Jing Zhang

Hexagon boron nitride (h‐BN) with excellent chemical, mechanical, and optical properties has the potential to replace the use of thick and stiff blocking dielectrics in two‐terminal or three‐terminal devices. Recent progress in h‐BN memories with volatile or nonvolatile properties is presented, expanding the memories to functional applications, and further challenges in the development of h‐BN‐based memories are discussed.

Abstract

The fast development of synthesis routes and preparation technology of 2D materials has motivated a rapid growth in the micro‐ and nanoelectronic memory devices, which gives rise to the breakthroughs in the semiconductor research area. Hexagon boron nitride (h‐BN) with excellent chemical, mechanical, and optical properties has been proven to have potential in overcoming the scaling limit to nanometer, and even sub‐nanometer lengths to replace the use of thick and stiff blocking dielectrics in two‐terminal or three‐terminal devices. The use of atomically thin h‐BN or h‐BN van der Waals heterostructures (vdWhs) can improve the reliability, capability, and functionality of memory devices. This is an encouraging strategy toward high‐density on‐chip integrated circuits, which has recently earned considerable interest. While the research in h‐BN material properties and characterization is comprehensively verified, specified mechanisms of resistive switching have not been analyzed in‐depth. Moreover, recent concern about novel structure design and expanding applications in electronics, optoelectronics, and spintronics has arisen. In this review, recent progress in h‐BN memories with volatile or nonvolatile properties is presented, expanding the memories to functional applications, and further challenges of the development of h‐BN‐based memories and logic circuits are discussed.

The dynamics of excitonic valley polarization in monolayer MoSe₂ under magnetic fields has been systematically studied. The revealed longer inter‐valley relaxation time of trion in a few nanoseconds reveals important aspects for the new physics of valley degree of freedom, and possibilities for the application of valleytronics.

Abstract

The valley polarization dynamics of excitonic states (excitons and trions) in monolayer (1L)‐MoSe₂ under strong magnetic field are investigated. The experimentally observed magnetic‐field‐induced valley polarizations of trions (excitons) are analyzed by the rate equation model. It is found that the magnetic‐field‐induced valley polarizations are attributed to the asymmetric valley scattering of trions from K+ to K− valley and vice versa due to the degeneracy lifting of K+ and K− valleys by valley Zeeman effect under magnetic field. Moreover, the feeding of the valley‐polarized excitons for trions formation and longer inter‐valley scattering time reaching to a few nanoseconds at 10 K contribute to the larger valley polarization of trions than that of excitons in 1L‐MoSe₂.

In article number 2005582, Fei Zhuge and co‐workers develop an all‐optically controlled (AOC) analog memristor based on the relatively mature material InGaZnO. The memconductance is reversibly tunable over a continuous range by varying only the wavelength of the controlling light. The light‐induced multiple memconductance states are nonvolatile. This device has promising applications in AOC spiking neural networks for highly efficient optoelectronic neuromorphic computing.

A room‐temperature 2D Bi₂O₂Se photodetector with ultrafast (476 ns) and ultralow noise (0.2 pW Hz^–1/2) is demonstrated for broadband detection. The photoresponse of the devices in the IR (940–1550 nm) and THz (0.168–0.17 and 0.02–0.04 THz) regions are driven by multi‐mechanisms. Meanwhile, the Bi₂O₂Se THz photodetector exhibits excellent optoelectronic properties, polarized sensitivity, and high‐resolution imaging.

Abstract

2D Bi₂O₂Se has shown great potential in photodetector from visible to infrared (IR) owing to its high mobility, ambient stability, and layer‐tunable bandgaps. However, for the terahertz (THz) band with longer wavelength and richer spectral information, there are few reports on the research of THz detection based on 2D materials. Herein, an antenna‐assisted Bi₂O₂Se photodetector is constructed to achieve broadband photodetection from IR to THz ranges driven by multi‐mechanism of electromagnetic waves to electrical conversion. The good tradeoff between the bandgap and high mobility results in a broad spectral detection. In the IR region, the nonequilibrium carriers result from photo‐induced electron‐hole pairs in the Bi₂O₂Se body. While in the THz region, the carriers are caused by the injected electrons from the metal electrodes by the electromagnetic‐induced well. The Bi₂O₂Se photodetector achieves a broadband responsivity of 58 A W^‐1 at 1550 nm, 2.7 × 10⁴ V W^‐1 at 0.17 THz, and 1.9 × 10⁸ V W^‐1 at 0.029 THz, respectively. Surprisingly, an ultrafast response time of 476 ns and a quite low noise equivalent power of 0.2 pW Hz^−1/2 are acquired at room temperature. Our researches exhibit promising prospects of Bi₂O₂Se in broadband detection, THz imaging, and ultrafast sensing.

2D hexagonally ordered covalent organic radical frameworks (2D hex‐CORFs) tend to exhibit an antiferromagnetic Mott insulating ground state. Through careful theoretical analysis, it is shown that modest out‐of‐plane pressures can be used to dynamically shift these low dimensional materials toward a transition to graphene‐like semimetallicity. 2D hex‐CORFs are thus established as a new class of single‐layer pressure‐tunable correlated materials.

Abstract

Quantum materials hold huge technological promise but challenge the fundamental understanding of complex electronic interactions in solids. The Mott metal–insulator transition on half‐filled lattices is an archetypal demonstration of how quantum states can be driven by electronic correlation. Twisted bilayers of 2D materials provide an experimentally accessible means to probe such transitions, but these seemingly simple systems belie high complexity due to the myriad of possible interactions. Herein, it is shown that electron correlation can be simply tuned in experimentally viable 2D hexagonally ordered covalent organic radical frameworks (2D hex‐CORFs) based on single layers of half‐filled stable radical nodes. The presented carefully procured theoretical analysis predicts that 2D hex‐CORFs can be varied between a correlated antiferromagnetic Mott insulator state and a semimetallic state by modest out‐of‐plane compressive pressure. This work establishes 2D hex‐CORFs as a class of versatile single‐layer quantum materials to advance the understanding of low dimensional correlated electronic systems.

Quantitative visualization of piezoelectric field spatial distribution is realized on an odd number of layers of 2D materials bubbles by measuring their surface potential with kelvin probe force microscope. Such technique also allows in situ mapping of piezoelectric potential tuned by photoillumination on MoS₂ nanobubbles. In addition, the work introduces nanobubbles as a platform of investigating piezoelectric properties of atomically thin crystals.

Abstract

2D crystals with noncentrosymmetric structures exhibit piezoelectric properties that show great potential for applications in energy conversion and electromechanical devices. Quantitative visualization of piezoelectric field spatial distribution is expected to offer a better understanding of macroscopic piezoelectricity, yet remains to be realized. Here, a technique of mapping piezoelectric potential on 2D materials bubbles based on the measurements of surface potential using kelvin probe force microscope is reported. By using odd number of layers hexagonal boron nitride and MoS₂ nanobubbles, strain‐induced piezoelectric potential profiles are quantitatively visualized on the bubbles. The obtained piezoelectric coefficient is 3.4 ± 1.2 × 10⁻¹⁰ C m⁻¹ and 3.3 ± 0.2 × 10⁻¹⁰ C m⁻¹ for hBN and MoS₂, in agreement with the values reported. On the contrary, homogeneous distribution of surface potential is measured on even number of layers crystals bubbles where the crystal's inversion symmetry is restored. Using such technique, in situ visualization of photogenerated charge carrier separation under piezoelectric potential is also achieved, which offers a platform of investigating the coupling between piezoelectricity and photoelectric effect, and an approach of tuning piezoelectric field. The present work should aid the understanding of local piezoelectric potential and its various affecting factors including substrate doping and external stimuli, and give insights for designing piezoelectric nanodevices based on 2D nanobubbles.

Precise control of phase transitions in polymorphic 2D transition metal dichalcogenides (TMDs) is expected to play a key role in modern intelligent devices. However, an atomic-scale understanding and thus control of the phase transitions in the atomically-thin TMDs have not been reached, especially in some metastable phases. Here, in metastable monolayer 1T′ WS 2 , we demonstrate the dynamics of a phase transition nucleated from atomic defects by the means of time-resolved annular dark-field imaging and atomic-resolution electron energy-loss spectroscopy. It is found that the atomic and electronic structure of the 1T′ phase is inhomogeneous, which is decided by zone-dependent W–S bond strengths due to a Peierls-like structure distortion. Meanwhile, the W–S bonding is flexible to allow large nonequilibrium atom shifts for phase transition. Thus, just a few atomic defects can stabilize the atomic-scale nucleus of the new phase to initialize the phase transition from 1T′ t...

Bilayer graphene (BLG) with 30°-twist (30°-tBLG) has been proven to possess a quasicrystal structure potentially providing novel applications. Despite the growth of BLG, especially the AB-stacking bilayer, has gained great attention, the growth of 30°-tBLG has been rarely achieved. Herein, for the first time, the decaborane-assisted synchronous growth of millimeter-sized single-crystalline 30°-tBLG was achieved on Cu foil by controlling the nucleation density and growth kinetics of graphene during chemical vapor deposition using diluted methane gas as the carbon source. The synchronous growth kinetics and decaborane-assisted co-catalysis mechanism are revealed by monitoring the growth process from the initial stage of graphene seeds to the millimeter-size scale. A 30°-tBLG based field effect transistor was fabricated and was found to possess a field-effect carrier mobility as high as 3671.3 cm 2 V −1 s −1 at room temperature. Thus, this work provide...

We analyze the lineshape of the quasiparticle photoluminescence of monolayer (ML) and bilayer (BL) molybdenum ditelluride in temperature- and excitation intensity-dependent experiments. We confirm the existence of a negatively charged trion in the BL based on its emission characteristics and find hints for a coexistence of intra- and interlayer trions with a few meV splitting in energy. From the lineshape analysis of exciton and trion emission we extract values for exciton and trion deformation potentials as well as acoustical and optical phonon-limited mobilities in MoTe 2 . We estimate an acoustical phonon limited mobility of 6000 and 4300 cm 2  Vs −1 for the exciton at low temperature for ML and BL, respectively, which corresponds to an electron mobility of 10 5 cm 2  Vs −1 . At higher temperatures, the optical phonons limit the mobility to 1100 and 250 cm 2  Vs −1 for ML and BL.

Understanding the evolution mechanisms of interlayer stacking structures, particularly at the atomic scale, is of great significance for modulating the physical properties and realizing the full potential of 2D materials in electronics and quantum information applications. Herein, by performing in situ experiments using aberration corrected scanning transmission electron microscopy, the evolution of diverse interlayer stacking sequences (from 3R to N, N to 3R and N(3R) to AB′-stacked) in bilayer PtSe 2 are directly observed. Furthermore, the interlayer rotational angles are tuned (e.g. 13.3° to 9.4°, 16.8° to 11° and 16.1° to 6°) in situ at real time in bilayer PtSe 2 . Density functional theory calculations reveal a small energy barrier (<0.2 eV per formula unit) for the kinetic evolution of interlayer structures. The illumination electron beam, while being as an atomic-scale probe for imaging, transfers enough energy initiating the transitio...

In this article, it is found that polarized deep eutectic solvent (DES) molecules can also act as an effective intercalant to delaminate MXene. Moreover, the hydrogen bond accepting and donating molecules in DESs, which have strong interactions with the oxygen functional groups and unsaturated Ti vacancies in the defect sites of Ti₃C₂T _x not only have the superb ability to disperse Ti₃C₂T _x sheets, but also prevent oxidation in both the solution and dried phase.

Abstract

2D transition metal carbides and nitrides, namely MXenes, are normally synthesized in acidic solutions and are delaminated in basic solutions. This results in versatile materials with unique physical/chemical properties suitable for various practical applications. However, solution‐based chemical treatments can affect the chemical structures of MXenes, which accelerates the oxidation reactions and degrades their intrinsic properties. Here, long‐term stable Ti₃C₂T _x dispersion in deep eutectic solvents (DESs) that resisted oxidation degradation for up to 28 weeks is demonstrated. As an anti‐oxidative dispersion medium, DESs helped prevent oxidation of Ti₃C₂T _x layers due to hydrogen bond accepting and donating molecules passivated surface of the Ti₃C₂T _x . In addition, DES molecules in bulk solution can also be hydrated in the presence of water, which stabilizes reactive oxygen by forming stable DES‐water cluster. Therefore, the use of DESs enhanced the delamination of the Ti₃C₂T _x nanosheets, while preventing oxidation of the nanosheets in solution and even in their dried state. As a result, thick and thin films of Ti₃C₂T _x fabricated using DESs exhibited stable sheet resistance in comparison with pristine‐Ti₃C₂T _x . In addition, Ti₃C₂T _x dispersed in DESs can be applied as electrodes for electrochemical capacitors, in which they showed higher chemical stability and better performance than pristine Ti₃C₂T _x .

Hexagon boron nitride (h‐BN) with excellent chemical, mechanical, and optical properties has the potential to replace the use of thick and stiff blocking dielectrics in two‐terminal or three‐terminal devices. Recent progress in h‐BN memories with volatile or nonvolatile properties is presented, expanding the memories to functional applications, and further challenges in the development of h‐BN‐based memories are discussed.

Abstract

The fast development of synthesis routes and preparation technology of 2D materials has motivated a rapid growth in the micro‐ and nanoelectronic memory devices, which gives rise to the breakthroughs in the semiconductor research area. Hexagon boron nitride (h‐BN) with excellent chemical, mechanical, and optical properties has been proven to have potential in overcoming the scaling limit to nanometer, and even sub‐nanometer lengths to replace the use of thick and stiff blocking dielectrics in two‐terminal or three‐terminal devices. The use of atomically thin h‐BN or h‐BN van der Waals heterostructures (vdWhs) can improve the reliability, capability, and functionality of memory devices. This is an encouraging strategy toward high‐density on‐chip integrated circuits, which has recently earned considerable interest. While the research in h‐BN material properties and characterization is comprehensively verified, specified mechanisms of resistive switching have not been analyzed in‐depth. Moreover, recent concern about novel structure design and expanding applications in electronics, optoelectronics, and spintronics has arisen. In this review, recent progress in h‐BN memories with volatile or nonvolatile properties is presented, expanding the memories to functional applications, and further challenges of the development of h‐BN‐based memories and logic circuits are discussed.

In article number 2005443, Fucai Liu, Zheng Liu, and co‐workers provide a comprehensive review of 2D material‐based synaptic devices involving the properties of materials and heterostructures, various multifunctional artificial synapses, and associated neuromorphic applications. The challenges and strategies for the future development of 2D synaptic devices are also discussed.

A systematic experimental and theoretical study of the in‐plane dielectric functions of 2D gallium and indium films consisting of two or three atomic metal layers shows a strong thickness and metal choice dependence of the light–matter interaction and epsilon near‐zero properties, making these 2D polar metals attractive for quantum engineered metal films, tunable (quantum‐)plasmonics, and nanophotonics.

Abstract

This work is a systematic experimental and theoretical study of the in‐plane dielectric functions of 2D gallium and indium films consisting of two or three atomic metal layers confined between silicon carbide and graphene with a corresponding bonding gradient from covalent to metallic to van der Waals type. k‐space resolved free electron and bound electron contributions to the optical response are identified, with the latter pointing towards the existence of thickness dependent quantum confinement phenomena. The resonance energies in the dielectric functions and the observed epsilon near‐zero behavior in the near infrared to visible spectral range, are dependent on the number of atomic metal layers and properties of the metal involved. A model‐based spectroscopic ellipsometry approach is used to estimate the number of atomic metal layers, providing a convenient route over expensive invasive characterization techniques. A strong thickness and metal choice dependence of the light–matter interaction makes these half van der Waals 2D polar metals attractive for quantum engineered metal films, tunable (quantum‐)plasmonics and nano‐photonics.

Chemical vapor deposition of multilayer graphene (MLG) on Cu‐phosphorus eutectic solid solution is proposed. At the growth temperature, the Cu‐phosphorus solid solution forms the liquid phase, which facilitates the formation of MLG. In addition, phosphorus atoms in the Cu catalyst adsorb on the basal plane graphene, and the phosphorus atoms enable the control of doping types of graphene from n‐type to p‐type.

Abstract

A one‐step chemical vapor deposition (CVD) is proposed to grow multilayer graphene (MLG) with tunable doping types using a copper–phosphorus eutectic system as a catalyst. At the growth temperature, the phosphorus‐dissolved copper forms a liquid phase, which promotes the formation of phosphorus‐doped MLG. With this method, the thickness and doping level of graphene are simultaneously controlled at the synthesis stage. Moreover, the proposed CVD method enables patterned growth of MLG at the microscale. The resultant phosphorus‐doped graphene demonstrates a tunable doping state from large n‐type doping to p‐type doping because of the high affinity of phosphorus to water molecules. Finally, stable n‐type doping of MLG by passivating it with a parylene thin film is demonstrated.

Nature Nanotechnology, Published online: 21 January 2021; doi:10.1038/s41565-020-00833-9

Highly stable and ultrapermeable membranes can be fabricated by the hybridization of zeolitic imidazolate framework-8 and graphene oxide.

Transition metal dichalcogenides, intercalated with transition metals, are studied for their potential applications as dilute magnetic semiconductors. We investigate the magnetic properties of WSe 2 doped with third-row transition metals (Co, Cr, Fe, Mn, Ti and V). Using density functional theory in combination with Monte Carlo simulations, we obtain an estimate of the Curie or Néel temperature. We find that the magnetic ordering is highly dependent on the dopant type. While Ti and Cr-doped WSe 2 have a ferromagnetic ground state, V, Mn, Fe and Co-doped WSe 2 are antiferromagnetic in their ground state. For Fe doped WSe 2 , we find a high Curie-temperature of 327 K. In the case of V-doped WSe 2 , we find that there are two distinct magnetic phase transitions, originating from a frustrated in-plane antiferromagnetic exchange interaction and a ferromagnetic out-of-plane interaction. We calculate the formation energy and reveal that, ...

Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long-lived valley polarization for potential valleytronic applications. However, the roles of the chemical composition and geometric alignment of the constituent layers in the underlying dynamics remain largely unexplored. Here we study spin–valley relaxation dynamics in heterobilayers with different structures and optical properties engineered via the use of alloyed monolayer semiconductors. Through a combination of time-resolved Kerr rotation spectroscopic measurements and theoretical modeling for Mo 1 −  x W x Se 2 /WSe 2 samples with different chemical compositions and stacking angles, we uncover the contributions of the interlayer exciton recombination and charge carrier spin depolarization to the overall valley dynamics. We show that the corresponding decay rates can be tuned in a wide range in transit...

Hexagonal boron nitride ( h -BN) has long been recognized as an ideal substrate for electronic devices due to its dangling-bond-free surface, insulating nature and thermal/chemical stability. These properties of the h -BN multilayer are mainly determined by its lattice structure. Therefore, to analyse the lattice structure and orientation of h -BN crystals becomes important. Here, the stacking order and wrinkles of h -BN are investigated by transmission electron microscopy. It is experimentally confirmed that the layers in the h -BN flakes are arranged in the AA′ stacking. The wrinkles in a form of threefold network throughout the h -BN crystal are oriented along the armchair direction, and their formation mechanism was further explored by molecular dynamics simulations. Our findings provide a deep insight about the microstructure of h -BN and shed light on the structural design/electronic modulations of two-dimensional crystals.

Atomic layers of black phosphorus (BP) present unique opto-electronic properties dominated by a direct tunable bandgap in a wide spectral range from visible to mid-infrared (IR). In this work, we investigate the IR photoluminescence (PL) of BP single crystals at very low temperature. Near-band-edge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is calculated with an anisotropic Wannier–Mott model and found equal to 9.1 meV. On the contrary, the PL intensity quenching of the 0.276 eV peak at high temperature is found with a much smaller activation energy, attributed to the localization of free excitons on a shallow impurity. This analysis leads us to attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons and free excitons in BP. As a result, the value of bulk BP bandgap is refined to 0.287 eV at 2 K.

Field-effect transistors (FETs) based on van der Waals heterostructures without the traditional lattice mismatch constraints are highly promising for electronics. As the devices’ scale decreases, it is crucial to find a dielectric layer with a high dielectric constant to ensure gate capacitance and low leakage current. Herein, a layered insulator VOCl single crystal synthesized by chemical vapor transport is demonstrated as a high-performance gate dielectric. Notably, the dielectric constant of VOCl can reach up to 11.7, estimated through measuring the capacitance of the metal–insulator-metal device. The MoSe 2 FET with VOCl dielectric exhibits a significant decrease in the subthreshold swing from 4906 to 169 mV dec −1 , indicating a low trap density at the interface of MoSe 2 /VOCl. Besides, the threshold voltage ( V th ) of bottom-gated MoSe 2 FET is as low as 0.2 V, further confirming the high potential of VOCl as an ideal...

Chirality emergence in biological systems is common but its nature in water‐based systems is poorly understood. In this work, a well‐defined periodic texture is observed in chiral induced chromonics confined in a curved geometry as a microsphere. An attempt to tune the texture is carried out using cations in view of possible biocompatible technological applications.

Abstract

Chirality emergence in biological systems is common but the chiral expression from the molecular to macroscopic level in water‐based systems is poorly understood. Among water‐based systems, chromonic liquid crystals have recently received a lot of attention due to the spontaneous chirality they show when confined in curved geometries. Confinement of chiral‐induced chromonics is not trivial since they are three component systems whose time stability is a delicate thermodynamic balance. In this work, a well‐defined periodic Frank–Pryce texture, typical of chiral thermotropic liquid crystals, is observed in microspheres of a chiral induced chromonic embedded in a poly(dimethylsiloxane) matrix. This texture slowly degrades in time and a possible mechanism behind the degradation process is suggested via X‐ray diffraction and atomic force microscopy measurements on thin chromonic films. To stabilize this texture and to control the structure periodicity, cations are added to the three components system in an attempt to tune the non‐covalent interactions between molecules and supramolecular stacks. The study of the effects of this addition allows for better insight into the molecular interactions that occur in the chiral induced mesophase. This is a crucial point in view of possible biocompatible technological applications.

Laser‐induced graphene (LIG) tape as stick‐on photothermal labels for developing light‐driven actuators based on Marangoni effect is reported. The LIG embedded PI tape can be either stuck on any desired objects or folded into 3D origami, forming a photothermal Marangoni actuator. The LIG tapes hold great promise for the facile development of light‐driven soft robots.

Abstract

Direct light‐to‐work conversion enables remote actuation through a non‐contact manner, among which the photothermal Marangoni effect is significant for developing light‐driven robots because of the diversity of applicable photothermal materials and light sources, as well as the high energy conversion efficiency. However, the lack of nanotechnologies that enable flexible integration of advanced photothermal materials with actuators of complex configurations significantly restricts their practical applications. In this paper, laser‐induced graphene (LIG) tape is reported as stick‐on photothermal labels for developing light‐driven actuators based on the Marangoni effect. With the help of direct laser writing technology, graphene patterns with superior photothermal properties are prepared on the PI tape. The patterned LIG tape can be stuck on any desired objects and generates an asymmetric photothermal field under light irradiation, forming a photothermal Marangoni actuator. Additionally, the PI tape with LIG patterns can be folded into 3D origami actuators that permit photothermal Marangoni actuation including both translation and rotation. The graphene‐based photothermal Marangoni actuators feature biocompatibility, which is confirmed by MDA‐MB‐231 cells proliferation experiments. Owing to the excellent photothermal property of LIG patterns, the as‐produced photothermal actuators can be manipulated by a variety of light sources, holding great promise for developing light‐driven soft robots.

This work makes a review of recent advances in 2D rare earth materials, which are the rising star in 2D areas. The crystal structure, synthesis methods, properties and applications of 2D rare earth materials are summarized. Finally, the problems, future challenges, and new opportunities of this area are also discussed.

Abstract

2D rare earth (RE) materials have received considerable attention in recent years due to the fascinating luminescence, magnetism, and electric properties originated from RE associated with sharp and various emission peaks, intrinsic 2D ferromagnetism, and incommensurate charge density wave. These materials might open up a new prospect in next‐generation lighting, magnetic devices, and phototransistors. Herein, a comprehensive review of 2D RE materials is presented, focusing on their recent progresses. First, the crystal structures of 2D RE materials are discussed. Then, typical synthesis methods such as mechanical exfoliation, molecular beam epitaxy, pulsed laser deposition, and chemical vapor deposition are introduced. Furthermore, various properties in luminescence, magnetism, and electronics are summarized. The recently reported RE‐based 2D novel photodetectors are outlined as three constructions: MoS₂/RE, graphene/RE, and perovskite/RE, which show promising applications for both narrow and broad band detection arised from the special absorption windows of different RE elements. Finally, the conclusions and outlook of this area are proposed, such as exploring novel 2D RE compounds, improving stability, and broadening applications.