Top Gold Layer Research Articles

Laser-initiated electron and heat transport in gold-skutterudite CoSb3 bilayers resolved by pulsed x-ray scattering

Abstract Electron and lattice heat transport have been investigated in bilayer thin films of gold and CoSb3 after photo-excitation of the nanometric top gold layer through picosecond x-ray scattering in a pump-probe setup. The kinetics of heat transfer are detected by thermal lattice expansion and compared to simulations based on the two-temperature model of coupling of electron and phonon degrees of freedom. The unexpected observation of a larger portion of the deposited heat being detected in the underlying CoSb3 layer before the topmost gold layer is heated supports the picture of transport of the photo-excited electrons from gold to the underlying layer to be converted into lattice heat. The change of partition of heat between the gold and CoSb3 layer with laser fluence and wavelength (either exciting intraband transitions or additionally interband transitions) is rooted in the amplitude of electron temperature. Higher electron temperatures result in a longer equilibration time with the lattice and thus a larger proportion of ballistic electron transport across the interface.

Investigations on fire‐gilding

AbstractFire‐gilding is a historic technique for the application of golden layers on a number of different base materials utilizing a gold amalgam. This technique leaves a significant amount of Hg in the golden layer, giving archeometrists a reliable indicator to identify fire‐gildings. Recent findings on presumably fire‐gilded objects have shown in several cases significantly lower Hg content than previously studied objects. This prompted a synchrotron‐based X‐ray fluorescence investigation into the Hg distribution along the material–gilding interface, as well as a series of measurements regarding the Hg content development in fire‐gilded samples during artificial aging. This work presents findings on laboratory‐prepared fire‐gildings, indicating an Hg enrichment at the interface of fire‐gilded silver samples. Notably, such an enrichment is missing in fire‐gilded copper samples. Further, it is confirmed that fire‐gilded layers typically do not undercut an Hg bulk content of 5%. In this light, it seems improbable that ancient samples that contain <5% Hg are fire‐gilded. The results presented in this study might lead to a non‐destructive method to identify the Hg enrichment at the interface. This might be obtained by a combination of different non‐destructive measurements and might also work unambiguously in samples in which the gold top layer is altered.

Structural characterization of Au/Cr bilayer thin films using combined X‐ray reflectivity and grazing incidence X‐ray fluorescence measurements

Gold thin‐film mirrors are widely used as X‐ray reflective optics to manipulate X‐rays especially in synchrotron beamlines. Application of a thin underlying buffer layer of chromium enhances the adhesion of gold film to the substrate material and provides long term structural stability. In the present work, Au/Cr bilayers deposited using the DC magnetron sputtering technique with different thickness values on a Si substrate are studied to better understand the correlation between micro‐structure properties and residual stress. In order to obtain precise structural properties of Cr binding layer and gold layer, as well as in‐between nanoscaled interfacial structure, nondestructive probing is carried out by simultaneously utilizing synchrotron‐based combined X‐ray reflectivity (XRR)–grazing incidence X‐ray fluorescence (GIXRF) techniques. It has been clearly observed that when thickness of Cr layer is relatively small, a composite interlayer structure is formed at the Au/Cr boundary and at the Cr/Si interface boundary due to the interdiffusion of two layer mediums. However, when the thickness of the Cr layer is relatively larger (~90 Å), emergence of such an interlayer medium does not take place. The grazing incidence X‐ray diffraction (GIXRD) measurements have also been performed to investigate crystalline structure and residual stress of the gold layer medium. Our results show that relatively large residual stress exists in the Au layer if thickness of Cr layer is small. The surface roughness morphology of the top gold layer obtained from the atomic force microscopy measurements showed the partial agreement with the results obtained from the XRR‐GIXRF technique.

Resonance modes of a metal-semiconductor-metal multilayer mediated by electric charge

Electromagnetic fields around metal–semiconductor–metal (MSM) multilayers with square island top layers were numerically simulated to elucidate the difference in physics between the circuit resonance and Fabry–Pérot interference mediated by the surface plasmon polaritons (SPP). In the current study, the top and bottom metal layers were made of gold, and the intermediate semiconductor layer was a gallium antimony (GaSb). The lumped-element and Fabry–Pérot interference models showed less accuracy when the island width of the MSM multilayer was comparatively smaller. Since the capacitor and SPP could not be supported between the top and bottom gold layers, the anti-reflection mode of the gold–GaSb bilayer mainly affected the absorptance. However, when the width of the island was sufficiently large, the time-lapse development of the electromagnetic fields at resonant wavelengths showed strong electric and magnetic responses relating to the circuit resonance. Simultaneously, the electric fields depicted the movement of the electric charge, which coupled to the short-range surface plasmon polariton (SRSP) existing at the thin GaSb layer sandwiched by two gold layers. The wavelength of the SRSP approximately corresponded to that of the Fabry–Pérot interference. It was revealed that the lumped-element and Fabry–Pérot interference models indicated the same resonant mode from two different perspectives in physics.

A polarization-independent highly sensitive hybrid plasmonic waveguide structure

In this paper, we proposed a novel design of a polarization-independent hybrid plasmonic waveguide. The transverse magnetic (TM) hybrid mode is confined in the low index material sandwiched between the silicon core and top gold layer whereas the transverse electric (TE) hybrid mode is supported in a narrow air gap on both sides between the silicon core and a gold layer. For sensing applications, two vital parameters such as the sensitivity of the hybrid mode (S mode ) and evanescent field ratio (EFR) are studied which depends on the geometric parameters of the waveguide. The geometric parameters of the waveguide are optimized using the finite element method. The highest S mode and EFR for both polarizations is greater than 0.91 and 0.6, respectively. This study shows that polarization-independent highly sensitive refractive index sensors or evanescent field absorption gas sensors can be realized by utilizing the proposed hybrid plasmonic waveguide design.

Next-generation on-chip plasmonic tweezer with a built-in light source

We are proposing next-generation lab-on-a-chip plasmonic tweezers with a built-in optical source that can be activated electrically. The building block of these tweezers is composed of an Au/p+-InAs/p+-AlAs0.16Sb0.84 Schottky diode, with a circular air-hole opened in the Au layer. Under an appropriate forward bias, the interband optical transitions in InAs, acting as a built-in optical source that can excite the localized surface plasmons (LSPs) around the edge of the hole. Numerical simulations show that the LSPs mode penetrates a chamber that is filled with water and electrically isolated from the top gold layer, providing the gradient force components desired for trapping the target nanoparticles suspended in the water. Moreover, we show that tweezers with air-holes of radius 90 nm under an applied bias of −1.6 V, can trap polystyrene nanoparticles of radius as small as 93 nm. The proposed structure provides a new platform for developing the next-generation compact on-chip plasmonic tweezers with no need for any external optical pump.

Period- and cavity-depth-dependent plasmonic metamaterial perfect absorber at visible frequency: design rule

Plasmonic perfect absorbers are artificially designed metal-dielectric-metal (MDM) structures to realize unity optical absorption (UOA) at the specific spectral wavelength. In the recent past, a variety of structures have been investigated theoretically and experimentally to achieve UOA. However, fabrication of a complex structure often requires sophisticated instruments and skills. A simple structural design may be an alternative solution. An MDM structure consisting of a square array of cylindrical holes in the top layer is proposed, and optical reflection/absorption is studied using 3-D finite-difference time-domain computation to achieve UOA. The structure has three sequential layers: a ground gold plane (100-nm thick) to block the transmission, a spacer layer of typical thickness of 20 nm of constant real value of dielectric permittivity, and a top gold layer structured with periodic square array of cylindrical holes. Numerical results show period- and hole-depth-dependent 100% optical absorption (UOA) at a specific spectral wavelength. A significant influence of hole packing density and refractive index of filled material on the resonant absorption and absorption bandwidth is also observed. Based on the results, a design rule is proposed to achieve UOA at an arbitrarily chosen spectral wavelength in the visible range.

On the Nonlinear Dynamics of a Doubly Clamped Microbeam Near Primary Resonance

This work aims to investigate theoretically and experimentally various nonlinear dynamic behaviors of a doubly clamped microbeam near its primary resonance. Mainly, we investigate the transition behavior from hardening, mixed, and then softening behavior. We show in a single frequency–response curve, under a constant voltage load, the transition from hardening to softening behavior demonstrating the dominance of the quadratic electrostatic nonlinearity over the cubic geometric nonlinearity of the beam as the motion amplitudes becomes large, which may lead eventually to dynamic pull-in. The microbeam is fabricated using polyimide as a structural layer coated with nickel from top and chromium and gold layers from the bottom. Frequency sweep tests are conducted for different values of direct current (DC) bias revealing hardening, mixed, and softening behavior of the microbeam. A multimode Galerkin model combined with a shooting technique are implemented to generate the frequency–response curves and to analyze the stability of the periodic motions using the Floquet theory. The simulated curves show a good agreement with the experimental data.

Multifrequency excitation of a clamped-clamped microbeam: Analytical and experimental investigation.

Using partial electrodes and a multifrequency electrical source, we present a large-bandwidth, large-amplitude clamped–clamped microbeam resonator excited near the higher order modes of vibration. We analytically and experimentally investigate the nonlinear dynamics of the microbeam under a two-source harmonic excitation. The first-frequency source is swept around the first three modes of vibration, whereas the second source frequency remains fixed. New additive and subtractive resonances are demonstrated. We illustrated that by properly tuning the frequency and amplitude of the excitation force, the frequency bandwidth of the resonator is controlled. The microbeam is fabricated using polyimide as a structural layer coated with nickel from the top and chromium and gold layers from the bottom. Using the Galerkin method, a reduced order model is derived to simulate the static and dynamic response of the device. A good agreement between the theoretical and experimental data are reported.

Wideband MEMS resonator using multifrequency excitation

We demonstrate the excitation of combination resonances of additive and subtractive types and their exploitations to realize a large bandwidth micro-machined resonator of large amplitude even at higher harmonic modes of vibrations. The investigation is conducted on a Microelectromechanical systems (MEMS) clamped-clamped microbeam fabricated using polyimide as a structural layer coated with nickel from top and chromium and gold layers from bottom. The microbeam is excited by a two-source harmonic excitation, where the first frequency source is swept around the targeted resonance (first or third mode of vibration) while the second source frequency is kept fixed. We report for the first time a large bandwidth and large amplitude response near the higher order modes of vibration. Also, we show that by properly tuning the frequency and amplitude of the excitation force, the frequency bandwidth of the resonator is controlled.

Tri-layered composite plasmonic structure with a nanohole array for multiband enhanced absorption at visible to NIR frequencies: plasmonic and metamaterial resonances

A tri-layered composite structure of gold/ZnS/gold, with the top gold layer patterned into a periodic array of circular holes, was fabricated by laser interference lithography and lift-off processes. This plasmonic composite absorbing structure showed a series of enhanced absorption peaks across the visible to NIR frequencies with an peak absorption exceeding 95% at 0.52 μm wavelength. These absorption peaks were reproduced in electromagnetic simulations of the structures. The peaks are shown to arise from the various resonances of the system: the localized surface plasmon resonances of the holes, the surface plasmon polaritons on the various interfaces and the shape dependent electromagnetic resonances of the holes. The measured angular dispersion of the absorption peaks indicated the SPP origin of the resonances while the computer simulations of the electromagnetic fields could be used to understand the nature of the localized resonances.

Higher order modes excitation of electrostatically actuated clamped–clamped microbeams: experimental and analytical investigation

In this study, we demonstrate analytically and experimentally the excitations of the higher order modes of vibrations in electrostatically actuated clamped–clamped microbeam resonators. The concept is based on using partial electrodes with shapes that induce strong excitation of the mode of interest. The devices are fabricated using polyimide as a structural layer coated with nickel from the top and chrome and gold layers from the bottom. Experimentally, frequency sweeps with different electro-dynamical loading conditions are shown to demonstrate the excitation of the higher order modes of vibration. Using a half electrode, the second mode is excited with high amplitude of vibration compared with almost zero response using the full electrode. Also, using a two-third electrode configuration is shown to amplify the third mode resonance amplitude compared with the full electrode under the same electrical loading conditions. An analytical model is developed based on the Euler–Bernollui beam model and the Galerkin method to simulate the device response. Good agreement between the simulation results and the experimental data is reported.

A plasmon-tuned ‘gold sandwich’ for metal enhanced fluorescence in silica coated NaYF4:Yb,Er upconversion nanoparticles

The low quantum yield of luminescence from lanthanide-doped up-conversion nanoparticles (UCNPs) has been enhanced by using an optimized ‘gold sandwich’ with a transparent top layer and a reflecting bottom layer at 980 nm excitation. Erbium (Er)-doped UCNPs, with a NaYF4:Yb,Er core, were synthesized by a thermal decomposition process and coated with silica to assist in metal-enhanced fluorescence (MEF). A bottom layer of thick coalesced gold island film, acting as a mirror, increases the optical path length of the 980 nm radiation through the UCNP layer dispersed on it. This layer enhances the UCNPs' 540 nm green emission by a factor of 5–8 compared to that in the absence of the gold reflector. A thin nanoparticle-like gold layer on top of the UCNPs, with a surface plasmon absorption around ∼550 nm, completes the sandwich, which augments the luminescence enhancement by another factor of ∼2.5, thus taking the net enhancement factor to ∼13–19 when compared to the luminescence in the absence of the gold-sandwich. The surface plasmon absorption in the top gold layer enhances the local electric field at the UCNPs to promote their radiative decay. Compared to previous reports, mostly for the solution state, the current case study is a solid state measurement.

Gold/ Polypyrrole Multilayer Electrode for Biochip Applications

Novel gold/ polypyrrole/ gold electrodes for biochip applications are presented. The electrodes demonstrated better conductivity than polypyrrole electrodes and better sensitivity than conventional gold electrodes. We describe the device fabrication and the electrical properties of the electrodes. Polypyrrole was deposited by electrochemical polymerization, and a thin top gold layer was electro-deposited on the Polypyrrole. A study of the nucleation and growth of the top gold using high resolution scanning electron microscope is presented. Under stagnant conditions a significant electro-crystallization was observed. Electrochemical characterization essential for future bio-sensing application was demonstrated. The electrochemical behavior of the metal/Ppy/metal electrodes was tested by CV and impedance spectroscopy. The novel electrode system was tested for their application as whole cell biosensors using substrate - enzyme interaction by product electrochemical activity. We studied the response of the new electrode in biological solutions (PBS). We found that depositing Ppy on thin metal seed layer dramatically increases the measured current of the electrodes, as compared to Au electrode. This effect was also demonstrated by CV measurements as well as in amperometric detection of alkaline-phosphatase enzymatic activity using biological chips. This high current produces noise that interferes with the proper operation. However, when sufficient metal is electrodeposited on the Ppy, the behavior of the electrode becomes similar to that of a metal electrode, as demonstrated by CV and by amperometric detection of enzymatic activity. The CV curve has the same characteristics as that of the planar gold electrode, with a considerably higher current peak (Ip). The modified Au/Ppy/Au electrode yielded an improved signal, compared with that from a flat gold electrode. The electrodeposition of Ppy followed by Au, on the surface of the biochip’s Au seed layer, affected the electrocrystallization of the Au particles on the Ppy (Fig 1) resulting in a high surface area. This electrode can serve as a highly sensitive, high surface area and flexible sensor for biological species. Figure 1

Optical Salisbury screen with design-tunable resonant absorption bands

A thin-film selective absorber at visible and near infra-red wavelengths is demonstrated. The structure consists of an optically thick layer of gold, a SiO2 dielectric spacer and a partially transparent gold film on top. The optical cavity so formed traps and absorbs light at a resonance wavelength determined by the film thicknesses. Observed fundamental-resonance absorption strengths are in the range 93%–97%. The absorption red-shifts and broadens as the thickness of the top gold layer is decreased with little change in absorption strength. Thus, strong absorption with design-tunable wavelength and width is achieved easily by unstructured blanket depositions. Observed angle-dependent spectra agree well with the recent three-layer analytical model of Shu et al. [Opt. Express 21, 25307 (2013)], if effective medium approximation is used to calculate the permittivity of the top gold film when it becomes discontinuous at the lowest thicknesses.

Novel electrochemical approach to study corrosion mechanism of Al–Au wire–bond pad interconnections

A gold–aluminium material combination is typically employed as an interconnection for microelectronic devices. One of the reliability risks of such devices is that of corrosion of aluminium bond pads resulting from the galvanic coupling between an aluminium bond pad and a gold wire. The research presented in this manuscript focuses on studying bond pad corrosion by selecting an appropriate model system and a dedicated set of electrochemical and analytical experimental tools. Taking into account the complex three-dimensional structure and the small dimensions of Au–Al interconnections (around 50–100 μm), a dedicated and novel experimental approach was developed. Au–Al covered silicon chips were developed under clean room conditions. Three-dimensional electrodes were mimicked as flat, two-dimensional bond pad model systems, allowing the use of microelectrochemical local probe techniques. Thin gold films were applied on Ti–Al covered silicon surfaces, and their morphology and electrochemical behaviour were analysed using the localised electrochemical cell, scanning vibrating electrode technique, scanning electron microscopy–energy dispersive X-ray spectroscopy, scanning electron microscopy–focused ion beam and Auger electron spectroscopy. The results revealed that the electrochemical behaviour of thin gold films was influenced by the underlying metal layers and the microstructural changes during heat treatment. The effect of the underlying layers on the electrochemical properties of the top gold layer was attributed to a mixed potential behaviour and microgalvanic interactions, which was more pronounced after heat treatment. Overall, the current experimental approach can be employed to study the mechanism of electrochemical events that are occurring on metallic interconnections in different electronic devices.

Fabrication of a heterostructure device with Au/PPani–TiO2/ITO configuration and study of device parameters including current conduction mechanism

Polyaniline based composites incorporating titanium dioxide have been synthesized by an alternative pathway using reactive magnetron sputtering of titanium and plasma polymerization of aniline monomer. Structural, optical and morphological characterizations of plasma polymerized aniline (PPani) and titanium dioxide (TiO2) composites (PPani–TiO2) reveal the evidence for the incorporation of TiO2 in the PPani matrix. A hybrid heterostructure device having PPani–TiO2 composite with a top gold (Au) layer and bottom indium-tin oxide (ITO) layer is fabricated. The developed heterostructure device exhibits rectifying behaviour indicating the formation of a Schottky contact between Au and PPani–TiO2. The detailed electrical measurement of the device is performed under different temperatures. The ideality factor (n) and barrier height (φB) of the heterojunction diode at room temperature (300 K) are found to be 1.28 and 0.43 eV, respectively. Possible conduction mechanisms are examined using various plotting and curve fitting methods for space charge limited conduction mechanism (SCLC), Schottky emission mechanism and Poole–Frenkel (PF) emission mechanism. The heterostructure device shows best fit of SCLC process as compared to the other mechanisms including Schottky emission and PF emission.

A superhydrophobic layer formed by fluoro-derivative-treated gold sheets on grown-up zinc oxide nanoparticles for a spherical DNA hydrogel

A facile deposition process was developed for the fabrication of a new superhydrophobic layer composed of an underlying zinc oxide nanoparticle support and a gold top layer doped with the hydrophobic chemical, heptadecafluoro-1-decanethiol (HDFT). The resulting microscaled and spherical DNA-based hydrogels could serve as a platform for pseudo-nucleus mimics.

High-temperature gold metallization for ZnO nanowire device on a SiC substrate

Gold is commonly used nowadays in metal contacts to nanowire devices. Due to their small size, nanowire devices often get heat up enough to cause a reaction of the contact and substrate, whether during operation or as a result of a spontaneous pulse of an electrostatic discharge. In most cases, the point of failure is the metallization, as is the case studied here. Gold is useful not only for its good electrical conductance but also because it is a good heat conductor and inert to the ambient. To improve the survivability of a gold metallization for nanowire devices incorporating ZnO nanowire atop a SiC substrate, we used a sputter-deposited Ti-Si-N ternary diffusion barrier layer and a Ti adhesion layer between the top gold layer and a 4H-SiC substrate that survives 30 min of vacuum annealing at 850 °C and 5 days of annealing at 500 °C in Ar. Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy were used to test the integrity of the layers before and after annealing both with and without the diffusion barrier. Current-voltage characteristics were measured up to 75 V in air to test the metallization.

Understanding the effects of dielectric medium, substrate, and depth on electric fields and SERS of quasi-3D plasmonic nanostructures

The local electric field distribution and the effect of surface-enhanced Raman spectroscopy (SERS) were investigated on the quasi-3D (Q3D) plasmonic nanostructures formed by gold nanohole and nanodisc array layers physically separated by a dielectric medium. The local electric fields at the top gold nanoholes and bottom gold nanodiscs as a function of the dielectric medium, substrate, and depth of Q3D plasmonic nanostructures upon the irradiation of a 785 nm laser were calculated using the three-dimensional finite-difference time-domain (3D-FDTD) method. The intensity of the maximum local electric fields was shown to oscillate with the depth and the stronger local electric fields occurring at the top or bottom gold layer strongly depend on the dielectric medium, substrate, and depth of the nanostructure. This phenomenon was determined to be related to the Fabry-Pérot interference effect and the interaction of localized surface plasmons (LSPs). The enhancement factors (EFs) of SERS obtained from the 3D-FDTD simulations were compared to those calculated from the SERS experiments conducted on the Q3D plasmonic nanostructures fabricated on silicon and ITO coated glass substrates with different depths. The same trend was obtained from both methods. The capabilities of tuning not only the intensity but also the location of the maximum local electric fields by varying the depth, dielectric medium, and substrate make Q3D plasmonic nanostructures well suited for highly sensitive and reproducible SERS detection and analysis.