Two-Dimensional Multifunctional Metal-Organic Frameworks with Simultaneous Ferro-/Ferrimagnetism and Vertical Ferroelectricity

Designing Chiral Organometallic Nanosheets with Room-Temperature Multiferroicity and Topological Nodes

Two-dimensional (2D) room-temperature chiral multiferroic and magnetic topological materials are essential for constructing functional spintronic devices, yet their number is extremely limited. Here, by using the chiral and polar HPP (HPP = 4-(3-hydroxypyridin-4-yl)pyridin-3-ol) as an organic linker and transition metals (TM = Cr, Mo, W) as nodes, we predict a class of 2D TM(HPP)2 organometallic nanosheets that incorporate homochirality, room-temperature magnetism, ferroelectricity, and topological nodes. The homochirality is introduced by chiral HPP linkers, and the change in structural chirality induces a topological phase transition of Weyl phonons. The room-temperature magnetism arises from the strong d-p spin coupling between TM cations and HPP doublet anions. The ferroelectricity is attributed to the breaking of spatial inversion symmetry in the lattice structure. Additionally, by adjusting the type of TMs, these nanosheets show rich and tunable band structures. Notably, all predicted materials are topologically nontrivial, featuring a quadratic nodal point around the Fermi level.

A Route to Two-Dimensional Room-Temperature Organometallic Multiferroics: The Marriage of d-p Spin Coupling and Structural Inversion Symmetry Breaking

Two-dimensional (2D) room-temperature multiferroic materials are highly desirable but still very limited. Herein, we propose a potential strategy to obtain such materials in 2D metal-organic frameworks (MOFs) by utilizing the d-p direct spin coupling in conjunction with center-symmetry-breaking six-membered heterocyclic rings. Based on this strategy, a screening of 128 2D MOFs results in the identification of three multiferroics, that is, Cr(1,2-oxazine)2, Cr(1,2,4-triazine)2, and Cr(1,2,3,4-trazine)2, simultaneously exhibiting room-temperature ferrimagnetism and ferroelectricity/antiferroelectricity. The room-temperature ferrimagnetic order (306-495 K) in these MOFs originates from the strong d-p direct magnetic exchange interaction between Cr cations and ligand anions. Specifically, Cr(1,2-oxazine)2 exhibits ferroelectric behavior with an out-of-plane polarization of 4.24 pC/m, whereas the other two manifest antiferroelectric characteristics. Notably, all three materials present suitable polarization switching barriers (0.18-0.31 eV). Furthermore, these MOFs are all bipolar magnetic semiconductors with moderate band gaps, in which the spin direction of carriers can be manipulated by electrical gating.

2D antiferromagnetic semiconducting FeCN with interesting properties

Two-dimensional magnetic materials have demonstrated favorable properties (e.g., large spin polarization and net magnetization) for the development of next-generation spintronic devices. The discovery of such materials and insight into their magnetic coupling mechanism has become a research focus. Here, on the basis of first-principles structural search calculations, we have identified a fresh FeCN monolayer consisting of edge-sharing Fe triangle sublattices and FeC3N2 rings, which integrates antiferromagnetism, semiconductivity, and planarity. Interestingly, it possesses a large magnetic anisotropy energy (MAE) of 614 μeV per Fe atom, a narrow band gap (Eg) of 0.47 eV, a large magnetic moment of 3.15 μB, and a proper Néel temperature (TN) of 97 K. The direct exchange between the nearest-neighbor Fe atoms in the triangle sublattice is mainly responsible for the AFM ordering. Its high structural stability, stemming from the collective contribution of covalent C-C and C-N bonds, ionic Fe-N bonds, and metallic Fe-Fe bonds, provides a strong feasibility for experimental synthesis.

Expanding the horizons of porphyrin metal-organic frameworks via catecholate coordination: exploring structural diversity, material stability and redox properties

Porphyrin based Metal-Organic Frameworks (MOFs) have generated high interest because of their unique combination of light absorption, electron transfer and guest adsorption/desorption properties. In this study, we expand the range of available MOF materials by focusing on the seldom studied porphyrin ligand H10TcatPP, functionalized with tetracatecholate coordinating groups. A systematic evaluation of its reactivity with M(iii) cations (Al, Fe, and In) led to the synthesis and isolation of three novel MOF phases. Through a comprehensive characterization approach involving single crystal and powder synchrotron X-ray diffraction (XRD) in combination with the local information gained from spectroscopic techniques, we elucidated the structural features of the solids, which are all based on different inorganic secondary building units (SBUs). All the synthesized MOFs demonstrate an accessible porosity, with one of them presenting mesopores and the highest reported surface area to date for a porphyrin catecholate MOF (>2000 m2 g-1). Eventually, the redox activity of these solids was investigated in a half-cell vs. Li with the aim of evaluating their potential as electrode positive materials for electrochemical energy storage. One of the solids displayed reversibility during cycling at a rather high potential (∼3.4 V vs. Li+/Li), confirming the interest of redox active phenolate ligands for applications involving electron transfer. Our findings expand the library of porphyrin-based MOFs and highlight the potential of phenolate ligands for advancing the field of MOFs for energy storage materials.

Chemically Controlled Reversible Magnetic Phase Transition in Two-Dimensional Organometallic Lattices

Developing an efficient method to reversibly control materials' spin order is urgently needed but challenging in spintronics. Though various physical field control methods have been advancing, the chemical control of spin is little exploited. Here, we propose a chemical means for such spin manipulation, i.e., utilizing the well-known lactim-lactam tautomerization to reversibly modulate the magnetic phase transition in two-dimensional (2D) organometallic lattices. The proposal is verified by theoretically designing several 2D organometallic frameworks with antiferromagnetic to ferrimagnetic spin order transformation modulated by lactim-lactam tautomerization on organic linkers. The transition originates from the change in spin states of organic linkers (from singlet to doublet) via tautomerization. Such a transition further switches materials' electronic structures from normal semiconductors with zero spin polarization to bipolar magnetic semiconductors with valence and conduction band edges 100% spin polarized in opposite spin channels. Moreover, the magnitude of magnetic anisotropy energy also enhances by 5- to 9-fold.

Two-Dimensional Metal-Organic Frameworks Towards Spintronics

The intrinsic properties of predesignable topologies and tunable electronic structures, coupled with the increase of electrical conductivity, make two-dimensional metal-organic frameworks (2D MOFs) highly prospective candidates for next-generation electronic/spintronic devices. In this Minireview, we present an outline of the design principles of 2D MOF-based spintronics materials. Then, we highlight the spin-transport properties of 2D MOF-based organic spin valves (OSVs) as a notable achievement in the progress of 2D MOFs for spintronics devices. After that, we discuss the potential for spin manipulation in 2D MOFs with bipolar magnetic semiconductor (BMS) properties as a promising field for future research. Finally, we provide a brief summary and outlook to encourage the development of novel 2D MOFs for spintronics applications.

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.

Quintuple Function Integration in Two-Dimensional Cr(II) Five-Membered Heterocyclic Metal Organic Frameworks via Tuning Ligand Spin and Lattice Symmetry

Two-dimensional (2D) semiconductors (SCs) integrated with two or more functions are the cornerstone for constructing multifunctional nanodevices but remain largely limited. Here, by tuning the spin state of organic linkers and the symmetry/topology of crystal lattices, we predict a class of unprecedented multifunctional SCs in 2D Cr(II) five-membered heterocyclic metal organic frameworks that simultaneously possess auxetic effect, room-temperature ferrimagnetism, chiral ferroelectricity (FE), electrically reversible spin polarization, and topological nodal lines/points. Taking 2D Cr(TDZ)₂ (TDZ = 1.2.5-thiadiazole) as an exemplification, the auxetic effect is produced by the antitetra-chiral lattice structure. The high temperature ferrimagnetism originates from the strong d-p direct magnetic exchange interaction between Cr cations and TDZ doublet radical anions. Meanwhile, the clockwise-counterclockwise alignment of TDZ's dipoles results in unique 2D chiral FE with atomic-scale vortex-antivortex states. 2D Cr(TDZ)₂ is an intrinsic bipolar magnetic SC where half-metallic conduction with switchable spin-polarization direction can be induced by applying a gate voltage. In addition, the symmetry of the little group C₄ of the lattice structure endows 2D Cr(TDZ)₂ with topological nodal lines and a quadratic nodal point in the Brillouin zone near the Fermi level.

A review of bipolar magnetic semiconductors from theoretical aspects

Spintronics, which employs electrons' spin degree of freedom in data storage and transmission, acts as a promising candidate for next-generation information technology owing to its improved processing speed and reduced power consumption. To seek and design materials with highly spin polarized carriers and find an efficient way to control the spin polarization direction of carriers are critical and urgent to spintronics applications. In this aspect, the bipolar magnetic semiconductor (BMS) serves as an ideal solution since it can generate currents with 100% spin polarization, and the direction of spin polarization is easily tunable by an external gate voltage. Up to now, there have been lots of BMSs predicted by first-principles calculations, however, most of them are extrinsically induced by chemical or physical modifications, and a generalized scheme for designing BMS materials is still lacking. This paper is aimed to briefly review the existing BMS materials designed by theoretical simulations, analyze the main obstacles to experimental realization, and put forward suggestions for future development.

Transition Metal-Free Half-Metallicity in Two-Dimensional Gallium Nitride with a Quasi-Flat Band

Two-dimensional half-metallicity without a transition metal is an attractive attribute for spintronics applications. On the basis of first-principles calculation, we revealed that a two-dimensional gallium nitride (2D-GaN), which was recently synthesized between graphene and SiC or wurtzite GaN substrate, exhibits half-metallicity due to its half-filled quasi-flat band. We found that graphene plays a crucial role in stabilizing a local octahedral structure, whose unusually high density of states due to a flat band leads to a spontaneous phase transition to its half-metallic phase from normal metal. It was also found that its half-metallicity is strongly correlated to the in-plane lattice constants and thus subjected to substrate modification. To investigate the magnetic property, we simplified its magnetic structure with a two-dimensional Heisenberg model and performed Monte Carlo simulation. Our simulation estimated its Curie temperature (T_C) to be ∼165 K under a weak external magnetic field, suggesting that transition metal-free 2D-GaN exhibiting p orbital-based half-metallicity can be utilized in future spintronics.

Two-dimensional multiferroics

Due to unprecedented application prospects such as high-density and low-power multistate storage, spintronics and nanoelectronics, two-dimensional (2D) multiferroics, coupled with at least two ferroic orders, have gotten a lot of interest in recent years. Multiple functions can be achieved in 2D multiferroics via coupling phenomena such as magnetoelectricity, piezoelectricity, and magnetoelasticity, which offers technical support for the creation of multifunctional devices. The research progress of 2D ferromagnetic-ferroelectric multiferroic materials, ferroelectric-ferroelastic multiferroic materials, and ferromagnetic-ferroelastic materials in recent years is reviewed in this paper. The categorization of 2D multiferroics is explored in terms of the multiple sources of ferroelectricity. The top-down approaches and the bottom-up methods used to fabricate 2D multiferroics materials are introduced. Finally, the authors outline potential research prospects and application scenarios for 2D multiferroic materials.

Robust Intrinsic Multiferroicity in a FeHfSe3 Layer

As a consequence of the mutually exclusive origins of ferroelectricity and magnetism, multiferroic materials with electromagnetic coupling are rare. In this work, stable two-dimensional FeHfSe₃ with experimental accessibility is however demonstrated to harbor robust electromagnetic coupling. FeHfSe₃ illustrates spontaneous in-plane polarization of 1.29 × 10^-10 C/m, and the energy barrier of 116.54 meV ensures easy switching and a high Curie temperature. In addition, semiconducting FeHfSe₃ possesses a stable antiferromagnetic ground state with a Néel temperature of approximately 300 K. In the case of applying strain, ferroelectricity and magnetism coexist stably, and uniaxial tensile strain can effectively enhance the ferroelectricity.

Polymerization-Induced Reassembly of Gemini Molecules toward Generating Porous Two-Dimensional Polymers

In situ polymerization of preorganized amphiphilic monomers on various substrates provides a flexible synthetic route to construct high-quality two-dimensional polymers (2DPs) with designed functionalities. However, the detailed polymerization kinetics of these monomers in 2D confinement and their impact on the structural features of 2DPs have not been efficiently explored. Here, using dissipative particle dynamics (DPD) simulations, we unveil the similarity of the polymerization kinetics of the amphiphilic Gemini molecules in both a 2D-confined space and solution and emphasize the key role of the initiator concentration in modifying the morphology of 2DPs. More interestingly, introducing a spacer group into the Gemini monomer facilitates the formation of porous 2DPs. The size and periodic arrangement of pores in these 2DPs could be directly controlled by the Gemini molecular geometries and polymerization kinetics. The insights based on our DPD simulations provide valuable guidelines for the rational design and synthesis of 2DPs from a wider range of amphiphilic molecules.

Two-dimensional conjugated metal–organic frameworks (2D c-MOFs): chemistry and function for MOFtronics

Two-dimensional conjugated MOFs are emerging for multifunctional electronic devices that brings us “MOFtronics”, such as (opto)electronics, spintronics, energy devices.

First-principles screening of novel ferroelectric MXene phases with a large piezoelectric response and unusual auxeticity

Out-of-plane ferroelectricity in the two-dimensional (2D) regime shows great promise for applications in next-generation electronic devices but has been rarely reported. Herein, by using high-throughput search and density functional theory (DFT) calculations, three types of ferroelectric MXene phases (type-I: Nb2CS2 and Ta2CS2; type-II: Sc2CO2 and Y2CO2; and type-III: Sc2CS2 and Y2CS2) are for the first time predicted to be achievable by surface functionalization of 2D MXenes. The identified 2D ferroelectric MXenes not only demonstrate remarkable and reversible spontaneous electric polarization along both the out-of-plane and in-plane orientations but also exhibit giant out-of-plane and in-plane piezoelectric response to the external strain. More excitingly, the type-III ferroelectric MXenes possess rare out-of-plane auxeticity that can be triggered by the tensile strain along both the zig-zag and the arm-chair directions. The intriguing ferroelectricity, large piezoelectric response, and auxeticity render the ferroelectric MXenes versatile candidates for nanoscale electronic and mechanic devices.