Gas Adsorption Response of Piezoelectrically Driven Microcantilever Beam Gas Sensors: Analytical, Numerical, and Experimental Characterizations.

This work presents an approach for the estimation of the adsorbed mass of 1,5-diaminopentane (cadaverine) on a functionalized piezoelectrically driven microcantilever (PD-MC) sensor, using a polynomial developed from the characterization of the resonance frequency response to the known added mass. This work supplements the previous studies we carried out on the development of an electronic nose for the measurement of cadaverine in meat and fish, as a determinant of its freshness. An analytical transverse vibration analysis of a chosen microcantilever beam with given dimensions and desired resonance frequency (>10 kHz) was conducted. Since the beam is considered stepped with both geometrical and material non-uniformity, a modal solution for stepped beams, extendable to clamped-free beams of any shape and structure, is derived and used for free and forced vibration analyses of the beam. The forced vibration analysis is then used for transformation to an equivalent electrical model, to address the fact that the microcantilever is both electronically actuated and read. An analytical resonance frequency response to the mass added is obtained by adding simulated masses to the free end of the beam. Experimental verification of the resonance frequency response is carried out, by applying known masses to the microcantilever while measuring the resonance frequency response using an impedance analyzer. The obtained response is then transformed into a resonance frequency to the added mass response polynomial using a polynomial fit. The resulting polynomial is then verified for performance using different masses of cantilever functionalization solution. The functionalized cantilever is then exposed to different concentrations of cadaverine while measuring the resonance frequency and mass of cadaverine adsorbed estimated using the previously obtained polynomial. The result is that there is the possibility of using this approach to estimate the mass of cadaverine gas adsorbed on a functionalized microcantilever, but the effectiveness of this approach is highly dependent on the known masses used for the development of the response polynomial model.

Surface Modification Enabling Reproducible Cantilever Functionalization for Industrial Gas Sensors.

Micro-cantilever sensors are a known reliable tool for gas sensing in industrial applications. We have demonstrated the application of cantilever sensors on the detection of a meat freshness volatile biomarker (cadaverine), for determination of meat and fish precise expiration dates. For achieving correct target selectivity, the cantilevers need to be functionalized with a cadaverine-selective binder, based on a cyclam-derivative. Cantilever surface properties such as surface energy strongly influence the binder morphology and material clustering and, therefore, target binding. In this paper, we explore how chemical and physical surface treatments influence cantilever surface, binder morphology/clustering and binding capabilities. Sensor measurements with non-controlled surface properties are presented, followed by investigations on the binder morphology versus surface energy and cadaverine capture. We demonstrated a method for hindering binder crystallization on functionalized surfaces, leading to reproducible target capture. The results show that cantilever surface treatment is a promising method for achieving a high degree of functionalization reproducibility for industrial cantilever sensors, by controlling binder morphology and uniformity.

Tailoring optical discs for surface plasmon polaritons generation.

The article reports on an optimization of gold submicron structures based on modified recordable blank digital versatile discs for surface plasmon polaritions excitation, mainly in near-infrared region. We have examined internal layers of commercially available DVD+R, DVD-R, DVD+RW and DVD-RW optical discs and we have elaborated a simple, inexpensive approach providing sharp resonances with efficiency reaching 95% for collimated excitation laser beams. We have experimentally and numerically confirmed the SPPs intensity being up to 220 times the intensity of the excitation laser beam. We have also directly measured thermal energy loss accompanying SPPs excitation.

Diamond like carbon nanocomposites with embedded metallic nanoparticles.

In this work we present an overview on structure formation, optical and electrical properties of diamond like carbon (DLC) based metal nanocomposites deposited by reactive magnetron sputtering and treated by plasma and laser ablation methods. The influence of deposition mode and other technological conditions on the properties of the nanosized filler, matrix components and composition were studied systematically in relation to the final properties of the nanocomposites. Applications of the nanocomposites in the development of novel biosensors combining resonance response of wave guiding structures in DLC based nanocomposites as well as plasmonic effects are also presented.

Highly Stable Monocrystalline Silver Clusters for Plasmonic Applications.

Plasmonic sensor configurations utilizing localized plasmon resonances in silver nanostructures typically suffer from the rapid degradation of silver under ambient atmospheric conditions. In this work, we report on the fabrication and detailed characterization of ensembles of monocrystalline silver nanoparticles (NPs), which exhibit a long-term stability of optical properties under ambient conditions without any protective treatments. Ensembles with different densities (surface coverages) of size-selected NPs (mean diameters of 12.5 and 24 nm) on quartz substrates are fabricated using the cluster-beam technique and characterized by linear spectroscopy, two-photon-excited photoluminescence, surface-enhanced Raman scattering microscopy, and transmission electron, helium ion, and atomic force microscopies. It is found that the fabricated ensembles of monocrystalline silver NPs preserve their plasmonic properties (monitored with optical spectroscopy) and strong field enhancements (revealed by surface-enhanced Raman spectroscopy) at least 5 times longer as compared to chemically synthesized silver NPs with similar sizes. The obtained results are of high practical relevance for the further development of sensors, resonators, and metamaterials utilizing the plasmonic properties of silver NPs.

ITO with embedded silver grids as transparent conductive electrodes for large area organic solar cells.

In this work, development of semi-transparent electrodes for efficient large area organic solar cells (OSCs) has been demonstrated. Electron beam evaporated silver grids were embedded in commercially available ITO coatings on glass, through a standard negative photolithography process, in order to improve the conductivity of planar ITO substrates. The fabricated electrodes with embedded line and square patterned Ag grids reduced the sheet resistance of ITO by 25% and 40%, respectively, showing optical transmittance drops of less than 6% within the complete visible light spectrum for both patterns. Solution processed bulk heterojunction OSCs based on PTB7:[70]PCBM were fabricated on top of these electrodes with cell areas of 4.38 cm2, and the performance of these OSCs was compared to reference cells fabricated on pure ITO electrodes. The Fill Factor (FF) of the large-scale OSCs fabricated on ITO with embedded Ag grids was enhanced by 18% for the line grids pattern and 30% for the square grids pattern compared to that of the reference OSCs. The increase in the FF was directly correlated to the decrease in the series resistance of the OSCs. The maximum power conversion efficiency (PCE) of the OSCs was measured to be 4.34%, which is 23% higher than the PCE of the reference OSCs. As the presented method does not involve high temperature processing, it could be considered a general approach for development of large area organic electronics on solvent resistant, flexible substrates.

Surface plasmon polariton excitation by second harmonic generation in single organic nanofibers.

Coherent local excitation of surface plasmon polaritons (SPPs) by second-harmonic generation (SHG) in individual aligned crystalline organic functionalized para-phenylene nanofibers deposited on a thin silver film is demonstrated. The SH-SPP generation is considered theoretically and investigated experimentally with angular-resolved leakage radiation spectroscopy for normal incidence of the excitation beam. Both measurements and simulations show asymmetric excitation of left- and right-propagating SH-SPPs, which is explained as an effect of fiber molecules being oriented at an angle relative to the silver film surface.

Compact wavelength add-drop multiplexers using Bragg gratings in coupled dielectric-loaded plasmonic waveguides.

We report a novel design of a compact wavelength add-drop multiplexer utilizing dielectric-loaded surface plasmon-polariton waveguides (DLSPPWs). The DLSPPW-based configuration exploits routing properties of directional couplers and filtering abilities of Bragg gratings. We present practical realization of a 20-µm-long device operating at telecom wavelengths that can reroute optical signals separated by approximately 70 nm in the wavelength band. We characterize the performance of the fabricated structures using scanning near-field optical microscopy as well as leakage-radiation microscopy and support our findings with numerical simulations.

The interplay between localized and propagating plasmonic excitations tracked in space and time.

In this work, the mutual coupling and coherent interaction of propagating and localized surface plasmons within a model-type plasmonic assembly is experimentally demonstrated, imaged, and analyzed. Using interferometric time-resolved photoemission electron microscopy the interplay between ultrashort surface plasmon polariton wave packets and plasmonic nanoantennas is monitored on subfemtosecond time scales. The data reveal real-time insights into dispersion and localization of electromagnetic fields as governed by the elementary modes determining the functionality of plasmonic operation units.

Optical dipole mirror for cold atoms based on a metallic diffraction grating.

We report on the realization of a plasmonic dipole mirror for cold atoms based on a metallic grating coupler. A cloud of atoms is reflected by the repulsive potential generated by surface plasmon polaritons (SPPs) excited on a reflection gold grating by a 780 nm laser beam. Experimentally and numerically determined mirror efficiencies are close to 100%. The intensity of SPPs above a real grating coupler and the atomic trajectories, as well as the momentum dispersion of the atom cloud being reflected, are computed. A suggestion is given as to how the plasmonic mirror might serve as an optical atom chip.

Substrate steered crystallization of naphthyl end-capped oligothiophenes into nanofibers: the influence of methoxy-functionalization.

Naphthyl end-capped oligothiophenes are a class of materials well suited for high-performance organics based devices. The formation of nanofibers on muscovite mica from 2,5-bis(naphth-2-yl)thiophene (NaT), 5,5'-bis(naphth-2-yl)-2,2'-bithiophene (NaT2), and 5,5''-bis(naphth-2-yl)-2,2':5',2''-terthiophene (NaT3) as well as of the methoxy-functionalized variants MONaT, MONaT2, and MONaT3 is investigated via atomic force microscopy, X-ray diffraction, polarized fluorescence microscopy, and fluorescence spectroscopy. From polarized fluorescence microscopy spatially resolved molecular orientations are deduced revealing a profound anisotropy. Fibers from lying molecules grow along distinct substrate directions. Methoxy-functionalization substantially increases the crystallization into aligned fibers. In air Ostwald ripening is observed. The morphological variations of the aggregates result in specific optical signatures, disclosed by temperature dependent and spatially resolved fluorescence spectra.

Modeling Nonlinear Dynamics of Functionalization Layers: Enhancing Gas Sensor Sensitivity for Piezoelectrically Driven Microcantilever.

This article presents a parametrized response model that enhances the limit of detection (LOD) of piezoelectrically driven microcantilever (PD-MC) based gas sensors by accounting for the adsorption-induced variations in elastic properties of the functionalization layer (binder) and the nonlinear motional dynamics of the PD-MC. The developed model is demonstrated for quantifying cadaverine, a volatile biogenic diamine whose concentration is used to assess the freshness of meat. At low concentrations of cadaverine, an increase in the resonance frequency is observed, contrary to the expected reduction due to mass added by adsorption. The study explores the variations in the elastic modulus vis-à-vis the adsorbed mass of cadaverine and derives the resonance frequency to the adsorbed mass response function. We advance a blended technique involving the analysis of atomic force microscopy (AFM) force-distance (f-d) curves and fitting of the quartz crystal microbalance (QCM) impedance response spectrum to deduce the adsorption-induced changes in the viscoelastic properties of the functionalization layer. The findings obtained are subsequently employed in modeling the response function for a structurally nonhomogenous PD-MC, highlighting the significance of the functionalization layer to the global elastic properties. The structural composition of the PD-MC beam adopted herein features a trapezoidal base hosting the actuating piezoelectric stratum and a rectangular free end with a functionalization layer. The Euler-Bernoulli beam theory coupled with Hamilton's principle is used to develop the equation of motion, which is subsequently discretized into a set of nonlinear ordinary differential equations via Galerkin expansion, and the solutions to the first fundamental mode of vibration are determined using the method of multiple scales. The obtained solutions provide a basis for deducing the nonlinear response function model to the adsorbed mass. The derived model is validated by recorded resonance frequency changes resulting from exposure to known concentrations of cadaverine. We demonstrate that the increase in resonance frequency for low concentrations of cadaverine is due to the dominance of the variation of the elastic modulus of the functionalization layer originating from the initial binder-analyte interactions over damping due to added mass. It is concluded that the developed nonlinear response function model can reliably be used to quantify the cadaverine concentration at low concentrations with an elevated Limit of Detection.

Tuning Surface Defect States in Sputtered Titanium Oxide Electron Transport Layers for Enhanced Stability of Organic Photovoltaics.

Nonfullerene acceptors (NFAs) have dramatically improved the power conversion efficiency (PCE) of organic photovoltaics (OPV) in recent years; however, their device stability currently remains a bottleneck for further technological progress. Photocatalytic decomposition of nonfullerene acceptor molecules at metal oxide electron transport layer (ETL) interfaces has in several recent reports been demonstrated as one of the main degradation mechanisms for these high-performing OPV devices. While some routes for mitigating such degradation effects have been proposed, e.g., through a second layer integrated on the ETL surface, no clear strategy that complies with device scale-up and application requirements has been presented to date. In this work, it is demonstrated that the development of sputtered titanium oxide layers as ETLs in nonfullerene acceptor based OPV can lead to significantly enhanced device lifetimes. This is achieved by tuning the concentration of defect states at the oxide surface, via the reactive sputtering process, to mitigate the photocatalytic decomposition of NFA molecules at the metal oxide interlayers. Reduced defect state formation at the oxide surface is confirmed through X-ray photoelectron spectroscopy (XPS) studies, while the reduced photocatalytic decomposition of nonfullerene acceptor molecules is confirmed via optical spectroscopy investigations. The PBDB-T:ITIC organic solar cells show power conversion efficiencies of around 10% and significantly enhanced photostability. This is achieved through a reactive sputtering process that is fully scalable and industry compatible.

Organic surface-grown nanowires for functional devices.

Discontinuous organic thin film growth on the surface of single crystals results in crystalline nanowires with extraordinary morphological and optoelectronic properties. By way of being generated at the interface of organic and inorganic materials, these nanowires combine the advantages of flexible organic films with the defectless character of inorganic crystalline substrates. The development of destruction-free transfer and direct growth methods allows one to integrate the organic nanowires into semiconductor, metallic electronic or photonic platforms. This article details the mechanisms that lead to the growth of these nanowires and exemplifies some of the linear as well as non-linear photonic properties, such as optical wave guiding, lasing and frequency conversion. The article also highlights future potential by showing that organic nanowires can be integrated into optoelectronic devices or hybrid photonic/plasmonic platforms as passive and active nanoplasmonic elements.

Measurement of surface plasmon autocorrelation functions.

In this paper we demonstrate the realization of an autocorrelator for the characterization of ultrashort surface plasmon polariton (SPP) pulses. A wedge shaped structure is used to continuously increase the time delay between two interfering SPPs. The autocorrelation signal is monitored by non-linear two-photon photoemission electron microscopy. The presented approach is applicable to other SPP sensitive detection schemes that provide only moderate spatial resolution and may therefore be of general interest in the field of ultrafast plasmonics.

Surface plasmon polariton propagation in organic nanofiber based plasmonic waveguides.

Plasmonic wave packet propagation is monitored in dielectric-loaded surface plasmon polariton waveguides realized from para-hexaphenylene nanofibers deposited onto a 60 nm thick gold film. Using interferometric time resolved two-photon photoemission electron microscopy we are able to determine phase and group velocity of the surface plasmon polariton (SPP) waveguiding mode (0.967c and 0.85c at λ(Laser) = 812nm) as well as the effective propagation length (39 µm) along the fiber-gold interface. We furthermore observe that the propagation properties of the SPP waveguiding mode are governed by the cross section of the waveguide.

Morphological tuning of the plasmon dispersion relation in dielectric-loaded nanofiber waveguides.

Understanding the impact of lateral mode confinement in plasmonic waveguides is of fundamental interest regarding potential applications in plasmonic devices. The knowledge of the frequency-wave vector dispersion relation provides the full information on electromagnetic field propagation in a waveguide. This Letter reports on the measurement of the real part of the surface plasmon polariton dispersion relation in the near infrared spectral regime for individual nanoscale plasmonic waveguides, which were formed by deposition of para-hexaphenylene (p-6P) based nanofibers on top of a gold film. A detailed structural characterization of the nanofibers provides accurate information on the dimensions of the investigated waveguides and enables us to quantify the effect of mode confinement by comparison with experimental results from continuous p-6P films and calculations based on the effective index method.

Flexible organic solar cells including efficiency enhancing grating structures.

In this work, a new method for the fabrication of organic solar cells containing functional light-trapping nanostructures on flexible substrates is presented. Polyimide is spin-coated on silicon support substrates, enabling standard micro- and nanotechnology fabrication techniques, such as photolithography and electron-beam lithography, besides the steps required for the bulk-heterojunction organic solar cell fabrication. After the production steps, the solar cells on polyimide are peeled off the silicon support substrates, resulting in flexible devices containing nanostructures for light absorption enhancement. Since the solar cells avoid using brittle electrodes, the performance of the flexible devices is not affected by the peeling process. We have investigated three different nanostructured grating designs and conclude that gratings with a 500 nm pitch distance have the highest light-trapping efficiency for the selected active layer material (P3HT:PCBM), resulting in an enhancement of about 34% on the solar cell efficiency. The presented method can be applied to a large variety of flexible nanostructured devices in future applications.

Helium Ion Microscopy and Sectioning of Spider Silk.

Focused ion beams have recently emerged as a powerful tool for ultrastructural imaging of biological samples. In this article, we show that helium ion microscopy (HIM), in combination with ion milling, can be used to visualize the inner structure of both major and minor ampullate silk fibers of the orb-web weaving spider Nephila madagascariensis. The internal nanofibrils were imaged in pristine silk fibers, with little or no damage to the sample structure observed. Furthermore, a method to cut/rupture the fibers using He+ ions combined with internal sample tension is presented. This showed that the stretching and rupturing of spider silk is a highly dynamic process with considerable material reorganization.

Nanoscale imaging of major and minor ampullate silk from the orb-web spider Nephila Madagascariensis.

Spider silk fibres have unique mechanical properties due to their hierarchical structure and the nanoscale organization of their proteins. Novel imaging techniques reveal new insights into the macro- and nanoscopic structure of Major (MAS) and Minor (MiS) Ampullate silk fibres from pristine samples of the orb-web spider Nephila Madagascariensis. Untreated threads were imaged using Coherent Anti-Stokes Raman Scattering and Confocal Microscopy, which revealed an outer lipid layer surrounding an autofluorescent protein core, that is divided into two layers in both fibre types. Helium ion imaging shows the inner fibrils without chemical or mechanical modifications. The fibrils are arranged parallel to the long axis of the fibres with typical spacing between fibrils of 230 nm ± 22 nm in the MAS fibres and 99 nm ± 24 nm in the MiS fibres. Confocal Reflection Fluorescence Depletion (CRFD) microscopy imaged these nano-fibrils through the whole fibre and showed diameters of 145 nm ± 18 nm and 116 nm ± 12 nm for MAS and MiS, respectively. The combined data from HIM and CRFD suggests that the silk fibres consist of multiple nanoscale parallel protein fibrils with crystalline cores oriented along the fibre axes, surrounded by areas with less scattering and more amorphous protein structures.