Positive magnetoresistance (PMR) and negative magnetoresistance (NMR) describe two opposite responses of resistance induced by a magnetic field. Materials with giant PMR are usually distinct from those with giant NMR due to different physical natures. Here, we report the unusual photomagnetoresistance in the van der Waals heterojunctions of WSe 2 /quasi-two-dimensional electron gas, showing the coexistence of giant PMR and giant NMR. The PMR and NMR reach 1,007.5% at – 9 T and – 93.5% at 2.2 T in a single device, respectively. The magnetoresistance spans over two orders of magnitude on inversion of field direction, implying a giant unidirectional magnetoresistance (UMR). By adjusting the thickness of the WSe 2 layer, we achieve the maxima of PMR and NMR, which are 4,900,000% and – 99.8%, respectively. The unique magnetooptical transport shows the unity of giant UMR, PMR, and NMR, referred to as giant bipolar unidirectional photomagnetoresistance. These features originate from strong out-of-plane spin splitting, magnetic field–enhanced recombination of photocarriers, and the Zeeman effect through our experimental and theoretical investigations. This work offers directions for high-performance light-tunable spintronic devices.NMR)

Doping is an effective approach to tune the structure of materials at an atomic level, optimizing their performance toward various energy conversion applications. Herein, we show that nickel (Ni) and iron (Fe) dual doping activates the electrochemical inert W18O49 into a highly active electrocatalyst toward the oxygen evolution reaction (OER). Compared to monodoping, dual doping of Ni and Fe in the lattice of W18O49 results in the synergistic modulation of the electronic structure and physicochemical properties of tungsten oxides. The Ni and Fe dual-doped W18O49 (NiFe–W18O49) achieves a low overpotential of 325 mV at a current density of 10 mA cm–2 and a Tafel slope of 42 mV dec–1 for the OER in 0.1 M potassium hydroxide (KOH) solution, comparable with those of state-of-the-art IrO2. The Zn–air battery based on a NiFe–W18O49 cathode displays a long-term cycling durability of over 180 h, superior to the battery with a commercial Pt/C–IrO2 cathode. Combined experimental analysis and density functional theory calculations unveil that the distorted geometric structure and regulated electronic structure of W18O49 contribute crucially to the activation of its inert catalytic activity.

Reversible regulation of ferroelectric polarization possesses great potentials recently in bionic neural networks. Photoinduced cis−trans isomers have changeable dipole moments, but they cannot be directed to some specific orientation. Here, we construct a host−guest composite structure which consists of a porous ferroelectric metal (Ni)−organic framework [Ni(DPA) 2 ] as host and photoisomer, azobenzene (AZB), as guest molecules. When AZB molecules are embedded in the nanopores of Ni(DPA) 2 in the form of a single molecule, polarization strength tunable regulation is realized after ultraviolet irradiation of 365 and 405 nm via cis−trans isomerism transformation of AZB. An intrinsic built-in field originating from the distorted {NiN 2 O 4 } octahedra in Ni(DPA) 2 directs the dipole moments of AZB to the applied electric field. As a result, the overlapped ferroelectric polarization strength changes with content of cis-AZB after ultraviolet and visible irradiation. Such a connection of ferroelectric Ni(DPA) 2 structure with cis−trans isomers provides an important strategy for regulating the ferroelectric polarization strength.

Despite the demonstrated high efficiency of solar cells and light-emitting devices based on two-dimensional (2D) perovskites, intrinsic stability of the 2D perovskites is yet far from satisfactory. In this work, we find the 2D (BA)2PbI4 perovskite crystals rapidly degrade in the ambient conditions and the photoluminescence (PL) nearly completely quenches in 6 days. Moreover, the PL shoulder band due to defects and absorption band of PbI2 gradually rise during degradation, suggesting the precipitation of PbI2. Besides, rod structures are observed in the degraded crystals, which are attributed to the formation of one-dimensional (1D) (BA)3PbI5 perovskites. And the degradation can be largely retarded by decreasing the humidity during storage. Therefore, a chemical reaction for the degradation of (BA)2PbI4 is proposed, revealing the interactions between water molecules and undercoordinated defects are very critical for understanding the degradation. Enlightened by these findings, dimethyl itaconate (DI) treatment is developed to passivate the defects and block the intrusion of moisture to improve the stability of the (BA)2PbI4. After storage in the ambient environment for 16 days, the DI treated (BA)2PbI4 only shows a slight surface degradation without formation of any nanorod-like structures, and the PL intensity retains about 70%. Therefore, our systematic study provides a comprehensive understanding on the degradation dynamics of 2D perovskites, which will promote future development of intrinsically stable 2D perovskites.

Deep insight into the ultrafast charge carrier dynamics is fundamentally significant for the development of high-performance optoelectronic devices. The CsPbBr 3 /Cs 4 PbBr 6 nanocomposite, as an important member of the perovskite family, has attracted extensive research interest. However, the underlying ultrafast carrier dynamics is still elusive. Herein, an endeavor has been made to investigate the impact of Cs 4 PbBr 6 encapsulation on the ultrafast carrier dynamics of CsPbBr 3 nanoinclusions. The hot-carrier cooling time is about 190−302 fs, relying on the pump fluence, which is relatively shorter than that of pure CsPbBr 3 . Thus, the encapsulation of Cs 4 PbBr 6 shortens the hot-carrier cooling time, which is attributed to the participation of longitudinal optical (LO) phonons from Cs 4 PbBr 6 and dissipation of the energy via coupling to surface encapsulation. The biexciton binding energy is estimated to be 69 meV, with a biexciton lifetime of 40 ps. The biexciton binding energy is a relatively high value compared to those of CsPbBr 3 nanostructures with similar biexciton lifetimes, which is mainly due to the smaller sizes of the CsPbBr 3 inclusions in the composites.