RESUMEN
The metal halide perovskite nanocrystals exhibit a remarkable tolerance to midgap defect states, resulting in high photoluminescence quantum yields. However, the potential of these nanocrystals for applications in display devices is hindered by the suppression of biexcitonic emission due to various Auger recombination processes. By adopting single-particle photoluminescence spectroscopy, herein, we establish that the biexcitonic quantum efficiency increases with the increase in the number of facets on cesium lead bromide perovskite nanocrystals, progressing from cube to rhombic dodecahedron to rhombicuboctahedron nanostructures. The observed enhancement is attributed mainly to an increase in their surface polarity as the number of facets increases, which reduces the Coulomb interaction of charge carriers, thereby suppressing Auger recombination. Moreover, Auger recombination rate constants obtained from the time-gated photon correlation studies exhibited a discernible decrease as the number of facets increased. These findings underscore the significance of facet engineering in fine-tuning biexciton emission in metal halide perovskite nanocrystals.
RESUMEN
Understanding the mechanism of chirality transfer from a chiral surface to an achiral molecule is essential for designing molecular systems with tunable chiroptical properties. These aspects are explored herein using l- and d-isomers of alkyl valine amphiphiles, which self-assemble in water as nanofibers possessing a negative surface charge. An achiral chromophore, acridine orange, upon electrostatic binding on these surfaces displays mirror-imaged bisignated circular dichroism and red-emitting circularly polarized luminescence signals with a high dissymmetry factor. Experimental and computational investigations establish that the chiroptical properties emerge from surface-bound asymmetric H-type dimers of acridine orange, further supported by fluorescence lifetime imaging studies. Specifically, atomistic molecular dynamics simulations show that the experimentally observed chiral signatures have their origin in van der Waals interactions between acridine orange dimers and the amphiphile head groups as well as in the extent of solvent exposure of the chromophore.
RESUMEN
The design of cube-connected nanorods is accomplished by connecting seed nanocrystals of a defined shape in a particular orientation or by etching selective facets of preformed nanorods. In lead halide perovskite nanostructures, which retain mostly a hexahedron cube shape, such patterned nanorods can be designed with the anisotropic direction along the edge, vertex, or facet of seed cubes. Combining the Cs-sublattice platform for transforming metal halides to halide perovskites with facet-specific ligand binding chemistry, herein, vertex-oriented patterning of nanocubes in one-dimensional (1D) rod structures is reported. By tuning the length of host metal halides, their lengths could also be tuned from 100 nm to nearly 1000 nm. The symmetry of the hexagonal phase of host halide CsCdBr3 and product orthorhombic CsPbBr3 helped in maintaining the vertex [201] as the anisotropic direction. Neutral exciton recombination rates, extracted from photoluminescence blinking traces, showed a systematic increase from isolated cubes to cube-connected nanorods of various lengths. Efficient coupling of wave functions in vertex-oriented cube assemblies permits exciton delocalization. Our findings on carrier delocalization in cube-connected nanorods along their vertex direction having minimum interfacial contacts provide valuable insights into the fundamental chemistry of assembling anisotropic halide perovskite nanostructures as conducting wires.
RESUMEN
Silicon-based light-emitting materials have emerged as a favorable substitute to various organic and inorganic systems due to silicon's high natural abundance, low toxicity, and excellent biocompatibility. However, efforts on the design of free-standing silicon nanoparticles with chiral non-racemic absorption and emission attributes are rather scare. Herein, we unravel the structural requirements for ligand-induced chirality in silicon-based nanomaterials by functionalizing with D- and L-isomers of a bifunctional ligand, namely, tryptophan. The structural aspects of these systems are established using high-resolution high-angle annular dark-field imaging in the scanning transmission electron microscopy mode, solid-state nuclear magnetic resonance, Fourier transform infrared, and X-ray photoelectron spectroscopy. Silicon nanoparticles capped with L- and D-isomers of tryptophan displayed positive and negative monosignated circular dichroic signals and circularly polarized luminescence indicating their ground- and excited-state chirality. Various studies supported by density functional theory calculations signify that the functionalization of indole ring nitrogen on the silicon surface plays a decisive role in modifying the chiroptical characteristics by generating emissive charge-transfer states. The chiroptical responses originate from the multipoint interactions of tryptophan with the nanoparticle surface through the indole nitrogen and -CO2- groups that can transmit an enantiomeric structural imprint on the silicon surface. However, chiroptical properties are not observed in phenylalanine- and alanine-capped silicon nanoparticles, which are devoid of Si-N bonds and chiral footprints. Thus, the ground- and excited-state chiroptics in tryptophan-capped silicon nanoparticles originates from the collective effect of ligand-bound emissive charge-transfer states and chiral footprints. Being the first report on the circularly polarized luminescence in silicon nanoparticles, this work will open newer possibilities in the field of chirality.