Ferroic orders in two-dimensional transition/rare-earth metal halides

The fascinating two-dimensional (2D) materials are being unconsciously applied in various fields from science to engineering, which is benefited from the glamorous physical and chemical properties of mechanics, optics, electronics, and magnetism. The representative 2D thermoelectric/piezoelectric materials can directly convert thermal/mechanical energy into electrical energy, which can resolve the energy issues and avoid further environmental deterioration. The thermoelectric or piezoelectric properties of various 2D materials, such as graphene, hexagonal boron nitride, black phosphorus, transition metal dichalcogenides (TMDs), arsenene, metal carbides and nitrides (MXenes), and so on, have been investigated in detail. Although tremendous progress has been achieved in the past few years, these properties still need to be improved for their practical application by designing new 2D materials, strain engineering, chemical functionalization, etc. In addition to this, in 2D materials, there are many other novel physical properties, such as magnetism, topology, valley, and so on. The combination of thermoelectricity/piezoelectricity with other unique properties may lead to novel device applications or scientific breakthroughs in new physics. Overall, the emergence of 2D thermoelectric and piezoelectric materials has expanded energy conversion research dramatically. By combing this new device concept with the novel 2D materials, original devices should have potential applications in energy harvesting.

Two-dimensional (2D) ferromagnetic materials are considered as promising candidates for the future generations of spintronic devices. Yet, 2D materials with intrinsic ferromagnetism are scarce. Hereby, high-throughput first-principles simulations are performed to screen 2D materials that present a non-magnetic to a ferromagnetic transition upon hole doping. A global evolutionary search is subsequently performed to identify alternative possible atomic structures of the eligible candidates, and 122 materials exhibiting a hole-doping induced ferromagnetism are identified. Their energetic and dynamic stability, as well as magnetic properties under hole doping are investigated systematically. Half of these 2D materials are metal halides, followed by chalcogenides, oxides, and nitrides, some of them having predicted Curie temperatures above 300 K. The exchange interactions responsible for the ferromagnetic order are also discussed. This work not only provides theoretical insights into hole-doped 2D ferromagnetic materials, but also enriches the family of 2D magnetic materials for possible spintronic applications.

Superior performance of ultrathin metal organic framework nanosheets for antiwear and antifriction testing

The recently discovered two-dimensional magnetic materials have attracted tremendous attention and become a cutting-edge research topic due to their long-range magnetic ordering at a single-unit-cell thickness, which not only provide an ideal platform for studying the magnetism in the two-dimensional limit and other novel physical effects, but also open up a new way to develop low-power spintronics/magnetic storage devices. In this review, first, we introduce the crystal structures, magnetic structures and magnetic properties of various recently discovered intrinsic two-dimensional magnetic materials. Second, we discuss the research progress of controlling the magnetic properties of two-dimensional magnetic materials by magnetic field, electric field, electrostatic doping, ion intercalation, stacking, strain, interface, etc. Finally, we give a perspective of possible research directions of the two-dimensional magnetic materials. We believe that an in-depth understanding of the origin and mechanism of magnetism of the two-dimensional magnetic materials as well as the study of the relationship between magnetic properties and microstructures are of great significance in exploring a magnetic material with a substantially high Curie temperature (Néel temperature), and designing multifunctional new concept devices.

Flexoelectricity in hexagonal boron nitride monolayers

Photonic platforms with excellent nonlinear optical characteristics are very important to improve the devices' performance parameters such as integration, modulation speeds and working bandwidths for all-optical signal processing. The traditional processing technology of photonic platforms based on silicon, silicon nitride and silicon oxide is mature, but the nonlinear function of these optical platforms is limited due to the characteristics of materials; Although two-dimensional (2D) materials possess excellent nonlinear optical properties, their nonlinear potentials cannot be fully utilized because of their atomic layer thickness. Integrating 2D materials with mature photonic platforms can significantly improve the interaction between light and matter, give full play to the potentials of 2D materials in the field of nonlinear optics, and improve the nonlinear optical performances of the integrated platforms on the basis of fully utilizing the mature processing technology of the photonic platforms. Based on the above ideas, starting from the basic principle of nonlinear optics (Section 2), this review combs the research progress of various nonlinear photonic platforms (resonators, metasurfaces, optical fibers, on-chip waveguides, etc.) heterogeneously integrated with 2D materials, realized by traditional transfer methods (Section 3) and emerging direct-growth methods (Section 4) in recent years, and the introduction is divided into second-order and third-order nonlinearity. Comparing with the transfer methods, the advantages of using direct-growth methods to realize the heterogeneous integration of 2D materials and photonic platforms for the study of nonlinear optics are expounded, and the technical difficulties to be overcome in preparing the actual devices are also pointed. In the future, we can try to grow 2D materials directly onto the surfaces of various cavities to study the enhancement of second-order nonlinearity; we can also try to grow 2D materials directly onto the on-chip waveguides or microrings to study the enhancement of third-order nonlinearity. Generally speaking, the research on integrated nonlinearity by directly growing 2D materials onto various photonic structures has aroused great interest of researchers in this field. As time goes on, breakthrough progress will be made in this field, and technical problems such as continuous growth of high-quality 2D materials onto photonic structures and wafer-level large-scale preparation will be broken through, further improving the performance parameters of chips and laying a good foundation for optical communication, signal processing, optical sensing, all-optical computing, quantum technology and so on.

Two‐dimensional (2D) materials have disrupted materials science due to the development of van der Waals technology. It enables the stacking of ultrathin layers of materials characterized by vastly different electronic structures to create man‐made heterostructures and devices with rationally tailored properties, circumventing limitations of matching crystal structures, lattice constants, and geometry of constituent materials and supporting substrates. 2D materials exhibit extraordinary mechanical flexibility, strong light‐matter interactions driven by their excitonic response, single photon emission from atomic centers, stable ferromagnetism in sub‐nm thin films, fractional quantum Hall effect in high‐quality devices, and chemoselectivity at ultrahigh surface‐to‐volume ratio. Consequently, van der Waals heterostructures with atomically flat interfaces demonstrate an unprecedented degree of intertwined mechanical, chemical, optoelectronic, and magnetic properties. This constitutes a foundation for multiproperty sensing, based on complex intra‐ and intermaterial interactions, and a robust response to external stimuli originating from the environment. Here, recent progress are reviewed in the development of sensing applications with 2D materials, highlighting the areas where van der Waals heterostructures offer the highest sensitivity, simultaneous responses to multiple distinct externalities due to their atomic thickness in conjunction with unique material combinations, and conceptually new sensing methodology.

This book summarizes the current knowledge of two-dimensional oxide materials. The fundamental properties of 2-D oxide systems are explored in terms of atomic structure, electronic behavior and surface chemistry. The concept of polarity in determining the stability of 2-D oxide layers is examined, charge transfer effects in ultrathin oxide films are reviewed as well as the role of defects in 2-D oxide films. The novel structure concepts that apply in oxide systems of low dimensionality are addressed, and a chapter giving an overview of state-of-the-art theoretical methods for electronic structure determination of nanostructured oxides is included. Special emphasis is given to a balanced view from the experimental and the theoretical side. Two-dimensional materials, and 2-D oxides in particular, have outstanding behavior due to dimensionality and proximity effects. Several chapters treat prototypical model systems as illustrative examples to discuss the peculiar physical and chemical properties of 2-D oxide systems. The chapters are written by renowned experts in the field

Two-dimensional (2D) materials have emerged as a forefront of research in materials science, driven by their exceptional and diverse properties. These materials, often with atomic thickness, exhibit phase transitions with small energy barrier, which can enable the development of energy-efficicient devices. Understanding these phase transitions is not only a fascinating scientific endeavor but also crucial for technological innovation. In this presentation, we will cover phase transitions in 2D materials, encompassing both intralayer and interlayer phenomena. We will dissect the mechanisms by which phase transitions can radically modify the electrical, optical, and mechanical properties of these materials, and discuss the engineering approaches designed to control energy barrier and transition process. Special attention will be given to the devices developed from phase transition phenomena in 2D materials, highlighting their potential applications in the realms of memory storage and sensory technology.

Recently, the observation of atomic thin film SnTe with a Curie temperature (Tc) higher than that of the bulk [Chang et al., Science 353, 274 (2016)] has boosted the research on two-dimensional (2D) ferroic materials tremendously. However, the origin of such a phenomenon is yet to be thoroughly investigated, which hinders the understanding and design of materials with ferroic orders at the 2D limit. By using the density functional theory, we investigated the structural and ferroelectrical properties of 2D SnTe to reveal the thickness dependence. The calculated results demonstrate that 2D SnTe automatically transforms into a periodical bilayer structure, resulting from the surface effect. Moreover, based on the double-well potential and atomic distortion analysis, we found that the Tc of 2D SnTe is higher than that of the bulk counterpart, and more surprisingly, Tc exhibits a non-monotonous dependence of thickness, featuring a pronounced atomic distortion and Curie temperature maximum at 8 atomic-layers (4 unit cells). In addition, this non-monotonous dependence is sensitive to the external strain and it can be easily tuned by the external compressive strain.

With the rapid development of microelectronic integration technology, the miniaturization, integration and multifunction of electronic devices are becoming a general trend. Two-dimensional materials are a class of layered material with atomic layer thickness, and have unique electrical, magnetic, optical and mechanical properties. The co-existence of the weak van der Waals force between layers and the strong covalent bonding within layers makes the two-dimensional material very suitable for the miniature design of new-generation multifunctional electronic devices. Two-dimensional materials, represented by graphene and transition metal chalcogenides, exhibit high mobility, adjustable energy band and high visible light transmittance, and thus having become the frontier hotspots in the field of micro-nanoscience in recent years. Synergy between two-dimensional materials and various functional materials such as SiO<sub>2</sub> insulator, semiconductor, metal and organic compound may lead to new properties and device applications, thus can deepen and expand the basic research and application of two-dimensional materials. Among them, ferroelectric materials have received much attention because of their spontaneous polarizations, high dielectric constants, and high piezoelectric coefficients. The two-dimensional ferroelectric composites well have the advantages of the two, i.e. they not only contain a variety of rich phenomena such as the magnetoelectric coupling effect, ferroelectric field effect and lattice strain effect, tunneling effect, photoelectric effect, and photoluminescence effect, but also have broad applications in devices such as multi-state memories, tunneling transistors, photoelectric diodes, solar cells, super capacitors, and pyroelectric infrared detectors, which have attracted wide concern from academia and industry. To better understand the combination of two-dimensional thin films with ferroelectric substrates and provide a holistic view, we review the researches of several typical two-dimensional film/ferroelectrics heterostructures in this article. First, two-dimensional materials and ferroelectric materials are introduced. Then, the physical mechanism at the interface is briefly illustrated. After that, several typical two-dimensional film/ferroelectrics heterostructures are mainly introduced. The ferroelectric materials including Pb(Zr<sub>1–<i>x</i></sub>Ti<i><sub>x</sub></i>)O<sub>3</sub>, (1–<i>x</i>)PbMg<sub>1/3</sub>Nb<sub>2/3</sub>O<sub>3</sub>–<i>x</i>PbTiO<sub>3</sub>, P(VDF-TrFE), are mainly summarized, and other ferroelectric materials such as P(VDF-TrFE-CFE), BaTiO<sub>3</sub>, BiFeO<sub>3</sub>, PbTiO<sub>3</sub>, CuInP<sub>2</sub>S<sub>6</sub>, HfO<sub>2</sub> are briefly involved. The future research emphasis of the two-dimensional materials/ferroelectrics composites is also suggested at the end of the article. This review will present a significant reference to the future design of miniature and multifunctional devices.

The scalable preparation of high‐quality and low‐cost two‐dimensional (2D) materials is critical to achieving their potential applications in various fields. Chemical vapor deposition (CVD) method is considered the most promising method for producing ultrathin 2D materials and has continued to develop in recent years. First‐principles calculations have provided important theoretical guidance for the CVD synthesis of 2D materials, and have played an increasingly important role in the field of material synthesis in recent years. In this review, we present recent advances in the growth mechanism of 2D materials, focusing on the theoretical research progress of four typical 2D materials: graphene, hexagonal boron nitride (hBN), transition metal dichalcogenide (TMDC), and phosphorene. Several aspects of the growth process are discussed in detail, including the decomposition of precursors, nucleation, growth kinetics, domain shape, and epitaxial and alignment of 2D crystals. Based on the understanding of these atomic‐scale growth processes, strategies toward the wafer‐scale growth of continuous and homogeneous 2D thin films are proposed and confirmed by experiments. In the final section, we summarize future challenges and opportunities in the computational studies of the growth mechanism of 2D materials.This article is categorized under: Structure and Mechanism > Computational Materials Science Electronic Structure Theory > Density Functional Theory Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics

Field-effect transistor (FET) biosensors based on two-dimensional (2D) materials have drawn significant attention due to their outstanding sensitivity. However, the Boltzmann distribution of electrons imposes a physical limit on the subthreshold swing (SS), and a 2D-material biosensor with sub-60 mV/dec SS has not been realized, which hinders further increase of the sensitivity of 2D-material FET biosensors. Here, we report tunnel FETs (TFETs) based on a SnSe2/WSe2 heterostructure and observe the tunneling effect of a 2D material in aqueous solution for the first time with an ultralow SS of 29 mV/dec. A bilayer dielectric (Al2O3/HfO2) and graphene contacts, which significantly reduce the leakage current in solution and contact resistance, respectively, are crucial to the realization of the tunneling effect in solution. Then, we propose a novel biosensing method by using tunneling current as the sensing signal. The TFETs show an extremely high pH sensitivity of 895/pH due to ultralow SS, surpassing the sensitivity of FET biosensors based on a single 2D material (WSe2) by 8-fold. Specific detection of glucose is realized, and the biosensors show a superb sensitivity (3158 A/A for 5 mM), wide sensing range (from 10-9 to 10-3 M), low detection limit (10-9 M), and rapid response rate (11 s). The sensors also exhibit the ability of monitoring glucose in complex biofluid (sweat). This work provides a platform for ultrasensitive biosensing. The discovery of the tunneling effect of 2D materials in aqueous solution may stimulate further fundamental research and potential applications.

Direct liquid-phase exfoliation of layered materials by means of ultrasound, shear forces or electrochemical intercalation holds enormous promise as a convenient, cost-effective approach to the mass production of two-dimensional (2D) materials, particularly in the form of colloidal suspensions of high quality and micrometer- and submicrometer-sized flakes. Of special relevance due to environmental and practical reasons is the production of 2D materials in aqueous medium, which generally requires the use of certain additives (surfactants and other types of dispersants) to assist in the exfoliation and colloidal stabilization processes. In this context, biomolecules have received, in recent years, increasing attention as dispersants for 2D materials, as they provide a number of advantages over more conventional, synthetic surfactants. Here, we review research progress in the use of biomolecules as exfoliating and dispersing agents for the production of 2D materials. Although most efforts in this area have focused on graphene, significant advances have also been reported with transition metal dichalcogenides (MoS2, WS2, etc.) or hexagonal boron nitride. Particular emphasis is placed on the specific merits of different types of biomolecules, including proteins and peptides, nucleotides and nucleic acids (RNA, DNA), polysaccharides, plant extracts and bile salts, on their role as efficient colloidal dispersants of 2D materials, as well as on the potential applications that have been explored for such biomolecule-exfoliated materials. These applications are wide-ranging and encompass the fields of biomedicine (photothermal and photodynamic therapy, bioimaging, biosensing, etc.), energy storage (Li- and Na-ion batteries), catalysis (e.g., catalyst supports for the oxygen reduction reaction or electrocatalysts for the hydrogen evolution reaction), or composite materials. As an incipient area of research, a number of knowledge gaps, unresolved issues and novel future directions remain to be addressed for biomolecule-exfoliated 2D materials, which will be discussed in the last part of this review.

Two-dimensional (2D) materials, such as graphene and related transition metal dichalcogenides (TMDC), have attracted much attention for their potential applications. Ferroelectrics, one of the special and traditional dielectric materials, possess a spontaneous electric polarization that can be reversed by the application of an external electric field. In recent years, a new type of device, combining 2D materials with ferroelectrics, has been fabricated. Many novel devices have been fabricated, such as low power consumption memory devices, highly sensitive photo-transistors, etc. using this technique of hybrid systems incorporating ferroelectrics and 2D materials. This paper reviews two types of devices based on field effect transistor (FET) structures with ferroelectric gate dielectric construction (termed FeFET). One type of device is for logic applications, such as a graphene and TMDC FeFET for fabricating memory units. Another device is for optoelectric applications, such as high performance phototransistors using a graphene p-n junction. Finally, we discuss the prospects for future applications of 2D material FeFET.