Reciprocating Thermal Behavior in Multichannel Relaxation of Cobalt(II) Based Single Ion Magnets

A series of mononuclear Co(II) complexes showing slow magnetic relaxation is assessed from the point of view of relaxation mechanisms. In certain cases, the reciprocating thermal behavior is detected: On cooling, the slow relaxation time is prolonged until a certain limit and then, unexpectedly, is accelerated. The low-temperature magnetic data can be successfully fitted by assuming Raman and/or phonon bottleneck mechanisms of the slow magnetic relaxation for the high-frequency relaxation channel. An additional term with the negative temperature exponent is capable of reproducing the whole experimental dataset.

The Effect of Tin Substitution on the Excitonic Properties of Two Dimensional Metal Halide Perovskites

<div>With growing interest in the lead-free derivatives of the metal-halide perovskites (MHP), it is imperative to fully understand the contribution of the metal cation to their desirable excitonic characteristics. Here, we explore this question by performing an in-depth spectroscopic and theoretical analysis of phenethylammonium tin iodide ((PEA)₂SnI₄), a prototypical tin based MHP, and rigorously compare it with its lead counterpart. We elaborate on the origin of multiple excitonic resonances uniquely observed in the linear absorption spectrum of (PEA)₂SnI₄ at energies about 200-300meV above the primary exciton. By performing calculations based on density functional theory and many- body perturbation theory, we suggest that the excitonic series at these higher energies are composed of electronic transitions from a lower lying valence band. Importantly, the valence band splitting is driven by the octahedral conformations that follow subtle variations in the organic-inorganic interactions within the crystal lattice. We experimentally show that the presence of the higher energy excitonic resonance results in a relatively slow nanosecond component in the formation dynamics of the primary exciton, in addition to the ultrafast phonon-driven hot carrier thermalization. While the presence of such slow relaxation channel for the excitons might be beneficial to many optoelectronic applications, our work suggests its possible control via systematic design of the organic cation. Moreover, our observations indicate that spin-orbit coupling does not play a primary role in the intricate yet crucial changes in the excitonic characteristics imparted by the tin substitution.</div>

The Effect of Tin Substitution on the Excitonic Properties of Two Dimensional Metal Halide Perovskites

<div>With growing interest in the lead-free derivatives of the metal-halide perovskites (MHP), it is imperative to fully understand the contribution of the metal cation to their desirable excitonic characteristics. Here, we explore this question by performing an in-depth spectroscopic and theoretical analysis of phenethylammonium tin iodide ((PEA)₂SnI₄), a prototypical tin based MHP, and rigorously compare it with its lead counterpart. We elaborate on the origin of multiple excitonic resonances uniquely observed in the linear absorption spectrum of (PEA)₂SnI₄ at energies about 200-300meV above the primary exciton. By performing calculations based on density functional theory and many- body perturbation theory, we suggest that the excitonic series at these higher energies are composed of electronic transitions from a lower lying valence band. Importantly, the valence band splitting is driven by the octahedral conformations that follow subtle variations in the organic-inorganic interactions within the crystal lattice. We experimentally show that the presence of the higher energy excitonic resonance results in a relatively slow nanosecond component in the formation dynamics of the primary exciton, in addition to the ultrafast phonon-driven hot carrier thermalization. While the presence of such slow relaxation channel for the excitons might be beneficial to many optoelectronic applications, our work suggests its possible control via systematic design of the organic cation. Moreover, our observations indicate that spin-orbit coupling does not play a primary role in the intricate yet crucial changes in the excitonic characteristics imparted by the tin substitution.</div>

Crossover from Deformation Twinning to Lattice Dislocation Slip in Metal–Graphene Composites with Bimodal Structures

Theoretical model is suggested, which describes of a new micromechanism of crossover from deformation twinning to lattice dislocation slip in metal–graphene nanocomposite with a bimodal structure. In the framework of the model, the lattice dislocation slip occurs through emission of lattice dislocations from the disclinated grain boundary fragments between a nanocrystalline metal–matrix and large (micrometer-size) grains providing the plastic deformation of bimodal metal–graphene nanocomposite. It is shown that the lattice dislocation emission serves as an effective stress relaxation channel being in competition with nanocrack generation.

Electron detachment from anions in aqueous solutions studied by two- and three-pulse femtosecond spectroscopy

The electron photodetachment of the aqueous halides and hydroxide is studied after resonant excitation in the lowest charge-transfer-to-solvent (CTTS) state. The initially excited state is followed by an intermediate assigned to a donor-electron pair that displays a competition of recombination and separation. Using pump–repump–probe (PREP) spectroscopy, the pair species is verified via a secondary excitation with separation of the pairs so that the yield of released electrons is increased. The observed recombination process on the one hand and the similar absorptions of the intermediate and the hydrated electron on the other hand suggest that the donor-electron pairs incorporate only few if not just one water molecule. The geminate dynamics measured in the various CTTS systems reveal a strong influence of the parent radical. The electron survival probability decreases significantly from 0.77 to 0.29 going from F– to OH–. The extracted dissociation rates of the halogen-electron pairs seem to be proportional to the mutual diffusion coefficients of the geminate particles, while such a relation between the recombination rate and the diffusion coefficient is not found. Results for I– show that excitation of a higher-lying CTTS state opens a new relaxation channel, which directly leads to a fully hydrated electron, while the relaxation channel discussed above is not significantly affected.