Controlled Growth and Thickness-Dependent Conduction-Type Transition of 2D Ferrimagnetic Cr2 S3 Semiconductors

Controllable Syntheses, Structure Identifications, and Property Explorations of Self-Intercalated 2D Transition Metal Chalcogenides

2D transition metal dichalcogenides (2D TMDCs) have attracted intensive interest in physics and materials science-related fields, due to their exotic properties (e.g., superconductivity, charge density wave (CDW) phase transition, magnetism, electrocatalytic property). Intercalation of native metal atoms in the layered 2D TMDCs (e.g., from VS2 to V5S8 by V intercalation) can afford new stoichiometric ratios, phase states, and thus rich properties. This review hereby summarizes the recent progress in the controllable syntheses, structure characterizations, and property explorations of self-intercalated 2D transition metal chalcogenides (TMCs), with the metal elements focusing on group-V, VI, and VIII metals. The self-intercalation-related synthetic strategies will be introduced via chemical vapor deposition (CVD) and molecule beam epitaxy (MBE), especially by tuning the chemical potentials of intercalated metal elements, growth promoters, substrates, etc. Additionally, the structure/phase identifications of the self-intercalated 2D TMCs through various characterization techniques will be overviewed. More significantly, the intriguing properties in such 2D TMCs will be thoroughly discussed, such as the thickness- or composition-dependent magnetism, CDW phase transition, electrocatalytic property, etc. Finally, challenges and prospects are proposed for developing new self-intercalated 2D materials and their heterostructures and exploring their unique properties and applications.

Ultrahigh Exchange Bias Field/Coercive Field Ratio in In Situ Formed Two-Dimensional Magnetic Te-Cr2O3/Cr5Te6 Heterostructures

The exchange bias (EB) effect is a fundamental magnetic phenomenon, in which the exchange bias field/coercive field ratio (|HEB/HC|) can improve the stability of spintronic devices. Two-dimensional (2D) magnetic heterostructures have the potential to construct low-power and high-density spintronic devices, while their typically air unstable and |HEB/HC| lesser, limiting the possibility of applications. Here, 2D Cr5Te6 nanosheets have been systematically synthesized with an in situ formed ≈2 nm-thick Te doped Cr2O3 layer (Te-Cr2O3) on the upper surface by chemical vapor deposition (CVD) method. The strong and air stable EB effect, achieving a |HEB/HC| of up to 80% under an ultralow cooling field of 0.01 T, which is greater than that of the reported 2D magnetic heterostructures. Meanwhile, the uniformity of thickness and chemical composition of the Te-Cr2O3 layer can be controlled by the growth conditions which are highly correlated with the EB effect of 2D Te-Cr2O3/Cr5Te6 heterostructures. First-principles calculations show that the Te-Cr2O3 can provide uncompensated spins in the Cr2O3, thus forming strong spin pinning effect. The systematical investigation of the EB effect in 2D Te-Cr2O3/Cr5Te6 heterostructures with high |HEB/HC| will open up exciting opportunities in low-power and high-stability 2D spintronic devices.

Dramatic switchable polarities in conduction type and self-driven photocurrent of BiI3 via pressure engineering

The intentional manipulation of carrier characteristics serves as a fundamental principle underlying various energy-related and optoelectronic semiconductor technologies. However, achieving switchable and reversible control of the polarity within a single material to design optimized devices remains a significant challenge. Herein, we successfully achieved dramatic reversible p-n switching during the semiconductor‒semiconductor phase transition in BiI3 via pressure, accompanied by a substantial improvement in their photoelectric properties. Carrier polarity flipping was monitored by measuring the photocurrent dominated by the photothermoelectric (PTE) effect in a zero-bias two-terminal device. Accompanying the p-n transition, a switch between positive and negative photocurrents was observed in BiI3, providing a feasible method to determine the conduction type of materials via photoelectric measurements. Furthermore, the combined effects of the photoconductivity and PTE mechanism improved the photoresponse and extended the detection bandwidth to encompass the optical communication waveband (1650 nm) under an external bias. The remarkable photoelectric properties were attributed to the enhanced energy band dispersion and increased charge density of BiI3 under pressure. These findings highlight the effective and flexible modulation of carrier properties through pressure engineering and provide a foundation for designing and implementing multifunctional logic circuits and optoelectronic devices.

Ferroelectric Perovskite/MoS2 Channel Heterojunctions for Wide-Window Nonvolatile Memory and Neuromorphic Computing

Ferroelectric materials commonly serve as gate insulators in typical field-effect transistors, where their polarization reversal enables effective modulation of the conductivity state of the channel material, thereby realizing non-volatile memory. Currently, novel 2D ferroelectrics unlock new prospects in next-generation electronics and neuromorphic computation. However, the advancement of these materials is impeded by limited selectivity and narrow memory windows. Here, new concepts of 2D ferroelectric perovskite/MoS2 channel heterostructures field-effect transistors are presented, in which 2D ferroelectric perovskite features customizable band structure, few-layered ferroelectricity, and submillimeter-size monolayer wafers. Further studies reveal that these devices exhibit unique charge polarity modulation (from n- to p-type channel) and remarkable nonvolatile memory behavior, especially record-wide hysteresis windows up to 177 V, which enables efficient imitation of biological synapses and achieves high recognition accuracy for electrocardiogram patterns. This result provides a device paradigm for future nonvolatile memory and artificial synaptic applications.

Lithium Ion Intercalation-Induced Metal-Insulator Transition in Inclined-Standing Grown 2D Non-Layered Cr2S3 Nanosheets

Gate-controlled ionic intercalation in the van der Waals gap of 2D layered materials can induce novel phases and unlock new properties. However, this strategy is often unsuitable for densely packed 2D non-layered materials. The non-layered rhombohedral Cr2S3 is an intrinsic heterodimensional superlattice with alternating layers of 2D CrS2 and 0D Cr1/3. Here an innovative chemical vapor deposition method is reported, utilizing strategically modified metal precursors to initiate entirely new seed layers, yields ultrathin inclined-standing grown 2D Cr2S3 nanosheets with edge instead of face contact with substrate surfaces, enabling rapid all-dry transfer to other substrates while ensuring high crystal quality. The unconventional ordered vacancy channels within the 0D Cr1/3 layers, as revealed by cross-sectional scanning transmission electron microscope, permitting the insertion of Li+ ions. An unprecedented metal-insulator transition, with a resistance modulation of up to six orders of magnitude at 300 K, is observed in Cr2S3-based ionic field-effect transistors. Theoretical calculations corroborate the metallization induced by Li-ion intercalation. This work sheds light on the understanding of growth mechanism, structure-property correlation and highlights the diverse potential applications of 2D non-layered Cr2S3 superlattice.

A Newly Predicted Magnetic TMB6 Monolayer for Efficient Nitrogen Fixation

Developing electrocatalysts for the N2 reduction reaction (NRR) with high activity, high selectivity, and low cost is urgently required to enhance the NH3 yield rate. Based on first-principles calculations, we predict a series of new transition metal boride TMB6 (TM = Ti, V, Cr, Mn, Fe, and Co) monolayers and investigate their magnetoelectronic and electrocatalytic properties. The results reveal that VB6 and CoB6 favor ferromagnetic coupling, while TiB6, CrB6, MnB6, and FeB6 display antiferromagnetic ordering. Furthermore, TiB6 exhibits a high Néel temperature of 344 K and a large magnetic anisotropy energy of 614 μeV per Ti atom. Most interestingly, TiB6 and VB6 exhibit superior NRR catalytic activity with a limiting potential of -0.50 and -0.19 V, respectively, and favorable NRR selectivity over the HER. Finally, the structural stability of TMB6 monolayers has been confirmed by a set of phonon dispersion, molecular dynamics, and elastic constant calculations. Our results highlight the use of the newly designed two-dimensional (2D) TM borides as promising candidates for spintronic devices and nitrogen fixation applications.

Recent Progress in Two-Dimensional Magnetic Materials

The giant magnetoresistance effect in two-dimensional (2D) magnetic materials has sparked substantial interest in various fields; including sensing; data storage; electronics; and spintronics. Their unique 2D layered structures allow for the manifestation of distinctive physical properties and precise performance regulation under different conditions. In this review, we present an overview of this rapidly developing research area. Firstly, these 2D magnetic materials are catalogued according to magnetic coupling types. Then, several vital effects in 2D magnets are highlighted together with theoretical investigation, such as magnetic circular dichroism, magneto-optical Kerr effect, and anomalous Hall effect. After that, we forecast the potential applications of 2D magnetic materials for spintronic devices. Lastly, research advances in the attracting magnons, skyrmions and other spin textures in 2D magnets are discussed.

Anisotropic nonlinear optical responses of Ta2NiS5 flake towards ultrafast logic gates and secure all-optical information transmission

Optical logic gates based on nonlinear optical property of material with ultrafast response speed and excellent computational processing power can break the performance bottleneck of electronic transistors. As one of the layered 2D materials, Ta2NiS5 exhibits high anisotropic mobility, exotic electrical response, and intriguing optical properties. Due to the low-symmetrical crystal structures, it possesses in-plane anisotropic physical properties. The optical absorption information of Ta2NiS5 is investigated by anisotropic linear absorption spectra, femtosecond laser intensity scanning (I-scan), and non-degenerate pump-probe technology. The I-scan results show a distinct maximum of ∼4.9 % saturable absorption (SA) and ∼4 % reverse saturable absorption (RSA) at different polarization directions of the incident laser. And, these unique nonlinear optical (NLO) properties originate from the anisotropic optical transition probability. Furthermore, the novel Ta2NiS5-based all-optical logic gates are proposed by manipulating the NLO absorption processes. And, the all-optical OR and NOR logic gates possess an ultrafast response speed approaching 1.7 THz. Meanwhile, an all-optical information transmission method with higher security and accuracy is achieved, which has promising potential to avoid the disclosure of information. This work provides a new path for designing versatile and novel optical applications based on Ta2NiS5 materials.

Polarized Raman spectroscopy study of CVD-grown Cr2S3 flakes: unambiguous identification of phonon modes

We report a systematic Raman spectroscopy investigation of chemical vapor deposited 2D nonlayered Cr2S3, with both linearly and circularly polarized light over a wide temperature range (5-300 K). Temperature-dependent Raman spectra exhibit a good linear relationship between the peak positions of the phonon modes and temperature. Angle-resolved polarized Raman spectra reveal the polarization-dependent optical response of in-plane and out-of-plane phonon modes. Helicity-dependent Raman investigations complete definite assignment of all the phonon modes observed in the Raman spectra of 2D nonlayered Cr2S3 by the optical selection rule based on a Raman tensor. Our work realizes clear phonon mode identification over a wide temperature range for the emerging material 2D Cr2S3, an important representative of nonlayered 2D system with unique properties for optoelectronic and spintronic applications.

Epitaxial growth of quasi-2D van der Waals ferromagnets on crystalline substrates

Intrinsic magnetism in van der Waals materials has instigated interest in exploring magnetism in the 2D limit for potential applications in spintronics and also in understanding novel control of 2D magnetism via variation of layer thickness, gate tunability and magnetoelectric effects. The chromium telluride (CrxTey) family is an interesting subsection of ferromagnetic materials with highTCvalues, also presenting diverse stoichiometry arising from self-intercalation of Cr. Apart from the layered CrTe2system, the other non-layered CrxTeycompounds also offer exceptional magnetic properties, and a novel growth technique to grow thin films of these non-layered compounds offers exciting possibilities for ultra-thin spin-based electronics and magnetic sensors. In this work, we discuss the role of crystalline substrates in chemical vapor deposition growth of non-layered 2D ferromagnets, where the crystal symmetry of the substrate as well as the misfit and strain are the key players governing the growth mechanism of ultra-thin Cr5Te8, a non-layered ferromagnet. The magnetic studies of the as-grown Cr5Te8reveal the signatures of co-existing soft and hard ferromagnetic phases, which makes this system an intriguing system to search for emergent topological phases, such as magnetic skyrmions.

Liquid Phase Exfoliation of Few-Layer Non-Van der Waals Chromium Sulfide

Exfoliation of 2D non-Van der Waals (non-vdW) semiconductor nanoplates (NPs) from inorganic analogs presents many challenges ahead for further exploring of their advanced applications on account of the strong bonding energies. In this study, the exfoliation of ultrathin 2D non-vdW chromium sulfide (2D Cr2S3) by means of a combined facile liquid-phase exfoliation (LPE) method is successfully demonstrated. The morphology and structure of the 2D Cr2S3 material are systematically examined. Magnetic studies show an obvious temperature-dependent uncompensated antiferromagnetic behavior of 2D Cr2S3. The material is further loaded on TiO2 nanorod arrays to form an S-scheme heterojunction. Experimental measurements and density functional theory (DFT) calculations confirm that the formed TiO2@Cr2S3 S-scheme heterojunction facilitates the separation and transmission of photo-induced electron/hole pairs, resulting in a significantly enhanced photocatalytic activity in the visible region.

Microscopy aided detection of the self-intercalation mechanism and in situ electronic properties in chromium selenide

Two-dimensional (2D) chromium-based self-intercalated materials Cr1+nX2 (0 ≤ n ≤ 1, X = S, Se, Te) have attracted much attention because of their tunable magnetism with good environmental stability. Intriguingly, the magnetic and electrical properties of the materials can be effectively tuned by altering the coverage and spatial arrangement of the intercalated Cr (ic-Cr) within the van der Waals gap, contributing to different stoichiometries. Several different Cr1+nX2 systems have been widely investigated recently; however, those with the same stoichiometric ratio (such as Cr1.25Te2) were reported to exhibit disparate magnetic properties, which still lacks explanation. Therefore, a systematic in situ study of the mechanisms with microscopy techniques is in high demand to look into the origin of these discrepancies. Herein, 2D self-intercalated Cr1+nSe2 nanoflakes were synthesized as a platform to conduct the characterization. Combining scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM), we studied in depth the microscopic structure and local electronic properties of the Cr1+nSe2 nanoflakes. The self-intercalation mechanism of ic-Cr and local stoichiometric-ratio variation in a Cr1+nSe2 ultrathin nanoflake is clearly detected at the nanometer scale. Scanning tunneling spectroscopy (STS) measurements indicate that Cr1.5Se2/Cr2Se2 and Cr1.25Se2 exhibit conductive and semiconductive behaviors, respectively. The STM tip manipulation method is further applied to manipulate the microstructure of Cr1+nSe2, which successfully produces clean zigzag-type boundaries. Our systematic microscopy study paves the way for the in-depth study of the magnetic mechanism of 2D self-intercalated magnets at the nano/micro scale and the development of new magnetic and spintronic devices.

Stoichiometry-Tunable Synthesis and Magnetic Property Exploration of Two-Dimensional Chromium Selenides

Emerging 2D chromium-based dichalcogenides (CrXn (X = S, Se, Te; 0 < n ≤ 2)) have provoked enormous interests due to their abundant structures, intriguing electronic and magnetic properties, excellent environmental stability, and great application potentials in next generation electronics and spintronics devices. Achieving stoichiometry-controlled synthesis of 2D CrXn is of paramount significance for such envisioned investigations. Herein, we report the stoichiometry-controlled syntheses of 2D chromium selenide (CrxSey) materials (rhombohedral Cr2Se3 and monoclinic Cr3Se4) via a Cr-self-intercalation route by designing two typical chemical vapor deposition (CVD) strategies. We have also clarified the different growth mechanisms, distinct chemical compositions, and crystal structures of the two type materials. Intriguingly, we reveal that the ultrathin Cr2Se3 nanosheets exhibit a metallic feature, while the Cr3Se4 nanosheets present a transition from p-type semiconductor to metal upon increasing the flake thickness. Moreover, we have also uncovered the ferromagnetic properties of 2D Cr2Se3 and Cr3Se4 below ∼70 K and ∼270 K, respectively. Briefly, this research should promote the stoichiometric-ratio controllable syntheses of 2D magnetic materials, and the property explorations toward next generation spintronics and magneto-optoelectronics related applications.

Coexistence of Ferroelectricity and Ferromagnetism in Atomically Thin Two-Dimensional Cr2S3/WS2 Vertical Heterostructures

Two-dimensional (2D) heterostructures with ferromagnetism and ferroelectricity provide a promising avenue to miniaturize the device size, increase computational power, and reduce energy consumption. However, the direct synthesis of such eye-catching heterostructures has yet to be realized up to now. Here, we design a two-step chemical vapor deposition strategy to growth of Cr2S3/WS2 vertical heterostructures with atomically sharp and clean interfaces on sapphire. The interlayer charge transfer and periodic moiré superlattice result in the emergence of room-temperature ferroelectricity in atomically thin Cr2S3/WS2 vertical heterostructures. In parallel, long-range ferromagnetic order is discovered in 2D Cr2S3 via the magneto-optical Kerr effect technique with the Curie temperature approaching 170 K. The charge distribution variation induced by the moiré superlattice changes the ferromagnetic coupling strength and enhances the Curie temperature. The coexistence of ferroelectricity and ferromagnetism in 2D Cr2S3/WS2 vertical heterostructures provides a cornerstone for the further design of logic-in-memory devices to build new computing architectures.

Controlled Growth of Submillimeter-Scale Cr5Te8 Nanosheets and the Domain Wall Nucleation Governed Magnetization Reversal Process

Two-dimensional (2D) ferromagnets have attracted widespread attention for promising applications in compact spintronic devices. However, the controlled synthesis of high-quality, large-sized, and ultrathin 2D magnets via facile, economical method remains challenging. Herein, we develop a hydrogen-tailored chemical vapor deposition approach to fabricating 2D Cr5Te8 ferromagnetic nanosheets. Interestingly, the time period of introducing hydrogen was found to be crucial for controlling the lateral size, and a Cr5Te8 single-crystalline nanosheet of lateral size up to ∼360 μm with single-unit-cell thickness has been obtained. These samples exhibit a leading role of domain wall nucleation in governing the magnetization reversal process, providing important references for optimizing the performances of associated devices. The nanosheets also show notable magnetotransport response, including nonmonotonous magnetic-field-dependent magnetoresistance and sizable anomalous Hall resistivity, demonstrating Cr5Te8 as a promising material for constructing high-performance magnetoelectronic devices. This study presents a breakthrough of large-sized CVD-grown 2D magnetic materials, which is indispensable for constructing 2D spintronic devices.

Enhancing Metallicity and Basal Plane Reactivity of 2D Materials via Self-Intercalation

Intercalation (ic) of metal atoms into the van der Waals (vdW) gap of layered materials constitutes a facile strategy to create materials whose properties can be tuned via the concentration of the intercalated atoms. Here we perform systematic density functional theory calculations to explore various properties of an emergent class of crystalline 2D materials (ic-2D materials) comprising vdW homobilayers with native metal atoms on a sublattice of intercalation sites. From an initial set of 1348 ic-2D materials, generated from 77 vdW homobilayers, we find 95 structures with good thermodynamic stability (formation energy within 200 meV/atom of the convex hull). A significant fraction of the semiconducting host materials are found to undergo an insulator to metal transition upon self-intercalation, with only PdS2, PdSe2, and GeS2 maintaining a finite electronic gap. In five cases, self-intercalation introduces magnetism. In general, self-intercalation is found to promote metallicity and enhance the chemical reactivity on the basal plane. Based on the calculated H binding energy, we find that self-intercalated SnS2 and Hf3Te2 are promising candidates for hydrogen evolution catalysis. All the stable ic-2D structures and their calculated properties can be explored in the open C2DB database.

Robust multiferroic in interfacial modulation synthesized wafer-scale one-unit-cell of chromium sulfide

Multiferroic materials offer a promising avenue for manipulating digital information by leveraging the cross-coupling between ferroelectric and ferromagnetic orders. Despite the ferroelectricity has been uncovered by ion displacement or interlayer-sliding, one-unit-cell of multiferroic materials design and wafer-scale synthesis have yet to be realized. Here we develope an interface modulated strategy to grow 1-inch one-unit-cell of non-layered chromium sulfide with unidirectional orientation on industry-compatible c-plane sapphire. The interfacial interaction between chromium sulfide and substrate induces the intralayer-sliding of self-intercalated chromium atoms and breaks the space reversal symmetry. As a result, robust room-temperature ferroelectricity (retaining more than one month) emerges in one-unit-cell of chromium sulfide with ultrahigh remanent polarization. Besides, long-range ferromagnetic order is discovered with the Curie temperature approaching 200 K, almost two times higher than that of bulk counterpart. In parallel, the magnetoelectric coupling is certified and which makes 1-inch one-unit-cell of chromium sulfide the largest and thinnest multiferroics.

van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

Surface Stability and Exfoliability of Non-van der Waals Magnetic Chromium Tellurides

Exfoliation of two-dimensional (2D) magnetic materials from non-van der Waals (non-vdW) materials has attracted increasing attention because it provides a great platform for the construction of 2D magnetic materials. For non-vdW magnetic chromium tellurides with high Curie temperatures, their few-layer samples show promising applications in the field of spintronics. However, there is still no consensus on whether the surface structures of few-layer chromium tellurides should be terminated by Cr or Te atoms. By calculating the surface and exfoliation energy, we find that which structure is more stable depends greatly on the value of the chemical potential of Te atoms, and the few-layer sample with a Cr-terminated surface is easier to exfoliate than that with both Te-terminated surfaces. Finally, we propose that different exfoliated structures can be identified by using the atomic number ratio of Cr to Te and the average magnetic moment of Cr atoms in few-layer samples.

Phase Engineering of 2D Spinel-Type Manganese Oxides

2D magnetic materials have been of interest due to their unique long-range magnetic ordering in the low-dimensional regime and potential applications in spintronics. Currently, most studies are focused on strippable van der Waals magnetic materials with layered structures, which typically suffer from a poor stability and scarce species. Spinel oxides have a good environmental stability and rich magnetic properties. However, the isotropic bonding and close-packed nonlayered crystal structure make their 2D growth challenging, let alone the phase engineering. Herein, a phase-controllable synthesis of 2D single-crystalline spinel-type oxides is reported. Using the van der Waals epitaxy strategy, the thicknesses of the obtained tetragonal and hexagonal manganese oxide (Mn3 O4 ) nanosheets can be tuned down to 7.1 nm and one unit cell (0.7 nm), respectively. The magnetic properties of these two phases are evaluated using vibrating-sample magnetometry and first-principle calculations. Both structures exhibit a Curie temperature of 48 K. Owing to its ultrathin geometry, the Mn3 O4 nanosheet exhibits a superior ultraviolet detection performance with an ultralow noise power density of 0.126 pA Hz-1/2 . This study broadens the range of 2D magnetic semiconductors and highlights their potential applications in future information devices.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Precise Construction and Growth of Submillimeter Two-Dimensional WSe2 and MoSe2 Monolayers

Currently, as shown by large-scale research on two-dimensional materials in the field of nanoelectronics and catalysis, the construction of large-area two-dimensional materials is crucial for the development of devices and their application in photovoltaics, sensing, optoelectronics, and energy generation/storage. Here, using atmospheric-pressure chemical vapor deposition, we developed a method to regulate growth conditions according to the growth mechanism for WSe₂ and MoSe₂ materials. By accurately controlling the hydrogen flux within the range of 1 sccm and the distance between the precursor and the substrate, we obtained large-size films of single atomic layers with thicknesses of only about 1 nm. When growing the samples, we could not only obtain a 100 percent proportion of samples with the same shape, but the samples could also be glued into pieces of 700 μm and above in size, changing the shape and making it possible to reach the millimeter/submillimeter level visible to the naked eye. Our method is an effective method for the growth of large-area films with universal applicability.

Controllable CVD Growth of 2D Cr5Te8 Nanosheets with Thickness-Dependent Magnetic Domains

As a unique 2D magnetic material with self-intercalated structure, Cr₅Te₈ exhibits many intriguing magnetic properties. While its ferromagnetism of Cr₅Te₈ has been previously reported, the research on its magnetic domain remains unexplored. Herein, we have successfully fabricated 2D Cr₅Te₈ nanosheets with controlled thickness and lateral size by chemical vapor deposition (CVD). Then magnetic property measurement system revealed Cr₅Te₈ nanosheets exhibiting intense out-of-plane ferromagnetism with a Curie temperature (T_C) of 176 K. Significantly, we reported for the first time two magnetic domains: magnetic bubbles and thickness-dependent maze-like magnetic domains in our Cr₅Te₈ nanosheets by cryogenic magnetic force microscopy (MFM). The domain width of the maze-like magnetic domains increases rapidly with decreasing sample thickness; meanwhile, the domain contrast decreases. This indicates the dominant role of ferromagnetism shifts from dipolar interactions to magnetic anisotropy. Our research not only establishes a pathway for the controllable growth of 2D magnetic materials but also points toward novel avenues for regulating magnetic phases and methodically tuning domain characteristics.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Even-Odd-Layer-Dependent Ferromagnetism in 2D Non-van-der-Waals CrCuSe2

Van der Waals (vdW) layered materials with strong magnetocrystalline anisotropy have attracted significant interest as the long-range magnetic order in these systems can survive even when their thicknesses is reduced to the 2D limit. Even though the interlayer coupling between the neighboring magnetic layers is very weak, it has a determining effect on the magnetism of these atomic-thickness materials. Herein, a new 2D ferromagnetic material, namely, non-vdW CuCrSe₂ nanosheets with even-odd-layer-dependent ferromagnetism when laminated from an antiferromagnetic bulk is reported. Monolayer and even-layer CuCrSe₂ exhibit the anomalous Hall effect and a significantly enhanced magnetic ordering temperature of more than 125 K. In contrast, the linear Hall effect exists in the odd-layer samples. Theoretical calculations indicate that the layer-dependent magnetic coupling is attributable to the orbital shift of the Cr atoms in the CrSe₂ layers owing to the Cu-induced breaking of the centrosymmetry. Thus, this work sheds light on the exotic magnetic properties of layered materials that exhibit phenomena beyond weak interlayer interactions.

Two-Dimensional Iron Silicide (FeSix) Alloys with Above-Room-Temperature Ferromagnetism

Two-dimensional (2D) materials with intrinsic room-temperature ferromagnetism have gathered tremendous interest as promising candidates for next-generation spintronics. Here, on the basis of first-principles calculations, we report a family of stable 2D iron silicide (FeSi_x) alloys via dimensional reduction of their bulk counterparts. Our results demonstrate that 2D Fe₄Si₂-hex, Fe₄Si₂-orth, Fe₃Si₂, and FeSi₂ nanosheets are lattice-dynamically and thermally stable, confirmed by the calculated phonon spectra and Born-Oppenheimer dynamic simulation up to 1000 K. 2D FeSi_x nanosheets are ferromagnetic metals with estimated Curie temperatures ranging from 547 to 971 K due to strong direct exchange interaction between Fe sites. In addition, the electronic properties of 2D FeSi_x alloys can be maintained on silicon substrates, providing an ideal platform for spintronics applications in the nanoscale.

Interisland-Distance-Mediated Growth of Centimeter-Scale Two-Dimensional Magnetic Fe3O4 Arrays with Unidirectional Domain Orientations

Two-dimensional (2D) nanosheet arrays with unidirectional orientations are of great significance for synthesizing wafer-scale single crystals. Although great efforts have been devoted, the growth of atomically thin magnetic nanosheet arrays and single crystals is still unaddressed. Here we design an interisland-distance-mediated chemical vapor deposition strategy to synthesize centimeter-scale atomically thin Fe₃O₄ arrays with unidirectional orientations on mica. The unidirectional alignment of nearly all the Fe₃O₄ nanosheets is driven by a dual-coupling-guided growth mechanism. The Fe₃O₄/mica interlayer interaction induces two preferred antiparallel orientations, whereas the interisland interaction of Fe₃O₄ breaks the energy degeneracy of antiparallel orientations. The room-temperature long-range ferrimagnetic order and thickness-tunable magnetic domain evolution are uncovered in atomically thin Fe₃O₄. This strategy to tune the orientations of nanosheets through the an interisland interaction can guide the synthesis of other 2D transition-metal oxides, thereby laying a solid foundation for future spintronic device applications at the integration level.

General Synthesis of 2D Magnetic Transition Metal Dihalides via Trihalide Reduction

Two-dimensional (2D) transition metal dihalides (TMDHs) have been receiving extensive attention due to their diversified magnetic properties and promising applications in spintronics. However, controlled growth of 2D TMDHs remains challenging owing to their extreme sensitivity to atmospheric moisture. Herein, using a home-built nitrogen-filled interconnected glovebox system, a universal chemical vapor deposition synthesis route of high-quality 2D TMDH flakes (1T-FeCl₂, FeBr₂, VCl₂, and VBr₂) by reduction of their trihalide counterparts is developed. Representatively, ultrathin (∼8.6 nm) FeCl₂ flakes are synthesized on SiO₂/Si, while on graphene/Cu foil the thickness can be down to monolayer (1L). Reflective magnetic circular dichroism spectroscopy shows an interlayer antiferromagnetic ordering of FeCl₂ with a Neel temperature at ∼17 K. Scanning tunneling microscopy and spectroscopy further identify the atomic-scale structures and band features of 1L and bilayer FeCl₂ on graphene/Cu foil.

Composition-Controllable Syntheses and Property Modulations from 2D Ferromagnetic Fe5 Se8 to Metallic Fe3 Se4 Nanosheets

Exploring new-type 2D magnetic materials with high magnetic transition temperature and robust air stability has attracted wide attention for developing innovative spintronic devices. Recently, intercalation of native metal atoms into the van der Waals gaps of 2D layered transition metal dichalcogenides (TMDs) has been developed to form 2D non-layered magnetic TMDs, while only succeeded in limited systems (e.g., Cr₂ S₃ , Cr₅ Te₈ ). Herein, composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe₅ Se₈ and 50% Fe-intercalated monoclinic Fe₃ Se₄ ) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe₅ Se₈ exhibits intrinsic room-temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'-FeSe₂ to non-layered Fe-intercalated Fe₅ Se₈ based on density functional theory calculations. In contrast, the ultrathin Fe₃ Se₄ presents novel metallic features comparable with that of metallic TMDs. This work hereby sheds light on the composition-controllable synthesis and fundamental property exploration of 2D self-intercalation induced novel TMDs compounds, by propelling their application explorations in nanoelectronics and spintronics-related fields.

Two-dimensional chalcogenide-based ferromagnetic semiconductors

Two-dimensional (2D) magnetic materials draw an enormous amount of attention due to their novel physical properties and potential spintronics device applications. Room-temperature ferromagnetic (FM) semiconductors have long been pursued in 2D magnetic materials, which show a long range magnetic order down to atomic-layer thickness. The intrinsic ferromagnetism has been predicted in a series of 2D materials and verified in experiments and the magnetism can be modulated by multiple physical fields, exhibiting promising application prospects. In this review, we overview several types of 2D chalcogenide-based FM semiconductors discovered in recent years. We summary and compare their basic physical properties, including the crystal structures, electronic structures, and mechanical stability. The 2D magnetism can be described by several physical models. We also focus on the recent progresses about theoretical prediction of FM semiconductors and experimental observation of external-field regulation. Most of investigations have shown that 2D chalcogenide-based FM semiconductors have relatively high Curie temperature (Tc) and structural stability. These materials are promising to realize the room-temperature ferromagnetism in atomic-layer thickness, which is significant to design spintronics devices.

Demonstration of conventional soliton, bound-state soliton, and noise-like pulse based on chromium sulfide as saturable absorber

Ferromagnetic semiconductor chromium sulfide (Cr2S3), as a member of transition metal chalcogenide (TMC), exhibits the narrow bandgap value of 0.45 eV theoretically and has been applied in photoelectric field. However, the application in ultrafast fiber laser of Cr2S3 has not been investigation at present. In this work, the Cr2S3-based SA was successfully prepared by depositing nanosheets onto tapered fiber. The conventional soliton (CS) operation, three pulse bound-state (BS) soliton operation, and noise-like pulse (NLP) operation around 1531 nm are observed from 80 mW to 147 mW in an EDFL. The experimental results demonstrated that Cr2S3 as a promising 2D material has tremendous potential in designing ultrafast photonics device.

Atomic Imaging and Thermally Induced Dynamic Structural Evolution of Two-Dimensional Cr2S3

In this study, facile salt-assisted chemical vapor deposition (CVD) was used to synthesize ultrathin non-van der Waals chromium sulfide (Cr₂S₃) with a thickness of ∼1.9 nm. The structural transformation of as-grown Cr₂S₃ was studied using advanced in situ heating techniques combined with transmission electron microscopy (TEM). Two-dimensional (2D) and quasi-one-dimensional (1D) samples were fabricated to investigate the connection between specific planes and the dynamic behavior of the structural variation. The rearrangement of atoms during the phase transition was driven by the loss of sulfur atoms at elevated temperatures, resulting in increased free energy. A decrease in the ratio of the (001) plane led to an overall increase in surface energy, thus lowering the critical phase transition temperature. Our study provides detailed insight into the mechanism of structural transformation and the critical factors governing transition temperature, thus paving the way for future studies on intriguing Cr-S compounds.

Emerging Phases of Layered Metal Chalcogenides

Layered metal chalcogenides, as a "rich" family of 2D materials, have attracted increasing research interest due to the abundant choices of materials with diverse structures and rich electronic characteristics. Although the common metal chalcogenide phases such as 2H and 1T have been intensively studied, many other unusual phases are rarely explored, and some of these show fascinating behaviors including superconductivity, ferroelectrics, ferromagnetism, etc. From this perspective, the unusual phases of metal chalcogenides and their characteristics, as well as potential applications are introduced. First, the unusual phases of metal chalcogenides from different classes, including transition metal dichalcogenides, magnetic element-based chalcogenides, and metal phosphorus chalcogenides, are discussed, respectively. Meanwhile, their excellent properties of different unusual phases are introduced. Then, the methods for producing the unusual phases are discussed, specifically, the stabilization strategies during the chemical vapor deposition process for the unusual phase growth are discussed, followed by an outlook and discussions on how to prepare the unusual phase metal dichalcogenides in terms of synthetic methodology and potential applications.

Chemical Vapor Deposition of Ferrimagnetic Fe3O4 Thin Films

Ultrathin magnetic materials with room-temperature ferromagnetism/ferrimagnetism hold great potential in spintronic applications. In this work, we report the successful controllable growth of Fe3O4 thin films using a facile chemical vapor deposition method. Room-temperature ferrimagnetism was maintained in the as-grown Fe3O4 thin films down to 4 nm. Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy analysis were conducted to reveal the structure and quality of the Fe3O4 film. Magnetization measurement showed ferrimagnetic hysteresis loops in all Fe3O4 thin films. A saturation magnetization of 752 emu/cm3 was observed for the 4 nm Fe3O4 film, which was higher than that of bulk Fe3O4 materials (480 emu/cm3). Additionally, the Verwey transition at ~120 K was visible for the Fe3O4 thin films. This work provides an alternative method of synthesizing ferrimagnetic ultrathin films for electronic, spintronic, and memory device applications.

Antiferromagnetic Phase Induced by Nitrogen Doping in 2D Cr2S3

Exploration for the new members of air-stable 2D antiferromagnetic magnets to widen the magnetic families has drawn great attention due to its potential applications in spintronic devices. In addition to seeking the intrinsic antiferromagnets, externally introducing antiferromagnetic ordering in existing 2D materials, such as structural regulation and phase engineering, may be a promising way to modulate antiferromagnetism in the 2D limit. In this work, the in situ nitrogen doping growth of ultrathin 2D Cr2S3 nanoflakes has been achieved. Antiferromagnetic ordering in 2D Cr2S3 nanoflakes can be triggered by nitrogen doping induced new phase (space group P3¯1c). This work provides a new route to realize antiferromagnetism in atomically thin 2D magnets and greatly extend applications of 2D magnets in valleytronics and spintronics.

Ag0 decorated Cr2S3 NPs embedded on PVP matrix: A colorimetric probe for selective and rapid detection of sulphide ions from environmental samples

Globally, the environmental pollution is one of the major issues causing toxicity towards human and aquatic life. We have developed a facile and innovative sensing approach for detection of sulphide ions (S^2-) present in the aqueous media using Ag⁰ decorated Cr₂S₃ NPs embedded on PVP matrix (Ag/Cr₂S₃-PVP). Based on the SPR phenomena, the detection of S^2- ions was established. The nanohybrid was characterized using various techniques such as UV-vis spectrophotometer, High-Resolution Transmission Electron Microscopy (HR-TEM), Thermal Gravimetric Analysis (TGA), X-ray diffraction analysis(XRD), Energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The yellowish colour of Ag/Cr₂S₃-PVP nanohybrid turned to brown colour in presence of S^2- ions. The selectivity and sensitivity of the prepared probe was studied against the other interfering metal ions. In addition, the effect of different concentration of S^2- ions in the nanohybrid solution was investigated and the Limit of detection (LOD) was found to be 6.6 nM. The good linearity was found over the range of 10 nM to 100 μM with R² value of 0.981. The paper strip based probe was developed for rapid onsite monitoring of S^2- ions. The proposed method is found to be cost-effective, rapid, and simple. We have validated the practical applicability of the prepared probe for determining the concentration of S^2- ions in real water samples.

Epitaxial Growth of Step-Like Cr2 S3 Lateral Homojunctions Towards Versatile Conduction Polarities and Enhanced Transistor Performances

For expanding the applications of 2D transition metal dichalcogenides (TMDCs), integrating functional devices with diverse conduction polarities in the same parent material is a very promising direction. Improving the contact issue at the metal-semiconductor interface also holds fundamental significance. To achieve these concurrently, step-like Cr₂ S₃ vertical stacks with varied thicknesses are achieved via a one-step chemical vapor deposition (CVD) method route. Various types of 2D Cr₂ S₃ lateral homojunctions are thus naturally evolved, that is, p_m -ambipolar/n, p/ambipolar, ambipolar/n, and n_m -ambipolar/n junctions, allowing the integration of diverse conduction polarities in single Cr₂ S₃ homojunctions. Significantly, on-state current density and field-effect mobility of the thinner 2D Cr₂ S₃ flakes stacked below are detected to be ≈5 and ≈6 times increased in the lateral homojunctions, respectively. This work should hereby provide insights for designing 2D functional devices with simpler structures, for example, multipolar field-effect transistors, photodetectors, and inverters, and provide fundamental references for optimizing the electrical performances of 2D materials related devices.

Air-Stable 2D Cr5 Te8 Nanosheets with Thickness-Tunable Ferromagnetism

2D magnetic materials have aroused widespread research interest owing to their promising application in spintronic devices. However, exploring new kinds of 2D magnetic materials with better stability and realizing their batch synthesis remain challenging. Herein, the synthesis of air-stable 2D Cr₅ Te₈ ultrathin crystals with tunable thickness via tube-in-tube chemical vapor deposition (CVD) growth technology is reported. The importance of tube-in-tube CVD growth, which can significantly suppress the equilibrium shift to the decomposition direction and facilitate that to the synthesis reaction direction, for the synthesis of high-quality Cr₅ Te₈ with accurate composition, is highlighted. By precisely adjusting the growth temperature, the thickness of Cr₅ Te₈ nanosheets is tuned from ≈1.2 nm to tens of nanometers, with the morphology changing from triangles to hexagons. Furthermore, magneto-optical Kerr effect measurements reveal that the Cr₅ Te₈ nanosheet is ferromagnetic with strong out-of-plane spin polarization. The Curie temperature exhibits a monotonic increase from 100 to 160 K as the Cr₅ Te₈ thickness increases from 10 to 30 nm and no apparent variation in surface roughness or magnetic properties after months of exposure to air. This study provides a robust method for the controllable synthesis of high-quality 2D ferromagnetic materials, which will facilitate research progress in spintronics.

Micrometer Sized Hexagonal Chromium Selenide Flakes for Cryogenic Temperature Sensors

Nanoscale thermometers with high sensitivity are needed in domains which study quantum and classical effects at cryogenic temperatures. Here, we present a micrometer sized and nanometer thick chromium selenide cryogenic temperature sensor capable of measuring a large domain of cryogenic temperatures down to tenths of K. Hexagonal Cr-Se flakes were obtained by a simple physical vapor transport method and investigated using scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray photoelectron spectroscopy measurements. The flakes were transferred onto Au contacts using a dry transfer method and resistivity measurements were performed in a temperature range from 7 K to 300 K. The collected data have been fitted by exponential functions. The excellent fit quality allowed for the further extrapolation of resistivity values down to tenths of K. It has been shown that the logarithmic sensitivity of the sensor computed over a large domain of cryogenic temperature is higher than the sensitivity of thermometers commonly used in industry and research. This study opens the way to produce Cr-Se sensors for classical and quantum cryogenic measurements.

Room Temperature Ferromagnetism of Monolayer Chromium Telluride with Perpendicular Magnetic Anisotropy

The realization of long-range magnetic ordering in 2D systems can potentially revolutionize next-generation information technology. Here, the successful fabrication of crystalline Cr₃ Te₄ monolayers with room temperature (RT) ferromagnetism is reported. Using molecular beam epitaxy, the growth of 2D Cr₃ Te₄ films with monolayer thickness is demonstrated at low substrate temperatures (≈100 °C), compatible with Si complementary metal oxide semiconductor technology. X-ray magnetic circular dichroism measurements reveal a Curie temperature (T_c ) of v344 K for the Cr₃ Te₄ monolayer with an out-of-plane magnetic easy axis, which decreases to v240 K for the thicker film (≈7 nm) with an in-plane easy axis. The enhancement of ferromagnetic coupling and the magnetic anisotropy transition is ascribed to interfacial effects, in particular the orbital overlap at the monolayer Cr₃ Te₄ /graphite interface, supported by density-functional theory calculations. This work sheds light on the low-temperature scalable growth of 2D nonlayered materials with RT ferromagnetism for new magnetic and spintronic devices.

Recent progress on emergent two-dimensional magnets and heterostructures

Intrinsic two-dimensional (2D) magnetic materials own strong long-range magnetism while their characteristics of the ultrathin thickness and smooth surface provide an ideal platform for manipulating the magnetic properties at 2D limit. This makes them to be potential candidates in various spintronic applications compared to their corresponding bulk counterparts. The discovery of magnetic ordering in 2D CrI₃and Gr₂Ge₂Te₆nanostructures stimulated tremendous research interest in both experimental and theoretical studies on various intrinsic magnets at 2D limit. This review gives a comprehensive overview of the recent progress on the emergent 2D magnets and heterostructures. Firstly, several kinds of typical 2D magnetic materials discovered in the last few years and their fabrication methods are summarized in detail. Secondly, the current strategies for manipulating magnetic properties in 2D materials are further discussed. Then, the recent advances on the construction of representative van der Waals magnetic heterostructures and their respective performance are provided. With the hope of motivating the researchers in this area, we finally offered the challenges and outlook on 2D magnetism.

Diverse Structures and Magnetic Properties in Nonlayered Monolayer Chromium Selenide

Thickness-dependent magnetic behavior has previously been observed in chemical vapor deposition-grown chromium selenide. However, the low-dimensional structure in nonlayered chromium selenide, which plays a crucial role in determining the low-dimensional magnetic order, needs further study. Here, we report the structure-dependent magnetic properties in monolayer CrSe₂ and Cr₂Se₃ grown by molecular beam epitaxy. In the monolayer CrSe₂, 1T-CrSe₂ with a lattice constant of 3.3 Å has a metallic character, coexisting with the 1T″ phase with 2 × 2 surface periodicity. Monolayer CrSe₂ can be transformed into Cr₂Se₃ with a lattice constant of 3.6 Å by annealing at 300 °C. X-ray magnetic circular dichroism (XMCD) measurements combined with DFT calculations reveal that while the MBE-grown monolayer CrSe₂ is antiferromagnetic, monolayer Cr₂Se₃ is ferromagnetic with a Curie temperature of ∼200 K. This work demonstrates the structural diversity in nonlayered chromium selenide and the critical effect of different structures on its electronic and magnetic properties.

Phase-Selective Synthesis of Ultrathin FeTe Nanoplates by Controllable Fe/Te Atom Ratio in the Growth Atmosphere

Phase controllable synthesis of 2D materials is of significance for tuning related electrical, optical, and magnetic properties. Herein, the phase-controllable synthesis of tetragonal and hexagonal FeTe nanoplates has been realized by a rational control of the Fe/Te ratio in a chemical vapor deposition system. Using density functional theory calculations, it has been revealed that with the change of the Fe/Te ratio, the formation energy of active clusters changes, causing the phase-controllable synthesis of FeTe nanoplates. The thickness of the obtained FeTe nanoplates can be tuned down to the 2D limit (2.8 nm for tetragonal and 1.4 nm for hexagonal FeTe). X-ray diffraction pattern, transmission electron microscopy, and high resolution scanning transmission electron microscope analyses exhibit the high crystallinity of the as-grown FeTe nanoplates. The two kinds of FeTe nanoflakes show metallic behavior and good electrical conductivity, featuring 8.44 × 10⁴ S m^-1 for 9.8 nm-thick tetragonal FeTe and 5.45 × 10⁴ S m^-1 for 7.6 nm-thick hexagonal FeTe. The study provides an efficient and convenient route for tailoring the phases of FeTe nanoplates, which benefits to study phase-sensitive properties, and may pave the way for the synthesis of other multiphase 2D nanosheets with controllable phases.

Controllable Synthesis of 2D Nonlayered Cr2S3 Nanosheets and Their Electrocatalytic Activity Toward Oxygen Evolution Reaction

The design of oxygen evolution reaction (OER) electrocatalysts based on Earth-abundant materials holds great promise for realizing practically viable water-splitting systems. In this regard, two-dimensional (2D) nonlayered materials have received considerable attention in recent years owing to their intrinsic dangling bonds which give rise to the exposure of unsaturated active sites. In this work, we solved the synthesis challenge in the development of a 2D nonlayered Cr2S3 catalyst for OER application via introducing a controllable chemical vapor deposition scheme. The as-obtained catalyst exhibits a very good OER activity requiring overpotentials of only 230 mV and 300 mV to deliver current densities of 10 mA cm−2 and 30 mA cm−2, respectively, with robust stability. This study provides a general approach to optimize the controllable growth of 2D nonlayered material and opens up a fertile ground for studying the various strategies to enhance the water splitting reactions.

Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order

The discovery of intrinsic ferromagnetism in ultrathin two-dimensional van der Waals crystals opens up exciting prospects for exploring magnetism in the ultimate two-dimensional limit. Here, we show that environmentally stable CrSe₂ nanosheets can be readily grown on a dangling-bond-free WSe₂ substrate with systematically tunable thickness down to the monolayer limit. These CrSe₂/WSe₂ heterostructures display high-quality van der Waals interfaces with well-resolved moiré superlattices and ferromagnetic behaviour. We find no apparent change in surface roughness or magnetic properties after months of exposure in air. Our calculations suggest that charge transfer from the WSe₂ substrate and interlayer coupling within CrSe₂ play a critical role in the magnetic order in few-layer CrSe₂ nanosheets. The highly controllable growth of environmentally stable CrSe₂ nanosheets with tunable thickness defines a robust two-dimensional magnet for fundamental studies and potential applications in magnetoelectronic and spintronic devices.

Morphology-Tunable Synthesis of Intrinsic Room-Temperature Ferromagnetic γ-Fe2O3 Nanoflakes

Intrinsic two-dimensional (2D) magnetic materials with room-temperature ferromagnetism and air stability are highly desirable for spintronic applications. However, the experimental observations of such 2D or ultrathin ferromagnetic materials are rarely reported owing to the scarcity of these materials in nature and for the intricacy in their synthesis. Here, we report a successful controllable growth of ultrathin γ-Fe₂O₃ nanoflakes with a variety of morphologies tunable by the growth temperature alone using a facile chemical vapor deposition method and demonstrate that all ultrathin nanoflakes still show intrinsic room-temperature ferromagnetism and a semiconducting nature. The γ-Fe₂O₃ nanoflakes epitaxially grown on α-Al₂O₃ substrates take a triangular shape at low temperature and develop gradually in lateral size, forming eventually a large-scale γ-Fe₂O₃ thin film as the growth time increases due to a thermodynamic control process. The morphology of the nanoflakes could be tuned from triangular to stellated, petaloid, and dendritic crystalloids in sequence with the rise of precursor temperature, revealing a growth process from thermodynamically to kinetically dominated control. Moreover, the petaloid and dendritic nanoflakes exhibit enhanced coercivity compared with the triangular and stellated nanoflakes, and all the nanoflakes with diverse shapes possess differing electrical conductivity. The findings of such ultrathin, air-stable, and room-temperature ferromagnetic γ-Fe₂O₃ nanoflakes with tunable shape and multifunctionality may offer guidance in synthesizing other non-layered magnetic materials for next-generation electronic and spintronic devices.

Conversation from antiferromagnetic MnBr2 to ferromagnetic Mn3Br8 monolayer with large MAE

A pressing need in low energy spintronics is two-dimensional (2D) ferromagnets with Curie temperature above the liquid-nitrogen temperature (77 K), and sizeable magnetic anisotropy. We studied Mn3Br8 monolayer which is obtained via inducing Mn vacancy at 1/4 population in MnBr2 monolayer. Such defective configuration is designed to change the coordination structure of the Mn-d5 and achieve ferromagnetism with sizeable magnetic anisotropy energy (MAE). Our calculations show that Mn3Br8 monolayer is a ferromagnetic (FM) half-metal with Curie temperature of 130 K, large MAE of - 2.33 meV per formula unit, and atomic magnetic moment of 13/3μB for the Mn atom. Additionally, Mn3Br8 monolayer maintains to be FM under small biaxial strain, whose Curie temperature under 5% compressive strain is 160 K. Additionally, both biaxial strain and carrier doping make the MAE increases, which mainly contributed by the magneto-crystalline anisotropy energy (MCE). Our designed defective structure of MnBr2 monolayer provides a simple but effective way to achieve ferromagnetism with large MAE in 2D materials.

Controlled synthesis and Raman study of a 2D antiferromagnetic P-type semiconductor: α-MnSe

Two-dimensional (2D) non-van der Waals magnetic materials have attracted considerable attention due to their high-temperature ferromagnetism, active surface/interface properties originating from dangling bonds, and good stability under ambient conditions. Here, we demonstrate the controlled synthesis and systematic Raman investigation of ultrathin non-van der Waals antiferromagnetic α-MnSe single crystals. Square and triangular nanosheets with different growth orientations can be achieved by introducing different precursors via the atmospheric chemical vapor deposition (APCVD) method. The temperature-dependent resonant enhancement in the Raman intensity of two peaks at 233.8 cm^-1 and 459.9 cm^-1 gives obvious evidence that the antiferromagnetic spin-ordering is below T_N∼ 160 K. Besides, a new peak located at 254.2 cm^-1, gradually appearing as the temperature decreased from 180 K to 100 K, may also be a signature of phase transition from paramagnetic to antiferromagnetic. The phonon dispersion spectra of α-MnSe simulated by density functional perturbation theory (DFPT) match well with the observed Raman signals. Moreover, a fabricated α-MnSe phototransistor exhibits p-type conducting behavior and high photodetection performance. We believe that these findings will be beneficial for the applications of 2D α-MnSe in magnetic and semiconducting fields.