Plasmonic Polarization Rotation in SERS Spectroscopy.

Surface-enhanced Raman optical activity (SEROA) has been extensively investigated due to its ability to directly probe stereochemistry and molecular structure. However, most works have focused on the Raman optical activity (ROA) effect arising from the chirality of the molecules on isotropic surfaces. Here, we propose a strategy for achieving a similar effect: i.e., a surface-enhanced Raman polarization rotation effect arising from the coupling of optically inactive molecules with the chiral plasmonic response of metasurfaces. This effect is due to the optically active response of metallic nanostructures and their interaction with molecules, which could extend the ROA potential to inactive molecules and be used to enhance the sensibility performances of surface-enhanced Raman spectroscopy. More importantly, this technique does not suffer from the heating issue present in traditional plasmonic-enhanced ROA techniques, as it does not rely on the chirality of the molecules.

Fabrication of a Novel Electrochemical Sensor Based on Carbon Cloth Matrix Functionalized with MoO3 and 2D-MoS2 Layers for Riboflavin Determination.

The preparation and characterization of a hybrid composite, based on carbon cloth (CC) matrix functionalized with two-dimensional (2D) MoS2 flakes and MoO3, and its use for developing an electrochemical sensor for the determination of riboflavin (RF) is here reported. The 2D-MoS2-MoO3CC composite was prepared by depositing 2D-MoS2 nanosheets, obtained by liquid phase exfoliation (LPE), on the surface of a carbon cloth fiber network, previously functionalized with a layer of molybdenum oxide (α-MoO3) by radio-frequency magnetron reactive sputtering technique. The 2D-MoS2-MoO3CC composite was characterized by scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDX), and Raman spectroscopy. An electrochemical sensor has been then fabricated by fixing a slice of the 2D-MoS2-MoO3CC composite on the working electrode of a screen-printed carbon electrode (SPCE). The 2D-MoS2-MoO3-CC/SPCE sensor display good electrochemical characteristics which have been exploited, for the first time, in the electroanalytical determination of riboflavin (RF). The sensitivity to RF, equal to 0.67 µA mM-1 in the linear range from 2 to 40 µM, and a limit of detection (LOD) of 1.5 µM at S/N = 3, demonstrate the promising characteristics of the proposed 2D-MoS2-MoO3-CC/SPCE electrochemical sensor for the determination of riboflavin.

Gold Nanocylinders on Gold Film as a Multi-spectral SERS Substrate.

The surface enhanced Raman scattering (SERS) efficiency of gold nanocylinders deposited on gold thin film is studied. Exploiting the specific plasmonic properties of such substrates, we determine the influence of the nanocylinder diameter and the film thickness on the SERS signal at three different excitation wavelengths (532, 638 and 785 nm). We demonstrate that the highest signal is reached for the highest diameter of 250 nm due to coupling between the nanocylinders and for the lowest thickness (20 nm) as the excited plasmon is created at the interface between the gold and glass substrate. Moreover, even if we show that the highest SERS efficiency is obtained for an excitation wavelength of 638 nm, a large SERS signal can be obtained at all excitation wavelengths and on a wide spectral range. We demonstrate that it can be related with the nature of the plasmon (propagative plasmon excited through the nanocylinder grating) and with its angular dependence (tuning of the plasmon position with the excitation angle). Such an effect allows the excitation of plasmon on nearly the whole visible range, and paves the way to multispectral SERS substrates.

Low cost tips for tip-enhanced Raman spectroscopy fabricated by two-step electrochemical etching of 125 µm diameter gold wires.

Tip-enhanced Raman spectroscopy (TERS) has become a well-applied technique for nanospectroscopy, allowing for single molecule sensitivity with sub-nanometer spatial resolution. The demand for efficient, reproducible and cost-effective probes for TERS is increasing. Here we report on a new electrochemical etching protocol to fabricate TERS tips starting from 125 µm diameter gold wires in a reproducible way. The process is reliable (50% of the tips have radius of curvature <35 nm, 66% <80 nm), fast (less than 2 min) and 2.5 times cheaper than the etching of standard 250 µm diameter wires. The TERS performance of the tips is tested on dyes, pigments and biomolecules and enhancement factors higher than 105 are observed. TERS mapping with a spatial resolution of 5 nm is demonstrated.

Double-Wall Nanotubes and Graphene Nanoplatelets for Hybrid Conductive Adhesives with Enhanced Thermal and Electrical Conductivity.

Improving the electrical and thermal properties of conductive adhesives is essential for the fabrication of compact microelectronic and optoelectronic power devices. Here we report on the addition of a commercially available conductive resin with double-wall carbon nanotubes and graphene nanoplatelets that yields simultaneously improved thermal and electrical conductivity. Using isopropanol as a common solvent for the debundling of nanotubes, exfoliation of graphene, and dispersion of the carbon nanostructures in the epoxy resin, we obtain a nanostructured conducting adhesive with thermal conductivity of ∼12 W/mK and resistivity down to 30 µΩ cm at very small loadings (1% w/w for nanotubes and 0.01% w/w for graphene). The low filler content allows one to keep almost unchanged the glass-transition temperature, the viscosity, and the curing parameters. Die shear measurements show that the nanostructured resins fulfill the MIL-STD-883 requirements when bonding gold-metalized SMD components, even after repeated thermal cycling. The same procedure has been validated on a high-conductivity resin characterized by a higher viscosity, on which we have doubled the thermal conductivity and quadrupled the electrical conductivity. Graphene yields better performances with respect to nanotubes in terms of conductivity and filler quantity needed to improve the resin. We have finally applied the nanostructured resins to bond GaN-based high-electron-mobility transistors in power-amplifier circuits. We observe a decrease of the GaN peak and average temperatures of, respectively, ∼30 °C and ∼10 °C, with respect to the pristine resin. The obtained results are important for the fabrication of advanced packaging materials in power electronic and microwave applications and fit the technological roadmap for CNTs, graphene, and hybrid systems.

Photonic Torque Microscopy of the Nonconservative Force Field for Optically Trapped Silicon Nanowires.

We measure, by photonic torque microscopy, the nonconservative rotational motion arising from the transverse components of the radiation pressure on optically trapped, ultrathin silicon nanowires. Unlike spherical particles, we find that nonconservative effects have a significant influence on the nanowire dynamics in the trap. We show that the extreme shape of the trapped nanowires yields a transverse component of the radiation pressure that results in an orbital rotation of the nanowire about the trap axis. We study the resulting motion as a function of optical power and nanowire length, discussing its size-scaling behavior. These shape-dependent nonconservative effects have implications for optical force calibration and optomechanics with levitated nonspherical particles.

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.

Silicon nanowire and carbon nanotube hybrid for room temperature multiwavelength light source.

The realization of an innovative hybrid light source operating at room temperature, obtained by embedding a carbon nanotube (CNT) dispersion inside a Si nanowire (NW) array is reported. The NW/CNT system exhibits a peculiar photoluminescence spectrum, consisting of a wide peak, mainly observed in the visible range, due to quantum confined Si NWs, and of several narrower IR peaks, due to the different CNT chiralities present in the dispersion. The detailed study of the optical properties of the hybrid system evidences that the ratio between the intensity of the visible and the IR emissions can be varied within a wide range by changing the excitation wavelength or the CNT concentration; the conditions leading to the prevalence of one signal with respect to the other are identified. The multiplicity of emission spectra obtainable from this composite material opens new perspectives for Si nanostructures as active medium in light sources for Si photonics applications.

Fast and reliable fabrication of gold tips with sub-50 nm radius of curvature for tip-enhanced Raman spectroscopy.

We have developed a new electrochemical etching procedure to fabricate gold tips with sub-50 nm apical radius of curvature with a production yield of 80% and production time lower than 5 min. The technique is based on a two-step self-terminating process in which a gold wire is first quickly (<1 min) pre-etched in an hydrochloric acid (HCl)∕ethanol solution at high voltage (10 VDC), and then slowly (2-4 min) etched at lower voltages (<2.5 VDC). The first step occurs under intense bubbling conditions and allows us to thin rapidly the wire. This reduces the time required by subsequent low-voltage process during which the tips are formed at the liquid∕air interface. A statistical analysis of the surface morphology has been carried out on a set of 60 tips by scanning electron microscopy. The results show that the surface roughness and the sharpness of the final tip are critically influenced by the intrinsic granularity of the gold wires. Moreover, there is a correlation between the tip quality and the time elapse required to complete the low-voltage etching step. Tips featuring smooth surfaces and radii of curvature <50 nm are produced whenever the etching times are lower than 250 s, while etching times larger than 300 s typically yield rough, blunt tips. Such a correlation can be used as a screening criterion to select sharp tips during production with an 80% yield. The high quality of the gold tips produced with such method is confirmed by the electromagnetic field enhancement measured both in tip-enhanced Raman scattering and surface-enhanced Raman scattering on the tip apex experiments.

Re-radiation enhancement in polarized surface-enhanced resonant Raman scattering of randomly oriented molecules on self-organized gold nanowires.

We explore the effect of re-radiation in surface-enhanced Raman scattering (SERS) through polarization-sensitive experiments on self-organized gold nanowires on which randomly oriented Methylene Blue molecules are adsorbed. We provide the exact laws ruling the polarized, unpolarized, and parallel- and cross-polarized SERS intensity as a function of the field polarizations. We show that SERS is polarized along the wire-to-wire nanocavity axis, independently from the excitation polarization. This proves the selective enhancement of the Raman dipole component parallel to the nanocavity at the single molecule level. Introducing a field enhancement tensor to account for the anisotropic polarization response of the nanowires, we work out a model that correctly predicts the experimental results for any excitation/detection polarization and goes beyond the E(4) approximation. We also show how polarization-sensitive SERS experiments permit one to evaluate independently the excitation and the re-radiation enhancement factors accessing the orientation-averaged non-diagonal components of the molecular Raman polarizability tensor.