The rise of two-dimensional MoS2 for catalysis

Two-dimensional (2D) MoS₂ is used as a catalyst or support and has received increased research interest because of its superior structural and electronic properties compared with those of bulk structures. In this article, we illustrate the active sites of 2D MoS₂ and various strategies for enhancing its intrinsic catalytic activity. The recent advances in the use of 2D MoS₂-based materials for applications such as thermocatalysis, electrocatalysis, and photocatalysis are discussed. We also discuss the future opportunities and challenges for 2D MoS₂-based materials, in both fundamental research and industrial applications.     

Light–matter interaction of 2D materials:Physics and device applications

In the last decade, the rise of two-dimensional(2D) materials has attracted a tremendous amount of interest for the entire field of photonics and opto-electronics. The mechanism of light–matter interaction in 2D materials challenges the knowledge of materials physics, which drives the rapid development of materials synthesis and device applications. 2D materials coupled with plasmonic effects show impressive optical characteristics, involving efficient charge transfer, plasmonic hot electrons doping, enhanced light-emitting, and ultrasensitive photodetection. Here, we briefly review the recent remarkable progress of 2D materials, mainly on graphene and transition metal dichalcogenides, focusing on their tunable optical properties and improved opto-electronic devices with plasmonic effects. The mechanism of plasmon enhanced light–matter interaction in 2D materials is elaborated in detail, and the state-of-the-art of device applications is comprehensively described. In the future, the field of 2D materials holds great promise as an important platform for materials science and opto-electronic engineering, enabling an emerging interdisciplinary research field spanning from clean energy to information technology.     

Toward Quantum Paraelectric,Paraelastic, and Paramagnetic 2D Materials

Quantum fluctuations are known to affect the finite-temperature properties of materials made out of light elements such as hydrogen and helium. More recently, it has also been realized that quantum effects may play a role on structural transformations of ferroic materials containing heavier atoms, provided the energy barrier separating two different phases is small when compared to thermal fluctuations. Herein, 2D ferroelectric and ferroelastic materials are showcased as potential candidates to experience quantum effects on their structural conformation at liquid helium temperatures. A brief literature overview of the path integral molecular dynamics approach, which could be useful for the discovery of quantum paraelectric, paraelastic, and paramagnetic behavior in 2D materials, is also provided.     

Electronic Structure and Electromagnetic Properties for 2D Electromagnetic Functional Materials in Gigahertz Frequency

Two‐dimensional (2D) materials exhibit massive potential for research and development in the scientific world due to the unique electrical, optical, thermal, and mechanical properties. Particularly, they have received intensive attentions in the microwave absorption (MA) and electromagnetic interference (EMI) shielding. Herein, the structure and electronic state of 2D materials are examined systematically, and the origination of electromagnetic (EM) response is shown, including electron transport, dipole relaxation, magnetic resonance, and eddy current. Furthermore, the latest research progress of 2D materials in EM field is introduced, and the application prospect and limitation are highlighted.     

纳米材料热传导中的新奇物理效应

纳米尺度热传导是物理科学、材料科学和工程热物理等相关学科的研究热点。除基础研究上的意义外,这个方向的研究在微纳米器件温度控制、新能源、热防护等重大工程技术领域也有着重要的应用价值。文章主要介绍一维、二维纳米材料(包括纳米管、纳米线、石墨烯及其他二维材料)的热传导性质。由于篇幅所限,文章集中讨论在低维体系热传导中的新奇物理效应,如碳纳米管热导率随长度的发散行为,硅纳米线中的声子相干性,以及石墨烯热传导性质的尺寸效应。文章侧重强调低维纳米材料热传导与宏观体材料热传导特性的本质区别。     

单元素二维原子晶体材料研究进展

非碳元素构成的二维单质原子晶体材料,具有与石墨烯相似的单层结构和物理性质,对它们的研究有望发掘一些崭新的性能及相关的应用,是当前凝聚态物理及新材料等相关领域的重要研究方向之一。文章详细介绍了近几年发展起来的单一元素所形成的二维原子晶体材料,分别对其结构、物性、应用前景等方面的研究进展进行了综述介绍。     

Structures,mobilities, electronic and optical properties of two-dimensional α-phase group-VI binary compounds: α-Se2Te and α-SeTe2

Based on the first-principles calculations, we confirm the geometry and electronic structures of two binary group-VI compounds: monolayer α-Se₂Te and α-SeTe₂. The stabilities are confirmed by the cohesive energies, phonon dispersions, and elastic constants. The mechanical properties, strain-stress relationships, and strain-dependent variations of band gaps and band structures are investigated detailed. Furthermore, the high carrier mobilities (up to $5.4\times {10}^3$ cm² V⁻¹ s⁻¹) and optical absorption coefficients (several 10⁵ cm⁻¹) are also exhibited, demonstrating the great application potentials in optoelectronics.     

具有负泊松比效应的2D碳纳米结构设计及其性能的理论计算研究

本文利用第一性原理PBE密度泛函理论计算的方法设计了一种由炔基链、吡啶环及少量氢原子组成的具有内凹六边形结构单元的新型理想二维碳纳米结构,并对其平面内负泊松比效应等力学性能和光学性能与电子结构进行了预测.计算表明,该2D材料具有较好的结构稳定性和特殊的力学性能.当将该2D结构在面内(bc面)沿c方向压缩时,其在b方向收缩;当沿c方向拉伸时,其在b方向伸长,即该2D结构同样具有期望的负泊松比效应.材料的泊松比为-3.26;将该2D结构沿b方向拉伸时,c方向将随之伸长;沿b方向压缩时,c方向将随之收缩.沿b方向拉伸或压缩时,泊松比约为-1.951.即该2D材料在面内具有非常显著的负泊松比效应.此外,该2D材料表现出半导体材料的电子结构特征和良好的光反射和折射性能.希望本工作能为具有本征负泊松比效应和优良电子与光学功能的理想二维碳纳米材料的开发提供一种理想的结构设计策略.     

Recent advances in two-dimensional layered and non-layered materials hybrid heterostructures

With the development of Moore's law, the future trend of devices will inevitably be shrinking and integration to further achieve size reduction. The emergence of new two-dimensional non-layered materials (2DNLMs) not only enriches the 2D material family to meet future development, but also stimulates the global enthusiasm for basic research and application technologies in the 2D field. Van der Waals (vdW) heterostructures, in which two-dimensional layered materials (2DLMs) are physically stacked layer by layer, can also occur between 2DLMs and 2DNLMs hybrid heterostructures, providing an alternative platform for nanoelectronics and optoelectronic applications. Here, we outline the recent developments of 2DLMs/2DNLMs hybrid heterostructures, with particular emphasis on major advances in synthetic methods and applications. And the categories and crystal structures of 2DLMs and 2DNLMs are also shown. We highlight some promising applications of the heterostructures in electronics, optoelectronics, and catalysis. Finally, we provide conclusions and future prospects in the 2D materials field.     

Synthesis,Properties, and Applications of 2-D Materials: A Comprehensive Review

Recent advances in atomically thin two-dimensional (2-D) materials have led to a variety of promising future technologies for post-CMOS nanoelectronics and energy generation. This review is an attempt to thoroughly illustrate the current status and future prospects for 2-D materials other than graphene (e.g., BN nanosheets, MoS₂, NbSe₂, WS₂, etc.), which have already been contemplated for both low-end and high-end technological applications. An overview of the different synthesis techniques for 2-D materials is presented here, with an exploration of the potential for developing methods of controllable large scale synthesis. Furthermore, we summarize the underlying theories which correlate the structural and physical properties of 2-D materials with their state-of-the-art applications. Finally, we show that utilizing the unprecedented properties arising from these materials would lead to innovative devices. Such devices would significantly reduce both device dimensions and power consumption, as necessary for the creation of tomorrow's sustainable technology.     

Tuning band alignment and optical properties of 2D van der Waals heterostructure via ferroelectric polarization switching

Favourable band alignment and excellent visible light response are vital for photochemical water splitting. In this work, we have theoretically investigated how ferroelectric polarization and its reversibility in direction can be utilized to modulate the band alignment and optical absorption properties. For this objective, 2D van der Waals heterostructures (HTSs) are constructed by interfacing monolayer MoS₂ with ferroelectric In2Se3. We find the switch of polarization direction has dramatically changed the band alignment, thus facilitating different type of reactions. In In₂Se₃/MoS₂/In₂Se₃ heterostructures, one polarization direction supports hydrogen evolution reaction and another polarization direction can favour oxygen evolution reaction. These can be used to create tuneable photocatalyst materials where water reduction reactions can be selectively controlled by polarization switching. The modulation of band alignment is attributed to the shift of reaction potential caused by spontaneous polarization. Additionally, the formed type-II van der Waals HTSs also significantly improve charge separation and enhance the optical absorption in the visible and infrared regions. Our results pave a way in the design of van der Waals HTSs for water splitting using ferroelectric materials.     

Prospects and Opportunities of 2D van der Waals Magnetic Systems

The existence of spontaneous magnetization in low dimensional magnetic systems has attracted intensive studies since the early 60s and research remains very active even now. Only recently, magnetic van der Waals (vdW) systems down to a few layers have been broadly discussed for their magnetic order ground states at finite temperature. The naturally inherited layered structure of the vdW magnetic systems possessing onsite magnetic anisotropy from band electrons can suppress the long-range fluctuations. This provides an excellent vehicle to study the transition of magnetism to 2D limits both theoretically and experimentally. Here the current status of 2D vdW magnetic system and its potential applications are briefly summarized and discussed.     

Recent advances in MXene: Preparation,properties, and applications

Owing to the exceptional properties of graphene, intensive studies have been carried out on novel two-dimensional (2D) materials. In the past several years, an elegant exfoliation approach has been used to successfully create a new family of 2D transition metal carbides, nitrides, and carbonitrides, termed MXene, from layered MAX phases. More recently, some unique properties of MXene have been discovered leading to proposals of potential applications. In this review, we summarize the latest progress in development of MXene from both a theoretical and experimental view, with emphasis on the possible applications.     

Strained 2D Layered Materials and Heterojunctions

2D layered materials and heterojunctions with excellent ductility and controllable atomic‐layer thicknesses have shown promise for use in advanced electronics and optical functional devices. Tailoring of nanoscale configurations and physical properties is essential and required for bespoke fabrication of advanced devices based on 2D materials. Due to the high strain tolerance of 2D layered materials, strain engineering is an effective method to tune their behaviors of electrons and phonons. A wide variety of 2D materials are available with tunable bandgaps from interface coupling effects, making 2D layered heterojunctions a versatile platform for understanding fundamental physical issues. Most physical properties and functional applications can be tailored by applying strain to 2D layered materials and heterostructures to realize a scheduled target in carrier concentration, mobility, and barrier height. Herein, the latest research on the roles of strain in modulating the physical properties of 2D layered materials and heterojunctions is introduced, focusing on the physical properties behind strain modulation in 2D materials. Understanding and manipulating strain in 2D layered materials and heterojunctions is important and beneficial for creating tunable electronic and optoelectronic constructions with advanced components, including functional flexible and wearable devices.     

Ti Impurity Effect on the Optical Coefficients in 2D Cu_2Si: A DFT Study

The electronic and optical properties of 2D Cu_2Si and Cu_2Si:Ti are investigated based on the density functional theory(DFT) using the FP-LAPW method and GGA approximation. The 2D Cu_2Si has metallic and non magnetic properties, whereas adding Ti impurity to its structure changes the electronic behavior to the half-metallic with 3.256μB magnetic moment. The optical transition is not occurred in the infrared and visible area for the 2D Cu_2Si in x-direction and by adding Ti atom, the real part of dielectric function in the x-direction, Re(ε(ω))_x is reached to a Dirac peak at this energy range. Moreover, the absorption gap tends to zero in x-direction of the 2D Cu_2Si:Ti.     

High-Speed Quantum Transducer with a Single-Photon Emitter in a 2D Resonator

Quantum transducers can transfer quantum information between different systems. Microwave–optical photon conversion is important for future quantum networks to interconnect remote superconducting quantum computers with optical fibers. Here, a high-speed quantum transducer based on a single-photon emitter in an atomically thin membrane resonator, that can couple single microwave photons to single optical photons, is proposed. The 2D resonator is a freestanding van der Waals heterostructure (which may consist of hexagonal boron nitride, graphene, or other 2D materials) that hosts a quantum emitter. The mechanical vibration (phonon) of the 2D resonator interacts with optical photons by shifting the optical transition frequency of the single-photon emitter with strain or the Stark effect. The mechanical vibration couples to microwave photons by shifting the resonant frequency of an LC circuit that includes the membrane. Thanks to the small mass of the 2D resonator, both the single-photon optomechanical coupling strength and the electromechanical coupling strength can reach the strong coupling regime. This provides a way for high-speed quantum state transfer between a microwave photon, a phonon, and an optical photon.     

On the use of 2D correlation and exchange NMR spectroscopy in organic porous materials

Two-dimensional (2D) nuclear magnetic resonance (NMR) methods for the investigation of correlation and exchange have been introduced in recent years and have been applied to a range of different systems. Here, we report on the use of 2D NMR diffusion-diffusion correlation spectroscopy for the investigation of diffusion anisotropy in cellular plant tissues and of diffusion-diffusion exchange spectroscopy for the study of the diffusive exchange of dextran in a dispersion of polyelectrolyte multilayer hollow capsules. Furthermore, diffusion-relaxation correlation spectroscopy was applied to both systems.     

Role of surface defects and microstructure in infrared optical properties of thermochromic VO2 materials

Thermochromic vanadium dioxide VO₂ exhibits a semi-conducting to metallic phase transition at T_c=68 °C, involving strong variations in optical transmittance, reflectance and emissivity. However, the optical contrasts observed in thin films or nanostructured compacted samples seem to depend on both surface microstructure and surface crystal texture. In the case of opaque materials, surface defects might play a drastic role in optical reflectivity. As the high temperature metallic phase of VO₂ is opaque for infrared radiations, we used aluminum samples as standards allowing us to correlate reflectivity responses with porosity and surface defects. Then, various polycrystalline and nanostructured VO₂ samples compacted at various pressures and presenting variable surface roughness were prepared. Thin films were deposited by radio frequency sputtering process. The samples were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Optical properties (reflectance and emissivity) were analyzed above and below the transition temperature, making use of specific FTIR equipments. In thin films, the deposited VO₂ phase was systematically oriented and surface porosity was very weak. In polycrystalline samples, as the compaction pressure increased, surface porosity decreased, and infrared optical contrast increased. In such samples, preferred orientations were favored for low applied pressures. These features clearly show that the main parameters conditioning the optical contrast should be the surface defects and porosity, not the preferred crystal orientations. As an additional interesting result, the surfaces formed from compacted nanocrystalline VO₂ powders present improved optical contrast for reflectance and emissivity properties.