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

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.

Controllable Synthesis of Magnetic 2D Non-Layered Cobalt Sulfide Nanocrystals Using Chemical Vapor Deposition

Among 2-dimensional (2D) non-layered transition-metal chalcogenides (TMCs), cobalt sulfides are highly interesting because of their diverse structural phases and unique properties. The unique magnetic properties of TMCs have generated significant interest in their potential applications in future spintronic devices. In addition, their high conductivity, large specific surface area, and abundant active sites have attracted attention in the field of catalysis. However, the synthesis of phase-controllable 2D non-layered cobalt sulfide nanocrystals remains challenging. In the present study, a method is reported in which ambient-pressure chemical vapor deposition (APCVD) is used to synthesize 2D non-layered cobalt sulfide nanocrystals on insulating substrates. By controlling the growth temperature, the transition of nanocrystal phases from pyrite-structured CoS2 to cubic Co3S4 and hexagonal CoS is achieved. Magnetotransport studies revealed metallic and ferromagnetic behaviors at temperatures below the Curie temperature for CoS2. In addition, electrical measurements of Co3S4- and CoS-based devices showed conventional metallic behaviors, including temperature- and magnetic field-dependent ordinary Hall effects. These findings demonstrate the potential of APCVD for synthesizing high-quality 2D non-layered cobalt sulfide nanocrystals with controllable phases, paving the way for their application in spintronics and catalysis.

In Situ Manipulation of Surface Spin Configurations for Enhanced Performance in Oxygen Evolution Reactions

In situ studies of the relationship between surface spin configurations and spin-related electrocatalytic reactions are crucial for understanding how magnetic catalysts enhance oxygen evolution reaction (OER) performance under magnetic fields. In this work, 2D Fe7Se8 nanosheets with rich surface spin configurations are synthesized via chemical vapor deposition. In situ magnetic force microscopy and Raman spectroscopy reveal that a 200 mT magnetic field eliminates spin-disordered domain walls, forming a spin-ordered single-domain structure, which lowers the OER energy barrier, as confirmed by theoretical calculations. Electrochemical tests show that under a 200 mT magnetic field, the OER overpotential of multidomain Fe7Se8 nanosheets at 10 mA cm-2 decreases from 346 mV to 259 mV, while the magnetic field has minimal effect on single-domain nanosheets. These findings highlight the critical role of spin configurations in enhancing electrocatalytic performance, offering new insights into the design of magnetic catalysts for industrial applications.

Monolayer Magnetic Metal with Scalable Conductivity

2D magnets have emerged as a class of materials highly promising for studies of quantum phenomena and applications in ultra-compact spintronics. Current research aims at design of 2D magnets with particular functional properties. A formidable challenge is to produce metallic monolayers: the material landscape of layered magnetic systems is strongly dominated by insulators; rare metallic magnets, such as Fe3GeTe2, become insulating as they approach the monolayer limit. Here, electron transport measurements demonstrate that the recently discovered 2D magnet GdAlSi - graphene-like AlSi layers coupled to layers of Gd atoms - remains metallic down to a single monolayer. Band structure analysis indicates the material to be an electride, which may stabilize the metallic state. Remarkably, the sheet conductance of 2D GdAlSi is proportional to the number of monolayers - a manifestation of scalable conductivity. The GdAlSi layers are epitaxially integrated with silicon, facilitating applications in electronics.

Unconventional Anomalous Hall Effect Driven by Self-Intercalation in Covalent 2D Magnet Cr2Te3

Covalent 2D magnets such as Cr2Te3, which feature self-intercalated magnetic cations located between monolayers of transition-metal dichalcogenide material, offer a unique platform for controlling magnetic order and spin texture, enabling new potential applications for spintronic devices. Here, it is demonstrated that the unconventional anomalous Hall effect (AHE) in Cr2Te3, characterized by additional humps and dips near the coercive field in AHE hysteresis, originates from an intrinsic mechanism dictated by the self-intercalation. This mechanism is distinctly different from previously proposed mechanisms such as topological Hall effect, or two-channel AHE arising from spatial inhomogeneities. Crucially, multiple Weyl-like nodes emerge in the electronic band structure due to strong spin-orbit coupling, whose positions relative to the Fermi level is sensitively modulated by the canting angles of the self-intercalated Cr cations. These nodes contribute strongly to the Berry curvature and AHE conductivity. This component competes with the contribution from bands that are less affected by the self-intercalation, resulting in a sign change in AHE with temperature and the emergence of additional humps and dips. The findings provide compelling evidence for the intrinsic origin of the unconventional AHE in Cr2Te3 and further establish self-intercalation as a control knob for engineering AHE in complex magnets.

Band structure database of layered intercalation compounds with various intercalant atoms and layered hosts

Here we provide a database comprising electronic band structures of 9,004 layered intercalation compounds, where atoms are intercalated into a host layered compound with different intercalant atoms, along with 468 structures related to the layered host compounds. Additionally, we provide properties derived from the electronic states such as band gap as well as stability-related properties like formation energies. Direct comparison of the band structures before and after intercalation is generally challenging due to changes in their space group and k-path. However, in this study, we developed new k-paths consistent with the host materials, allowing for the direct comparison of band structures before and after intercalation. This enables direct and quantitative discussion of the band structure changes induced by the intercalations and provides a valuable database for intercalant-driven band engineering. Layered intercalation compounds are widely used in many fields, including superconductivity and energy applications, and understanding of electronic structures is necessary. The feature of our database holds promises for the development of layered compounds with enhanced functionalities through database utilization.

High-Active Surface of Centimeter-Scale β-In2S3 for Attomolar-Level Hg2+ Sensing

Recognition layer materials play a crucial role in the functionality of chemical sensors. Although advancements in two-dimensional (2D) materials have promoted sensor development, the controlled fabrication of large-scale recognition layers with highly active sites remains crucial for enhancing sensor sensitivity, especially for trace detection applications. Herein, we propose a strategy for the controlled preparation of centimeter-scale non-layered ultrathin β-In2S3 materials with tailored high-active sites to design ultrasensitive Hg2+ sensors. Our results reveal that the highly active sites of non-layered β-In2S3 materials are pivotal for achieving superior sensing performance. Selective detection of Hg2+ at the 1 aM level is achieved via selective Hg-S bonding. Additionally, we evaluate that this sensor exhibits excellent performance in detecting Hg2+ in the tap water matrix. This work provides a proof-of-concept for utilizing non-layered 2D films in high-performance sensors and highlights their potential for diverse analyte sensing applications.

Atomically Thin Kagome-Structured Co9Te16 Achieved through Self-Intercalation and Its Flat Band Visualization

Kagome materials have recently garnered substantial attention due to the intrinsic flat band feature and the stimulated magnetic and spin-related many-body physics. In contrast to their bulk counterparts, two-dimensional (2D) kagome materials feature more distinct kagome bands, beneficial for exploring novel quantum phenomena. Herein, we report the direct synthesis of an ultrathin kagome-structured Co-telluride (Co9Te16) via a molecular beam epitaxy (MBE) route and clarify its formation mechanism from the Co-intercalation in the 1T-CoTe2 layers. More significantly, we unveil the flat band states in the ultrathin Co9Te16 and identify the real-space localization of the flat band states by in situ scanning tunneling microscopy/spectroscopy (STM/STS) combined with first-principles calculations. A ferrimagnetic order is also predicted in kagome-Co9Te16. This work should provide a novel route for the direct synthesis of ultrathin kagome materials via a metal self-intercalation route, which should shed light on the exploration of the intriguing flat band physics in the related systems.

Two-Dimensional Ultrathin Fe3Sn2 Kagome Metal with Defect-Dependent Magnetic Property

Two-dimensional (2D) Fe3Sn2, which is a room-temperature ferromagnetic kagome metal, has potential applications in spintronic devices. However, the systematic synthesis and magnetic study of 2D Fe3Sn2 single crystals have rarely been reported. Here we have synthesized 2D hexagonal and triangular Fe3Sn2 nanosheets by controlling the amount of FeCl2 precursors in the chemical vapor deposition (CVD) method. It is found that the hexagonal Fe3Sn2 nanosheets exist with Fe vacancy defects and show no obvious coercivity. While the triangular Fe3Sn2 nanosheet has obvious hysteresis loops at room temperature, its coercivity first increases and then remains stable with an increase in temperature, which should result from the competition of the thermal activation mechanism and spin direction rotation mechanism. A first-principles calculation study shows that the Fe vacancy defects in Fe3Sn2 can increase the distances between Fe atoms and weaken the ferromagnetism of Fe3Sn2. The resulting 2D Fe3Sn2 nanosheets provide a new choice for spintronic devices.

2D Air-Stable Nonlayered Ferrimagnetic FeCr2S4 Crystals Synthesized via Chemical Vapor Deposition

The discovery of intrinsic 2D magnetic materials has opened up new opportunities for exploring magnetic properties at atomic layer thicknesses, presenting potential applications in spintronic devices. Here a new 2D ferrimagnetic crystal of nonlayered FeCr2S4 is synthesized with high phase purity using chemical vapor deposition. The obtained 2D FeCr2S4 exhibits perpendicular magnetic anisotropy, as evidenced by the out-of-plane/in-plane Hall effect and anisotropic magnetoresistance. Theoretical calculations further elucidate that the observed magnetic anisotropy can be attributed to its surface termination structure. By combining temperature-dependent magneto-transport and polarized Raman spectroscopy characterizations, it is discovered that both the measured Curie temperature and the critical temperature at which a low energy magnon peak disappeared remains constant, regardless of its thickness. Magnetic force microscopy measurements show the flipping process of magnetic domains. The exceptional air-stability of the 2D FeCr2S4 is also confirmed via Raman spectroscopy and Hall hysteresis loops. The robust anisotropic ferrimagnetism, the thickness-independent of Curie temperature, coupled with excellent air-stability, make 2D FeCr2S4 crystals highly attractive for future spintronic devices.

Growth of Quasi-Two-Dimensional CrTe Nanoflakes and CrTe/Transition Metal Dichalcogenide Heterostructures

Two-dimensional (2D) van der Waals layered materials have been explored in depth. They can be vertically stacked into a 2D heterostructure and represent a fundamental way to explore new physical properties and fabricate high-performance nanodevices. However, the controllable and scaled growth of non-layered quasi-2D materials and their heterostructures is still a great challenge. Here, we report a selective two-step growth method for high-quality single crystalline CrTe/WSe2 and CrTe/MoS2 heterostructures by adopting a universal CVD strategy with the assistance of molten salt and mass control. Quasi-2D metallic CrTe was grown on pre-deposited 2D transition metal dichalcogenides (TMDC) under relatively low temperatures. A 2D CrTe/TMDC heterostructure was established to explore the interface's structure using scanning transmission electron microscopy (STEM), and also demonstrate ferromagnetism in a metal-semiconductor CrTe/TMDC heterostructure.

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.

CO2 Stress-Driven Room Temperature Ferromagnetism of Ultrathin 2D Gallium Oxide

Spintronic devices work by manipulating the spin of electrons other than charge transfer, which is of revolutionary significance and can largely reduce energy consumption in the future. Herein, ultrathin two-dimensional (2D) non-van der Waals (non-vdW) γ-Ga2O3 with room temperature ferromagnetism is successfully obtained by using supercritical CO2 (SC CO2). The stress effect of SC CO2 under different pressures selectively modulates the orientation and strength of covalent bonds, leading to the change of atomic structure including lattice expansion, introduction of O vacancy, and transition of Ga-O coordination (GaO4 and GaO6). Magnetic measurements show that pristine γ-Ga2O3 is nonferromagnetic, whereas the SC CO2 treated γ-Ga2O3 exhibits obvious ferromagnetic behavior with an optimal magnetization of 0.025 emu g-1 and a Curie temperature of 300 K.

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.

Phase and Composition Engineering of Self-Intercalated 2D Metallic Tantalum Sulfide for Second-Harmonic Generation

Self-intercalation in two-dimensional (2D) materials is significant, as it offers a versatile approach to modify material properties, enabling the creation of interesting functional materials, which is essential in advancing applications across various fields. Here, we define ic-2D materials as covalently bonded compounds that result from the self-intercalation of a metal into layered 2D compounds. However, precisely growing ic-2D materials with controllable phases and self-intercalation concentrations to fully exploit the applications in the ic-2D family remains a great challenge. Herein, we demonstrated the controlled synthesis of self-intercalated H-phase and T-phase Ta1+xS2 via a temperature-driven chemical vapor deposition (CVD) approach with a viable intercalation concentration spanning from 10% to 58%. Atomic-resolution scanning transmission electron microscopy-annular dark field imaging demonstrated that the self-intercalated Ta atoms occupy the octahedral vacancies located at the van der Waals gap. The nonperiodic Ta atoms break the centrosymmetry structure and Fermi surface properties of intrinsic TaS2. Therefore, ic-2D T-phase Ta1+xS2 consistently exhibit a spontaneous nonlinear optical (NLO) effect regardless of the sample thickness and self-intercalation concentrations. Our results propose an approach to activate the NLO response of centrosymmetric 2D materials, achieving the modulation of a wide range of optoelectronic properties via nonperiodic self-intercalation in the ic-2D family.

Epitaxial Growth of Two-Dimensional Nonlayered AuCrS2 Materials via Au-Assisted Chemical Vapor Deposition

Two-dimensional (2D) nonlayered transition metal dichalcogenide (TMD) materials are emergent platforms for various applications from catalysis to quantum devices. However, their limited availability and nonstraightforward synthesis methods hinder our understanding of these materials. Here, we present a novel technique for synthesizing 2D nonlayered AuCrS2 via Au-assisted chemical vapor deposition (CVD). Our detailed structural analysis reveals the layer-by-layer growth of [AuCrS2] units atop an initial CrS2 monolayer, with Au binding to the adjacent monolayer of CrS2, which is in stark contrast with the well-known metal intercalation mechanism in the synthesis of many other 2D nonlayered materials. Theoretical calculations further back the crucial role of Cr in increasing the mobility of Au species and strengthening the adsorption energy of Au on CrS2, thereby aiding the growth throughout the CVD process. Additionally, the resulting free-standing nanoporous AuCrS2 (NP-AuCrS2) exhibits exceptional electrocatalytic properties for the hydrogen evolution reaction.

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.

Mechanism Regulating Self-Intercalation in Layered Materials

Recent experimental breakthrough demonstrated a powerful synthesis approach for intercalating the van der Waals gap of layered materials to achieve property modulation, thereby opening an avenue for exploring new physics and devising novel applications, but the mechanism governing intercalant assembly patterns and properties remains unclear. Based on extensive structural search and energetics analysis by ab initio calculations, we reveal a Sabatier-like principle that dictates spatial arrangement of self-intercalated atoms in transition metal dichalcogenides. We further construct a robust descriptor quantifying that strong intercalant-host interactions favor a monodispersing phase of intercalated atoms that may exhibit ferromagnetism, while weak interactions lead to a trimer phase with attenuated or quenched magnetism, which further evolves into tetramer and hexagonal phases at increasing intercalant density. These findings elucidate the mechanism underpinning experimental observations and paves the way for rational design and precise control of self-intercalation in layered materials.

Advanced Strategies in Synthesis of Two-Dimensional Materials with Different Compositions and Phases

In recent years, 2D materials-M_a X_b with different compositions and phases have attracted tremendous attention due to their diverse structures and electronic features. The common thermodynamically stable 2H and metastable 1T phases have been extensively studied, however, there are many unusual compositions and phases with novel physical properties that have yet to be explored. Therefore, summarization of the synthesis strategies, atomic structures, and the unique physical properties of 2D materials with different compositions and phases is very important for their development. In this review, the strategies including chemical vapor deposition, intercalation, atomic layer deposition, chemical vapor transport, and electrostatic gating for synthesizing various 2D materials with different phases and compositions are first summarized. Specially, the intercalation strategies including heterogeneous- and self-intercalation for controllable phases and compositions fabrication are mainly discussed. Then, the novel atomic structures of 2D materials are analyzed, followed by the fascinating physical properties including ferroelectricity, ferromagnetism, superconductivity, and so on. Finally, the conclusion and outlook are offered including the challenges and future prospects of 2D materials with different compositions and phases.