Jiuxiang Dai

Nature Electronics, Published online: 28 June 2021; doi:10.1038/s41928-021-00602-z

The bandgaps of bilayers of two-dimensional C3N can be modulated by controlling the stacking order of the layers or by applying an electric field.

Publication date: November 2021

Source: Materials Today, Volume 50

Author(s): Aparna Murali, Giriraj Lokhande, Kaivalya A. Deo, Anna Brokesh, Akhilesh K. Gaharwar

Nature Chemistry, Published online: 21 June 2021; doi:10.1038/s41557-021-00721-2

Although monolayers of N-heterocyclic carbenes (NHCs) readily form on metals, surface reactivity usually hinders their self-assembly on semiconductors. Now, it has been shown that thermally stable, well-ordered monolayers of NHCs can be formed on silicon surfaces. A large reduction in work function is observed and steric effects enable sufficient diffusivity of the NHCs.

Photodetectors based on nano-structured superconducting thin films are currently some of the most sensitive quantum sensors and are key enabling technologies in such broad areas as quantum information, quantum computation and radio-astronomy. However, their broader use is held back by the low operation temperatures which require expensive cryostats. Here, we demonstrate a high- T c superconducting photodetector, which shows orders of magnitude improved performance characteristics of any superconducting detector operated above 77 K, with a responsivity of 9.61 × 10 4 V W −1 , theoretically achievable noise equivalent power of 15.9 fW Hz 1/2 and nanosecond relaxation times. At 15 K the detector reaches an ultra-high performance of 2.33 × 10 7 V W −1 and 55.2 aW Hz 1/2 . It is based on van der Waals heterostructures of the high temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+

Among the large variety of two-dimensional (2D) materials discovered to date, elemental monolayers that host superconductivity are very rare. Using ab initio calculations we show that recently synthesized gallium monolayers, coined gallenene, are intrinsically superconducting through electron–phonon coupling. We reveal that Ga-100 gallenene, a planar monolayer isostructural with graphene, is the structurally simplest 2D superconductor to date, furthermore hosting topological edge states due to its honeycomb structure. Our anisotropic Eliashberg calculations show distinctly three-gap superconductivity in Ga-100, in contrast to the alternative buckled Ga-010 gallenene which presents a single anisotropic superconducting gap. Strikingly, the critical temperature ( T c ) of gallenene is in the range of 7–10 K, exceeding the T c of bulk gallium from which it is exfoliated. Finally we explore chemical functionalization of galle...

Graphene is a material displaying exceptional properties; however, its full potential emerges when it is combined with other nanomaterials and molecules forming hybrid materials. This review presents a comprehensive overview on the graphene-based hybrids, with a focus on the main synthetic methods and on the applications of these hybrids in the most diverse fields.

Abstract

Graphene is a 2D material combining numerous outstanding physical properties, including high flexibility and strength, extremely high thermal conductivity and electron mobility, transparency, etc., which make it a unique testbed to explore fundamental physical phenomena. Such physical properties can be further tuned by combining graphene with other nanomaterials or (macro)molecules to form hybrid functional materials, which by design can display not only the properties of the individual components but also exhibit new properties and enhanced characteristics arising from the synergic interaction of the components. The implementation of the hybrid approach to graphene also allows boosting the performances in a multitude of technological applications. This review reports the hybrids formed by graphene combined with other low-dimensional nanomaterials of diverse dimensionality (0D, 1D, and 2D) and (macro)molecules, with emphasis on the synthetic methods. The most important applications of these hybrids in the fields of sensing, water purification, energy storage, biomedical, (photo)catalysis, and opto(electronics) are also reviewed, with a special focus on the superior performances of these hybrids compared to the individual, nonhybridized components.

Soda-lime glass presents a smooth “liquid-phase” surface at high temperature (≈900 °C), which is helpful for the fast growth of large-size monolaye MoSe₂ single crystals. The unique “beam-bridge” assisted metal-precursor feeding strategy contributes to sufficient, homogeneous release of Mo-based precursor, thereby leading to the ultrafast growth (≈50 μm s⁻¹) of monolayer MoSe₂ single crystals of millimeter-size, as presented in article number 2100415 by Xiaolong Zou, Fenghua Chen, Yanfeng Zhang, and co-workers.

A trace amount of Na cations, which remained at the SiO₂ substrate of a NaCl-assisted large-scale uniform MoS₂ monolayer, can be the origin of the n-type doping. The residual Na cations are electrically moved toward the bottom side of the MoS₂ monolayer to passivate the interfacial defects achieving the quasi-intrinsic semiconducting state.

Abstract

Alkali metal halide-assisted chemical vapor deposition (CVD) methods can produce wafer-scale uniform monolayer transition metal dichalcogenides (TMDs). Further defect engineering is necessary to obtain high-performance functional devices. While defect engineering has focused on the surface of the monolayer TMDs or the contact property, interface defect engineering is rare and non-trivial. Based on a NaCl-assisted CVD-grown large-scale uniform MoS₂ monolayer on SiO₂/Si substrate, a trace amount of Na cations is present, residing at the SiO₂ substrate during the CVD-growth process and contributes to the n-type doping into the supported monolayer MoS₂. Furthermore, the residual Na cations are electrically moved toward the bottom side of monolayer MoS₂ to passivate the interfacial defects.

A thinnest light disk with encryption functionality is realized on WS₂ monolayers. The writing-in and reading-out of information are enabled by the directly controlling of fluorescence contrast of WS₂ monolayers. The writing speed can be ≈6.25 MB s⁻¹, while the data storage density can reach ≈62.5 MB cm⁻² within the thickness of <1 nm.

Abstract

The thinnest light disk is demonstrated at the atomic level by developing an erasable method to directly write encrypted information onto WS₂ monolayers. The write-in is realized by precise control of photoluminescence emission by means of ozone functionalization and scanning focused laser beam. The visual decryption and reading-out of information are enabled by fluorescence contrast. The high encryption level is ensured by the threshold power upon which the data deletion will be triggered. Owing to the high spatial resolution and power sensitivity, the storage capacity within <1 nm thickness can be up to ≈62.5 MB cm⁻², and the writing speed can reach ≈6.25 MB s⁻¹. Density functional theory calculations suggest that the disk formatting is realized by ozone molecule adsorption induced localized unoccupied states, while the read-in relies on the passivation of defects via substitution of the sulfur vacancies with oxygen atoms. The results of this study promote data storage and encryption on the atomic scale.

Recent progress in fabricating 2D multi-heterostructures by chemical vapor deposition method is presented.

Abstract

The past few years have witnessed significant development in the controlled growth of 2D heterostructures. Among those kinds of heterostructures, vertical and lateral ones have drawn the most attention. Vertical heterostructures can be created in the mode of layer by layer. The layer number and sequence in the vertical orientation can be modulated and thus leading to customized properties. However, the fabrication of lateral heterostructures has been met with challenges. The most concerning issue is related to the consistency at the atomic scale when two layers are stitched in a lateral direction. Adhering to the concept of epitaxial growth, chemical vapor deposition (CVD) has exerted significant impact in forming 2D lateral heterostructures. In this review, recent academic breakthroughs involving controlled growth of multi-heterostructures by CVD are present. The CVD technique in terms of growth parameters, choice of catalysts, and mechanism is fully emphasized, offering guidelines for shaping novel 2D heterostructures. Several novel multi-heterostructures attained by the CVD method are exhibited. Further, the properties and devices are described to demonstrate the unique features of multi-heterostructures. The great advances in precisely constructing multi-heterostructure are expected to push forward the way for 2D materials to industrialization and commercialization.

This review summarizes recent research on TMNs-based materials in various photocatalytic applications including water splitting, CO₂ reduction, and dye degradation. Different roles of TMNs materials in photocatalytic systems such as semiconductor active components, co-catalysts, inter-band excitation, and surface plasmon resonance components are systematically discussed and summarized.

Abstract

Photocatalysis is a promising and convenient strategy to convert solar energy into chemical energy for various fields. However, photocatalysis still suffers from low solar energy conversion efficiency. Developing state of the art photocatalysts with high efficiency and low cost is a huge challenge. Transition metal nitrides (TMNs) as a class of metallic interstitial compounds have attracted significant attention in photocatalytic applications. In fact, TMNs exhibit multifunctional properties in various photocatalytic systems. This review is the first attempt that summarizes recent research on TMNs-based materials in various photocatalytic applications. Different roles of TMNs materials in photocatalytic systems including semiconductor active components, co-catalysts, inter-band excitation, and surface plasmon resonance components are systematically discussed and summarized. The fundamentals, latest progress, and emerging opportunities for further improving the performances of TMNs-based materials for photocatalysis are also discussed. Finally, some challenges facing TMNs, and perspectives on their future that are relevant for furthering research in the area of photocatalysis are also proposed.

In field-effect transistors with solid ionic conductors as the gate dielectric, the easy-axis of the ferromagnetism of Cr₂Ge₂Te₆ thin flakes can be uniformly tuned from the out-of-plane direction to the in-plane direction by an electric field, coinciding with a significant increase of the Curie temperature. The surface of the sample is fully exposed in this type of devices, making further heterostructure-engineering possible.

Abstract

The discovery of magnetism in 2D materials offers new opportunities for exploring novel quantum states and developing spintronic devices. In this work, using field-effect transistors with solid ion conductors as the gate dielectric (SIC-FETs), we have observed a significant enhancement of ferromagnetism associated with magnetic easy-axis switching in few-layered Cr₂Ge₂Te₆. The easy axis of the magnetization, inferred from the anisotropic magnetoresistance, can be uniformly tuned from the out-of-plane direction to an in-plane direction by electric field in the few-layered Cr₂Ge₂Te₆. Additionally, the Curie temperature, obtained from both the Hall resistance and magnetoresistance measurements, increases from 65 to 180 K in the few-layered sample by electric gating. Moreover, the surface of the sample is fully exposed in the SIC-FET device configuration, making further heterostructure-engineering possible. This work offers an excellent platform for realizing electrically controlled quantum phenomena in a single device.

High-entropy transition metal dichalcogenide alloys containing 4 or 5 transition metals are synthesized based on first-principles stability predictions. The 5-component alloy (MoWVNbTa)S₂ is shown to be an excellent electrocatalyst for the conversion of CO₂ into CO. First-principles calculations suggest that a small concentration of highly active sites is responsible for the high activity.

Abstract

High-entropy alloys combine multiple principal elements at a near equal fraction to form vast compositional spaces to achieve outstanding functionalities that are absent in alloys with one or two principal elements. Here, the prediction, synthesis, and multiscale characterization of 2D high-entropy transition metal dichalcogenide (TMDC) alloys with four/five transition metals is reported. Of these, the electrochemical performance of a five-component alloy with the highest configurational entropy, (MoWVNbTa)S₂, is investigated for CO₂ conversion to CO, revealing an excellent current density of 0.51 A cm⁻² and a turnover frequency of 58.3 s⁻¹ at ≈ −0.8 V versus reversible hydrogen electrode. First-principles calculations show that the superior CO₂ electroreduction is due to a multi-site catalysis wherein the atomic-scale disorder optimizes the rate-limiting step of CO desorption by facilitating isolated transition metal edge sites with weak CO binding. 2D high-entropy TMDC alloys provide a materials platform to design superior catalysts for many electrochemical systems.

In article number 2003832 by Jian Zhou, Ju Li, and co-workers, optomechanical theory is derived and applied to predict terahertz optics driven ultrafast and reversible topological phase transitions in transition metal dichalcogenide monolayers, which is a damage-free and non-contacting approach that compensates other strategies. The cover displays such a novel optical strategy to trigger a thermal topological phase transition.

This work provides a facile strategy for efficient high-temperature thermal camouflage using ultrathin MXene films/coatings; the performance of the MXene films/coatings is almost comparable to that of stainless steel and superior to that of other 2D nanomaterials as well as other reported film-/coating-based thermal camouflage materials/systems. The results of this work demonstrate the great promise of MXene materials for thermal camouflage, infrared stealth, counter-surveillance, and security protection.

Abstract

Thermal camouflage has attracted increasing attention owing to the rapid development of infrared (IR) surveillance technologies. Various materials and systems have been developed to date, but the realization of high-temperature thermal camouflage using ultrathin film/coating remains a great challenge; this is of great significance, especially for IR stealth in military equipment. This work demonstrates a series of ultrathin Ti₃C₂T _x MXene films (as low as 1 µm) with superior high-temperature indoor/outdoor thermal camouflage performance: wide camouflage temperature range (from below −10 °C to over 500 °C), large reduction in radiation temperature (exceeding 300 °C for objects with temperatures over 500 °C), long-term high-temperature or fire stability, multifunctionality including disguised Joule heating capability, and high electromagnetic interference shielding efficiency. The superior high-temperature thermal camouflage performance of the ultrathin MXene film is attributed to its low mid-IR emissivity (0.19), which is comparable to that of stainless steel but far below that of other 2D nanomaterials, such as graphene. The multifunctional ultrathin MXene films prepared through simple vacuum-assisted filtration provide a feasible method for efficient high-temperature thermal camouflage using ultrathin films, demonstrating the great promise of MXene materials for thermal camouflage, IR stealth, counter-surveillance, and security protection.

2D g-C₃N₄ features a short charge/mass transfer path, abundant reactive sites, and easy functionalization, which are promising for energy conversion and storage. Based on the inherent properties and structural advantages of 2D g-C₃N₄, the advancements in the diverse applications of 2D g-C₃N₄ in photocatalysis, electrocatalysis, batteries, and supercapacitors are elaborately summarized.

Abstract

Graphitic carbon nitride (g-C₃N₄) have attracted great attention in the field of energy conversion and storage due to its unique layered structure, tunable bandgap, metal-free characteristic, high physicochemical stability, and easy accessibility. 2D g-C₃N₄ nanosheets have the features of short charge/mass transfer path, abundant reactive sites and easy functionalization, which are beneficial to optimizing their performance in different fields. However, the reviews of the comprehensive applications of 2D g-C₃N₄ for energy conversion and storage are rare. Herein, this review first introduces the physicochemical properties of bulk g-C₃N₄ and g-C₃N₄ nanosheets, and then summarizes the synthetic strategies of 2D g-C₃N₄ nanosheets in detail, such as thermal oxidation etching, chemical exfoliation, ultrasonication-assisted liquid phase exfoliation, chemical vapor deposition, and others. Emphasis is focused on the rational design and development of 2D g-C₃N₄ nanosheets for the diversified applications in energy conversion and storage, including photocatalytic H₂ evolution, CO₂ reduction, electrocatalytic H₂ evolution, O₂ evolution, O₂ reduction, alkali-metal ion batteries, lithium-metal batteries, lithium–sulfur batteries, metal-air batteries, and supercapacitors. Finally, the current challenges and perspectives of 2D g-C₃N₄ nanosheets for energy conversion and storage applications are discussed.

2D materials for nonlinear photonics and electro-optical applications are reviewed. First, the fabrication methods of 2D materials are introduced systematically. The nonlinear optical properties of 2D materials including Kerr effect, saturable absorption, carrier dynamics, and other nonlinear effects are summarized in detail. Based on this, several electro-optical applications are reviewed.

Abstract

2D materials have received significant attention from the scientific community due to their unique structures and excellent physical properties. A lot of applications are explored based on graphene, topological insulators, and transition metal dichalcogenides. With further development of 2D materials, other Group-VA materials such as titanium carbide and MXenes also show potential advantages in several important applications. Therefore, it is indispensable to investigate the properties and applications of 2D materials comprehensively. In this review, the fabrication methods of 2D materials are introduced systematically. The nonlinear optical properties of 2D materials including Kerr effect, saturable absorption, carrier dynamics, and other nonlinear effects are summarized in detail. Based on this, several electro-optical applications are reviewed. Finally, future perspectives of 2D materials in electro-optics are presented.

Large-scale transitional metal dichalcogenides (TMDCs) development is a crucial challenge in the horizons of materials science; this review provides suitable routes to commercialize TMDCs. The dynamics and chemistry of liquid-phase exfoliation provide ease of scaling-up and suitability for various applications with authors’ remarks. Finally, insight on challenges posed by exfoliated TMDCs and their possible solutions with authors’ recommendations for future opportunities are provided.

Abstract

A host of innovative developments in technology have led by 2D materials owing to their remarkable electronic and physical properties which opens doors for advanced research areas and horizons of material science. Among these materials, transitional metal dichalcogenides (TMDCs) e.g., MoS₂, WS₂, MoSe₂, and WSe₂, are reflected as promising candidates of 2D family. Despite significant achievements, the primary challenge is to produce these 2D materials with high purity, massive yield, and well-controlled structure that lead to fundamental research as well as industrial applications in an efficient and scalable way. A variety of techniques have been employed to develop 2D-TMDCs, such as mechanical exfoliation, chemical vapor deposition, and chemical exfoliations. Among state-of-the-art synthetic protocols, chemical exfoliations including Liquid-phase and intercalation isolations of TMDCs are deliberated as promising solutions for high yield, great performance, low cost, and excellent up-scalability. Herein, a succinct and comprehensive survey of recent progress in chemical exfoliation routes is presented with the processing techniques, strategic design for exfoliations, and mechanisms of individual approaches. The focus of this review is to fulfill the gap in recent reviews such as underlying mechanisms, chemistries, and critical hurdles with effective solutions in performing chemical exfoliations and suggesting a framework for future studies of TMDCs’ advanced applications.

A novel van der Waals stencil lithography technique based on dry polymer mask lamination process is developed. The soft stencil lithography ensures pristine contact region of 2D materials without any high-energy electron/photon radiation, polymer residues, or chemical doping effects in conventional lithography process and leads to ultra-high resolution down to 60 nm.

Abstract

2D semiconductors have attracted tremendous attention as an atomically thin channel for transistors with superior immunity to short-channel effects. However, with atomic thin structure, the delicate 2D lattice is not fully compatible with conventional lithography processes that typically involve high-energy photon/electron radiation and unavoidable polymer residues, posing a key limitation for high performance 2D transistors. Here, a novel van der Waals (vdW) stencil lithography technique based on dry mask lamination process is developed. By pre-fabricating polymethyl methacrylate (PMMA) resist with designed patterns, the whole PMMA mask layers could be mechanically released from the sacrifice wafer and physically laminated on top of various 2D semiconductors. The vdW stencil lithography ensures pristine 2D surface without any high-energy electron/photon radiation, polymer residues, or chemical doping effects in conventional lithography process; and the soft nature of PMMA enables intimate contact between the mask and the 2D materials without physical gap, leading to ultra-high resolution down to 60 nm. Together, by applying vdW stencil lithography for 2D semiconductors, high performance transistors are demonstrated. Our method not only demonstrates improved 2D transistor performance without lithography induced damages, but also provides a new vdW stencil lithography technique for 2D materials with high resolution.