543 research outputs found
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
Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods
We report a method of growing site controlled InGaN multiple quantum discs (QDs) at uniform wafer scale on coalescence free ultra-high density (>80%) nanorod templates by metal organic chemical vapour deposition (MOCVD). The dislocation and coalescence free nature of the GaN space filling nanorod arrays eliminates the well-known emission problems seen in InGaN based visible light sources that these types of crystallographic defects cause. Correlative scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) mapping and cathodoluminescence (CL) hyperspectral imaging illustrates the controlled site selection of the red, yellow and green (RYG) emission at these nano tips. This article reveals that the nanorod tips’ broad emission in the RYG visible range is in fact achieved by manipulating the InGaN QD’s confinement dimensions, rather than significantly increasing the In%. This article details the easily controlled method of manipulating the QDs dimensions producing high crystal quality InGaN without complicated growth conditions needed for strain relaxation and alloy compositional changes seen for bulk planar GaN templates
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