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.

Flory-Huggins Photonic Sensors for the Optical Assessment of Molecular Diffusion Coefficients in Polymers.

The lack of cost-effective systems for the assessment of air pollutants is a concern for health and safety in urban and industrial areas. The use of polymer thin films as label-free colorimetric sensors featuring specific interactions with pollutants would then represent a paradigm shift in environmental monitoring and packaging technologies, allowing to assess air quality, formation of byproducts in closed environment, and the barrier properties of the polymers. To this end, all-polymer distributed Bragg reflectors represent a promising approach toward a reliable and cost-effective transduction of chemical stimuli and effective colorimetric label-free selective detectors. We show selectivity attained by specific interactions between the polymer and analytes. Such interactions drive the analyte intercalation through the polymer structure and its kinetics, converting it in a dynamic optical response which is at the basis of the Flory-Huggins photonic sensors. The multivariate analyses of the response kinetics also allow distinguishing binary mixtures. Additionally, we demonstrate that such optical responses can be used to esteem the diffusion coefficients of small molecules within polymer media via simple UV-vis spectroscopy retrieving data comparable to those obtained with state-of-the-art gravimetric procedures. Last, we assess the figures of merit of the sensors in terms of lower detection limit, sensitivity, and reversibility, demonstrating that such devices can pave the way to an innovative, simple, and low-cost detection method integrable to in situ assessment of barrier polymers used for the encapsulation of optoelectronic devices, food packaging, and goods storage in general.

Designer Shape Anisotropy on Transition-Metal-Dichalcogenide Nanosheets.

MoS2 and generally speaking, the wide family of transition-metal dichalcogenides represents a solid nanotechnology platform on which to engineer a wealth of new and outperforming applications involving 2D materials. An even richer flexibility can be gained by extrinsically inducing an in-plane shape anisotropy of the nanosheets. Here, the synthesis of anisotropic MoS2 nanosheets is proposed as a prototypical example in this respect starting from a highly conformal chemical vapor deposition on prepatterend substrates and aiming at the more general purpose of tailoring anisotropy of 2D nanosheets by design. This is envisioned to be a suitable configuration for strain engineering as far as strain can be spatially redistributed in morphologically different regions. With a similar approach, both the optical and electronic properties of the 2D transition-metal dichalcogenides can be tailored over macroscopic sample areas in a self-organized fashion, thus paving the way for new applications in the field of optical metasurfaces, light harvesting, and catalysis.

Anisotropic MoS2 Nanosheets Grown on Self-Organized Nanopatterned Substrates.

Manipulating the anisotropy in 2D nanosheets is a promising way to tune or trigger functional properties at the nanoscale. Here, a novel approach is presented to introduce a one-directional anisotropy in MoS2 nanosheets via chemical vapor deposition (CVD) onto rippled patterns prepared on ion-sputtered SiO2 /Si substrates. The optoelectronic properties of MoS2 are dramatically affected by the rippled MoS2 morphology both at the macro- and the nanoscale. In particular, strongly anisotropic phonon modes are observed depending on the polarization orientation with respect to the ripple axis. Moreover, the rippled morphology induces localization of strain and charge doping at the nanoscale, thus causing substantial redshifts of the phonon mode frequencies and a topography-dependent modulation of the MoS2 workfunction, respectively. This study paves the way to a controllable tuning of the anisotropy via substrate pattern engineering in CVD-grown 2D nanosheets.

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.