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

Emerging two-dimensional ferromagnetic semiconductors

Two-dimensional (2D) semiconductors have attracted considerable attention for their potential in extending Moore's law and advancing next-generation electronic devices. Notably, the discovery and development of 2D ferromagnetic semiconductors (FMSs) open exciting opportunities in manipulating both charge and spin, enabling the exploration of exotic properties and the design of innovative spintronic devices. In this review, we aim to offer a comprehensive summary of emerging 2D FMSs, covering their atomic structures, physical properties, preparation methods, growth mechanisms, magnetism modulation techniques, and potential applications. We begin with a brief introduction of the atomic structures and magnetic properties of novel 2D FMSs. Next, we delve into the latest advancements in the exotic physical properties of 2D FMSs. Following that, we summarize the growth methods, associated growth mechanisms, magnetism modulation techniques and spintronic applications of 2D FMSs. Finally, we offer insights into the challenges and potential applications of 2D FMSs, which may inspire further research in developing high-density, non-volatile storage devices based on 2D FMSs.

Versatile Method for Preparing Two-Dimensional Metal Dihalides

Ever since the ground-breaking isolation of graphene, numerous two-dimensional (2D) materials have emerged with 2D metal dihalides gaining significant attention due to their intriguing electrical and magnetic properties. In this study, we introduce an innovative approach via anhydrous solvent-induced recrystallization of bulk powders to obtain crystals of metal dihalides (MX2, with M = Cu, Ni, Co and X = Br, Cl, I), which can be exfoliated to 2D flakes. We demonstrate the effectiveness of our method using CuBr2 as an example, which forms large layered crystals. We investigate the structural properties of both the bulk and 2D CuBr2 using X-ray diffraction, along with Raman scattering and optical spectroscopy, revealing its quasi-1D chain structure, which translates to distinct emission and scattering characteristics. Furthermore, microultraviolet photoemission spectroscopy and electronic transport reveal the electronic properties of CuBr2 flakes, including their valence band structure. We extend our methodology to other metal halides and assess the stability of the metal halide flakes in controlled environments. We show that optical contrast can be used to characterize the flake thicknesses for these materials. Our findings demonstrate the versatility and potential applications of the proposed methodology for preparing and studying 2D metal halide flakes.

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.

Toward Clean 2D Materials and Devices: Recent Progress in Transfer and Cleaning Methods

Two-dimensional (2D) materials have tremendous potential to revolutionize the field of electronics and photonics. Unlocking such potential, however, is hampered by the presence of contaminants that usually impede the performance of 2D materials in devices. This perspective provides an overview of recent efforts to develop clean 2D materials and devices. It begins by discussing conventional and recently developed wet and dry transfer techniques and their effectiveness in maintaining material "cleanliness". Multi-scale methodologies for assessing the cleanliness of 2D material surfaces and interfaces are then reviewed. Finally, recent advances in passive and active cleaning strategies are presented, including the unique self-cleaning mechanism, thermal annealing, and mechanical treatment that rely on self-cleaning in essence. The crucial role of interface wetting in these methods is emphasized, and it is hoped that this understanding can inspire further extension and innovation of efficient transfer and cleaning of 2D materials for practical applications.

Evidence of Ferromagnetism and Ultrafast Dynamics of Demagnetization in an Epitaxial FeCl2 Monolayer

The development of two-dimensional (2D) magnetism is driven not only by the interest of low-dimensional physics but also by potential applications in high-density miniaturized spintronic devices. However, 2D materials possessing a ferromagnetic order with a relatively high Curie temperature (Tc) are rare. In this paper, the evidence of ferromagnetism in monolayer FeCl2 on Au(111) surfaces, as well as the interlayer antiferromagnetic coupling of bilayer FeCl2, is characterized by using spin-polarized scanning tunneling microscopy. A Curie temperature (Tc) of ∼147 K is revealed for monolayer FeCl2, based on our static magneto-optical Kerr effect measurements. Furthermore, temperature-dependent magnetization dynamics is investigated by the time-resolved magneto-optical Kerr effect. A transition from one- to two-step demagnetization occurs as the lattice temperature approaches Tc, which supports the Elliott-Yafet spin relaxation mechanism. The findings contribute to a deeper understanding of the underlying mechanisms governing ultrafast magnetization in 2D ferromagnetic materials.

High spin polarization, large perpendicular magnetic anisotropy and room-temperature ferromagnetism by biaxial strain and carrier doping in Janus MnSeTe and MnSTe

The emerging two-dimensional (2D) Janus systems with broken symmetry provide a new platform for designing ultrathin multifunctional spintronic materials. Recently, based on experimental monolayer MnSe2, ferromagnetism was predicted in Janus MnXY (X ≠ Y = S, Se, Te) monolayers; however, they exhibit low Curie temperatures and small magnetic anisotropic energies. To improve the Curie temperature and magnetic anisotropy, herein, we systemically explore the stability and electronic and magnetic properties of Janus MnSeTe and MnSTe monolayers under strain and carrier-doping using first-principles calculations and Monte Carlo simulations. It is found that both MnSeTe and MnSTe monolayers possess robustly high spin polarization with rational strain and carrier-doping. Both tensile strain and hole doping strengthen the ferromagnetic super-exchange interactions of the two nearest Mn atoms mediated by chalcogen atoms and exceedingly improve the perpendicular magnetic anisotropic energies (by up to 3.1 meV per f.u. for MnSeTe and 2.0 meV per f.u. for MnSTe). The Te-5p intraorbital hybridizations contributed to the main magnetic anisotropy. More remarkably, the tensile strain and hole doping collectively increase the Curie temperatures of MnSeTe and MnSTe to above and near room temperature (345 and 290 K, respectively). The present study reveals that Janus MnSeTe and MnSTe monolayers with robustly high spin polarization, room-temperature ferromagnetism and large perpendicular magnetic anisotropy are promising candidates for ultrathin multifunctional spintronic materials. This study will be of great interest for further experimental and theoretical explorations of 2D Janus manganese dichalcogenides.