Pressure-Induced Emission toward Harvesting Cold White Light from Warm White Light.

The pressure-induced emission (PIE) behavior of halide perovskites has attracted widespread attention and has potential application in pressure sensing. However, high-pressure reversibility largely inhibits practical applications. Here, we describe the emission enhancement and non-doping control of the color temperature in two-dimensional perovskite (C6 H5 CH2 CH2 NH3 )2 PbCl4 ((PEA)2 PbCl4 ) nanocrystals (NCs) through high-pressure processing. A remarkable 5 times PIE was achieved at a mild pressure of 0.4 GPa, which was highly associated with the enhanced radiative recombination of self-trapped excitons. Of particular importance is the retention of the 1.6 times emission of dense (PEA)2 PbCl4 NCs upon the complete release of pressure, accompanied by a color change from "warm" (4403 K) to "cold" white light with 14295 K. The irreversible pressure-induced structural amorphization, which facilitates the remaining local distortion of inorganic Pb-Cl octahedra with respect to the steric hindrance of organic PEA+ cations, should be greatly responsible for the quenched high-efficiency photoluminescence.

Influence of framework disordering on the luminescence performance of Cr3+-doped near-infrared phosphors: a case study of A3B6O14-type hosts.

The luminescence efficiency and thermal stability are enduring topics in the realm of phosphors. It is acknowledged that the structural transformation from disorder to order results in increased lattice rigidity, consequently inducing heightened efficiency and enhanced thermal stability. In this case study of the structural evolution of Ca3Ga2Ge4O14:Cr3+, NaCa2GaGe5O14:Cr3+ and Na2CaGe6O14:Cr3+ near-infrared (NIR) phosphors, a significant paradox is revealed: the incongruent relationship between the fluctuating degrees of disorder and the simultaneous improvements in efficiency and thermal stability. By drawing on insights gained from structural analysis, optical investigations, and theoretical calculations, a notable revelation surfaces: the primary factor affecting rigidity and optical performance is not the disordering of the entire lattice, but rather the disordering of the framework itself. The findings elucidate the principle of framework-order engineering for crafting high-performance NIR phosphors.

Facile fabrication of hierarchical ultrathin Rh-based nanosheets for efficient hydrogen evolution.

Rational design of efficient and stable electrocatalysts for the hydrogen evolution reaction (HER) has attracted wide attention. Noble metal-based electrocatalysts with ultrathin structures and highly exposed active surfaces are essential to boost the HER performance, while the simple synthetic strategies remain challenging. Herein, we reported a facile urea-mediated method to synthesize hierarchical ultrathin Rh nanosheets (Rh NSs) without using toxic reducing agents and structure directing agents in the reaction. The hierarchical ultrathin nanosheet structure and grain boundary atoms endow Rh NSs with excellent HER activities, which only requires a lower overpotential of 39 mV in 0.5 M H2SO4 compared to the 80 mV of Rh nanoparticles (Rh NPs). Extending the synthesis method to alloys, hierarchical ultrathin RhNi nanosheets (RhNi NSs) can be also obtained. Benefiting from the optimization of electronic structure and abundant active surfaces, RhNi NSs only require an overpotential of 27 mV. This work provides a simple and promising method to construct ultrathin nanosheet electrocatalysts for highly active electrocatalytic performance.

Pressure-Induced Metallization of Lead-Free Halide Double Perovskite (NH4 )2 PtI6.

Metallization has recently garnered significant interest due to its ability to greatly facilitate chemical reactions and dramatically change the properties of materials. Materials displaying metallization under low pressure are highly desired for understanding their potential properties. In this work, the effects of the pressure on the structural and electronic properties of lead-free halide double perovskite (NH4 )2 PtI6 are investigated systematically. Remarkably, an unprecedented bandgap narrowing down to the Shockley-Queisser limit is observed at a very low pressure of 0.12 GPa, showing great promise in optoelectronic applications. More interestingly, the metallization of (NH4 )2 PtI6 is initiated at 14.2 GPa, the lowest metallization pressure ever reported in halide perovskites, which is related to the continuous increase in the overlap between the valence and conduction band of I 5p orbital. Its structural evolution upon compression before the metallic transition is also tracked, from cubic Fm-3m to tetragonal P4/mnc and then to monoclinic C2/c phase, which is mainly associated with the rotation and distortions within the [PtI6 ]2- octahedra. These findings represent a significant step toward revealing the structure-property relationships of (NH4 )2 PtI6 , and also prove that high-pressure technique is an efficient tool to design and realize superior optoelectronic materials.

Pressure-Tuned Core/Shell Configuration Transition of Shell Thickness-Dependent CdSe/CdS Nanocrystals.

Pressure is adopted as a "clean" tool to achieve a core/shell configuration transition of CdSe/CdS nanocrystals (NCs) from quasi-type II to type I. The pressure-dependent photoluminescence (PL) spectra demonstrate a sudden decrease in PL intensity, because of the enhanced rate of exciton-exciton annihilation of type I structured CdSe/CdS NCs. Likewise, the large decrease in the PL lifetime with pressure confirms that the electron wave function mainly localizes into the CdSe core, indicating the decreased separation of electrons and holes in type I band alignment. We propose that pressure increases the conduction band energy of the CdS shell but hardly changes that of the CdSe core with almost both unchanged valence band energies, thus ultimately increasing the conduction band offsets between the CdSe core and CdS shell to form the type I core/shell configuration. Our studies elucidate the significance of external pressure in determining the electronic and optical properties of core/shell nanomaterials.

Pressure-induced structural transition and band gap evolution of double perovskite Cs2AgBiBr6 nanocrystals.

Lead-free double halide perovskite nanocrystals (NCs) are attracting increasing attention due to their non-toxic nature and exceptional stability as a substitute material for lead-based perovskites. Herein, we investigate the relationship between the structural and optical properties of double halide perovskite Cs2AgBiBr6 NCs under high pressure. In situ synchrotron high-pressure powder X-ray diffraction and Raman experiments indicated that the structure of Cs2AgBiBr6 NCs transformed into a tetragonal from a cubic system at 2.3 GPa. Pressure-dependent absorption demonstrated that the band gap changes in the sequence red-shift → blue-shift. First-principles calculations further indicated that the band gap evolution was highly related to the orbital interactions, associated with the tilting and distortion of [AgBr6]5- and [BiBr6]3- octahedra under pressure. It is worth noting that the quenched absorption peak of Cs2AgBiBr6 NCs was slightly blue-shifted compared with that of the initial one under ambient conditions, which is in stark contrast to that of the corresponding bulk counterparts. This is because the structure of the sample was not yet recovered and maintained a certain degree of distortion after fully releasing the pressure. What's more, the NCs after decompression are a mixture of cubic and tetragonal phases, which leads to a larger quenched band gap than that of the initial value. Our results improve the understanding of the structural and optical properties of nanostructured double halide perovskites, thus providing a basis for their application in optoelectronic devices.