Ultrafast Interfacial Self-Assembly of 2D Transition Metal Dichalcogenides Monolayer Films and Their Vertical and In-Plane Heterostructures

Interfacial Self-Assembly of Oriented Semiconductor Monolayer for Chemiresistive Sensing

Semiconductor nanofilm fabrication with advanced technology is of great importance for next-generation electronics/optoelectronics. Fabrication of high-quality and perfectly oriented semiconductor thin films and integration into high-performance electronic devices with low cost and high efficiency are huge challenges. Here we exquisitely utilized the Marangoni effect to perfectly guide tin disulfide (SnS2) nanocoins into an ordered assembly in milliseconds, resulting in an uniaxial-oriented monolayer semiconductor film. Further exploration revealed that the formed "crumple zone" at the interface caused by the Marangoni force endows the nanofilm with a rapid healable capability, which can be easily transferred to arbitrary substrates. As a proof of concept, the nanocoin-monolayer was transferred onto a micro-interdigitated electrode substrate to form a high-performance chemiresistive sensor that can effectively monitor the trace amounts of toxic gases. In addition, the assembled monolayer nanofilms can be conformally printed on freeform surfaces: both flat and nonflat substrates. This efficient and low-cost Marangoni force-assisted surface self-assembly (MFA-SSA) strategy is promising for advanced microelectronics and real industrial applications.

Robust Ti3C2Tx MXene foam modified with natural antioxidants for long-term effective electromagnetic interference shielding

MXenes have been proven to be outstanding lossy phase of advanced electromagnetic interference (EMI) shielding materials. However, their poor tolerance to oxygen and water results in fast degradation of the pristine two-dimensional (2D) nanostructure and fading of the functional performance. Herein, in this research, natural antioxidants (e.g., melatonin, tea polyphenols, and phytic acid) were employed to protect the Ti3C2Tx MXene from its degradation in order to achieve a long-term stability of the EMI shielding performance. The results showed that the synthesized composites comprised of antioxidants and Ti3C2Tx exhibited a decelerating degradation rate resulting in an improved EMI shielding effective (SE) stability. The antioxidation mechanism of the applied antioxidants is discussed with respect to the nanostructure evolution of the Ti3C2Tx MXene. This work contributes to the basic foundations for the further development of advanced MXenes for stable applications in the EM field.

Application of 2D Materials for Adsorptive Removal of Air Pollutants

Air pollution is on the priority list of global safety issues, with the concern of fatal environmental and public health deterioration. 2D materials are potential adsorbent materials for environmental decontamination, owing to their high surface area, manageable interlayer binding, large surface-to-volume ratio, specific binding capability, and chemical, thermal, and mechanistic stability. Specifically, graphene oxide and reduced graphene oxide have been attracting attention, taking advantage of their low cost synthesis, excessive oxygen containing surface functionalities, and intrinsic aqueous dispersibility, making them desirable for the development of cost-effective, high performance air filters. Many different material designs have been proposed to expand their filtration capability, including the functionalization and integration with other metals and metal oxides, which act not only as binding agents to the target pollutants but also as antimicrobial agents. This review highlights the advantages and drawbacks of 2D materials for air filtration and summarizes the interrelationships among various strategies and the resultant filtration performance in terms of structural engineering, morphology control, and material compositions. Finally, potential future directions are suggested toward the idealized designs of 2D material based air filters.

Homogeneously niobium-doped MoS2 for rapid and high-sensitive detection of typical chemical warfare agents

Rapid detection of Chemical Warfare Agents (CWAs) is of great significance in protecting civilians in public places and military personnel on the battlefield. Two-dimensional (2D) molybdenum disulfide (MoS2) nanosheets (NSs) can be integrated as a gas sensor at room temperature (25°C) due to their large specific surface area and excellent semiconductor properties. However, low sensitivity and long response-recovery time hinder the pure MoS2 application in CWAs gas sensors. In this work, we developed a CWAs sensor based on in-situ niobium-doped MoS2 NSs (Nb-MoS2 NSs) via direct chemical-vapor-deposition (CVD) growth. Characterization results show that the high content of Nb elements (7.8 at%) are homogeneously dispersed on the large-area 2D structure of MoS2. The Nb-MoS2 NSs-based CWAs sensor exhibits higher sensitivity (−2.09% and −3.95% to 0.05 mg/m3 sarin and sulfur mustard, respectively) and faster response speed (78 s and 30 s to 0.05 mg/m3 sarin and sulfur mustard, respectively) than MoS2 and other 2D materials at room temperature. And the sensor has certain specificity for sarin and sulfur mustard and is especially sensitive to sulfur mustard. This can be attributed to the improvement of adsorption properties via electronic regulation of Nb doping. This is the first report about CWAs detection based on two-dimensional (2D) transition metal dichalcogenides (TMDs) sensing materials, which demonstrates that the high sensitivity, rapid response, and low limit of detection of 2D TMDs-based CWAs sensor can meet the monitoring needs of many scenarios, thus showing a strong application potential.

Recent progress in 2D hybrid heterostructures from transition metal dichalcogenides and organic layers: properties and applications in energy and optoelectronics fields

Atomically thin transition metal dichalcogenides (TMDs) present extraordinary optoelectronic, electrochemical, and mechanical properties that have not been accessible in bulk semiconducting materials. Recently, a new research field, 2D hybrid heteromaterials, has emerged upon integrating TMDs with molecular systems, including organic molecules, polymers, metal-organic frameworks, and carbonaceous materials, that can tailor the TMD properties and exploit synergetic effects. TMD-based hybrid heterostructures can meet the demands of future optoelectronics, including supporting flexible, transparent, and ultrathin devices, and energy-based applications, offering high energy and power densities with long cycle lives. To realize such applications, it is necessary to understand the interactions between the hybrid components and to develop strategies for exploiting the distinct benefits of each component. Here, we provide an overview of the current understanding of the new phenomena and mechanisms involved in TMD/organic hybrids and potential applications harnessing such valuable materials in an insightful way. We highlight recent discoveries relating to multicomponent hybrid materials. Finally, we conclude this review by discussing challenges related to hybrid heteromaterials and presenting future directions and opportunities in this research field.

Structural evolution of MXenes and their composites for electromagnetic interference shielding applications

Nowadays, the extensive utilization of electronic devices and equipment inevitably leads to severe electromagnetic interference (EMI) issues. Therefore, EMI shielding materials have drawn considerable attention, and great effort has been devoted to the exploration of high-efficiency EMI shielding materials. As a novel kind of 2D transition metal carbide material, MXenes have been widely investigated for EMI shielding in the past few years due to their extraordinary electrical conductivity, large specific surface area, light weight, and easy processability. In view of the great achievements in MXene-based materials for EMI shielding, herein, we reviewed the recent studies on the structural design and evolution of MXenes and their composites for EMI shielding. First, the methods for structural control of MXenes, including HF etching, in situ HF etching, fluorine-free etching, electrochemical etching, and molten salt etching, are systematically summarized. Then we illustrate the fundamental relationship between the microstructure of MXenes and the EMI shielding mechanism. In the following, the effects of different synthesis methods and structures of MXene-based composite materials as well as their EMI shielding performances are comprehensively discussed. Lastly, future prospects for the development of MXene-based composite materials in EMI shielding applications are commented on.

Layer-dependent nonlinear optical properties of two-dimensional InSe and its applications in waveguide lasers

The thickness-dependent third-order nonlinear optical properties of two-dimensional β-InSe and its potential applications as a saturable absorber in pulsed laser generation are investigated. InSe sheets with different layers are prepared by the chemical vapor deposition. Using open-aperture femtosecond Z-scan technique at 1030 nm, the modulation depth and nonlinear absorption coefficient are obtained to be 36% and -1.6 × 10⁴ cm·GW^-1, respectively. The intrinsic mechanism of the layer-dependent energy band structure evolution is analyzed based on density functional theory, and the theoretical analysis is consistent with the experimental results. Based on a waveguide cavity, a Q-switched mode-locked laser at 1 µm with a repetition frequency of 8.51 GHz and a pulse duration of 28 ps is achieved by utilizing the layered InSe as a saturable absorber. This work provides an in-depth understanding of layer-dependent properties of InSe and extends its applications in laser technology for compact light devices.

Lateral layered semiconductor multijunctions for novel electronic devices

Layered semiconductors, represented by transition metal dichalcogenides, have attached extensive attention due to their unique and tunable electrical and optical properties. In particular, lateral layered semiconductor multijunctions, including homojunctions, heterojunctions, hybrid junctions and superlattices, present a totally new degree of freedom in research on electronic devices beyond traditional materials and their structures, providing unique opportunities for the development of new structures and operation principle-based high performance devices. However, the advances in this field are limited by the precise synthesis of high-quality junctions and greatly hampered by ambiguous device performance limits. Herein, we review the recent key breakthroughs in the design, synthesis, electronic structure and property modulation of lateral semiconductor multijunctions and focus on their application-specific devices. Specifically, the synthesis methods based on different principles, such as chemical and external source-induced methods, are introduced stepwise for the controllable fabrication of semiconductor multijunctions as the basics of device application. Subsequently, their structure and property modulation are discussed, including control of their electronic structure, exciton dynamics and optical properties before the fabrication of lateral layered semiconductor multijunction devices. Precise property control will potentially result in outstanding device performances, including high-quality diodes and FETs, scalable logic and analog circuits, highly efficient optoelectronic devices, and unique electrochemical devices. Lastly, we focus on several of the most essential but unresolved debates in this field, such as the true advantages of few-layer vs. monolayer multijunctions, how sharp the interface should be for specific functional devices, and the superiority of lateral multijunctions over vertical multijunctions, highlighting the next-phase strategy to enhance the performance potential of lateral multijunction devices.

Exfoliation of Al-Residual Multilayer MXene Using Tetramethylammonium Bases for Conductive Film Applications

This study describes the concise exfoliation of multilayer Ti₃C₂T_x MXene containing residual aluminum atoms. Treatment with tetramethylammonium base in a co-solvent of tetrahydrofuran and H₂O produced single-layer Ti₃C₂T_x, which was confirmed via atomic force microscopy observations, with an electrical conductivity 100+ times that of Ti₃C₂T_x prepared under previously reported conditions. The scanning electron microscopy and X-ray diffraction measurements showed that the exfoliated single-layer Ti₃C₂T_x MXenes were reconstructed to assembled large-domain layered films, enabling excellent macroscale electric conductivity. X-ray photoelectron spectroscopy confirmed the complete removal of residual Al atoms and the replacement of surface fluorine atoms with hydroxy groups. Using the exfoliated dispersion, a flexible transparent conductive film was formed and demonstrated in an electrical application.

Optical-Transparent Self-Assembled MXene Film with High-Efficiency Terahertz Reflection Modulation

The modulation of the terahertz (THz) wave is fundamental for its applications in next-generation communications, biological imaging, sensing, and so forth. Searching for higher efficient modulation is still in progress, although plenty of materials have been explored for tuning THz wave. In this work, optical-transparent self-assembled MXene films are used to modulate the THz reflection at the SiO₂/MXene/air interface based on the impedance matching mechanism. By adjusting the number of stacked MXene layers/concentrations of MXene dispersions, the sheet conductivity of the MXene films will be changed so that the impedance at the SiO₂/MXene/air interface can be tuned and lead to a giant modulation of THz reflection. Particularly, we demonstrate that the MXene films have highly efficient THz modulation from antireflection to reflection-enhancing with a relative reflection of 27% and 406%, respectively. This work provides a new pathway for developing the MXene films with the combination of optical-transparency and high smart THz reflection characteristics, and the films can be applied for THz antireflection or reflection-enhancing.

Wafer-Scale Lateral Self-Assembly of Mosaic Ti3C2Tx MXene Monolayer Films

Bottom-up assembly of two-dimensional (2D) materials into macroscale morphologies with emergent properties requires control of the material surroundings, so that energetically favorable conditions direct the assembly process. MXenes, a class of recently developed 2D materials, have found new applications in areas such as electrochemical energy storage, nanoscale electronics, sensors, and biosensors. In this paper, we present a lateral self-assembly method for wafer-scale deposition of a mosaic-type 2D MXene flake monolayer that spontaneously orders at the interface between two immiscible solvents. ReaxFF molecular dynamics simulations elucidate the interactions of a MXene flake with the solvents and its stability at the liquid/liquid interface, the prerequisite for MXene flakes self-assembly at the interface. Moreover, facile transfer of this monolayer onto a flat substrate (Si, glass) results in high-coverage monolayer films with uniform thickness and homogeneous optical properties. Multiscale characterization of the resulting films reveals the mosaic structure and sheds light on the electronic properties of the films, which exhibit good electrical conductivity over cm-scale areas.

Nanoscale Assembly of 2D Materials for Energy and Environmental Applications

Rational design of 2D materials is crucial for the realization of their profound implications in energy and environmental fields. The past decade has witnessed significant developments in 2D material research, yet a number of critical challenges remain for real-world applications. Nanoscale assembly, precise control over the orientational and positional ordering, and complex interfaces among 2D layers are essential for the continued progress of 2D materials, especially for energy storage and conversion and environmental remediation. Herein, recent progress, the status, future prospects, and challenges associated with nanoscopic assembly of 2D materials are highlighted, specifically targeting energy and environmental applications. Geometric dimensional diversity of 2D material assembly is focused on, based on novel assembly mechanisms, including 1D fibers from the colloidal liquid crystalline phase, 2D films by interfacial tension (Marangoni effect), and 3D nanoarchitecture assembly by electrochemical processes. Relevant critical advantages of 2D material assembly are highlighted for application fields, including secondary batteries, supercapacitors, catalysts, gas sensors, desalination, and water decontamination.

Electromagnetic Shielding of Monolayer MXene Assemblies

Miniaturization of electronics demands electromagnetic interference (EMI) shielding of nanoscale dimension. The authors report a systematic exploration of EMI shielding behavior of 2D Ti₃ C₂ T_x MXene assembled films over a broad range of film thicknesses, monolayer by monolayer. Theoretical models are used to explain the shielding mechanism below skin depth, where multiple reflection becomes significant, along with the surface reflection and bulk absorption of electromagnetic radiation. While a monolayer assembled film offers ≈20% shielding of electromagnetic waves, a 24-layer film of ≈55 nm thickness demonstrates 99% shielding (20 dB), revealing an extraordinarily large absolute shielding effectiveness (3.89 × 10⁶ dB cm² g^-1 ). This remarkable performance of nanometer-thin solution processable MXene proposes a paradigm shift in shielding of lightweight, portable, and compact next-generation electronic devices.

Intact Crystalline Semiconducting Graphene Nanoribbons from Unzipping Nitrogen-Doped Carbon Nanotubes

Unzipping carbon nanotubes (CNTs) may offer a valuable route to synthesize graphene nanoribbon (GNR) structures with semiconducting properties. Unfortunately, currently available unzipping methods commonly rely on a random harsh chemical reaction and thereby cause significant degradation of the crystalline structure and electrical properties of GNRs. Herein, crystalline semiconducting GNRs are achieved by a synergistic, judiciously designed two-step unzipping method for N-doped CNTs (NCNTs). NCNTs are effectively unzipped by damage-minimized, dopant-specific electrochemical unzipping and subsequent sonochemical treatment into long ribbon-like nanostructures with crystalline basal planes. Owing to the nanoscale dimension originating from the dense nucleation of the unzipping reaction at highly NCNTs, the resultant GNRs demonstrate semiconducting properties, which can be exploited for chemiresistor-type gas-sensing devices and many other applications.

Hydrophilic/Hydrophobic Interphase-Mediated Bubble-like Stretchable Janus Ultrathin Films toward Self-Adaptive and Pneumatic Multifunctional Electronics

As promising candidates for intelligent biomimetic applications similar to living organisms, smart soft materials have aroused extensive interest due to their extraordinarily designable structures and functionality. Herein, a bubble-like elastomer-based electronic skin that can be pneumatically actuated is achieved through hydrophilic/hydrophobic interphase mediated asymmetric functionalization. The asymmetric and controllable introduction of elastic polydimethylsiloxane into the carbon nanotube film at the air/water interface can endow the Janus ultrathin film with tunable conductivity, self-adhesivity, self-adaptivity, and even self-sealing properties. As a result, the Janus films can be employed as multifunctional electronics, including self-adhesive strain sensing/thermal managing devices and even noncontact mechanical sensors as artificial eardrums for tiny air-pressure detection. Significantly, these excellent features can further enable the integration of actuating and sensing functions. As a proof of concept, the Janus film can serve as a self-supported device to simultaneously imitate the controllable contracting/expanding behaviors of the vocal sac of frog and monitor the real-time current change in this process, demonstrating significant potential in smart bionic applications.

High-Throughput Computational Screening of Vertical 2D van der Waals Heterostructures for High-efficiency Excitonic Solar Cells

As an effort to identify van der Waals heterostructures for efficient excitonic solar cell application, high-throughput computational screening was carried out to study the band alignments of 1540 vertical heterostructures formed by 56 two-dimensional semiconducting/insulating materials. More than 90 heterostructures with estimated power conversion efficiency (PCE) higher than 15% have been identified, of which 17 heterostructures are predicted to have PCE higher than the best value (20%) reported or proposed in the literature.

Superlubricity between MoS2 Monolayers

The ultralow friction between atomic layers of hexagonal MoS₂ , an important solid lubricant and additive of lubricating oil, is thought to be responsible for its excellent lubricating performances. However, the quantitative frictional properties between MoS₂ atomic layers have not been directly tested in experiments due to the lack of conventional tools to characterize the frictional properties between 2D atomic layers. Herein, a versatile method for studying the frictional properties between atomic-layered materials is developed by combining the in situ scanning electron microscope technique with a Si nanowire force sensor, and the friction tests on the sliding between atomic-layered materials down to monolayers are reported. The friction tests on the sliding between incommensurate MoS₂ monolayers give a friction coefficient of ≈10^-4 in the regime of superlubricity. The results provide the first direct experimental evidence for superlubricity between MoS₂ atomic layers and open a new route to investigate frictional properties of broad 2D materials.

Kinetically Controlled Layer-by-Layer Stacking of Metal Oxide 2D Nanosheets

An efficient chemical way to finely control the layer-by-layer stacking of inorganic nanosheets (NS) is developed by tuning the type and composition of intercalant ion, and the reaction temperature for restacking process. The finely controlled stacking of NS relies on a kinetic control of the self-assembly of NS in the presence of coordinating organic cations. A critical role of organic cations in this assembly highlights the importance of the appropriate activation energy. Of prime importance is that a fine-control of the interstratification of 2D NS is highly effective not only in tailoring its pore structure but also in enhancing its electrode activity. The present study clearly demonstrates that the kinetically controlled restacking of NS provides a facile and powerful method to tailor their stacking number and functionality.