Separating Crystal Growth from Nucleation Enables the In Situ Controllable Synthesis of Nanocrystals for Efficient Perovskite Light-Emitting Diodes.

Colloidal perovskite nanocrystals (PNCs) display bright luminescence for light-emitting diode (LED) applications; however, they require post-synthesis ligand exchange that may cause surface degradation and defect formation. In situ-formed PNCs achieve improved surface passivation using a straightforward synthetic approach, but their LED performance at the green wavelength is not yet comparable with that of colloidal PNC devices. Here, it is found that the limitations of in situ-formed PNCs stem from uncontrolled formation kinetics: conventional surface ligands confine perovskite nuclei but fail to delay crystal growth. A bifunctional carboxylic-acid-containing ammonium hydrobromide ligand that separates crystal growth from nucleation is introduced, leading to the formation of quantum-confined PNC solids exhibiting a narrow size distribution. Controlled crystallization is further coupled with defect passivation using deprotonated phosphinates, enabling improvements in photoluminescence quantum yield to near unity. Green LEDs are fabricated with a maximum current efficiency of 109 cd A-1 and an average external quantum efficiency of 22.5% across 25 devices, exceeding the performance of their colloidal PNC-based counterparts. A 45.6 h operating half-time is further documented for an unencapsulated device in N2 with an initial brightness of 100 cd m-2 .

Variant Plateau's law in atomically thin transition metal dichalcogenide dome networks.

Since its fundamental inception from soap bubbles, Plateau's law has sparked extensive research in equilibrated states. However, most studies primarily relied on liquids, foams or cellular structures, whereas its applicability has yet to be explored in nano-scale solid films. Here, we observed a variant Plateau's law in networks of atomically thin domes made of solid two-dimensional (2D) transition metal dichalcogenides (TMDs). Discrete layer-dependent van der Waals (vdWs) interaction energies were experimentally and theoretically obtained for domes protruding in different TMD layers. Significant surface tension differences from layer-dependent vdWs interaction energies manifest in a variant of this fundamental law. The equivalent surface tension ranges from 2.4 to 3.6 N/m, around two orders of magnitude greater than conventional liquid films, enabling domes to sustain high gas pressure and exist in a fundamentally variant nature for several years. Our findings pave the way towards exploring variant discretised states with applications in opto-electro-mechanical devices.

Physical Mechanism and Chemical Trends in the Thermal Expansion of Inorganic Halide Perovskites.

The considerable thermal expansion of halide perovskites is one of the challenges to device stability, yet the physical origin and modulation strategy remain unclear. Herein, we report first-principles calculations of the thermal properties of halide perovskites at 300 K using oxides as a reference. We found that the large thermal expansion of halide perovskites can mainly be attributed to their low bulk modulus and volumetric heat capacity because of the soft crystal lattice, whereas composition-dependent anharmonicity emerges as the most important factor in determining thermal expansion with the same structure. We discovered that thermal expansion of halide perovskites can be decreased by weakening the B-X bond to promote the octahedral anharmonicity. We further proposed an effective thermal expansion coefficient descriptor of halide perovskites with a Pearson correlation coefficient of nearly -80%. Our findings provide insights into the underlying mechanisms and chemical trends in the thermal expansion behavior of halide perovskites.

Electronic property modulation in two-dimensional lateral superlattices of monolayer transition metal dichalcogenides.

Fabricating lateral heterostructures (HSs) and superlattices (SLs) provides a unique degree of freedom for modulating the physical properties of two-dimensional (2D) materials by varying the chemical component, geometric size and interface structure in the ultra-thin atomic thickness limit. While a variety of 2D lateral HSs/SLs have been synthesized, especially for transition metal dichalcogenides (TMDs), how such structures affect quantitatively the physical properties of 2D materials has not yet been established. We herein explore electronic property modulation in 2D lateral SLs of monolayer TMDs through first-principles high-throughput calculations. The dependence of the electronic structure, bandgap, carrier effective masses, charge density overlap on chemical components, interface type, and sub-lattice size of lateral TMD-SLs are investigated. We find that by comparison with their random alloy counterparts, the lateral TMD-SLs exhibit generally type-II band alignment, a wider range of bandgap tunability, larger carrier effective masses, and stronger electron-hole charge separation tendency. The bandgap variation with a sub-lattice size shows larger bowing parameters for the SLs with heterogeneous anions, by comparison with the homogeneous anion cases. A similar behavior is observed for the SLs with an armchair-type interface, by comparison with the zigzag-type interface cases. Further analyses reveal that the underlying physical mechanism can be attributed to the synergistic interplay among the band offset of sub-lattices, quantum confinement effect, and existing internal strain.

Entropy-Driven Stabilization of Multielement Halide Double-Perovskite Alloys.

Currently, a major obstacle restricting the commercial application of halide perovskites is their low thermodynamic stability. Herein, inspired by the high-stability high-entropy alloys, we theoretically investigated a variety of multielement double-perovskite alloys. First-principles calculations show that the entropy contribution to Gibbs free energy, which offsets the positive enthalpy contribution by up to 35 meV/f.u., can significantly enhance the material stability of double-perovskite alloys. We found that the electronic properties of bandgaps (1.04-2.21 eV) and carrier effective masses (0.34 to greater than 2 m0) of the multielement double-perovskite alloys can be tuned over a wide range. Meanwhile, the parity-forbidden condition of optical transitions in the Cs2AgInCl6 perovskite can be broken because of the lower symmetry of the configurational disorder, leading to enhanced transition intensity. This work demonstrates a promising strategy by utilizing the alloy entropic effect to further improve the material stability and optoelectronic performance of halide perovskites.