Thin Au Research Articles

Electrodeposition of the MnO2 on the Ag/Au Core-Shell Nanowire and Its Application to the Flexible Supercapacitor.

Supercapacitors have received considerable attention as energy storage devices owing to their high power density, fast charge/discharge rate, and long cyclic life. Especially with an increasing demand for flexible and wearable devices, research on flexible supercapacitors has surged in recent years. The silver nanowire (Ag NW) network has been used as a flexible electrode owing to its excellent mechanical and electrical properties; however, its use as an electrode for flexible supercapacitors has been limited due to insufficient electrochemical stability. In this study, we proposed a method to resolve this issue. We employed a solution process that enabled the coating of the surface of Ag NW by a thin Au shell of ≈ 5 nm thickness, which significantly improved the electrochemical stability of the Ag NW network electrodes. Furthermore, we confirmed for the first time that MnO2, which is one of the most widely used capacitive materials, can be directly electroplated on the AACS NW network electrode. Finally, we fabricated a high-performance and flexible solid-state supercapacitor using the suggested Ag/Au/MnO2 core–shell NW network electrodes.

Formation and evolution of Au-SiOx Heterostructures: From nanoflowers to nanosprouts

This work reports the formation of circular cavities and Au-SiOx nanoflowers after annealing of thin Au film deposited on SiO2/Si substrates, and the transformation of nanoflowers to nanosprouts with increasing the annealing time. Two reference experiments indicate that both H2 and Si are indispensable for the above structures. The formation of cavities can be attributed to the SiO2 layer decomposition and the product, volatile SiO, provides a Si source for the formation of nanoflowers at the early stage. A model is proposed to indicate that SiO gas produced at the Si/SiO2 interface can diffuse to the surface assisted by the defects in the SiO2 layer before the decomposed cavities are exposed. Then the exposing of those cavities introduces another volatile SiO from the active oxidation of Si substrate, provoking a change in the direction of the main Si source, which in turn makes the one nanoparticle of the nanoflower split in two and finally form the nanosprout. The model about the escape of SiO further details SiO2 decomposition process, and the transformation mechanism from nanoflowers to nanosprout sheds light on a feasible nanofabrication method to design tunable size and shape of nanoparticles.

Spectral tuning of double resonant nanolaminate plasmonic nanoantennas with a fixed size

Multiresonant plasmonic nanoantennas can enhance nanolocalized multiphoton processes or enable wavelength-multiplexed nano-optic operations by supporting multiple spatially overlapped plasmonic modes. Nevertheless, current multiresonant plasmonic nanoantenna designs do not consider engineering multiresonant spectral responses with strict size and footprint constraints. Developing a strategy to engineer fixed-size nanoantennas with tunable multiresonant responses is highly desirable for maintaining controlled cellular responses at the nano-bio interface and achieving seamless integration with other nanodevices with predefined footprints. Here, we report that fixed-size tapered nanolaminate nanoantennas (TNLNAs) can achieve a wide double-resonance spectral tunability by only changing the metal-to-insulator thickness ratio (t/h). Three separate TNLNAs' samples (8/38 , 20/20, and 28/8 nm) with a nominal total height of ∼100 nm are created from a high-throughput nanofabrication technique. Specifically, we fabricated TNLNAs' samples by exploiting a nanohole array membrane from soft interference lithography as a deposition mask for electron-beam evaporation of alternating Au and SiO2 layers. Transmission and dark field scattering measurements show that TNLNAs support two distinct resonant features with t/h-dependent tunable resonant wavelengths in the range of 730–850 and 840–1050 nm, respectively. Numerical simulations reveal that (i) a bianisotropy-induced magnetoelectric response in top and bottom nanogaps due to the asymmetric tapered shape can enhance light trapping and achieve optical near-field intensity enhancements up to 1000-fold and (ii) while TNLNAs consisting of thin Au nanodisks at low t/h primarily support spatial overlap between modes with enhanced electric polarizability, TNLNAs consisting of thick Au nanodisks at high t/h support spatial overlap between modes with enhanced magnetic polarizability, evoking higher-order multipolar behaviors.

In Situ Stress Evolution on an Au Thin Film Cathode during Charging/Discharging of a Lithium-Oxygen Battery

The theoretical energy density of lithium-oxygen batteries is by far the closest competitor to gasoline compared to other battery configurations. Unfortunately, the commercial implementation of this technology is hindered by the poor cycling performance, mostly due to the slow oxygen evolution/reduction kinetics. The sluggish oxygen evolution kinetics are typically more severe and cause the slow oxidation of lithium peroxide. This can cause the irreversible formation of a passivating lithium peroxide layer on the cathode surface. The interfacial dynamics due to the growth of lithium peroxide and the formation of cathode-electrolyte interface (CEI) layers remains largely unknown. It is crucial to elucidate how the nucleation of discharge products on the cathode surface affects the electrochemical performance in order to design new electrolytes and cathodes for lithium-oxygen batteries.In this study, we propose to investigate the surface stress generation associated with the formation of discharge products and CEI layers on the cathode. Thin Au films were deposited via e-beam evaporation onto a borosilicate glass substrate. These substrates were used as a cathode material. The electrolyte used was 1 M LiTFSI in DMSO, and it was saturated with oxygen prior to performing electrochemical measurements. In situ stress measurements were conducted using a multiple beam optical sensor (MOS). A custom cell was designed to monitor the physical response of the cathode during cycling while enabling optical access to the back of the substrate for stress measurements.The in situ curvature evolution on an Au coated borosilicate glass cantilever is monitored during one discharge/charge cycle at 10 µA/cm2. Tensile stress generation is observed during the formation of the lithium peroxide discharge product on the Au cathode. A large stress relaxation into the compressive regime is observed at the onset of charging. From our results, the nucleation of lithium peroxide onto the cathode surface causes tensile stress, while the oxidation of lithium peroxide causes compressive stress. Revealing the mechanical response of the charge/discharge products on the cathode will shed light into the formation mechanisms of these products, which is crucial to design both new electrolytes and cathode materials for lithium-oxygen batteries.Acknowledgement: This work was supported by the Binational Science Foundation (#2018327), and we are thankful to both Dr. Malachi Noked and Rosy for fruitful discussions.

Microfabricated fiducial markers for digital image correlation-based micromechanical testing of LIGA Ni alloys

Photolithographically defined thin film Au dots were used as micro fiducial markers for digital image correlation (DIC), to enable two-dimensional strain measurement of 200 μm-thick LIGA (Lithographie, Galvanformung, Abformung) nickel alloys. Due to the sensitivity of electrodeposited films’ microstructure and properties on the processing conditions, characterization of LIGA mechanical properties continues to be necessary for microsystems commercialization. DIC offers advantages over laser-based strain measurement techniques but creating suitable speckle patterns on specimens with dimensions under a millimeter is challenging. The material surface roughness itself is often used as the speckle pattern, or micro- or nanoparticles are applied to the surface. But for materials with highly polished surfaces, such as commercial LIGA alloys, the surface roughness is not always suitable, while application of particles still poses technical challenges in uniformity and reproducibility. We fabricated freestanding tensile specimens, with gauge sections 700 μm wide × 3 mm long × 200 μm thick, from electrodeposited Ni-10% Co using a commercial LIGA process, and conducted microtensile tests at strain rate 0.001 s−1. Designing and fabricating arrays of randomly oriented 1.5 μm-thick Au dots on the specimens provided a suitable way to obtain full-field surface strains over the entire gauge lengths and was reproducible from one specimen to another. Microfabricated fiducial markers therefore can be a useful surface-preparation approach for investigating micromechanical behavior, particularly plasticity and fracture, of LIGA films using DIC.

Harnessing Wrinkling Patterns Using Shape Memory Polymer Microparticles.

Shape memory polymers (SMPs) are the simplest and most attractive alternatives for soft substrates of typical bilayer wrinkle systems because of shape fixity and recovery capabilities. Herein, we have successfully programmed large compressive strains in chemical cross-linking shape memory polystyrene (PS) microparticles via nanoimprint lithography, which acted as the substrate of a wrinkle system using a gold nanoparticle (Au NP) film as the top layer. When triggered by two different stimuli (direct heating and toluene vapors), the thin Au NP film could transform into various wrinkle structures atop the recovered PS particles. In addition, we also investigated the evolution mechanisms of wrinkling by heating and toluene vapors and tuned the wrinkled surfaces through altering the Au NP thickness and stimulation methods (direct heating and toluene vapors), which utilized the structural adjustability of Au NPs to program the amplitude, wavelength, and morphology of the wrinkles. The concept presented here provides a cost-effective approach to realize the surface wrinkling and can be extended to other available SMPs.

Nanobowls-assisted broadband absorber for unbiased Si-based infrared photodetection.

Hot electrons from the nonradiative decay of surface plasmons have drawn extensive attention due to the outstanding performance in realizing below-bandgap photodetection. However, the widely employed metallic nanostructures are normally complex and delicate with a great challenge in large-area fabrication, and there is a great limitation to achieve substantial photoresponse at relatively long wavelengths (e.g., 2000nm) with polarization- and incident-angle independence. In this study, we theoretically and experimentally demonstrate a broadband, omnidirectional, and polarization-insensitive absorber based on wafer-scale silicon honeycomb nanobowls with 20-nm-thick gold overlayer. The average absorption across the long wave near infrared band (LW-NIR, i.e., 1100-2500 nm) is higher than 82%, which is contributed from the random nature and multimode localized plasmonic resonances excited on the side walls of nanobowls. Benefitted from the well-connected thin Au film and relatively low Schottky barrier, the generated hot electrons have a high transport probability to reach Schottky interface and participate in the interfacial charge transfer process. As a result, the hot-electron photodetector under no bias realizes a broadband photodetection up to 2000nm wavelength with a responsivity of 0.145 mA/W, and its cutoff wavelength is predicted up to 3300 nm by fitting the experimental result with Fowler theory. Our proposed Au/Si nanobowls photodetector could open a pathway to further extend the detection wavelength of Si-based photodetectors with a large-area and low-cost fabrication process, which promotes practical hot-electron applications.

Lasing action of ZnO nanowires grown by mist chemical vapor deposition using thin Au layer on c-plane sapphire substrate

ZnO nanowires were grown by mist chemical vapor deposition at a growth temperature of approximately 900 °C (tube furnace temperature) on a c-plane sapphire substrate sputtered with a thin Au layer with a designed thickness of 3 nm. ZnO nanowires of various lengths were grown, and their optical properties were investigated by room-temperature photoluminescence (RT-PL) measurements under high excitation conditions. The RT-PL spectra having sharp peaks may be capable of lasing actions. The result shows that this fabrication technique is useful for obtaining optical microcavities based on ZnO nanowires.

Highly Efficient Plasmonic Membrane Flow Reactor

AbstractA plasmonic flow reactor, consisting of thin Au film at exits of monolithic anodized aluminum oxide (AAO) membranes under LED illumination is demonstrated. The system shows over 200% quantum efficiency (QE) for peroxide activation and the ability to limit to single oxidation reaction by controlling residence time with flow rate and pore geometry. Periodic pore arrays (20–200 nm diameter) with 25 nm thick Au on AAO are modeled by finite‐difference time‐domain (FDTD) simulations and predicted largest E‐field enhancements for the larger 200 nm pore diameters. Peroxide activation, as measured by O2 generation is most efficient with a 200 nm pore diameter system under 523 nm LED illumination. The optimal wavelength falls near the absorption peak of Au@AAO with 200 nm pore diameter suggesting that hot electron generated from gold plasmonic response is the primary mechanism for activation of H2O2. QE for gold plasmonic flow system calculated from O2 generation experiments is as high as 250%, which indicates a mechanism of hot‐electron activation of peroxide that leaves a still energetic hot‐electron to catalytically activate multiple reactions. The formation of Au surface oxides that are catalytically active in dark is also observed and must be accounted for in Au plasmonic photochemical studies.

Development of residual strains and their relaxation processes in atomically thin layers of core-shell structured nanoparticles

Residual elastic strains imposed on atomically thin films coated onto substrate materials cause changes in the electronic structures of such films. This knowledge has been employed in various industrial sectors as a means of tuning the optical, electrical, magnetic, and chemical properties of materials. Although the magnitudes of residual strains and properties of materials are closely related to the thickness of coating layers, the measurement of residual strains that develop in atomically thin films is challenging owing to the experimental complexities of conventional measurement methods. In this work, the residual strains developed in atomically thin Au overlayers grown epitaxially on Pd nanoparticles were measured using transmission electron microscopy based on nanobeam precession electron diffraction. Experiments revealed that a 1-nm-thick Au film can withstand an exceptionally high compressive strain of 4.3%, which is reduced to nearly zero as the film thickness increases beyond 5 nm. The microstructural evolution of Au films was monitored using high-resolution transmission electron microscopy and the large strain storage capacity and plastic relaxation behavior of Au films were interpreted by comparing them with the previous computer simulations and theoretical models.

Three-Dimensional Stretchable Microelectronics by Projection Microstereolithography (PμSL).

Stretchable and flexible electronics conformal to human skin or implanted into biological tissues has attracted considerable interest for emerging applications in health monitoring and medical treatment. Although various stretchable materials and structures have been designed and manufactured, most are limited to two-dimensional (2D) layouts for interconnects and active components. Here, by using projection microstereolithography (PμSL)-based three-dimensional (3D) printing, we introduce a versatile microfabrication process to push the manufacturing limit and achieve previously inaccessible 3D geometries at a high resolution of 2 μm. After coating the printed microstructures with thin Au films, the 3D conductive structures offer exceptional stretchability (∼130%), conformability, and stable electrical conductivity (<5% resistance change at 100% tensile strain). This fabrication process can be further applied to directly create complicated 3D interconnect networks of sophisticated active components, as demonstrated with a stretchable capacitive pressure sensor array here. The proposed scheme allows a simple, facile, and scalable manufacturing route for complex, integrated 3D flexible electronic systems.

Ultrasensitive Optical Label-Free Biosensing and Fluorescent Measurements using UV Surface Plasmons in Gold Nanolayers Launched with the Support of a Specially Designed Photonic Crystal

Recently, we used Photonic Crystal (PC) - supported surface plasmons (SP) propagating along thin Pd, Co (in red spectral range) and Au (in blue, 405 nm) nanolayers for ultrasensitive gas sensing (Pd, Au) and magnetoplasmonic researches (Co). In all these cases, without the exploitation of a specially designed PC, the propagation length of the plasmons is just of the order of the wavelength hence there would be no reason to speak about SP at all. Here we experimentally show that the same concept is applicable even in UV spectral range at 375 nm despite the extremely non-favorable conditions of SP excitation at bulk gold - dielectric interface: the real part of the dielectric permittivity of gold at this wavelength is minus 1.5, and its imaginary part 5.6. Nevertheless, the use of a specially designed PC, containing seven pairs of transparent Ta2O5/SiO2 layers plus an additional Ta2O5 and 8 nm-thick gold layer, enabled us to attain the angular width of the surface plasmon excitation around 0.250 and thus the UV SP propagation length of tens of microns. This same structure can be used in quite usual fashion as an ultrasensitive optical label-free biosensor. Besides, the short UV wavelength exploited enables also to use it for excitation of fluorescence for very broad class of studied compounds. The perspectives of the application of this approach to deeper UV, at 280 nm, will be discussed. Such structures will enable direct label-free excitation and measurement of fluorescence from many proteins and DNA/RNA molecules/complexes.

Soldered joints of Ag electrode to ultra-thin metallic Au layer on ceramic substrate

Among various methods of microjoining applied to electronic equipment soldering technologies are of great importance. In the construction of sensors a technique of a reflow soldering, common for microelectronic devices assembly, is rather not applicable because of the unique design of the sensor. Connecting external leads to thin metal layer of contact pads is a particular challenge. Special requirements are placed on devices and joints operating at cryogenic temperatures. The joints operating in cryo-temperatures should be characterized by good and stable electrical conductivity, tightness and the required mechanical strength. In the paper the new method of soldering of Ag leads to thin Au layer on ceramic substrate is presented. A special feature of the new soldering method is the very short time (2-3 seconds) in which the solder (In) is in the liquid state. The manufactured joints are characterized by a very low resistance as well as a linear and continuous current-voltage characteristic in the temperature range of 15 to 300 K.

Au and Pt Diffusion in Electrodeposited Amorphous Sb2Te3 Thin Films

Amorphous Sb2Te3 thin films are synthesized using electrodeposition in different thicknesses to explore their solid electrolytic nature with respect to some noble metals. Au and Pt are used as the substrate materials and thermally diffused into the Sb2Te3 films under passivated and nonpassivated conditions. Rutherford backscattering spectrometry is used to study the Au and Pt diffusion into the films as a function of depth. It is found that Au diffuses into the Sb2Te3 thin films even at room temperature. In contrast, there is no observable diffusion of Pt at room temperature. At a higher temperature (i.e., 200 °C), both Au and Pt diffuse into the Sb2Te3 thin films and Au diffusion is more significant than Pt. The findings suggest that amorphous Sb2Te3 can be used as a good solid electrolyte to permit the diffusion of some noble metals such as Au and Pt.

Optically‐Controlled Quantum Size Effect in a Hybrid Nanocavity Composed of a Perovskite Nanoparticle and a Thin Gold Film

AbstractMetal halide perovskites have attracted great interest in recent years and their emission wavelength can be adjusted either by doping impurities or by exploiting quantum size effect. Here, it is reported that the realization of optically‐controlled quantum size effect in a hybrid nanocavity composed of a perovskite (CsPbBr3) nanoparticle and a thin gold (Au) film. Such nanocavities are created by synthesizing polycrystalline CsPbBr3 nanoparticles composed of quantum dots on a thin Au film via chemical vapor deposition, which emit luminescence at ≈488 nm under the excitation of femtosecond laser pulses with a low repetition rate. The phase transition from polycrystalline to monocrystalline, which quenches the quantum size effect and shifts the emission wavelength to ≈515 nm, can be introduced in CsPbBr3 nanoparticles by simply increasing the laser power. Interestingly, such a phase transition is reversible provided that the laser power is lower than a threshold. Consequently, four optical states including dual‐wavelength emission, can be achieved by deliberately setting the laser power. The underlying physical mechanism is unveiled by the static and transient temperature distributions simulated for the hybrid nanocavity. The findings open a new avenue for designing novel photonic devices based on perovskite nanoparticles and plasmonic nanostructures.

Optical outcoupling in monochromatic top-emitting organic light-emitting diodes with an Au/Ag semi-transparent electrode

Top-emitting organic light-emitting diodes can achieve high efficiencies due to the strong cavity effect resulting from the relatively thick semi-transparent metallic top electrode. The strong cavity resonance, however, simultaneously brings along negative side effects such as pronounced angular-dependent emission and spectral narrowing. In this work, through numerical simulations, we demonstrate that top-emitting organic light-emitting diodes using a thin Au(2 nm)/Ag(7 nm) top electrode can achieve light-outcoupling efficiency comparable to a thick silver electrode, while reducing spectral narrowing. This can be realized by tuning the organic capping layer thickness without affecting the electrical properties, which can be applied to diodes based on either intrinsic or efficiently doped charge transport layers.

New method of SPP excitation based on metal photoluminescence

A new method of surface plasmon waves excitation based on the photoluminescence of a nanostructured metal surface is proposed. The method was demonstrated using a plasmonic crystal formed by an array of nanoholes in 200 nm thick Ag film (supporting SPP resonances) covered by a 10 nm thin Au layer (for efficient excitation of photo-induced luminescence). It was shown that using this method it is possible: (i) to excite SPP waves in ultra-large spectral range, (ii) to measure plasmonic crystal optical properties, (iii) to measure optical characteristics of the SPP waves.

Electrochemical glucose sensor based on the glucose oxidase entrapped in chitosan immobilized onto laser-processed Au-Ti electrode

Abstract Glucose is essential to keep the human body in well order. Therefore, the control of its level is of key importance. The study on novel electrode material composed of structured titanium foil with embedded Au nanoparticles and modified with chitosan with entrapped glucose oxidase is presented. In the first step, Ti foil undergoes anodization followed by chemical etching resulting in formation of homogenously distributed inverted caps of diameter of 85 nm. After deposition of thin Au layer, the rapid and well optimized laser dewetting is applied to form Au nanoparticles of size limited by the dimensions of cavities. Finally, the surface is modified using enzyme and chitosan mixture. To confirm the successful immobilization of glucose oxidase FT-IR analysis was performed. The response of electrodes were tested towards glucose in 0.1 M PBS containing most different interference compounds and biological fluids. It is proven that prepared material exhibits excellent performance for glucose detection with a wide linear range of 0.04–15.05 and 15.05–40.00 mM, with sensitivity of 23.47 ± 1.36 and 10.63 ± 1.28 μAcm−2 mM-1, respectively, and very low limit of detection 1.75 ± 0.30 μM. The perfect selectivity and stability promotes the material selection as the main biosensor component.

Thermal stability of thin Au films deposited on salt whiskers

Thin metal films deposited on patterned or rough substrates play an increasing role in microelectronics, sensing, catalysis, and other areas of nanotechnology. However, the thermal stability and solid state dewetting of thin metal films with complex three-dimensional architecture is still poorly understood. In this work we employed a model system of nanocrystalline Au thin films deposited on prismatic single crystalline KCl whiskers to study the solid state dewetting of thin films in a three-dimensional setting. The arrays of KCl whiskers were grown on porous substrates under well-defined humidity and temperature conditions. Single crystalline prismatic KCl whiskers with a very high aspect ratio, [001] axis and {100} side facets were obtained. The whiskers were coated with thin conformal Au films of 20-30 nm in thickness. The annealing of these core-shell whiskers at the temperature of 350oC resulted in solid state dewetting of the Au film, with the dewetting processes occurring much faster along the whisker edges than on the side facets. The orientation relationships between Au and KCl were determined by employing similarly prepared thin Au films deposited on the flat KCl (100) substrates. Inspired by our experimental results, we developed a numerical model describing the curvature-gradient driven and surface diffusion-controlled growth of a hole in the thin film deposited on a curved substrate. The model predicted the growth of anisotropic elliptical holes elongated along the whisker axis. We discuss the experimental results in terms of the proposed model, indicating the importance of the change in orientation relationship between the Au grains and KCl whisker along the whisker edges.

Laser Printing of Plasmonic Nanosponges.

Three-dimensional porous nanostructures made of noble metals represent novel class of nanomaterials promising for nonlinear nanooptics and sensors. Such nanostructures are typically fabricated using either reproducible yet time-consuming and costly multi-step lithography protocols or less reproducible chemical synthesis that involve liquid processing with toxic compounds. Here, we combined scalable nanosecond-laser ablation with advanced engineering of the chemical composition of thin substrate-supported Au films to produce nanobumps containing multiple nanopores inside. Most of the nanopores hidden beneath the nanobump surface can be further uncapped using gentle etching of the nanobumps by an Ar-ion beam to form functional 3D plasmonic nanosponges. The nanopores 10–150 nm in diameter were found to appear via laser-induced explosive evaporation/boiling and coalescence of the randomly arranged nucleation sites formed by nitrogen-rich areas of the Au films. Density of the nanopores can be controlled by the amount of the nitrogen in the Au films regulated in the process of their magnetron sputtering assisted with nitrogen-containing discharge gas.