Critical Roles of Octahedron Bilayer Surface/Interior Bromide Defects in Photodynamics of Multi-Quantum-Well-Structured Cesium Bismuth Bromide.

We investigate theoretically the roles of the intrinsic point defects in the photophysics of wide-bandgap multi-quantum-well-structured Cs3Bi2Br9 based on the Shockley-Read-Hall statistics and multiphonon recombination theory. The GW plus Bethe-Salpeter equation calculation reveals that there is a prominent exciton peak below the interband absorption edge, and it clarifies the experimental debate. The most energetically favorable native defects possess deep thermodynamic transition levels. The bromide self-interstitials within the octahedron bilayers exhibit as efficient carrier trapping centers through the non-radiative multiphonon recombination, with a lifetime of 184 ns being on the same order of magnitude as the experimental value. The octahedron bilayer surface bromide self-interstitials account for the experimentally observed dominant blue luminescence in Cs3Bi2Br9. These results reveal that the intrinsic point defects at different sites of the multi-quantum-well-like octahedron bilayers play different roles in the photodynamics of such unique layer-structured semiconductors.

Cooccurrence of pH-sensitive shifting blue and immobile green triple surface-state fluorescence in ultrasmall super body-centered cubic carbon quantum dots.

Carbon quantum dots are widely used in various fields owing to excellent optical properties and outstanding biocompatibility. We synthesize rare super body-centered cubic (C8) structured carbon quantum dots by using cheap source materials and simple preparation method. They exhibit one shifting blue emission band and two close immobile green bands. They have large Stokes shifts ranging from 0.68 to 1.01 eV and large quantum yields as high as 60%. The three types of emissions are competitive and their intensities vary sensitively and differently with pH. Moreover, their emission intensity versus excitation power curves followI(P)∝Pkwithkvalues significantly smaller than unity. The blue emission follows the stretched exponential decay law with an intermediate lifetime of ∼3.9 ns and a lifetime-dispersion factor of ∼0.85 whereas the two green emissions exhibit faster and slower decays with respective lifetimes of around 2.0 and 13.0 ns. The results reveal that the blue emission originates from an ensemble of emission sites exhibiting quantum confinement-like effect and two green emissions stem from pH-sensitive surface functional groups-associated fluorophores.

Resonant defect recombination-localized surface plasmon energy transfer and exciton dominated fluorescence in ZnO-Au-ZnO multi-interfaced heteronanocrystals.

The semiconductor-metal heteronanocrystals (HNCs) that possess a perfect epitaxial interface can accommodate novel and interesting physical phenomena owing to the strong interaction and coupling between the semiconductor excitons and metal plasmons at the interface. Here, we fabricate the pyramidal ZnO-Au HNCs and study their unique photophysical properties. Several Au nanospheres are perfectly epitaxially bound with a single ZnO NC owing to the small lattice mismatch between them and there are also ZnO-Au-ZnO sandwiched HNCs. There is a strong coupling between the green defect-associated recombination in the ZnO NC and the localized surface plasmon resonance (LSPR) of the Au nanosphere at the interface of the HNC. This leads to resonant defect recombination-LSPR energy transfer and resultant nearly complete quenching of the green defect luminescence of the ZnO NCs in the HNCs, leaving only the UV exciton luminescence. The lifetimes of both the green and UV emission bands decrease significantly in the ZnO-Au HNCs relative to that of the pure ZnO NCs owing to the combined effect of resonance energy transfer and surface plasmon enhanced radiative transition. The exponent of the luminescence intensity-excitation intensity power function for the green emission band is remarkably smaller than unity, and this suggests that the involved defects have an intermediate concentration.

Native surface oxidation yields SiC-SiO2 core-shell quantum dots with improved quantum efficiency.

Silicon carbide is an important wide-bandgap semiconductor with wide applications in harsh environments and its applications rely on a reliable surface, with dry or wet oxidation to form an insulating layer at temperatures ranging from 850 to 1250 °C. Here, we report that the SiC quantum dots (QDs) with dimensions lying in the strong quantum confinement regime can be naturally oxidized at a much lower temperature of 220 °C to form core/shell and heteroepitaxial SiC/SiO2 QDs with well crystallized silica nanoshells. The surface silica layer enhances the radiative transition rate of the core SiC QD by offering an ideal carrier potential barrier and diminishes the nonradiative transition rate by reducing the surface dangling bonds, and, as a result, the quantum yield is highly improved. The SiC/SiO2 QDs are very stable in air, and they have better biocompatibility for cell-labeling than the bare SiC QDs. These results pave the way for constructing SiC-based nanoscale electronic and photonic devices.

One-Center and Two-Center Self-Trapped Excitons in Zero-Dimensional Hybrid Copper Halides: Tricolor Luminescence with High Quantum Yields.

The organic-inorganic hybrid copper halides exhibit intriguing and complex photophysical properties, and the underlying mechanisms are far from clear. Here, we study the photodynamics of six novel types of low-dimensional hybrid copper halides, which have a maximum quantum yield of 98.6%. They exhibit two origins of photon emission with distinct temperature dependence and quantum transition rates. The experiments in junction with first-principles calculations indicate that they stem from two kinds of self-trapped excitons (STEs): one-center a-STE (localized on Cu+ monomer) and two-center m-STE (localized on Cu22+ dimer). There is phase transition between a-STE and m-STE when enough thermal energy is acquired for crossing the potential barrier between them. The degree of softness of the compositional organic cations of the copper halide plays a key role in determining the self-trapping type of the STEs.

Fabry-Perot Mode-Limited High-Purcell-Enhanced Spontaneous Emission from In Situ Laser-Induced CsPbBr3 Quantum Dots in CsPb2Br5 Microcavities.

The patterned metal halide perovskites exhibit novel photophysical properties and high performance in photonic applications. Here, we show that a UV continuous wave laser can induce in situ crystallization of individual and patterned CsPbBr3 quantum dots (QDs) inside the CsPb2Br5 microplatelets. The microplatelet acts as a natural Fabry-Perot cavity and causes the high-Purcell-effect-enhanced (by 287 times) cavity mode spontaneous emission of the embedded CsPbBr3 QDs. The luminescence exhibits a superlinear emission intensity-excitation intensity relation I(p) ∝ p2.83, and the exponent is much bigger than that of the free-space exciton spontaneous emission, suggesting arising of stimulated emission at higher photon concentrations. These laser-driven crystallized and patterned cavity mode luminescent perovskite QDs in a waterproof wider-bandgap perovskite microcavity act as an ideal platform for studying the cavity quantum electrodynamics phenomena and for applications in information storage and encryption, anticounterfeiting, and low-threshold lasers.

Role of Polyhedron Unit in Distinct Photophysics of Zero-Dimensional Organic-Inorganic Hybrid Tin Halide Compounds.

The zero-dimensional (0D) metal halides comprising isolated metal-halide polyhedra are the smallest inorganic quantum systems and accommodate quasi-localized Frenkel excitons with unique photophysics of broadband luminescence, huge Stokes shift, and long lifetime. Little is known about the role of polyhedron type in the characteristics of 0D metal halides. We comparatively study three novel kinds of 0D hybrid tin halides having identical organic groups. They are efficient light emitters with a maximal quantum yield of 92.3%. Their most stable phases are composed of octahedra for the bromide and iodide but disphenoids for the chloride. They separately exhibit biexponential and monoexponential luminescence decays due to different symmetries and electronic structures. The chloride has the largest absorption and smallest emission photon energies. A proposed model regarding unoccupied-energy-band degeneracy explains well the experimental phenomena and reveals the crucial role of polyhedron type in determining optical properties of the 0D tin halides.