ABSTRACT
Nanocrystal superlattices (NC SLs) have long been sought as promising metamaterials, with nanoscale-engineered properties arising from collective and synergistic effects among the constituent building blocks. Lead halide perovskite (LHP) NCs come across as outstanding candidates for SL design, as they demonstrate collective light emission, known as superfluorescence, in single- and multicomponent SLs. Thus far, LHP NCs have only been assembled in single-component SLs or coassembled with dielectric NC building blocks acting solely as spacers between luminescent NCs. Here, we report the formation of multicomponent LHP NC-only SLs, i.e., using only CsPbBr3 NCs of different sizes as building blocks. The structural diversity of the obtained SLs encompasses the ABO6, ABO3, and NaCl structure types, all of which contain orientationally and positionally locked NCs. For the selected model system, the ABO6-type SL, we observed efficient NC coupling and Förster-like energy transfer from strongly confined 5.3 nm CsPbBr3 NCs to weakly confined 17.6 nm CsPbBr3 NCs, along with characteristic superfluorescence features at cryogenic temperatures. Spatiotemporal exciton dynamics measurements reveal that binary SLs exhibit enhanced exciton diffusivity compared to single-component NC assemblies across the entire temperature range (from 5 to 298 K). The observed coherent and incoherent NC coupling and controllable excitonic transport within the solid NC SLs hold promise for applications in quantum optoelectronic devices.
ABSTRACT
Lead halide perovskite nanocrystals (NCs) are highly suitable active media for solution-processed lasers in the visible spectrum, owing to the wide tunability of their emission from blue to red via facile ion-exchange reactions. Their outstanding optical gain properties and the suppressed nonradiative recombination losses stem from their defect-tolerant nature. In this work, we demonstrate flexible waveguides combining the transparent, bioplastic, polymer cellulose acetate with green CsPbBr3 or red-emitting CsPb(Br,I)3 NCs in simple solution-processed architectures based on polymer-NC multilayers deposited on polymer micro-slabs. Experiments and simulations indicate that the employment of the thin, free-standing membranes results in confined electrical fields, enhanced by 2 orders of magnitude compared to identical multilayer stacks deposited on conventional, rigid quartz substrates. As a result, the polymer structures exhibit improved amplified emission characteristics under nanosecond excitation, with amplified spontaneous emission (ASE) thresholds down to â¼95 µJ cm-2 and â¼70 µJ cm-2 and high net modal gain up to â¼450 and â¼630 cm-1 in the green and red parts of the spectrum, respectively. The optimized gain properties are accompanied by a notable improvement of the ASE operational stability due to the low thermal resistance of the substrate-less membranes and the intimate thermal contact between the polymer and the NCs. Their application potential is further highlighted by the membrane's ability to sustain dual-color ASE in the green and red parts of the spectrum through excitation by a single UV source, activate underwater stimulated emission, and operate as efficient white light downconverters of commercial blue LEDs, producing high-quality white light emission, 115% of the NTSC color gamut.
ABSTRACT
Nanocrystal (NC) self-assembly is a versatile platform for materials engineering at the mesoscale. The NC shape anisotropy leads to structures not observed with spherical NCs. This work presents a broad structural diversity in multicomponent, long-range ordered superlattices (SLs) comprising highly luminescent cubic CsPbBr3 NCs (and FAPbBr3 NCs) coassembled with the spherical, truncated cuboid, and disk-shaped NC building blocks. CsPbBr3 nanocubes combined with Fe3O4 or NaGdF4 spheres and truncated cuboid PbS NCs form binary SLs of six structure types with high packing density; namely, AB2, quasi-ternary ABO3, and ABO6 types as well as previously known NaCl, AlB2, and CuAu types. In these structures, nanocubes preserve orientational coherence. Combining nanocubes with large and thick NaGdF4 nanodisks results in the orthorhombic SL resembling CaC2 structure with pairs of CsPbBr3 NCs on one lattice site. Also, we implement two substrate-free methods of SL formation. Oil-in-oil templated assembly results in the formation of binary supraparticles. Self-assembly at the liquid-air interface from the drying solution cast over the glyceryl triacetate as subphase yields extended thin films of SLs. Collective electronic states arise at low temperatures from the dense, periodic packing of NCs, observed as sharp red-shifted bands at 6 K in the photoluminescence and absorption spectra and persisting up to 200 K.
ABSTRACT
The slowdown of carrier cooling in lead halide perovskites (LHP) may allow the realization of efficient hot carrier solar cells. Much of the current effort focuses on the understanding of the mechanisms that retard the carrier relaxation, while proof-of-principle demonstrations of hot carrier harvesting have started to emerge. Less attention has been placed on the impact that the energy and momentum relaxation slowdown imparts on the spontaneous and stimulated light-emission process. LHP nanocrystals (NCs) provide an ideal testing ground for such studies as they exhibit bright emission and high optical gain, while the carrier cooling bottleneck is further pronounced compared to their bulk analogues due to confinement. Herein, the luminescent properties of CsPbBr3, FAPbBr3, and FAPbI3 NCs in the strong photoexcitation regime are investigated. In the former two NC systems, amplified spontaneous emission is found to dominate over the radiative recombination at average carrier occupancy per nanocrystal larger than 5-10. On the other hand, under the same photoexcitation conditions in the FAPbI3 NCs, a longer lived population of hot carriers results in a competition between hot luminescence, stimulated emission, and defect recombination. The dynamic interplay between the aforementioned three emissive channels appears to be influenced by various experimental and material parameters that include temperature, material purity, film morphology, and excitation pulse width and wavelength.
ABSTRACT
Advances in the technology and processing of flexible optical materials have paved the way toward the integration of semiconductor emitters and polymers into functional light emitting fabrics. Lead halide perovskite nanocrystals appear as highly suitable optical sensitizers for such polymer fiber emitters due to their ease of fabrication, versatile solution-processing and highly efficient, tunable, and narrow emission across the visible spectrum. A beneficial byproduct of the nanocrystal incorporation into the polymer matrix is that it provides a facile and low-cost method to chemically and structurally stabilize the perovskite nanocrystals under ambient conditions. Herein, we demonstrate two types of robust fiber composites based on electrospun hydrophobic poly(methyl methacrylate) (PMMA) or hydrophilic polyvinylpyrrolidone (PVP) fibrous membranes sensitized by green-emitting all-inorganic CsPbBr3 or hybrid organic-inorganic FAPbBr3 nanocrystals. We perform a systematic investigation on the influence of the nanocrystal-polymer relative content on the structural and optical properties of the fiber nanocomposites and we find that within a wide content range, the nanocrystals retain their narrow and high quantum yield emission upon incorporation into the polymer fibers. Quenching of the radiative recombination at the higher/lower bound of the nanocrystal:polymer mass ratio probed is discussed in terms of nanocrystal clustering/ligand desorption due to dilution effects, respectively. The nanocomposite's optical stability over an extended exposure in air and upon immersion in water is also discussed. The studies confirm the demonstration of robust and bright polymer-fiber emitters with promising applications in backlighting for LCD displays and textile-based light emitting devices.
