Showing papers by "Loredana Protesescu published in 2018"
05 Mar 2018
TL;DR: Wet ball milling of bulk APbBr3 (A = Cs, FA) mixed with solvents and capping ligands yields green luminescent colloidal NCs with a high overall reaction yield and optoelectronic quality on par with that of NCs of the same composition obtained by hot-injection method.
Abstract: Lead halide perovskites of APbX3 type [A = Cs, formamidinium (FA), methylammonium; X = Br, I] in the form of ligand-capped colloidal nanocrystals (NCs) are widely studied as versatile photonic sources. FAPbBr3 and CsPbBr3 NCs have become promising as spectrally narrow green primary emitters in backlighting of liquid-crystal displays (peak at 520–530 nm, full width at half-maximum of 22–30 nm). Herein, we report that wet ball milling of bulk APbBr3 (A = Cs, FA) mixed with solvents and capping ligands yields green luminescent colloidal NCs with a high overall reaction yield and optoelectronic quality on par with that of NCs of the same composition obtained by hot-injection method. We emphasize the superiority of oleylammonium bromide as a capping ligand used for this procedure over the standard oleic acid and oleylamine. We also show a mechanically induced anion-exchange reaction for the formation of orange-emissive CsPb(Br/I)3 NCs.
142 citations
TL;DR: Droplet-based microfluidics can successfully tackle the problem of solving the practical utility and rational synthesis of multinary colloidal NCs in both a time- and cost-efficient manner, and demonstrates the excellent transference of reaction parameters from microfluidity to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices.
Abstract: Hybrid organic–inorganic and fully inorganic lead halide perovskite nanocrystals (NCs) have recently emerged as versatile solution-processable light-emitting and light-harvesting optoelectronic materials. A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. In this context, all three archetypal A-site monocationic perovskites—CH3NH3PbI3, CH(NH2)2PbI3, and CsPbI3—suffer from either chemical or thermodynamic instabilities in their bulk form. A promising approach toward the mitigation of these challenges lies in the formation of multinary compositions (mixed cation and mixed anion). In the case of multinary colloidal NCs, such as quinary CsxFA1–xPb(Br1–yIy)3 NCs, the outcome of the synthesis is defined by a complex interplay between the bulk thermodynamics of the solid solutions, crystal surface energies, energetics, dynamics of capping ligands, and the multiple effects of the reagents in solution. Accordingly, the rat...
125 citations
TL;DR: It is demonstrated that the controlled synthesis of Cl-rich perovskites with stable emission remains a challenge due to fast segregation of halide ions, and the controlled formation of blue emitting FAPb(Cl1-xBrx)3 NCs lags behind their green and red counterparts.
Abstract: Hybrid organic–inorganic perovskites and in particular formamidinium lead halide (FAPbX3, X = Cl, Br, I) perovskite nanocrystals (NCs) have shown great promise for their implementation in optoelectronic devices. Specifically, the Br and I counterparts have shown unprecedented photoluminescence properties, including precise wavelength tuning (530–790 nm), narrow emission linewidths (<100 meV) and high photoluminescence quantum yields (70–90%). However, the controlled formation of blue emitting FAPb(Cl1–xBrx)3 NCs lags behind their green and red counterparts and the mechanism of their formation remains unclear. Herein, we report the formation of FAPb(Cl1–xBrx)3 NCs with stable emission between 440 and 520 nm in a fully automated droplet-based microfluidic reactor and subsequent reaction upscaling in conventional laboratory glassware. The thorough parametric screening allows for the elucidation of parametric zones (FA-to-Pb and Br-to-Cl molar ratios, temperature, and excess oleic acid) for the formation of n...
100 citations
TL;DR: Temperature-dependent PL spectra reveal a line broadening of the emission caused by exciton phonon interaction as well an unusual energy shift which is attributed to a crystal phase transition within the single QD.
Abstract: Formamidinium lead bromide (FAPbBr3) quantum dots (QDs) are promising materials for light emitting applications in the visible spectral region because of their high photoluminescence (PL) quantum yield (QY) and the enhanced chemical stability as compared to, for instance, methylammonium based analogues. Toward practical harnessing of their compelling optical characteristics, the exciton recombination process, and in particular the exciton–phonon interaction and the impact of crystal phase transition, has to be understood in detail. This is addressed in this contribution by PL studies on single colloidal FAPbBr3 QDs. Polarization-resolved PL measurements reveal a fine structure splitting of excitonic transitions due to the Rashba effect. Distinct phonon replica have been observed within energetic distances of 4.3 ± 0.5, 8.6 ± 0.9, and 13.2 ± 1.1 meV from the zero phonon line, which we attribute to vibrational modes of the lead bromide lattice. Additional vibrational modes of 18.6 ± 0.3 and 38.8 ± 1.1 meV a...
48 citations
TL;DR: Multidimensional 113Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces, which enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size- and shape-engineered CdSe NCs.
Abstract: Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a long-standing goal of paramount importance for all their potential optoelectronic applications, which remains unsolved largely because of the lack of site-specific physical techniques. Here, we show that multidimensional 113Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size- and shape-engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc-blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets), as well as magic-sized CdSe clusters. Assignments of the NMR signals to spec...
39 citations
TL;DR: In this paper, the authors demonstrate highly efficient amplified spontaneous emission (ASE) from close-packed films of formamidinium lead iodide perovskite (FAPbI3) nanocrystals under excitation in the nanosecond regime.
Abstract: Lead halide perovskite nanocrystals (NCs) were recently found to exhibit extraordinary optical amplification properties. The great majority of such studies implemented ultrashort photon pulses in the femtosecond regime to initiate the stimulated emission process. Yet the realization of practical lasing applications based on such materials is crucially dependent on their ability to sustain optical amplification at significantly longer time scales, at which major losses associated with spontaneous emission and nonradiative recombination occur. Herein we demonstrate highly efficient amplified spontaneous emission (ASE) from close-packed films of formamidinium lead iodide perovskite (FAPbI3) NCs under excitation in the nanosecond regime. Systematic optimization of the NC processing and thermal treatment yields solids that exhibit high ASE net modal gain up to 604 cm–1 and weakly temperature sensitive ASE thresholds with room-temperature values as low as 140 μJ cm–2. The efficient optical amplification using e...
28 citations
TL;DR: A high density of planar defects in oleate-terminated zincblende ZB CdSe colloidal quantum dots (QDs) and size-dependent faceting within a square-cuboid morphology is unveiled by combining X-ray scattering techniques in the wide- and small-angle regions and using the Debye scattering equation (DSE) method of analysis.
Abstract: Surface chemistry and core defects are known to play a prominent role in governing the photophysical properties of nanocrystalline semiconductors. Nevertheless, investigating them in small nanocrystals remains a complex task. Here, by combining X-ray scattering techniques in the wide- and small-angle regions and using the Debye scattering equation (DSE) method of analysis, we unveil a high density of planar defects in oleate-terminated zincblende (ZB) CdSe colloidal quantum dots (QDs) and size-dependent faceting within a square-cuboid morphology. Atomistic models of faulted ZB nanocrystals, based on the probabilistic stacking of CdSe layers in cubic and hexagonal sequences, and data analysis point to the preferential location of faults near the center of nanocrystals. By finely modeling faulting and morphological effects on the X-ray scattering pattern, a relaxation of the Cd–Se bond distance parallel to the stacking direction, up to +3% (2.71 A) with respect to the reference bulk value (2.63 A), is detec...
27 citations