Thick Gold Film Research Articles

Simple Method to Fabricate Au Nanoparticle-Decorated TiO2 Nanotube Arrays for Enhanced Visible Light Photocurrent**Supported by the National Natural Science Foundation of China under Grant Nos 11474215 and 21204058, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Au nanoparticle-decorated TiO2 nanotube arrays are prepared by a simple method, which is a thermal annealing thin gold film deposited on anodic oxidized TiO2 nanotube arrays. These electron microscope images present that Au nanoparticles are well dispersed within the wall and on the surface of the TiO2 nanotubes. Meanwhile, the morphologies of Au nanoparticles can be controlled by changing the thickness of the deposited gold film. Associated with the excitation of localized surface plasmon resonances, the prepared Au nanoparticle-decorated TiO2 nanotube arrays could work as visible light responsive photocatalysts to produce a greatly enhanced photocurrent density. By varying the initial gold film thickness, such Au nanoparticle-decorated TiO2 nanotube arrays could be optimized to obtain the highest photocurrent generation efficiency in the visible and UV light regions.

Improving Stitch Bond on Hybrid Thick Film Substrate using Stand-Off Stitch Wire Bond Technique

Thick film technology is based on a paste containing glass frit that is screen printed and fused at high temperature onto various ceramic substrate materials. Softening or melting this glassy frit forms a cohesive layer, binding the conductors, resistors or dielectric materials to the ceramic. The dynamics of the printing process and inherent number of associated variables negatively impact the uniformity of the fired surface on a micro scale, which can lead to variation in the wire bonding process. Other processes associated with thick film substrate fabrication can cause problems as well. Laser trimming is used to adjust the value of printed resistors to meet design requirements. This ablation of printed resistors by high–powered pulse laser leaves a halo of debris and contamination on the ceramic substrate, which can cause wire bond lifting. In this paper, we will demonstrate a way to eliminate these problems using a bonding technique called Stand- Off Stitch bonding (SOS). This wire bond type is formed by first placing a ball bump at the second bond, or stitch, location on the thick film substrate, and then forming a normal wire that terminates on that bump. This places two ball bumps at each end of the wire, similar to a security bond. However, the ball bump is located under the stitch instead of on top. This SOS wire bond technique is compliant with the MIL-STD- 883 for a compound bond, where one bond is placed on top of another bond. With the gold bump placed on top of the gold thick film pad, the bump acts as a foundation for the stitch bond, providing a wider contact area and clean bond surface to secure a reliable stitch bond interconnect. With this change, an abrupt improvement to the resultant destruct wire pull tests can be achieved, promoting a robust, controlled process for wire bond interconnects.

Measurement of gold nanofilm dose enhancement using unlaminated radiochromic film.

Bombarding high-Z material with x-ray radiation releases Auger electrons and Coster-Kronig electrons, along with deeper penetrating fluorescent x-rays and photoelectrons. The Auger and Coster-Kronig electron penetration distance is on the order of nanometers to micrometers in water or tissue, creating a large dose enhancement accompanied by a RBE greater than 1 at the cellular level. The authors' aim is to measure the gold nanofilm dose enhancement factor (DEF) at the cellular level with unlaminated radiochromic film via primary 50 kVp tungsten x-ray spectrum interaction, similar to an electronic brachytherapy spectrum. Unlaminated Gafchromic(®) EBT2 film and Monte Carlo modeling were combined to derive DEF models. Gold film of thickness 23.1 ± 4.3 nm and surface roughness of 1.2 ± 0.2 nm was placed in contact with unlaminated radiochromic film in a downstream orientation and exposed to a 50 kVp tungsten bremsstrahlung, mean energy 19.2 keV. Film response correction factors were derived by Monte Carlo modeling of electron energy deposition in the film's active layer, and by measuring film energy dependence from 4.5 keV to 50 kVp. The measured DEF within a 13.6 μm thick water layer was 0.29 with a mean dose of 94 ± 9.4 cGy from Au emissions and 324 ± 32.4 cGy from the 50 kVp primary beam. Monte Carlo derived correction factors allowed determination of Au contributed dose in shallower depths at 0.25 μm intervals. Maximum DEF of 18.31 was found in the first 0.25 μm water depth. Dose enhancement from Au nanofilm can be measured at the cellular level using unlaminated radiochromic film. Complementing the measured dose value with Monte Carlo calculations allows estimation of dose enhancement at depth increments within the cellular range.

A transparent silica colloidal crystal/PDMS composite and its application for crack suppression of metallic coatings

A silica colloidal crystal (SCC)–polydimethylsiloxane (PDMS) composite with a heterogeneous surface of silica and PDMS was prepared by spreading a premixed PDMS solution on the 3D structured SCCs and curing the solution in-situ. Although the SCCs had a light blue color, the obtained composite of SCC and PDMS, due to the close effective refractive indexes of the materials, was colorless and transparent; the UV–vis spectra indicated a negligible effect of the added SCC on the transmittance of the PDMS sheet (1% reduction). Interestingly, the transparent composite sheet became translucent under stress and became clear again when relaxed. It was found that the wrinkles formed on the surface under stress were responsible for the optical change; and, the formation of the wrinkles was ascribed to the rigid nature of the SCC layer embedded in PDMS. We had applied this SCC/PDMS composite as a substrate to support a thin gold film of nanoscale thickness and found that the embedded SCC layer worked well as a transitional interface for bonding materials of mismatched mechanical properties. The incorporation of SCC layer significantly suppressed the crack generation and propagation of the gold film. The results demonstrated a potential approach for fabricating compliant and crackfree metallic films on polymeric substrates.

Integration of silicon-loaded nanoplasmonic waveguides onto a micro-machined characterization beam for nonlinear optics applications

Silicon-loaded nanoplasmonic waveguides were integrated onto a micron-scale characterization beam to allow for accurate and efficient nonlinear optical characterization. The waveguides consist of a 95nm×340nm silicon core that is capped by a 60nm thick gold film. The characterization beam is formed by precision cleaving one waveguide end facet and by deep silicon etching the substrate area adjacent to the other end facet. This configuration allows input radiation to be coupled directly to the waveguides using a microscope objective and output radiation to be out-coupled with a lensed single-mode optical fiber. The fabrication steps are characterized via scanning electron microscopy at various points throughout the process. The fabricated devices are optically characterized using an ultrafast nonlinear pump–probe time-domain spectroscopy setup. Ultrafast all-optical modulation is measured in the waveguides on two timescales: τ1=1.98±0.40ps and τ2=17.9±6.8ps.

Localized and nonlocalized plasmon resonance enhanced light absorption in metal-insulator-metal nanostructures

Multiple absorption bands in a metal-insulator-metal (MIM) nanostructure are comprehensively investigated in the visible and near-infrared (near-IR) regime. The MIM nanostructure consists of patterned gold squares with systematically varying size and period atop a thin aluminum nitride dielectric layer on a thick gold film. Both the transverse-electric (TE) slab-waveguide mode and the transverse-magnetic (TM) surface plasmon-polariton mode can be excited in the nanostructure. Grating coupling of the incident light into nonlocalized traveling waves is found from electromagnetic field patterns and from the linear dispersion of the absorption modes with period. A localized mode strengthens the near-IR absorption band for small square sizes, and the resonance shifts to red for large square sizes.

Evaluating indent pile-up with metallic films on ceramic-like substrates

Abstract

Cooperative Enhancement of Second-Harmonic Generation from a Single CdS Nanobelt-Hybrid Plasmonic Structure

Semiconductor nanostructures (e.g., nanowires and nanobelts) hold great promise as subwavelength coherent light sources, nonlinear optical frequency converters, and all-optical signal processors for optoelectronic applications. However, at such small scales, optical second-harmonic generation (SHG) is generally inefficient. Herein, we report on a straightforward strategy using a thin Au layer to enhance the SHG from a single CdS nanobelt by 3 orders of magnitude. Through detailed experimental and theoretical analysis, we validate that the augmented SHG originates from the mutual intensification of the local fields induced by the plasmonic nanocavity and by the reflections within the CdS Fabry-Pérot resonant cavity in this hybrid semiconductor-metal system. Polarization-dependent SHG measurements can be employed to determine and distinguish the contributions of SH signals from the CdS nanobelt and gold film, respectively. When the thickness of gold film becomes comparable to the skin depth, SHG from the gold film can be clearly observed. Our work demonstrates a facile approach for tuning the nonlinear optical properties of mesoscopic, nanostructured, and layered semiconductor materials.

Demonstration of a 3D Radar‐Like SERS Sensor Micro‐ and Nanofabricated on an Optical Fiber

A 3D surface‐enhanced Raman scattering (SERS) sensor is realized on the facet of an optical fiber. A SERS radar configuration consisting of a parabolic mirror and a 3D spherical SERS body is designed. The parabolic mirror is used to focus the excitation laser light onto the surface of the SERS body, and to collect and reflect the Raman scattered light back into the optical fiber. The SERS body's surface consists of microscale trenches and is covered with gold nanogratings and nanoparticles on a thick gold film with a thin SiO2 spacer. Nanoscale 3D printing technology, thermal evaporation, and UV laser pulse irradiation are used to fabricate and metalize the mold of the 3D SERS radar configuration. The performance of this Au‐based SERS fiber sensor is competitive with those of Ag‐based SERS fiber sensors reported in the literature. It exhibits a detection limit of 10−6m for crystal violet in ethanol using 30 mW of 785 nm excitation (off resonance with the crystal violet electronic absorption) and 10 s acquisition time. Such a 3D scheme expands the design possibilities of SERS nanostructures on the facets of optical fibers enormously.

Ageing Effects on Sputtered YSZ Thin Films and Gold Electrodes for Electrochemical Sensor Applications

8mol% Yttria Stabilized Zirconia (8YSZ) is one of the most studied solid electrolytes. With increasing demand to miniaturize electrochemical sensors and solid oxide fuel cells using 8YSZ thin film electrolytes, it is vital to study the ageing effects of 8YSZ thin films. Although ageing effects in bulk YSZ had been previously studied, no work has been reported on YSZ thin films. This paper reports the fabrication, characterization of RF sputtered, polycrystalline 8YSZ thin films and the effect of prolonged heat treatment on the ionic conductivity and surface morphology. The films are annealed at 10000C for three hours before starting the aging experiment. With electrochemical sensor applications in mind, the ageing temperature was chosen to be closer to exhaust gas temperature (i.e, 6500C). The results show that there is no significant variation in the ionic conductivity and activation energy values of 8YSZ thin films (250 nm) before and after 15000 minutes of heat treatment at 6500C. The conductivity of YSZ film is constant at 0.057Scm-1 throughout the course of high temperature soaking in spite of significant grain growth. The grain size of the film is found to increase from 15nm in as deposited film to 60nm in aged film. The activation energy value before and after heat treatment was 0.98eV. After 15000 minutes, the YSZ film started developing cracks in the vicinity of thick film gold electrode and the conductivity values deteriorated. However, no cracks are present in the (bare) YSZ sample without any electrodes. Hence the conductivity deterioration is attributed to the cracks on YSZ triggered by the presence of thick film gold electrode. YSZ samples are aged with three types of thin film gold electrodes on them i) 200nm Au without Ti adhesion layer ii) 200nm Au with 10nm Ti adhesion layer and ii) 200nm Au with 10nm Ti adhesion layer in 45nm trench on YSZ. The electrodes without adhesion layer completely de-wetted itself in 48 hours. Ti/Au layers showed re-crystallization induced porosity after 48 hours and gradually disintegrated around 100hours. In the third type where Ti/Au layers were deposited on a 45nm trench on 250nm YSZ thin film. This buried electrode seemed more promising and continuous up to at least 15000 minutes. Figure 1

Actively controllable EIT-like resonance between localized and propagating surface plasmons for optical switching

We theoretically investigate the plasmonic interaction between radiative localized surface plasmon resonances and subradiative propagating surface plasmon modes in a nanostructure consisting of a periodic array of gold nanobars and an optically thick gold film, separated by a silica dielectric spacer layer. A controllable transparency window within the broad dipole resonance profile is observed clearly in the reflectance spectra via tailoring the length of the bar, the periodicity of the nanoparticle array, or the incident angle of applied field, respectively, a classic analog of electromagnetically induced transparency (EIT). We believe that the last excitation configuration is particularly beneficial for the realization of active manipulation of plasmonic optical switching without using coupling/control fields required in the conventional EIT scheme.

Growth temperature dependent surface plasmon resonances of densely packed gold nanoparticles’ films and their role in surface enhanced Raman scattering of Rhodamine6G

Localized surface plasmon resonance (LSPR) characteristics of gold nanoparticles films grown at different substrate temperatures and mass thicknesses with and without alumina capping were studied. At different film mass thicknesses, the LSPR response was observed mainly in the films grown at high substrate temperatures. About 300°C substrate temperature was found to be optimum for producing narrow and strong LSPR band in both uncapped and alumina capped gold nanoparticles films. The LSPR wavelength could be tuned in the range of 600–750nm by changing either number of ablation pulses or decreasing target to substrate distance (TSD) and alumina layer capping. Though the alumina capping re-sputtered the gold films still these films exhibited stronger LSPR response compared to the uncapped films. Atomic force microscopic analysis revealed formation of densely packed nanoparticles films exhibiting strong LSPR response which is consistent with the package density of the nanoparticles predicted by the theoretical calculations. The average size of nanoparticles increased with substrate temperature, number of ablation pulses and decreasing the TSD. For the same mass thickness of gold films grown at different substrate temperatures the surface enhanced Raman scattering (SERS) intensity of Rhodamine6G dye was found to be significantly different which had direct correlation with the LSPR strength of the films at the excitation wavelength.

Surface plasmon resonance based integrable micro spectrometer

We propose and realize the concept of a surface plasmon resonance based integrable and planar micro spectrometer that is suitable to determine the wavelength of incident light with sub-nanometer resolution over a broad range of frequencies in the infrared, visible, and ultraviolet regime. A dielectric grating structure with an ascending effective refractive index in the direction perpendicular to the periodicity is applied on top of a 20 nm thick gold film. Monochromatic light in normal incidence excites surface plasmons at confined areas on the sample so that the transmission is reduced locally. Depending on the wavelength, this area of low transmission shifts along the grating. The spatial transmission pattern provides information on the wavelength of the incident light.

Nanoscale boiling during single-shot femtosecond laser ablation of thin gold films

A nanoscale chaotic relief structure appears as a result of subthreshold single-shot femtosecond laser ablation of gold films in the regimes of fabrication of microbumps and nanospikes, but only for a relatively thick film. The observed nanoablation tendency versus film thickness makes it possible to suppose the existence of a sub-surface temperature maximum in thicker gold films and its absence within thinner film, which results from competing evaporative cooling and electronic heat conduction, as demonstrated by numerical simulations of the thermal dynamics.

Tapered Fiber-Optic Based Surface Plasmon Resonance Sensor

We propose a tapered fiber-optic based surface plasmon resonance (SPR) sensor. The plasmonic sensing is designed by coating the waist of tapered fiber-optic with gold. The transmission spectrum of SPR wavelength were investigated by 2D finite element method (FEM). The calculation shows that the dips of the resonance wavelength shift toward long wave direction with the thickness of gold film decreasing. And increasing the diameters of the waist core of tapered fiber-optic also makes the resonance wavelength shift long wave direction. Furthermore, changing the refractive index of the external samples from 1.333 to 1.343 with step of ~0.002, the SPR wavelength shifts linearly from 575.05nm to 472.5nm. Owing to its compact and simple configuration, it also provides a feasible program for the refractive index high sensitivity detection.Keywords: surface plasmon resonance (SPR); tapered fiber-optic sensor; surface plasma; finite element method (FEM); optical fiber sensor

Gold Photoluminescence Wavelength and Polarization Engineering

We demonstrate engineering of the spectral content and polarization of photoluminescence (PL) from arrayed gold nanoparticles atop a subwavelength-thin dielectric spacer and optically thick gold film, a configuration that supports gap-surface plasmon resonances (GSPRs). Choice of shapes and dimensions of gold nanoparticles influences the GSPR wavelength and polarization characteristics, thereby allowing us to enhance and spectrally mold the plasmon-assisted PL while simultaneously controlling its polarization. In order to understand the underlying physics behind the plasmon-enhanced PL, we develop a simple model that faithfully reproduces all features observed in our experiments showing also good quantitative agreement for the PL enhancement.

Design and optimization of surface plasmon resonance sensor based on multimode fiber

We optimized the design of surface plasmon resonance sensor based on side polished multimode fiber using the method of finite element. The effects of the residual fiber thickness, gold layer thickness and the guiding modes on the depth, the full width at half maximum of the resonance dips were investigated numerically. The sensitivity to the variation in refractive index of analyte and the figure of merit are evaluated for design optimization. The results show that the fiber propagation modes have negligible influence on the resonant wavelength but have effect on the resonance strength. The optimal design was achieved at the residual fiber thickness of 75 μm and the gold film thickness of 50 nm. The optimized sensor has an average sensitivity of 4989 nm/RIU in the refractive index range from 1.33 to 1.40.

Improved polarization contrast method for surface plasmon resonance imaging sensors by inert background gold film extinction

To improve the performance of high-throughput surface plasmon resonance (SPR) imaging sensors, the imperfections of the conventional gold film polarization contrast method and the bare prism polarization contrast method are discussed, and a background extinction method is proposed. Thick gold film is coated as the background area, surrounding gold sensing spots, to form a “gold microwell array”. By blocking the light reflected from the background area with appropriate parameters, an opposite-oriented SPR curve is obtained, which can be exploited to achieve a high signal-to-noise ratio in trace amount detection. The contrast between the sensing spots and the background is effectively improved, and the background is inert to the change of solution. Influences of parameter errors on the SPR curve are also investigated, and results show that extinction adjustment in this method is easy to be realized. The practicability of the background extinction method is proved by an experiment using a home-built SPR imaging sensor.

Fixed Escherichia coli bacterial templates enable the production of sensitive SERS-based gold nanostructures

We show herein for the first time the ability to fabricate highly sensitive SERS substrates using fixed Escherichia coli bacterial cells, used as templates after glutaraldehyde fixing, whereupon stable gold nanostructures of a suitable size for SERS generation were found lying on top of bacteria after an evaporation deposition procedure, followed by high temperature annealing. Thanks to the template effect provided by the bacterial cells, gold nanoparticles had been produced forming controlled agglomerate patterns containing high density plasmonic “hot spots”, thereby contributing to a remarkable enhancement of Raman signals. Moreover, a TEM copper grid marker used during the gold evaporation process, enabled to precisely correlate the optical properties (including LSPR and SERS spectra) and SEM morphology of a pre-defined area of gold nanostructures. It was observed that modulating either gold film thickness or annealing temperature, enabled to influence the LSPR properties of the produced Au NPs, and this was demonstrated by obtaining a highly sensitive SERS detection of a model compound, trans-1,2-bis(4-pyridyl)ethylene (BPE), down to 10−12M.

Self-Referenced Plasmon Waveguide Resonance Sensor Using Different Waveguide Modes

We report a plasmon waveguide resonance (PWR) sensing structure supporting two waveguide modes for self-referenced measurement. We show theoretically the dual mode PWR sensing structure owns unique opportunities for self-referenced measurement, and the accuracy and cross sensitivity can be optimized by simply varying the gold film thickness or dielectric layer thickness or dielectric film refractive index. This structure may provide an approach owning good compatibility with the surface plasmon resonance and PWR biosensors for self-referenced biosensing.