Adhesion Of Au Research Articles

Mesoporous alloy chiral nanoparticles with high production yield and strong optical activities.

Applying galvanic replacement reactions (GRRs) to the host chiral nanoparticles (CNPs) is an exclusive method to generate alloy CNPs with mesoporous structures through chirality transfer. However, the GRR-mediated chirality transfer is too inefficient to impose strong optical activities on the alloy mesoporous CNPs (or m-CNPs). Here we dope the host with gold (Au) to significantly enhance the chirality transfer, and additionally employ the Au adhesion layer to increase the production yield (PY) of binary m-CNPs.

Mitigating Re-Entrant Etch Profile Undercut in Au Etch with an Aqua Regia Variant

We investigate the re-entrant undercut profile resulting from Au wet etching for patterning micron range thick films using an aqua regia-based solution in comparison with an iodine-iodide-based commercial etchant. Our work discriminates between two undercutting mechanisms: galvanic acceleration of etch rate at the Au adhesion or barrier layer, and delamination-based undercutting. We tracked etch outcomes of feature size reduction from photoresist size, undercut Au in cross-section and lift-off of small (5–10 μm) features. Results indicate that galvanic undercutting is well-mitigated by the aqua regia solution compared to commercial etchant results. Good Au adhesion eliminates undercut for 500 nm-thick Au and mitigates it by ∼80% for 1 μm-thick Au. We discuss the electrochemical origin of this mitigated galvanic undercut.

Nonconventional nucleation and growth of Au nanoparticles with improved adhesion on oxygen-excessive oxide surfaces

Controlling the number and size of nanoparticles on oxides is essential for manipulating the chemical and physical properties of Au to manifest its catalytic and sensing activities. This task requires a drastic deviation from the current understanding of the inherent nucleation behavior of Au nanoparticles weakly adhered on oxides. Herein, we report experimental and computational evidence supporting the effectiveness of incorporating excessive atomic O at ZnO surfaces on enhancing the adhesion of Au − ZnO. This result is highly contrary to current beliefs, emphasizing the advantage of O-deficient oxide surfaces for enhancing the nucleation. The use of atomic O-excessive ZnO surfaces facilitated the implementation of extremely high nucleation densities (>4 × 1012 cm−2) of Au nanoparticles, primarily with reduced diameters of 2–3 nm. These structural features were primarily attributed to the reduction in the free energy at the Au − ZnO interface due to the diffusion and participation of atomic O in Au oxidation. This result offers new insights into the crucial role played by atomic O in enhancing the adhesion of Au with oxides and the development of unique structural features of Au nanoparticles.

Solid-State Dewetting of Gold on Stochastically Periodic SiO2 Nanocolumns Prepared by Oblique Angle Deposition.

Solid-state dewetting (SSD) on patterned substrates is a straightforward method for fabricating ordered arrays of metallic nanoparticles on surfaces. However, a drawback of this procedure is that the patterning of substrates usually requires time-consuming and expensive two-dimensional (2D) fabrication methods. Nanostructured thin films deposited by oblique angle deposition (OAD) present at the surface a form of stochastically arranged periodic bundles of nanocolumns that might act as a patterned template for fabricating arrays of nanoparticles by SSD. In this work, we explore this concept and investigate the effect of three different types of OAD SiO2 thin films on the SSD of Au deposited on their surface. We demonstrate that the size and spatial distribution of the particles can be tailored through the surface morphology of these OAD film substrates. It has been found that the SSD of the evaporated Au layer gives rise to a bimodal size distribution of particles. A majority of them appeared as mesoparticles with sizes ≳100 nm and the rest as nanoparticles with ∼10 nm, respectively, located either on top of the nanocolumns following their lateral distribution (i.e., resulting from a patterning effect) or incorporated inside the open mesopores existing among them. Moreover, on the SiO2-OAD thin films where interconnected nanocolumnar bundles arrange in the form of discrete motifs, the patterning effect gave rise to the formation of approximately one Au mesoparticle per motif, which is one of the assets of patterned SSD. The morphological, optical (i.e., plasmon resonance), and crystalline structural characteristics of Au mesoparticles suggest that the interplay between a discontinuous nanocolumnar surface acting as a template and the poor adhesion of Au onto SiO2 are key factors for the observed template effect controlling the SSD on the surface of OAD thin films.

Simultaneous photoadhesion and photopatterning technique for passivation of flexible neural electrodes based on fluoropolymers

Herein, we introduce a method to simultaneously photoadhere a photocrosslinkable polymer to a plasma-treated fluoropolymer while photopatterning the photocrosslinkable polymer via a single-photo-exposure as a new electrode passivation technique. Photoadhesion was determined to result from plasma-generated radicals of the plasma-treated fluoropolymer. Crystallinity of the fluoropolymer was analysed to determine the photoadhesion strength through its effects on both the formation of radicals and the etching of fluoropolymers. Passivation feasibility of simultaneous photoadhesion and photopatterning (P&P) technique were demonstrated by fabricating an Au electrocorticography electrode array and modifying the electrode with electro-deposited metallic nanoparticles. Adhesion of sputter-deposited Au to the fluoropolymer was dependent on mechanical interlocking, indicated by the formation of Au clusters which are typically influenced by the surface temperature during the sputter-deposition and the glass transition temperature of the fluoropolymer. The adhesion of Au to the fluoropolymer without an additional adhesion promotor and the proposed P&P passivation technique would help prevent detachment of the electrode and the delamination of the passivation layer in fluoropolymer-based neural electrode.

Equilibrium shape of core(Fe)–shell(Au) nanoparticles as a function of the metals volume ratio

The equilibrium shape of nanoparticles is investigated to elucidate the various core–shell morphologies observed in a bimetallic system associating two immiscible metals, iron and gold, that crystallize in the bcc and fcc lattices, respectively. Fe–Au core–shell nanoparticles present a crystalline Fe core embedded in a polycrystalline Au shell, with core and shell morphologies both depending on the Au/Fe volume ratio. A model is proposed to calculate the energy of these nanoparticles as a function of the Fe volume, Au/Fe volume ratio, and the core and shell shape, using the density functional theory-computed energy densities of the metal surfaces and of the two possible Au/Fe interfaces. Three driving forces leading to equilibrium shapes were identified: the strong adhesion of Au on Fe, the minimization of the Au/Fe interface energy that promotes one of the two possible interface types, and the Au surface energy minimization that promotes a 2D–3D Stranski–Krastanov-like transition of the shell. For a low Au/Fe volume ratio, the wetting is the dominant driving force and leads to the same polyhedral shape for the core and the shell, with an octagonal section. For a large Au/Fe ratio, the surface and interface energy minimizations can act independently to form an almost cube-shaped Fe core surrounded by six Au pyramids. The experimental nanoparticle shapes are well reproduced by the model, for both low and large Au/Fe volume ratios.

Exposed-core localized surface plasmon resonance biosensor

We propose and numerically characterize an exposed-core photonic crystal fiber for biosensing applications. Surface plasmons are excited within gold (Au) strips, and titanium dioxide is employed to support adhesion of Au with glass. In consideration of ease of fabrication, only four air holes are used to simplify the sensor structure. Simulation results show an improved wavelength and amplitude sensitivity of 34,000 nm/RIU and 1170 RIU−1, respectively, that comes with a low confinement loss of 0.79 dB/cm. Results also indicate a low full width at half-maximum that contributes to high figure of merit of 310.

Plasmonic nanomaterial structuring for SERS enhancement.

Unique structures of a gold island over nanospheres (AuIoN) featuring a three-dimensional (3D) nanostructure on a highly ordered two-dimensional (2D) array of nanospherical particles with different adhesion layers were fabricated as surface-enhanced Raman scattering (SERS) substrates. Ultra-thin Au was thermally evaporated onto PS nanospheres while aluminum oxide (Al2O3) was applied as an Au adhesion layer. The outcomes demonstrate that the higher metallic particle density and surface roughness supplied by the Al2O3 provided larger interatomic bonding than a conventional adhesion layer, the highly-dispersive Cr. Nanosphere lithography (NSL) to deposit templating particles as small as ∼100 nm successfully created a simple initial roughening process which in turn boosted the localized surface plasmon resonance (LSPR) efficiency. So far, PS template deposition of a size less than 200 nm has been challenging, but here, through the use of a simple solvent ratio adjustment on drop-casting NSL, the novelty of natural lithography with downscaled properties as an alternative to the complexity of photolithography which is mostly conducted in the strict ambience of a clean room, is presented. SERS activity was primarily attributed to the synergistic effect of collective LSPRs from the AuIoN structure reinforcing the electromagnetic field, particularly in the crevices of two neighboring AuIoNs, as simulated by FDTD (Finite-Difference Time-Domain) computation. An AuIoN fabricated by the integration of Al2O3 with thinner Au particles showed the optimum SERS activities with an improved enhancement factor of 1.51 × 106. Overall, a non-lithographic technique in tuning SERS hotspots and favorable characteristics of Al2O3 for ultra-thin Au adhesion support, which can potentially be used in the fabrication of various devices, was demonstrated.

A Hi-Bi Ultra-Sensitive Surface Plasmon Resonance Fiber Sensor

In this paper, a simple, miniature, and highly sensitive photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor is proposed. The target analyte and the plasmonic material are at the outer surface of the fiber making practical applications feasible. A 30-nm gold (Au) layer supports surface plasmons. A thin titanium dioxide (TiO 2 ) layer is used to assist adhesion of Au on the glass fiber. The fiber cross section is formed purely by circular-shaped holes simplifying the preform manufacturing process. A high-birefringence (hi-bi) fiber is obtained by means of an array of air holes at the center of the fiber. A finite element method (FEM) is employed to analyze the surface plasmon properties of the proposed PCF-SPR sensor. By optimizing the geometric parameters, a maximum wavelength sensitivity (WS) of 25 000 nm/RIU and an amplitude sensitivity (AS) of 1411 RIU -1 for a dielectric refractive index (RI) range of 1.33-1.38 are obtained. Moreover, an estimated maximum resolution of 4 × 10 -6 and a figure of merit (FOM) of 502 are obtained that ensures high detection accuracy of small refractive index (RI) changes. Owing to its sensitivity and simple architecture, the proposed sensor has potential application in a range of sensing application, including biosensing.

Impact of the Metal Adhesion Layer on the Radiation Power of Plasmonic Photoconductive Terahertz Sources

The use of plasmonic contact electrodes in a photoconductive terahertz source offers high optical-to-terahertz conversion efficiencies. The high efficiency is because plasmonic contact electrodes concentrate a large portion of the incident optical pump beam in close proximity to the contact electrodes. By reducing the average transport path length of the photo-generated carriers from the contact electrodes, a larger number of the photocarriers drift to the terahertz radiating elements of the photoconductive source within a sub-picosecond time scale. Therefore, higher terahertz radiation power levels are achieved compared to a similar photoconductive source without plasmonic contact electrodes. Au is a preferred metal for plasmonic contact electrodes because of the strong plasmonic enhancement factors it offers at near-infrared optical wavelengths. However, it requires an adhesion layer to stick well to most III–V semiconductor substrates used in photoconductive terahertz sources. In this paper, we analyze the impact of the Au adhesion layer on the performance of plasmonic photoconductive sources fabricated on a GaAs substrate. Our analysis suggests that Cr is the most promising adhesion layer for plasmonic contact electrodes. We show that the use of a Cr adhesion layer instead of Ti, which is used in previously demonstrated plasmonic photoconductive sources, offers up to an 80% enhancement in the generated terahertz powers. We report record-high terahertz power emissions of up to 6.7 mW from plasmonic photoconductive sources with Cr/Au contacts.

Adsorption and Adhesion of Au on Reduced CeO2(111) Surfaces at 300 and 100 K

The adsorption of vapor-deposited Au onto CeO2-x(111) thin films (x = 0.05 and 0.2) at 300 and 100 K was studied using single crystal adsorption calorimetry (SCAC). The morphology of Au on these films was investigated using He+ low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS) by monitoring the changes in substrate and adsorbate signals with Au coverage. Both techniques indicate that Au grows on CeO1.95(111) as three-dimensional particles in the approximate shape of hemispherical caps with a density of 2.8 × 1012 particles/cm2 at 300 K and 7.8 × 1012 particles/cm2 at 100 K. At 300 K, Au initially grows on CeO1.80(111) with a shape similar to hemispherical caps with a density of 5.4 × 1012 particles/cm2 until ∼1.6 ML Au coverage, above which the Au particles become thicker than hemispherical caps. At 300 K, the initial heat of adsorption of Au onto CeO1.95(111) is 259 kJ/mol, which is 37 kJ/mol lower than that on CeO1.80(111). This indicates stronger binding of Au to ...

Improved surface plasmon resonance biosensing using silanized optical fibers

Coupling surface plasmon resonance (SPR) to optical fiber (FO) technology has brought tremendous advancements in the field by offering attractive advantages over the traditional prism-based SPR platforms, such as simplicity, cost-effectiveness and miniaturization. However, the performance of the existing FO-SPR sensors widely depends on the adhesion of the gold (Au) layer to the FO silica core, thereby often representing a major limiting factor in achieving the properties of the benchmark SPR systems. In this paper, we used (3-marcaptopropyl)trimethoxysilane (MPTMS) as an adhesion promoter for developing robust Au surfaces on the three-dimensional (3D) FO-SPR sensing probe. Carefully prepared FO substrates were first silanized using a wet chemistry approach, with MPTMS concentrations ranging from 2.5 to 24mM, and subsequently exposed to a drying treatment at room temperature (RT) or at 100°C, before coating them with a ∼50nm Au plasmonic film. Differently prepared silanized FOs were next used for evaluating their sensitivities, by performing refractive index (RI) measurements in sucrose dilutions. Advanced statistical analysis of the obtained data indicated that using 8mM MPTMS solution coupled with a RT post-drying treatment is an efficient way of producing FOs with dramatically improved Au adhesion properties. The role of the MPTMS underlayer was further investigated by exposing the reference and silanized FOs to stress conditions, such as strong mechanical (adhesion tape tests), chemical (piranha solution treatments) and thermal variations. Although additional studies using scanning electron microscopy (SEM) revealed changes in the Au film morphology after these endurance tests, the silanized FOs exhibited an enhanced robustness while retaining the overall sensor's capabilities. In contrast, the reference FOs consistently failed the mechanical and chemical tests, while only resisting under thermal variations. Moreover, the improved resistance of the silanized FO-SPR probes allowed them to be reused up to three times with no significant loss in the sensor performance, while implementing bioassays based on two types of bioreceptors (a DNA aptamer against thrombin protein and a polyclonal antibody against human immunoglobulin E – hIgE). All these results might represent a step forward in the fabrication of more robust and reusable FO-SPR biosensors, featuring great potential for developing highly-sensitive biochemical assays.

Ultrathin, Ultrasmooth Gold Layer on Dielectrics without the Use of Additional Metallic Adhesion Layers

With advances in the plasmonics and metamaterials research field, it has become more and more important to fabricate thin and smooth Au metal films in a reliable way. Here, by thin films we mean that their average height is below 10 nm and their average roughness is below 5% of the total thickness. In this article, we investigated the use of amino- and mercaptosilanes to increase the adhesion of Au on Si wafers, thus obtaining a smooth and thin layer. This method does not include the use of other metals to improve the adhesion of gold, like Ti or Cr, since they would reduce the optical characteristics of the structure. Our results show that layers having 6 nm thickness and below 0.3 nm roughness can be reproducibly obtained using aminosilanes. Layers having a nominal thickness of 5 nm have a yield of 58%; thus, this thickness is the limit for the process that we investigated.

Dynamics of ultrathin gold layers on vitreous silica probed by density functional theory.

The structure and properties of Au ultrathin films on hydroxyl-free and hydroxylated silica glass surfaces are investigated using ab initio molecular dynamics simulations. Substantial surface structure dependence of Au agglomeration behavior (solid-state dewetting) is found. On hydroxyl-free surfaces, the Au film virtually undergoes instantaneous agglomeration accompanied by the formation of voids exposing a bare silica glass surface. In contrast, simulated annealing of the Au film on hydroxylated surface models leaves its structure unchanged within the simulation time. This points to a key role of reactive defect sites in the kinetics of solid-state dewetting processes of metals deposited on the glass surface. Such sites are important for initial void nucleation and formation of metal clusters. In addition, our calculations demonstrate the crucial role of the appropriate inclusion of dispersion interactions in density functional theory simulations of metals deposited on glass surfaces. For defective, hydroxyl-free glass surfaces the dispersion correction accounts for 35% of the total adhesion energy. The effect is even more dramatic for hydroxylated glass surfaces, where adhesion energies are almost entirely due to dispersion interactions. The Au adhesion energies of 200 and 160 kJ (mol nm(2))(-1) calculated for hydroxylated glass surfaces are in good agreement with the experimental data.

Sonochemical Deposition of Au Nanoparticles on Different Facets-Dominated Anatase TiO2 Single Crystals and Resulting Photocatalytic Performance

The dependence of Au adhesion and photocatalytic performance on the support’s exposed facets in the Au/TiO2 system is an important consideration in the design of Au/TiO2 heterogeneous catalysts. In this article, we prepared anatase TiO2 single crystals with different percentages of exposed {101} and {001} facets by a hydrothermal process. The uniform deposition of crystalline metallic Au nanoparticles with the mean diameter of 10 to 11 nm onto both exposed {101} and {001} facets of the prepared anatase TiO2 single crystals has been achieved with the aid of power ultrasound. The Au-deposited TiO2 single crystals with the dominant {001} facet have been demonstrated to have much higher plasmon-induced photocatalytic activity than those with the dominant {101} facet for the degradation of 2,4-dichlorophenol. It was revealed that the dominant facet of anatase TiO2 crystal plays a critical role in the plasmon-induced photocatalytic performance of Au/TiO2 system. The {001} facet enhances the efficiency of the in...

Growth of Vertically Aligned ZnO Nanowire Arrays Using Bilayered Metal Catalysts

Vertically aligned, high‐density ZnO nanowires (NWs) were grown for the first time on c‐plane sapphire using binary alloys of Ni/Au or Cu/Au as the catalyst. The growth was performed under argon gas flow and involved the vapor‐liquid‐solid (VLS) growth process. We have investigated various ratios of catalyst components for the NWs growth and results indicate that very thin adhesion layers of Ni or Cu deposited prior to the Au layer are not deleterious to the ZnO NW array growth. Significant improvement of the Au adhesion on the substrate was noted, opening the potential for direct catalyst patterning of Au and subsequent NW array growth. Additionally, we found that an increase of in thickness of the Cu adhesion layer results in the simultaneous growth of NWs and nanoplates (NPs), indicating that in this case the growth involves both the VLS and vapor‐solid (VS) growth mechanisms. Energy dispersive X‐ray spectroscopy (EDX) and surface‐enhanced Raman scattering (SERS) studies were also performed to characterize the resulting ZnO NW arrays, indicating that the NWs grown using a thin adhesion layer of Ni or Cu under the Au show comparable SERS enhancement to those of the pure Au‐catalyzed NWs.

Characterization of LTCC-Thick Film Technology for 300°C Packaging

Economic geothermal well construction and reservoir characterization requires high temperature logging tools and sensors with the long-term operation capability of 300°C. Low temperature co-fired ceramic (LTCC) and thick film technology make them a potential candidate for building interconnects to SiC device and functional analog or digital circuits to operate at that temperature. In this work, single conductor layer LTCC substrates have been fabricated to characterize the thick film Au adhesion under 300°C aging. Via reliability has been evaluated on the multi-layer LTCC substrates under temperature cycling of 35–325°C. In addition, die attach, wire bonding and stud bumping assembly techniques has been investigated.

Modulation of bonding between noble metal monomers and CNTs by B-, N-doping

Abstract By means of density functional theory calculations, we demonstrate that the bonding between noble metal monomers and CNTs can be modulated by the B-, N-doping. But the modulation mechanisms are strongly dependent on the doping element and metal. B-doping significantly enhances the adhesion of Au, Ag on CNTs mainly via the formation the metal-B ionic bond. While N-doping enhance the adhesion of Au, Ag and Pt monomer on CNTs due to the formation of stronger metal carbon covalent bond than that on pristine CNTs, which is caused by electron rich N-CNTs.

Effect of source/drain undercut on the performance of pentacene thin-film transistors on plastic

We studied the electrical properties of bottom contact pentacene thin-film transistors (TFTs) on plastic with and without source/drain undercut. The undercut depends on the Cr thickness in Au∕Cr source/drain electrodes, where Au is used for the ohmic contact with pentacene, and the Cr is to enhance the adhesion of Au to the cross-linked poly-4-vinyphenol gate insulator. The undercut can be removed by reducing the Cr thickness less than 5nm. The TFTs with and without undercut exhibited the field-effect mobility in the saturation region of 0.96 and 1.3cm2∕Vs, respectively, but in the linear region the field-effect mobility of the TFT with source/drain undercut is ∼1∕9 that of the organic TFT without undercut.

Integration of benzocyclobutene polymers and silicon micromachined structures using anisotropic wet etching

Integration of thick, low-k dielectric benzocyclobutene (BCB) film with deep etched structures in silicon allows the fabrication of microelectromechanical systems (MEMS) devices with low parasitic loss. A fabrication process is developed for integration of 1-μm-thick BCB low-k dielectric film and 200-μm-deep anisotropically etched grooves in silicon with potassium hydroxide (KOH). In order to protect the low-k film during the highly corrosive, long, high-temperature KOH etching process, gold (Au) is used as an etch mask. Chromium (Cr) is used to improve the adhesion of Au to the underlying BCB layer. Metal-BCB adhesion is the key parameter in this masking design. Partial cure of BCB at 210 °C for 40 min with appropriate surface treatment (adhesion promoter) prior to metallization and full cure at 250 °C for 1 h after metallization, together with Cr/Au sputtering at 200 °C, improves the adhesion dramatically. The adhesion strength of metal films to BCB was experimentally verified in a qualitative manner. V grooves were etched into silicon in 20 wt%, 80 °C KOH solution for 8 h in the presence of 1 μm BCB film. BCB was protected and kept intact with an Au mask layer during the etch process. In order to understand the mechanism of the adhesion improvement, the interface between BCB and the Cr/Au layer was studied using secondary ion mass spectroscopy and Auger electron spectroscopy. Adhesion improvement which is mainly due to cure management and use of adhesion promoter is associated with (1) the diffusion of silicon and carbon from the polymer structure into the Cr layer, and (2) the chemical interaction of BCB and Cr at the interface mainly in the form of the oxidation of Cr. The integration of BCB and the KOH etching process which was obtained by improving the adhesion of metal etch mask to the BCB film, together with the study of the interfaces, allow us to use thick low dielectric constant BCB film for fabrication of MEMS devices with very low parasitic loss.