Size effect of Au seeds on structure of Au–Pt bimetallic nanoparticles

Au–Pt bimetallic nanoparticles (NPs) were synthesized by a seeded growth method. Au NPs with different sizes were obtained by reducing HAuCl₄ with butyllithium, and AuPt bimetallic NPs were synthesized by reducing H₂PtCl₆ with oleylamine using the pre-synthesized Au NPs as seeds. The size of Au seeds was found to be a key factor on the structure of Au–Pt bimetallic NPs. Using big Au NP seeds (8 nm or 12 nm) resulted in the formation of Au–Pt dendritic structures. While relatively small Au NPs (3 nm) were used as seeds, the fast atomic diffusion inside relatively small bimetallic NPs will result in an Au–Pt alloy formation.     

Distribution of Au atom number in the Au agglomerates generated during the out-diffusion process of supersaturated high-temperature substitutional Au in Si at 900 °C

Au atom number in the Au agglomerates generated during the annealing of supersaturated high-temperature substitutional Au in Si at 900 °C are measured by SIMS and their distributions have been investigated. The annealing time is chosen as 22.5, 90 or 360 h, which corresponds to initial, middle or near final stage of the annealing. Many “initial agglomerates” containing about 2.0 × 10⁵ Au atoms are generated and the distributions show an abrupt one with the peak at the atom number in the initial and middle stages. The “initial agglomerates” have absorbed supersaturated Au atoms within 0.86 μm. The “initial agglomerates” are generated even in the near final stage and grow up to containing about 4 × 10⁶ Au atoms by absorbing the Au atoms within 2.3 μm, finally. As the consequence, many agglomerates contain 5 × 10⁴–1.3 × 10⁶ Au atoms resulting in a broad distribution at the near final stage. Schematic models of agglomerations corresponding to each stage are proposed.     

Biosynthesis and stabilization of Au and Au–Ag alloy nanoparticles by fungus,Fusarium semitectum

Abstract

Crystallized and spherical-shaped Au and Au–Ag alloy nanoparticles have been synthesized and stabilized using a fungus, F . semitectum in an aqueous system. Aqueous solutions of chloroaurate ions for Au and chloroaurate and Ag⁺ ions (1 : 1 ratio) for Au–Ag alloy were treated with an extracellular filtrate of F . semitectum biomass for the formation of Au nanoparticles (AuNP) and Au–Ag alloy nanoparticles (Au–AgNP). Analysis of the feasibility of the biosynthesized nanoparticles and core–shell alloy nanoparticles from fungal strains is particularly significant. The resultant colloidal suspensions are highly stable for many weeks. The obtained Au and Au–Ag alloy nanoparticles were characterized by the surface plasmon resonance (SPR) peaks using a UV-vis spectrophotometer, and the structure, morphology and size were determined by Fourier transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), and transmission electron microscopy (TEM). Possible optoelectronics and medical applications of these nanoparticles are envisaged.     

TEM microstructural analysis of As-Bonded Al–Au wire-bonds

In this study the interface morphology of a model 99.999% (5N) Au wire bonded to Al pads in the as-bonded state was examined by scanning/transmission electron microscopy with energy dispersive spectroscopy. Specimens for transmission electron microscopy were prepared using the lift-out method in a dual-beam focused ion beam system. Analysis of the bond microstructure was conducted as a function of the Al pad content and as a function of the bonding temperature. Additions of Si and Cu to the Al pad affect the morphology and the uniformity of the interface. A characteristic-void line is formed between two intermetallic regions with different morphologies in the as-bonded samples. According to the morphological analysis it was concluded that a liquid phase forms during the bonding stage, and the void-line formed in the intermetallic region is the result of shrinkage upon solidification and not the Kirkendall effect.     

Constitution of Au–AuSn–Pb partial ternary system

Abstract

In the light of recent changes made to the phase diagrams of the Au–Sn and Au–Pb binary systems, the constitution of the Au–AuSn–Pb partial ternary system has been redetermined using thermal analysis, metallography, and X-ray techniques. The equilibria consist of four ternary transition reactions at 63·5Au–26·0Pb–10·5Sn, 382·5°C; 42·5Au–47·5Pb–10·0Sn, 254°C; 30·5Au–60·5Pb–9·0Sn, 224°C; and 20·5Au–75·5Pb–4·0Sn, 214°C; a ternary eutectic equilibrium at 15·0Au–84·0Pb–1·0Sn, 211°C; and a four-phase monotectic equilibrium at 64·0Au–9·0Pb–27·0Sn, 257·5°C (compositions are given in atomic per cent). The phase assemblage of the partial ternary system is characterized by a region of true ternary liquid immiscibility that covers much of the composition triangle. The composition of the second liquid phase at the monotectic reaction temperature is 45·0Au–40·0Pb–15·0Sn. Solid solubilities were not determined.

MST/620     

Temperature dependence of current—voltage characteristics of Au/n-GaAs epitaxial Schottky diode

The influence of temperature on current-voltage (I-V) characteristics of Au/n-GaAs Schottky diode formed on n-GaAs epitaxial layer grown by metal organic chemical vapour deposition technique has been investigated. The dopant concentration in the epitaxial layer is 1 X 10¹⁶ cm^-3. The change in various parameters of the diode like Schottky barrier height (SBH), ideality factor and reverse breakdown voltage as a function of temperature in the range 80–300 K is presented. The variation of apparent Schottky barrier height and ideality factor with temperature has been explained considering lateral inhomogeneities in the Schottky barrier height in nanometer scale lengths at the metal-semiconductor interface     

Synthesis of bio-inspired Ag–Au nanocomposite and its anti-biofilm efficacy

In the present study, bio-inspired Ag–Au nanocomposite was synthesized using banana peel extract (BPE) powder. The Ag–Au nanocomposite was characterized using various techniques such as UV–vis spectrophotometry, transmission electron microscopy (TEM) attached with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Efficiency of AuNPs, AgNPs and Ag–Au nanocomposite was tested for their antibacterial activity against Pseudomonas aeruginosa NCIM 2948. The Ag–Au nanocomposite exhibits enhanced antimicrobial activity over its monometallic counterparts. Anti-biofilm activity of AgNPs, AuNPs and Ag–Au nanocomposite against P. aeruginosa was evaluated on glass surfaces. The Ag–Au nanocomposite exhibited the highest biofilm reduction (70–80%) when compared with individual AgNPs and AuNPs. Effect of AuNPs, AgNPs and Ag–Au nanocomposite on biofilm formation was evaluated in 96 wells microtiter plates. The percentage of biofilm inhibition was sharply increased with increasing concentration of AuNPs, AgNPs and Ag–Au composite. However, Au–Ag nanocomposite showed the highest biofilm inhibition when compared with individual AuNPs and AgNPs. This synergistic anti-biofilm activity of Ag–Au nanocomposite has an importance in the development of novel therapeutics against multidrug-resistant bacterial biofilm.     

High rectification ratios of Fe–porphyrin molecules on Au facets

We report room temperature measurements of current vs. voltage (I–V) from self-assembled Fe porphyrin [Fe(III) 5,15-di[4-(s-acetylthio)phenyl]-10,20-diphenyl porphine] molecular layers formed on annealed gold crystal facets on glass substrates. I–V curves were measured using an atomic force microscope with a conductive platinum tip. We observed a rectifier effect that shows asymmetric I–V curves from a monolayer of molecules. The majority rectification ratios at ±1 V obtained from hundreds of I–V lie in between 20 and 200, with the highest up to 9000. This is in contrast to the symmetric I–V curves measured from a few nm thick multilayer molecular islands. We contribute the observed rectification in ultrathin FeP molecular layers from asymmetric Schottky barriers that result from molecules in different bonding strengths to electrodes of gold and platinum.     

Structure and magnetic properties of nano-structured heterogeneous Au—Co alloys

High-resolution and analytical transmission electron microscopy, X-ray diffraction, magnetic and magnetoresistance measurements were used to investigate nanostructures of melt-spun Au₈₆Co₁₄ and Au₇₈Co₂₂ alloys. The microstructure of the Au₈₆Co₁₄ alloy was composed of very small Co precipitates inside the Au grains with some larger Co precipitates (20–35 nm) dispersed at the grain boundaries, while the microstructure of the Au₇₈Co₂₂ alloy consisted of Au/Co lamellar eutectic grains with Co precipitates (50–70 nm) dispersed at the grain boundaries. A few grains had very small (4 nm) Co precipitates. Annealing at 773 K for 10 min caused Co depletion in the Au matrix from 5.4–10 at.% to 0.9–2.0 at.%. Annealing also caused transitions from superparamagnetic to ferromagnetic and from single to multiple domain magnetic structure of some of the small and some of the larger Co precipitates, respectively. The MR ratios (/, in magnetic field of 14.5 kOe) of the as-cast Au₈₆Co₁₄ and Au₇₈Co₂₂ alloys were 2.5% and 2.6%, respectively. Annealing of the alloys at 673 K for 1 hr reduced / to 0.9–1%.     

Composition control of co-electroplating Au–Sn deposits using experimental strategies

The optimal electroplating parameters for a pulse-current co-electroplating system of Au–Sn deposits in a non-cyanide electrolyte were investigated using experimental strategies, including fractional factorial design (FFD) and central composite design (CCD) coupled with the response surface methodology. pH value, ethylene diamine tetraacetic acid (EDTA) concentration, catechol concentration and metallic ions molar ratio (i.e., [Au]/[Sn]) were identified as the key factors affecting the composition of Au–Sn deposits in the FFD study. A reliable model between the response variable and the key factors of pH value, EDTA concentration and catechol concentration was established for the composition control of Au–Sn alloys in the CCD study. The standard deviation of the response variable (tin content) was set at the minimum level to determine the optimal co-electroplating parameters for the predicted composition value of Au–Sn deposits. Pair T test was conducted to validate both predicted and observed composition values under the optimal electroplating parameters, and the composition of Au–Sn deposits can be precisely controlled based on the established model. Scanning electron microscope observation and X-ray diffractometer analysis revealed that the morphology and crystalline of the Au–Sn deposits were composition-dependent.     

Wear behavior of Au–ZnO nanocomposite films for electrical contacts

Electrical contact switches require low contact resistance for efficient passage of signals, while withstanding repetitive cycling. Hard gold with alloy additions of Ni, Co, or Ag can increase the wear resistance of Au films, however, this causes a significant decrease in conductivity and alloying elements can segregate during long-term aging leading to property evolution. The current work demonstrates that Au–zinc oxide (ZnO) nanocomposites can create a hard Au coating with a uniform, stable structure under frictional loading. Addition of ZnO particles decreases the grain size and texture of the film by 35 and 40–75 %, respectively, indicating a change in growth behavior of the film. The nanoindentation hardness increased directly with increasing ZnO concentration. Atomic force microscopy examination of wear-tested films demonstrated morphological stability after frictional contact and thus showed the potential for these films to replace current hard Au used on contact terminals.     

Theoretical study of the optical absorption behavior of Au/Ag core–shell nanoparticles

The dependence of linear optical response properties of bimetallic core–shell spherical nanoparticles is investigated as a function of size and relative composition. Two kinds of schematic models have been tested for describing the dielectric behavior of bimetallic particles and the related linear electromagnetic response: (i) Drude model, in conjunction with bulk dielectric data relative to the pure metals, in the assumption of a simple combination law; (ii) DFT-based approach to the dynamic polarizability of a binary particle, with the nature of the metals involved taken into account through their Wigner–Seitz radius.     

Factors affecting the optical properties of Pd-free Au–Pt-based dental alloys

The optical properties of experimental Au-Pt-based alloys containing a small amount of In, Sn, and Zn were investigated by spectrophotometric colorimetry to extract factors affecting color of Au-Pt-based high-karat dental alloys. It was found that the optical properties of Au-Pt-based alloys are strongly affected by the number of valence electrons per atom in an alloy, namely, the electron:atom ratio, e/a. That is, by increasing the e/a-value, activities of reflection in the long-wavelength range and absorption in the short-wavelength range in the visible spectrum apparently increased. As a result, the maximum slope of the spectral reflectance curve at the absorption edge, which is located near 515 nm (approximately 2.4 eV), apparently increased with e/a-value. Due to this effect, the b*-coordinate (yellow-blue) in the CIELAB color space considerably increased and the a*-coordinate (red-green) slightly increased with e/a-value. The addition of a third element with a higher number of valence electrons to the binary Au-Pt alloy is, therefore, effective in giving a gold tinge to the parent Au-Pt alloy. This information may be useful in controlling the color of Au-Pt-based dental alloys.     

One-pot synthesis of Polyindole–Au nanocomposite and its nanoscale electrical properties

Nanosized gold (Au) and polyindole (PIn) composite was prepared via in-situ polymerization of indole, using metal salt chloro-auric acid as an oxidant, in a microemulsion system. The oxidization of indole and the reduction of Au³⁺ ions occurred simultaneously in a single step, which resulted in a core shell structure having a coating of polyindole over monodispersed, size-controlled, highly populated, and stable gold nanoparticles. Indole polymerization governed by chloro-auric acid, was monitored using UV–vis absorption spectroscopy. Nanoscale electrical characterization of polyindole nanocomposite was performed using current-sensing atomic force microscopy. The investigated properties of the composite proved its enormous potential in electronic applications and fabrication of nanoscale organic devices.     

Influence of plasmonic Au nanoparticles on the photoactivity of Fe₂O₃ electrodes for water splitting

An experimental study of the influence of gold nanoparticles on α-Fe(2)O(3) photoanodes for photoelectrochemical water splitting is described. A relative enhancement in the water splitting efficiency at photon frequencies corresponding to the plasmon resonance in gold was observed. This relative enhancement was observed only for electrode geometries with metal particles that were localized at the semiconductor-electrolyte interface, consistent with the observation that minority carrier transport to the electrolyte is the most significant impediment to achieving high efficiencies in this system.     

Mechanical behavior of Au–In intermetallics for low temperature solder diffusion bonding

In this study, gold (Au)–indium (In) intermetallic compounds (IMCs) formation for low temperature solder bonding was investigated by imbedding a gold wire into the annealing indium solder. According to available research on liquid–solid reaction of gold and indium, experiments were only conducted at an annealing temperature in the range of 200–300 °C. To investigate the feasibility of forming the Au–In IMCs at lower temperature, a low annealing temperature of 160 °C was applied in this study, which is just above the melting point of indium of 156 °C. AuIn₂ precipitates were confirmed to be predominately formed in the IMCs by X-ray diffraction. Different annealing times of 10, 40, and 120 min were applied to study the stabilization time of IMC AuIn₂. With thermal considerations, AuIn₂ was confirmed to form with a low annealing temperature of 160 °C, and a short annealing time of 10 min. In addition, the microstructure of the cross-sections in the interfacial region of the gold wire and indium solder was investigated by scanning electron microscopy. The mechanical behavior of gold, indium, and their IMCs with different annealing times were studied by nanoindentation. Mechanical properties including reduced modulus and hardness were extracted after taking into account of the pile-up effect. Increased reduced modulus and hardness were observed with increasing annealing times, due to the strengthening of the atomic bonding in the compounds. The reduced modulus and hardness measured from nanoindentation indicate a significant strengthening of the indium solder by the AuIn₂ nanoparticles.     

A simple process for electrodeposition of Sn-rich, Au–Sn solder films

The Sn-rich eutectic alloy (90 wt% Sn) in the Au–Sn system offers a potentially cheaper alternative to the Au-rich eutectic alloy (20 wt% Sn) for optoelectronic and microelectromechanical systems device packaging and may be applicable as a Pb-free solder for microelectronic packaging. A simple electrodeposition method was utilized to fabricate Sn-rich, Au–Sn solder films, including the eutectic composition for this purpose. The electrolyte consisted of a solution of Sn chloride and ammonium citrate. Gold was added to the electrolyte in the form of either a Au nanoparticle (<20 nm) suspension, prepared with Na citrate, or by directly adding Au powder (500–800 nm particles). The resultant suspensions were used to electrodeposit eutectic and near-eutectic alloy films. Uniform thicknesses and compositions were obtained with the latter approach, i.e., direct addition of Au powder. Gold content in the deposits increased with increasing Au particle loading in the electrolyte and increasing current density. Room temperature aging led to the formation of AuSn₄ at the Au particle-Sn matrix interface. Reflow of deposits with near-eutectic compositions resulted in the formation of the two eutectic phases, Sn and AuSn₄.