Van Der Waals Epitaxial Growth and Phase Transition of Layered FeSe2 Nanocrystals

Phase-Modulated Elastic Properties of 2D Magnetic FeTe: Hexagonal and Tetragonal Polymorphs

2D layered magnets, such as iron chalcogenides, have emerged these years as a new family of unconventional superconductors and provided the key insights to understand the phonon-electron interaction and pairing mechanism. Their mechanical properties are of strategic importance for the potential applications in spintronics and optoelectronics. However, there is still a lack of efficient approach to tune the elastic modulus despite the extensive studies. Herein, the modulated elastic modulus of 2D magnetic FeTe and its thickness-dependence is reported via phase engineering. The grown 2D FeTe by chemical vapor deposition can present various polymorphs, that is tetragonal FeTe (t-FeTe, antiferromagnetic) and hexagonal FeTe (h-FeTe, ferromagnetic). The measured Young's modulus of t-FeTe by nanoindentation method shows an obvious thickness-dependence, from 290.9 ± 9.2 to 113.0 ± 8.7 GPa when the thicknesses increased from 13.2 to 42.5 nm, respectively. In comparison, the elastic modulus of h-FeTe remains unchanged. These results can shed light on the efficient modulation of mechanical properties of 2D magnetic materials and pave the avenues for their practical applications in nanodevices.

Phase Engineering of Nanomaterials: Transition Metal Dichalcogenides

Crystal phase, a critical structural characteristic beyond the morphology, size, dimension, facet, etc., determines the physicochemical properties of nanomaterials. As a group of layered nanomaterials with polymorphs, transition metal dichalcogenides (TMDs) have attracted intensive research attention due to their phase-dependent properties. Therefore, great efforts have been devoted to the phase engineering of TMDs to synthesize TMDs with controlled phases, especially unconventional/metastable phases, for various applications in electronics, optoelectronics, catalysis, biomedicine, energy storage and conversion, and ferroelectrics. Considering the significant progress in the synthesis and applications of TMDs, we believe that a comprehensive review on the phase engineering of TMDs is critical to promote their fundamental studies and practical applications. This Review aims to provide a comprehensive introduction and discussion on the crystal structures, synthetic strategies, and phase-dependent properties and applications of TMDs. Finally, our perspectives on the challenges and opportunities in phase engineering of TMDs will also be discussed.

Two-Dimensional Room-Temperature Magnetism in Janus Mn2I3S3 and Cr2I3Se3 Monolayers with Tunable Magnetic Properties by Strain Engineering

Exploring room-temperature intrinsic magnetism in two-dimensional (2D) materials for nanoscale spintronic devices has garnered significant interest. Achieving a high Curie temperature and substantial spin polarization in 2D ferromagnetic materials remains challenging. Drawing inspiration from the substantial enhancement of the Curie temperature observed in ferromagnetic CrIS monolayers by manipulating the covalent nature of Cr-S bonds, our study systematically delves into the electronic structure and magnetic properties of Janus M2X3Y3 (M = V, Cr, Mn, Fe, and Co; X = Cl, Br, I; Y = S, Se, and Te) monolayers through first-principles calculations. Our findings reveal that 15 kinds of these monolayers exhibit dynamic and thermodynamic stability while displaying diverse electronic and ferromagnetic characteristics. Notably, Mn2I3S3 demonstrates half-metallicity and in-plane magnetic anisotropy, while Cr2I3Se3 exhibits a half-semiconductor and perpendicular magnetic anisotropy. Consequently, Mn2I3S3 transforms from in-plane to perpendicular magnetic anisotropy through strain manipulation. Cr2I3Se3, under strain, transforms from a half-semiconductor to a bipolar magnetic semiconductor. The strong coupling caused by the M-Y bonds makes them have a Curie temperature higher than room temperature. The unique magnetic properties exhibited by the 2D Janus Mn2I3S3 and Cr2I3Se3 magnets hold promise for applications in spintronics. Our study provides a foundational understanding for future experimental explorations in this exciting research area.

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.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Van der Waals Epitaxy Growth of 2D Single-Element Room-Temperature Ferromagnet

2D single-element materials, which are pure and intrinsically homogeneous on the nanometer scale, can cut the time-consuming material-optimization process and circumvent the impure phase, bringing about opportunities to explore new physics and applications. Herein, for the first time, the synthesis of ultrathin cobalt single-crystalline nanosheets with a sub-millimeter scale via van der Waals epitaxy is demonstrated. The thickness can be as low as ≈6 nm. Theoretical calculations reveal their intrinsic ferromagnetic nature and epitaxial mechanism: that is, the synergistic effect between van der Waals interactions and surface energy minimization dominates the growth process. Cobalt nanosheets exhibit ultrahigh blocking temperatures above 710 K and in-plane magnetic anisotropy. Electrical transport measurements further reveal that cobalt nanosheets have significant magnetoresistance (MR) effect, and can realize a unique coexistence of positive MR and negative MR under different magnetic field configurations, which can be attributed to the competition and cooperation effect among ferromagnetic interaction, orbital scattering, and electronic correlation. These results provide a valuable case for synthesizing 2D elementary metal crystals with pure phase and room-temperature ferromagnetism and pave the way for investigating new physics and related applications in spintronics.

Self-Intercalated 1T-FeSe2 as an Effective Kagome Lattice

In kagome lattice, with the emergence of Dirac cones and flat band in electronic structure, it provides a versatile ground for exploring intriguing interplay among frustrated geometry, topology and correlation. However, such engaging interest is strongly limited by available kagome materials in nature. Here we report on a synthetic strategy of constructing kagome systems via self-intercalation of Fe atoms into the van der Waals gap of FeSe₂ via molecular beam epitaxy. Using low-temperature scanning tunneling microscopy, we unveil a kagome-like morphology upon intercalating a 2 × 2 ordered Fe atoms, resulting in a stoichiometry of Fe₅Se₈. Both the bias-dependent STM imaging and theoretical modeling calculations suggest that the kagome pattern mainly originates from slight but important reconstruction of topmost Se atoms, incurred by the nonequivalent subsurface Fe sites due to the intercalation. Our study demonstrates an alternative approach of constructing artificial kagome structures, which envisions to be tuned for exploring correlated quantum states.

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.

Controlled Synthesis of a Two-Dimensional Non-van der Waals Ferromagnet toward a Magnetic Moiré Superlattice

Two-dimensional (2D) magnetic materials provide an ideal platform for spintronics, magnetoelectrics, and numerous intriguing physical phenomena in 2D limits. Moiré superlattices based on 2D magnets offer an avenue for controlling the spin degree of freedom and engineering magnetic properties. However, the synthesis of high-quality, large-grain, and stable 2D magnets, much less obtaining a magnetic moiré superlattice, is still challenging. We synthesize 2D ferromagnets (trigonal Cr₅Te₈) with controlled thickness and robust stability through chemical vapor deposition. Single-unit-cell-thick flakes with lateral sizes of tens of micrometers are obtained. We observe the layer-by-layer growth mode for the crystal formation in non-van der Waals Cr₅Te₈. The robust anomalous Hall signal confirms that Cr₅Te₈ of varying thickness have a long-range ferromagnetic order with an out-of-plane easy axis. There is no obvious change of the Curie temperature when the thickness of Cr₅Te₈ decreases from 52.1 to 7.2 nm. Here, we construct diverse 2D non-van der Waals/van der Waals vertical heterostructures (Cr₅Te₈/graphene, Cr₅Te₈/h-BN, Cr₅Te₈/MoS₂). A uniform moiré superlattice is formed in the heterostructure through a lattice mismatch. The successful growth of 2D Cr₅Te₈ and a related moiré superlattice introduces 2D non-van der Waals ferromagnets into moiré superlattice research, thus highlighting prospects for property investigation of a non-van der Waals magnetic moiré superlattice and massive applications which require a scalable approach to magnetic moiré superlattices.

Controllable Synthesis Quadratic-Dependent Unsaturated Magnetoresistance of Two-Dimensional Nonlayered Fe7S8 with Robust Environmental Stability

Two-dimensional (2D) iron chalcogenides (FeX, X = S, Se, Te) are emerging as an appealing class of materials for a wide range of research topics, including electronics, spintronics, and catalysis. However, the controlled syntheses and intrinsic property explorations of such fascinating materials still remain daunting challenges, especially for 2D nonlayered Fe₇S₈ with mixed-valence states and high conductivity. Herein, we design a general and temperature-mediated chemical vapor deposition (CVD) approach to synthesize ultrathin and large-domain Fe₇S₈ nanosheets on mica substrates, with the thickness down to ∼4.4 nm (2 unit-cell). Significantly, we uncover a quadratic-dependent unsaturated magnetoresistance (MR) with out-of-plane anisotropy in 2D Fe₇S₈, thanks to its ultrahigh crystalline quality and high conductivity (∼2.7 × 10⁵ S m^-1 at room temperature and ∼1.7 × 10⁶ S m^-1 at 2 K). More interestingly, the CVD-synthesized 2D Fe₇S₈ nanosheets maintain robust environmental stability for more than 8 months. These results hereby lay solid foundations for synthesizing 2D nonlayered iron chalcogenides with mixed-valence states and exploring fascinating quantum phenomena.

Atomically Thin 2D van der Waals Magnetic Materials: Fabrications, Structure, Magnetic Properties and Applications

Two-dimensional (2D) van der Waals (vdW) magnetic materials are considered to be ideal candidates for the fabrication of spintronic devices because of their low dimensionality, allowing the quantization of electronic states and more degrees of freedom for device modulation. With the discovery of few-layer Cr2Ge2Te6 and monolayer CrI3 ferromagnets, the magnetism of 2D vdW materials is becoming a research focus in the fields of material science and physics. In theory, taking the Heisenberg model with finite-range exchange interactions as an example, low dimensionality and ferromagnetism are in competition. In other words, it is difficult for 2D materials to maintain their magnetism. However, the introduction of anisotropy in 2D magnetic materials enables the realization of long-range ferromagnetic order in atomically layered materials, which may offer new effective means for the design of 2D ferromagnets with high Curie temperature. Herein, current advances in the field of 2D vdW magnetic crystals, as well as intrinsic and induced ferromagnetism or antiferromagnetism, physical properties, device fabrication, and potential applications, are briefly summarized and discussed.

Phase-Tunable Synthesis and Etching-Free Transfer of Two-Dimensional Magnetic FeTe

Two-dimensional (2D) Fe-chalcogenides (e.g., FeS, FeSe, and FeTe, etc.) have sparked extensive interest due to their rich phase diagrams including superconductivity, magnetism, and topological state, as well as versatile applications in electronic devices and energy related fields. However, the phase-tunable synthesis and green transfer of such fascinating materials still remain challenging. Herein, we develop a temperature-mediated chemical vapor deposition (CVD) approach to grow ultrathin nonlayered hexagonal and layered tetragonal FeTe nanosheets on mica substrates, with their thicknesses down to ∼2.3 and ∼4.0 nm, respectively. Interestingly, we have observed exciting ferromagnetism with the Curie temperature approaching ∼300 K and high conductivity (∼1.96 × 10⁵ S m^-1) in 2D hexagonal FeTe. More significantly, we have designed a swift, high-efficiency, and etching-free method for the transfer of 2D FeTe nanosheets onto arbitrary substrates, and such a transfer strategy enables the cyclic utilization of growth substrates. These results should propel the further development of phase-tunable synthesis and green transfer of 2D Fe-chalcogenides, as well as their potential applications in spintronic devices.

The Photodetectors Based on Lateral Monolayer MoS2/WS2 Heterojunctions

Monolayer transition metal dichalcogenides (TMDs) show promising potential for next-generation optoelectronics due to excellent light capturing and photodetection capabilities. Photodetectors, as important components of sensing, imaging and communication systems, are able to perceive and convert optical signals to electrical signals. Herein, the large-area and high-quality lateral monolayer MoS2/WS2 heterojunctions were synthesized via the one-step liquid-phase chemical vapor deposition approach. Systematic characterization measurements have verified good uniformity and sharp interfaces of the channel materials. As a result, the photodetectors enhanced by the photogating effect can deliver competitive performance, including responsivity of ~ 567.6 A/W and detectivity of ~ 7.17 × 1011 Jones. In addition, the 1/f noise obtained from the current power spectrum is not conductive to the development of photodetectors, which is considered as originating from charge carrier trapping/detrapping. Therefore, this work may contribute to efficient optoelectronic devices based on lateral monolayer TMD heterostructures.