Au Thin Films Research Articles

Shape evolution and optical response of ZnO nanostructures grown on thermally evaporated ZnO and Au thin films

ZnO thin films of 100–400 nm thickness and 100 nm thickness Au thin films were deposited on glass substrates by thermal evaporation. Shape control of ZnO nanostructures on these films is achieved by microwave assisted hydrothermal synthesis at 90 °C for 30 min using zinc nitrate and zinc acetate as the precursors. There is a hierarchy of shapes comprising star shaped tetrapods and ball-shapes at the microscale made up of rice grains at the nanoscale on the ZnO and Au thin films respectively, when the precursor is zinc nitrate. The typical dimensions of the nano rice grains are 1 μm length, 100 nm width and tips of the order of 10 nm. In contrast, when the precursor is zinc acetate the nanostructures grow as hexagonal rods on the ZnO films and hexagonal tetrapods on the Au films with dimensions of 1–3 μm length and hexagon side of the order of 100–150 nm. Raman spectroscopy studies indicate the presence of defect related peaks in the nanostructures that are suppressed with increase in thickness of the ZnO films. The intensity of the defect peaks is much higher in the nanostructures grown on the Au films and is dependent on the precursor used to produce the nanostructures. Photoluminescence studies indicate that the ultraviolet-near band edge emission is the strongest on ZnO while defect related emission is prominent on the Au films. Shape control by microwave assisted hydrothermal synthesis is, thus, a facile low temperature method to produce nanostructures with different properties by simply changing the precursor and substrate on which they are grown.

Numerical Study of Graphene/Au/SiC Waveguide-Based Surface Plasmon Resonance Sensor.

A new waveguide-based surface plasmon resonance (SPR) sensor was proposed and investigated by numerical simulation. The sensor consists of a graphene cover layer, a gold (Au) thin film, and a silicon carbide (SiC) waveguide layer on a silicon dioxide/silicon (SiO2/Si) substrate. The large bandgap energy of SiC allows the sensor to operate in the visible and near-infrared wavelength ranges, which effectively reduces the light absorption in water to improve the sensitivity. The sensor was characterized by comparing the shift of the resonance wavelength peak with change of the refractive index (RI), which mimics the change of analyte concentration in the sensing medium. The study showed that in the RI range of 1.33~1.36, the sensitivity was improved when the graphene layers were increased. With 10 graphene layers, a sensitivity of 2810 nm/RIU (refractive index unit) was achieved, corresponding to a 39.1% improvement in sensitivity compared to the Au/SiC sensor without graphene. These results demonstrate that the graphene/Au/SiC waveguide SPR sensor has a promising use in portable biosensors for chemical and biological sensing applications, such as detection of water contaminations (RI = 1.33~1.34), hepatitis B virus (HBV), and glucose (RI = 1.34~1.35), and plasma and white blood cells (RI = 1.35~1.36) for human health and disease diagnosis.

Development of a High-Resolution Acoustic Sensor Based on ZnO Film Deposited by the RF Magnetron Sputtering Method.

In the study, an acoustic sensor for a high-resolution acoustic microscope was fabricated using zinc oxide (ZnO) piezoelectric ceramics. The c-cut sapphire was processed into a lens shape to deposit a ZnO film using radio frequency (RF) magnetron sputtering, and an upper and a lower electrode were deposited using E-beam evaporation. The electrode was a Au thin film, and a Ti thin film was used as an adhesion layer. The surface microstructure of the ZnO film was observed using a scanning electron microscope (SEM), the thickness of the film was measured using a focused ion beam (FIB) for piezoelectric ceramics deposited on the sapphire wafer, and the thickness of ZnO was measured to be 4.87 μm. As a result of analyzing the crystal growth plane using X-ray diffraction (XRD) analysis, it was confirmed that the piezoelectric characteristics were grown to the (0002) plane. The sensor fabricated in this study had a center frequency of 352 MHz. The bandwidth indicates the range of upper (375 MHz) and lower (328 MHz) frequencies at the −6 dB level of the center frequency. As a result of image analysis using the resolution chart, the resolution was about 1 μm.

Pseudoequilibrium between Etching and Selective Grain Growth: Chemical Conversion of a Randomly Oriented Au Film into a (111)-Oriented Ultrathin Au Film

Metal thin films with a specific orientation play vital roles in electronics, catalysts, and epitaxial templates. Although oriented metal films have been produced in the recent years, ultrathin oriented metal films (<10 nm) have not been achieved owing to the interfacial instability of the ultrathin films during the thermal annealing process. This study investigates chemical conversion of randomly oriented multigrain Au ultrathin films into (111)-oriented Au ultrathin films. A novel chemical process, termed pseudoequilibrium of etching and selective grain growth, is presented for the chemical conversion by using a quaternary ammonium halide. The reaction variables (reaction time, reaction temperature, species of halide ions) for the chemical conversion process are systematically investigated. This study reveals the in-plane rotational degeneracy in the Au(111) thin film epitaxially grown on a Si(111) substrate. The chemical process can be applied to a broad range of thicknesses from 9 to 100 nm.

The role of positively charge poly-L-lysine in the formation of high yield gold nanoplates on the surface for plasmonic sensing application.

An anisotropic structure, gold (Au) nanoplates was synthesized using a two-step wet chemical seed mediated growth method (SMGM) directly on the substrate surface. Prior to the synthesis process, poly-l-lysine (PLL) as a cation polymer was used to enhance the yield of grown Au nanoplates. The electrostatic interaction of positive charged by PLL with negative charges from citrate-capped gold nanoseeds contributes to the yield increment. The percentage of PLL was varied from 0% to 10% to study the morphology of Au nanoplates in term of shape, size and surface density. 5% PLL with single layer treatment produce a variety of plate shapes such as hexagonal, flat rod and triangular obtained over the whole substrate surface with the estimated maximum yield up to ca. 48%. The high yield of Au nanoplates exhibit dual plasmonic peaks response that are associated with transverse and longitudinal localized surface plasmon resonance (TSPR and LSPR). Then, the PLL treatment process was repeated twice resulting the increment of Au nanoplates products to ca. 60%. The thin film Au nanoplates was further used as sensing materials in plasmonic sensor for detection of boric acid. The anisotropic Au nanoplates have four sensing parameters being monitored when the medium changes, which are peak position (wavelength shift), intensity of TSPR and LSPR, and the changes on sensing responses. The sensor responses are based on the interaction of light with dielectric properties from surrounding medium. The resonance effect produces by a collection of electron vibration on the Au nanoparticles surface after hit by light are captured as the responses. As a conclusion, it was found that the PLL treatment is capable to promote high yield of Au nanoplates. Moreover, the high yield of the Au nanoplates is an indication as excellent candidate for sensing material in plasmonic sensor.

Au-assisted polymerization of conductive poly(N-phenylglycine) as high-performance positive electrodes for asymmetric supercapacitors

Herein, a novel conductive poly(N-phenylglycine) (PNPG) polymer was successfully prepared, by in situ electrochemical polymerization method (+0.75 V versus Ag/AgCl) for 10 min, on flexible stainless-steel plate coated with a thin Au film (Au/SS) to serve as a binder-free pseudocapacitive PNPG/Au/SS electrode for energy storage devices. Compared to the electrode without Au coating, PNPG/Au/SS electrode exhibited better electrochemical performance with larger specific capacitance (495 F g−1 at a current density of 2 A g−1), higher rate performance and lower resistance, which are good indications to act as a positive electrode for asymmetric supercapacitor devices. Combined with activated carbon as a negative electrode, an asymmetric supercapacitor device was constructed. It displayed a specific capacitance of 38 F g−1 at a current density of 0.5 A g−1 and an energy density of 5.3 Wh kg−1 at a power density of 250 W kg−1. Experimentally, two asymmetric supercapacitor devices were connected in series to power a home-made windmill continuously for 8 s, revealing the high potential of this novel conductive polymer material for energy storage application.

Si-Based Infrared Detector Using Organic Nano-Dot Array

Silicon-based infrared photodetector is proposed using an organic nano-dot array covered with an Au thin film. The nano-dots were self-assembled structures composed of crystalline CuPc and PTCDA organic semiconductors, and they enhanced the absorption of the incident infrared light. The annealing process at the CuPc and PTCDA deposition formed these semiconductor materials to be a nano-dot array shape. The Au-coated nano-dots presented broadband reflectance decrease in the near- to mid-infrared region. The Arrhenius plot investigation revealed that the proposed photodetector had 0.4-0.43 eV activation energy, a sub-Si-bandgap energy barrier. The photodetection experiments in the near-infrared region presented the proposed photodetector discriminated on and off of 2.07- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -long super continuum light. In addition, band-pass filtered emission from a black body cavity with a wavelength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.05~\mu \text{m}$ </tex-math></inline-formula> could be measured. Since the derivation of the activation energy and the cut-off wavelength showed a good agreement, the organic semiconductor generated a carrier at the photodetection. A possible detection mechanism was thus attributed to carrier excitations at an interface between organic semiconductors, CuPc and PTCDA. The proposed structure contributes to fabricate mid-infrared silicon-based photodetectors with high light absorption performance.

Influence of plasmon resonance on the luminescence of titanium dioxide thin films doped with rare earth ions

In this work the study of the optical properties of europium doped titanium dioxide thin films (TiO2:Eu) enhanced by gold plasmonic nanostructures are presented. Plasmonic platforms were manufactured by thermal annealing of thin film of Au, deposited on a Corning glass substrate. As a result of thermal treatment, gold spherical nanostructures with average dimensions of 50 nm were obtained. Luminescent TiO2:Eu film was deposited by RF magnetron sputtering method, from mosaic target. Morphology of gold nanostructures was investigated by SEM and TEM microscopes, while composition of oxides film was analysed by XPS methods. Luminescence properties were studied on the basis of excitation and emission spectra. Experiments have shown that such structures exhibit interesting luminescent properties and could be potential candidates for optoelectronics applications.

Theoretical Analysis of the LRSPR Sensor With Enhance FOM for Low Refractive Index Detection Using MXene and Fluorinated Graphene

In the present study, a novel heterostructure of surface plasmon resonance (SPR) sensors has been proposed based on two-dimensional (2D) materials. 2D materials have attracted a huge response in the area of SPR biosensors, for the sensor performance analysis sensitivity and figure of merit (FOM) are the important parameters. Recently, SPR sensors are getting significant research interest due to their variety of applications. Fluorinated graphene (fluorographene) and MXene 2D materials have been theoretically investigated in this work, and also discussed about the detection accuracy (DA), sensitivity, FOM, and Full width half maximum (FWHM) for the proposed SPR sensors. It is investigated that the FOM found very high i.e., (347 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ) in this proposed sensor which is ten times greater as per the conventional Au thin film (50 nm) in the visible range SPR sensors at wavelength of 1550 nm. Our study using the novel proposed heterostructure can be developed into a new Surface Plasmon Polarization technique (SPP) SPR sensor material.

In Situ Stress Measurements on Thin Film Au Positive Electrode during the First Discharge of Li-O2 Batteries

The formation and growth of the Li2O2 discharge product impacts the reversibility of the oxygen evolution and reduction reactions in Li-O2 batteries which may lead to a shorter cycle life. A clear understanding of the surface reactions and the growth mechanism of Li2O2 requires probing dynamic changes on the surface of the positive electrodes in situ during the discharge of a Li-O2 battery. To investigate this, we establish an experimental system by adopting a multi-beam optical sensor (MOS) and developing a custom-made battery cell. First, the accuracy and reliability of the system was demonstrated by analyzing the stress accumulation on the Au negative electrode during Li plating/stripping, and the results were consistent with an earlier single-beam scanning deflectometry report. Then, the Li-O2 battery was discharged in LiNO3 in diglyme electrolyte by applying either linear sweep voltammetry or by applying constant current under an O2 environment. Control experiments in Argon-saturated electrolytes indicate surface stress generation due to charge-induced stress. The stress generation on Au positive electrode is attributed to the formation of Li2O2 reaction products on the Au surface and charge-induced stress.

Microstructures of magnetron sputtered Fe–Au thin films

Abstract Freestanding films of highly pure iron and gold multilayers were fabricated and characterized for their intended use as biodegradable implant materials. These samples were deposited using magnetron sputtering on unheated substrates. This technology allows the combination of various non-compounding materials. After annealing for 2 h at 685 °C and 850 °C, respectively to homogenize the multilayer, the microstructures were investigated using X-ray diffraction, energy dispersive X-ray spectroscopy and scanning transmission electron microscopy. Due to the annealing, the multilayered microstructure converts into a new multiphase system consisting of an iron matrix and two different kinds of gold morphologies: segregations along grain boundaries and nanosized core–shell like precipitates.

Development of Operando Analytical System for Simultaneous Measurements of Electrochemical Reaction Rate and Haxpes

To quantitatively understand the electrochemical reaction, the reaction rate must be measured such as by hydrodynamic voltammetry. At the same time, the electronic states of the electrode surface closely related to the bonding of atoms and molecules, as well as to the reaction mechanisms, must be understood. We have designed and constructed an operando analytical system for the simultaneous measurements of the electrochemical reaction rate and the hard X-ray photoemission spectrum.1) In this system, a solution containing reactants was continuously supplied in a Hagen-Poiseuille flow with a channel flow electrode (CFE) method to a specimen, such as catalyst, under the control of temperature, pressure, concentration, and solution flow rate, so that the electrochemical reaction rate is able to be accurately measured or controlled. Two different environments, solution and ultra-high vacuum, were connected with an ultra-thin Au/SixNy window with a thickness of 20 nm (Fig. 1(a)) to make the operando hard X-ray photoemission spectroscopy (HAXPES) possible during the steady-state electrochemical reaction. In this report, the Au thin film 5 nm in thickness was used as a model electrode.Figure 1(b) shows a schematic illustration of the operando reaction rate-HAXPES measurement system. This system was installed to a UHV chamber at the beamline BL46XU of SPring-8, Japan.2) The energy of the synchrotron X-ray was 14 keV. The analyzer was Focus HV-CSA 300/15. Figure 1(c) shows the chronoamperometry result of the oxygen reduction reaction obtained at 0.4 V vs. Ag/AgCl sat. KCl in 0.1 M HClO4 at room temperature. From the current, the reaction rate was calculated as 0.9 nmol s-1 cm-2.1) Figure 1(d) shows the Au3d5/2 HAXPES spectrum of the Au thin film electrode measured during electrochemical reaction (blue circles for data and solid line for fitting). The horizontal axis indicates the kinetic energy of photoelectrons. The green triangles and solid line are of the spectrum of the reference metal Au. When the kinetic energy of the photoelectrons of Au3d5/2 was 11,809 eV, the inelastic mean free path estimated from the TPP-2M equation was 8.7 nm.3) Therefore, the HAXPES signal was from the entire Au film electrode.With this high energy of X-ray, HAXPES of the specimens located on this Au electrode can be measured as expected. Separately, a Pt film was deposited onto the Au thin film electrode. After the measurement of the Au peak, only the scanning energy was changed to observe clearly the Pt3d5/2 peak from the Pt film through the underlying Au thin film. In the future, specimens, such as fuel cell catalysts, can be located on the Au thin film electrode for the operando measurements. References 1) K. Suda et al., J. Electrochem. Soc., (2021) https://doi.org/10.1149/1945-7111/abf9712) H. Oji et al., J. Phys. Conf. Ser., 502, 012006 (2014).3) S. Tanuma et al., Surf. Interface Anal., 21, 165 (1994). Figure 1

Fabrication of Plasmonic Nanoslit Surrounded with Hourglass Structure

Horn structure has been utilized to enhance the sound wave intensity via various hour Glass type structures fro ancient ages such as Viking ages. In this report, Previously we reported the optical enhancement from the nanometer size circular aperture via hour glass type nanostructure. We will discuss the optical horn effect from microfabricated hourglass type nanoslits. In this report, the naoslit nanostructures are fabricated via conventional Si technology, and its optical characterizations are carried out using ND YAG 532 nm laser along with Nikon Ti Halogen lamp equipped with Princeton Spectrophotometer. The huge optical enhancement from the fabricated nanoslit with its ranging from ~ 1 nm to 30 nm will be reported and discussed. In addition, the plasmonic nanoslit array was fabricated on Au thin film with each slit of a ~ 10 nm width and a 300 nm length, and its optical characterization will be reported.

A Wedge-Shaped Au Thin Film: Integrating Multiple Surface Plasmon Resonance Sensors in a Single Chip and Enhancing the Figure of Merit

We show here the design, fabrication, and characterization of a wedge-shaped Au thin film with an enhanced figure of merit (FOM). This is achieved by using a reflectivity change in an attenuated total reflection (ATR) setup by slightly modulating the wavenumber of the surface plasmon polariton by means of the varying thickness of the Au thin film. The wedge-shaped Au thin film is equivalent to multiple surface plasmon resonance (SPR) transducers integrated in a single chip and was fabricated by an electron-beam evaporation process with the position of the shutter controlled during the deposition. The FOM, defined as the difference between the maximum and minimum values of the normalized reflectivity change (ΔR/R) divided by the corresponding difference of the incident angles, was 8.0-times larger than that based on the reflectivity R. Also, we demonstrated that the wedge-shaped Au thin film was able to detect ethanol gas at a concentration of 0.2%, corresponding to a refractive index change of 2 × 10−5, without any surface functionalization. Since the sensing signal can be obtained with a single image from the wedge-shaped Au thin film without precise thickness control of the metal thickness, no other materials or modulation equipment is necessary, and the sensing chip can be employed in simple and highly sensitive systems.

Localized and Surface Plasmons Coupling for Ultrasensitive Dopamine Detection by means of SPR‐Based Perylene Bisimide/Au Nanostructures Thin Film

AbstractDopamine (DA) is an important catecholamine neurotransmitter involved in neural and hormonal functions. Neural diseases, as Parkinson's, are connected to a significant lowering of DA concentration to picomolar levels in cerebrospinal fluid. So, in this contribution, a hybrid active layer based on a perylene bisimide (PBI) derivative and multishaped Au nanostructures is proposed for the development of surface plasmon resonance (SPR) dopamine detection system. PBI molecules, transferred as thin film by the dip coating approach, are demonstrated to be able to detect dopamine down to 10–12 m by SPR method. The presence of Au nanostructures promotes an important surface enhanced Raman scattering effect of Raman PBI signals highlighting a strong energetic communication between the two species. In this way, when PBI/Au nanostructures (AuNS) films are deposited by the layer‐by‐layer method and used for DA SPR detection, the sensitivity toward dopamine 10−12 m is doubled. The PBI/AuNS hybrid layer is able to detect picomolar concentration of dopamine, due to the coupling of localized (AuNS) and propagating surface plasmons on SPR slide. In particular, an interfacial coupling among localized surface plasmon resonance on AuNS and the propagating plasmon on Au thin film (of SPR slide) can be promoted.

Exploiting eutectic formation and phase separation to produce plasmonic metamaterials

Novel Au mesostructures with a polyhedron shape, henceforth referred to as pyramids, are produced by annealing in vacuo an Au thin film on a Si(100) substrate. Gold diffusion and incorporation into pyramids is a function of the thickness of the Au film, the annealing temperature, and the vacuum pressure. The Au pyramids have unique surface morphologies due to the presence of channels and plateaus, where channels are cut into the surface of the pyramids and plateaus are the surfaces between adjacent channels. The bulk of the pyramids consists of Au with cavities that are devoid of Au or Si. Normalized energy dispersive spectroscopy of intact regions of the surface are 98.1 wt. % Au and 1.9 wt. % Si, while the bottom of the channels are 85.7 wt. % Au and 14.3 wt. % Si. Therefore, one step in the growth process is the formation of an Au–Si eutectic. The low concentration of Si in the solid regions of the pyramid and its high concentration in the walls of the cavities are indicative of phase separation of the Au–Si eutectic. The pyramids are oriented in the same direction relative to one another and are a consequence of eutectic formation-induced etching of the Si(100) surface. The scattering spectrum (non-specular reflectivity) of the Au pyramids consists of two very strong surface plasmon polariton states that correspond to excitation from the Au d-bands to the sp conduction bands. The surface morphology produces linearly polarized reflected light.

Simulation on surface roughness variation of Au thin films by microwave post annealing

A simulation study was performed in order to interpret the experimental results in the previous report on roughness reduction in Au thin films by microwave post annealing in comparison with an electric furnace. The simulation model of this study is based on one-dimensional capillary driven surface diffusion with incorporation of electromigration force due to microwave irradiation. It was shown that alternating microwave electric field does not cancel the atomic migration completely, because of the capillary driven (thermal) effects simultaneously occurring within the alternation period, but is not adequate to account for the variation of the film profiles and the observed wave number spectra. In this study, capillary model was applied to the furnace annealing, and an anisotropic factor in atomic transport by electromigration was introduced in microwave annealing simulations. We reproduced the profiles and spectra changes observed in the experiments by both annealing methods.

Mechanisms of long-range edge retraction of metal bilayer films

The agglomeration of thin films on substrates is driven by minimization of the free surface and film/substrate interface energies and has been studied extensively for single component metal films. Only a few studies have investigated the agglomeration of kinetically constrained metal bilayer films, for which unusual long-range edge retraction was recently reported. This study has explored the agglomeration of kinetically constrained thin films of Au and Ni that were subsequently deposited on SiO2/Si substrates and annealed under high vacuum conditions at 545, 675, and 730 °C. Long-range edge retraction of the metal bilayer films revealed seven regions across the receding edge that are microstructurally distinct. The absolute and relative widths of the regions depend on the deposition sequence of the two metal films and annealing temperature. Arrhenius analysis of growth rates for different regions was used to identify energy barriers for mass transport mechanisms. The presence of native nickel oxide was found to have a significant effect on the kinetics of long-range edge retraction. The experimental results suggest that the formation of multiple regions across the receding edge is part of the kinetic evolution of long-range edge retraction of metal bilayer films.

Wavelength modulation based surface plasmon resonance sensor for detection of cardiac marker proteins troponin I and troponin T

A wavelength modulation based surface plasmon resonance (SPR) sensor setup was developed with inexpensive components. The developed setup is utilized for the detection of cardiac marker proteins, cardiac troponin I (CTnI), and cardiac troponin T (CTnT). A simple approach was used for the fabrication of CTnI/CTnT antibody sensor chip. A self-assembled monolayer (SAM) of 1-octanethiol and 11-mercaptoundecanoic acid (11-MUA) was formed on annealed Au thin film. The −COOH group of 11-MUA was modified with EDC-NHS for amine coupling to monoclonal antibodies of CTnI and CTnT. The developed SPR setup and the sensor chip with monoclonal antibodies showed good performance for the detection of CTnI and CTnT in direct assay. For CTnI, the lowest concentration detected was 0.03125 ng/mL whereas for CTnT, it was 0.0625 μg/mL. The detected concentrations are well below the onset concentration of these two proteins in blood following the acute myocardial infarction (AMI). The performance of this direct assay based on the developed setup and sensor chip therefore can be used easily for detection of CTnI and CTnT during their onset period.

Preparation of a highly-reflective gold thin film by a sputtering technique using ionic coating system

Metallic mirror coatings with high reflectivity using Au thin films deposited onto a Borosilicate BK7 glass substrates by a sputtering technique using ionic coating system (KIC-1A) were investigated. In order to facilitate the process of calculating the relationship between deposition duration and thin film thickness to reach to a certain thickness, so we have described and calculated the relationship between deposition duration and thin film thickness by theoretical analysis and then simulated by polynomial function. This study showed the relationship between the reflection and thin film thickness, where the highest value of the reflective coatings of 90.5% was obtained at thin film thickness of 153 nm.