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.
We report the simultaneous measurement of the structural and electronic components of the metal-insulator transition of VO2 using electron and photoelectron spectroscopies and microscopies. We show that these evolve over different temperature scales, and are separated by an unusual monoclinic-like metallic phase. Our results provide conclusive evidence that the new monoclinic-like metallic phase, recently identified in high-pressure and nonequilibrium measurements, is accessible in the thermodynamic transition at ambient pressure, and we discuss the implications of these observations on the nature of the MIT in VO2.PACS numbers: 71.30.+h, 71.27.+a, 79.60.-i The metal-insulator transition (MIT) of VO 2 is one of the most intensively studied examples of its kind, and yet it continues to surprise and inform us: some recent examples include the observation of its solid-state triplepoint, which is remarkably found to lie at the ambient pressure transition temperature, 1 and the peculiar nanosized striped topographical pattern that has been found in strained VO 2 films.2,3 Moreover, the phase transition itself faces renewed questions as to its origin and mechanism following the discovery at high pressure, and in nonequilibrium experiments, of a metallic state of monoclinic symmetry, 4-6 which beforehand had universally been the reserve of the insulating state in experiments. Very recently, the decoupling of the structural and electronic phase transitions has been confirmed in the related compound, V 2 O 3 .7 In part, the widespread interest that VO 2 has attracted is owed to the accessibility of its sharp, 8 ultrafast 9 transition, occurring in the bulk at 65• C at ambient pressures, coupled with the rich tunability of its properties with alloying and strain 10-12 and flexibility in fabrication 13 that make it a promising candidate for device application. 14In the bulk, the MIT of VO 2 is accompanied by a large structural distortion that has added to the difficulties in unraveling its origins. The high temperature metallic phase resides in the tetragonal rutile structure (isostructural with TiO 2 ). Below the first-order transition temperature, V-V dimers form, accompanied by the twisting of the VO 6 octahedra, as the structure is distorted into the insulating monoclinic M 1 phase. A second insulating monoclinic structure (M 2 ), in which one-half of the V atoms dimerize, is accessible through Cr doping 10 and strain.12 On the one hand, the dimerization has been considered a hallmark of the Peierls transition, in which the rearrangement of the lattice plays the key role. On the other hand, several experiments have made it clear that electron-electron correlations cannot be ignored, 15 and should be considered on at least an equal footing. 16We report the direct observation of the structural and electronic components of the transition in strained VO 2 by simultaneously combining powerful spatial and energy resolved probes of the crystal and electronic structure. We further show that the recently-observed monoclinic ...
Echoing the roaring success of their bulk counterparts, nano-objects built from organolead halide perovskites (OLHP) present bright prospects for surpassing the performances of their conventional organic and inorganic analogues in photodriven technologies. Unraveling the photoinduced charge dynamics is essential for optimizing the optoelectronic functionalities. However, mapping the carrier-lattice interactions remains challenging, owing to their manifestations on multiple length scales and time scales. By correlating ultrafast time-resolved optical and X-ray absorption measurements, this work reveals the photoinduced formation of strong-coupling polarons in CHNHPbBr nanoparticles. Such polarons originate from the self-trapping of electrons in the Coulombic field caused by the displaced inorganic nuclei and the oriented organic cations. The transient structural change detected at the Pb L X-ray absorption edge is well-captured by a distortion with average bond elongation in the [PbBr] motif. General implications for designing novel OLHP nanomaterials targeting the active utilization of these quasi-particles are outlined.
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