A series of lead‐free double perovskite nanocrystals (NCs) Cs2AgSb1−yBiyX6 (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs2AgSbBr6 NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag–Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag–Bi or Ag–Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge‐carrier relaxation. The two fast trapping processes are dominated by intrinsic self‐trapping (ca. 1–2 ps) arising from giant exciton–phonon coupling and surface‐defect trapping (ca. 50–100 ps). Slow hot‐carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot‐carrier relaxation are also discussed.

No abstract

Lead halide perovskite nanocrystals (NCs) developed in recent years are strong light absorbers and fast-yet-efficient emitters with many potential optoelectronic or quantum-optical applications. In terms of photochemistry, however, while strong light absorption is a desired property for a photosensitizer, the short exciton lifetime of these NCs (on the order of a few nanoseconds) strongly limits the efficiency of charge/energy extraction from these NCs and hence their photochemical reactivity. Herein, we report that in quantum-confined CsPbBr 3 NCs surface-functionalized with phenanthrene ligands, triplet energy transfer from photoexcited NCs to phenanthrene is followed by thermally activated repopulation of NC excitons, leading to delayed NC emission as long as ∼80 μs at room temperature, four orders of magnitude longer than that of unmodified CsPbBr 3 NCs (∼5 ns). Leveraging the exceptionally long lifetime, phenanthrene-functionalized CsPbBr 3 NCs efficiently drive steady-state photoreduction reactions via diffusioncontrolled electron transfer. This study establishes a general strategy for applying strongly light-absorbing NC materials to photochemical transformations.

Two-dimensional (2D) borophene has attracted tremendous interest due to its fascinating properties, which have potential applications in catalysts, energy storage devices, and high-speed transistors. In the past few years, borophene was theoretically predicted as an ideal electrode material for lithium− sulfur (Li−S) batteries because of its low-density, metallic conductivity, high Li-ion surface mobility, and strong interface bonding energy to polysulfide. But until now, borophene-based Li−S batteries have not yet been achieved in experiments due to the absence of a large-scale synthetic method of freestanding borophene nanostructures with a high enough structural stability, conductivity, and uniformity. Herein, we developed a lowtemperature liquid exfoliation (LTLE) method to synthesize freestanding few-layer β 12 -borophene single-crystalline sheets with a P m 6 2 ̅ symmetry in tens of milligrams. The as-synthesized 2D sheets were used as the polysulfide immobilizers and electrocatalysts of Li−S batteries. The resulting borophene-based Li− S battery delivered an extralarge areal capacity of 5.2 mAh cm −2 at a high sulfur loading of 7.8 mg cm −2 , an excellent rate performance of 8 C (@721 mAh g −1 ), and an ultralow capacity fading rate of 0.039% in 1000 cycles, outperforming commercial Li-ion batteries and many other 2D material-based Li−S batteries. Based on the density functional theory model, the excellent electrochemical performances of the borophene-based Li−S batteries should originate from the enormous enhancement of β 12 -borophene sheets for both the surface migration of the Li-ions and the adsorption energy of Li 2 S n clusters. Our results thus demonstrate a great potential for scalable production of freestanding β 12 -borophene single-crystalline sheets in future high-performance Li−S batteries.

Vertical GaAs nanowires on Si (111) substrate were grown by metal organic chemical vapor deposition via Au-catalyst vapor-liquid-solid mechanism. Stacking-faults-free zinc blende nanowires were realized by using AlGaAs/GaAs buffer layers and growing under the optimized conditions, that the alloy droplet act as a catalyst rather than an adatom collector and its size and composition would keep stable during growth. The stable droplet contributes to the growth of stacking-faults-free nanowires. Moreover, by using the buffer layers, epitaxial growth of well-aligned NWs was not limited by the misfit strain induced critical diameter, and the unintentional doping of the GaAs nanowires with Si was reduced.

A series of lead‐free double perovskite nanocrystals (NCs) Cs2AgSb1−yBiyX6 (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs2AgSbBr6 NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag–Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag–Bi or Ag–Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge‐carrier relaxation. The two fast trapping processes are dominated by intrinsic self‐trapping (ca. 1–2 ps) arising from giant exciton–phonon coupling and surface‐defect trapping (ca. 50–100 ps). Slow hot‐carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot‐carrier relaxation are also discussed.

InAs quantum dots (QDs) are grown epitaxially on Au-catalyst-grown GaAs nanowires (NWs) by metal organic chemical vapor deposition (MOCVD). These QDs are about 10-30 nm in diameter and several nanometers high, formed on the {112} side facets of the GaAs NWs. The QDs are very dense at the base of the NW and gradually sparser toward the top until disappearing at a distance of about 2 μm from the base. It can be concluded that these QDs are formed by adatom diffusion from the substrate as well as the sidewalls of the NWs. The critical diameter of the GaAs NW that is enough to form InAs QDs is between 120 and 160 nm according to incomplete statistics. We also find that these QDs exhibit zinc blende (ZB) structure that is consistent with that of the GaAs NW and their edges are faceted along particular surfaces. This hybrid structure may pave the way for the development of future nanowire-based optoelectronic devices.

Metal-halide perovskites are promising optical gain materials because of their excellent photophysical properties. Recently, large perovskite single crystals with phase purity, less defects, and over millimeter dimensions have been successfully synthesized. However, the optical gain effect from these large-size single crystals has not yet been realized. Herein, we for the first time report efficient two-photon pumped amplified spontaneous emission (ASE) from millimeter-sized CsPbBr 3 single crystals (SCs) with a low threshold of 0.65 mJ cm −2 and an optical gain of 38 cm −1 . Furthermore, the CsPbBr 3 SCs also exhibit ultrastable ASE under continuous laser irradiation for more than 40 h (corresponds to 1.5 × 10 8 laser shots) at ambient condition. This work suggests the potential application of large-size perovskite single crystals in practical nonlinear optical devices.