Au-Sn Alloy Research Articles

Thermal chemical vapor deposition of tin oxide nanowires in a hydrogen reduction atmosphere

We fabricated tin oxide nanowires (NWs) on Au/Si3N4/Si(001) substrates using thermal chemical vapor deposition (CVD) in a hydrogen reduction atmosphere. Unlike typical thermal CVD conditions, a mixture of H2 and Ar gas was used as the carrier gas, and a catalyst powder such as graphite was not used. The hydrogen gas maintained the reducing atmosphere and easily vaporized the SnO2 powder without a catalyst, to supply vapors of Sn and O. This process enables vapor-liquid-solid (VLS) growth of SnO2 NWs at temperatures below 600 °C. The formation of a eutectic AuSn alloy that acts as a liquid seed during the VLS process contributes to lowering the growth temperature. X-ray diffraction and transmission electron microscopy analysis show that SnO2 NWs and SnO NWs coexist in the as-prepared sample. The oxygen-deficient atmosphere results in nucleation and growth of SnO NWs.

Complexation Behavior and Co-Electrodeposition Mechanism of Au-Sn Alloy in Highly Stable Non-Cyanide Bath

The complexation behavior and co-electrodeposition mechanism of Au-Sn alloy in a highly stable non-cyanide bath were studied by electrochemical analysis and quantum chemical calculation based on density functional theory (DFT). The interactions between metal ions and multiple complexing agents were revealed, and the mechanism on the high stability of the Au-Sn bath was clarified. The complexing agents potassium pyrophosphate (K4P2O7) and ethylene diamine tetraacetic acid (EDTA) have a synergetic complexation to Sn ions, which exist in one valence state of Sn2+ to form three complex ions, i.e., [Sn(P2O7)]2−, [Sn(P2O7)2]6− and Sn-EDTA; while the complexing agents of 5,5-dimethylhydantoin (DMH) and sodium sulfite (Na2SO3) have a complexation to Au ions, which exist in two different valence states (i.e., Au3+ and Au+) to form two complex ions of [Au(DMH)4]− and Au[(SO3)2]3−. Moreover, the existence of [Au(DMH)4]− inhibits the decomposition of [Au(SO3)2]3−. Meanwhile, EDTA participates in some form of coordination with Au+, inhibites the decomposition of [Au(SO3)2]3− and facilitates the reduction of [Au(SO3)2]3−. The co-electrodeposition of Au-Sn alloy is a diffusion-controlled process, and the nucleation of Au-Sn alloy is a progressive nucleation process. The antioxidant catechol significantly increases the overpotential of Au-Sn co-electrodeposition and improves the brightness of electrodeposited films of Au-Sn alloys.

Defect-free SnTe topological crystalline insulator nanowires grown by molecular beam epitaxy on graphene.

SnTe topological crystalline insulator nanowires have been grown by molecular beam epitaxy on graphene/SiC substrates. The nanowires have a cubic rock-salt structure, they grow along the [001] crystallographic direction and have four sidewalls consisting of {100} crystal planes known to host metallic surface states with a Dirac dispersion. Thorough high resolution transmission electron microscopy investigations show that the nanowires grow on graphene in the van der Waals epitaxy mode induced when the catalyzing Au nanoparticles mix with Sn delivered from a SnTe flux, providing a liquid Au-Sn alloy. The nanowires are totally free from structural defects, but their {001} sidewalls are prone to oxidation, which points out the necessity of depositing a protective capping layer in view of exploiting the magneto-electric transport phenomena involving charge carriers occupying topologically protected states.

Ni(OH)2 nanoflakes supported on 3D hierarchically nanoporous gold/Ni foam as superior electrodes for supercapacitors

The increasing demand for portable electronic devices and hybrid electric vehicles stimulates the development of supercapacitors as an advanced energy storage system. Here, we demonstrate a binder-free nickel hydroxide@nanoporous gold/Ni foam (Ni(OH)2@NPG/Ni foam) electrode for high-performance supercapacitors, which is prepared by a facile three-step fabrication route including electrodeposition of Au-Sn alloy on Ni foam, chemical dealloying of Sn and electrodepostion of Ni(OH)2 on NPG/Ni foam. Such Ni(OH)2@ NPG/Ni foam electrode is composed of a thin layer of conformable Ni(OH)2 nanoflakes supported on three-dimensional (3D) hierarchically porous NPG/Ni foam substrate. The resulting Ni(OH)2@NPG/Ni foam electrode can offer highways for both electron transfer and ion transport and lead to an excellent electrochemical performance with an ultrahigh specific capacitance of 3,380 F g–1 at a current density of 2 A g–1. Even when the current density was increased to 50 A g–1, it still retained a high capacitance of 1,927 F g–1. The promising performance of the Ni(OH)2@NPG/Ni foam electrode is mainly ascribed to the 3D hierarchical porosity and the highly conductive network on the NPG/Ni foam composite current collector, as well as the conformal electrodeposition of Ni(OH)2 active material on the NPG/Ni foam, which induces the formation of interconnected porosity both on the top surface and on the inner surface of the electrode. This inspiring electrochemical performance would make the as-designed electrode material become one of the most promising candidates for future electrochemical energy storage systems.

Hot embossing of Au- and Pb-based alloys for x-ray grating fabrication

Grating-based X-ray phase-contrast interferometry has a high application impact in materials science and medicine for imaging of weakly absorbing (low Z) materials and soft tissues. For absorbing gratings, casting of highly X-ray absorbing metals, such as Au and Pb alloys, has proven to be a viable way to generate large area periodic high aspect ratio microstructures. In this paper, the authors review the grating fabrication strategy with a special focus on a novel approach of casting low temperature melting alloys (Au-Sn and Pb-based alloys) into Si grating templates using hot embossing. This process, similar to nanoimprint lithography, requires particular adjusting efforts of process parameters as a function of the metal alloy and the grating feature size. The transition between the solid and liquid state depends on the alloy phase diagram, the applied pressure can damage the high aspect ratio Si lamellas, and the microstructure of the solid metal can affect the grating structure. The authors demonstrate that metal casting by hot embossing can be used to fabricate gratings on a large area (up to 70 × 70 mm2) with an aspect ratio of up to 50:1 and a pitch in the range of 1–20 μm.

Study on Co-Electrodeposition Mechanism of Au-30at.%Sn Eutectic in Non-Cyanide Bath by Electrochemical Methods

The co-deposition of Au-30at.%Sn alloy in a non-cyanide alkaline Au+-Sn2+ bath was successfully achieved with sodium sulfite (Na2SO3) and potassium pyrophosphate (K4P2O7) as green complexing agents, ethylenediamine tetraacetic acid (EDTA) as bath stabilizer and catechol as additive. The Au(I) is mainly complexed with sulfite in the form of [Au(SO3)2]3−, while the Sn(II) is mainly complexed in the form of [Sn(P2O7)2]6−. The reduction potentials of both Sn(II) and Au(I) shift to a negative value (more negative than −0.70 V vs. SCE) in the presence of sulfite and pyrophosphate complexing agents, resulting in the onset reduction potential gap between Au(I) and Sn(II) decreased from 1.828 V (for standard deposition potential) to 100 mV, and consequently the co-deposition of Au and Sn is realized. The cyclic voltammetry and chronoamperometry characterizations revealed that the co-deposition of Au-Sn alloy is an irreversible and diffusion-controlled process and the nucleation mechanism is represented by the progressive model. The addition of catechol, which can be adsorbed on the cathode surface, increased the co-deposition overpotential, inhibited the co-deposition process and refined the grain size of the electrodeposits. Finally, Au-Sn eutectic film consisting of AuSn and Au5Sn phases was successfully co-deposited.

Formation of alloys upon the simultaneous electrochemical deposition of gold and tin from ethylene glycol and aqueous electrolytes

An ethylene glycol (EG) solution containing Au(III) and Sn(IV) compounds, and conditions for the electrochemical deposition of Au–Sn alloy based on AuSn and Au5Sn intermetallics with total tin content of 30–55 at % are proposed. Fundamental difficulties of the deposition of alloys with high tin content, (including eutectic Au–Sn alloy) from aqueous electrolytes are revealed. It is determined via voltammetry that the simultaneous deposition of gold and tin from aqueous and EG electrolytes proceeds with the depolarization effect of both Au(III) and Sn(IV) as a result of the formation of the alloy, the increase in the rate of tin cathodic reduction being more noticeable in case of EG solution. Formation of SnCl2EG(H2O)2+ complex upon the dissolution of SnCl4 · 5H2O in glycol, the stability of the composition of tetracyanoaurate ions upon the dissolution of K[Au(CN)4], and the weakening of intermolecular interactions in EG with small amounts of water were revealed via IR spectroscopy. It is suggested that the depolarization effect is due not only to alloy formation, but also to the formation of SnCl2EG(H2O)2+ cations, their association with Au(CN)4- anions, and a change in the mechanism of Au(III) and Sn(IV) reduction.

Ion beam engineering of morphological, structural and optical properties of Au/SnO2 hybrid nanostructured thin films

Hybrid nanostructured thin films of Au/SnO2 consisting of SnO2 nanostructures decorated with Au nanoparticles were synthesized by thermal evaporation of nanostructured SnO2 thin films combined with deposition of ultrathin Au films by sputtering. The synthesized Au/SnO2 hybrid nanostructured thin films were irradiated with 8 MeV Si3+ ions at room temperature to fluences varying from 2 × 1014 to 5 × 1015 ions/cm2. Effects of MeV ion irradiation on the morphological, structural and optical properties of Au/SnO2 hybrid nanostructured thin films were studied using atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, UV–Visible absorption spectroscopy and photoluminescence spectroscopy (PL). XRD studies revealed the presence of nanostructures of SnO2, Au, Sn and Au-Sn alloy in the as-deposited thin films. XRD and AFM studies showed growth in size of nanostructures in the Au/SnO2 hybrid nanostructured thin films upon ion irradiation. PL studies on as-deposited Au/SnO2 hybrid nanostructured thin film revealed intense peaks at 382, 399, 419, 455 and 542 nm. MeV ion irradiation resulted in strong enhancement in UV emission and visible emissions from SnO2 nanostructures in the Au/SnO2 hybrid nanostructured thin films. Gas sensing studies showed that the sensor prepared using Au/SnO2 hybrid nanostructured thin film exhibits highly enhanced CO gas sensing properties as compared to pure SnO2 nanostructured thin film sensor and shows excellent selectivity for CO gas with negligible interference from CH4, NH3, CO2 and SO2. The mechanism underlying the enhanced gas sensing properties of Au/SnO2 nanostructured thin film is tentatively proposed.

Three-dimensional nanoporous gold–cobalt oxide electrode for high-performance electroreduction of hydrogen peroxide in alkaline medium

Using a simple hydrothermal method combined with a post-annealing treatment, cobalt oxide (Co3O4) nanosheet arrays are grown on three-dimensional (3D) nanoporous gold (NPG) film supported on Ni foam substrates, in which NPG is fabricated by chemically dealloying electrodeposited Au–Sn alloy films. The morphology and structure of the Co3O4@NPG/Ni foam hybrids are characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical activity of the Co3O4@NPG/Ni foam electrode toward hydrogen peroxide electroreduction in alkaline medium is studied by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA). The results demonstrate that the Co3O4@NPG/Ni foam electrode possesses exceptionally high catalytic activity and excellent stability for the peroxide electroreduction, resulting mainly from the unique electrode architecture. The combined 3D hierarchical porous structures of NPG/Ni foam with the open and porous structures of Co3O4 nanosheet arrays facilitate the mass transport and charge transfer. Therefore, the metal oxides supported on 3D hierarchical porous NPG/Ni foam framework may hold great promise to be effective electrodes for electrocatalytic reduction of peroxide and other electrochemical reactions.

Three-dimensional nanoporous Au films as high-efficiency enzyme-free electrochemical sensors

Herein we develop an enzyme-free electrochemical sensor for hydrogen peroxide (H2O2) and hydrazine (N2H4) by preparing three-dimensional nanoporous gold (NPG) supported on three dimensional Ni foam. The NPG@Ni foam hybrid electrode was prepared via electrodeposition of an AuSn alloy film on a Ni foam substrate, followed by a chemical dealloying process to leach the Sn component. The morphology and structure of the as-prepared NPG@Ni foam hybrid were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrum (EDS). The as-prepared hybrid electrodes showed high sensitivity and selectivity for the detection of both H2O2 and N2H4. The non-enzymatic sensor exhibits a linear response for H2O2 and N2H4 in the range of 20–9740 and 0.2–110.1μM, with a sensitivity of 2.88 and 10.687mAmM−1cm−2, and a detection limit (S/N=3) of 10μM and 33nM, respectively. The excellent sensitivity and selectivity make the NPG@Ni foam electrodes very attractive for electrochemical sensing applications.

Interfacial reactions and diffusion path in gold–tin–nickel system during eutectic or thermo-compression bonding for 200 mm MEMS wafer level hermetic packaging

In this work, Au–Sn eutectic bonding and Au–Sn thermo-compression bonding are studied for applications in hermetic packaging at wafer level. Eutectic bonding experiments were performed under vacuum or pure nitrogen at temperatures between 310 and 400 °C while thermo-compression bonding experiments were performed under vacuum at 270 °C. Nickel layers 350 nm thick are used as wetting layers on both cap and device sides. During these experiments, the interaction of electrodeposited Au–Sn alloy with nickel layers is studied either in solid or liquid state. The diffusion paths at 270 °C as well as during isothermal solidification of the joint at 310, 350 and 400 °C are determined in relation with the thicknesses of the reaction layers formed at the interfaces and the microstructure of the joint. The effective interdiffusion coefficient in the ζ-Au5Sn phase is deduced at 270 and 310 °C. Moreover, the melting and solidification process of electrodeposited Au/Sn layers with an overall eutectic composition were studied by Differential Scanning Calorimetry in a specific configuration that does not involve the interaction of Au–Sn alloy with the nickel layer. Bonded wafers with good mechanical properties were characterized by cross-section Scanning Electron Microscopy using Energy Dispersive X-ray mode. The mechanical strength of the seal was checked by blade plate insertion technique.

Fabrication of nanoporous gold and the application for substrate bonding at low temperature

MEMS-compatible fabrication of nanoporous gold and the application for low temperature bonding are demonstrated. A cyanide-free electroplating solution is prepared for the Au–Sn alloy deposition. To investigate the influence of electroplating on Au–Sn alloy and nanoporous gold, different plating parameters and various sizes of patterns are designed and discussed. The optimized electroplating condition realizes 40 to 720 µm line width patterns fabricated on the same substrate. Low temperature substrate bonding at 200 °C is achieved with nanoporous gold and gold film, which has shear bond strength more than 60 MPa. The fracture inspection of the bonded area after shear tests verifies the bonding success. This study gives a study for fabricating on-chip nanostructure, and the results indicate the high feasibility of nanoporous gold for low temperature substrate bonding.

A strategy for fabricating nanoporous gold films through chemical dealloying of electrochemically deposited Au-Sn alloys

We report a novel strategy for the fabrication of nanoporous gold (NPG) films. The fabrication process involves the electrodeposition of a gold–tin alloy, followed by subsequent chemical dealloying of tin. Scanning electron microscopy (SEM) images show a bicontinuous nanoporous structure formed on the substrates after chemical dealloying. Energy dispersive x-ray (EDX) analysis indicates that there are no impurities in the Au–Sn alloy film with an average composition of 58 at. % Au and 42 at. % Sn. After dealloying, only gold remains in the NPG film indicating the effectiveness of this technique. X-ray diffraction (XRD) results reveal that the as-prepared Au–Sn alloy film is composed of two phases (Au5Sn and AuSn), while the NPG film is composed of a single phase (Au). We demonstrate that this approach enables the fabrication of NPG films, either freestanding or supported on various conductive substrates such as copper foil, stainless steel sheet and nickel foam. The resulting NPG electrode exhibits enhanced electrocatalytic activity toward both H2O2 reduction and methanol oxidation compared to the polished Au disc electrode. Our strategy provides a general method to fabricate high quality NPG films on conductive substrates, which will broaden the application potential of NPG or NPG-based materials in various fields such as catalysis, optics and sensor technology.

Pd-decorated three-dimensional nanoporous Au/Ni foam composite electrodes for H2O2reduction

Nanoporous gold (NPG) films were fabricated on Ni foam substrates via a two-step procedure which involves electrodeposition of Au–Sn alloy films on the Ni foam surface, followed by selectively leaching the Sn component through a chemical dealloying process. Pd nanoparticles were then electrochemically deposited on the NPG/Ni foam electrode. The morphology of the Pd-decorated NPG/Ni foam composite electrode (Pd@NPG/Ni foam) was characterized by scanning electron microscopy (SEM). The catalytic activity of the Pd@NPG/Ni foam composite electrode toward H2O2 electroreduction in acid media was evaluated by means of linear scan voltammetry and chronoamperometry. The Pd@NPG/Ni foam composite electrode exhibited high activity and excellent stability for the H2O2 electroreduction, generating a current density of 178 mA cm−2 at 0 V in a 0.5 M H2SO4 + 0.6 M H2O2 electrolyte, in comparison to 98 mA cm−2 on the Pd@Ni foam electrode and 36 mA cm−2 on the NPG/Ni foam electrode. The developed three-dimensional (3D) hierarchical porous Pd@NPG/Ni foam can therefore be considered as a promising type of electrode for fuel cell applications.

Nanoporous gold on three-dimensional nickel foam: An efficient hybrid electrode for hydrogen peroxide electroreduction in acid media

A hybrid structure of nanoporous gold (NPG) on three-dimensional (3D) macroporous Ni foam has been synthesized by electrodeposition of Au–Sn alloy film followed by a facile chemical dealloying process under free corrosion conditions. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to characterize the morphology and structure of the NPG/Ni foam hybrids. It is shown that the Ni foam skeletons are uniformly wrapped by the NPG film which is composed of bicontinuous nanostructures consisting of interconnected ligaments and nanopores. Electroreduction of H2O2 on the NPG/Ni foam hybrid electrode in acid media is investigated by linear scan voltammetry, chronoamperometry and electrochemical impedance spectroscopy. It is found that such hierarchical porous electrode displays superior activity, durability and mass transport property for H2O2 electroreduction. These results demonstrate the potential of the NPG/Ni foam hybrid electrodes for the applications in fuel cell technology.

Thermodynamic reassessment of Au–Cu–Sn ternary system

The Au-20wt.%Sn|Cu diffusion couples were annealed at 320°C for 100s and 2500s and at 150°C for 6600h. The microstructures of these samples were studied with SEM+EDS technique. It was found out that the interface between Cu and near eutectic AuSn alloy consisted of AuCu3/AuCu/Au5Sn layers after annealing at 150°C. For having a better understanding on the formation and evolution of metal bonding interconnection microstructures, the Au–Cu–Sn ternary system was thermodynamically reassessed on the basis of the experimental results from this work as well as by using the data available in the literature and the thermodynamic descriptions of the binary systems. The thermodynamic parameters from all the three binary systems were directly cited in this work, except for those of Cu6Sn5_HT and Cu6Sn5_LT. In the present work, the compounds AuSn and Cu6Sn5_HT were described as a one phase due to the reported complete solid solubility at 200°C and 360°C. A set of self-consist thermodynamic parameters for Au–Cu–Sn system was obtained in the present work. This thermodynamic description can reproduce most of the experimentally determined values, i.e. both the thermochemical quantities and phase equilibria.

Thermodynamic reassessment of Au–Ni–Sn ternary system

Abstract For having a better understanding on the formation and evolution of metal bonding interconnection microstructures the Au–Ni–Sn ternary system was reassessed on the basis of experimental results and the recently reported thermodynamical description for Au–Sn and Ni–Sn systems. In this paper, the thermodynamic parameters of Ni 3 Sn 4 phase were modified in order to achieve a better agreement with experimentally determined phase boundaries. Further, a self-consistent set of thermodynamic parameters for the Au–Ni–Sn system were obtained, which were able to reproduce most of the available experimental data. The Ni|80Au20Sn (wt%) diffusion couples were annealed at 320 °C for 10,000 s and at 150 °C for 2500 h. The microstructures of these samples were studied with SEM+EDS technique. The reaction interface between Ni and near eutectic AuSn alloy consisted of Ni 3 Sn/(Ni, Au) 3 Sn 2 /AuSn/Au 5 Sn layers. This experimentally observed diffusion path of Ni against Au–20 wt% Sn solder at 150 °C was rationalized on the basis of the thermodynamically calculated isothermal section.

Solidification and interfacial interactions in gold–tin system during eutectic or thermo-compression bonding for 200 mm MEMS wafer level hermetic packaging

In this work, Au–Sn eutectic bonding and Au–Sn thermo-compression bonding are studied for applications in hermetic packaging at wafer level. Eutectic bonding experiments were performed under vacuum or pure nitrogen at temperatures between 300 and 350 °C while thermo-compression bonding experiments were performed under vacuum at 270 °C. During these experiments, the solidification of electrodeposited Au–Sn alloy as well as the interaction of this alloy with W2N layers are studied. Some supplementary specific brazing experiments were performed using commercial sheets of eutectic Au–Sn alloy in order to understand the mechanisms of interactions between the Au–Sn alloy and the W2N layer and of solidification of the Au–Sn eutectic alloy. The melting and solidification process of eutectic Au–Sn alloy were studied by differential scanning calorimetry under different geometrical configurations such as commercial eutectic Au–Sn sheets alone, brazing joints performed by commercial eutectic Au–Sn alloy and samples made by thermo-compression bonding. Bonded wafers with good mechanical properties were characterized by cross-section scanning electron microscopy using energy dispersive X-ray mode. Some samples were characterized by transmission electron microscopy. The mechanical strength of the seal was checked by shear tests.

Low-Temperature Pressure-Less Silver Direct Bonding

A light-emitting diode (LED) die and a glass substrate, both of which were coated with a few-micrometers-thick silver layer, was successfully direct-bonded above 200°C in air without any pressure. In this silver direct bonding, the following three factors are essential: 1) keeping surfaces of silver layers clean; 2) bonding with a suitable solvent that can be reduced and has a boiling point near the sintering temperature; and 3) avoiding excessive heating. Oxygen concentration in a sintering atmosphere has a great impact on bonding. The surface of a silver layer shows roughening by abnormal grain growth as increasing oxygen resulting in increase of bonding strength. The abnormal grain growth on the surface of silver effectively contributes to the increase of junction area. Sintering at too high temperature, however, causes the formation of large voids, which migrated towards silver/glass interfaces resulting in the reduction of bonding strength. An LED die bonded at 240°C for 2 h can maintain high bonding strength through a heat run test at 350°C for 2 h. Furthermore, transient thermal resistance of an LED die bonded with the silver direct bonding is lower than that of an LED die bonded by traditional eutectic bonding with Au-Sn alloy. This bonding method can realize high thermal conductivity with low-temperature pressure-less process and can contribute to high-power semiconductor devices.

Synthesis of Au–Sn alloy nanoparticles for lead-free electronics with unique combination of low and high melting temperatures

Au–Sn alloy nanoparticles (ANPs) with 16–67wt% Sn were successfully synthesized in the size range of 2–10nm using chemical reduction method. X-ray diffraction confirmed the formation of AuSn phase with hexagonal structure and was further validated by microscopy and spectroscopy techniques. Chemical reduction is a more scalable and economical technique relative to other synthesis methods for solder alloy particles; it can produce particles in nanoscale range and can be conducted at room temperature. A significant depression in the melting transition (starting from 187°C) was observed for the Au–Sn ANPs relative to the bulk eutectic point (Tm=280°C at 30% Au). Au-Sn nanoparticles offer a unique set of advantages as lead-free soldering material. In nanoparticle state, they can reflow at lower temperatures leading to less thermal stresses in adjacent electronic components during the manufacturing process. After the initial reflow and solder joint formation (bulk state), they can reclaim bulk Au-Sn properties including permanent high melting temperatures (up to 420°C) which can potentially lead to superior thermal and mechanical performance suitable for high temperature electronic applications such as military and aerospace.