Exploiting plasmonic enhancement with light-emitting diode excitation in surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) is a well-established technique that enables the detection of very low molecular concentrations down to single molecules. Typical applications of SERS are the consistent identification of various samples used in chemistry, biology, and physics among others. In contrast to common SERS setups, where lasers are used as excitation source, we exploit SERS to perform Raman spectroscopy with a light-emitting diode (LED). We demonstrate the applicability of our approach on four different Raman reporters. We unambiguously distinguish two similar designer molecules 4-nitrothiophenol (p-NTP) and 5,5-dithio-bis-(2-nitrobenzoic acid) (DTNB) that are often used in SERS experiments. Additionally, we probe Rhodamine 6G that is used in many different applications and carbon nanotubes as a one-dimensional solid state nanosystem. The LED excited surface-enhanced Raman spectra reproduce the characteristic Raman modes of the different samples. We compare the LED spectra to Raman spectra excited with a laser at the same wavelength. We envision the combination of LED sources with SERS substrates in the next generation of handheld devices and low-cost Raman setups

Dark interlayer plasmons in colloidal gold nanoparticle bi- and few-layers

We demonstrate the excitation of dark plasmon modes with linearly polarized light at normal incidence in self-assembled layers of gold nanoparticles. Because of field retardation, the incident light field induces plasmonic dipoles that are parallel within each layer but antiparallel between the layers, resulting in a vanishing net dipole moment. Using microabsorbance spectroscopy we measured a pronounced absorbance peak and reflectance dip at 1.5 eV for bi- and trilayers of gold nanoparticles with a diameter of 46 nm and 2 nm interparticle gap size. The excitations were identified as dark interlayer plasmons by finite-difference time-domain simulations. The dark plasmon modes are predicted to evolve into standing waves when further increasing the layer number, which leads to 90% transmittance of the incident light through the nanoparticle film. Our approach is easy to implement and paves the way for large-area coatings with tunable plasmon resonance

Spatial clustering of defect luminescence centers in Si-doped low resistivity Al0.82Ga0.18N

A series of Si-doped AlN-rich AlGaN layers with low resistivities was characterized by a combination of nanoscale imaging techniques. Utilizing the capability of scanning electron microscopy to reliably investigate the same sample area with different techniques, it was possible to determine the effect of doping concentration, defect distribution, and morphology on the luminescence properties of these layers. Cathodoluminescence shows that the dominant defect luminescence depends on the Si-doping concentration. For lower doped samples, the most intense peak was centered between 3.36 eV and 3.39 eV, while an additional, stronger peak appears at 3 eV for the highest doped sample. These peaks were attributed to the (VIII-ON)2− complex and the V3−III vacancy, respectively. Multimode imaging using cathodoluminescence, secondary electrons, electron channeling contrast, and atomic force microscopy demonstrates that the luminescence intensity of these peaks is not homogeneously distributed but shows a strong dependence on the topography and on the distribution of screw dislocations.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeBMBF, 13N12587, Photonische Plattformtechnologie zur ultrasensitiven und hochspezifischen biochemischen Sensorik auf Basis neuartiger UV-LEDs (UltraSens

Mean-field analysis of a dynamical phase transition in a cellular automaton model for collective motion

A cellular automaton model is presented for random walkers with biologically motivated interactions favoring local alignment and leading to collective motion or swarming behavior. The degree of alignment is controlled by a sensitivity parameter, and a dynamical phase transition exhibiting spontaneous breaking of rotational symmetry occurs at a critical parameter value. The model is analyzed using nonequilibrium mean field theory: Dispersion relations for the critical modes are derived, and a phase diagram is constructed. Mean field predictions for the two critical exponents describing the phase transition as a function of sensitivity and density are obtained analytically.Comment: 4 pages, 4 figures, final version as publishe

Hybrid top-down/bottom-up fabrication of regular arrays of AlN nanorods for deep-UV core-shell LEDs

Core–shell nanostructures are predicted to highly improve the efficiency of deep-UV light emitting diodes (LEDs), owing to their low defect density, reduced quantum-confined Stark effect, high-quality non-polar growth and improved extraction efficiency. In this paper, we report on the nanofabrication of high-quality AlN nanorod arrays using a hybrid top-down/bottom-up approach for use as a scaffold for UV LED structures. We describe the use of Displacement Talbot Lithography to fabricate a metallic hard etch mask to allow AlN nanorod arrays to be dry etched from a planar AlN template. In particular, we investigate the impact of etching parameters on the nanorod etch rate, tapering profile and mask selectivity in order to achieve vertical-sided nanorod arrays with high aspect ratios. AlN facet recovery is subsequently explored by means of regrowth using Metal Organic Vapor Phase Epitaxy. Low pressure and high V/III ratio promote straight and smooth sidewall faceting, which results in an improvement of the optical quality compared to the initial AlN template. The promising results open new perspectives for the fabrication of high-efficiency deep-UV-emitting core–shell LEDs

Resonant anti-Stokes Raman scattering in single-walled carbon nanotubes

The dependence of the anti-Stokes Raman intensity on the excitation laser energy in carbon nanotubes is studied by resonant Raman spectroscopy. The complete resonant anti-Stokes and Stokes Raman profiles of the high-energy longitudinal phonon (G+) are obtained for (8,3), (7,5), (6,4), and (6,5) single chirality enriched samples. A high asymmetry between the intensity of the incoming and outgoing resonance is observed in the resonant Raman profiles. In contrast to Stokes scattering, anti-Stokes scattering is more intense at the outgoing resonance then at the incoming resonance. The resonance profiles are explained by a Raman process that includes the phonon- mediated interactions with the dark excitonic state. The chirality dependence of the Raman profiles is due to the variation in the exciton-phonon matrix elements, in agreement with tight-binding calculations. Based on the asymmetric Raman profiles we present the resonance factors for the Stokes /anti-Stokes ratios in carbon nanotubes

Stacking fault-associated polarized surface-emitted photoluminescence from zincblende InGaN/GaN quantum wells

Zincblende InGaN/GaN quantum wells offer a potential improvement to the efficiency of green light emission by removing the strong electric fields present in similar structures. However, a high density of stacking faults may have an impact on the recombination in these systems. In this work, scanning transmission electron microscopy and energy-dispersive x-ray measurements demonstrate that one dimensional nanostructures form due to indium segregation adjacent to stacking faults. In photoluminescence experiments these structures emit visible light which is optically polarised up to 86% at 10K and up to 75% at room temperature. The emission redshifts and broadens as the well width increases from 2nm to 8nm. Photoluminescence excitation measurements indicate that carriers are captured by these structures from the rest of the quantum wells and recombine to emit light polarised along the length of these nanostructures