Observation of 3D Biexcitons in Pristine-Quality CH3 NH3 PbBr3 Single Crystals.

Halide perovskites (HPs) are fascinating materials whose optoelectronic properties are arguably excitonic. In the HP family, biexcitons are known to exist only in low dimensions where exciton-exciton binding is strongly enhanced by quantum and dielectric confinements. In this paper, however, it is shown that they indeed do exist in 3D bulk CH3 NH3 PbBr3 (MAPbBr3 ) single crystals if the pristine crystal quality is ensured for subtle binding of two excitons. The existence of biexcitons is clearly evidenced below 30 K with a binding energy of ≈3.9 ± 0.3 meV according to i) exciton-biexciton population dynamics, ii) giant resonant two-photon excitation of biexcitons, iii) inverted Boltzmann-type spectral feature, and iv) zero degree of circular polarization in the biexciton photoluminescence. Because of the polariton effect, the two-photon resonance occurs at the excited biexciton state from which longitudinal-transverse splitting is calculated to be 3.7 meV. The discovery of the 3D biexcitons underscores the very quality of HP crystals for generating various many-body excitonic phases in MAPbBr3 and its analogues toward the improved understanding of their fundamental properties and highly efficient optoelectronic applications.

Simultaneous Enhancement of Charge Separation and Hole Transportation in a W:α-Fe2O3/MoS2 Photoanode: A Collaborative Approach of MoS2 as a Heterojunction and W as a Metal Dopant.

In this study, a facile approach has been successfully applied to synthesize a W-doped Fe2O3/MoS2 core-shell electrode with unique nanostructure modifications for photoelectrochemical performance. A two-dimensional (2D) structure of molybdenum disulfide (MoS2) and tungsten (W)-doped hematite (W:α-Fe2O3) overcomes the drawbacks of the α-Fe2O3 and MoS2 semiconductor through simple and facile processes to improve the photoelectrochemical (PEC) performance. The highest photocurrent density of the 0.5W:α-Fe2O3/MoS2 photoanode is 1.83 mA·cm-2 at 1.23 V vs reversible hydrogen electrode (RHE) under 100 mW·cm2 illumination, which is higher than those of 0.5W:α-Fe2O3 and pure α-Fe2O3 electrodes. The overall water splitting was evaluated by measuring the H2 and O2 evolution, which after 2 h of irradiation for 0.5W:α-Fe2O3/MoS2 was determined to be 49 and 23.8 µmol.cm-2, respectively. The optimized combination of the heterojunction and metal doping on pure α-Fe2O3 (0.5W:α-Fe2O3/MoS2 photoanode) showed an incident photon-to-electron conversion efficiency (IPCE) of 37% and an applied bias photon-to-current efficiency (ABPE) of 26%, which are around 5.2 and 13 times higher than those of 0.5W:α-Fe2O3, respectively. Moreover, the facile fabrication strategy can be easily extended to design other oxide/carbon-sulfide/oxide core-shell materials for extensive applications.

Role of the A-Site Cation in Low-Temperature Optical Behaviors of APbBr3 (A = Cs, CH3NH3).

APbBr3 (A = Cs, CH3NH3) are prototype halide perovskites having bandgaps of 2.30-2.35 eV at room temperature, rendering their apparent color nearly identical (bright orange but opaque). Upon optical excitation, they emit bright photoluminescence (PL) arising from carrier recombination whose spectral features are also similar. At 10 K, however, the apparent color of CsPbBr3 becomes transparent yellow, whereas that of CH3NH3PbBr3 does not change significantly due to the presence of an indirect Rashba gap. With increasing the excitation level, evolution of the PL spectra, which are excitonic at 10 K, reveals the emergence of P-band emission arising from inelastic exciton-exciton scattering. Based on the spectral location of the P-band, exciton binding energies are determined to be 21.6 ± 2.0 and 38.3 ± 3.0 meV for CsPbBr3 and CH3NH3PbBr3, respectively. Intriguingly, upon further increase in the exciton density, electron-hole plasma appears in CsPbBr3 as evidenced by both red-shift and broadening of the PL. This phase, however, does not occur in CH3NH3PbBr3 presumably due to polaronic effects. Although the A-site cation is believed not to directly impact optical properties of APbBr3, our results underscore its critical role, which destines different high-density phases and apparent color at low temperatures.

Static Rashba Effect by Surface Reconstruction and Photon Recycling in the Dynamic Indirect Gap of APbBr3 (A = Cs, CH3NH3) Single Crystals.

Recently, halide perovskites have gained significant attention from the perspective of efficient spintronics owing to the Rashba effect. This effect occurs as a consequence of strong spin-orbit coupling under a noncentrosymmetric environment, which can be dynamic and/or static. However, there exist intense debates on the origin of broken inversion symmetry since the halide perovskites typically crystallize into a centrosymmetric structure. In order to clarify the issue, we examine both dynamic and static effects in the all-inorganic CsPbBr3 and organic-inorganic CH3NH3PbBr3 (MAPbBr3) perovskite single crystals by employing temperature- and polarization-dependent photoluminescence excitation spectroscopy. The perovskite single crystals manifest the dynamic effect by photon recycling in the indirect Rashba gap, causing dual peaks in the photoluminescence. However, the effect vanishes in CsPbBr3 at low temperatures (<50 K) accompanied by a striking color change of the crystal, arising presumably from lower degrees of freedom for inversion symmetry breaking associated with the thermal motion of the spherical Cs cation compared with the polar MA cation in MAPbBr3. We also show that the static Rashba effect occurs only in MAPbBr3 below 90 K, presumably due to surface reconstruction via MA-cation ordering, which likely extends across a few layers from the crystal surface to the interior. We further demonstrate that this static Rashba effect can be completely suppressed upon surface treatment with polymethyl methacrylate (PMMA) coating. We believe that our results provide a rationale for the Rashba effects in halide perovskites.

Perovskite-like Framework Encapsulated with Fe-Based Magnetic Units in Ba10Fe3Sb7Se24.

Ba10Fe3Sb7Se24 was synthesized using a KBr flux at 850 °C (Crystal Data: orthorhombic, Cmc21, a = 9.3412(2) Å, b = 44.6666(10) Å, c = 12.5496(3) Å, V = 5236.2(2) Å3, and Z = 4). The compound adopts a new three-dimensional framework constructed by the layer to include Fe2Se6 dimers and FeSe4 tetrahedra in the linkage motifs of [Fe2SbSe10] and [FeSb6Se14], respectively. Alternatively, the all Sb-based polyhedra are assembled as a semiconducting, perovskite-like framework lacking an inversion center where these Fe-based magnetic units are trapped within the interstices. The strong antiferromagnetic interaction is revealed by a high Curie constant of -113 K, but the curvature of field-cooled and zero-field-cooled magnetic susceptibilities bifurcating at ∼19 K is observed. The critical temperature is well verified by a broad peak of χM″ signal showing a rapid increase below 19 K under an alternating current field. The Fe2Se6 dimer featuring distorted edge-sharing tetrahedra to induce the spin-canted antiferromagnetic ordering strongly dominates such magnetic ordering. Finally, a weak hysteresis loop is clearly observed at 2.0 K. This dilute magnetic selenide displays a direct bandgap at ∼1.54 eV, analyzed by the Tauc equation. Interestingly, the use of second-harmonic-generation temperature dependence shows a turning point at ∼20 ± 1 K, which precisely corresponds to the magnetic ordering temperature within the error bar, thereby demonstrating the versatility of the technique for probing magnetic phase transition.