Europium chalcogenide magnetic semiconductor nanostructures

Synthesis, Crystal Structure, and Optical and Magnetic Properties of the New Quaternary Erbium Telluride EuErCuTe3: Experiment and Calculation

This paper reports for the first time on a new layered magnetic heterometallic erbium telluride EuErCuTe3. Single crystals of the compound were obtained from the elements at 1120 K using CsI as a flux. The crystal structure of EuErCuTe3 was solved in the space group Cmcm (a = 4.3086(3) Å, b = 14.3093(9) Å, and c = 11.1957(7) Å) with the KZrCuS3 structure type. In the orthorhombic structure of erbium telluride, distorted octahedra ([ErTe6]9-) form two-dimensional layers (Er(Te1)2/2e(Te2)4/2k-)∞2, while distorted tetrahedra ([CuTe4]7-) form one-dimensionally connected substructures (Cu(Te1)2/2e(Te2)2/1t5-∞1) along the [100] direction. The distorted octahedra and tetrahedra form parallel two-dimensional layers (CuErTe32-∞2) between which Eu2+ ions are located in a trigonal-prismatic coordination environment (EuTe610-). The trigonal prisms are connected by faces, forming chains (Eu(Te1)2/2(Te2)4/22-∞1) along the [100] direction. Regularities in the variations in structural parameters were established in the series of erbium chalcogenides (EuErCuCh3 with Ch = S, Se, and Te) and tellurides (EuLnCuTe3 with Ln = Gd, Er, and Lu). Ab-initio calculations of the crystal structure, phonon spectrum, and elastic properties of the compound EuErCuTe3 were performed. The types and wavenumbers of fundamental modes were determined, and the involvement of ions in the IR and Raman modes was assessed. The experimental Raman spectra were interpreted. The telluride EuErCuTe3 at temperatures below 4.2 K was ferrimagnetic, as were the sulfide and selenide derivatives (EuErCuCh3 with Ch = S and Se). Its experimental magnetic characteristics were close to the calculated ones. The decrease in the magnetic phase transition temperature in the series of the erbium chalcogenides was discovered.

The Influence of Yb Doping and Sintering Conditions on the Magnetocaloric and Mechanical Properties of EuS

For this work, europium monosulfide (EuS) powders were prepared by sulfurizing Eu₂O₃ powder with CS₂ gas. The synthesized EuS powders were sintered by SPS at temperatures in the 800-1600 °C range for 0.33-1 h at 50 MPa under vacuum conditions. The influences of Yb doping and sintering conditions on the magnetocaloric and mechanical properties of EuS were investigated systematically. An increase in sintering temperature caused the rise of lattice parameters of EuS, whereas Yb doping caused them to drop. SEM showed that the grain size of the EuS increased with sintering temperatures in the 1000-1400 °C range. Higher sintering temperatures can enlarge the magnetizability and saturation magnetization of EuS compact. On the contrary, Yb doping can weaken the magnetizability and saturation magnetization of EuS compact. All sintered polycrystalline EuS compacts had weaker thermomagnetic irreversibility and lower magnetic anisotropy.

Sulfur-source-dependent phase-selective preparation of Cu3NiInSnS6 nanocrystals and their optical and magnetic properties

Multifunctional multinary metal chalcogenides have long been a research hotspot in the field of materials chemistry due to their rich composition, flexible structure, excellent properties and wide range of applications. However, the exploration of complex quinary chalcogenides is still challenging. In this work, for the first time, we have developed the controlled synthesis of quinary Cu₃NiInSnS₆ nanocrystals, realizing the selective preparation of hexagonal wurtzite and cubic zinc blende metastable phases by simply tuning the sulfur source. The phase structure analysis reveals that both metastable phases possess a disordered structure with a random distribution of metal atoms in the unit cells. The fabricated wurtzite and zinc blende-structure Cu₃NiInSnS₆ nanocrystals have a direct band gap of 1.82 and 1.94 eV, respectively, and both exhibit superparamagnetic behavior at low temperatures. This work is of great significance for the development of novel multifunctional materials based on metastable multinary metal chalcogenide phases.

Ferromagnetism and Spin-Polarized Luminescence in Lead-Free CsEuCl3 Perovskite Nanocrystals and Thin Films

The emergence of next-generation spintronic and spin-photonic technologies will be aided by the development of materials showing strongly coupled magnetic, electronic, and optical properties. Through a combination of magneto-photoluminescence and magnetic circular dichroism spectroscopies we demonstrate strong magneto-optical responses from CsEuCl₃ perovskite nanocrystals and thin films in the near-UV/visible region, stemming from the f-d transitions centered at the B-site Eu²⁺ cations. We show that this material undergoes a ferromagnetic phase transition at ∼3 K in both the nanocrystal and thin-film samples, resulting in complete spin alignment and indicating intrinsic ferromagnetism. We also report the observation of spin-polarized photoluminescence in the presence of a magnetic field at cryogenic temperatures, saturating with a large polarization ratio (ΔI/I = (I_L - I_R)/(I_L + I_R)) of nearly 30% at modest magnetic fields (∼2 T). These results highlight CsEuCl₃ as an intrinsically ferromagnetic, luminescent metal-halide perovskite with potentially interesting implications for future spin-based technologies using perovskites.

Synthesis of Mixed-Valent Lanthanide Sulfide Nanoparticles

In targeting reduced valent lanthanide chalcogenides, we report the first nanoparticle synthesis of the mixed-valent ferromagnets Eu₃ S₄ and EuSm₂ S₄ . Using divalent lanthanide halides with bis(trimethylsilyl)sulfide and oleylamine, we prepared nanoparticles of EuS, Eu₃ S₄ , EuSm₂ S₄ , SmS_1.9 , and Sm₃ S₄ . All nanoparticle phases were identified using powder X-ray diffraction, transmission electron microscopy was used to confirm morphology and nanoparticle size, and magnetic susceptibility measurements for determining the ordering temperatures and valence. The UV/Vis, Raman and X-ray photoelectron spectroscopies for each phase were compared. Surprisingly, the phase is influenced by the halide and the reaction temperature, where EuCl₂ formed EuS while EuI₂ formed Eu₃ S₄ , highlighting the role of kinetics in phase stabilization. Interestingly, at lower temperatures EuI₂ initially forms EuS, and converts over time to Eu₃ S₄ .

Recent Progress in Proximity Coupling of Magnetism to Topological Insulators

Inducing long-range magnetic order in 3D topological insulators can gap the Dirac-like metallic surface states, leading to exotic new phases such as the quantum anomalous Hall effect or the axion insulator state. These magnetic topological phases can host robust, dissipationless charge and spin currents or unique magnetoelectric behavior, which can be exploited in low-energy electronics and spintronics applications. Although several different strategies have been successfully implemented to realize these states, to date these phenomena have been confined to temperatures below a few Kelvin. This review focuses on one strategy: inducing magnetic order in topological insulators by proximity of magnetic materials, which has the capability for room temperature operation, unlocking the potential of magnetic topological phases for applications. The unique advantages of this strategy, the important physical mechanisms facilitating magnetic proximity effect, and the recent progress to achieve, understand, and harness proximity-coupled magnetic order in topological insulators are discussed. Some emerging new phenomena and applications enabled by proximity coupling of magnetism and topological materials, such as skyrmions and the topological Hall effect, are also highlighted, and the authors conclude with an outlook on remaining challenges and opportunities in the field.

Dithiocarbamate Complexes as Single Source Precursors to Nanoscale Binary, Ternary and Quaternary Metal Sulfides

Nanodimensional metal sulfides are a developing class of low-cost materials with potential applications in areas as wide-ranging as energy storage, electrocatalysis, and imaging. An attractive synthetic strategy, which allows careful control over stoichiometry, is the single source precursor (SSP) approach in which well-defined molecular species containing preformed metal-sulfur bonds are heated to decomposition, either in the vapor or solution phase, resulting in facile loss of organics and formation of nanodimensional metal sulfides. By careful control of the precursor, the decomposition environment and addition of surfactants, this approach affords a range of nanocrystalline materials from a library of precursors. Dithiocarbamates (DTCs) are monoanionic chelating ligands that have been known for over a century and find applications in agriculture, medicine, and materials science. They are easily prepared from nontoxic secondary and primary amines and form stable complexes with all elements. Since pioneering work in the late 1980s, the use of DTC complexes as SSPs to a wide range of binary, ternary, and multinary sulfides has been extensively documented. This review maps these developments, from the formation of thin films, often comprised of embedded nanocrystals, to quantum dots coated with organic ligands or shelled by other metal sulfides that show high photoluminescence quantum yields, and a range of other nanomaterials in which both the phase and morphology of the nanocrystals can be engineered, allowing fine-tuning of technologically important physical properties, thus opening up a myriad of potential applications.

An air-stable anionic two-dimensional semiconducting metal-thiolate network and its exfoliation into ultrathin few-layer nanosheets

Metal-thiolate networks are topical electronic materials, but hard to crystallize: this one makes big single crystals, and boasts small band gap, stable radical organic linkers, and facile exfoliation into nanosheets.

Synthesis, Crystal Structure, and Liquid Exfoliation of Layered Lanthanide Sulfides KLn2CuS6 (Ln = La, Ce, Pr, Nd, Sm)

Among the great amount of known lanthanide nanoparticles, reports devoted to chalcogenide ones are deficient. The properties of such nanoparticles remain almost unknown due to the lack of simple and proper synthetic methods avoiding hydrolysis and allowing preparation of oxygen-free lanthanide nanoparticles. A liquid exfoliation method was used to select the optimum strategy for the preparation of quaternary lanthanide sulfide nanoparticles. Bulk KLn₂CuS₆ (Ln = La-Sm) materials were obtained via a reactive flux method. The crystal structures of three new members of the KLn₂CuS₆ series were determined for Pr, Nd, and Sm as well as for known KLa₂CuS₆. KLn₂CuS₆ (Ln = La, Pr, Nd) compounds crystallize in the monoclinic C2 /c space group, whereas KSm₂CuS₆ crystallizes in the orthorhombic Fddd space group. The analysis of their electronic structures confirms that the main bonding interactions occur within the anionic {Ln₂CuS₆}^- layers. Due to their layered structure, exfoliation of these compounds is possible using ultrasonic treatment in appropriate solvents with the formation of colloidal solutions. Colloidal particles show a plate-like morphology with a lateral size of 100-200 nm and a thickness of 2-10 nm. Highly negative or positive charges found in isopropanol and acetonitrile dispersions, respectively, are associated with high stability and concentration of the dispersions.

Facile synthesis of highly monodisperse EuSe nanocubes with size-dependent optical/magnetic properties and their electrochemiluminescence performance

We reported a facile and robust method for the synthesis of highly monodisperse EuSe nanocubes (EuSe NCs) with controllable edge lengths in the range of 8-70 nm. The EuSe NCs were formed through the aggregation of EuSe small particles (cores) and then their surface reconstruction under the influence of 1-dodecanethiol (DDT) that acted as a capping surfactant. DDT was not only found to be critical to the nucleation temperature of preparing EuSe NCs, but also played a decisive role in the formation of structurally well-defined nanocubes. The results indicated that the remarkable monodispersity and high shape consistency of EuSe NCs were highly controlled by the change in the DDT concentration. Furthermore, the size-dependent optical/magnetic properties based on the quantum size effect and the influence of edge lengths of EuSe NCs were also investigated and discussed. More importantly, the electrochemiluminescence (ECL) performance of EuSe NCs was first reported. This will make possible more biomedical applications in future.

Characterization of Luminescent Materials with 151Eu Mössbauer Spectroscopy

The application of Mössbauer spectroscopy to luminescent materials is described. Many solids doped with europium are luminescent, i.e., when irradiated with light they emit light of a longer wavelength. These materials therefore have practical applications in tuning the light output of devices like light emitting diodes. The optical properties are very different for the two possible valence states Eu 2 + and Eu 3 + , the former producing ultraviolet/visible light that shifts from violet to red depending on the host and the latter red light, so it is important to have a knowledge of their behavior in a sample environment. Photoluminescence spectra cannot give a quantitative analysis of Eu 2 + and Eu 3 + ions. Mössbauer spectroscopy, however, is more powerful and gives a separate spectrum for each oxidation state enabling the relative amount present to be estimated. The oxidation state can be identified from its isomer shift which is between - 12 and - 15 mm/s for Eu 2 + compared to around 0 mm/s for Eu 3 + . Furthermore, within each oxidation state, there are changes depending on the ligands attached to the europium: the shift is more positive for increased covalency of the bonding ligand X, or Eu concentration, and decreases for increasing Eu⁻X bond length.

Nanomorphology Effects in Semiconductors with Native Ferromagnetism: Hierarchical Europium (II) Oxide Tubes Prepared via a Topotactic Nanostructure Transition

Semiconductors with native ferromagnetism barely exist and defined nanostructures are almost unknown. This lack impedes the exploration of a new class of materials characterized by a direct combination of effects on the electronic system caused by quantum confinement effects with magnetism. A good example is EuO for which currently no reliable routes for nanoparticle synthesis can be established. Bottom-up approaches applicable to other oxides fail because of the labile oxidation state +II. Instead of targeting a direct synthesis, the two steps-"structure control" and "chemical transformation"-are separated. The generation of a transitional, hybrid nanophase is followed by its conversion into EuO under full conservation of all morphological features. Hierarchical EuO materials are now accessible in the shape of oriented nanodisks stacked to tubular particles. Magnetically, the coupling of either vortex or onion states has been found. An unexpected temperature dependence is governed by thermally activated transitions between these states.

Size- and dimensionality-dependent optical, magnetic and magneto-optical properties of binary europium-based nanocrystals: EuX (X = O, S, Se, Te)

Europium chalcogenides (EuX, X = O, S, Se, Te), a class of prototypical Heisenberg magnetic semiconductors, exhibit intriguing properties in optics, magnetism, and magneto-optics at the nanoscale, and have broad application potential in optical/magnetic sensors, spintronics, optical isolators, etc. EuX nanocrystals (NCs) exhibit enhanced properties, such as high saturation magnetization, a strong magneto-optic effect (Faraday rotation), and high magneto resistance, which are all unanimously dependent on the NC's size, shape, and surface information. In this report, we give an overview of the fundamental properties of bulk EuX, and illustrate the quantum confinement effects on the optical, magnetic and magneto-optical properties of EuX nanostructures. We then focus on doping and self-assembly-two efficient methods that enhance magnetic properties by manipulating magnetic coupling in EuX nanostructures. In particular, we look towards future research on Eu(2+) NCs, which along with the overview provides an up-to-date platform for evaluating the fundamental properties and application potential of Eu-based semiconductors.

Proximity-Driven Enhanced Magnetic Order at Ferromagnetic-Insulator-Magnetic-Topological-Insulator Interface

Magnetic exchange driven proximity effect at a magnetic-insulator-topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. Here we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS/Sb(2-x)V(x)Te3 hybrid heterostructure, where V doping is used to drive the TI (Sb2Te3) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

Solvent assisted and solvent free orientation of growth of nanoscaled lanthanide sulfides: tuning of morphology and manifestation of photocatalytic behavior

We observed solvent-mediated and solvent-free growth of lanthanide sulfides where EuS emerged as a promising candidate for visible-light induced photocatalysis towards the degradation of toxic organic dyes.