A Noncentrosymmetric Metal-Free Borophosphate: Achieving a Large Birefringence and Excellent Stability by Covalent-Linkage

Organic-inorganic hybrid linear and nonlinear optical (NLO) materials have received increasingly wide spread attention in recent years. Herein, the first hybrid noncentrosymmetric (NCS) borophosphate, (C5H6N)2B2O(HPO4)2 (4PBP), is rationally designed and synthesized by a covalent-linkage strategy. 4-pyridyl-boronic acid (4 PB) is considered as a bifunctional unit, which may effectively improve the optical properties and stability of the resultant material. On the one hand, 4 PB units are covalently linked with PO3(OH) groups via strong B-O-P connections, which significantly enhances the thermal stability of 4PBP (decomposition at 321, vs lower 200 °C of most of hybrid materials). On the other hand, the planar π-conjugated C5H6N units and their uniform layered arrangements represent large structural anisotropy and hyperpolarizability, achieving the largest birefringence (0.156 @ 546 nm) in the reported borophosphates and a second-harmonic generation response (0.7 × KDP). 4PBP also exhibits a wide transparency range (0.27-1.50 µm). This work not only provides a promising birefringent material, but also offers a practical covalent-attachment strategy for the rational design of new high-performance optical materials.

Sb4O3(TeO3)2(HSO4)(OH): An Antimony Tellurite Sulfate Exhibiting Large Optical Anisotropy Activated by Lone Pair Stereoactivity

A new birefringent crystal of Sb₄O₃(TeO₃)₂(HSO₄)(OH) was achieved by incorporating two stereochemically active lone pair (SCALP) cations of Sb(III) and Te(IV) into sulfates simultaneously. The Sb³⁺ and Te⁴⁺ ions display highly distorted coordination environments due to the SCALP effect. Sb₄O₃(TeO₃)₂(HSO₄)(OH) displays a 3D structure composed of [Sb₄O₃(TeO₃)₂(OH)]_∞⁺ layers bridged by [SO₃(OH)]^- tetrahedra. It possesses a large birefringence and a wide optical transparent range, making it a new UV birefringent crystal. First-principles calculation analysis suggests that the synergistic effect of the cooperation of SCALP effect of Sb³⁺ and Te⁴⁺ cations make a dominant contribution to the birefringence. The work highlights that units with SCALP cations have advantages in generating large optical anisotropy and are preferable structural units for designing novel birefringent materials.

A Dual-Functional Fibrous Skeleton Implanted with Single-Atomic Co-Nx Dispersions for Longevous Li-S Full Batteries

Although lithium-sulfur (Li-S) batteries have long been touted as next-generation energy storage devices, the rampant dendrite growth at the anode side and sluggish redox kinetics at the cathode side drastically impede their practical application. Herein, a dual-functional fibrous skeleton implanted with single-atom Co-N_x dispersion is devised as an advanced modificator to realize concurrent regulation of both electrodes. The rational integration of single-atomic Co-N_x sites could convert the fibrous carbon skeleton from lithiophobic to lithiophilic, helping assuage the dendritic formation for the Li anode. Meanwhile, the favorable electrocatalytic activity from the Co-N_x species affording a lightweight feature effectively enables expedited bidirectional conversion kinetics of sulfur electrochemistry, thereby inhibiting the polysulfide shuttle. Moreover, the interconnected porous framework endows the entire skeleton with good mechanical robustness and fast electron/ion transportation. Benefiting from the synergistic effects between atomically dispersed Co-N_x sites and three-dimensional conductive networks, the integrated Li-S full batteries can achieve a reversible areal capacity (>7.0 mAh cm^-2) at a sulfur loading of 6.9 mg cm^-2. This work might be beneficial to the development of practically viable Li-S batteries harnessing single-atom mediators.

Linear and Nonlinear Optical Properties of Centrosymmetric Sb4O5SO4 and Noncentrosymmetric Sb4O4(SO4)(OH)2 Induced by Lone Pair Stereoactivity

Introducing stereochemically active lone-pair Sb³⁺ cations into sulfates, two three-dimensional (3D) antimony-sulfates, Sb₄O₅SO₄ (1) and Sb₄O(SO₄)(OH)₂ (2), were achieved under moderate hydrothermal conditions. Both structures are constructed by tetranuclear-{Sb₄}-clusters-based layers and SO₄ tetrahedra. However, owing to the different packing patterns of the layers, they display different characteristics: 1 exhibits a centrosymmetric structure while 2 possesses a noncentrosymmetric structure. UV-vis spectra show that they possess wide band gaps. Sb₄O(SO₄)(OH)₂ is nonlinear optical (NLO) active with a second-harmonic generation (SHG) response of ∼1.2 times of KH₂PO₄, together with the phase-matchable capacity, endowing it a promising UV NLO material. The first-principle calculations were performed to elucidate the structure-property relationships. The results indicate that the lone pair stereoactivity of Sb³⁺ provides the large contribution to the macroscopic SHG effect.

Valley Polarization and Valleyresistance in a Monolayer Transition Metal Dichalcogenide Superlattice

A significant, fundamental challenge in the field of valleytronics is how to generate and regulate valley-polarized currents in practical ways. Here, we discover a new mechanism for producing valley polarization in a monolayer transition metal dichalcogenide superlattice, in which valley-resolved gaps are formed at the supercell Brillouin zone boundaries and centers due to intervalley scattering. When the incident energy of the electron lies in the gaps, the available states are valley polarized, thus providing a valley-polarized current from the superlattice. We show that the direction and strength of the valley polarization may be further tuned by varying the potential applied to the superlattice. The transmission can have a net valley polarization of 55% for a four-period heterostructure. Moreover, two such valley filters in series may function as an electrostatically controlled giant valleyresistance device, representing a zero-magnetic field counterpart to the familiar giant magnetoresistance device.

One-step synthesis, electronic structure, and photocatalytic activity of earth-abundant visible-light-driven FeAl2O4

The development of inexpensive visible-light-driven photocatalysts is an important prerequisite for realizing the industrial application of photocatalysis technology. In this paper, an earth-abundant FeAl₂O₄ photocatalyst is prepared via facile solution combustion synthesis. Density functional theory and the scanning Kelvin probe technique are employed to ascertain the positions of the energy bands and the Fermi level. Phenol is taken as a model pollutant to evaluate the photocatalytic activity of FeAl₂O₄. The scavenger experiment results, ˙OH-trapping fluorescence technique, and electron spin resonance measurements confirm that the superoxide anion radical is the main active species generated in the photocatalytic process, which also further corroborates the proposed electronic structure of FeAl₂O₄. The degradation experiments and O₂ temperature programmed desorption results over various samples verify that the crystallinity degree is a more important factor than the oxygen adsorption ability in determining photocatalytic activity.

Realization of Valley and Spin Pumps by Scattering at Nonmagnetic Disorders

The recent success in optical pumping of valley polarization in two-dimensional transition metal dichalcogenides (TMDs) has greatly promoted the concept of valley-based informatics and electronics. However, between the demonstrated valley polarization of transient electron-hole pair excitations and practical valleytronic operations, there exist obvious gaps to fill, among which is the valley pump of long-lived charge carriers. Here we discover that the quested valley pump of electrons or holes can be realized simply by scattering at the ubiquitous nonmagnetic disorders, not relying on any specific material property. The mechanism is rooted in the nature of the valley as a momentum space index: the intervalley backscattering in general has a valley contrasted rate due to the distinct momentum transfers, causing a net transfer of population from one valley to another. As examples, we numerically demonstrate the sizable valley pump effects driven by charge current in nanoribbons of monolayer TMDs, where the spin-orbit scattering by nonmagnetic disorders also realizes a spin pump for the spin-valley locked holes. Our finding points to a new opportunity towards valley spintronics, turning disorders from a deleterious factor to a resource of valley and spin polarization.

Measurable spin-polarized current in two-dimensional topological insulators

We propose a simple method for generating a spin-polarized current in a two-dimensional topological insulator. As z-component magnetic impurities exist on one edge of the Kane-Mele model, a subgap is opened in the corresponding pair of edge states, but another pair of gapless edge states is still protected by the time reversal symmetry. Thus the conductance plateau with the value e(2)/h in the subgap corresponds to a single-edge and spin-polarized current. We also find that the spin-polarized current is insensitive to weak non-magnetic disorder. This mechanism for generating spin-polarized currents is independent of the concrete theoretical model and can be generalized to two-dimensional topological insulators, such as HgTe/CdTe quantum wells and silicene nanoribbons.