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Register a new userLong-Range Exciton Diffusion in Two-Dimensional Assemblies of Cesium Lead Bromide Perovskite Nanocrystals
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Forster Resonant Energy Transfer (FRET)-mediated exciton diffusion through artificial nanoscale building block assemblies could be used as a new optoelectronic design element to transport energy. However, so far nanocrystal (NC) systems supported only diffusion length of 30 nm, which are too small to be useful in devices. Here, we demonstrate a FRET-mediated exciton diffusion length of 200 nm with 0.5 cm2/s diffusivity through an ordered, two-dimensional assembly of cesium lead bromide perovskite nanocrystals (PNC). Exciton diffusion was directly measured via steady-state and time-resolved photoluminescence (PL) microscopy, with physical modeling providing deeper insight into the transport process. This exceptionally efficient exciton transport is facilitated by PNCs high PL quantum yield, large absorption cross-section, and high polarizability, together with minimal energetic and geometric disorder of the assembly. This FRET-mediated exciton diffusion length matches perovskites optical absorption depth, opening the possibility to design new optoelectronic device architectures with improved performances, and providing insight into the high conversion efficiencies of PNC-based optoelectronic devices.
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Electronic states of the hybrid perovskite enable their promising applications as distinctive optoelectronic materials. The understanding of their electronic structures and charge characters remains highly controversial. The electronic mechanism such as reabsorption, Urbach tail and indirect band for interpreting dual-peak emissions is one of the controversial focuses. Herein, we report that through heterojunction enhanced exciton dissociation and global tracing of multiple radiative electronic states across wide temperature regions, we have succeeded in directly observing free carrier emissions from the hybrid lead bromide perovskite and clearly discriminating the direct continuum band and exciton states. The widely-concerned dual-peak emissions are clarified to be excitonic, arising from two types of exciton states of the perovskite. These excitons possess giant binding energies and superior phase stability compared to conventional inorganic semiconductors, providing important implications for exploiting the excitonic mechanism for realizing novel optoelectronic applications.
Halide perovskites have emerged as disruptive semiconductors for applications including photovoltaics and light emitting devices, with modular optoelectronic properties realisable through composition and dimensionality tuning. Layered Ruddlesden-Popper perovskites of the form BA2MAn-1PbnI3n+1, where n is the number of lead-halide and methylammonium (MA) sheets spaced by longer butylammonium (BA) cations, are particularly interesting due to their unique two-dimensional character and charge carrier dynamics dominated by strongly bound excitons. However, long-range energy transport through exciton diffusion in these materials is not understood or realised. Here, we employ local time-resolved luminescence mapping techniques to visualise exciton transport in high-quality exfoliated flakes of the BA2MAn-1PbnI3n+1 perovskite family. We uncover two distinct transport regimes, depending on the temperature range studied. At temperatures above 100 K, diffusion is mediated by thermally activated hopping processes between localised states. At lower temperatures, a non-uniform energetic landscape emerges in which exciton transport is dominated by energy funnelling processes to lower energy states, leading to long range transport over hundreds of nanometres even in the absence of exciton-phonon coupling and in the presence of local optoelectronic heterogeneity. Efficient, long-range and switchable excitonic funnelling offers exciting possibilities of controlled directional long-range transport in these 2D materials for new device applications.
Previous theoretical calculations show azetidinium has the right radial size to form a 3D perovskite with lead halides [1], and has been shown to impart, as the A-site cation of ABX3 unit, beneficial properties to ferroelectric perovskites [2]. However, there has been very limited research into its use as the cation in lead halide perovskites to date. In this communication we report the synthesis and characterization of azetidinium-based lead mixed halide perovskite colloidal nanocrystals. The mixed halide system is iodine and chlorine unlike other reported nanocrystals in the literature where the halide systems are either iodine/bromine or bromine/chlorine. UV-visible absorbance data, complemented with photoluminescence spectroscopy, reveals an indirect-bandgap of about 1.96 eV for our nanocrystals. Structural characterization using TEM shows two distinct interatomic distances (2.98 +/- 0.15 Angstroms and 3.43 +/- 0.16 Angstroms) and non-orthogonal lattice angles (approximately 112 degrees) intrinsic to the nanocrystals with a probable triclinic structure revealed by XRD. The presence of chlorine and iodine within the nanocrystals is confirmed by EDS spectroscopy. Finally, light-induced electron paramagnetic resonance (LEPR) spectroscopy with PCBM confirms the photoinduced charge transfer capabilities of the nanocrystals. The formation of such semiconducting lead mixed halide perovskite using azetidinium as the cation suggests a promising subclass of hybrid perovskites holding potential for optoelectronic applications such as in solar cells and photodetectors.
The surprising recent observation of highly emissive triplet-states in lead halide perovskites accounts for their orders-of-magnitude brighter optical signals and high quantum efficiencies compared to other semiconductors. This makes them attractive for future optoelectronic applications, especially in bright low-threshold nano-lasers. Whilst non-resonantly pumped lasing from all-inorganic lead-halide perovskites is now well-established as an attractive pathway to scalable low-power laser sources for nano-optoelectronics, here we showcase a resonant optical pumping scheme on a fast triplet-state in CsPbBr3 nanocrystals. The scheme allows us to realize a polarized triplet-laser source that dramatically enhances the coherent signal by one order of magnitude whilst suppressing non-coherent contributions. The result is a source with highly attractive technological characteristics including a bright and polarized signal, and a high stimulated-to-spontaneous emission signal contrast that can be filtered to enhance spectral purity. The emission is generated by pumping selectively on a weakly-confined excitonic state with a Bohr radius ~10 nm in the nanocrystals. The exciton fine-structure is revealed by the energy-splitting resulting from confinement in nanocrystals with tetragonal symmetry. We use a linear polarizer to resolve two-fold non-degenerate sub-levels in the triplet exciton and use photoluminescence excitation spectroscopy to determine the energy of the state before pumping it resonantly.
Fully-inorganic cesium lead halide perovskite nanocrystals (NCs) have shown to exhibit outstanding optical properties such as wide spectral tunability, high quantum yield, high oscillator strength as well as blinking-free single photon emission and low spectral diffusion. Here, we report measurements of the coherent and incoherent exciton dynamics on the 100 fs to 10 ns timescale, determining dephasing and density decay rates in these NCs. The experiments are performed on CsPbBr$_{2}$Cl NCs using transient resonant three-pulse four-wave mixing (FWM) in heterodyne detection at temperatures ranging from 5 K to 50 K. We found a low-temperature exciton dephasing time of 24.5$pm$1.0 ps, inferred from the decay of the photon-echo amplitude at 5 K, corresponding to a homogeneous linewidth (FWHM) of 54$pm$5 {mu}eV. Furthermore, oscillations in the photon-echo signal on a picosecond timescale are observed and attributed to coherent coupling of the exciton to a quantized phonon mode with 3.45 meV energy.
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