Long-Range Ballistic Propagation of 80% Excitonic Fraction Polaritons in a Perovskite Metasurface at Room Temperature.

Exciton-polaritons, hybrid light-matter excitations arising from the strong coupling between excitons in semiconductors and photons in photonic nanostructures, are crucial for exploring the physics of quantum fluids of light and developing all-optical devices. Achieving room temperature propagation of polaritons with a large excitonic fraction is challenging but vital, e.g., for nonlinear light transport. We report on room temperature propagation of exciton-polaritons in a metasurface made from a subwavelength lattice of perovskite pillars. The large Rabi splitting, much greater than the optical phonon energy, decouples the lower polariton band from the phonon bath of the perovskite. These cooled polaritons, in combination with the high group velocity achieved through the metasurface design, enable long-range propagation, exceeding hundreds of micrometers even with an 80% excitonic component. Furthermore, the design of the metasurface introduces an original mechanism for unidirectional propagation through polarization control, suggesting a new avenue for the development of advanced polaritonic devices.

Taming Friedrich-Wintgen Interference in a Resonant Metasurface: Vortex Laser Emitting at an On-Demand Tilted Angle.

Friedrich-Wintgen (FW) interference is an atypical coupling mechanism that grants loss exchange between leaky resonances in non-Hermitian classical and quantum systems. Intriguingly, such a mechanism makes destructive interference possible for scenarios in which a radiating wave becomes a bound state in the continuum (BIC) by giving away all of its losses. Here we propose and demonstrate experimentally an original concept to tailor FW-BICs with polarization singularity at on-demand wavevectors in an optical metasurface. As a proof-of-concept, using hybrid organic-inorganic halide perovskite as an active material, we empower this novel polarization singularity to obtain lasing emission, exhibiting both highly directional emission at oblique angles and a polarization vortex in momentum space. Our results pave the way to steerable coherent emission with a tailored polarization pattern for applications in optical communication/manipulation in free space, high-resolution imaging/focusing, and data storage.

Electron Dynamics in Hybrid Perovskites Reveal the Role of Organic Cations on the Screening of Local Charges.

The large tolerance of hybrid perovksites to the trapping of electrons by defects is a key asset in photovoltaic applications. Here, the ionic surface terminations of CH3NH3PbI3 are employed as a testbed to study the effect of electrostatic fields on the dynamics of excited carriers. We characterize the transition across the tetragonal to orthorhombic phase. The observed type II band offset and drift of the excited electrons highlight the important role that organic cations have on the screening of local electrostatic fields. When the orientation of organic cations is frozen in the orthorhombic phase, the positively charged termination induces a massive accumulation of excited electrons at the surface of the sample. Conversely, no electron accumulation is observed in the tetragonal phase. We conclude that the local fields cannot penetrate in the sample when the polarizability of freely moving cations boosts the dielectric constant up to ε = 120.

Tailoring Dispersion of Room-Temperature Exciton-Polaritons with Perovskite-Based Subwavelength Metasurfaces.

Exciton-polaritons represent a promising platform for studying quantum fluids of light and realizing prospective all-optical devices. Here we report on the experimental demonstration of exciton-polaritons at room temperature in resonant metasurfaces made from a sub-wavelength two-dimensional lattice of perovskite pillars. The strong coupling regime is revealed by both angular-resolved reflectivity and photoluminescence measurements, showing anticrossing between photonic modes and the exciton resonance with a Rabi splitting in the 200 meV range. Moreover, by tailoring the photonic Bloch mode to which perovskite excitons are coupled, polaritonic dispersions are engineered exhibiting linear, parabolic, and multivalley dispersions. All of our results are perfectly reproduced by both numerical simulations based on a rigorous coupled wave analysis and an elementary model based on a quantum theory of radiation-matter interaction. Our results suggest a new approach to study exciton-polaritons and pave the way toward large-scale and low-cost integrated polaritonic devices operating at room temperature.

Directing random lasing emission using cavity exciton-polaritons.

Random lasing is an intriguing phenomenon occurring in disordered structures with optical gain in which light scattering provides the necessary feedback for lasing action. Unlike conventional lasers, random lasing systems emit in all directions due to light scattering. While this property can be desired in some cases, directional emission remains required for most applications. In a vertical microcavity containing the hybrid perovskite CH3NH3PbBr3, we report here the coupling of the emission of a random laser with a cavity polaritonic resonance, resulting in a directional random lasing, whose emission angles can be tuned by varying the cavity detuning and reach values as large as 15.8° and 22.4°.

Multinuclear NMR as a tool for studying local order and dynamics in CH3NH3PbX3 (X = Cl, Br, I) hybrid perovskites.

We report on 207Pb, 79Br, 14N, 1H, 13C and 2H NMR experiments for studying the local order and dynamics in hybrid perovskite lattices. 207Pb NMR experiments conducted at room temperature on a series of MAPbX3 compounds (MA = CH3NH3+; X = Cl, Br and I) showed that the isotropic 207Pb NMR shift is strongly dependent on the nature of the halogen ions. Therefore 207Pb NMR appears to be a very promising tool for the characterisation of local order in mixed halogen hybrid perovskites. 207Pb NMR on MAPbBr2I served as a proof of concept. Proton, 13C and 14N NMR experiments confirmed the results previously reported in the literature. Low temperature deuterium NMR measurements, down to 25 K, were carried out to investigate the structural phase transitions of MAPbBr3. Spectral lineshapes allow following the successive phase transitions of MAPbBr3. Finally, quadrupolar NMR lineshapes recorded in the orthorhombic phase were compared with simulated spectra, using DFT calculated electric field gradients (EFG). Computed data do not take into account any temperature effect. Thus, the discrepancy between the calculated and experimental EFG evidences the fact that MA cations are still subject to significant dynamics, even at 25 K.

Using Low Temperature Photoluminescence Spectroscopy to Investigate CH3NH3PbI3 Hybrid Perovskite Degradation.

Investigating the stability and evaluating the quality of the CH3NH3PbI3 perovskite structures is quite critical both to the design and fabrication of high-performance perovskite devices and to fundamental studies of the photophysics of the excitons. In particular, it is known that, under ambient conditions, CH3NH3PbI3 degrades producing some PbI2. We show here that low temperature Photoluminescence (PL) spectroscopy is a powerful tool to detect PbI2 traces in hybrid perovskite layers and single crystals. Because PL spectroscopy is a signal detection method on a black background, small PbI2 traces can be detected, when other methods currently used at room temperature fail. Our study highlights the extremely high stability of the single crystals compared to the thin layers and defects and grain boundaries are thought to play an important role in the degradation mechanism.

Influence of arylalkyl amines on the formation of hybrid CsPbBr3 nanocrystals via a modified LARP method.

Perovskite nanocrystals have attracted much attention in the last ten years due to their different applications, especially in the photovoltaic domain and LED performance. In this large family of perovskite nanocrystals, CsPbBr3 nanocrystals are attractive nanomaterials because they are good candidates for obtaining green emissions and exploring new synthesis routes. In this context, controlling the nanometric scale's morphology, particularly the size and monodispersity, is fundamental for exploring their photophysical properties and final applications. Currently, the nanometric size of nanocrystals is ensured by the presence of oleic acid and oleylamine molecules, in using Hot Injection (HI) or ligand-assisted reprecipitation (LARP) methods. If oleic acid plays a fundamental role, oleylamine can be easily substituted by other amino molecules, opening the way for the functionalization of CsPbBr3 nanocrystals and the obtention of new hybrid perovskite nanocrystal families. In this article, we describe the synthesis, by soft chemistry, of a new family of hybrid organic-inorganic CsPbBr3 nanocrystals, functionalized by aryl-alkylamine (AAA) molecules, through the modified LARP method. We highlight the mechanism for cutting submicron crystals into nanocrystals, using aryl-alkylamine molecules like scissors. The impact of these amino molecules on the final nanocrystals leads to different nanocrystal morphologies (nanocubes, nanosheets, or nanorods) and structures (monoclinic, rhombohedral, or tetragonal). In addition, this modified LARP method highlights, under certain experimental conditions, an unexpected formation of PbO ribbons.

Exciton Cooling in 2D Perovskite Nanoplatelets: Rationalized Carrier-Induced Stark and Phonon Bottleneck Effects.

Using femtosecond transient absorption (fs-TA), we investigate the hot exciton relaxation dynamics in strongly confined lead iodide perovskite nanoplatelets (NPLs). The large quantum and dielectric confinement leads to discrete excitonic transitions and strong Stark features in the TA spectra. This prevents the use of conventional relaxation analysis methods extracting the carrier temperature or measuring the buildup of the band-edge bleaching. Instead, we show that the TA spectral line shape near the band-edge reflects the state of the system, which can be used to probe the exciton cooling dynamics. The ultrafast hot exciton relaxation in one- to three- monolayer-thick NPLs confirms the absence of intrinsic phonon bottleneck. However, excitation fluence-dependent measurements reveal a hot phonon bottleneck effect, which is found to be independent of the nature of the internal cations but strongly affected by the ligands and/or sample surface state. Together, these results suggest a role of the surface ligands in the cooling process.

Unexpected Anisotropy of the Electron and Hole Landé g-Factors in Perovskite CH3NH3PbI3 Polycrystalline Films.

In this work, we studied, at low temperature, the coherent evolution of the localized electron and hole spins in a polycrystalline film of CH3NH3PbI3 (MAPI) by using a picosecond-photo-induced Faraday rotation technique in an oblique magnetic field. We observed an unexpected anisotropy for the electron and hole spin. We determined the electron and hole Landé factors when the magnetic field was applied in the plane of the film and perpendicular to the exciting light, denoted as transverse ⟂ factors, and when the magnetic field was applied perpendicular to the film and parallel to the exciting light, denoted as parallel ∥ factors. We obtained |ge,⟂|=2.600 ± 0.004, |ge,∥|=1.604 ± 0.033 for the electron and |gh,⟂|=0.406 ± 0.002, |gh,∥|=0.299 ± 0.007 for the hole. Possible origins of this anisotropy are discussed herein.

In Vivo Fast Nonlinear Microscopy Reveals Impairment of Fast Axonal Transport Induced by Molecular Motor Imbalances in the Brain of Zebrafish Larvae.

Cargo transport by molecular motors along microtubules is essential for the function of eukaryotic cells, in particular neurons in which axonal transport defects constitute the early pathological features of neurodegenerative diseases. Mainly studied in motor and sensory neurons, axonal transport is still difficult to characterize in neurons of the brain in absence of appropriate in vivo tools. Here, we measured fast axonal transport by tracing the second harmonic generation (SHG) signal of potassium titanyl phosphate (KTP) nanocrystals (nanoKTP) endocytosed by brain neurons of zebrafish (Zf) larvae. Thanks to the optical translucency of Zf larvae and to the perfect photostability of nanoKTP SHG, we achieved a high scanning speed of 20 frames (of ≈90 µm × 60 µm size) per second in Zf brain. We focused our study on endolysosomal vesicle transport in axons of known polarization, separately analyzing kinesin and dynein motor-driven displacements. To validate our assay, we used either loss-of-function mutations of dynein or kinesin 1 or the dynein inhibitor dynapyrazole and quantified several transport parameters. We successfully demonstrated that dynapyrazole reduces the nanoKTP mobile fraction and retrograde run length consistently, while the retrograde run length increased in kinesin 1 mutants. Taking advantage of nanoKTP SHG directional emission, we also quantified fluctuations of vesicle orientation. Thus, by combining endocytosis of nanocrystals having a nonlinear response, fast two-photon microscopy, and high-throughput analysis, we are able to finely monitor fast axonal transport in vivo in the brain of a vertebrate and reveal subtle axonal transport alterations. The high spatiotemporal resolution achieved in our model may be relevant to precisely investigate axonal transport impairment associated with disease models.

Synthesis of highly calibrated CsPbBr3 nanocrystal perovskites by soft chemistry.

A new synthetic method for preparing highly calibrated CsPbBr3 nanocrystal perovskites is described and analyzed using high-resolution scanning transmission electron microscopy. This new method based on soft chemistry leads to the large-scale production of nanocrystals. Such monodisperse nanocrystals allow for the deposition of homogeneous films, which provides new opportunities for the next generation of optoelectronic devices.

Energy Tuning of Electronic Spin Coherent Evolution in Methylammonium Lead Iodide Perovskites.

We investigated the coherent evolution of the electronic spin at low temperature in high-quality CH3NH3PbI3 polycrystalline films by picosecond-resolved photoinduced Faraday rotation. We show that this coherent evolution can be tuned by choosing the pump-probe energy within the lowest optical-absorption band, and we explain it as the result of two main contributions: the localized electron and the localized hole. Their corresponding amplitude ratios are not constant across the lowest absorption band-an observation which disqualifies a free exciton from being at the origin of the electronic spin coherent evolution. We measured a spin coherence time of localized electrons (holes) of 4.4 ns (3.7 ns) at 1.635 eV, which evolves to about 7 ns at 1.612 eV (the hole coherence time remains almost constant at lower energies). Finally, we provide a global image of the spin coherent evolution in bulk metal halide perovskite, which overcomes recent controversies.

Tetrazine molecules as an efficient electronic diversion channel in 2D organic-inorganic perovskites.

Taking advantage of an innovative design concept for layered halide perovskites with active chromophores acting as organic spacers, we present here the synthesis of two novel two-dimensional (2D) hybrid organic-inorganic halide perovskites incorporating for the first time 100% of a photoactive tetrazine derivative as the organic component. Namely, the use of a heterocyclic ring containing a nitrogen proportion imparts a unique electronic structure to the organic component, with the lowest energy optical absorption in the blue region. The present compound, a tetrazine, presents several resonances between the organic and inorganic components, both in terms of single particle electronic levels and exciton states, providing the ideal playground to discuss charge and energy transfer mechanisms at the organic/inorganic interface. Photophysical studies along with hybrid time-dependent DFT simulations demonstrate partial energy transfer and rationalise the suppressed emission from the perovskite frame in terms of different energy-transfer diversion channels, potentially involving both singlet and triplet states of the organic spacer. Periodic DFT simulations also support the feasibility of electron transfer from the conduction band of the inorganic component to the LUMO of the spacer as a potential quenching mechanism, suggesting the coexistence and competition of charge and energy transfer mechanisms in these heterostructures. Our work proves the feasibility of inserting photoactive small rings in a 2D perovskite structure, meanwhile providing a robust frame to rationalize the electronic interactions between the semiconducting inorganic layer and organic chromophores, with the prospects of optimizing the organic moiety according to the envisaged application.

Revealing Excitonic Phonon Coupling in (PEA)2(MA)n-1PbnI3n+1 2D Layered Perovskites.

The family of 2D Ruddlesden-Popper perovskites is currently attracting great interest of the scientific community as highly promising materials for energy harvesting and light emission applications. Despite the fact that these materials are known for decades, only recently has it become apparent that their optical properties are driven by the exciton-phonon coupling, which is controlled by the organic spacers. However, the detailed mechanism of this coupling, which gives rise to complex absorption and emission spectra, is the subject of ongoing controversy. In this work we show that the particularly rich, absorption spectra of (PEA)2(CH3NH3)n-1PbnI3n+1 (where PEA stands for phenylethylammonium and n = 1, 2, 3), are related to a vibronic progression of excitonic transition. In contrast to other two-dimensional perovskites, we observe a coupling to a high-energy (40 meV) phonon mode probably related to the torsional motion of the NH3+ head of the organic spacer.

Grafting oligothiophenes on surfaces by diazonium electroreduction: a step toward ultrathin junction with well-defined metal/oligomer interface.

The functionalization of electrode materials through diazonium electroreduction using a heteroaromatic compound, without phenyl groups, has been investigated for the first time. The electrochemical reduction of 2-aminoterthiophenyldiazonium cation, generated in situ, coats the electrode (glassy carbon (GC), gold or platinum) with an ultrathin organic layer, shown by X-ray photoelectron spectroscopy (XPS) of that deposited on gold to consist of terthiophene or oligothiophene. The coating is electroactive at potential close to that of terthiophene in solution. The electrochemical response of the modified GC electrode in the presence of various reversible redox couples shows that the attached layer acts as a conductive switch. It behaves as a barrier to electron transfer when the standard redox potential is below 0.5 V/SCE; in this case diode-like behavior is observed. However, for more oxidizing redox probes the layer can be considered as transparent and no barrier effect is observed. The layer deposited on a platinum ultramicroelectrode (UME) behaves similarly to that obtained on the large GC electrode. Scanning electrochemical microscopy (SECM) can be performed using this electroswitchable modified platinum UME which can act as a filter toward competitive redox exchange pathways.

Exciton-Exciton Annihilation in Two-Dimensional Halide Perovskites at Room Temperature.

Recently, Ruddlesden-Popper 2D perovskite (RPP) solar cells and light-emitting diodes (LEDs) have shown promising efficiencies and improved stability in comparison to 3D halide perovskites. Here, the exciton recombination dynamics is investigated at room temperature in pure-phase RPP crystals (C6H5C2H4NH3)2(CH3NH3)n-1PbnI3n+1 (n = 1, 2, 3, and 4) by time-resolved photoluminescence (TRPL) in a large range of power excitations. As the number of perovskite layers increases, we detect the presence of an increasing fraction of out-of-equilibrium free carriers just after photoexcitation, on a picosecond time scale, while the dynamics is characterized by the recombination of excitons with long lifetime spanning several tens of nanoseconds. At low excitation power, the TRPL decays are nonexponential because of defect-assisted recombination. At high fluence, defects are filled and many-body interactions become important. Similar to other 2D systems, exciton-exciton annihilation (EEA) is then the dominant recombination path in a high-density regime below the Mott transition.

Atomic contacts via electrochemistry in water/cyclodextrin media: a step toward protected atomic contacts.

Atomic contacts are nanoscience devices proposed for applications such as single-atom switches in nanoelectronic circuits or one-molecule sensing devices. The conductance of such contacts varies in a stepwise fashion with a tendency to quantize near integer multiples of the conductance quantum (G0) but can also deviate significantly from integer values upon molecular adsorption. However, for sensing applications it is first necessary to coat the contact permanently to avoid nonspecific adsorption. Here, we show that marked differences are observed between atomic contacts generated in water, and in water/beta-CD. In this latter medium, atomic contacts with unusual properties can be generated. They have below 1 G0 conductance, low conductance fluctuation with time, and appear to be protected or partially protected from salicylate external molecular probes. Such contacts are not obtained in water, in water/glucose, or when beta-CD is added after 1 G0 contacts have been generated in water. These results indicate specific adsorption of beta-cyclodextrin on the atomic contacts and are compatible with the formation of encapsulated atomic contacts. However, direct independent structural evidence is still needed to confirm or infirm this interpretation.

Impact of Reabsorption on the Emission Spectra and Recombination Dynamics of Hybrid Perovskite Single Crystals.

Understanding the surface properties of organic-inorganic lead-based perovskites is of high importance to improve the device's performance. Here, we have investigated the differences between surface and bulk optical properties of CH3NH3PbBr3 single crystals. Depth-resolved cathodoluminescence was used to probe the near-surface region on a depth of a few microns. In addition, we have studied the transmitted luminescence through thicknesses between 50 and 600 µm. In both experiments, the expected spectral shift due to the reabsorption effect has been precisely calculated. We demonstrate that reabsorption explains the important variations reported for the emission energy of single crystals. Single crystals are partially transparent to their own luminescence, and radiative transport is the dominant mechanism for propagation of the excitation in thick crystals. The transmitted luminescence dynamics are characterized by a long rise time and a lengthening of their decay due to photon recycling and light trapping.

Surface and Electrochemical Properties of Polymer Brush-Based Redox Poly(Ionic Liquid).

Redox-active poly(ionic liquid) poly(3-(2-methacryloyloxy ethyl)-1-(N-(ferrocenylmethyl) imidazolium bis(trifluoromethylsulfonyl)imide deposited onto electrode surfaces has been prepared using surface-initiated atom transfer radical polymerization SI-ATRP. The process starts by electrochemical immobilization of initiator layer, and then methacrylate monomer carrying ferrocene and imidazolium units is polymerized in ionic liquid media via SI-ATRP process. The surfaces analyses of the polymer exhibit a well-defined polymer brushlike structure and confirm the presence of ferrocene and ionic moieties within the film. Furthermore, the electrochemical investigations of poly(redox-active ionic liquid) in different media demonstrate that the electron transfer is not restricted by the rate of counterion migration into/out of the polymer. The attractive electrochemical performance of these materials is further demonstrated by performing electrochemical measurement, of poly(ferrocene ionic liquid), in solvent-free electrolyte. The facile synthesis of such highly ordered electroactive materials based ionic liquid could be useful for the fabrication of nanostructured electrode suitable for performing electrochemistry in solvent free electrolyte. We also demonstrate possible applications of the poly(FcIL) as electrochemically reversible surface wettability system and as electrochemical sensor for the catalytic activity toward the oxidation of tyrosine.