Meifeng Liu

Along with their improved stability and tunable quantum well structures, two-dimensional (2D) perovskites have become potential candidates for developing high-efficiency optoelectronic devices, while the atomic configuration of the materials governs their optoelectronic properties. In this work, direct experimental evidence for conformational ordering in 2D (PEA)2PbBr4 single crystals driven by thermal annealing was obtained. Upon post-annealing at 160 °C, an ordered phase with a larger d-spacing emerges, which was unambiguously confirmed by x-ray diffraction patterns. The organic cations exhibit a perpendicular orientation relative to the 2D plane, as evidenced by the infrared-active C–C stretching mode. In addition, the reduced in-plane anisotropy of two-photon excited photoluminescence indicates that the tilting of [PbBr6]4− octahedra in the inorganic layers was suppressed after post-annealing. The observed structural ordering induced by thermal annealing furnishes important scientific support for the application of 2D perovskites in optoelectronic devices.

The strong spin-orbital coupling and significant metal–oxygen covalency in 5 d oxides SrIrO ₃ have stimulated a range of intriguing phenomena in recent years, rendering it attractive in both physics and chemistry. Modulation of SrIrO ₃ properties through defect engineering is expected to further impact its most-studied spintronic and electrochemical functionalities. Here, we report that the transport properties of SrIrO ₃ films can be effectively tuned by immersion in molecular liquids, which are typically considered inert toward transition metal oxides. The modulated transport is attributed to the formation of cation defects resulting from Sr leaching, facilitated by the enhanced ionic character of the Sr–O bond due to the strong Ir–O covalency. Comparative experiments with other transition metal oxides highlight the critical role of high covalency between the transition metal and ligand oxygen in driving these changes. This work demonstrates that bond covalency will offer a key to modify the lattice microstructures and physical properties in 5 d oxides via molecular liquid immersion, providing a strategy for introducing cation defects in the post-treatment, and potentially guiding the design of 5 d oxide-based catalysts or electronic devices.

Non-precious catalysts for alkaline hydrogen evolution reaction (HER) face a fundamental multi-scale challenge: lack of synergy between electronic structure tuning for balancing H adsorption and water dissociation, active site stabilization for boosting intrinsic turnover frequency (TOF), and mass transport. Even Pt loses 2–3 orders of magnitude activity in alkaline media due to inefficient water dissociation, a synergistic gap unresolved by the Sabatier principle alone. Existing strategies only address isolated aspects: phase engineering optimizes electronic structure but not active site stability; heteroatom doping introduces defects unlinked to mass transport; and nanostructuring enhances mass transfer but not atomic-level activity. None of them address multi-scale mechanistic synergy. Herein, we design a hierarchically porous P-doped NiCo alloy (hpP-NiCo) with an aim of achieving this synergy via integrating α-FCC/ε-HCP phases, P-induced defects, and 3D porosity. The formed α/ε interface tunes the d-band center to balance H adsorption and water dissociation; and the doped P stabilizes metal-vacancy sites to boost TOF. In addition, porosity matches mass transport with active site accessibility. In 1 M KOH, hpP-NiCo reaches 1000 mA cm⁻² at 185 mV overpotential and has a Tafel slope of 43.1 mV dec⁻¹, corresponding to electrochemical desorption as the rate-limiting step and verifying Volmer acceleration. Moreover, it also exhibits bifunctional oxygen evolution reaction (OER), achieving 100 mA cm⁻² at potential of 1.55 V. This work establishes a mechanistic synergy model for non-precious HER catalysts.

Due to their superior electronic and optical properties, halide perovskite single crystals have been potential candidates for developing high-efficiency optoelectronic devices. Defect states located on the surface or within the bulk of single crystals play a crucial role in determining the performance of devices. In this work, the surface and bulk states in CsPbBr3 single crystals were investigated by polarization-resolved second harmonic generation (SHG) measurements. Onefold anisotropy of SHG intensity indicates that the polycrystalline Cs4PbBr6 and CsPb2Br5 phases dominate the surface, which exhibits Cs symmetry. Moreover, an unpolarized and strong luminescence was observed induced by two-photon absorption, which displays a twofold symmetry. Br vacancies within the bulk were identified as the radiative recombination centers according to their energy level position. These experimental results not only establish a method for distinguishing between surface and bulk states but also facilitate a deeper understanding of the optical properties of defect states in CsPbBr3 single crystals.

It is known that rare-earth-based pyrochlore oxide may accommodate the well-defined two-in-two-out spin ice state with its tetrahedral unit. Low-energy excitation is argued to favor the highly concerned monopole state which attracts essential attention. However, such an excitation cannot be directly tracked and imaged using advanced characterizations, raising challenges to our understanding of the physics of monopoles. In this work, we propose an improved two-dimensional artificial spin ice structure on the Shastry–Sutherland lattice to restore the degeneracy of realistic pyrochlore systems. Such a structure avoids the deficiency of inequivalent nearest and next-nearest exchanges in the planar quadrate unit, which, however, is equivalent to the tetrahedral unit of realistic pyrochlore oxides. Therefore, this spin ice model restores state degeneracy that is lost in conventional planar artificial spin ice structures, representing an improved simulator of real spin ice systems. Our careful investigations of such improved structures reveal the rich physics of spin ice excitations, including the phase diagram, which allows different ordered phases and interesting critical phase transitions between spin ice phase I and phase II. Energy spectrum analysis suggests that restoration of state degeneracy substantially reduces monopole excitation energy, resulting in a striking monopole emergency at the critical point. Furthermore, the emergent spin dimer phase in this improved model allows high-density monopole excitations and exhibits high-correlated monopole fluid states.

The multiferroic Y-type hexaferrites BaxSr2−xMe2Fe12−yAlyO22 (Me = Zn2+, Co2+, Mg2+, etc.) have attracted much attention due to their giant magnetoelectric (ME) effect up to room temperature and low modulated magnetic field by the chemical doping control of the complex magnetic phases. However, the research of substitution between the Me ions is rare. As doping at the Me ion site can combine the advantages of both, e.g., higher magnetic ordering temperature in Co2 and stronger ME coefficient in Mg2 Y-type hexaferrites, herein, we report the stability and switching of magnetoelectric states in the Y-type hexaferrites Ba0.5Sr1.5CoMgFe11AlO22 single crystals. Our results demonstrate that substituting half of the Mg2+ with Co2+ enhances the transition temperature of the alternating longitudinal conical phase to proper screw spin order up to room temperature compared to those Mg2 Y-type hexaferrites. Simultaneous occurrence of in-plane and out-of-plane ferroelectric polarization is observed, alongside comparable spontaneous magnetization. It was found that the in-plane spin-driven polarization can be reversible below 50 K with a substantial ME coefficient α = −8000 ps/m but becomes irreversible at 100 K. This reversal in the sign of the ME coefficient signifies the transition between two distinct ME states at high temperature. The reversibility and irreversibility of spin-induced polarization are discussed within the framework of free energy based on the ferroelectric phase, which prevail in numerous Y-type hexaferrites. Our results provide insights into understanding the role of the Me ions in the magnetoelectric coupling in Y-type hexaferrites.

With the discovery of two-dimensional (2D) ferroelectricity and ferromagnetism in van der Waals (vdW) materials, there has been significant interest in 2D multiferroics. Herein, we report the occurrence of weak ferromagnetism and magnetoelectricity in vdW antiferromagnet MnPSe3 single crystals. Our results demonstrate that MnPSe3 undergoes an antiferromagnetic transition at the Néel temperature TN = 70 K with weak ferromagnetism along the [1–10] direction. Detailed magnetoelectric (ME) data show that MnPSe3 exhibits a linear ME tensor αij with nine nonzero components. Additionally, a magnetically induced electric polarization as large as 98.5 μC/m2 is observed along the [110] direction, with a ME coefficient of 13.5 ps/m at 10 K for a magnetic field of 9 T applied along the [110] direction. Importantly, we discuss our experiments based on symmetry and microscopic analysis, thereby suggesting that the spin-dependent p-d hybridization mechanism plays an important role in the emergence of magnetic-field-induced ferroelectricity. Hence, our findings provide insights for exploring the ME coupling in vdW materials.