arXiv subjects

Umberto Filippi

Publications and source records attributed to Umberto Filippi.

4 recordsLinked to original sources

arXiv2026

Deterministic nucleation of nanocrystal superlattices on 2D perovskites for light-funneling heterostructures

Semiconductor heterostructures that combine components with different dimensionality provide an interesting way to manipulate the physical properties of the resulting material. Two-dimensional lead halide perovskites crystallize as flat microcrystals and have efficient in-plane exciton mobility, while perovskite nanocrystals are efficient emitters with a tunable bandgap that can self-assemble into microscopic superlattices. However, combining such intricate architectures into heterostructures has been challenging due to the mismatch in solubility properties and the challenging transfer procedures. Here we realize heterostructures where CsPbBr3 nanocrystal superlattices are deterministically grown along the faces of PEA2PbBr4 two-dimensional layered perovskite microcrystals. The growth can be limited to the lateral faces of the microcrystals and result in core-crown epitaxial heterostructures, or extended to the vertical direction leading to core-shell-like structures. The growth method is simple yet effective and versatile, and promises to be expanded to a large variety of other materials. We demonstrate that these heterostructures can be employed as efficient light-harvesting systems. In fact, energy can be transferred from the two-dimensional microcrystal domain to the superlattices, enabling switching between linear and non-linear carrier recombination regimes by tuning the excitation fluence. Moreover, by exploiting the lifetime shortening of CsPbBr3 nanocrystal emission upon sample cooling, we ensure that energy transfer occurs after the biexcitonic and single-excitonic decays of the nanocrystals, effectively extending the radiative recombination of superlattices.

cond-mat.mtrl-sci↗

arXiv2025

Sinusoidal Displacement Describes Disorder in CsPbBr3 Nanocrystal Superlattices

Disorder is an intrinsic feature of all solids, from crystals of atoms to superlattices of colloidal nanoparticles. Unlike atomic crystals, in nanocrystal superlattices a single misplaced particle can affect the positions of neighbors over long distances, leading to cumulative disorder. This elusive form of collective particle displacement leaves clear signatures in diffraction, but little is known about how it accumulates and propagates throughout the superlattice. Here we rationalize propagation and accumulation of disorder in a series of CsPbBr3 nanocrystal superlattices by using synchrotron grazing incidence small- and wide-angle X-ray scattering. CsPbBr3 nanocrystals of colloidal softness S in the range of 0.3-0.7 were obtained by preparing particles with different sizes and ligand mixtures, consisting of oleic acid and primary amines of variable lengths. Most diffraction patterns showed clear signatures of anisotropic disorder, with multilayer diffraction characteristics of high structural coherence visible only for the {100} axial directions and lost in all other directions. As the softness decreased, the superlattices transitioned to a more ordered regime where small-angle diffraction peaks became resolution-limited, and superlattice multilayer diffraction appeared for the (110) diagonal reflections. To rationalize these anisotropies in structural coherence and their dependence on superlattice softness, we propose a sinusoidal displacement model where longitudinal and transverse displacements modulate nanocrystal positions. The model explains experimental observations and advances the understanding of disorder in mesocrystalline systems as they approach the limits of structural perfection.

cond-mat.mtrl-sci↗

arXiv2024

The fate of optical excitons in FAPbI3 nanocube superlattices

Understanding the nature of the photoexcitation and ultrafast charge dynamics pathways in organic halide perovskite nanocubes and their aggregation into superlattices is key for the potential applications as tunable light emitters, photon harvesting materials and light-amplification systems. In this work, we apply two-dimensional coherent electronic spectroscopy (2DES) to track in real time the formation of near-infrared optical excitons and their ultrafast relaxation in CH(NH2)2PbI3 nanocube superlattices. Our results unveil that the coherent ultrafast dynamics is limited by the combination of the inherent short exciton decay time ~40 fs and the dephasing due to the coupling with selective optical phonon modes at higher temperatures. On the picosecond timescale, we observe the progressive formation of long-lived localized trap states. The analysis of the temperature dependence of the excitonic intrinsic linewidth, as extracted by the anti-diagonal components of the 2D spectra, unveils a dramatic change of the excitonic coherence time across the cubic to tetragonal structural transition. Our results offer a new way to control and enhance the ultrafast coherent dynamics of photocarrier generation in hybrid halide perovskite synthetic solids.

cond-mat.mes-hall↗

arXiv2023

Halide perovskite artificial solids as a new platform to simulate collective phenomena in doped Mott insulators

The development of Quantum Simulators, artificial platforms where the predictions of many-body theories of correlated quantum materials can be tested in a controllable and tunable way, is one of the main challenges of condensed matter physics. Here we introduce artificial lattices made of lead halide perovskite nanocubes as a new platform to simulate and investigate the physics of correlated quantum materials. The ultrafast optical injection of quantum confined excitons plays the role of doping in real materials. We show that, at large photo-doping, the exciton gas undergoes an excitonic Mott transition, which fully realizes the magnetic-field-driven insulator-to-metal transition described by the Hubbard model. At lower photo-doping, the long-range interactions drive the formation of a collective superradiant state, in which the phases of the excitons generated in each single perovskite nanocube are coherently locked. Our results demonstrate that time-resolved experiments span a parameter region of the Hubbard model in which long-range and phase-coherent orders emerge out of a doped Mott insulating phase. This physics is relevant for a broad class of phenomena, such as superconductivity and charge-density waves in correlated materials whose properties are captured by doped Hubbard models.

cond-mat.str-el↗