated via a bidirectional freezing method for lightweight EMI shielding application. The synergism of the lamellar and porous structure of the MXene/CNT hybrid aerogels contributed extensively to their excellent electrical conductivity (9.43 S cm-1) and superior electromagnetic shielding effectiveness (EMI SE) value of 103.9 dB at 3 mm thickness at the X-band frequency, the latter of which is the best value reported for synthetic porous nanomaterials. The CNT reinforcement in the MXene/CNT hybrid aerogels enhanced the mechanical robustness and increased the compressional modulus by 9661% relative to that of the pristine MXene aerogel. The hybrid aerogel with high electrical conductivity, good mechanical strength, and superior EMI shielding performance is a promising material for inhibiting EMI pollution.

Ultralight MXene-Coated, Interconnected SiCnws Three-Dimensional Lamellar Foams for Efficient Microwave Absorption in the X-Band.

Two-dimensional (2D) few-layered Ti3C2T X MXene (f-Ti3C2T X) has been proved to be one of the most promising electromagnetic interference (EMI) materials, but its electromagnetic (EM) absorption properties and loss mechanism have not been studied so far. Herein, for the first time, ordered lamellar f-Ti3C2T X/SiCnws hybrid foams with ultralow density are synthesized by a combination of self-assembly and bidirectional freezing processes. The freestanding foams exhibit excellent EM absorption properties superior to most of the current foam-based counterparts. The effective absorption bandwidth is always able to cover the whole X-band, when the sample thicknesses of f-Ti3C2T X/SiCnws hybrid foams distribute in any value between 3.5 and 3.8 mm, and the minimum reflection coefficient reaches -55.7 dB at an ultralow density of only about 0.029 g·cm-3. The fundamental mechanism associated with optimized impedance matching, enhanced polarization loss, and conductive loss is discussed in detail. Our results evidence that 2D flexible f-Ti3C2T X MXene has great potential in EM absorption field like graphene.

Broadband Microwave Absorbing Composites with a Multi-Scale Layered Structure Based on Reduced Graphene Oxide Film as the Frequency Selective Surface.

A broadband microwave absorbing composite with a multi-scale layered structure is proposed, in which a reduced graphene oxide (RGO) film sandwiched between two layers of epoxy glass fiber laminates serves as the frequency selective surface (FSS). RGO films with the desired electrical properties were synthesized directly by hydrothermal reaction, vacuum filtration, and heat treatment without subsequent processing. With the novel layer-by-layer structure ranging from micro to macro scale, the optimized composite exhibits excellent microwave absorption performance with a total thickness of 3.2 mm. Its reflection coefficient (RC) is less than -10 dB in the entire X and Ku band, reaching a minimum value of -32 dB at 10.2 GHz and an average RC of -22.8 dB from 8 to 18 GHz. Enhanced microwave absorption of the composites is achieved through the optimization of layer thickness in the sandwich structure to promote destructive interference. Improved impedance matching by the introduction of FSS along with the polarization and conduction loss of layered graphene films also contribute to the increased absorption.

Laminated and Two-Dimensional Carbon-Supported Microwave Absorbers Derived from MXenes.

Microwave absorbers with layered structures that can provide abundant interfaces are highly desirable for enhancing electromagnetic absorbing capability and decreasing the thickness. The atomically thin layers of two-dimensional (2D) transition-metal carbides (MXenes) make them a convenient precursor for synthesis of other 2D and layered structures. Here, laminated carbon/TiO2 hybrid materials composed of well-aligned 2D carbon sheets with embedded TiO2 nanoparticles were synthesized and showed excellent microwave absorption. Disordered 2D carbon layers with an unusual structure were obtained by annealing multilayer Ti3C2 MXene in a CO2 atmosphere. The minimum reflection coefficient of laminated carbon/TiO2 composites reaches -36 dB, and the effective absorption bandwidth ranges from 3.6 to 18 GHz with the tunable thickness from 1.7 to 5 mm. The effective absorption bandwidth covers the whole Ku band (12.4-18 GHz) when the thickness of carbon/TiO2/paraffin composite is 1.7 mm. This study is expected to pave the way to the synthesis of carbon-supported absorbing materials using a large family of 2D carbides.

Flexible and Thermostable Graphene/SiC Nanowire Foam Composites with Tunable Electromagnetic Wave Absorption Properties.

Three-dimensional (3D) flexible foams consisting of reduced graphene oxides (rGO) and in situ grown SiC nanowires (NWs) were prepared using freeze-drying and carbothermal reduction processes. By means of incorporating SiC nanowires into rGO foams, both the thermostability and electromagnetic absorption of the composites were improved. It was demonstrated that rGO/SiC NW foams were thermostable beyond ∼630 °C (90% weight retention in air atmosphere). As expected, rGO/SiC NW foams in the poly(dimethylsiloxane) matrix achieved effective absorption in the entire X-band (8.2-12.4 GHz) with a thinner thickness (3 mm) in comparison with those of the pure rGO foams. It is revealed that SiC nanowires with abundant stacking faults, twinning interfaces, and bridged junctions play an important role in the enhanced electromagnetic absorption performance, in addition to the contribution of interconnected graphene networks. Hierarchical rGO/SiC NW foams not only are efficient absorbers in the critical environments but also can be applied in photocatalytic and thermal-management fields.

Carbon Hollow Microspheres with a Designable Mesoporous Shell for High-Performance Electromagnetic Wave Absorption.

In this work, mesoporous carbon hollow microspheres (PCHMs) with designable mesoporous shell and interior void are constructed by a facile in situ stöber templating approach and a pyrolysis-etching process. The PCHMs are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectra, Raman spectroscopy, and nitrogen adsorption and desorption system. A uniform mesoporous shell (pore size 4.7 nm) with a thickness of 55 nm and a cavity size of 345 nm is realized. The composite of paraffin mixed with 20 wt % PCHMs exhibits a minimum reflection coefficient (RCmin) of -84 dB at 8.2 GHz with a sample thickness of 3.9 mm and an effective absorption bandwidth (EAB) of 4.8 GHz below -10 dB (>90% electromagnetic wave is attenuated). Moreover, the composite of phenolic resin mixed with 20 wt % PCHMs exhibits an ultrawide EAB of 8 GHz below -10 dB with a thinner thickness of 2.15 mm. Such excellent electromagnetic wave absorption properties are ascribed to the large carbon-air interface in the mesoporous shell and interior void, which is favorable for the matching of characteristic impedance as compared with carbon hollow microspheres and carbon solid microspheres. Considering the excellent performance of PCHMs, we believe the as-fabricated PCHMs can be promising candidates as highly effective microwave absorbers, and the design philosophy can be extended to other spherical absorbers.

Ti3C2 MXenes with Modified Surface for High-Performance Electromagnetic Absorption and Shielding in the X-Band.

Electromagnetic (EM) absorbing and shielding composites with tunable absorbing behaviors based on Ti3C2 MXenes are fabricated via HF etching and annealing treatment. Localized sandwich structure without sacrificing the original layered morphology is realized, which is responsible for the enhancement of EM absorbing capability in the X-band. The composite with 50 wt % annealed MXenes exhibits a minimum reflection loss of -48.4 dB at 11.6 GHz, because of the formation of TiO2 nanocrystals and amorphous carbon. Moreover, superior shielding effectiveness with high absorption effectiveness is achieved. The total and absorbing shielding effectiveness of Ti3C2 MXenes in a wax matrix with a thickness of only 1 mm reach values of 76.1 and 67.3 dB, while those of annealed Ti3C2 MXenes/wax composites are 32 and 24.2 dB, respectively. Considering the promising performance of Ti3C2 MXenes with the modified surface, this work is expected to open the door for the expanded applications of MXenes family in EM absorbing and shielding fields.