opportunities for structural engineering by the diverse choices of chemical composition, [3] and thus, interlayer spacing, and lattice distortions. Ultimately, a controlled variation of these parameters could effectively guide the tailoring of the electronic band structure of RPLP semiconductors. In this context, metal-halide RPLP are of particular interest, as they manifest unique properties, such as quantum and dielectric confinement, [4] narrow-and broadband emission, [5,6] fast radiative recombination, [7] long carrier diffusion lengths, [8] and large exciton binding energies. [9] These properties come along with low-cost processing and simplified device architectures. Besides, the presence of the organic layers provides structural stability and protection from moisture, making them more robust against environmental conditions, which is critical for device performance and lifetime. [10] Due to the stacking configuration of RPLP-with relatively strong organicinorganic interconnectivity-the type of organoammonium molecule plays a fundamental role in their photophysics. [11] For example, by introducing phenethylammonium in the Pb-Br system, the resulting RPLP can show four times higher emission efficiency in the deep blue region compared to butylammonium, a behavior that is related to the high lattice rigidity provided by Pb-Br-π stacking and reduced electron-phonon interaction in the phenethylammonium-basedThe unique combination of organic and inorganic layers in 2D layered perovskites offers promise for the design of a variety of materials for mechatronics, flexoelectrics, energy conversion, and lighting. However, the potential tailoring of their properties through the organic building blocks is not yet well understood. Here, different classes of organoammonium molecules are exploited to engineer the optical emission and robustness of a new set of Ruddlesden-Popper metal-halide layered perovskites. It is shown that the type of molecule regulates the number of hydrogen bonds that it forms with the edge-sharing [PbBr 6 ] 4octahedra layers, leading to strong differences in the material emission and tunability of the color coordinates, from deep-blue to pure-white. Also, the emission intensity strongly depends on the length of the molecules, thereby providing an additional parameter to optimize their emission efficiency. The combined experimental and computational study provides a detailed understanding of the impact of lattice distortions, compositional defects, and the anisotropic crystal structure on the emission of such layered materials. It is foreseen that this rational design can be extended to other types of organic linkers, providing a yet unexplored path to tailor the optical and mechanical properties of these materials and to unlock new functionalities.

Nanoelectronic devices based on ferroelectric domain walls (DWs), such as memories, transistors, and rectifiers, have been demonstrated in recent years. Practical high‐speed electronics, on the other hand, usually demand operation frequencies in the gigahertz (GHz) regime, where the effect of dipolar oscillation is important. Herein, an unexpected giant GHz conductivity on the order of 103 S m−1 is observed in certain BiFeO3 DWs, which is about 100 000 times greater than the carrier‐induced direct current (dc) conductivity of the same walls. Surprisingly, the nominal configuration of the DWs precludes the alternating current (ac) conduction under an excitation electric field perpendicular to the surface. Theoretical analysis shows that the inclined DWs are stressed asymmetrically near the film surface, whereas the vertical walls in a control sample are not. The resultant imbalanced polarization profile can then couple to the out‐of‐plane microwave fields and induce power dissipation, which is confirmed by the phase‐field modeling. Since the contributions from mobile‐carrier conduction and bound‐charge oscillation to the ac conductivity are equivalent in a microwave circuit, the research on local structural dynamics may open a new avenue to implement DW nano‐devices for radio‐frequency applications.

Using scanning probe microscopy, we measure the out-of-plane mechanical response of ferroelectric 180°domain walls and observe that, despite separating domains that are mechanically identical, the walls appear mechanically distinct-softer-compared to the domains. This effect is observed in different ferroelectric materials (LiNbO 3 , BaTiO 3 , and PbTiO 3) and with different morphologies (from single crystals to thin films), suggesting that the effect is universal. We propose a theoretical framework that explains the domain wall softening and justifies that the effect should be common to all ferroelectrics. The lesson is, therefore, that domain walls are not only functionally different from the domains they separate, but also mechanically distinct.

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