Noncollinear Magnetic Order in Two-Dimensional NiBr2 Films Grown on Au(111)

Intrinsically Patterned Two-Dimensional Transition Metal Halides

Patterning and defect engineering are key methods for tuning the properties and enabling distinctive functionalities in two-dimensional (2D) materials. However, generating 2D periodic patterns of point defects in 2D materials, such as vacancy lattices that can serve as antidot lattices, has been elusive until now. Herein, we report on 2D transition metal dihalides epitaxially grown on metal surfaces featuring periodically assembled halogen vacancies that result in alternating coordination of the transition metal atom. Using low-temperature scanning probe microscopy and low-energy electron diffraction, we identified the structural properties of intrinsically patterned FeBr2 and CoBr2 monolayers grown epitaxially on Au(111). Density functional theory reveals that Br vacancies are facilitated by low formation energies, and the formation of a vacancy lattice results in a substantial decrease in the lattice mismatch with the underlying Au(111). We demonstrate that interfacial strain engineering presents a versatile strategy for controlled patterning in two dimensions with atomic precision over several hundred nanometers to solve a long-standing challenge of growing atomically precise antidot lattices. In particular, patterning of 2D materials containing transition metals provides a versatile method to achieve unconventional spin textures with noncollinear spin.

Ferromagnetic Order in 2D Layers of Transition Metal Dichlorides

Magnetism in two dimensions is traditionally considered an exotic phase mediated by spin fluctuations, but far from collinearly ordered in the ground state. Recently, 2D magnetic states have been discovered in layered van der Waals compounds. Their robust and tunable magnetic state by material composition, combined with reduced dimensionality, foresee a strong potential as a key element in magnetic devices. Here, a class of 2D magnets based on metallic chlorides is presented. The magnetic order survives on top of a metallic substrate, even down to the monolayer limit, and can be switched from perpendicular to in-plane by substituting the metal ion from iron to nickel. Using functionalized STM tips as magnetic sensors, local exchange fields are identified, even in the absence of an external magnetic field. Since the compounds are processable by molecular beam epitaxy techniques, they provide a platform with large potential for incorporation into current device technologies.

Stacking engineering in layered homostructures: transitioning from 2D to 3D architectures

Artificial materials, characterized by their distinctive properties and customized functionalities, occupy a central role in a wide range of applications including electronics, spintronics, optoelectronics, catalysis, and energy storage. The emergence of atomically thin two-dimensional (2D) materials has driven the creation of artificial heterostructures, harnessing the potential of combining various 2D building blocks with complementary properties through the art of stacking engineering. The promising outcomes achieved for heterostructures have spurred an inquisitive exploration of homostructures, where identical 2D layers are precisely stacked. This perspective primarily focuses on the field of stacking engineering within layered homostructures, where precise control over translational or rotational degrees of freedom between vertically stacked planes or layers is paramount. In particular, we provide an overview of recent advancements in the stacking engineering applied to 2D homostructures. Additionally, we will shed light on research endeavors venturing into three-dimensional (3D) structures, which allow us to proactively address the limitations associated with artificial 2D homostructures. We anticipate that the breakthroughs in stacking engineering in 3D materials will provide valuable insights into the mechanisms governing stacking effects. Such advancements have the potential to unlock the full capability of artificial layered homostructures, propelling the future development of materials, physics, and device applications.

Ultra-Thin 2D Ionic Salt Supported with Strong Hydrogen-Bonding Assisted Ionic Interaction

2D materials have attracted great interest since the report of graphene. However, because of the fragile stability of ultra-thin nanosheets, most studies are restricted to sheets maintained by strong covalent or coordination bonds. The research on which kind of bonds can maintain the free-standing existence of 2D nanosheets is still of great significance. Recently, 2D ionic salts are successfully synthesized on substrates, but whether 2D ionic salts can free-stand is still a problem. Herein this problem is addressed by a free-standing 2D ionic salt (thickness: ≈2 nm) exfoliated from a 4,4'-bipyridinium hydrochloride salt crystal. The stability of this 2D salt is supported by a strong NH···Cl hydrogen (H)-bonding assisted ionic interaction (17.99 kcal mol^-1 ), which is verified by density functional theory calculation and natural bond orbital analysis. The salt crystal has strong air-stable radicals inside and the 2D ionic salt exhibits red fluorescence in solution and in solid-state, especially in solution the stokes shifts are very large (≈ 386 nm). This breakthrough work is not only beneficial for the construction of novel 2D materials but also for the understanding of H-bonding interactions.

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.

2D magnetic phases of Eu on Ge(110)

2D magnetic materials are at the forefront of research on fundamentals of magnetism; they exhibit unconventional phases and properties controlled by external stimuli. 2D magnets offer a solution to the problem of miniaturization of spintronic devices. A technological target of materials science is to find suitable magnetic materials and scale their thickness down as much as possible, a single monolayer being a natural limit. However, magnetism does not halt at one monolayer - it may persist beyond this boundary, to sparse but regular lattices of magnetic atoms. Here, we report 2D magnetic phases of Eu on the Ge(110) surface. We synthesized two submonolayer structures Eu/Ge(110) employing molecular beam epitaxy. The phases, identified by electron diffraction, differ in the surface density of Eu atoms. At low temperature, they exhibit magnetic ordering with magnetic moments lying in-plane. Strong dependence of the effective magnetic transition temperature on weak magnetic fields points at the 2D nature of the observed magnetism. The results are set against those on the Eu/Si system. The study of Eu/Ge(110) magnets demonstrates that a variety of substrates of different structure and symmetry can host submonolayer 2D magnetic phases, suggesting the phenomenon to be rather general.

Epitaxial growth and electronic properties of an antiferromagnetic semiconducting VI2 monolayer

The van der Waals materials down to the monolayer (ML) limit provide a fertile platform for exploring low-dimensional magnetism and developing the novel applications of spintronics. Among them, due to the absence of the net magnetic moment, antiferromagnetic (AFM) materials have received much less attention than ferromagnetic ones. Here, by combining scanning tunneling microscopy and state-of-the-art first-principles calculations, we investigate the preparation, and electronic and magnetic properties of a vanadium(II) iodide (VI₂) ML. Single-layer VI₂ has been grown by molecular beam epitaxy on Au(111) surfaces. A band gap of 2.8 eV is revealed, indicating the semiconducting nature of the VI₂ ML. Vanadium and iodine vacancy defects give rise to additional feature states within the bandgap. A typical 120° AFM spin ordering is maintained in the ML limit of VI₂, as revealed by the first-principles calculations. Besides, the AFM coupling is greatly enhanced by slightly decreasing lattice constants. Our work provides an ideal platform for further studying two-dimensional magnetism with non-collinear AFM ordering and for investigating the possibility of realizing the spin Hall effect in the ML limit.