Au Thin Films Research Articles

Monolayer Capping Provides Close to Optimal Resistance to Laser Dewetting of Au Films.

Next-generation heat-assisted magnetic recording (HAMR) relies on fast, localized heating of the magnetic medium during the write process. Au plasmonic near-field transducers are an attractive solution to this challenge, but increased thermal stability of Au films is required to improve long-term reliability. This work compares the effect of nanoscale Al, AlOx, and Ta capping films on Au thin films with Ti or Ta adhesion layers for use in HAMR and other high-temperature plasmonic applications. Thermal stability is investigated using a bespoke laser dewetting system, and SEM and AFM are extensively used to interrogate the resulting dewet areas. The most effective capping layers are found to be 0.5-1 nm of Al or AlOx, which can eliminate dewetting under certain conditions. Even one monolayer of AlOx is shown to be highly effective in reducing dewetting. In the case of thicker capping layers of Ta and AlOx, the Au film can easily dewet underneath, leaving an intact capping layer. It is concluded that thinner capping layers are most effective against dewetting as the Au cannot dewet without breaking them and pulling them apart during the dewetting process. A simple model based on energetics considerations is developed, which explains how thinner capping layers can more effectively protect the metal from pore or fissure creation. The model provides some convenient guidelines for choosing both the substrate and capping layer, for a given metal, to maximize the resistance to laser-induced damage.

Plasmon resonance-enhanced superior 2D MoS2 photodetector via single-layer gold nanoparticle film with ultra-high area density

Molybdenum disulfide (MoS2) has been considered as a promising candidate material for photodetectors due to its unrivaled transistor behavior and strong light absorption. However, due to the ultra-thin nature of monolayer or few-layer MoS2, it exhibits a low optical cross-section, resulting in a very weak light–matter interaction and accordingly limiting the photoelectric conversion efficiency. In this work, we report a facile method to prepare single-layer gold (Au) nanoparticles with ultra-high area density according to the annealing of thin Au films. By transferring the single-layer Au nanoparticle film onto a prefabricated MoS2 photodetector, we demonstrate a photodetector with a responsivity as high as 1120 A W−1. Moreover, it is found that the response time is not affected by the Au nanoparticle decoration. This method provides an easy but effective way to fabricate high-performance two-dimensional material-based photodetectors.

Spectral Tuning of a Nanoparticle-on-Mirror System by Graphene Doping and Gap Control with Nitric Acid.

Nanoparticle-on-mirror systems are a stable, robust, and reproducible method of squeezing light into sub-nanometer volumes. Graphene is a particularly interesting material to use as a spacer in such systems as it is the thinnest possible 2D material and can be doped both chemically and electrically to modulate the plasmonic modes. We investigate a simple nanoparticle-on-mirror system, consisting of a Au nanosphere on top of an Au mirror, separated by a monolayer of graphene. With this system, we demonstrate, with both experiments and numerical simulations, how the doping of the graphene and the control of the gap size can be controlled to tune the plasmonic response of the coupled nanosphere using nitric acid. The coupling of the Au nanosphere and Au thin film reveals multipolar modes which can be tuned by adjusting the gap size or doping an intermediate graphene monolayer. At high doping levels, the interaction between the charge-transfer plasmon and gap plasmon leads to splitting of the plasmon energies. The study provides evidence for the unification of theories proposed by previous works investigating similar systems.

Pump-probe reflectivity studies of ultrashort laser-induced acousto-mechanical strains in ZnO films

In the current work we report on the generation of acoustic strains in thin ZnO layers using optoacoustic transduction of ultrashort laser pulses into acoustic waves on an Au thin film transducer. After absorption of energy by the electron system of the metal, energy conversion, thermal expansion and mechanical deformation takes place. The generation and propagation of the induced acoustic strains are monitored in time via a degenerate pump-probe transient reflectivity optical setup at 800 nm, as opposed to most commonly used schemes that employ different wavelengths for the pump and probe beams, mostly in the vicinity of ZnO maximum absorption. The experimental results include energy relaxation times and phonon scattering frequencies and are supported by a thermal vibro-acoustic finite element model. The model is based on the combination of a revised two-temperature approach and elasticity theory, and considers anisotropic properties for the ZnO film and the computation of the elastic wave velocity for the first time. The outcomes are discussed in the context of electron–phonon coupling factors and other material properties. A good agreement between the experimental findings and the results from the numerical simulations has been established, regarding outcomes like the mean velocity of the strain waves, establishing a novel characterization method applicable to a variety of materials and structures.

Design of a Biaxially Stretchable Microstrip Antenna Enabled by Buckled Au Thin Film on an Elastomeric Substrate

Wearable wireless biomedical electronics enable monitoring and wireless transmission of patient physiological and pathological signals to provide remote guidance for appropriate diagnosis and treatment. As a core component, the antenna must be flexible and stretchable to adapt to the complex mechanical deformations (e.g., stretching, bending, and twisting) induced by human motions. This work proposes a biaxially stretchable microstrip antenna based on buckled gold thin films bonded on an elastomeric substrate. A simplified analytic model validated by simulations and experiments is established to investigate the biaxial buckling behaviors of the thin films within 10% tensile strains. The properties, including resonance frequency, bandwidth, and radiation pattern of the fabricated biaxially stretchable microstrip antenna under various stretched states, are studied by combining experiments and finite element analysis. The effects of biaxial tensile deformations on the resonance frequency, bandwidth, and radiation properties are discussed. Results show that the designed microstrip antenna has a relatively stable performance under both natural and deformed states within 10% of uniaxial and biaxial tensile strains, which enables the designed antenna to have broad application prospects in wearable wireless medical devices for stable transmission of signals between body-worn sensors and terminals, especially for situations accompanied with complex deformations.

Visible light‐enhanced and degradable SERS substrates based on g‐C3N4/Au NP hybrid structures

AbstractIn this study, a visible light‐activated and degradable surface‐enhanced Raman spectroscopy (SERS) substrate was fabricated based on graphite carbon nitride (g‐C3N4) and gold nanoparticle (Au NP) hybrid structures. By using porous structure of g‐C3N4 as the support, Au NP arrays can be formed through sputtering of a thin Au film. Typical surface Raman enhancement effect can be observed by using p‐toluenethiol as the probe molecule. Interestingly, it was found that the Raman signal of probe molecules could be enhanced 2.7 times by pre‐illumination of visible light for a short time within 2 min, while the Raman signal decreased significantly about five times with further long‐term irradiation of more than 60 min. The electromagnetic enhancement effect of Au NP array on semiconductor materials with various extinction coefficients was studied by FDTD simulations. Furthermore, a charge transfer enhancement mechanism was proposed based on band structure of g‐C3N4/Au NP hybrid substrate and molecular orbitals. Our experimental and simulation results demonstrated an approach to construct a visible light‐activated SERS substrate with enhancement and degradation properties based on porous organic semiconductors.

Unveiling localized electronic properties of ReS2 thin layers at nanoscale using Kelvin force probe microscopy combined with tip-enhanced Raman spectroscopy

Electronic properties of two-dimensional (2D) materials can be strongly modulated by localized strain. The typical spatial resolution of conventional Kelvin probe force microscopy (KPFM) is usually limited in a few hundreds of nanometers, and it is difficult to characterize localized electronic properties of 2D materials at nanoscales. Herein, tip-enhanced Raman spectroscopy (TERS) is proposed to combine with KPFM to break this restriction. TERS scan is conducted on ReS2 bubbles deposited on a rough Au thin film to obtain strain distribution by using the Raman peak shift. The localized contact potential difference (CPD) is inversely calculated with a higher spatial resolution by using strain measured by TERS and CPD-strain working curve obtained using conventional KPFM and atomic force microscopy. This method enhances the spatial resolution of CPD measurements and can be potentially used to characterize localized electronic properties of 2D materials.

Effect of Au thin films thickness on the wetting characteristics of eutectic Pb-Sn solder on Ni/Ag solder bath

Wetting of Pb-Sn solder on Au/Ni surface is experimentally studied as a function of the thickness of Au coating on Ni buffer layer. While Au coating is known to increase the wettability of Pb-Sn solder on Ni surface, the effect of Au layer thickness was not examined systematically. This study shows that i) an increase in Au layer thickness from 0.15 to 0.3 µm decreases the surface coverage (spread) of the solder and ii) there is a compositional change in Pb-Sn solder on Au/Ni surface due to formation of a Sn rich region at the solder - Ni interface during a reflow process of the solder. The thickness of Sn rich region near the interface increases with increasing the Au layer thickness. A driving force for the formation of the Sn rich phase is explained using the negative mixing enthalpy of Au-Sn. Since an increase in the Sn content of Pb-Sn solder increases the surface energy and viscosity of the solder melt, an increase in the thickness of the Sn rich region at the solder-Ni interface suppressed the wetting of the solder.

Structural, compositional, morphological and electrical characteristics of thermally evaporated Au Ohmic Contact on p-type HgCdTe substrate for possible infrared detectors

Gold (Au) thin films were deposited over the HgCdTe (mercury cadmium telluride) surface by a thermal-evaporation deposition technique. The thermally grown Au films were investigated using different characterization techniques, X-ray diffraction (XRD), energy dispersive X-ray (EDX) analysis, atomic force microscopy (AFM), scanning electron microscopy (SEM), and current-voltage (I–V)/resistance measurement. The maximum film growth occurs along with the preferred orientation (111). The grown Au film possesses a cubic fcc structure, desired elemental compositions, and purity. The AFM confirms that the films were uniform, dense, and had very low roughness. The films attained the desired grain size of film-forming particles. The optimized Au films were established and further employed to fabricate the transfer length method (TLM) structure with configuration (Au/p-HgCdTe). The I–V and resistance measurements of the as-deposited TLM structure produced the Ohmic Au-contact with low specific contact resistance (ρc) of 2.73 × 10−3 Ω cm2. Annealing treatment (contacts annealed for 2 h at 80 °C in the air) was applied to the as-deposited TLM structure and the measured value of specific contact resistance (ρc) for the annealed Au contact (Ohmic behaviour) is 7.11 × 10−4 Ω cm2 that is relatively low than the as-deposited contacts. Thermal annealing has reduced the magnitude of ρc by about one order. TLM structures (as-deposited and annealed) exposed the efficiency of Au thin films as an effective Ohmic contact. Thus, Au thin films could be used to create a compelling and appropriate Ohmic p-contact to fabricate HgCdTe infrared detectors.

Controlled Growth of Indium Oxide Nanowires for Gas Sensing Application.

The In2O3 nanowires have attracted enormous attention for gas sensor application due to their advantageous features. However, the controlled synthesis of In2O3 nanowires for gas sensors is vital and challenging because the gas sensing performance of the nanowires is strongly dependent on their characteristics. Here in this patent, we fabricated In2O3 nanowires on SiO2/Si substrate via a simple thermal vapor deposition method with the Au thin film as the catalyst. The growth temperatures were controlled to obtain desired nanowires of small size. The grown In2O3 nanowires were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The ethanol gas sensing properties were tested under the dynamic flow of dry air and analytic gas. The synthesized In2O3 nanowires have the potential for use in ethanol gas sensor application. In2O3 nanostructures grown at different temperatures ranging from 600 to 900oC have different morphologies. The sample grown at 600oC had a morphology of nanowire, with a diameter of approximately 80 nm and a length of few micrometers. Nanowires grown at 600°C were composed of oxygen (O) and indium (In) elements, with the atomic ratio of [O]/[In] = 3/5. The nanowire was a single phase cubic structure of In2O3 crystal. The In2O3 nanowire sensor showed typical n-type semiconducting sensing properties. The response decreased from 130 to 75 at 100 ppm when the working temperature decreased from 450°C to 350°C. The nanowires grown at 600°C by the thermal vapor deposition method had the best morphology with a small diameter of about 80 nm and a length of few micrometers. The In2O3 nanowires had a good ability to sense ethanol at varying concentrations in the range of 20 ppm to 100 ppm. The In2O3 nanowires can be used as building blocks for future nanoscale gas sensors.

Modification of Weak Localization in Metallic Thin Films Due to the Adsorption of Chiral Molecules.

We perform low-temperature magneto-conductance measurements on Cu and Au thin films with adsorbed chiral molecules and investigate their phase-coherent transport properties. Upon adsorption of chiral molecules, the spin-orbit coupling strength in Cu decreases and the Au films become ferromagnetic as evident from weak localization and antilocalization data. A theoretical model indicates that anisotropy in the molecular tilt angles, provided that the chiral molecules act as magnetic moments, induces a nonvanishing magnetic exchange interaction, causing changes in the spin-orbit coupling strength in Cu and Au. Our work adds a new viewpoint to the plethora of unique phenomena emerging from chiral molecule adsorption on materials.

Pulsed laser-induced dewetting to fabricate Au-Pd nanoparticles supported on ZnO films and its application for the photocatalytic degradation of indigo carmine

Gold-palladium (Au-Pd) bimetallic alloy nanoparticles (NPs) were obtained by the interaction of a laser pulse with a stack of Au and Pd thin films, these films were previously and sequentially deposited on a zinc oxide (ZnO) thin films grown on a glass substrate. This method can produce, with a single laser pulse, spherical NPs of different compositions which remain attached to the surface of the ZnO film. X-ray diffraction analysis showed that the NPs are composed of a solid solution of the metals whose relative content was modified by varying the deposition time of the starting metal films. The higher amount of Pd in the NPs, the smaller particle size being the minimum around 50 nm. To demonstrate the usefulness of this method to produce Au-Pd NPs directly on surfaces, the decorated ZnO layers were tested as photocatalysts material for the decomposition of indigo carmine dye. The bimetallic NPs supported on ZnO showed to have a better photocatalytic performance to decompose the dye than their mono-metallic counterpart and bare ZnO thin film. The bimetallic NPs richer in Pd achieved a degradation of 99% of the dye while bare ZnO film reached just 32%.

The effect of cavities produced by focused ion beam milling on solid state dewetting of thin Au films

The goal of this study was to explore the possibility to control the nucleation and expansion of holes during initial stages of solid state dewetting of thin polycrystalline Au film deposited on sapphire substrate. To this end, we fabricated isolated triangular cavities in the film employing Ga+ ion milling in the focused ion beam instrument. The following dewetting heat treatments indicated that the cavities do promote early nucleation and expansion of dewetting holes, yet these holes develop tortuous shapes dominated by a random distribution of nearby hillocks in the film. Deep holes exhibited a triangular patch of amorphous Al-Ga-O phase at their bottom which retained its size and morphology during subsequent thermal treatments. This amorphous phase caused some delay in holes expansion at high temperatures. We proposed a geometrical model of the retracting metal film edge pinning by the amorphous phase. The observed pinning phenomenon can be utilized for the design of patterned thin films with increased thermal stability.

SERS substrates based on rose petal replicas for the oxidative stress detection

Oxidative stress leads to multiple changes in properties of cells and of biological liquids that can cause various pathologies including cardiovascular and metabolic diseases. Modern test systems make it possible to assess the concentration of oxidized molecules in the blood plasma, but do not provide direct information about oxidation of cell membranes. In this work, we presented an effective, scalable, and inexpensive technique for manufacturing biomimetic SERS sensors for the detection of biomembrane oxidation. The main feature of this technique is the use of rose petals as templates for scalable manufacturing of SERS substrates with a cavity structure that promotes the capture and immobilization of blood cells on the grooved inner surfaces of microcavities. To obtain the SERS effect in these microcavities, the silicone replicas of rose petals were covered with an Au thin film (100 nm) and spherical Au nanoparticles by the sequential deposition and annealing processes. The application of SERS substrates with a cavity structure enables to perform studies on individual erythrocytes and detect first signs of their membrane oxidation, which can be used for early diagnostics of various blood and metabolic diseases.

Direct Observation of Grain-Boundary-Migration-Assisted Radiation Damage Healing in Ultrafine Grained Gold under Mechanical Stress.

Nanostructured metals are a promising class of radiation-tolerant materials. A large volume fraction of grain boundaries (GBs) can provide plenty of sinks for radiation damage, and understanding the underlying healing mechanisms is key to developing more effective radiation tolerant materials. Here, we observe radiation damage absorption by stress-assisted GB migration in ultrafine-grained Au thin films using a quantitative in situ transmission electron microscopy nanomechanical testing technique. We show that the GB migration rate is significantly higher in the unirradiated specimens. This behavior is attributed to the presence of smaller grains in the unirradiated specimens that are nearly absent in the irradiated specimens. Our experimental results also suggest that the GB mobility is decreased as a result of irradiation. This work implies that the deleterious effects of irradiation can be reduced by an evolving network of migrating GBs under stress.

Method for a Tube Inner Surface Coating

There are applications where the deposition of a coating must be applied on the inner surface of a tube part. In this paper, we describe a method for deposition of coatings on a tube inner wall. The coating method is based on ion beam sputtering, which is a well-established technology that enables the precise control of coating composition, thickness, and uniformity. This technique also offers the opportunity to deposit coatings selectively on desired locations using shadow masking. As an example, a study of Au and Al thin films is presented.

Print‐Light‐Synthesis of Gold Thin Film Electrodes for Electrochemical Sensing

AbstractThe one‐step fabrication of gold films by inkjet printing of a gold precursor ink and its photochemical reduction by exposure to UV light is presented. Inkjet printing creates on a substrate with high control micrometer‐thin reaction volumes in which upon direct high‐intensity light irradiation, the gold precursor reduces to pure and well‐adhered Au particles, while all other ink components escape in the gas phase, without the need for any further post‐treatment. The Au precursor ink does neither contain stabilizing agents, such as polymers or surfactants, nor sacrificial compounds, such as photoinitiators, to initiate and accelerate the reduction. This economic and green process is known as Print‐Light‐Synthesis (PLS) and is herein used to create gold patterns of thin compact Au films and separated Au nanoparticles. Gold loadings are in the µg cm−2 range and precisely controlled, thanks to the inkjet printing parameters. The gold films are characterized by spectroscopic and electrochemical methods. Finally, the applicability of Au films as electrochemical sensors is demonstrated for the detection of 1,4‐butanediol in comparison to a commercial screen‐printed Au electrode. The PLS Au electrode shows a 20 times higher sensitivity and opens new possibilities for disposable electrode production.

Efficient all-optical MoS2/Au hybrid plasmon-exciton competitive response towards selective gas sensors

In this study, we explore the realization of the optical concept of hybrid surface plasmon resonance (SPR) and lossy mode resonance (LMR), which facilitates plasmonic hot electron transfer from an Au thin film to MoS2 layers coated on a polymer optical fiber. The purpose of this experiment is to address the selective detection of humidity and volatile organic compounds in ambient conditions. By altering the properties of the surrounding environment, we can induce changes in the resonance dip wavelength stemming from the surface plasmon and lossy modes of the Au/MoS2 bilayer thin film coated on the optical fiber, acting as the transducing medium. Comparative experimental studies of transmitted light through the coated fiber reveal evidence of plasmon-enhanced charge injection from Au to a sensing surface, which relies on the competitive behavior of environment-induced conformational charge transfer. This provides substantial sensing enhancement towards optical devices based on hot-electrons technology. By optimizing the Au thickness and the MoS2 synthesis period, we obtained the most enhanced sensing performance, with a sensitivity of − 1.21 Δλ/RH% and a correlation factor of R2 = 0.99.

Resolving Interface Barrier Deviation from the Schottky–Mott Rule: A Mitigation Strategy via Engineering MoS2–Metal van der Waals Contact

The Schottky barrier (SB) in the ultraclean van der Waals contact between two-dimensional (2D) MoS2 and three-dimensional (3D) indium (In) strikingly deviates from the Schottky-Mott limit (SML). Herein, on the basis of first-principles calculation, the origin of the SB deviation is brought to bear, as well as a strategy for mitigating the SB deviation. In light of the good agreement between the SB and the corrected SB by interface potential difference (ΔV) and Fermi -level shift (ΔEF) based on the SML, the SB deviation is attributed to the combined effects of ΔV and ΔEF. Furthermore, when a Au, Sc, or Ti thin film is coated on the back side of In, the SB deviation and the sum of ΔV and ΔEF decrease similarly. Importantly, in the Ti coating situation, the SB is reduced to 0.12 eV, notably smaller than the value of 0.30 eV in the Au coating case. This interface engineering can be generalized to regulate the SB between a 2D semiconductor and a 3D alloy.

A correlation between fractal growth, water contact angle, and SERS intensity of R6G on ion beam nanostructured ultra-thin gold (Au) films

Introduction: This study focuses on the detection of rhodamine-6G using surface-enhanced Raman scattering (SERS) on gold nanostructures (AuNS) of different sizes. Ion beam irradiation has been carried out to tune the size of AuNS and investigate the underlying mechanisms of sputtering and diffusion that govern their growth. Additionally, the study established a correlation between fractal growth parameters, water contact angle, and SERS detection of R6G. The results of this study offer new insights into the mechanisms of SERS detection on roughened metallic surfaces.Methods: Thermal evaporation was used to deposit an Au thin film on a glass substrate. Subsequent 10 keV Ar+ irradiation was done on Au thin film for fluences ranging from 3×1014 to 3×1016 ions/cm2 to tune the size of AuNS. Rutherford backscattering spectroscopy (RBS) was used to confirm that the decrease in Au concentration under ion beam sputtering was responsible for the tuning in size and structure of AuNS. Fractal dimension (Df) and interface width (w) were used as statistical parameters to control the wettable characteristics of the AuNS surfaces.Results and discussion: The researchers found that the growth of AuNS was governed by ion beam induced sputtering and diffusion mechanisms. They established a correlation between fractal growth parameters, water contact angle, and SERS detection of R6G. They found that a higher surface coverage area of Au NPs with lower fractal dimensions and water contact angles favoured the SERS detection of R6G. This study provides new insights into the mechanisms of SERS detection on roughened metallic surfaces. It is found that the growth of AuNS was governed by ion beam-induced sputtering and diffusion mechanisms, and established a correlation between fractal growth parameters, water contact angle, and SERS detection of R6G. The findings of this study may have applications in the development of more sensitive and efficient SERS-based chemical sensors.