Self-Organized Plasmonic Nanowire Arrays Coated with Ultrathin TiO2 Films for Photoelectrochemical Energy Storage.

The strategic field of renewable energy production and storage requires novel nanoscale platforms that can feature competitive solar energy conversion properties. Photochemical reactions that promote energy storage, such as water splitting and oxygen-hydrogen evolution reactions, play a crucial role in this context. Here, we demonstrate a novel photoelectrochemical device based on large-area (cm2) self-organized Au nanowire (NW) arrays, uniformly coated with ultrathin TiO2 films. The NW arrays act both as transparent nanoelectrodes and as a plasmonic metasurface that resonantly enhances the very weak visible photocurrent generated by a prototype photoelectrochemical oxygen evolution reaction. We demonstrate a polarization-sensitive plasmon-enhanced photocurrent that reaches a gain of about 3.8 in the visible spectral range. This highlights the potential of our novel nanopatterned plasmonic platform in photochemistry and energy storage.

Thermal Scanning-Probe Lithography for Broad-Band On-Demand Plasmonic Nanostructures on Transparent Substrates.

Thermal scanning-probe lithography (t-SPL) is a high-resolution nanolithography technique that enables the nanopatterning of thermosensitive materials by means of a heated silicon tip. It does not require alignment markers and gives the possibility to assess the morphology of the sample in a noninvasive way before, during, and after the patterning. In order to exploit t-SPL at its peak performances, the writing process requires applying an electric bias between the scanning hot tip and the sample, thereby restricting its application to conductive, optically opaque, substrates. In this work, we show a t-SPL-based method, enabling the noninvasive high-resolution nanolithography of photonic nanostructures onto optically transparent substrates across a broad-band visible and near-infrared spectral range. This was possible by intercalating an ultrathin transparent conductive oxide film between the dielectric substrate and the sacrificial patterning layer. This way, nanolithography performances comparable with those typically observed on conventional semiconductor substrates are achieved without significant changes of the optical response of the final sample. We validated this innovative nanolithography approach by engineering periodic arrays of plasmonic nanoantennas and showing the capability to tune their plasmonic response over a broad-band visible and near-infrared spectral range. The optical properties of the obtained systems make them promising candidates for the fabrication of hybrid plasmonic metasurfaces supported onto fragile low-dimensional materials, thus enabling a variety of applications in nanophotonics, sensing, and thermoplasmonics.

Highly efficient sequestration of aqueous lead on nanostructured calcite substrates.

Following defocused ion beam sputtering, large area highly corrugated and faceted nanoripples are formed on calcite (10.4) faces in a self-organized fashion. High resolution atomic force microscopy (AFM) imaging reveals that calcite ripples are defined by facets with highly kinked (11.0) and (21¯.12) terminations.In situAFM imaging during the exposure of such modified calcite surfaces to PbCl2aqueous solution reveals that the nanostructured calcite surface promotes the uptake of Pb. In addition, we observed the progressive smoothing of the highly reactive calcite facet terminations and the formation of Pb-bearing precipitates elongated in registry with the underlying nanopattern. By SEM-EDS analysis we quantified a remarkable 500% increase of the Pb uptake rate, up to 0.5 atomic weight % per hour, on the nanorippled calcite in comparison to its freshly cleaved (10.4) surfaces. These results suggest that nanostructurated calcite surfaces can be used for developing future systems for lead sequestration from polluted waters.

Broadband Photon Harvesting in Organic Photovoltaic Devices Induced by Large-Area Nanogrooved Templates.

Thin-film organic photovoltaic (OPV) devices represent an attractive alternative to conventional silicon solar cells due to their lightweight, flexibility, and low cost. However, the relatively low optical absorption of the OPV active layers still represents an open issue in view of efficient devices that cannot be addressed by adopting conventional light coupling strategies derived from thick PV absorbers. The light coupling to thin-film solar cells can be boosted by nanostructuring the device interfaces at the subwavelength scale. Here, we demonstrate broadband and omnidirectional photon harvesting in thin-film OPV devices enabled by highly ordered one-dimensional (1D) arrays of nanogrooves. Laser interference lithography, in combination with reactive ion etching (RIE), provides the controlled tailoring of the height and periodicity of the silica grooves, enabling effective tuning of the anti-reflection properties in the active organic layer (PTB7:PCBM). With this strategy, we demonstrate a strong enhancement of the optical absorption, as high as 19% with respect to a flat device, over a broadband visible and near-infrared spectrum. The OPV device supported on these optimized nanogrooved substrates yields a 14% increase in short-circuit current over the corresponding flat device, highlighting the potential of this large-scale light-harvesting strategy in the broader context of thin-film technologies.

Flat-optics hybrid MoS2/polymer films for photochemical conversion.

Novel light harvesting platforms and strategies are crucial to develop renewable photon to energy conversion technologies that overcome the current global energy and environmental challenges. Two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductor layers are particularly attractive for photoconversion applications but new ultra-compact photon harvesting schemes are urgently required to mitigate their poor photon absorption properties. Here, we propose a flat-optics scheme based on nanogrooved ultra-thin MoS2 layers conformally grown onto large area (cm2 scale) nanopatterned templates. The subwavelength re-shaping of the 2D-TMD layers promotes the excitation of photonic Rayleigh anomaly (RA) modes, uniquely boosting a strong in-plane electromagnetic confinement. By tailoring the illumination conditions, we demonstrate effective tuning of the photonic anomalies over a broadband visible spectrum across the absorption band of relevant polluting dye molecules. Thanks to the strong photonic in-plane confinement, we achieve a resonant enhancement of the photodissociation rate of methylene blue (MB) molecules, well above a factor of 2. These results highlight the potential of flat-optics photon harvesting schemes for boosting photoconversion efficiency in large-scale hybrid 2D-TMD/polymer layers, with a strong impact in various applications ranging from new-generation photonics to waste water remediation and renewable energy storage.

Free-standing plasmonic nanoarrays for leaky optical waveguiding and sensing.

Flat optics nanogratings supported on thin free-standing membranes offer the opportunity to combine narrowband waveguided modes and Rayleigh anomalies for sensitive and tunable biosensing. At the surface of high-refractive index Si3N4 membranes we engineered lithographic nanogratings based on plasmonic nanostripes, demonstrating the excitation of sharp waveguided modes and lattice resonances. We achieved fine tuning of these optical modes over a broadband Visible and Near-Infrared spectrum, in full agreement with numerical calculations. This possibility allowed us to select sharp waveguided modes supporting strong near-field amplification, extending for hundreds of nanometres out of the grating and enabling versatile biosensing applications. We demonstrate the potential of this flat-optics platform by devising a proof-of-concept nanofluidic refractive index sensor exploiting the long-range waveguided mode operating at the sub-picoliter scale. This free-standing device configuration, that could be further engineered at the nanoscale, highlights the strong potential of flat-optics nanoarrays in optofluidics and nanofluidic biosensing.

Omnidirectional and broadband photon harvesting in self-organized Ge columnar nanovoids.

Highly porous Germanium surfaces with uniformly distributed columnar nanovoid structures are fabricated over a large area (wafer scale) by large fluence Sn+irradiation through a thin silicon nitride layer. The latter represents a one-step highly reproducible approach with no material loss to strongly increase photon harvesting into a semiconductor active layer by exploiting the moth-eye antireflection effect. The ion implantation through the nitride cap layer allows fabricating porous nanostructures with high aspect ratio, which can be tailored by varying ion fluence. By comparing the reflectivity of nanoporous Ge films with a flat reference we demonstrate a strong and omnidirectional reduction in the optical reflectivity by a factor of 96% in the selected spectral regions around 960 nm and by a factor of 67.1% averaged over the broad spectral range from 350 to 1800 nm. Such highly anti-reflective nanostructured Ge films prepared over large-areas with a self-organized maskless approach have the potential to impact real world applications aiming at energy harvesting.

Broadband and Tunable Light Harvesting in Nanorippled MoS2 Ultrathin Films.

Nanofabrication of flat optic silica gratings conformally layered with two-dimensional (2D) MoS2 is demonstrated over large area (cm2), achieving a strong amplification of the photon absorption in the active 2D layer. The anisotropic subwavelength silica gratings induce a highly ordered periodic modulation of the MoS2 layer, promoting the excitation of Guided Mode Anomalies (GMA) at the interfaces of the 2D layer. We show the capability to achieve a broadband tuning of these lattice modes from the visible (VIS) to the near-infrared (NIR) by simply tailoring the illumination conditions and/or the period of the lattice. Remarkably, we demonstrate the possibility to strongly confine resonant and nonresonant light into the 2D MoS2 layers via GMA excitation, leading to a strong absorption enhancement as high as 240% relative to a flat continuous MoS2 film. Due to their broadband and tunable photon harvesting capabilities, these large area 2D MoS2 metastructures represent an ideal scalable platform for new generation devices in nanophotonics, photo- detection and -conversion, and quantum technologies.

Ultra-broadband photon harvesting in large-area few-layer MoS2 nanostripe gratings.

Flat optics nanoarrays based on few-layer MoS2 are homogeneously fabricated over large-area (cm2) transparent templates, demonstrating effective tailoring of the photon absorption in two-dimensional (2D) transition-metal dichalcogenide (TMD) layers. The subwavelength subtractive re-shaping of the few-layer MoS2 film into a one-dimensional (1D) nanostripe array results in a pronounced photonic anomaly, tunable in a broadband spectral range by simply changing the illumination conditions (or the lattice periodicity). This scheme promotes efficient coupling of light to the 2D TMD layers via resonant interaction between the MoS2 excitons and the photonic lattice, with subsequent enhancement of absorption exceeding 400% relative to the flat layer. In parallel, an ultra-broadband absorption amplification in the whole visible spectrum is achieved, thanks to the non-resonant excitation of substrate guided modes promoted by MoS2 nanoarrays. These results highlight the potential of nanoscale re-shaped 2D TMD layers for large-area photon harvesting in layered nanophotonics, quantum technologies and new-generation photovoltaics.

Color Routing via Cross-Polarized Detuned Plasmonic Nanoantennas in Large-Area Metasurfaces.

Bidirectional nanoantennas are of key relevance for advanced functionalities to be implemented at the nanoscale and, in particular, for color routing in an ultracompact flat-optics configuration. Here we demonstrate a novel approach avoiding complex collective geometries and/or restrictive morphological parameters based on cross-polarized detuned plasmonic nanoantennas in a uniaxial (quasi-1D) bimetallic configuration. The nanofabrication of such a flat-optics system is controlled over a large area (cm2) by a novel self-organized technique exploiting ion-induced nanoscale wrinkling instability on glass templates to engineer tilted bimetallic nanostrip dimers. These nanoantennas feature broadband color routing with superior light scattering directivity figures, which are well described by numerical simulations and turn out to be competitive with the response of lithographic nanoantennas. These results demonstrate that our large-area self-organized metasurfaces can be implemented in real-world applications of flat-optics color routing from telecom photonics to optical nanosensing.

Anisotropic Nanoscale Wrinkling in Solid-State Substrates.

Pattern formation induced by wrinkling is a very common phenomenon exhibited in soft-matter substrates. In all these systems, wrinkles develop in the presence of compressively stressed thin films lying on compliant substrates. Here, the controlled growth of self-organized nanopatterns exploiting a wrinkling instability on a solid-state substrate is demonstrated. Soda-lime glasses are modified in the surface layers by a defocused ion beam, which triggers the formation of a compressively stressed surface layer deprived of alkali ions. When the substrate is heated up near its glass transition temperature, the wrinkling instability boosts the growth rate of the pattern by about two orders of magnitude. High-aspect-ratio anisotropic ripples bound by faceted ridges are thus formed, which represent an optimal template for guiding the growth of large-area arrays of functional nanostructures. The engineering over large square centimeter areas of quasi-1D arrays of Au nanostripe dimers endowed with tunable plasmonic response, strong optical dichroism, and high electrical conductivity is demonstrated. These peculiar functionalities allow these large-area substrates to be exploited as active metamaterials in nanophotonics, biosensing, and optoelectronics.

Near-field terahertz probes with room-temperature nanodetectors for subwavelength resolution imaging.

Near-field imaging with terahertz (THz) waves is emerging as a powerful technique for fundamental research in photonics and across physical and life sciences. Spatial resolution beyond the diffraction limit can be achieved by collecting THz waves from an object through a small aperture placed in the near-field. However, light transmission through a sub-wavelength size aperture is fundamentally limited by the wave nature of light. Here, we conceive a novel architecture that exploits inherently strong evanescent THz field arising within the aperture to mitigate the problem of vanishing transmission. The sub-wavelength aperture is originally coupled to asymmetric electrodes, which activate the thermo-electric THz detection mechanism in a transistor channel made of flakes of black-phosphorus or InAs nanowires. The proposed novel THz near-field probes enable room-temperature sub-wavelength resolution coherent imaging with a 3.4 THz quantum cascade laser, paving the way to compact and versatile THz imaging systems and promising to bridge the gap in spatial resolution from the nanoscale to the diffraction limit.

Template-assisted growth of transparent plasmonic nanowire electrodes.

Self-organized nanowire arrays are confined by glancing-angle Au deposition on nanopatterned glass templates prepared by ion beam sputtering. The semi-transparent 1D nanowire arrays are extended over large cm2 areas and are endowed with excellent electrical conductivity competitive with the best transparent conductive oxides (sheet resistance in the range of 5-20 Ohm sq-1). In addition, the nanowires support localized surface plasmon (LSP) resonances, which are easily tunable into the visible and near infrared spectrum and are selectively excited with incident light polarized perpendicularly to the wires. Such substrates, thus, behave as multifunctional nanoelectrodes, which combine good optoelectronic performance with dichroic plasmonic excitation. The electrical percolation process of the Au nanoelectrodes was monitored in situ during growth at glancing angle, both on flat and nanopatterned glass templates. In the first case, we observed a universal scaling of the differential percolation rate, independently of the glancing deposition angle, while deviations from the universal scaling were observed when Au was confined on nanopatterned templates. In the latter case, the pronounced shadowing effect promotes the growth of locally connected 1D Au nanosticks on the 'illuminated' ripple ridges, thus, introducing strong anisotropies with respect to the case of a 2D percolating network.

SERS Amplification from Self-Organized Arrays of Plasmonic Nanocrescents.

We report on the surface-enhanced Raman scattering (SERS) efficiency of self-organized arrays of Au nanocrescents confined on monolayers of polystyrene nanospheres. A dichroic SERS emission in the visible spectrum is observed due to the selective excitation of a localized surface plasmon (LSP) resonance along the "short axis" of the Au nanocrescents. Under these conditions SERS signal amplifications in the range of 10(3) have been observed with respect to a flat reference Au film. The far field and near field plasmonic response of Au nanocrescent arrays have been investigated as a function of the metal dose deposited onto the polymeric spheres. In this way, we show the possibility of simply tailoring the SERS emission by engineering the morphology of the plasmonic nanocrescents. We highlight the SERS activity of chains of satellite nanoclusters that decorate the border of each connected crescent and sustain isotropic high energy LSP resonances in the visible spectrum.

Channeling motion of gold nanospheres on a rippled glassed surface.

Gold nanospheres have been manipulated by atomic force microscopy on a rippled glass surface produced by ion beam sputtering and coated with an ultrathin (10 nm thick) graphitic layer. This substrate is characterized by irregular wavy grooves running parallel to a preferential direction. Measurements in ambient conditions show that the motion of the nanoparticles is confined to single grooves ('channels'), along which the particles move till they are trapped by local bottlenecks. At this point, the particles cross the ripple pattern in a series of consecutive jumps and continue their longitudinal motion along a different channel. Moreover, due to the asymmetric shape of the ripple profiles, the jumps occur in the direction of minimum slope, resembling a ratchet mechanism. Our results are discussed, extending a collisional model, which was recently developed for the manipulation of nanospheres on flat surfaces, to the specific geometry of this problem.

Optical properties of biaxial nanopatterned gold plasmonic nanowired grid polarizer.

Gold nanoparticles deposited on self-organized nano-ripple quartz substrates have been studied by spectroscopic Mueller matrix ellipsometry. The surface was found to have biaxial anisotropic optical properties. For electric field components normal to the ripples the periodic and disconnected nature of the in plane nanowires gives rise to an optical response dominated by the localized plasmon resonance. In the direction parallel to the ripples the gold nanoparticles are aligned closely leading to localized plasmon resonances in the infrared. As Au was deposited at an angle oblique to the surface normal, the gold nanoparticles were formed on the side of the ripples facing the incoming evaporation flux. This makes the gold particles slightly inclined, correspondingly the principal coordinate system of the biaxial dielectric tensor results tilted. The anisotropic plasmonic optical response results in a strong polarizing effect, making it suitable as a plasmonic nanowired grid polarizer.