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.

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.

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.

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.

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.

Temperature-Driven Transformation of CsPbBr3 Nanoplatelets into Mosaic Nanotiles in Solution through Self-Assembly.

Two-dimensional colloidal halide perovskite nanocrystals are promising materials for light-emitting applications. Recent studies have focused on nanoplatelets that are able to self-assemble and transform on solid substrates. However, the mechanism behind the process and the atomic arrangement of their assemblies remain unclear. Here, we present a detailed analysis of the transformation of self-assembled stacks of CsPbBr3 nanoplatelets in solution over a period of a few months by using ex situ transmission electron microscopy and surface analysis. We demonstrate that the transformation mechanism can be understood as oriented attachment, proceeding through the following steps: (i) desorption of the ligands from the surfaces of the particles, causing the seamless atomic merging of nanoplatelet stacks into nanobelts; (ii) merging of neighboring nanobelts that form more extended nanoplates; and (iii) attachment of nanobelts and nanoplates, forming objects with an atomic structure that resembles a mosaic made of broken nanotiles. We reveal that aged nanobelts and nanoplates, which are mainly stabilized by amine/ammonium ions, link through a bilayer of CsBr, with the atomic columns of neighboring perovskite lattices shifted by a half-unit-cell, forming Ruddlesden-Popper planar faults. We also show, via in situ monitoring of the nanocrystal photoluminescence combined with transmission electron microscopy analysis, that the transformation is temperature driven and that it can take place within tens of minutes in solution and in spin-coated films. Understanding this process gives crucial information for the design and fabrication of perovskite materials, where control over the type and density of defects is desired, stimulating the development of perovskite nanocrystal structures with tailored electronic properties.

Nano- and microscale apertures in metal films fabricated by colloidal lithography with perovskite nanocrystals.

We demonstrate patterning of metal surfaces based on lift-off of perovskite nanocrystals that enables the fabrication of nanometer-size features without the use of resist-based nanolithography. The perovskite nanocrystals act as templates for defining the shape of the apertures in metal layers, and we exploit the variety of sizes and shapes that can be controlled in the colloidal synthesis to demonstrate the fabrication of nanoholes, nanogaps and guides with size smaller than the wavelength of light in the visible spectrum. The process can be readily integrated with standard lithography and etching techniques for the creation of more complex structures.

A Semi-Classical View on Epsilon-Near-Zero Resonant Tunneling Modes in Metal/Insulator/Metal Nanocavities.

Metal/Insulator/Metal nanocavities (MIMs) are highly versatile systems for nanometric light confinement and waveguiding, and their optical properties are mostly interpreted in terms of surface plasmon polaritons. Although classic electromagnetic theory accurately describes their behavior, it often lacks physical insight, leaving some fundamental aspects of light interaction with these structures unexplored. In this work, we elaborate a quantum mechanical description of the MIM cavity as a double barrier quantum well. We identify the square of the imaginary part κ of the refractive index ñ of the metal as the optical potential and find that MIM cavity resonances are suppressed if the ratio n/κ exceeds a certain limit, which shows that low n and high κ values are desired for strong and sharp cavity resonances. Interestingly, the spectral regions of cavity mode suppression correspond to the interband transitions of the metals, where the optical processes are intrinsically non-Hermitian. The quantum treatment allows to describe the tunnel effect for photons and reveals that the MIM cavity resonances can be excited by resonant tunneling via illumination through the metal, without the need of momentum matching techniques such as prisms or grating couplers. By combining this analysis with spectroscopic ellipsometry on experimental MIM structures and by developing a simple harmonic oscillator model of the MIM for the calculation of its effective permittivity, we show that the cavity eigenmodes coincide with low-loss zeros of the effective permittivity. Therefore, the MIM resonances correspond to epsilon-near-zero (ENZ) eigenmodes that can be excited via resonant tunneling. Our approach provides a toolbox for the engineering of ENZ resonances throughout the entire visible range, which we demonstrate experimentally and theoretically. In particular, we apply our quantum mechanical approach to asymmetric MIM superabsorbers and use it for configuring broadly tunable refractive index sensors. Our work elucidates the role of MIM cavities as photonic analogues to tunnel diodes and opens new perspectives for metamaterials with designed ENZ response.

Extending the Colloidal Transition Metal Dichalcogenide Library to ReS2 Nanosheets for Application in Gas Sensing and Electrocatalysis.

Among the large family of transition metal dichalcogenides, recently ReS2 has stood out due to its nearly layer-independent optoelectronic and physicochemical properties related to its 1T distorted octahedral structure. This structure leads to strong in-plane anisotropy, and the presence of active sites at its surface makes ReS2 interesting for gas sensing and catalysts applications. However, current fabrication methods use chemical or physical vapor deposition (CVD or PVD) processes that are costly, time-consuming and complex, therefore limiting its large-scale production and exploitation. To address this issue, a colloidal synthesis approach is developed, which allows the production of ReS2 at temperatures below 360 °C and with reaction times shorter than 2h. By combining the solution-based synthesis with surface functionalization strategies, the feasibility of colloidal ReS2 nanosheet films for sensing different gases is demonstrated with highly competitive performance in comparison with devices built with CVD-grown ReS2 and MoS2 . In addition, the integration of the ReS2 nanosheet films in assemblies together with carbon nanotubes allows to fabricate electrodes for electrocatalysis for H2 production in both acid and alkaline conditions. Results from proof-of-principle devices show an electrocatalytic overpotential competitive with devices based on ReS2 produced by CVD, and even with MoS2 , WS2 , and MoSe2 electrocatalysts.

Benzoyl Halides as Alternative Precursors for the Colloidal Synthesis of Lead-Based Halide Perovskite Nanocrystals.

We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic-inorganic lead halide perovskite nanocrystals (NCs). The main limitation of the protocols that are currently in use, such as the hot injection and the ligand-assisted reprecipitation routes, is that they employ PbX2 (X = Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations (mainly in the form of metal carboxylates) for the synthesis of APbX3 NCs (in which A = Cs+, CH3NH3+, or CH(NH2)2+). In this way, it is possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX3 NCs that were prepared with our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission thresholds, and enhanced stability in air. It is noteworthy that CsPbI3 NCs, which crystallize in the cubic α phase, are stable in air for weeks without any postsynthesis treatment. The improved properties of our CsPbX3 perovskite NCs can be ascribed to the formation of lead halide terminated surfaces, in which Cs cations are replaced by alkylammonium ions.

Colloidal Monolayer ß-In2Se3 Nanosheets with High Photoresponsivity.

We report a low-temperature colloidal synthesis of single-layer, five-atom-thick, ß-In2Se3 nanosheets with lateral sizes tunable from ∼300 to ∼900 nm, using short aminonitriles (dicyandiamide or cyanamide) as shape controlling agents. The phase and the monolayer nature of the nanosheets were ascertained by analyzing the intensity ratio between two diffraction peaks from two-dimensional slabs of the various phases, determined by diffraction simulations. These findings were further backed-up by comparing and fitting the experimental X-ray diffraction pattern with Debye formula simulated patterns and with side-view high-resolution transmission electron microscopy imaging and simulation. The ß-In2Se3 nanosheets were found to be indirect band gap semiconductors (Eg = 1.55 eV), and single nanosheet photodetectors demonstrated high photoresponsivity and fast response times.

Confined Acoustic Phonons in Colloidal Nanorod Heterostructures Investigated by Nonresonant Raman Spectroscopy and Finite Elements Simulations.

Lattice vibrational modes in cadmium chalcogenide nanocrystals (NCs) have a strong impact on the carrier dynamics of excitons in such confined systems and on the optical properties of these nanomaterials. A prominent material for light emitting applications are CdSe/CdS core-shell dot-in-rods. Here we present a detailed investigation of the acoustic phonon modes in such dot-in-rods by nonresonant Raman spectroscopy with laser excitation energy lower than their bandgap. With high signal-to-noise ratio in the frequency range from 5-50 cm-1, we reveal distinct Raman bands that can be related to confined extensional and radial-breathing modes (RBM). Comparison of the experimental results with finite elements simulation and analytical analysis gives detailed insight into the localized nature of the acoustic vibration modes and their resonant frequencies. In particular, the RBM of dot-in-rods cannot be understood by an oscillation of a CdSe sphere embedded in a CdS rod matrix. Instead, the dot-in-rod architecture leads to a reduction of the sound velocity in the core region of the rod, which results in a redshift of the rod RBM frequency and localization of the phonon induced strain in vicinity of the core where optical transitions occur. Such localized effects potentially can be exploited as a tool to tune exciton-phonon coupling in nanocrystal heterostructures.

Single-mode lasing from colloidal water-soluble CdSe/CdS quantum dot-in-rods.

Core-shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution-phase synthesis is based on surface-stabilizing ligands that make them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties poses a major challenge. This work demonstrates how core-shell CdSe/CdS quantum dot-in-rods can be transferred into water using a ligand exchange method employing mercaptopropionic acid (MPA). Key to maintaining the light-emitting properties is an enlarged CdS rod diameter, which prevents potential surface defects formed during the ligand exchange from affecting the photophysics of the dot-in-rods. Films made from water-soluble dot-in-rods show amplified spontaneous emission (ASE) with a similar threshold (130 µJ/cm(2)) as the pristine material (115 µJ/cm(2)). To demonstrate feasibility for lasing applications, self-assembled microlasers are fabricated via the "coffee-ring effect" that display single-mode operation and a very low threshold of ∼10 µJ/cm(2). The performance of these microlasers is enhanced by the small size of MPA ligands, enabling a high packing density of the dot-in-rods.

Writing and functionalisation of suspended DNA nanowires on superhydrophobic pillar arrays.

Nanowire arrays and networks with precisely controlled patterns are very interesting for innovative device concepts in mesoscopic physics. In particular, DNA templates have proven to be versatile for the fabrication of complex structures that obtained functionality via combinations with other materials, for example by functionalisation with molecules or nanoparticles, or by coating with metals. Here, the controlled motion of the a three-phase contact line (TCL) of DNA-loaded drops on superhydrophobic substrates is used to fabricate suspended nanowire arrays. In particular, the deposition of DNA wires is imaged in situ, and different patterns are obtained on hexagonal pillar arrays by controlling the TCL velocity and direction. Robust conductive wires and networks are achieved by coating the wires with a thin layer of gold, and as proof of concept conductivity measurements are performed on single suspended wires. The plastic material of the superhydrophobic pillars ensures electrical isolation from the substrate. The more general versatility of these suspended nanowire networks as functional templates is outlined by fabricating hybrid organic-metal-semiconductor nanowires by growing ZnO nanocrystals onto the metal-coated nanowires.

Nanocrystal film patterning by inhibiting cation exchange via electron-beam or X-ray lithography.

In this Letter we report patterning of colloidal nanocrystal films that combines direct e-beam (electron beam) writing with cation exchange. The e-beam irradiation causes cross-linking of the ligand molecules present at the nanocrystal surface, and the cross-linked molecules act as a mask for further processing. Consequently, in the following step of cation exchange, which is performed by directly dipping the substrate in a solution containing the new cations, the regions that have not been exposed to the electron beam are chemically transformed, while the exposed ones remain unchanged. This selective protection allows the design of patterns that are formed by chemically different nanocrystals, yet in a homogeneous nanocrystal film. Spatially resolved compositional analysis by energy-dispersive X-ray spectroscopy (EDS) corroborates that the selective exchange occurs only in the nonirradiated regions. We demonstrate the utility of this lithography approach by fabricating conductive wires and luminescent patterns in CdSe/CdS nanocrystal films by converting nonirradiated regions to Cu2-xSe/Cu2-xS. Furthermore, we show that X-ray irradiation too can lead to protection from cation exchange.

Amyloid ß peptide conformational changes in the presence of a lipid membrane system.

Here we are presenting a comparative analysis of conformational changes of two amyloid ß peptides, Aß(25-35) and Aß(1-42), in the presence and absence of a phospholipid system, namely, POPC/POPS (1-palmitoyl-2-oleoylphospatidylcholine/palmitoyl-2-oleoylphospatidylserine), through Raman spectroscopy, synchrotron radiation micro Fourier-transform infrared spectroscopy, and micro X-ray diffraction. Ringlike samples were obtained from the evaporation of pure and mixed solutions of the proteins together with the POPC/POPS system on highly hydrophilic substrates. The results confirm the presence of a α-helical to ß-sheet transition from the internal rim of the ringlike samples to the external one in the pure Aß(25-35) residual, probably due to the convective flow inside the droplets sitting on highly hydrophilic substrates enhancing the local concentration of the peptide at the external edge of the dried drop. In contrast, the presence of POPC/POPS lipids in the peptide does not result in α-helical structures and introduces the presence of antiparallel ß-sheet material together with parallel ß-sheet structures and possible ß-turns. As a control, Aß(1-42) peptide was also tested and shows ß-sheet conformations independently from the presence of the lipid system. The µXRD analysis further confirmed these conclusions, showing how the absence of the phospholipid system induces in the Aß(25-35) a probable composite α/ß material while its coexistence with the peptide leads to a not oriented ß-sheet conformation. These results open interesting scenarios on the study of conformational changes of Aß peptides and could help, with further investigations, to better clarify the role of enzymes and alternative lipid systems involved in the amyloidosis process of Aß fragments.

Plasmon resonance tuning in metal nanostars for surface enhanced Raman scattering.

We report the fabrication of Au nanostar arrays by means of electron beam lithography, in which the plasmon resonance energy can be tuned via the nanostar size from the visible into the near-infrared region. The spectral response of the nanostar arrays was investigated by optical extinction (transmittance) experiments, and their surface enhanced Raman scattering performance has been tested at two different excitation wavelengths, 633 nm and 830 nm, using chemisorbed Cresyl violet molecules as analyte. The experimental results are supported by numerical simulations of the spatial and spectral electric field enhancement.

Heterostructures via a Solution-Based Anion Exchange in Microcrystalline 2D Layered Metal-Halide Perovskites.

Layered perovskites consist of stacks of inorganic semiconducting metal-halide octahedra lattices sandwiched between organic layers with typically dielectric behavior. The in-plane confinement of electrical carriers in such two-dimensional metal halide perovskites drives a large range of appealing electronic properties, such as strong exciton binding, anisotropic charge diffusion, and polarization-directionality. Heterostructures provide additional control on carrier diffusion and localization, and in-plane heterojunctions are interesting because of the associated high charge mobility. Here, this work demonstrates a versatile solution-based approach to fabricate in-plane heterostructures with different halide composition in two-dimensional lead-halide perovskite microcrystals. This leads to spatially separated halide phases with different band gap and light emission. Interestingly, the composition of the exchanged phase and the morphology of the phase boundary depends on the exchange route, which can be related to the preferred localization of the halides at the equatorial or axial octahedra positions that either leads to dissolution and recrystallization of the octahedra lattice (for bromide to iodide), or allows for ion diffusion within the lattice (for iodide to bromide). These detailed insights on the ion exchange processes in layered perovskites will stimulate the development of heterostructures that can be tailored for different applications such as photocatalysis, energy storage, and light emission.