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3D plasmonic hot spots network via gold decorated deep micro-porous silicon exhibiting ultrahigh-SERS enhancement with application to explosives detection

Microporous deep (12 µm depth) silicon substrate coated with optimum Au layer is fabricated for ultrahigh SERS enhancement factor and employed for explosives trace detection. Being microporous with quasi-periodicity, short lived extended plasmons can be excited via scattering and coupled to localized ones thus providing ultrahigh local optical field, and the SERS ultrahigh enhancement is achieved. SERS activity of the substrate is checked using 4-ATP monolayer immobilized over the proposed substrate and analyzed using two different methods including peak absolute intensity and peak ratios, to find best suitable method for higher reproducibility. The obtained enhancement factor for Raman signal is found to be the order of 106–107, explained due to the existence of 3D hot-spot network and coupling between localized to extended plasmons. The coupling is verified through the Rabi splitting observed in the reflectivity dip. A 2–3 times fewer signal variability is obtained using peak ratio between highest peak intensity of Raman receptor and that of the substrate. The substrate is then employed for picric acid sensing providing the detection limits of 7.48 nM and 2.87 nM, determined based on peaks height and ratio measurements respectively. The proposed work suggests deep micro-porous silicon coated with 200 nm Au layer deposited over a large area as promising candidate for SERS and sensing of ultra-trace amount of chemicals.

3D silver metallized nanotrenches fabricated by nanoimprint lithography as flexible SERS detection platform

We report the development of highly sensitive substrates with great potential as Surface-enhanced Raman scattering (SERS) spectroscopy detection platforms, consisting of nanoimprint lithography (NIL) fabricated nanotrenches in plastic and covered by nanostructured silver (Ag) films with thicknesses in the 10–100 nm range deposited by direct current (DC) sputtering. The Ag film thickness was increased by using sequential deposition times and its contribution to the obtained enhancement factor was determined. The morphological and structural properties of the metalized nanotrenches were assessed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. Crystal violet (CV) was used as analyte to test the SERS activity of the substrates prepared with or without the nanoimprinted pattern. Our original approach was to determine the resulted SERS enhancement from the synergy of three key aspects: the Ag metallization of cheap, flexible substrates, the effect of increasing the Ag film thickness and the periodic nanotrenches imprinted by NIL as substrate. We found a dramatical contribution in the SERS signal of the periodical Ag nanopattern in comparison to the Ag film quantified by a calculated enhancement factor (EF) up to 107 in case of the SERS detection platform with a 25 nm Ag layer on top of the periodic nanotrenches. The contribution of plasmonic nanostructures contained in the Ag films as well as the contribution of the periodical nanopatterned trenches was assessed, as a cumulative effect to the first contribution.This substrate showed a considerably lower limit of detection (LOD) for SERS, down to 10 pM, much better uniformity as well as more reproducible signals in comparison with the other thicknesses of the metallic film.

Germanium hollow nanodisk resonator for magnetic dipole decay rate enhancement in near‐infrared

AbstractEnhancement of spontaneous emission has a large potential for the development of nanophotonics based applications. Due to the weak interaction of matter with the magnetic component of light, most of the effort has been directed toward the development of the nanophotonic devices for the enhancement of the electric dipole emission. However, in recent years the enhancement of magnetic dipole emission has attracted the focus of the researchers, owing to the utilization of the dielectric nanophotonic structures which have a great potential to facilitate various exciting applications such as nanolasers and single‐photon sources. Despite the high interest on the magnetic dipole emission enhancement, most of the research has been focused on the visible spectrum and few studies have been reported up to present for the near infrared. In this context, a germanium hollow nanodisk resonator is presented and numerically investigated for magnetic dipole decay rate enhancement in the near‐infrared spectrum. The obtained enhancement factor is much more than the previous results in the literature which are obtained via utilization of the dielectric nanostructures. The origin of electromagnetic behavior of the proposed resonator is revealed via near‐electromagnetic‐field enhancement maps and the multipolar decomposition of the electromagnetic scattering modes. Additionally, dependence of the enhancement factor on the geometrical parameters of the germanium hollow nanodisk resonator is studied via parameter sweep simulations. The results of this study may provide a novel strategy for the engineering of chip‐scale nanophotonic applications.

Absorption of carbon dioxide in aqueous MDEA solution coupling dust suppression under atomization

ABSTRACTLarge amounts of CO2 and dust particles coming from power plant flue need to be captured and removed before flue is discharged into the air. In present work, absorption of carbon dioxide in aqueous N-methylidiethanolamine (MDEA) solution coupling dust suppression has been studied in an atomization absorption column, with MDEA concentrations ranging from 0.1 to 0.5mol/L, and with atomization frequencies ranging from 50 to 80 HZ. The obtained experimental results show that absorption rate of CO2 in aqueous MDEA solution can be enhanced when the absorption process couples a dust suppression one under the condition of atomization. The reason for it is attributed to the adsorption of droplets on the solid particles which restrains the amount of entrainment and makes more droplets contact with gas so as to increase effective mass transfer area, thus resulting in the increase of CO2 absorption rate. The range of obtained enhancement factor is from 1.1 to 1.7. Mass transfer enhancement factor increases with the increase of MDEA concentration and atomization frequency at a certain range. Effective mass transfer areas and entrainment ratios suppressed have been calculated based on theoretic research. The results calculated agree with our experimental phenomena, and support the enhancement mass transfer mechanism proposed.

Theoretical investigation of the enhancement factor for a single field emitter in close proximity to the counter electrode

Field emission of electrons enables the patterning process in field-emission scanning probe lithography. A nanotip mounted at an active cantilever is used as an electron field emitter. Since the field enhancement at the tip apex is crucial for the Fowler–Nordheim emission current, the electric field and its enhancement based on a finite-element method (FEM) calculation are studied. The influences of the tip radius, the tip opening angle, and the tip–sample distance are considered. The numerically obtained enhancement factor γp is compared to various models found in the literature. Only three of them agree with the FEM data well. For small ratios of tip–sample distance and tip radius, the data resemble a “hemisphere on a post” model, and for larger ratios, it is better modeled by a “hemiellipsoid on a plane” model despite its “hemisphere on a cone” shape.

Real-Time and Tunable Substrate for Surface Enhanced Raman Spectroscopy by Synthesis of Copper Oxide Nanoparticles via Electrolysis

Here we show the capability of copper oxide (CuO) nanoparticles formed on copper (Cu) electrodes by the electrolysis as a real time active substrate for surface enhanced Raman scattering (SERS). We have experimentally found that using just the ultra pure water as the electrolyte and the Cu electrodes, ions are extracted from the copper anode form copper oxide nanoparticles on the anode surface in matter of minutes. Average particle size on the anode reaches to 100 nm in ninety seconds and grows to about 300 nm in five minutes. This anode is used in Raman experiments in real time as the nanoparticles were forming and the maximum enhancement factor (EF) of Raman signals were over five orders of magnitude. Other metal electrodes made of brass, zinc (Zn), silver (Ag) and aluminum (Al) were also tried for the anode material for a possible real-time substrate for SERS applications. Experimentally obtained enhancement factors were above five orders of magnitude for brass electrodes like the copper but for the other metals no enhancement is observed. Electron microscope images show the cubic nanoparticle formation on copper and brass electrodes but none in the other metals studied.

Surface Enhancement in Ultrafast 2D ATR IR Spectroscopy at the Metal-Liquid Interface

We investigate surface enhancement in two-dimensional attenuated total reflectance infrared (2D ATR IR) spectroscopy from organic monolayers (MLs) at metal–liquid interfaces. We consider MLs from both aromatic and aliphatic organic samples equipped with nitrile and azide functional groups, both of which are widely used as local vibrational probes in ultrafast spectroscopy. Polarization-dependent 2D ATR IR spectroscopy indicates the excitation of local hot spots formed between gold (Au) nanoparticles as the dominant origin of signal enhancement. The highest enhancement factors (∼50) are observed in the case of aromatic nitrile MLs, whereas modest values (<10) are found for aliphatic azide and nitrile groups. Different contributions to signal enhancement are evaluated systematically and indicate the presence of both electromagnetic enhancement and contributions from molecular properties. The obtained enhancement factors are promising to allow 2D ATR IR spectroscopy to become applicable as a versatile technique for the detection of ultrafast structural dynamics in even low-absorbing organic MLs at solid–liquid interfaces.

Copper nanoparticles applied to the preconcentration and electrochemical determination of β-adrenergic agonist: An efficient tool for the control of meat production

A novel method for preconcentration and electrochemical detection of zinterol in bovine urine samples was developed. In order to improve the limit of detection, the surface of a screen-printed carbon electrode was modified with electrodeposited metal copper nanoparticles. The experimental electrodeposition optimization was performed using a central composite design (CCD), involving the variables: precursor concentration, potential and time applied. Copper nanoparticles were characterized by transmission electron microscopy, scanning electron microscopy, cyclic voltammetry, and energy dispersive X-ray spectroscopy. Mesoporous shuttle-like copper oxide nanoparticles were used for the preconcentration step to avoid interferences with many compounds present in the sample matrix. The optimal working conditions for the preconcentration approach were found by means of both two-level fractional factorial and CCD designs. The obtained enhancement factor for a sample volume of 30mL was 35 fold. The calibration curve showed linearity between 0.5 and 45ngmL−1 and the limit of detection was 0.16ngmL−1. The intra and inter assay coefficients of variability were below 4% and 5%; respectively.

Plasmon-enhanced fluorescence of labeled biomolecules on top of a silver sol-gel film

A simple chemical technique was devised for the fabrication of silver nanostructured substrates which can be used for plasmonic enhancement of labeled proteins fluorescence. For bovine serum albumin labeled with fluorescein isothiocyanate, the obtained enhancement factor ranges from three to seven, depending on metal-luminophore spacing and silver nanoparticle size. For excitation with linear polarized light, the enhancement factor increases noticeably for p -polarization and decreases for s -polarization. The experimental results were interpreted in terms of the theoretical model in which the enhancement factor depends on incident light polarization, luminophoremetal spacing and silver nanoparticle size. Proposed plasmonic substrates can be considered as an affordable replacement of standard ones in different types of fluorescent assays for the purpose of increasing sensitivity.

Experimental demonstration of the enhanced transmission through circular and rectangular sub-wavelength apertures using omega-like split-ring resonators

Enhanced transmission through circular and rectangular sub-wavelength apertures using omega-shaped split-ring resonator is numerically and experimentally demonstrated at microwave frequencies. We report a more than 150,000-fold enhancement through a deep sub-wavelength aperture drilled in a metallic screen. To the authors’ best knowledge, this is the highest experimentally obtained enhancement factor reported in the literature. In the paper, we address also the origins and the physical reasons behind the enhancement results. Moreover, we report on the differences occurring when using circular, rectangular apertures as well as double-sided and single-sided omega-like split ring resonator structures.

Theoretical Investigation on SERS of Pyridine Adsorbed on Cn Clusters Induced by Charge Transfer: A Hint that SERS Could be Applied on Many Materials

The SERS spectra of pyridine-C(n) (n=1-6) complexes are investigated theoretically. The obtained enhancement factors of about 10(2)-10(3) in the pre-resonance Raman spectrum calculations are attributed to charge-transfer transitions from the carbon clusters to pyridine, where a good match of band structures between substrates and probe molecules is essential.

Theoretical study on contribution of charge transfer effect to surface-enhanced Raman scattering spectra of pyridine adsorbed on Ag n ( n = 2–8) clusters

We investigate surface-enhanced Raman scattering (SERS) spectra of pyridine–Ag n ( n = 2–8) complexes by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In simulated normal Raman scattering (NRS) spectra, profiles of pyridine–Ag n ( n = 2–8) complexes are analogical with that of isolated pyridine. Nevertheless, calculated pre-SERS spectra are strongly dependent on electronic transition states of new complexes. Wavelengths at 335 nm, 394.8 nm, 316.9 nm and 342.6 nm, which are nearly resonant with pure charge transfer excitation states, are adopted as incident light when simulating pre-SERS spectra for pyridine–Ag n ( n = 2–8) complexes, respectively. We obtain enhancement factors from 10 3 to 10 5 in pre-SERS spectra compared with corresponding NRS spectra. The obvious increase in Raman intensities mainly result from charge transfer resonance Raman enhancement. A charge difference densities (CDDs) methodology is adopted in describing chemical enhancement mechanism. This methodology aims at visualizing charge transfer from Ag n ( n = 2–8) clusters to pyridine on resonant electronic transition, which is one of the most direct evidences for chemical enhancement mechanism.

Bright Unidirectional Fluorescence Emission of Molecules in a Nanoaperture with Plasmonic Corrugations

Controlling the fluorescence emission from nanoscale quantum emitters is a key element for a wide range of applications, from efficient analytical sensing to quantum information processing. Enhancing the fluorescence intensity and narrowing the emission directivity are both essential features to achieve a full control of fluorescence, yet this is rarely obtained simultaneously with optical nanoantennas. Here we report that gold nanoapertures surrounded by periodic corrugations transform standard fluorescent molecules into bright unidirectional sources. We obtain enhancement factors of the fluorescence count rate per molecule up to 120 fold simultaneously with a directional emission of the fluorescence into a narrow angular cone in the direction normal to the sample plane. The bright emission and narrow directionality enable the detection of single molecules with a low numerical aperture objective, and improve the effectiveness of fluorescence-based applications. We thoroughly quantify the increased light-matter coupling as well as the radiation pattern intensity. These results are highly relevant for the development of single molecule sensing, single-photon sources, and light emitting devices.

Enhanced Two-Photon Absorption in a GaAs/AlAs Multilayer Cavity

A strong enhancement of two-photon absorption (TPA) has been demonstrated in a 30-period GaAs/AlAs multilayer with a GaAs half-wavelength (λ/2) cavity layer. In order to measure the time-resolved TPA signals, we successfully developed a selective etching process using an etch-stopper structure consisting of 5-nm-thick AlAs and 200-nm-thick Al0.3Ga0.7As double layers for removing the GaAs substrate. An analysis of the observed TPA results reveals that the average light intensity of the multilayer cavity is 41 times larger than that of a GaAs bulk sample. The obtained enhancement factor is in good agreement with the simulated value (45 times larger), which is determined from the simulated light intensity distribution by the conventional transfer matrix method.

A HOMOGENEOUS MODEL FOR MASS TRANSFER ENHANCEMENT IN GAS-LIQUID-LIQUID SYSTEMS

ABSTRACT A modified homogeneous mass transfer model based on penetration theory is proposed for the mass transfer enhancement in gas-liquid-liquid systems. The present model assumes the existence of a shuttle mechanism between the organic dispersed phase and the continuous aqueous phase and considers the effect of diffusivity along with the solubility ratio and the dispersed phase holdup. The concept of effective diffusivity for gas in liquid-liquid emulsions has been used to consider the effect of diffusivity on mass transfer enhancement. The proposed model predicts the experimentally obtained enhancement factor with reasonable accuracy and numerical simplicity. Fresh experiments were also conducted to further validate the proposed model.

Study and formation of 2D microstructures of sapphire by focused ion beam milling

Dry etching of C-plane sapphire wafer has been studied using Ga+ focused ion beam milling (FIBM). Due to a much lower milling rate of sapphire compared to GaN, it has been proven that gas-assisted FIBM (GAFIBM) is a necessity. Furthermore, it needs to be determined whether XeF2- or I2-GAFIBM can improve the technique. We found that XeF2-GAFIBM gave the highest milling rate. The obtained enhancement factor of the XeF2-GAFIBM milling rate compared to FIBM rate varied from 2.3 to 1.2 for the ion beam current in the range 20pA–1nA. A favorable milling rate selectivity of sapphire to nickel film of about 1.5 was obtained by XeF2-GAFIBM at 350pA. We have successfully fabricated a variety of 2D sapphire-based submicron pillar or hole arrays in regular crystals, quasicrystals and aperiodic symmetries. The nearest neighbor distance was down to 230nm. The depth achieved was deeper than 400nm. The air filling factor ranged from 12% to 60%. These sapphire-based 2D microstructures were applied on flip-chip GaN-based light emitting diodes (LEDs) and more than 40% improvement in light extraction was obtained.

Fiber optic-linear array detection spectrophotometry in combination with cloud point extraction for simultaneous preconcentration and determination of cobalt and nickel

A new combined method including fiber optic-linear array detection spectrophotometry (FO-LADS) and cloud point extraction (CPE) was developed using a cylindrical micro cell for simultaneous preconcentration and determination of different species. The CPE and FO-LADS methods have good matching conditions for combination because FO-LADS is suitable as a detection technique for the low volume of remained phase obtained after CPE. This combination was carried out using 50 μL cylindrical micro cell and then employed for simultaneous preconcentration and determination of cobalt and nickel. Cloud point extraction method was based on the chromogenic reaction of metal ions and 1-(2-pyridylazo)-2-naphthol (PAN) and then preconcentration of formed complexes using octylphenoxypolyethoxyethanol (Triton X-114). The remained phase after CPE was transferred into cylindrical micro cell and located at the cell holder of FO-LADS. The spectra of cobalt and nickel complexes were collected by FO-LADS and processed for ordinary and first derivative spectrophotometry. Optimization of different parameters was evaluated. Under optimum conditions, calibration curves were linear in the range of 0.6–30.0 and 0.1–15.0 μg L −1 with detection limits of 0.2 and 0.04 μg L −1 for Co and Ni respectively. The relative standard deviations (R.S.D.s) were lower than 4%. The obtained enhancement factors were 198 and 199 for cobalt and nickel, respectively. The proposed method was compared with the other methods and applied to the analysis of several real and spiked samples.

Surface enhanced Raman scattering of p-aminothiophenol self-assembled monolayers in sandwich structure fabricated on glass

A sandwich structure consisting of Ag nanoparticles (NPs), p-aminothiophenol (p-ATP) self-assembled monolayers (SAMs), and Ag NPs was fabricated on glass and characterized by surface enhanced Raman scattering (SERS). The SERS spectrum of a p-ATP SAM in such sandwich structure shows that the electromagnetic enhancement is greater than that on Ag NPs assembled on glass. The obtained enhancement factors (EF) on solely one sandwich structure were as large as 6.0 +/- 0.62 x 10(4) and 1.2 +/- 0.62 x 10(7) for the 7a and 3b(b(2)) vibration modes, respectively. The large enhancement effect of p-ATP SAMs is likely a result of plasmon coupling between the two layers of Ag NP (localized surface plasmon) resonance, creating a large localized electromagnetic field at their interface, where p-ATP resides. Moreover, the fact that large EF values (approximately 1.9 +/- 0.7 x 10(4) and 9.4 +/- 0.7 x 10(6) for the 7a- and b(2)-type vibration modes, respectively) were also obtained on a single sandwich structure of Au NPsp-ATP SAMsAg NPs in the visible demonstrates that the electromagnetic coupling does not exist only between Ag NPs but also between Au and Ag NPs. The lower EF values on Au-to-Ag NPs compared to those on Ag-to-Ag NPs demonstrate that the Au-to-Ag coupling must be less effective than the Ag-to-Ag coupling for the induction of SERS in the visible.

On the modifications of oscillator strengths and damping constants of Fe I spectral lines from 500 nm to 510 nm

We modified oscillator strengths and enhancement factors of van der Waals damping constants for 84 moderately strong and weak neutral iron spectral lines between 500 nm and 510 nm, by fitting the solar synthetic spectrum to the observed one. We have found significant difference between the oscillator strengths and damping constants taken from an extensive spectral line list, frequently used for spectral synthesis, and their modified values. Our findings include: (1) the mean value of the distribution of the difference between our and listed oscillator strengths is -0.31dex and its width is 0.42 dex, (2) a decrease of this difference with increasing equivalent width of spectral lines, (3) unusually high values of the obtained enhancement factors in comparison with the results of other authors, (4) a decrease of enhancement factor with increasing excitation potential for lines with equivalent width greater than 7.5 pm, (5) a decrease of enhancement factors with increasing equivalent width and (6) a decrease of enhancement factors with increasing oscillator strengths.

Ultraviolet light amplification within a nanometer-sized layer

Light amplification in the ultraviolet region is confirmed in mixed rubidium-cesium chloride crystals at room temperature. The probe laser light, which falls on the 275-nm band of Auger-free luminescence (AFL) arising from radiative transition of the Cl $3p$ valence electrons into the Cs $5p$ core holes, is enhanced in intensity when the deep-lying Rb $4p$ core electrons are pumped into the conduction band by undulator radiation from an electron storage ring. The obtained enhancement factor roughly corresponds to an amplification coefficient of $7\ifmmode\times\else\texttimes\fi{}{10}^{3} {\mathrm{cm}}^{\ensuremath{-}1},$ which is much higher than those of typical solid-state lasers. It is emphasized that the amplification of AFL occurs in a surface layer as thin as about 20 nm, and that the inverted population between the valence and core bands is realized with any pump power. The present observation provides us with a new possibility of nanolaser fabrication.