Influence of non-local pseudopotential on interband optical conductivity spectra in metals

In this work we present an algorithm for calculating interband optical conductivity spectra in metals when electrons are subjected to a non-local pseudopotential field, the form-factors of which depend on the electron quasimomentum k. The k-dependence of the pseudopotential is treated within Heine-Abarenkov-Shaw model-pseudopotential scheme. Due to the k-dependence of the pseudopotential, the energy gap of electron energy spectrum in the Bragg-plane vicinities is dependent on the perpendicular component of k-vector. This leads to essential changes in a spectral shape of interband conductivity spectra when the Fermi level is within or above the energy gap. The interband absorption lines experience shifts of their positions and acquire a spectral form which is characterized by two critical points, which are dependent both on a position of the Fermi level and the non-locality parameter

Photo De-Mixing in Dion-Jacobson 2D Mixed Halide Perovskites

2D halide perovskites feature a versatile structure, which not only enables the fine-tuning of their optoelectronic properties but also makes them appealing as model systems to investigate the fundamental properties of hybrid perovskites. In this study, the authors analyze the changes in the optical absorption of 2D Dion-Jacobson mixed halide perovskite thin films (encapsulated) based on (PDMA)Pb(I0.5Br0.5)4 (PDMA: 1,4-phenylenedimethanammonium spacer) exposed to a constant illumination. It is demonstrated that these 2D mixed-halide perovskites undergo photo-induced phase segregation, where the pristine mixed-phase de-mixes into iodide-rich and bromide-rich phases (photo de-mixing). The de-mixed state is largely maintained in the dark at room temperature for several months, while at higher temperatures it shows complete reversibility to the mixed-phase in terms of optical and structural properties (dark re-mixing). The authors further investigate temperature-dependent absorption measurements under light to extract the photo de-mixed compositions and to map the photo-miscibility-gap. This work thereby reveals that reversible photo de-mixing occurs in Dion-Jacobson 2D hybrid perovskites and provides strategies to address the role of light in the thermodynamic properties of these materials

Reversible Pressure-Dependent Mechanochromism of Dion–Jacobson and Ruddlesden–Popper Layered Hybrid Perovskites

Layered Dion–Jacobson (DJ) and Ruddlesden–Popper (RP) hybrid perovskites are promising materials for optoelectronic applications due to their modular structure. To fully exploit their functionality, mechanical stimuli can be used to control their properties without changing the composition. However, the responsiveness of these systems to pressure compatible with practical applications (<1 GPa) remains unexploited. Hydrostatic pressure is used to investigate the structure–property relationships in representative iodide and bromide DJ and RP 2D perovskites based on 1,4-phenylenedimethylammonium (PDMA) and benzylammonium (BzA) spacers in the 0–0.35 GPa pressure range. Pressure-dependent X-ray scattering measurements reveal that lattices of these compositions monotonically shrink and density functional theory calculations provide insights into the structural changes within the organic spacer layer. These structural changes affect the optical properties; the most significant shift in the optical absorption is observed in (BzA)2PbBr4 under 0.35 GPa pressure, which is attributed to an isostructural phase transition. Surprisingly, the RP and DJ perovskites behave similarly under pressure, despite the different binding modes of the spacer molecules. This study provides important insights into how the manipulation of the crystal structure affects the optoelectronic properties of such materials, whereas the reversibility of their response expands the perspectives for future applications

Reversible Pressure-Dependent Mechanochromism of Dion–Jacobson and Ruddlesden–Popper Layered Hybrid Perovskites

Layered Dion–Jacobson (DJ) and Ruddlesden–Popper (RP) hybrid perovskites are promising materials for optoelectronic applications due to their modular structure. To fully exploit their functionality, mechanical stimuli can be used to control their properties without changing the composition. However, the responsiveness of these systems to pressure compatible with practical applications (<1 GPa) remains unexploited. Hydrostatic pressure is used to investigate the structure–property relationships in representative iodide and bromide DJ and RP 2D perovskites based on 1,4-phenylenedimethylammonium (PDMA) and benzylammonium (BzA) spacers in the 0–0.35 GPa pressure range. Pressure-dependent X-ray scattering measurements reveal that lattices of these compositions monotonically shrink and density functional theory calculations provide insights into the structural changes within the organic spacer layer. These structural changes affect the optical properties; the most significant shift in the optical absorption is observed in (BzA)2PbBr4 under 0.35 GPa pressure, which is attributed to an isostructural phase transition. Surprisingly, the RP and DJ perovskites behave similarly under pressure, despite the different binding modes of the spacer molecules. This study provides important insights into how the manipulation of the crystal structure affects the optoelectronic properties of such materials, whereas the reversibility of their response expands the perspectives for future applications