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Lead halide perovskites (LHPs) nanocrystals (NCs), owing to their outstanding photophysical properties, have recently emerged as a promising material not only for solar cells but also for lighting and display applications. The photophysical properties of these materials can be further improved by chemical engineering such as cation exchange. Hot carrier (HC) cooling, as one of the key photophysical processes in LHPs, can strongly influence performance of LHPs NCs based devices. Here, we study HC relaxation dynamics in LHP NCs with cesium (Cs), methylammonium (MA, CH3NH3 +), and formamidinium (FA, CH­(NH2)2 +) cations by using femtosecond transient absorption spectroscopy. The LHP NCs show excitation intensity and excitation energy-dependent HC cooling. We investigate the details of HC cooling in CsPbBr3, MAPbBr3, and FAPbBr3 at three different excitation energies with low excitation intensity. It takes longer time for the HCs at high energy to relax (cool) to the band edge, compared to the HCs generated by low excitation energy. At the same excitation energy (350 nm, 3.54 eV), all the three LHP NCs show fast HC relaxation (<0.4 ps) with the cooling time and rate in the following order: CsPbBr3 (0.39 ps, 2.9 meV/fs) > MAPbBr3 (0.27 ps, 4.6 meV/fs) > FAPbBr3 (0.21 ps, 5.8 meV/fs). The cation dependence can be explained by stronger interaction between the organic cations with the Pb–Br frameworks compared to the Cs. The revealed cation-dependent HC relaxation process is important for providing cation engineering strategies for developing high performance LHP devices.

In this work we investigate the variation of the crystal structure of gold-seeded III-V nanowires with growth parameters, in order to gain a cohesive understanding of these effects. We investigate six III-V materials: GaAs, InAs, GaP, InP, GaSb and InSb, over a variation of growth conditions. All six of these materials exhibit a cubic zinc blende structure in bulk, but twin planes and stacking faults, as well as a hexagonal wurtzite structure, are commonly observed in nanowires. Parameters which may affect the crystal structure include growth temperature and pressure, precursor molar fraction and V/III ratio, nanowire diameter and surface density, and impurity atoms. We will focus on temperature, precursor molar fraction and V/III ratio. Our observations are compared to previous reports in the literature of the III-V nanowire crystal structure, and interpreted in terms of existing models. We propose that changes in the crystal structure with growth parameters are directly related to changes in the stable side facets.

All-inorganic colloidal perovskite quantum dots (QDs) based on cesium, lead, and halide have recently emerged as promising light emitting materials. CsPbBr QDs have also been demonstrated as stable two-photon-pumped lasing medium. However, the reported two photon absorption (TPA) cross sections for these QDs differ by an order of magnitude. Here we present an in-depth study of the TPA properties of CsPbBr QDs with mean size ranging from 4.6 to 11.4 nm. By using femtosecond transient absorption (TA) spectroscopy we found that TPA cross section is proportional to the linear one photon absorption. The TPA cross section follows a power law dependence on QDs size with exponent 3.3 ± 0.2. The empirically obtained power-law dependence suggests that the TPA process through a virtual state populates exciton band states. The revealed power-law dependence and the understanding of TPA process are important for developing high performance nonlinear optical devices based on CsPbBr nanocrystals.

Characteristics of nanoscale materials are often different from the corresponding bulk properties providing new, sometimes unexpected, opportunities for applications. Here we investigate the properties of 8 nm colloidal nanoparticles of MAPbBr3 perovskites and contrast them to the ones of large microcrystallites representing a bulk. X-ray spectroscopies provide an exciton binding energy of 0.32 ± 0.10 eV in the nanoparticles. This is 5 times higher than the value of bulk crystals (0.084 ± 0.010 eV), and readily explains the high fluorescence quantum yield in nanoparticles. In the bulk, at high excitation concentrations, the fluorescence intensity has quadratic behavior following the Saha-Langmuir model due to the nongeminate recombination of charges forming the emissive exciton states. In the nanoparticles, a linear dependence is observed since the excitation concentration per particle is significantly less than one. Even the bulk shows linear emission intensity dependence at lower excitation concentrations. In this case, the average excitation spacing becomes larger than the carrier diffusion length suppressing the nongeminate recombination. From these considerations we obtain the charge carrier diffusion length in MAPbBr3 of 100 nm.