Mapping emission heterogeneity in layered halide perovskites using cathodoluminescence.

Recent advancements in the fabrication of layered halide perovskites and their subsequent modification for optoelectronic applications have ushered in a need for innovative characterisation techniques. In particular, heterostructures containing multiple phases and consequently featuring spatially defined optoelectronic properties are very challenging to study. Here, we adopt an approach centered on cathodoluminescence, complemented by scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy analysis. Cathodoluminescence enables assessment of local emission variations by injecting charges with a nanometer-scale electron probe, which we use to investigate emission changes in three different systems: PEA2PbBr4, PEA2PbI4and lateral heterostructures of the two, fabricated via halide substitution. We identify and map different emission bands that can be correlated with local chemical composition and geometry. One emission band is characteristic of bromine-based halide perovskite, while the other originates from iodine-based perovskite. The coexistence of these emissions bands in the halide-substituted sample confirms the formation of lateral heterostructures. To improve the signal quality of the acquired data, we employed multivariate analysis, specifically the non-negative matrix factorization algorithm, on both cathodoluminescence and compositional datasets. The resulting understanding of the halide replacement process and identification of potential synergies in the optical properties will lead to optimised architectures for optoelectronic applications.

Impact of the organic cation on the band-edge emission of two-dimensional lead-bromide perovskites.

Organic-inorganic low-dimensional layered metal-halide perovskites are semiconductors in which the optoelectronic properties can be tuned by the material composition and the design of the layered architecture. While the electronic band structure is mainly determined by the inorganic octahedra lattice, the binding and conformation of the organic cations induces related lattice distortions that can break the symmetry and lead to the splitting of the exciton energy levels, and influence the dielectric confinement. Furthermore, organic-induced lattice deformations lead to offsets in k-space (where k is the wavevector) that go along with the exciton energy level splitting. Hence, the electronic transitions between these levels require the momentum contribution of phonons, and contributions of phonons in the exciton recombination dynamics result in thermal broadening of the emission linewidth. In this work, we investigate the band-edge emission of two-dimensional Ruddlesden-Popper lead-bromide perovskites synthesized with different organic cations that vary in their binding head group and their alkyl chain length. We find several peaks in the low-temperature photoluminescence spectra, and the number of peaks in the band-edge emission and their decay dynamics depend strongly on the type of organic cation in the material, which we relate to the difference in the inorganic lattice distortions that the cations induce. For two-dimensional layered perovskites with mainly in-plane distortions, induced by short primary ammonium molecules, we find a two-fold splitting of the band edge emission at low temperatures. If also out-of-plane distortions are present, as for the long-chain primary ammoniums, a three-fold splitting is observed. Interestingly, the low-energy peaks of the split series merge into the highest energy peak with increasing temperature. Thermal broadening analysis of the temperature-dependent photoluminescence linewidth in the structures with out-of-plane distortions yields energies that are larger than those reported for the inorganic lattice phonons. This indicates the involvement of either high-frequency oscillations involving the organic cations, or the broadening might be related to higher order phonon scattering processes in the excitonic recombination process. The strong directionality of the phonon modes in the octahedral lattice could promote the involvement of multiple electron-phonon scattering processes in the exciton relaxation dynamics, for example involving modes with orthogonal directionality.

Mixed Organic Cations Promote Ambient Light-Induced Formation of Metallic Lead in Lead Halide Perovskite Crystals.

One major concern toward the performance and stability of halide perovskite-based optoelectronic devices is the formation of metallic lead that promotes nonradiative recombination of charge carriers. The origin of metallic lead formation is being disputed whether it occurs during the perovskite synthesis or only after light, electron, or X-ray beam irradiation or thermal annealing. Here, we show that the quantity of metallic lead detected in perovskite crystals depends on the concentration and composition of the precursor solution. Through a controlled crystallization process, we grew black-colored mixed dimethylammonium (DMA)/methylammonium (MA) lead tribromide crystals. The black color is suggested to be due to the presence of small lead clusters. Despite the unexpected black coloring, the crystals show higher crystallinity and less defect density with respect to the standard yellow-colored DMA/MAPbBr3 crystals, as indicated by X-ray rocking curve and dark current measurements, respectively. While the formation of metallic lead could still be induced by external factors, the precursor solution composition and concentration can facilitate the formation of metallic lead during the crystallization process. Our results indicate that additional research is required to fully understand the perovskite precursor solution chemistry.

In-Plane and Out-of-Plane Investigation of Resonant Tunneling Polaritons in Metal-Dielectric-Metal Cavities.

Polaritons can be generated by tuning the optical transitions of a light emitter to the resonances of a photonic cavity. We show that a dye-doped cavity generates resonant tunneling polaritons with Epsilon-Near-Zero (ENZ) effective permittivity. We studied the polariton spectral dispersion in dye-doped metal-dielectric-metal (MDM) cavities as a function of the in-plane (k||) and out-of-plane (k⊥) components of the incident wavevector. The dependence on k|| was investigated through ellipsometry, revealing the ENZ modes. The k⊥ dependence was measured by varying the cavity thickness under normal incidence using a Surface Force Apparatus (SFA). Both methods revealed a large Rabi splitting well exceeding 100 meV. The SFA-based investigation highlighted the collective nature of strong coupling by producing a splitting proportional to the square root of the involved photons. This study demonstrates the possibility of generating ENZ polaritons and introduces the SFA as a powerful tool for the characterization of strong light-matter interactions.

Transformable Plasmonic Helix with Swinging Gold Nanoparticles.

Control over multiple optical elements that can be dynamically rearranged to yield substantial three-dimensional structural transformations is of great importance to realize reconfigurable plasmonic nanoarchitectures with sensitive and distinct optical feedback. In this work, we demonstrate a transformable plasmonic helix system, in which multiple gold nanoparticles (AuNPs) can be directly transported by DNA swingarms to target positions without undergoing consecutive stepwise movements. The swingarms allow for programmable AuNP translocations in large leaps within plasmonic nanoarchitectures, giving rise to tailored circular dichroism spectra. Our work provides an instructive bottom-up solution to building complex dynamic plasmonic systems, which can exhibit prominent optical responses through cooperative rearrangements of the constituent optical elements with high fidelity and programmability.

Emerging Approaches to DNA Data Storage: Challenges and Prospects.

With the total amount of worldwide data skyrocketing, the global data storage demand is predicted to grow to 1.75 × 1014 GB by 2025. Traditional storage methods have difficulties keeping pace given that current storage media have a maximum density of 103 GB/mm3. As such, data production will far exceed the capacity of currently available storage methods. The costs of maintaining and transferring data, as well as the limited lifespans and significant data losses associated with current technologies also demand advanced solutions for information storage. Nature offers a powerful alternative through the storage of information that defines living organisms in unique orders of four bases (A, T, C, G) located in molecules called deoxyribonucleic acid (DNA). DNA molecules as information carriers have many advantages over traditional storage media. Their high storage density, potentially low maintenance cost, ease of synthesis, and chemical modification make them an ideal alternative for information storage. To this end, rapid progress has been made over the past decade by exploiting user-defined DNA materials to encode information. In this review, we discuss the most recent advances of DNA-based data storage with a major focus on the challenges that remain in this promising field, including the current intrinsic low speed in data writing and reading and the high cost per byte stored. Alternatively, data storage relying on DNA nanostructures (as opposed to DNA sequence) as well as on other combinations of nanomaterials and biomolecules are proposed with promising technological and economic advantages. In summarizing the advances that have been made and underlining the challenges that remain, we provide a roadmap for the ongoing research in this rapidly growing field, which will enable the development of technological solutions to the global demand for superior storage methodologies.

Correlating Symmetries of Low-Frequency Vibrations and Self-Trapped Excitons in Layered Perovskites for Light Emission with Different Colors.

The soft hybrid organic-inorganic structure of two-dimensional layered perovskites (2DLPs) enables broadband emission at room temperature from a single material, which makes 2DLPs promising sources for solid-state white lighting, yet with low efficiency. The underlying photophysics involves self-trapping of excitons favored by distortions of the inorganic lattice and coupling to phonons, where the mechanism is still under debate. 2DLPs with different organic moieties and emission ranging from self-trapped exciton (STE)-dominated white light to blue band-edge photoluminescence are investigated. Detailed insights into the directional symmetries of phonon modes are gained using angle-resolved polarized Raman spectroscopy and are correlated to the temperature-dependence of the STE emission. It is demonstrated that weak STE bands at low-temperature are linked to in-plane phonons, and efficient room-temperature STE emission to more complex coupling to several phonon modes with out-of-plane components. Thereby, a unique view is provided into the lattice deformations and recombination dynamics that are key to designing more efficient materials.

Mixed Dimethylammonium/Methylammonium Lead Halide Perovskite Crystals for Improved Structural Stability and Enhanced Photodetection.

The solvent acidolysis crystallization technique is utilized to grow mixed dimethylammonium/methylammonium lead tribromide (DMA/MAPbBr3 ) crystals reaching the highest dimethylammonium incorporation of 44% while maintaining the 3D cubic perovskite phase. These mixed perovskite crystals show suppression of the orthorhombic phase and a lower tetragonal-to-cubic phase-transition temperature compared to MAPbBr3 . A distinct behavior is observed in the temperature-dependent photoluminescence properties of MAPbBr3 and mixed DMA/MAPbBr3 crystals due to the different organic cation dynamics governing the phase transition(s). Furthermore, lateral photodetectors based on these crystals show that, at room temperature, the mixed crystals possess higher detectivity compared to MAPbBr3 crystals caused by structural compression and reduced surface trap density. Remarkably, the mixed-crystal devices exhibit large enhancement in their detectivity below the phase-transition temperature (at 200 K), while for the MAPbBr3 devices only insignificant changes are observed. The high detectivity of the mixed crystals makes them attractive for visible-light communication and for space applications. The results highlight the importance of the synthetic technique for compositional engineering of halide perovskites that governs their structural and optoelectronic properties.

Photonic Cavity Effects for Enhanced Efficiency in Layered Perovskite-Based Light-Emitting Diodes.

Layered architectures for light-emitting diodes (LEDs) are the standard approach for solution-processable materials such as metal-halide perovskites. Upon designing the composition and thicknesses of the layers forming the LED, the primary focus is typically on the optimization of charge injection and balance. However, this approach only considers the process until electrons and holes recombine to generate photons, while for achieving optimized LED performance, the generated light must also be efficiently outcoupled. Our work focuses on the latter aspect. We assume efficient photon generation and analyze the effects of the geometrical configuration together with the dipole orientation, mimicking the light emission, on the main characteristics defining the LED, such as the Purcell effect and the outcoupling efficiency. We find that in-plane dipoles result in significantly increased outcoupling efficiency. Furthermore, the mismatch in refractive index among the layers and their different thicknesses can be tuned to maximize the Purcell effect and minimize internal losses. The combined optimization of dipole orientation and layer thicknesses can improve the efficiency of the LED up to a factor 10, hence highlighting the importance of considering also the photonic properties of the LED structures if the objective is to maximize the LED performance.

Fast Intrinsic Emission Quenching in Cs4PbBr6 Nanocrystals.

Cs4PbBr6 (0D) nanocrystals at room temperature have both been reported as nonemissive and green-emissive systems in conflicting reports, with no consensus regarding both the origin of the green emission and the emission quenching mechanism. Here, via ab initio molecular dynamics (AIMD) simulations and temperature-dependent photoluminescence (PL) spectroscopy, we show that the PL in these 0D metal halides is thermally quenched well below 300 K via strong electron-phonon coupling. To unravel the source of green emission reported for bulk 0D systems, we further study two previously suggested candidate green emitters: (i) a Br vacancy, which we demonstrate to present a strong thermal emission quenching at room temperature; (ii) an impurity, based on octahedral connectivity, that succeeds in suppressing nonradiative quenching via a reduced electron-phonon coupling in the corner-shared lead bromide octahedral network. These findings contribute to unveiling the mechanism behind the temperature-dependent PL in lead halide materials of different dimensionality.

Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis.

The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the ultraviolet up to the infrared. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large specific surface area, distinctive optical properties, high electrical conductivity, and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compounds, and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.

Engineering the Optical Emission and Robustness of Metal-Halide Layered Perovskites through Ligand Accommodation.

The 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 [PbBr6 ]4- octahedra 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.

Mechanical switching of orientation-related photoluminescence in deep-blue 2D layered perovskite ensembles.

The synergy between the organic component of two-dimensional (2D) metal halide layered perovskites and flexible polymers offers an unexplored window to tune their optical properties at low mechanical stress. Thus, there is a significant interest in exploiting their PL anisotropy by controlling their orientation and elucidating their interactions. Here, we apply this principle to platelet structures of micrometre lateral size that are synthesized in situ into free-standing polymer films. We study the photoluminescence of the resulting films under cyclic mechanical stress and observe an enhancement in the emission intensity up to ∼2.5 times along with a switch in the emission profile when stretching the films from 0% to 70% elongation. All the films recovered their initial emission intensity when releasing the stress throughout ca. 15 mechanical cycles. We hypothesize a combined contribution from reduced reabsorption, changes on in-plane and out-of-plane dipole moments that stem from different orientation of the platelets inside the film, and relative sliding of platelets within oriented stacks while stretching the films. Our results reveal how low-mechanical stress affects 2D layered perovskite aggregation and orientation, an open pathway toward the design of strain-controlled emission.

Understanding and Controlling Mode Hybridization in Multicavity Optical Resonators Using Quantum Theory and the Surface Forces Apparatus.

Optical fields in metal-dielectric multilayers display typical features of quantum systems, such as energy level quantization and avoided crossing, underpinned by an isomorphism between the Helmholtz and Schrödinger wave equations. This article builds on the fundamental concepts and methods of quantum theory to facilitate the understanding and design of multicavity resonators. It also introduces the surface forces apparatus (SFA) as a powerful tool for rapid, continuous, and extensive characterization of mode dispersion and hybridization. Instead of fabricating many different resonators, two equal metal-dielectric-metal microcavities were created on glass lenses and displaced relative to each other in a transparent silicone oil using the SFA. The fluid thickness was controlled in real time with nanometer accuracy from more than 50 µm to less than 20 nm, reaching mechanical contact between the outer cavities in a few minutes. The fluid gap acted as a third microcavity providing optical coupling and producing a complex pattern of resonance splitting as a function of the variable thickness. An optical wave in this symmetric three-cavity resonator emulated a quantum particle with nonzero mass in a potential comprising three square wells. Interference between the wells produced a 3-fold splitting of degenerate energy levels due to hybridization. The experimental results could be explained using the standard methods and formalism of quantum mechanics, including symmetry operators and the variational method. Notably, the interaction between square wells produced bonding, antibonding, and nonbonding states that are analogous to hybridized molecular orbitals and are relevant to the design of "epsilon-near-zero" devices with vanishing dielectric permittivity.

Phase Transitions in Low-Dimensional Layered Double Perovskites: The Role of the Organic Moieties.

Halide double perovskites are an interesting alternative to Pb-containing counterparts as active materials in optoelectronic devices. Low-dimensional double perovskites are fabricated by introducing large organic cations, resulting in organic/inorganic architectures with one or more inorganic octahedra layers separated by organic cations. Here, we synthesized layered double perovskites based on 3D Cs2AgBiBr6, consisting of double (2L) or single (1L) inorganic octahedra layers, using ammonium cations of different sizes and chemical structures. Temperature-dependent Raman spectroscopy revealed phase transition signatures in both inorganic lattice and organic moieties by detecting variations in their vibrational modes. Changes in the conformational arrangement of the organic cations to an ordered state coincided with a phase transition in the 1L systems with the shortest ammonium moieties. Significant changes of photoluminescence intensity observed around the transition temperature suggest that optical properties may be affected by the octahedral tilts emerging at the phase transition.

Author Correction: Graphene Plasmonic Fractal Metamaterials for Broadband Photodetectors.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Biodegradable and Insoluble Cellulose Photonic Crystals and Metasurfaces.

The replacement of plastic with eco-friendly and biodegradable materials is one of the most stringent environmental challenges. In this respect, cellulose stands out as a biodegradable polymer. However, a significant challenge is to obtain biodegradable materials for high-end photonics that are robust in humid environments. Here, we demonstrate the fabrication of high-quality micro- and nanoscale photonic and plasmonic structures via replica molding using pure cellulose and a blended version with nonedible agro-wastes. Both materials are biodegradable in soil and seawater according to the ISO 17556 standard. The pure cellulose films are transparent in the vis-NIR spectrum, having a refractive index similar to glass. The microstructured photonic crystals show high-quality diffractive properties that are maintained under extended exposure to water. Nanostructuring the cellulose transforms it to a biodegradable metasurface manifesting bright structural colors. A subsequent deposition of Ag endowed the metasurface with plasmonic properties used to produce plasmonic colors and for surface-enhanced Raman scattering.

Directional Anisotropy of the Vibrational Modes in 2D-Layered Perovskites.

The vibrational modes in organic/inorganic layered perovskites are of fundamental importance for their optoelectronic properties. The hierarchical architecture of the Ruddlesden-Popper phase of these materials allows for distinct directionality of the vibrational modes with respect to the main axes of the pseudocubic lattice in the octahedral plane. Here, we study the directionality of the fundamental phonon modes in single exfoliated Ruddlesden-Popper perovskite flakes with polarized Raman spectroscopy at ultralow frequencies. A wealth of Raman bands is distinguished in the range from 15 to 150 cm-1 (2-15 meV), whose features depend on the organic cation species, on temperature, and on the direction of the linear polarization of the incident light. By controlling the angle of the linear polarization of the excitation laser with respect to the in-plane axes of the octahedral layer, we gain detailed information on the symmetry of the vibrational modes. The choice of two different organic moieties, phenethylammonium (PEA) and butylammonium (BA), allows us to discern the influence of the linker molecules, evidencing strong anisotropy of the vibrations for the (PEA)2PbBr4 samples. Temperature-dependent Raman measurements reveal that the broad phonon bands observed at room temperature consist of a series of sharp modes and that such mode splitting strongly differs for the different organic moieties and vibrational bands. Softer molecules such as BA result in lower vibrational frequencies and splitting into fewer modes, while more rigid molecules such as PEA lead to higher frequency oscillations and larger number of Raman peaks at low temperature. Interestingly, in distinct bands the number of peaks in the Raman bands is doubled for the rigid PEA compared to the soft BA linkers. Our work shows that the coupling to specific vibrational modes can be controlled by the incident light polarization and choice of the organic moiety, which could be exploited for tailoring exciton-phonon interaction, and for optical switching of the optoelectronic properties of such 2D layered materials.