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.
Here we report the experimental observation of circular dichroism in the second-harmonic field (800-400 nm conversion) generated by self-organized gold nanowire arrays with subwavelength periodicity (160 nm). Such circular dichroism, raised by a nonlinear optical extrinsic chirality, is the evident signature of the sample morphology. It arises from the curvature of the self-assembled wires, producing a lack of symmetry at oblique incidence. The results were compared, both in the optical linear and nonlinear regime, with a reference sample composed of straight wires. Despite the weak extrinsic optical chirality of our samples (not observable by our optical linear measurements), high visibility (more than 50%) was obtained in the second-harmonic generated field.
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 MoS nanosheets via chemical vapor deposition (CVD) onto rippled patterns prepared on ion-sputtered SiO /Si substrates. The optoelectronic properties of MoS are dramatically affected by the rippled MoS 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 MoS workfunction, respectively. This study paves the way to a controllable tuning of the anisotropy via substrate pattern engineering in CVD-grown 2D nanosheets.
Large-scale integration of MoS2 in electronic devices requires the development of reliable and cost-effective deposition processes, leading to uniform MoS2 layers on a wafer scale. Here we report on the detailed study of the heterogeneous vapor-solid reaction between a pre-deposited molybdenum solid film and sulfur vapor, thus resulting in a controlled growth of MoS2 films onto SiO2/Si substrates with a tunable thickness and cm(2)-scale uniformity. Based on Raman spectroscopy and photoluminescence, we show that the degree of crystallinity in the MoS2 layers is dictated by the deposition temperature and thickness. In particular, the MoS2 structural disorder observed at low temperature (<750 °C) and low thickness (two layers) evolves to a more ordered crystalline structure at high temperature (1000 °C) and high thickness (four layers). From an atomic force microscopy investigation prior to and after sulfurization, this parametrical dependence is associated with the inherent granularity of the MoS2 nanosheet that is inherited by the pristine morphology of the pre-deposited Mo film. This work paves the way to a closer control of the synthesis of wafer-scale and atomically thin MoS2, potentially extendable to other transition metal dichalcogenides and hence targeting massive and high-volume production for electronic device manufacturing.
The exotic electrodynamics properties of graphene come from the linearly dispersive electronic bands that host massless Dirac electrons. A similar behavior was predicted to manifest in freestanding silicene, the silicon counterpart of graphene, thereby envisaging a new route for a silicon photonics. However, the access to silicene exploitation in photonics was hindered so far by the use of optically inappropriate substrates in experimentally realized silicene. Here we report on the optical conductivity of silicon nanosheets epitaxially grown on the optically transparent Al2O3(0001) from a thickness of a few tens of nanometers down to the extreme twodimensional (2D) limit. When approaching a 2D regime, a Dirac-like electrodynamics can be deduced from the observation of a low-energy optical conductivity feature owing to a silicene-based interfacing to the substrate.
ince its discovery, surface-enhanced Raman spectroscopy (SERS) has pushed researchers' interest to develop different kinds of active substrates for high sensitivity molecular detection. Defocused ion beam sputtering (IBS) represents a viable route for the production of large scale, highly reproducible SERS-active substrates consisting of near-field coupled nanowires featuring localized surface plasmon resonances in the visible and the near-infrared. Here we investigate the field enhancement and spatial confinement in the visible and the near-infrared of arrays of optically resonant gold nanowires, using two complementary techniques: SERS and scanning near-field optical microscopy (SNOM). While SERS allows us to quantify the field enhancement factor, SNOM is used to image the localization of the enhanced electromagnetic fields. We show that in the visible (633 nm) the nanowires are SERS active only for excitation polarized parallel to the wire-to-wire nanocavities, yielding enhancement factors of 7 × 103. In the near-infrared (785 nm) we observe a 2-fold larger SERS enhancement (1.3 × 104) for excitation parallel to the nanocavities and detect the onset of SERS amplification for excitation polarization parallel to the nanowires long axis. Polarization-sensitive SNOM in the near-infrared (830 nm) enables the correlation of the scattered intensity with the sample morphology at the local scale. We demonstrate that the field enhancement stems from the wire-to-wire nanocavity regions when the excitation field is polarized parallel to the wire-to-wire nanocavity, while we observe more complex field confinement patterns related to the partially inhomogeneous morphology of the substrate when the polarization is parallel to the nanowires long axis. Our experiments strongly suggest IBS-fabricated nanowires as novel substrates for plasmon-enhanced spectroscopie
The growth of atomically thin MoS2 films is achieved by sulfurization of molybdenum oxide precursor films grown by atomic layer deposition. The quality features of the MoS2 films are engineered controlling the stoichiometry, morphology, and thickness of the precursors. The interface interaction between the precursor films and the substrates (SiO2 or sapphire) plays a key role in the MoS2 formation.
Hybrid structures composed by self-ordered dielectric nanospheres and partially covered by gold nanocrescents, produce efficient second harmonic generation signals due to the cooperative effect of localized plasmon excitation and 2D Bragg gratings. Asymmetries in the geometric shape of the gold nanocrescents induce a circularly polarized optical response
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