Multiferroic Materials Based on Organic Transition-Metal Molecular Nanowires

Understanding the motional dynamics of the ammonium ion in the mechanism of multiferroicity of Cr(V) peroxychromates: a 1H NMR study

Cr(5+)-based peroxychromates, M3Cr(O2)4, with M = NH4 or a mixed NH4-alkali metal are a new class of multiferroics for potential use in molecular memory devices, with the NH4+ being a key element, but the underlying chemical mechanism is not fully understood. The NH4+ ion occupies two different sites, but their specific roles are not known. We thus performed detailed 1H NMR spin-relaxation (T1) measurements on (NH4)3Cr(O2)4 over a wide temperature range (120-300 K) to probe the displacive as well as hindered rotational dynamics of the NH4+ ions with the view of understanding their specific roles in the phase transitions. The NH4+ dynamics is seen to consist of at least three different processes with varying activation energies. The sharp jump in the T1 at around 250 K is assigned to the change in the displacive motion at one of the two sites, while a kink around 140 K is ascribed to motional slowing at the second site. Interestingly, the slowing down starts around 250 K, well above the structural phase transition at 140 K. Taken together, these results provide a clue to the role of the site and symmetry of the NH4+ ion in the mechanism of solid-solid phase transitions.

Locally Strained 2D Materials: Preparation, Properties, and Applications

2D materials are promising for strain engineering due to their atomic thickness and exceptional mechanical properties. In particular, non-uniform and localized strain can be induced in 2D materials by generating out-of-plane deformations, resulting in novel phenomena and properties, as witnessed in recent years. Therefore, the locally strained 2D materials are of great value for both fundamental studies and practical applications. This review discusses techniques for introducing local strains to 2D materials, and their feasibility, advantages, and challenges. Then, the unique effects and properties that arise from local strain are explored. The representative applications based on locally strained 2D materials are illustrated, including memristor, single photon emitter, and photodetector. Finally, concluding remarks on the challenges and opportunities in the emerging field of locally strained 2D materials are provided.

First-principles calculations of inorganic metallocene nanowires

Inspired by the recently synthesized inorganic metallocene derivatives Fe(P4)22-, we have identified four stable inorganic metallocene nanowires, MP4 (M = Sc, Ti, Cr and Fe) in configurations of either regular quadrangular prism (Q-type) or anticube (A-type), and further investigated their magnetic and electronic characteristics utilizing the first-principles calculation. It shows that CrP4 is a ferromagnetic metal, while other nanowires are semiconducting antiferromagnets with bandgaps of 0.44, 1.88, and 2.29 eV within the HSE06 level. It also shows that both ScP4 and TiP4 can be stabilized in the Q-type and A-type, corresponding to the antiferromagnetic and ferromagnetic ground states, respectively, indicating a configuration-dependent magnetism. The thermodynamic and lattice stabilities are confirmed by the ab initio molecular dynamics and phonon spectra. This study has unmasked the structural and physical properties of novel inorganic metallocene nanowires, and revealed their potential application in spintronics.

Oxocarbon Acids and their Derivatives in Biological and Medicinal Chemistry

The biological and medicinal chemistry of the oxocarbon acids 2,3- dihydroxycycloprop-2-en-1-one (deltic acid), 3,4-dihydroxycyclobut-3-ene-1,2-dione (squaric acid), 4,5-dihydroxy-4-cyclopentene-1,2,3-trione (croconic acid), 5,6-dihydroxycyclohex- 5-ene-1,2,3,4-tetrone (rhodizonic acid) and their derivatives is reviewed and their key chemical properties and reactions are discussed. Applications of these compounds as potential bioisosteres in biological and medicinal chemistry are examined. Reviewed areas include cell imaging, bioconjugation reactions, antiviral, antibacterial, anticancer, enzyme inhibition, and receptor pharmacology.

Metal cyclopropenylidene sandwich cluster and nanowire: structural, electronic, and magnetic properties

Organometallic sandwich clusters and nanowires can offer prototypes for molecular ferromagnet and nanoscale spintronic devices due to the strong coupling of local magnetic moments in the nanowires direction and experimental feasibility. Here, on the basis of first-principles calculations, we reportTM_n(c-C₃H₂)_n+1(TM= Ti, Mn;n= 1-4) sandwich clusters and 1D [TM(c-C₃H₂)]_∞sandwich nanowires building from transitional metal and the smallest aromatic carbene of cyclopropenylidene (c-C₃H₂). Based on the results of lattice dynamic and thermodynamic studies, we show that the magnetic moment of Mn_n(c-C₃H₂)_n+1clusters increases linearly with the number ofn, and 1D [Mn(c-C₃H₂)]_∞nanowire is a stable ferromagnetic semiconductor, which can be converted into half metal with carrier doping. In contrary, both Ti_n(c-C₃H₂)_n+1and 1D [Ti(c-C₃H₂)]_∞nanowire are nonmagnetic materials. This study reveals the potential application of the [TM(c-C₃H₂)]_∞nanowire in spintronics.

0D/1D organic ferroelectrics/multiferroics for ultrahigh density integration: Helical hydrogen-bonded chains, multi-mode switching, and proton synaptic transistors

In recent years, room-temperature ferroelectricity has been experimentally confirmed in a series of two-dimensional (2D) materials. Theoretically, for isolated ferroelectricity in even lower dimensions such as 1D or 0D, the switching barriers may still ensure the room-temperature robustness for ultrahigh-density non-volatile memories, which has yet been scarcely explored. Here, we show ab initio designs of 0D/1D ferroelectrics/multiferroics based on functionalized transition-metal molecular sandwich nanowires (SNWs) with intriguing properties. Some functional groups such as -COOH will spontaneously form into robust threefold helical hydrogen-bonded chains around SNWs with considerable polarizations. Two modes of ferroelectric switching are revealed: when the ends of SNWs are not hydrogen-bonded, the polarizations can be reversed via ligand reorientation that will reform the hydrogen-bonded chains and alter their helicity; when both ends are hydrogen-bonded, the polarizations can be reversed via proton transfer without changing the helicity of chains. The combination of those two modes makes the system the smallest proton conductor with a moderate migration barrier, which is lower compared with many prevalent proton-conductors for higher mobility while still ensuring the robustness at ambient conditions. This desirable feature can be utilized for constructing nanoscale artificial ionic synapses that may enable neuromorphic computing. In such a design of synaptic transistors, the migration of protons through those chains can be controlled and continuously change the conductance of MXene-based post-neuron for nonvolatile multilevel resistance. The success of mimicking synaptic functions will make such designs promising in future high-density artificial neutral systems.

Ultra-high piezoelectric coefficients and strain-sensitive Curie temperature in hydrogen-bonded systems

We propose a new approach to obtain ultra-high piezoelectric coefficients that can be infinitely large theoretically, where ferroelectrics with strain-sensitive Curie temperature are necessary. We show the first-principles plus Monte Carlo simulation evidence that many hydrogen-bonded ferroelectrics (e.g. organic PhMDA) can be ideal candidates, which are also flexible and lead-free. Owing to the specific features of hydrogen bonding, their proton hopping barrier will drastically increase with prolonged proton transfer distance, while their hydrogen-bonded network can be easily compressed or stretched due to softness of hydrogen bonds. Their barriers as well as the Curie temperature can be approximately doubled upon a tensile strain as low as 2%. Their Curie temperature can be tuned exactly to room temperature by fixing a strain in one direction, and in another direction, an unprecedented ultra-high piezoelectric coefficient of 2058 pC/N can be obtained. This value is even underestimated and can be greatly enhanced when applying a smaller strain. Aside from sensors, they can also be utilized for converting either mechanical or thermal energies into electrical energies due to high pyroelectric coefficients.

Developing the Pressure-Temperature-Magnetic Field Phase Diagram of Multiferroic [(CH3)2NH2]Mn(HCOO)3

We combined Raman scattering and magnetic susceptibility to explore the properties of [(CH₃)₂NH₂]Mn(HCOO)₃ under compression. Analysis of the formate bending mode reveals a broad two-phase region surrounding the 4.2 GPa critical pressure that becomes increasingly sluggish below the order-disorder transition due to the extensive hydrogen-bonding network. Although the paraelectric and ferroelectric phases have different space groups at ambient-pressure conditions, they both drive toward P1 symmetry under compression. This is a direct consequence of how the order-disorder transition changes under pressure. We bring these findings together with prior magnetization work to create a pressure-temperature-magnetic field phase diagram, unveiling entanglement, competition, and a progression of symmetry-breaking effects that underlie functionality in this molecule-based multiferroic. That the high-pressure P1 phase is a subgroup of the ferroelectric Cc suggests the possibility of enhanced electric polarization as well as opportunity for strain control.

Electrical Control of Magnetic Phase Transition in a Type-I Multiferroic Double-Metal Trihalide Monolayer

Controlling magnetism of two-dimensional multiferroics by an external electric field provides special opportunities for both fundamental research and future development of low-cost electronic nanodevices. Here, we report a general scheme for realizing a magnetic phase transition in 2D type-I multiferroic systems through the reversal of ferroelectric polarization. Based on first-principles calculations, we demonstrate that a single-phase 2D multiferroic, namely, ReWCl_{6} monolayer, exhibits two different low-symmetric (C_{2}) phases with opposite in-plane electric polarization and different magnetic order. As a result, an antiferromagnetic-to-ferromagnetic phase transition can be realized by reversing the in-plane electric polarization through the application of an external electric field. These findings not only enrich the 2D multiferroic family, but also uncover a unique and general mechanism to control magnetism by electric field, thus stimulating experimental interest.

Single-phase multiferroics: new materials, phenomena, and physics

Multiferroics, where multiple ferroic orders coexist and are intimately coupled, promise novel applications in conceptually new devices on one hand, and on the other hand provide fascinating physics that is distinctly different from the physics of high-T_C superconductors and colossal magnetoresistance manganites. In this mini-review, we highlight the recent progress of single-phase multiferroics in the exploration of new materials, efficient roadmaps for functionality enhancement, new phenomena beyond magnetoelectric coupling, and underlying novel physics. In the meantime, a slightly more detailed description is given of several multiferroics with ferrimagnetic orders and double-layered perovskite structure and also of recently emerging 2D multiferroics. Some emergent phenomena such as topological vortex domain structure, non-reciprocal response, and hybrid mechanisms for multiferroicity engineering and magnetoelectric coupling in various types of multiferroics will be briefly reviewed.

Surface multiferroics in silicon enabled by hole-carrier doping

We predict a coexistence of magnetic and electric orders on clean Si(0 0 1) surfaces by first-principles calculations. Upon hole-carrier doping, the Si surfaces can be ferromagnetic, with polarized spins concentrated in an atom-thick space near the surface, due to an exchange splitting of localized s-like surface states on surface Si dimers. The surface magnetization can be controlled by reorienting the electric polarization of Si dimers, manifested as a transition from the magnetic antiferroelectric ground state to ferroelectric p(2 × 1) reconstruction that can be driven by an in-plane external electric field. The coupling between magnetic and electric orders can be further enhanced by strain silicon technology, rendering the Si surfaces as the first metal-free material displaying a multiferroic behavior.

Polar Metallocenes

Crystalline polar metallocenes are potentially useful active materials as piezoelectrics, ferroelectrics, and multiferroics. Within density functional theory (DFT), we computed structural properties, energy differences for various phases, molecular configurations, and magnetic states, computed polarizations for different polar crystal structures, and computed dipole moments for the constituent molecules with a Wannier function analysis. Of the systems studied, Mn₂(C₉H₉N)₂ is the most promising as a multiferroic material, since the ground state is both polar and ferromagnetic. We found that the predicted crystalline polarizations are 30–40% higher than the values that would be obtained from the dipole moments of the isolated constituent molecules, due to the local effects of the self-consistent internal electric field, indicating high polarizabilities.

Theoretical investigation on the electronic structure of one dimensional infinite monatomic gold wire: insights into conducting properties

Mixed-valence metal-organic nanostructures show unusual electronic properties. In our pervious investigation, we have designed and predicted a unique one-dimensional infinite monatomic gold wire (1D-IMGW) with excellent conductivity and the interesting characteristic of mixed valency (Auc 3+ and Au0 i). For further exploring its conduction properties and stability in conducting state, here we select one electron as a probe to explore the electron transport channel and investigate its electronic structure in conducting state. Density functional theory (DFT) calculations show the 1D-IMGW maintains its original structure in conducting state illustrating its excellent stability. Moreover, while adding an electron, 1D-IMGW is transformed from a semiconductor to a conductor with the energy band mixed with Auc (5d) and Aui (6s) through the Fermi level. Thus 1D-IMGW will conduct along its gold atom chain demonstrating good application prospect in nanodevices.

Design of Single-Molecule Multiferroics for Efficient Ultrahigh-Density Nonvolatile Memories

It is known that an isolated single-molecule magnet tends to become super-paramagnetic even at an ultralow temperature of a few Kelvin due to the low spin switching barrier. Herein, single-molecule ferroelectrics/multiferroics is proposed, as the ultimate size limit of memory, such that every molecule can store 1 bit data. The primary strategy is to identify polar molecules that possess bistable states, moderate switching barriers, and polarizations fixed along the vertical direction for high-density perpendicular recording. First-principles computation shows that several selected magnetic metal porphyrin molecules possess buckled structures with switchable vertical polarizations that are robust at ambient conditions. When intercalated within a bilayer of 2D materials such as bilayer MoS2 or CrI3, the magnetization can alter the spin distribution or can be even switched by 180° upon ferroelectric switching, rendering efficient electric writing and magnetic reading. It is found that the upper limit of areal storage density can be enhanced by four orders of magnitude, from the previous super-paramagnetic limit of ≈40 to ≈106 GB in.-2, on the basis of the design of cross-point multiferroic tunneling junction array and multiferroic hard drive.

Magnetism, stability and electronic properties of a novel one-dimensional infinite monatomic copper wire: a density functional study

In the present work, the structural, magnetic and electronic properties of a novel one-dimensional infinite monatomic copper wire (1D-IMCW) have been investigated using first-principles computational calculation.

Origin of Two-Dimensional Vertical Ferroelectricity in WTe2 Bilayer and Multilayer

In a recent report, room-temperature vertical ferroelectricity was experimentally shown in WTe₂ bilayer, while its mechanism of ferroelectric switching without vertical ion displacements remains unclarified. In this work, we reveal its origin by first-principles calculations that the polarization stems from uncompensated interlayer vertical charge transfer depending on in-plane translation, which can be switched upon interlayer sliding. The calculated results are consistent with experimental data, and a similar switching mechanism can be applied to a multilayer counterpart. Despite its small ferroelectric switching barrier and polarization, the in-plane rigidity of WTe₂ layer gives rise to a high Curie temperature. A moire pattern of ferroelectric domain superlattice can be formed and tuned upon a small-angle twist of bilayer, which is unique compared with traditional ferroelectrics. Similar interlayer translational ferroelectricity may exist in a series of van der Waals bilayers or even bulk phases.

Unusual Ferroelectricity of Trans-Unitcell Ion-Displacement and Multiferroic Soliton in Sodium and Potassium Hydroxides

We show the first-principles evidence of a hitherto unreported type of ferroelectricity with ultralong ion-displacement in sodium and potassium hydroxides. Even a small number of proton vacancies can completely change the mode of proton-transfer from intra-unitcell to trans-unitcell, giving rise to multiferroic soliton with "mobile" magnetism and a tremendous polarization that can be orders of magnitude higher compared with most perovskite ferroelectrics. Their vertical polarizations of thin-film are robust against a depolarizing field, rendering various designs of two-dimensional ferroelectric field-transistors with nondestructive readout and ultrahigh on/off ratio via sensing the switchable metallic/insulating state.

Theoretical investigation of multiferroic metal-organic framework magnet [CH3NH3][Co(HCOO)3]: Green's function method

For the first presented magnetic ordering-induced multiferroics with a metal-organic framework (MOF) of formula [CH₃NH₃][Co(HCOO)₃], we theoretically investigate its multiple ferroics. It is found that Dzyaloshinskii-Moriya interaction is a main cause that leads to non-zero magnetization, and electric polarization, and the induced electric polarization can be regulated by magnetic fields. As an assistant mechanism, magnon-magnon interaction and quantum fluctuation play an important role on ferroelectrics and magnetism. Our methods are based on the double-time Green's function and Holstein-Primakoff transformation. Theoretical results can be compared with experiments, though there are some discrepancies.

Transition-metal-doped group-IV monochalcogenides: a combination of two-dimensional triferroics and diluted magnetic semiconductors

We report the first-principles evidence of a series of two-dimensional triferroics (ferromagnetic + ferroelectric + ferroelastic), which can be obtained by doping transition-metal ions in group-IV monochalcogenide (SnS, SnSe, GeS, GeSe) monolayers, noting that a ferromagnetic Fe-doped SnS₂ monolayer has recently been realized (Li B et al 2017 Nat. Commun. 8 1958). The ferroelectricity, ferroelasticity and ferromagnetism can be coupled and the magnetization direction may be switched upon ferroelectric/ferroelastic switching, rendering electrical writing + magnetic reading possible. They can be also two-dimensional half-metals or diluted magnetic semiconductors, where p/n channels or even multiferroic tunneling junctions can be designed by variation in doping and incorporated into a monolayer wafer.

Proton transfer ferroelectricity/multiferroicity in rutile oxyhydroxides

Oxyhydroxide minerals such as FeOOH have been a research focus in geology for studying the Earth's interior, and also in chemistry for studying their oxygen electrocatalysis activity. In this paper the first-principle evidence of a new class of ferroelectrics/multiferroics is given. In this class are: β-CrOOH (guyanaite), ε-FeOOH, β-GaOOH, and InOOH, which are earth-abundant minerals which have been experimentally verified to possess distorted rutile structures, are ferroelectric with considerable polarizations (up to 24 μC cm-2) and piezoelectric coefficients. Their atomic-thick layer may possess vertical polarization will not be diminished by depolarizing field because of the formation of O-HO bonds that can be hardly symmetrized. Furthermore, β-CrOOH is revealed to be a combination of a high Curie temperature (TC) in-plane type-I multiferroics and vertical type-II multiferroics, which is strain tunable and may give a desirable coupling between magnetism and ferroelectricity. Supported by experimental evidence on reversible conversion between metal oxyhydroxides and dioxides and their good lattice match that gives convenient epitaxial growth, a heterostructure composed of oxyhydroxides and common metal dioxides (e.g., TiO2, SnO2 and CrO2) may be constructed for various applications such as ferroelectric field-effect transistors and multiferroic tunneling junctions.

Two-Dimensional Metal-Free Organic Multiferroic Material for Design of Multifunctional Integrated Circuits

Coexistence of ferromagnetism and ferroelectricity in a single 2D material is highly desirable for integration of multifunctional units in 2D material-based circuits. We report theoretical evidence of C₆N₈H organic network as being the first 2D organic multiferroic material with coexisting ferromagnetic and ferroelectric properties. The ferroelectricity stems from multimode proton-transfer within the 2D C₆N₈H network, in which a long-range proton-transfer mode is enabled by the facilitation of oxygen molecule when the network is exposed to the air. Such oxygen-assisted ferroelectricity also leads to a high Curie temperature and coupling between ferroelectricity and ferromagnetism. We also find that hydrogenation and carbon doping can transform the 2D g-C₃N₄ network from an insulator to an n-type/p-type magnetic semiconductor with modest bandgap. Akin to the dopant induced n/p channels in silicon wafer, a variety of dopant created functional units can be integrated into the g-C₃N₄ wafer by design for nanoelectronic applications.

Continuous Draw Spinning of Extra-Long Silver Submicron Fibers with Micrometer Patterning Capability

Ultrathin metal fibers can serve as highly conducting and flexible current and heat transport channels, which are essential for numerous applications ranging from flexible electronics to energy conversion. Although industrial production of metal fibers with diameters of down to 2 μm is feasible, continuous production of high-quality and low-cost nanoscale metal wires is still challenging. Herein, we report the continuous draw spinning of highly conductive silver submicron fibers with the minimum diameter of ∼200 nm and length of more than kilometers. We obtained individual AgNO₃/polymer fibers by continuous drawing from an aqueous solution at a speed of up to 8 m/s. With subsequent heat treatment, freestanding Ag submicron fibers with high mechanical flexibility and electric conductivity have been obtained. Woven mats of aligned Ag submicron fibers were used as transparent electrodes with high flexibility and high performance with sheet resistance of 7 Ω sq^-1 at a transparency of 96%. Continuous draw spinning opened new avenues for scalable, flexible, and ultralow-cost fabrication of extra-long conductive ultrathin metal fibers.

The degree of π electron delocalization and the formation of 3D-extensible sandwich structures

DFT B3LYP/6-31G(d) calculations were performed to examine the feasibility of graphene-like C42H18 and starbenzene C6(BeH)6 (SBz) polymers as ligands of 3D-extensible sandwich compounds (3D-ESCs) with uninterrupted sandwich arrays. The results revealed that sandwich compounds with three or more C42H18 ligands were not feasible. The possible reason may be the localization of π electrons on certain C6 hexagons due to π-metal interactions, which makes the whole ligand lose its electronic structure basis (higher degree of π electron delocalization) to maintain the planar structure. For comparison, with the aid of benzene (Bz) molecules, the SBz polymers can be feasible ligands for designing 3D-ESCs because the C-Be interactions in individual SBz are largely ionic, which will deter the π electrons on one C6 ring from connecting to those on neighbouring C6 rings. This means that high degree of π electron delocalization is not necessary for maintaining the planarity of SBz polymers. Such a locally delocalized π electron structure is desirable for the ligands of 3D-ESCs. Remarkably, the formation of a sandwich compound with SBz is thermodynamically more favourable than that found for bis(Bz)chromium. The assembly of 3D-ESCs is largely exothermic, which will facilitate future experimental synthesis. The different variation trends on the HOMO-LUMO gaps in different directions (relative to the sandwich axes) suggest that they can be developed to form directional conductors or semiconductors, which may be useful in the production of electronic devices.

Theoretical investigation of an ultrastable one dimensional infinite monatomic mixed valent gold wire with excellent electronic properties

A one-dimensional monatomic gold wire with mixed-valent Au³⁺ and Au⁰ exhibits excellent conductivity and strong visible absorption.

Charge-transfer magnetoelectrics of polymeric multiferroics

The renaissance of multiferroics has yielded a deeper understanding of magneto-electric coupling of inorganic single-phase multiferroics and composites. Here, we report charge-transfer polymeric multiferroics, which exhibit external field-controlled magnetic, ferroelectric, and microwave response, as well as magneto-dielectric coupling. The charge-transfer-controlled ferroic properties result from the magnetic field-tunable triplet exciton which has been validated by the dynamic polaron-bipolaron transition model. In addition, the temperature-dependent dielectric discontinuity and electric-field-dependent polarization confirms room temperature ferroelectricity of crystalline charge-transfer polymeric multiferroics due to the triplet exciton, which allows the tunability of polarization by the photoexcitation.

Ferroelectric mechanism of croconic acid: a first-principles and Monte Carlo study

The ferroelectric mechanism of croconic acid in terms of the electronic structure and the molecular structure was studied by first principles using the density functional theory with the generalized gradient approximation. The spontaneous polarization (Ps) was simulated by the Berry phase method. It is found that the large polarization originates from charge transfer due to the strong "push-pull" effect of electron-releasing and -withdrawing groups along the hydrogen bond. According to the characteristics of polarization of croconic acid, we constructed a one-dimensional ferroelectric Hamiltonian model to describe the ferroelectric properties of croconic acid. Based on the Hamiltonian model, the thermal properties of the ferroelectricity of croconic acid were studied by Monte Carlo method. The simulated Curie temperature is 756 K, and the spontaneous polarization keeps well temperature range stability up to 400 K. These results are in good agreement with the experimental data.