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Nanoporous metals are a class of nanostructured materials finding extensive applications in multiple fields thanks to their unique properties attributed to their high surface area and interconnected nanoscale ligaments. They can be prepared following different strategies, but the deposition of an arbitrary pure porous metal is still challenging. Recently, a dry synthesis of nanoporous films based on the plasma treatment of metal thin layers deposited by physical vapour deposition has been demonstrated, as a general route to form pure nanoporous films from a large set of metals. An interesting aspect related to this approach is the possibility to apply the same methodology to deposit the porous films as a multilayer. In this way, it is possible to explore the properties of different porous metals in close contact. As demonstrated in this paper, interesting plasmonic properties emerge in a nanoporous Au–Ag bi-layer. The versatility of the method coupled with the possibility to include many different metals, provides an opportunity to tailor their optical resonances and to exploit the chemical and mechanical properties of components, which is of great interest to applications ranging from sensing, to photochemistry and photocatalysis. [ABSTRACT FROM AUTHOR]

Bio‐inspired and biodegradable quantum optics scenarios constitute a pathway toward environmentally friendly front‐end technologies. Such an inspiring perspective necessitates the replacement of classic gain materials with a biological counterpart like photoluminescent bacteria. It is easy to imagine that, in this case, a planar and cell‐viable substitute of classic bulk solid‐states resonators can be highly beneficial. In this paper a micro‐ and nano‐photonic structuration of both a standard and a functionalized version of a typical bacterial growth medium (Luria‐Bertani Agar – LBA) is successfully realized. Three structures belonging to the categories of photonic crystals are replicated, such as quasi‐crystals and meta‐surfaces, demonstrating how the proposed media can be used as templates for high‐end photonic applications. The optical quality of the replicated structures is confirmed by far‐field diffraction measurements. The structured growth media allow for a broad control of the surface wettability by accessing a so‐called Wenzel state, in which the original hydrophilicity of a material is increased due to the photonic structuration. Finally, the suitability of the nano‐structured LBA as a plasmonic platform is evidenced. The proposed micro‐and nano‐structured photonic growth media constitute the first, fundamental step toward quantum optical frameworks from biological media. [ABSTRACT FROM AUTHOR]

Plasmonic solid‐state nanopores with tunable hole diameters can be prepared via a photocatalytic effect resulting from the enhanced electromagnetic (EM) field inside a metallic ring on top of a dielectric nanotube. Under white light illumination, the plasmon‐enhanced EM‐field induces a site‐selective metal nucleation and growth within the ring. This approach is used to prepare Au and bimetallic Au–Ag nano‐rings and demonstrate the reduction of the initial inner diameter of the nanopore down to 4 nm. The tunability of the nanopore diameter can be used to enable optimized detection of single entities with different sizes. As a proof‐of‐concept, single object detection of double stranded DNA (dsDNA) and Au nanoparticles (AuNPs) with a diameter down to 15 nm is performed. Numerical simulations provide insights into the EM‐field distribution and confinement, showing that a field intensity enhancement of up to 104 can be achieved inside the nanopores. This localized EM‐field can be used to perform enhanced optical measurements and generate local heating, thereby modifying the properties of the nanopore. Such a flexible approach also represents a valuable tool to investigate plasmon‐driven photochemical reactions, and it can represent an important step toward the realization of new plasmonic devices. [ABSTRACT FROM AUTHOR]

Ferrell and Berreman modes are absorption resonances in thin metal films and polar-dielectric media that arise from radiative bulk plasmon-polariton and phonon-polariton excitations. Compared to surface polaritons, Ferrell and Berreman modes occur due to volume charge oscillations across the medium and provide a unique pathway for light–matter interactions. Though the resonances are studied individually, stringent polarization and material requirements have prevented their observation in one host medium. Here, we show simultaneous excitation of Ferrell and Berreman absorption resonances in refractory epitaxial TiN/Al0.72Sc0.28N plasmonic metal/polar-dielectric hyperbolic metamaterials in the visible and far-infrared spectral ranges. The nanoscale periodicity of the superlattices enables the coupling of bulk plasmons (and longitudinal optical phonons) across different TiN (and Al0.72Sc0.28N) layers and allows polarization matching with free-space light that results in Ferrell (and Berreman) mode excitations. Ferrell and Berreman absorption resonances can be used for strong light confinement in radiative cooling, thermophotovoltaics, and other dual-band applications. [ABSTRACT FROM AUTHOR]

The strong coupling of optical resonators results in a mode splitting proportional to the coupling strength, which can be achieved with metal‐dielectric‐metal (MDM) cavities that have similar thickness and refractive index. However, an active control of the mode coupling is challenging. Here, an alternative configuration of an MDM cavity coupled with a Guided‐Mode‐Resonator (GMR) is explored. The GMR grating is fabricated on top of the MDM cavity, such that the coupling can be tuned by the thickness of the central metal. The typical modal anti‐crossing (with a splitting of ≈50–65 meV) detected in the angular dispersion of the GMR‐MDM is observed. Excellent agreement of the experimental reflectance with simulations allows to report the angular dispersion from −50° to 50°. The anisotropy of the GMR modes enables to switch in and out of the strong coupling by changing the polarization of the incident light. Moreover, the asymmetry of the GMR‐MDM architecture induces a side‐dependent response. The MDM cavity is only transparent at its resonances, which leads to a suppression of all modes outside the resonance bands when impinging from the MDM side. These features are highly interesting for optoelectronic applications such as optical switches and multi‐level optical multiplexing. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Light‐emitting diodes (LEDs) are presently not exploited at their full potential due to a multiple range of detrimental effects, such as generation of local modes, either surface plasmon‐like or optical modes, ohmic losses, or total internal reflection caused by refractive index mismatch. Herein, the authors focus on LEDs with layered architectures by investigating the effects of interface nanopatterning occurring in the electron‐transport layer on their overall performance. In particular, the integration of photonic nanostructures is investigated, while leaving the optical properties of the emitting material unchanged. The numerical results show that specifically designed nanopatterning can strongly contribute to the enhancement of the Purcell factor, outcoupling efficiency and thereby increase the external quantum efficiency by a factor of 2 with respect to the unpatterned structures. This demonstrates that, as in other light‐emission technologies, morphological patterning is a valid strategy to improve the performance of LEDs. [ABSTRACT FROM AUTHOR]

Metal‐halide perovskites (PSKs) are emergent materials for a large range of applications, and the layered double PSK architectures vastly enrich the opportunities to design their composition, structural properties, and optoelectronic behavior. The stability, crystal phase, and electronic bandgap depend strongly on the bonds and distortions of the octahedra lattice that are at the origin of the vibrational spectrum of these materials. This work investigates the structural dynamics of flakes of exfoliated layered Ag‐Bi bromide double PSKs by angle‐dependent polarized Raman spectroscopy and density functional theory modeling. The well‐defined orientation of the inorganic octahedra lattice with respect to the light polarization allows to correlate the angle‐dependent intensity of the Raman signal to the directionality and symmetry of the phonon modes. Low‐frequency vibrations are revealed for which a detailed microscopic and group theory assignment of the Raman modes is provided. The temperature‐dependent measurements across the phase transitions show marked changes in the phonon frequencies, reveal soft modes, and help to distinguish first from second‐order transitions as well as to determine their transition temperature. This provides highly valuable insights to improve the properties of this class of Pb‐free PSKs for applications in energy harvesting and optoelectronics. [ABSTRACT FROM AUTHOR]

All‐inorganic metal halide perovskites are highly attractive materials for optoelectronics and are typically implemented as thin films, nanocrystals, and single crystals with macroscopic and millimeter‐size dimensions. Single crystals with regular shapes in the micrometer range are less explored and present an interesting alternative when a well‐defined and spatially localized response is desired. Here, CsPbBr3 microcrystals are fabricated by simple and fast drop‐casting of a precursor solution on several substrates that for the growth process are heated to temperatures in the range from 80 to 150 °C. The microcrystals have a cubic shape and feature a pyramidal cavity on their top surface that forms due to faster growth at the edges once the growing crystal is attached to the substrate. Distinct heterogeneity in the photoluminescence (PL) and conductivity of the different facets are measured by µ‐PL and conductive atomic force microscopy experiments. Toward device applications, the authors contact single microcrystals on conductive substrates mechanically with a micromanipulator tip. They observe diode‐like current–voltage curves and good photodetector functionality, obtaining a high responsivity of 150–300 A W−1 in the blue–green spectral region under forward bias, and a sharp detection band centered around 540 nm with peak responsivity close to 0.7 A W−1 under reverse bias. [ABSTRACT FROM AUTHOR]

Strong light–matter interaction is usually achieved by embedding a gain medium in a high‐quality (Q)‐factor cavity made with thick external mirrors. The high reflectivity of the mirrors poses, therefore, a trade‐off between pump radiation coupling efficiency and high Q‐factor, thus preventing the use of optical cavities in photonic contexts where the photosensitive element necessitates to be readily accessible. Here, this long‐standing problem is overcome by engineering "Pseudo‐Cavity Modes" (PCM) in a Metal/Dielectric Open Cavity (MDOC) system that has been commonly thought to be incapable to show resonances. The MDOC system lets the embedded fluorophore exposed to radiation, opening to strong light/matter interaction. To prove this: i) exciton/PCM strong coupling regime, with 160 meV Rabi splitting, ii) photoluminescence enhancement by more than 7 times, and iii) polarized and angle‐dependent spontaneous emission are experimentally and theoretically demonstrated. The proposed system holds great potential envisioning scenarios like cavity‐enhanced photovoltaics, surface‐enhanced Raman scattering, open‐cavity lasers, photoluminescence steering and polarization. [ABSTRACT FROM AUTHOR]

Mixed‐dimensional systems host interesting phenomena that involve electron and ion transport along or across the interface, with promising applications in optoelectronic and electrochemical devices. Herein, a heterosystem consisting of a graphene monolayer with a colloidal Ag2S nanocrystal film atop, in which both ions and electrons are involved in photoelectrical effects, is studied. An investigation of the transport at the interface in different configurations by using a phototransistor configuration with graphene as a charge‐transport layer and semiconductor nanocrystals as a light‐sensitive layer is performed. The key feature of charge transfer is investigated as a function of gate voltage, frequency, and incident light power. A simple analytical model of the photoresponse is developed, to gain information on the device operation, revealing that the nanocrystals transfer electrons to graphene in the dark, but the opposite process occurs upon illumination. A frequency‐dependence analysis suggests a fractal interface between the two materials. This interface can be modified using solid‐state electrochemical reactions, leading to the formation of metallic Ag particles, which affect the graphene properties by additional doping, while keeping the photoresponse. Overall, these results provide analytical tools and guidelines for the evaluation of coupled electron/ion transport in hybrid systems. [ABSTRACT FROM AUTHOR]

Gallium selenide (GaSe), a layered semiconductor of Group‐III monochalcogenides, has been recognized by the scientific community in recent years as an appealing material in the fields of photonics and (opto)electronics. Thanks to its pseudodirect bandgap and its thickness‐dependent (opto)electronic properties, GaSe has emerged as a promising candidate for the implementation of thin‐film transistors (TFTs) and photodetectors with fast response and high sensitivity. Solution processing of 2D materials provides low‐cost inks that allow the design and realization of printed electronic devices, enabling this technology to move from the laboratory to the industry. In this work, a solution‐processed GaSe‐based light‐driven transistor is presented. Liquid phase exfoliation (LPE) is used to exfoliate bulk GaSe in isopropanol, formulating a functional ink that is subsequently deposited by spray coating onto Si/SiO2 substrates. The GaSe phototransistor exhibits a p‐channel behavior with a high on/off ratio (≈103) that is gate‐voltage dependent. Moreover, the device response also depends on the illumination with a maximum responsivity of 13 A W–1 to UV–visible light and a fast response time of 35 ms. This study demonstrates that liquid phase exfoliated GaSe is a promising candidate for the design and realization of next‐generation (opto)electronic devices. [ABSTRACT FROM AUTHOR]

Ultrafast control of light−matter interactions is fundamental in view of new technological frontiers of information processing. However, conventional optical elements are either static or feature switching speeds that are extremely low with respect to the time scales at which it is possible to control light. Here, we exploit the artificial epsilon-near-zero (ENZ) modes of a metal-insulator-metal nanocavity to tailor the linear photon absorption of our system and realize a nondegenerate all-optical ultrafast modulation of the reflectance at a specific wavelength. Optical pumping of the system at its high energy ENZ mode leads to a strong redshift of the low energy mode because of the transient increase of the local dielectric function, which leads to a sub-3-ps control of the reflectance at a specific wavelength with a relative modulation depth approaching 120%. All-optical switching allows control of one optical signal using another, holding potential to overcome the limitations of electrical switches via ultrafast manipulation of light. In this work, sub-3 ps all-optical switching is achieved in an epsilon-near-zero nanocavity, exhibiting a relative modulation depth of 120% at a specific wavelength. [ABSTRACT FROM AUTHOR]

Metamaterials have recently established a new paradigm for enhanced light absorption in state-of-the-art photodetectors. Here, we demonstrate broadband, highly efficient, polarization-insensitive, and gate-tunable photodetection at room temperature in a novel metadevice based on gold/graphene Sierpinski carpet plasmonic fractals. We observed an unprecedented internal quantum efficiency up to 100% from the near-infrared to the visible range with an upper bound of optical detectivity of 1011 Jones and a gain up to 106, which is a fingerprint of multiple hot carriers photogenerated in graphene. Also, we show a 100-fold enhanced photodetection due to highly focused (up to a record factor of |E/E0| ≈ 20 for graphene) electromagnetic fields induced by electrically tunable multimodal plasmons, spatially localized in self-similar fashion on the metasurface. Our findings give direct insight into the physical processes governing graphene plasmonic fractal metamaterials. The proposed structure represents a promising route for the realization of a broadband, compact, and active platform for future optoelectronic devices including multiband bio/chemical and light sensors. [ABSTRACT FROM AUTHOR]

Layered semiconductors of the IIIA–VIA group have attracted considerable attention in (opto)electronic applications thanks to their atomically thin structures and their thickness‐dependent optical and electronic properties, which promise ultrafast response and high sensitivity. In particular, 2D indium selenide (InSe) has emerged as a promising candidate for the realization of thin‐film field effect transistors and phototransistors due to its high intrinsic mobility (>102 cm2 V−1 s−1) and the direct optical transitions in an energy range suitable for visible and near‐infrared light detection. A key requirement for the exploitation of large‐scale (opto)electronic applications relies on the development of low‐cost and industrially relevant 2D material production processes, such as liquid phase exfoliation, combined with the availability of high‐throughput device fabrication methods. Here, a β polymorph of indium selenide (β‐InSe) is exfoliated in isopropanol and spray‐coated InSe‐based photodetectors are demonstrated, exhibiting high responsivity to visible light (maximum value of 274 A W−1 under blue excitation 455 nm) and fast response time (15 ms). The devices show a gate‐dependent conduction with an n‐channel transistor behavior. Overall, this study establishes that liquid phase exfoliated β‐InSe is a valid candidate for printed high‐performance photodetectors, which is critical for the development of industrial‐scale 2D material‐based optoelectronic devices. [ABSTRACT FROM AUTHOR]

The coupling between multiple nanocavities in close vicinity leads to the hybridization of their modes. Stacked metal-insulator-metal (MIM) nanocavities constitute a highly versatile and very interesting model system to study and engineer such mode coupling, as they can be realized by lithography-free fabrication methods with fine control on the optical and geometrical parameters. The resonant modes of such MIM cavities are epsilon-near-zero (ENZ) resonances, which are appealing for nonlinear photophysics and a variety of applications. Here, we study the hybridization of ENZ resonances in MIMIM nanocavities, obtaining a very large mode splitting reaching 0.477 eV, Q-factors of the order of 40 in the visible spectral range, and fine control on the resonance wavelength and mode linewidth by tuning the thickness of the dielectric and metallic layers. A semiclassical approach that analyzes the MIMIM structure as a double quantum well system allows to derive the exact analytical dispersion relation of the ENZ resonances, achieving perfect agreement with numerical simulations and experiments. Interestingly, the asymmetry of the mode splitting in a symmetric MIMIM cavity is not reflected in the classical model of coupled oscillators, which can be directly related to quantum mechanical tunneling for the coupling of the two cavities. Interpreting the cavity resonances as resonant tunneling modes elucidates that they can be excited without momentum matching techniques. The broad tunability of high-quality ENZ resonances together with their strong coupling efficiency makes such MIMIM cavities an ideal platform for exploring light-matter interaction, for example, by the integration of quantum emitters in dielectric layers. [ABSTRACT FROM AUTHOR]

Resistive switching memories allow electrical control of the conductivity of a material, by inducing a high resistance (OFF) or a low resistance (ON) state, using electrochemical and ion transport processes. As an alternative to high temperature and vacuum-based physical sulphurization methods of silver (Ag), here we propose, as a resistive switching medium, a layer built from colloidal Ag2−xS nanocrystals – compatible with solution-processed approaches. The effects of the electrode size (from macro- to micro-scale), composition (Ag, Ti and Pt) and geometry on the device performance together with the electrochemical mechanisms involved are evaluated. We achieved an optimized Ag/Ti bowtie proof-of-concept configuration by e-beam lithography, which fulfils the general requirements for ReRAM devices in terms of low power consumption and a reliable ION/IOFF ratio. This configuration demonstrates reproducible switching between ON and OFF states with data endurance of at least 20 cycles; and an ION/IOFF ratio up to 103 at low power consumption (0.1 V read-out), which outperforms previous results in the literature for devices with resistive layers fabricated from silver chalcogenide nanoparticles. [ABSTRACT FROM AUTHOR]

Cation exchange is a versatile tool to control the composition of nanocrystals, and recently deterministic patterning could be achieved by combining it with lithography techniques. Regarding single nanocrystal structures, such spatial control of cation exchange enables the design of heterostructures, which can be integrated in functional optoelectronic elements. In this work, we fabricate nanowire CdSe/Cu2Se heterojunctions by masking cation exchange via electron-beam irradiation, such that cation exchange proceeds only in the non-irradiated sections. Interestingly, the heterojunction interfaces are almost atomically sharp, and the adjacent CdSe and Cu2Se domains exhibit epitaxial relationships. We show that the cation exchange at the CdSe/Cu2Se interface is only possible if the displaced Cd2+ ions can radially out-diffuse to the solution phase. If this exit pathway is blocked, the cation exchange cannot occur. Our technique allows one to transform already contacted single nanowires, and the obtained heterojunction nanowires manifest a noticeable gain in conductance. [ABSTRACT FROM AUTHOR]

Perovskite-based optoelectronic devices have shown remarkable performances, especially in the field of photovoltaics. Still, a rapid solution-processing approach able to produce localized stable perovskite crystals remains a general challenge, and is a key step toward the miniaturization of such materials in on-chip components. This study presents the confined growth of methylammonium (MA) lead halide perovskite crystals that is thermally induced through localized laser irradiation. Importantly, such structures remain stable over time; that is, they neither dissolve back into the surrounding liquid nor detach from the substrate. This is attributed to a chemical reaction locally triggered by the induced heat on the substrate surface that is transferred to the perovskite precursors (liquid) layer, thus generating 'on-demand' MA ions from the N-methylformamide solvent. By tuning the laser parameters, such as power density or irradiation time, variations in shape and size of the crystals, from microcrystals of ≈50 µm to nanocuboids of ≈500 nm, are observed. This study also demonstrates that with an optimized distance between the irradiated regions and by controlling the relative laser displacement speed, luminescent and photoconductive MAPbBr3 wires and microplates can be generated. [ABSTRACT FROM AUTHOR]

The integration of colloidal nanocrystals with polymers adds optoelectronic functionalities to flexible and mechanically robust organic films. In particular, self-assembled structures of nanocrystals in polymers can act as functional components enhancing, for instance, transport or optical properties of the hybrid material. This study presents Cu2Te hexagonal nanodisks that assemble into ribbons with a face-to-face configuration in poly(3-hexylthiophene-2,5-diyl) through a controlled solvent evaporation process. The ribbons form weaving patterns that create 3D networks fully embedded in the thin polymer film at high nanodisk concentration. The photoresponse of these composite films measured in a layered vertical geometry demonstrates increased photocurrent with increasing nanocrystal loading. This study attributes this behavior to the presence of networks of Cu2Te nanodisks that form a bulk heterojunction with the semiconducting polymer, which improves exciton dissociation and the overall photoelectric response. [ABSTRACT FROM AUTHOR]

A novel colloidal synthesis of copper selenide nanosheets (NSs) with lateral dimensions of up to 3 μm is developed. This material is used for the fabrication of flexible conductive films prepared via simple drop-casting of the NS dispersions without any additional treatment. The electrical performance of these coatings is benchmarked against copper selenide spherical nanocrystals (SNCs) in order to demonstrate the advantage of 2D morphology of the NSs for flexible electronics. In this contest, Cu2−xSe SNC films exhibit higher conductivity but lower reproducibility due to the formation of cracks leading to discontinuous films. Furthermore, the electrical properties of the films deposited on different flexible substrates following their bending, stretching and folding are studied. A comparison of Cu2−xSe SNC and CuSe NS films reveals an increased stability of the CuSe NS films under mechanical stress applied to the samples and their improved long-term stability in air. [ABSTRACT FROM AUTHOR]

Layered perovskites are useful materials that contain sheets of a perovskite semiconductor enclosed by organic molecules. Crystals of layered perovskites that include sheets of a second inorganic lattice can now be grown from solution.Single crystals of a multilayered material grown from solution. [ABSTRACT FROM AUTHOR]

The deposition of water solution containing colloidal nanorods on superhydrophobic substrates leads to the formation of a suspended drop. After evaporation, a robust 3D nanocrystal assembly remains and can be handled independently from the substrate, for example for the use as color converters. Optical fluorescence imaging reveals a hollow dome‐shape of the microstructure, and X‐ray diffraction confirms the material as CdSe/CdS nanorods with low contamination. [ABSTRACT FROM AUTHOR]

Solution-processed inorganic and organic materials have been pursued for more than a decade as low-threshold, high-gain lasing media, motivated in large part by their tunable optoelectronic properties and ease of synthesis and processing. Although both have demonstrated stimulated emission and lasing, they have not yet approached the continuous-wave pumping regime. Two-dimensional CdSe colloidal nanosheets combine the advantage of solution synthesis with the optoelectronic properties of epitaxial two-dimensional quantum wells. Here, we show that these colloidal quantum wells possess large exciton and biexciton binding energies of 132 meV and 30 meV, respectively, giving rise to stimulated emission from biexcitons at room temperature. Under femtosecond pulsed excitation, close-packed thin films yield an ultralow stimulated emission threshold of 6 μJ cm-2, sufficient to achieve continuous-wave pumped stimulated emission, and lasing when these layers are embedded in surface-emitting microcavities. [ABSTRACT FROM AUTHOR]