Recent development in 2D materials beyond graphene

Discovery of graphene and its astonishing properties have given birth to a new class of materials known as “2D materials”. Motivated by the success of graphene, alternative layered and non-layered 2D materials have become the focus of intense research due to their unique physical and chemical properties. Origin of these properties ascribed to the dimensionality effect and modulation in their band structure. This review highlights the recent progress of the state-of-the-art research on synthesis, characterization and isolation of single and few layer nanosheets and their assembly. Electronic, magnetic, optical and mechanical properties of 2D materials have also been reviewed for their emerging applications in the area of catalysis, electronic, optoelectronic and spintronic devices; sensors, high performance electrodes and nanocomposites. Finally this review concludes with a future prospective to guide this fast evolving class of 2D materials in next generation materials science.     

Emerging 2D metal oxides and their applications

In the past decade, several different classes of two-dimensional (2D) materials beyond graphene such as layered polymorphs of group V elements (phophorene, arsenene), Metalenes (gallenene, stanene etc.), Transition Metal–Dichalcogenides (TMDs), group III monochalcogenides, transition metal carbides as well as nitrides have been thoroughly explored. These atomically thin materials have gathered significant focus due to their unique electronic, optical, and magnetic properties, which are seldom found in their bulk counterparts due to the high surface to volume ratios and quantum confined electronic structure. These properties have led to excitement in the research community due to their potential applications in various fields of optoelectronics, energy harvesting and storage, sensing, electronics, magneto-electronics, and thermo-electronic applications. However, there is another emerging class of layered oxide 2D materials, which has been sporadically explored and lacks a systematic compilation of the made progress, potential benefits and research opportunities that may lie ahead. This specific review provides a thorough and systematic summary of research carried out on layered 2D oxides both from an experimental and theoretical perspective. Due to ultra-thin nature of the 2D metal oxides, a majority of the atoms are accessible to the surfaces, which induces new properties and applications in comparison to traditional bulk oxides. We discuss several different classes of metal oxides in their 2D forms such as MO, MO_x, M_xO_y (where M stands for metals; x and y possible oxidation states) as well as Perovskite type oxides in this review specifically focusing on optoelectronics, sensing and electrochemical storage applications. We further make critical comparisons with bulk metal oxides, and elaborate the specific advantages of 2D metal oxides as compared to their bulk counterparts in respective applications. Finally, we conclude by providing a critical assessment and outlook of technical challenges and research opportunities for future development of layered 2D oxides.     

Robust Room-Temperature Ferromagnetism with Giant Anisotropy in Nd-Doped ZnO Nanowire Arrays

As an important class of spintronic material, ferromagnetic oxide semiconductors are characterized with both charge and spin degrees of freedom, but they often show weak magnetism and small coercivity, which limit their applications. In this work, we synthesized Nd-doped ZnO nanowire arrays which exhibit stable room temperature ferromagnetism with a large saturation magnetic moment of 4.1 μ(B)/Nd as well as a high coercivity of 780 Oe, indicating giant magnetic anisotropy. First-principles calculations reveal that the remarkable magnetic properties in Nd-doped ZnO nanowires can be ascribed to the intricate interplay between the spin moments and the Nd-derived orbital moments. Our complementary experimental and theoretical results suggest that these magnetic oxide nanowires obtained by the bottom-up synthesis are promising as nanoscale building blocks in spintronic devices.     

Van der Waals Heterostructures for High-Performance Device Applications: Challenges and Opportunities

The discovery of two-dimensional (2D) materials with unique electronic, superior optoelectronic, or intrinsic magnetic order has triggered worldwide interest in the fields of material science, condensed matter physics, and device physics. Vertically stacking 2D materials with distinct electronic and optical as well as magnetic properties enables the creation of a large variety of van der Waals heterostructures. The diverse properties of the vertical heterostructures open unprecedented opportunities for various kinds of device applications, e.g., vertical field-effect transistors, ultrasensitive infrared photodetectors, spin-filtering devices, and so on, which are inaccessible in conventional material heterostructures. Here, the current status of vertical heterostructure device applications in vertical transistors, infrared photodetectors, and spintronic memory/transistors is reviewed. The relevant challenges for achieving high-performance devices are presented. An outlook into the future development of vertical heterostructure devices with integrated electronic and optoelectronic as well as spintronic functionalities is also provided.     

Tailoring ferromagnetic chalcopyrites

If magnetic semiconductors are ever to find wide application in real spintronic devices, their magnetic and electronic properties will require tailoring in much the same way that bandgaps are engineered in conventional semiconductors. Unfortunately, no systematic understanding yet exists of how, or even whether, properties such as Curie temperatures and bandgaps are related in magnetic semiconductors. Here we explore theoretically these and other relationships within 64 members of a single materials class, the Mn-doped II-IV-V(2) chalcopyrites (where II, IV and V represent elements from groups II, IV and V, respectively); three of these compounds are already known experimentally to be ferromagnetic semiconductors. Our first-principles results reveal a variation of magnetic properties across different materials that cannot be explained by either of the two dominant models of ferromagnetism in semiconductors. On the basis of our results for structural, electronic and magnetic properties, we identify a small number of new stable chalcopyrites with excellent prospects for ferromagnetism.     

Effect of oxygen nonstoichiometry on the structure, 55Mn and 57Fe NMR,electromagnetic properties,and magnetoresistance of manganese zinc ferrites

Detailed studies of manganese zinc ferrites for electronic applications are used to assess the effect of oxygen nonstoichiometry on their structural, electrical, and magnetic properties. The optimal synthesis conditions and nonstoichiometry are determined which ensure the preparation of high-permeability manganese zinc ferrites with low electromagnetic energy losses. The manganese zinc ferrites are shown to be attractive as materials for high-performance magnetoresistive electric current and magnetic field sensors and spintronic devices.     

Defects-Driven Ferromagnetism in Undoped Dilute Magnetic Oxides:A Review

In the past several decades, dilute magnetic semiconductors, particularly the dilute magnetic oxides have evolved into an important branch of materials science due to their potential application in spintronic devices combining of properties of semiconductors and ferromagnets. In spite of a major effort devoted to the mechanism of ferromagnetism with a high Curie temperature in these materials, it still remains the most controversial research topic, especially given the unexpected d0 ferromagnetism in a series of undoped wide-band-gap oxides films or nanostructures. Recently, an abundance of research has shown the critical role of various defects in the origin and control of spontaneous magnetic ordering, but contradicting views from intertwined theoretical calculations and experiments require more in-depth systematic research. In our previous work, considerable efforts have been focused on two major oxides, i.e. ZnO and ZrO 2. This review will present a summary of current experimental status of this defect-driven ferromagnetism in dilute magnetic oxides(DMOs).     

Recent Progress of Two-Dimensional Thermoelectric Materials

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power.Moreover,the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades.Among these compounds,layered two-dimensional(2D)materials,such as graphene,black phosphorus,transition metal dichalcogenides,IVA–VIA compounds,and MXenes,have generated a large research attention as a group of potentially high-performance thermoelectric materials.Due to their unique electronic,mechanical,thermal,and optoelectronic properties,thermoelectric devices based on such materials can be applied in a variety of applications.Herein,a comprehensive review on the development of 2D materials for thermoelectric applications,as well as theoretical simulations and experimental preparation,is presented.In addition,nanodevice and new applications of 2D thermoelectric materials are also introduced.At last,current challenges are discussed and several prospects in this field are proposed.     

二维磁性材料与自旋系统研究进展

自旋系统是一类基于电子自旋工作的器件,具有高速、低功耗等优点,其中磁存储器被认为是下一代存储器的领跑者之一。二维材料有着天然的界面性能优势,接近微缩极限的尺寸特点,以及二维结构带来的物理特性,在自旋系统中有望得到广泛运用。本文概述了二维材料在自旋领域的最新研究进展,介绍了一些常见的二维磁性材料,讨论了基于二维材料的二维自旋器件,以及回顾了基于二维材料的磁性调控,最后总结了这一领域仍然面临的挑战,并对未来发展进行了展望。     

Spin Crossover Nanomaterials: From Fundamental Concepts to Devices

Nanoscale spin crossover materials capable of undergoing reversible switching between two electronic configurations with markedly different physical properties are excellent candidates for various technological applications. In particular, they can serve as active materials for storing and processing information in photonic, mechanical, electronic, and spintronic devices as well as for transducing different forms of energy in sensors and actuators. In this progress report, a brief overview on the current state‐of‐the‐art of experimental and theoretical studies of nanomaterials displaying spin transition is presented. Based on these results, a detailed analysis and discussions in terms of finite size effects and other phenomena inherent to the reduced size scale are provided. Finally, recent research devices using spin crossover complexes are highlighted, emphasizing both challenges and prospects.     

Molecular layer deposition of an organic-based magnetic semiconducting laminate

Organic-based magnets are intriguing materials with unique magnetic and electronic properties that can be tailored by chemical methodology. By using molecular layer deposition (MLD), we demonstrate the thin film fabrication of V[TCNE: tetracyanoethylene](x), of the first known room temperature organic-based magnet. The resulting films exhibit improvement in surface morphology, larger coercivity (80 Oe), and higher Curie temperature/thermal stability (up to 400 K). Recently, the MLD method has been widely studied to implement fine control of organic film growth for various applications. This work broadens its application to magnetic and charge transfer materials and opens new opportunities for metal-organic hybrid material development and their applications in various multilayer film device structures. Finally, we demonstrate the applicability of the multilayer V[TCNE](x) as a spin injector combining LSMO, an standard inorganic magnetic semiconductor, for spintronics applications.     

Probing magnetic anisotropy and spin polarization in spintronic materials

Magnetic anisotropy and spin polarization are fundamental parameters in ferromagnetic materials that have use in spintronic device applications. As the need for screening properties of new magnetic materials rises, it is important to have measurement probes for quantities such as anisotropy and spin polarization. We have developed two unconventional yet powerful techniques to study these parameters. A resonant RF transverse susceptibility method is used to map the characteristic anisotropy and switching fields over a wide range in temperature and magnetic fields. For studies of spin polarization, the phenomenon of Andreev reflection across ferromagnet-superconductor junctions is used to extract values of the transport spin polarization. The effectiveness of these approaches is demonstrated in candidate spintronic materials such as half-metallic CrO/sub 2/ thin films and arrays of monodisperse, single-domain Fe nanoparticles.     

Supramolecular spin valves

Magnetic molecules are potential building blocks for the design of spintronic devices. Moreover, molecular materials enable the combination of bottom-up processing techniques, for example with conventional top-down nanofabrication. The development of solid-state spintronic devices based on the giant magnetoresistance, tunnel magnetoresistance and spin-valve effects has revolutionized magnetic memory applications. Recently, a significant improvement of the spin-relaxation time has been observed in organic semiconductor tunnel junctions, single non-magnetic molecules coupled to magnetic electrodes have shown giant magnetoresistance and hybrid devices exploiting the quantum tunnelling properties of single-molecule magnets have been proposed. Herein, we present an original spin-valve device in which a non-magnetic molecular quantum dot, made of a single-walled carbon nanotube contacted with non-magnetic electrodes, is laterally coupled through supramolecular interactions to TbPc(2) single-molecule magnets (Pc=phthalocyanine). Their localized magnetic moments lead to a magnetic field dependence of the electrical transport through the single-walled carbon nanotube, resulting in magnetoresistance ratios up to 300% at temperatures less than 1 K. We thus demonstrate the functionality of a supramolecular spin valve without magnetic leads. Our results open up prospects of new spintronic devices with quantum properties.     

2D V‐V Binary Materials: Status and Challenges

2D phosphorene, arsenene, antimonene, and bismuthene, as a fast‐growing family of 2D monoelemental materials, have attracted enormous interest in the scientific community owing to their intriguing structures and extraordinary electronic properties. Tuning the monoelemental crystals into bielemental ones between group‐VA elements is able to preserve their advantages of unique structures, modulate their properties, and further expand their multifunctional applications. Herein, a review of the historical work is provided for both theoretical predictions and experimental advances of 2D V‐V binary materials. Their various intriguing electronic properties are discussed, including band structure, carrier mobility, Rashba effect, and topological state. An emphasis is also given to their progress in fabricated approaches and potential applications. Finally, a detailed presentation on the opportunities and challenges in the future development of 2D V‐V binary materials is given.     

Mn and other magnetic impurities in GaN and other III–V semiconductors – perspective for spintronic applications

III–V semiconductors doped with magnetic ions have been attracting interest of many laboratories all over the world during more than thirty years. At the beginning the reason was the will to understand influence of omnipresent unintentional, as well as intentionally introduced, impurities of transition metals or rare earths on electrical and optical properties of semiconductors commonly applied in electronic and optoelectronic devices. In the last years the subject of III–V semiconductors highly doped with magnetic ions, the so-called diluted magnetic semiconductors, has revived rapidly again in the context of the newborn branch of electronics, called spintronics. Diluted magnetic semiconductors based on III–V compounds are regarded as prospect candidates for applications in spintronic devices. The results of studies performed on III–V semiconductors, doped or diluted with different magnetic ions, are presented. Special attention is put to GaN because of a strong hope, based on theoretical calculations, for high temperature ferromagnetism. Reasons for difficulties with obtaining high temperature ferromagnetic semiconductors are shown. A possible mechanism of magnetic ordering in III–V semiconductors doped with Mn is presented.     

Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer

Spintronics has shown a remarkable and rapid development, for example from the initial discovery of giant magnetoresistance in spin valves to their ubiquity in hard-disk read heads in a relatively short time. However, the ability to fully harness electron spin as another degree of freedom in semiconductor devices has been slower to take off. One future avenue that may expand the spintronic technology base is to take advantage of the flexibility intrinsic to organic semiconductors (OSCs), where it is possible to engineer and control their electronic properties and tailor them to obtain new device concepts. Here we show that we can control the spin polarization of extracted charge carriers from an OSC by the inclusion of a thin interfacial layer of polar material. The electric dipole moment brought about by this layer shifts the OSC highest occupied molecular orbital with respect to the Fermi energy of the ferromagnetic contact. This approach allows us full control of the spin band appropriate for charge-carrier extraction, opening up new spintronic device concepts for future exploitation.     

Modulation of Metal and Insulator States in 2D Ferromagnetic VS2 by van der Waals Interaction Engineering

2D transition‐metal dichalcogenides (TMDCs) are currently the key to the development of nanoelectronics. However, TMDCs are predominantly nonmagnetic, greatly hindering the advancement of their spintronic applications. Here, an experimental realization of intrinsic magnetic ordering in a pristine TMDC lattice is reported, bringing a new class of ferromagnetic semiconductors among TMDCs. Through van der Waals (vdW) interaction engineering of 2D vanadium disulfide (VS₂), dual regulation of spin properties and bandgap brings about intrinsic ferromagnetism along with a small bandgap, unravelling the decisive role of vdW gaps in determining the electronic states in 2D VS₂. An overall control of the electronic states of VS₂ is also demonstrated: bond‐enlarging triggering a metal‐to‐semiconductor electronic transition and bond‐compression inducing metallization in 2D VS₂. The pristine VS₂ lattice thus provides a new platform for precise manipulation of both charge and spin degrees of freedom in 2D TMDCs availing spintronic applications.     

Magnetic phase transition in strained MnAs compound

The MnAs compound is presently of increasing interest for spintronic applications. By means of accurate first-principles pseudopotential calculations, we investigate its structural, electronic and magnetic properties under different strain/stress conditions, starting from its stable bulk free standingphase which has the hexagonal structure of NiAs. The other structures considered here simulate the pseudomorphic growth on substrates with different orientation and lattice mismatch.The effects of stress on the magnetic properties are linear over a limited range, which is typically accessible to measurements, but sizeable and strongly nonlinear for higher stress, suggesting a magnetic phase transition.     

Development of Rapidly Quenched Soft Magnetic Materials in China

1. IntroductionSince the first amorphous alloy quenched directlyfrom melt was obtained by Duwez et al. in 196011j, anumber of stable amorphous metal alloys have beenprepared and extensively investigated. In 1966, thefirst iron-based amorphous alloy, FePC[2] I with highsaturation induction was discovered, opening the possibility of using this class of materials as practical ferromagnetic materials.The research and development activities on suchmaterials in China were started in 1976. Even th…