Triplet Sensitization and Photon Upconversion Using InP-Based Quantum Dots.

Molecular triplet sensitization using colloidal semiconductor nanocrystals or quantum dots (QDs) is important for many photochemical and photonic applications. Current QD sensitizers often contain toxic elements such as Cd and Pb. In order to "go green" with these sensitizers, we investigate triplet energy transfer from InP-based QDs. Time-resolved spectroscopy studies revealed picosecond hole trapping in core-only QDs, which could complicate and/or inhibit energy transfer. We therefore developed InP/ZnSe/ZnS core/shell QDs that effectively suppressed hole trapping and meanwhile allowed for triplet energy transfer to surface-anchored anthracene acceptors with an efficiency of 84%. The sensitized molecular triplets participated in triplet-triplet-annihilation, enabling photon upconversion with a normalized quantum yield reaching 10.0% ± 0.1%. This study demonstrates that nontoxic InP-based QDs can be engineered to be on par with state-of-the-art Cd- or Pb-containing QDs for use as triplet sensitizers.

Doped Zero-Dimensional Cesium Zinc Halides for High-Efficiency Blue Light Emission.

All-inorganic zero-dimensional (0D) metal halides have recently received increasing attention due to their excellent photoluminescence (PL) performance and high stability. Herein, we present the successful doping of copper(I) into 0D Cs2 ZnBr4 . The incorporating of Cu+ cations enables the originally weakly luminescent Cs2 ZnBr4 to exhibit an efficient blue emission centered at around 465 nm, with a high photoluminescence quantum yield (PLQY) of 65.3 %. Detailed spectral characterizations, including ultrafast transient absorption (TA) techniques, were carried out to investigate the effect of Cu+ dopants and the origin of blue emission in Cs2 ZnBr4 :Cu. To further study the role of the A-site cation and halogen, A2 ZnCl4 :Cu (A=Cs, Rb) were also synthesized and found to generate intense sky-blue emission (PLQY≈73.1 %). This work represents an effective strategy for the development of environmentally friendly, low-cost and high-efficiency blue-emitting 0D all-inorganic metal halides.

Colloidal Synthesis and Optical Properties of All-Inorganic Low-Dimensional Cesium Copper Halide Nanocrystals.

Low-dimensional metal halides have recently attracted extensive attention owing to their unique structure and photoelectric properties. Herein, we report the colloidal synthesis of all-inorganic low-dimensional cesium copper halide nanocrystals (NCs) by adopting a hot-injection approach. Using the same reactants and ligands, but different reaction temperatures, both 1D CsCu2 I3 nanorods and 0D Cs3 Cu2 I5 NCs can be prepared. Density functional theory indicates that the reduced dimensionality in 1D CsCu2 I3 compared to 0D Cs3 Cu2 I5 makes the excitons more localized, which accounts for the strong emission of 0D Cs3 Cu2 I5 NCs. Subsequent optical characterization reveals that the highly luminescent, strongly Stokes-shifted broadband emission of 0D Cs3 Cu2 I5 NCs arises from the self-trapped excitons. Our findings not only present a method to control the synthesis of low-dimensional cesium copper halide nanocrystals but also highlight the potential of 0D Cs3 Cu2 I5 NCs in optoelectronics.

Colloidal Synthesis and Charge-Carrier Dynamics of Cs2 AgSb1-y Biy X6 (X: Br, Cl; 0 ≤y ≤1) Double Perovskite Nanocrystals.

A series of lead-free double perovskite nanocrystals (NCs) Cs2 AgSb1-y Biy X6 (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs2 AgSbBr6 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.

Direct bleaching of a Cr4+:YAG saturable absorber in a passively Q-switched Nd:YAG laser.

In this work, the anisotropy of nonlinear absorption in a crystal Q-switch was considered when we established coupled rate equations of a passively Q-switched laser. A [100]-cut Cr4+:YAG crystal, with initial transmission T0=40%, was used as the Q-switch to evaluate the theoretical model, and the results of the simulation were in good accordance with the experiment. In order to control timing jitter of the passively Q-switched laser, an actively Q-switched Nd:YAG laser was applied to directly bleach the [100]-cut Cr4+:YAG crystal. The timing jitter was more than 1 µs without bleaching light. While there was a bleaching light, the time lag between the laser pulse and the bleaching light was less than 100 ns, which meant the timing jitter decreased. The pulse width of the passively Q-switched laser was found to decrease from 45 to 35 ns due to the existing of bleaching light. As the peak power of bleaching light was increased, the laser pulse energy increased from 18.2 to 24.6 mJ, which meant a 35% increment in the pulse energy. The increase in pulse energy can be explained by the increase of α coefficient, and the results of simulation agreed well with the experiment.

Self-trapped exciton engineering for white-light emission in colloidal lead-free double perovskite nanocrystals.

Intrinsic broadband photoluminescence (PL) of self-trapped excitons (STEs) are systematically studied in lead-free double perovskite nanocrystals (NCs). It is clarified that bandgap (direct/indirect) has important influence on the PL properties of STEs: indirect bandgap NCs exhibit strong exciton-phonon coupling which results in non-radiative STEs, while direct bandgap NCs exhibit moderate exciton-phonon coupling, inducing bright STE PL. Furthermore, by alloying K+ and Li+ ions in Cs2AgInCl6 NCs, the NCs exhibit broadband white-light emission. Charge-carrier dynamics study indicates that the efficient white-light emission originates from the further suppressed non-radiative processes of the STEs in the direct bandgap structure. This work may deepen the understanding of STEs and guide the design of high-performance lead-free perovskites.