arXiv subjects

Rafael B. Araujo

Publications and source records attributed to Rafael B. Araujo.

4 recordsLinked to original sources

arXiv2026

Insights into the Structural and Orientational Ordering of 2D Metal Halide Perovskites

Metal halide perovskite nanoplatelets combine quantum confinement with structural anisotropy, yet how thickness and processing govern their internal crystal lattice versus collective organization remains unresolved. In this study, we separate internal structure, superlattice organization, and film texture using 2-5 monolayer CsPbBr3 nanoplatelets and larger nanocrystals across spin-coating and drop-casting routes. Grazing incidence X-ray diffraction analysis of the specimens indicates that orthorhombic-compatible signatures recur throughout the thickness and deposition series, with a preserved lattice-morphology relationship in which 020/040 and 101/202 lattice planes are associated with the platelet thickness and lateral dimensions, respectively. Upon deposition on the substrate, we notice a thickness-dependent crossover: 2-monolayer nanoplatelets remain predominantly face-on across deposition routes, whereas thicker nanoplatelets show marked processing sensitivity and orientational disorder. Processing therefore can be used as a tool to direct the NPL packing, establishing thickness as a key determinant of processing sensitivity.

cond-mat.mtrl-sci↗

arXiv2025

Confinement-Driven Exciton Behavior in 2D Halide Perovskites from Dielectric-Dependent Hybrid Methods

Understanding how dielectric anisotropy governs excitonic behavior in two-dimensional (2D) halide perovskites is critical for predicting and engineering their optoelectronic properties. In this work, we investigate Cs(n+1)PbnBr3n+1 nanoplatelets (n = 2-5) experimentally and theoretically and show that the interplay between dielectric confinement and anisotropic screening critically determines both their electronic structure and excitonic landscape. To incorporate the dielectric screening effects, the Coulomb kernel in the Fock exchange term is refined using a model dielectric function together with a model Bethe-Salpeter Equation approach. The exciton binding energies show a monotonic decrease from 0.26 eV to 0.21 eV from n = 2 to 5, with 20 meV decrease per layer up to n = 4, and thereafter less change. The relatively small change per layer is a consequence of the strong spatial localization of excitons. By analyzing directionally resolved dielectric tensors, we demonstrate that the in-plane dielectric constant predominantly dictates optical transitions and is close to converging to the bulk value already at n = 5, while the out-of-plane dielectric response reflects the confined nature of excitonic wave functions as expected. Our calculated absorption spectra capture experimental results within 0.02 eV throughout the confinement regime (n = 2-5). The effects of lattice dynamics on the dimensionally dependent dielectric response and subsequent exciton screening occurring on longer time-scales than the optical response are also analyzed, important for analysis and interpretation of exciton lifetime, diffusion, and band alignments. The results establish a clear correlation between dielectric anisotropy, electronic structure, and exciton binding energy at different timescales in layered perovskites, providing essential insight for the design of 2D optoelectronic materials and devices.

cond-mat.mtrl-sci↗

arXiv2025

Symmetry-driven phonon confinement in 2D halide perovskites

Quantum confinement not only reshapes electronic states but also reorganizes the vibrational landscape of low-dimensional semiconductors. In halide perovskites, however, the role of confinement in governing symmetry effects on vibrational modes has remained unresolved. Here we synthesize 2D CsPbBr3 nanoplatelets with atomically defined thicknesses for 2-5 monolayers (MLs) and perform exciton absorption and emission analysis, crystalline phase determination, and phonon analysis. The lowest dimensional structure (2 MLs) reveal a co-existence of cubic and orthorhombic structure, energetically converging to orthorhombic for 3 MLs and beyond. Through polarization-resolved Raman spectroscopy and first-principles theory for 2-5 MLs, a striking symmetry contrast is found: B1g modes intensify and evolve in line with the phonon-confinement model, while Ag modes deviate, reflecting their distinct spatial localization. First principles calculations show that B1g vibrational modes largely reside in the xy-plane, Pb-Br-Pb units connect octahedra along the xy-direction with increased lattice dynamics as inner layers accumulate, whereas A1g vibrations couple to out-of-plane distortions and remain susceptible to surface disorder and finite-size effects. This symmetry-driven dichotomy provides a general framework for understanding phonon localization in layered halide perovskites. Beyond mechanism, we establish Raman fingerprints, particularly the A1g/B1g intensity ratio in cross-polarized geometry, as a calibrated, non-destructive metrology for 2D nanoplatelet thickness through 2-5 MLs. These results bridge electronic and phonon confinement and highlight symmetry engineering as a route to understand and control phonons, excitons, and their interactions in low-dimensional optoelectronic materials.

cond-mat.mtrl-sci↗

arXiv2025

Dimensionality-dependent electronic and vibrational dynamics in low-dimensional organic-inorganic tin halides

Photo-induced dynamics of electronic processes in materials are driven by the coupling between electronic and nuclear degrees of freedom. Here we construct 1D and 2D organic-inorganic tin halides to investigate the functional role of dimensionality to exciton-phonon coupling (EPC) and exciton self-trapping. The results show that the 1D system has strong EPC leading to excitation-independent self-trapped exciton (STE) emission, while the 2D system exhibits over ten times weaker EPC resulting in free exciton emission. By performing femtosecond transient absorption experiments, we directly resolve the room-temperature vibrational wavepackets in the 1D system, some of which propagate along the STE potential energy surface. A combination of wagging and asymmetric stretching motions (~106 cm-1) in tin iodide is identified as such a mode inducing exciton self-trapping. While no room-temperature wavepackets are observed in the 2D system. These findings uncover the interplay between the dimensionality-dependent EPC and electronic/nuclear dynamics, offering constructive guidance to develop multifunctional organic-inorganic metal halides.

physics.chem-ph↗