Computational Design of One‐Dimensional Ferromagnetic Semiconductors in Transition Metal Embedded Stannaspherene Nanowires

Developing low dimensional semiconductors with moderate band gaps, intrinsic ferromagnetism and large magnetic anisotropy energies (MAEs) is very desirable for high‐speed nano‐spintronic devices, which, however, still remains a big challenge. Here, via first principles calculations, a potential route to realize such materials is proposed based on a new class of one‐dimensional transition metal (TM) embedded stannaspherene (Sn₁₂^2–) nanowires [TM₂(Sn₁₂)]_∞ (TM = Ti‐Ni). Three semiconductors with robust ferromagnetism are achieved with TM = V, Cr and Fe, which all exhibit direct or quasi‐direct band gaps around 1.0 eV, rendering their great potentials for visible light optoelectronic applications. Interestingly, [Cr₂(Sn₁₂)]_∞ and [Fe₂(Sn₁₂)]_∞ are both identified as bipolar magnetic semiconductors (BMS) with valence and conduction band edges spin polarized in the opposite directions, which are promising for realizing switch of carriers’ spin orientation by electrical gating, while [V₂(Sn₁₂)]_∞ exhibits a half semiconductor (HSC) property with valence and conduction band edges spin polarized in the same direction and can be used for spin‐polarized carriers generation. Moreover, sizable MAEs are discovered in these nanowires, which are at least two orders of magnitude larger than those of Fe, Co and Ni bulks and also significantly larger than current ferromagnetic semiconductors.     

Room temperature ferromagnetism in sol–gel deposited un-doped ZnO films

Dilute magnetic semiconductors are fast emerging spintronic materials where advantage of magnetic properties of semiconductor materials (usually doped with small quantities of magnetic ions) is being explored. Sol–gel technique, being low-cost simple and application oriented method, has been used in the present case. ZnO films of <150 nm thickness have been deposited by spin coating onto single crystal p-type Si substrates. The optimized sol is of paramagnetic nature, whereas, mixed para- dia-magnetic phase is observed for the as-prepared films. A complete ferromagnetic phase transition has been observed after heating the films in vacuum at a temperature of 300 °C. These sol–gel prepared films exhibit hexagonal wurtzite structure as observed by X-ray diffraction. After the magnetic field annealing in vacuum the films showed strengthened magnetic as well as structural properties. This work presents a clear evidence of ferromagnetic behavior of the un-doped ZnO films deposited by sol–gel at room temperature. It is also pointed out that Zn vacancies rather than oxygen deficiency are responsible for ferromagnetism in these sol–gel deposited ZnO thin films, whereas, the experimental evidence has been substantiated with the theoretical calculations using density functional theory.     

Ferromagnetic semiconductor nanostructures—future spintronics

The review analyzes and summarizes the principal results of research on the magnetic properties of ferromagnetic semiconductor nanostructures. These materials possess unique physical properties, which attracts great attention both from the scintific and practical viewpoints. The review shows that group A_IIB_VI and A_IIIB_V ferromagnetic semiconductors, as well as group IV elemental semiconductors hold the greatest promise for spintronic applications.     

Construction of Long-Range Magnetic Sequences on Different Surfaces of BaTiO3

Using the first-principles spin-density-functional theory calculations, we studied the origin of ferromagnetism from non-magnetic ferroelectric barium titanate (BaTiO₃) and found out vacancies in different surface can successfully contribute to the origin of ferromagnetism. Accurately, our findings demonstrate that both O and Ti vacancies induce ferromagnetism on the (001) and (010) surfaces of BaTiO₃, and the optimal Ti−O bond length can control the vacancy-induced spin density that is delocalized or concentrated in the real space outside the vacancy, and it helps to enhance our understanding on the long-range magnetic order induced by the vacancy. In addition, intrinsic magnetism is shown on the defect-free (110) surface, and the structure is found to be a near-ideal two-dimensional Ising ferromagnet with large magnetocrystalline anisotropy, and it supplies the platform for studying basic spin behavior of BaTiO₃ and more according materials.     

Physics,chemistry and synthesis methods of nanostructured bismuth ferrite (BiFeO3) as a ferroelectro-magnetic material

Materials that combine ferroic properties, such as ferromagnetism and ferroelectricity are highly desirable, yet rare. The number of candidate materials is limited and their effects are typically too small at room temperature to be useful in applications. Bismuth ferrite (BiFeO₃) is potentially the only material which is both magnetic and highly ferroelectric at room temperature. Nanostructured BiFeO₃ are promising materials for magnetoelectric and spintronic devices, especially the memories that can be addressed both electrically and magnetically. This review paper investigates the structural, microstructural, physical concepts and different synthesis methods of BiFeO₃.     

Substitutional doping of symmetrical small fullerene dimers

Magnetic carbon nano-structures have potential applications in the field of spintronics as they exhibit valuable magnetic properties. Symmetrically sized small fullerene dimers are substitutional doped with nitrogen (electron rich) and boron (electron deficient) atoms to visualize the effect on their magnetic properties. Interaction energies suggests that the resultant dimer structures are energetically favorable and hence can be formed experimentally. There is significant change in the total magnetic moment of dimers of the order of 0.5 μ_B after the substitution of C atoms with N and B, which can also be seen in the change of density of states. The HOMO-LUMO gaps of spin up and spin down electronic states have finite energy difference which confirm their magnetic behaviour, whereas for non-magnetic doped dimers, the HOMO-LUMO gaps for spin up and down states are degenerate. The optical properties show that the dimers behave as optical semiconductors and are useful in optoelectronic devices. The induced magnetism in these dimers makes them fascinating nanocarbon magnetic materials.     

Physicochemical foundations of synthesis of new ferromagnets from chalcopyrites A<Superscript>II</Superscript>B<Superscript>IV</Superscript>C<Stack><Subscript>2</Subscript><Superscript>V</Superscript></Stack>

The results of studying phase equilibria of ternary A^IIB^IVC^V systems have been reported. Physicochemical foundations have been developed for the synthesis of new ferromagnets with Curie temperatures above room temperature structurally compatible with basic semiconducting materials. Methods of synthesis and physicochemical properties of manganese-doped A^IIB^IVC₂^V ferromagnets have been described. The results of theoretical simulation of magnetic properties have been considered and basic approaches to the explanation of the emergence of ferromagnetism in A^IIB^IVC₂^V doped with 3d metals have been surveyed. The most promising ways to produce and study dilute magnetic semiconductors as spintronics materials have been presented.     

A Radical Polymer as a Two‐Dimensional Organic Half Metal

Given that half‐metals are promising futuristic materials for spintronics, organic materials showing half‐metal character are highly desirable for spintronic devices, not only owing to their weak spin‐orbit and hyperfine interactions, but also their light and flexible properties. We predict that a two‐dimensional organic 2,4,6‐tri‐(1,3,5‐triazinyl)methyl radical polymer has half‐metallic properties as well as a spontaneous magnetic ordering at ambient temperature. The quantum transmission is studied based on the nonequilibrium Green function theory coupled with density functional theory. The half‐metallic property in the triazine‐based polymer depends mainly on the nature of the p‐band in contrast to of conventional half metals in which the nature of the d‐band is more important.     

Spintronics: a challenge for materials science and solid-state chemistry

Spintronics is a multidisciplinary field involving physics, chemistry, and engineering, and is a new research area for solid-state scientists. A variety of new materials must be found to satisfy different demands. The search for ferromagnetic semiconductors and stable half-metallic ferromagnets with Curie temperatures higher than room temperature remains a priority for solid-state chemistry. A general understanding of structure-property relationships is a necessary prerequisite for the design of new materials. In this Review, the most important developments in the field of spintronics are described from the point of view of materials science.     

Magnetoelectronic properties of ferromagnetic compounds Rb2TaZ6 (Z = Cl,Br) for possible spintronic applications

Highly spin-polarized ferromagnetic materials are essential for efficient spintronic devices. Here, 100% spin-polarized compounds Rb₂TaZ₆ (Z = Cl, Br) studied via density functional theory are reported. These compounds show stability in the ferromagnetic phase with cubic symmetry and half metallic behavior, thereby exhibiting a nonzero direct band gap in the spin-down channel and zero band gap in the spin-up configuration. The Ta-d sates contribute mainly to the net magnetic moments as explained by the crystal field theory and density of states. High Curie temperatures of 960.35 and 1021.74 K for Ra₂TaCl₆ and Rb₂TaBr₆, along with maximum spin polarizability, make these compounds favorable for efficient spintronic applications.     

Heading to photoswitchable magnets

The review concerns the strategic approaches to the photomodulation of the magnetic properties of molecular hybrid compounds combining magnetic and optical properties. The approaches developed include photocontrol of intra- and intermolecular magnetic coupling and manipulation of the effect of the photochromic sublattice on the bulk behavior of molecular magnets. In the framework of the first approach, we consider photoinduced charge-transfer phase transitions, changes in the spin state of magnetic centers, photoswitching of intramolecular exchange interactions between magnetic centers connected by a photochromic bridge, and generation of high-spin organic molecules forming high-spin groups due to intermolecular exchange interactions. In the framework of the second approach, we discuss hybrid polyfunctional compounds combining magnetic and photochromic sublattices in the same crystal lattice, as well as intercalation of organic photochromes into voids or inter-layer space of organic magnetics (or vice versa). Creation of such materials allows not only several functions to be combined in the same lattice, which is important for reducing the size of hardware components, but also these properties to be controlled and modified through synergistic effects. The results of basic research suggest that novel materials for various practical applications can be created using principles of crystal chemical engineering. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 704–721, April, 2008.     

Hydrogen dangling bonds induce ferromagnetism in two-dimensional metal-free graphitic-C3N4 nanosheets

Ferromagnetic two-dimensional (2D) ultrathin nanosheets hold great promise for next generation electronics. Ferromagnetic metal-free materials that usually possess only an s/p electronic configuration with weak spin–orbit coupling and a large spin relaxation time, would play an important role in constructing future spintronic devices. However, the absence of an intrinsic spin ordering structure in most metal-free materials greatly hampers the widening scope of ferromagnetic 2D nanostructures as well as in-depth understanding of their ferromagnetic nature. Herein, the induction of intrinsic ferromagnetism in 2D metal-free g-C₃N₄ ultrathin nanosheets has been achieved through a new effective strategy whereby hydrogen dangling bonds are introduced. In our case, g-C₃N₄ ultrathin nanosheets with hydrogen dangling bonds showed obvious room temperature ferromagnetic behavior that could even be tuned by the concentration of hydrogen. This work will pave a new pathway to engineer the properties of 2D nanomaterial systems.     

Anionogenic ferromagnets

Magnetism in molecules and solids is understood to originate from atoms in that part of the periodic table where a particular value of the angular momentum appears first (i.e., the 2p, 3d, and 4f series). In contrast to the many magnetic compounds containing transition metal or lanthanide atoms, ferromagnetism based on atoms from the 2p series is very rare. We report density functional calculations that show the existing compound rubidium sesquioxide is a ferromagnet with an estimated Curie temperature of 300 K, unprecedented in p-electron magnetism. The magnetic moment is carried by the anion. Rubidium sesquioxide is a conductor, but only for the minority spin electrons (a so-called "half-metal"). Half-metals play an important role in spintronics, that is, electronics that exploits the electron spin. Since the magnetic moment resides on a light element (oxygen), spin-orbit interactions are considerably reduced compared to other half-metals. Consequently spin relaxation is expected to be suppressed by up to 2 orders of magnitude in comparison with materials presently used in spintronics.     

Room temperature ferromagnetism in undoped and Fe doped ZnO nanorods: Microwave-assisted synthesis

One-dimensional (1D) undoped and Fe doped ZnO nanorods of average length ∼1 μm and diameter ∼50 nm have been obtained using a microwave-assisted synthesis. The magnetization (M) and coercivity (H_c) value obtained for undoped ZnO nanorods at room temperature is ∼5×10⁻³ emu/g and ∼150 Oe, respectively. The Fe doped ZnO samples show significant changes in M -H loop with increasing doping concentration. Both undoped and Fe doped ZnO nanorods exhibit a Curie transition temperature (T_c) above 390 K. Electron spin resonance and Mössbauer spectra indicate the presence of ferric ions. The origin of ferromagnetism in undoped ZnO nanorods is attributed to localized electron spin moments resulting from surface defects/vacancies, where as in Fe doped samples is explained by F center exchange mechanism.     

Photomagnetic Response in Highly Conductive Iron(II) Spin‐Crossover Complexes with TCNQ Radicals

Co‐crystallization of a cationic Fe^II complex with a partially charged TCNQ^.δ? (7,7′,8,8′‐tetracyanoquinodimethane) radical anion has afforded molecular materials that behave as narrow band‐gap semiconductors, [Fe(tpma)(xbim)](X)(TCNQ)_1.5?DMF (X=ClO₄^? or BF₄^?; tpma=tris(2‐pyridylmethyl)amine, xbim=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐bisimidazole). Remarkably, these complexes also exhibit temperature‐and light‐driven spin crossover at the Fe^II center, and are thus the first structurally defined magnetically bistable semiconductors assembled with the TCNQ^.δ? radical anion. Transport measurements reveal the conductivity of 0.2 S cm^?1 at 300 K, with the low activation energy of 0.11 eV.     

Tuning Magnetic Properties of CeO2 by Fe Doping via an Electrochemical Deposition Route

Herein Ce_1?xFe_xO_2?δ nanocomposites were investigated for dilute magnetic semiconductor (DMS) properties. Ce_1?xFe_xO_2?δ nanospheres and porous nanostructures with high surface areas have been successfully prepared by electrochemical deposition at room temperature and atmospheric pressure. The structures and morphologies of Ce_1?xFe_xO_2?δ deposits were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N₂ adsorption–desorption techniques. The magnetic properties of the prepared Ce_1?xFe_xO_2?δ nanospheres and porous nanostructures were studied, and they showed room‐temperature ferromagnetism and giant magnetic moments. In addition, the effects of morphologies and compositions on the magnetic properties of Ce_1?xFe_xO_2?δ deposits were studied.     

Complex Long-Range Magnetic Ordering Behaviors in Anisotropic Cobalt(II)–Azide Multilayer Systems

The crystal structures and magnetic properties of two new Co^II molecular magnets, [Co(N₃)₂(btzb)] ( 1 ) and [Co(N₃)₂(btze)₂] ( 2 ), are described and discussed (btzb=1,4-bis(tetrazol-1-yl)butane and btze=1,4-bis(tetrazol-1-yl)ethane). In the materials, (4,4) layers with μ-1,3-azide bridges are cross-linked by the monolayered btzb bridging ligands or spaced by bilayered btze terminal ligands to give a 3D ( 1 ) or 2D ( 2 ) coordination network with significantly different interlayer separations (10.6 vs. 15.2 Å). The observation that the layers in 1 and 2 are almost identical have not only allowed us to determine how the interlayer separation imposes its influences on their magnetic behavior, but also helps us understand the complex magnetic behavior of each structure. In the high-temperature range (>25 K), almost identical magnetic behaviors, typical of 2D antiferromagnetic systems, are observed for 1 and 2 . At low temperature they exhibit unusual and different behaviors that combine spin canting (weak ferromagnetism), metamagnetism, and stepped hysteresis. It has been found that the interlayer separation has little influence on the ordering temperature (23 vs. 22 K), but imposes very-strong influence on the metamagnetic critical field (6500 vs. 450 Oe), the coercivity (7500 vs. 650 Oe), and the hysteresis-step size. It may also play an adjusting role in determining the canting angle. Taking into account the strong anisotropy of the systems and the interlayer dipolar interactions, we have reasonably interpreted the unusual metamagnetic and hysteresis behaviors and the differences between 1 and 2 . In particularly, the stepped hysteresis loops have been explained by two weak ferromagnetic states.     

高性能双极性聚合物半导体材料与晶体管器件

有机聚合物半导体材料与晶体管器件是融合了化学、材料、半导体以及微电子等学科的前沿交叉研究方向.聚合物半导体材料分子是该领域研究的重要内容,其中双极性聚合物分子半导体材料,兼具了电子和空穴的双重载流子输运能力而受到学术界的广泛关注.本文总结了双极性聚合物半导体材料与器件的研究进展,重点介绍了我们在D-A型双极性聚合物分子半导体材料设计、加工技术与器件制备以及功能应用方面的研究工作,并论述了双极性聚合物分子半导体材料与器件研究过程中存在的科学问题及发展方向.