Epitaxial growth of binary and ternary metallic strained superlattices and their magnetic properties

Since the structure at/near the interface of superlattices influences physical properties such as magnetic property, it is important to investigate details of the structure. The interface structure is characterized by the factors like atomic species, strain, mixing and roughness. The reflection high-energy electron diffraction (RHEED) system installed in our molecular-beam epitaxy (MBE) system enables us to observe, continuously, the change of the surface in-plane lattice constant, which is affected by atomic species, strain and/or mixing, on a real-time basis. Ternary superlattices consisting of three elements can clarify the effect of stacking sequence by comparison between the two types of superlattices with the reverse deposition sequences, since the effect caused by the combination of the same atomic species is cancelled out and the effect caused by the different stacking sequences remains. In the present paper, we review growth behaviors of binary and ternary metallic strained superlattices, especially magnetic ones, investigated mainly by our group, and summarize the discussion on their magnetic properties, mainly on the magnetic anisotropy, in terms of their structural characteristics. First, we introduce our RHEED system that works efficiently under a magnetic field arising from evaporation sources for low vapor-pressure materials. Then, MBE-grown binary strained superlattices, Co/Au, Co/Pt and Cu/Au, are discussed, with comparing to incoherent superlattices of Co/Ag and Cu/Ag having nearly the same lattice mismatch of constituents. Next, we review ternary strained superlattices with immiscible constituents with reverse deposition order, Au/Co/Ag and Ag/Co/Au superlattices, and Au/Co/Cu and Cu/Co/Au superlattices, in relation to the growth behaviors of binary superlattices. Finally, ternary strained superlattices containing both miscible and immiscible constituents, Pt/Co/Ag and Ag/Co/Pt superlattices, and Au/Ni/Ag and Ag/Ni/Au superlattices, are reviewed.     

Thermodynamics and phase diagrams of lead-free solder materials

Many of the existing and most promising lead-free solders for electronics contain tin or tin and indium as a low melting base alloy with small additions of silver and/or copper. Layers of nickel or palladium are frequently used contact materials. This makes the two quaternary systems Ag–Cu–Ni–Sn and Ag–In–Pd–Sn of considerable importance for the understanding of the processes that occur during soldering and during operation of the soldered devices. The present review gives a brief survey on experimental thermodynamic and phase diagram research in our laboratory. Thermodynamic data were obtained by calorimetric measurements, whereas phase equilibria were determined by X-ray diffraction, thermal analyses and metallographic methods (optical and electron microscopy). Enthalpies of mixing for liquid alloys are reported for the binary systems Ag–Sn, Cu–Sn, Ni–Sn, In–Sn, Pd–Sn, and Ag–Ni, the ternary systems Ag–Cu–Sn, Cu–Ni–Sn, Ag–Ni–Sn, Ag–Pd–Sn, In–Pd–Sn, and Ag–In–Sn, and the two quaternary systems themselves, i.e. Ag–Cu–Ni–Sn, and Ag–In–Pd–Sn. Enthalpies of formation are given for solid intermetallic compounds in the three systems Ag–Sn, Cu–Sn, and Ni–Sn. Phase equilibria are presented for binary Ni–Sn and ternary Ag–Ni–Sn, Ag–In–Pd and In–Pd–Sn. In addition, enthalpies of mixing of liquid alloys are also reported for the two ternary systems Bi–Cu–Sn and Bi–Sn–Zn which are of interest for Bi–Sn and Sn–Zn solders.     

Vapor generation by UV irradiation for sample introduction with atomic spectrometry

Volatile species of the conventional hydride-forming elements (As, Bi, Sb, Se, Sn, Pb, Cd, Te), Hg, transition metals (Ni, Co, Cu, Fe), noble metals (Ag, Au, Rh, Pd, Pt), and nonmetals (I, S) were generated following UV irradiation of their aqueous solutions to which low molecular weight carboxylic acids (formic, acetic, propionic) had been added. Free radicals arising from photodissociation of the latter provide a new and useful alternative to the common methods of chemical/electrochemical vapor generation techniques for the determination of these analytes by atomic spectrometry. Quantitative estimates of the efficiencies of these generation processes were not undertaken, although calculated signal-to-background ratios (>1500 for 5 ng/mL As, Sb, Bi, Se, and Te; 20 for 10 ng/mL Sn, Cu, Rh, Au, Pd, Pt, and Cd; 2400 for 1 ng/mL Hg; and 1000 for Co using ICP-TOF-MS detection) do provide clear evidence of the efficacy of this approach for sample introduction. In the case of Ni and Se, the tetracarbonyl and alkylated selenium compounds have been identified, respectively.     

Ion release from gold/platinum dental alloy: could release of other elements be accountable in the contact allergy attributed to the gold?

Objectives: The release of metal ions (Al, Ag, Au, Ca, Cd, Co, Cr, Cu, Mg, Mo, Ni, Pd, Pt, Ti, and Zn) from the commercial gold/platinum (Au/Pt) dental alloy of declared composition was studied. Methods: Au/Pt was soaked in pH 6.0 phosphate buffer, 3.5 pH phosphate buffer and pH 3.5 mixture of lactic, formic and acetic acid, and incubated at 37 °C for 1, 2, 3, 4, 5, 6, 7, 14, 21, and 30 days. Six samples (n = 6) of every solution were prepared for any time period. Inductively coupled plasma atomic emission spectroscopy was used for analysis of the released elements. Results: Results demonstrated release of only Cr, Cu, Fe, and Zn from the tested Au/Pt dental alloy (ANOVA, p < 0.001 for buffer, time, and interaction, respectively); however, only Cu and Zn were declared. Conclusions: The undeclared chromium from Au/Pt dental alloy, or some other element might be responsible for the contact allergy thus far attributed to the gold.     

Thermal spikes in Ag/Fe and Cu/Fe ion beam mixing

Ion beam mixing has been studied since 1980, and since then a lot of experimental and theoretical work has been done and knowledge has been gathered. Nevertheless, there are still many fundamental aspects that need to be clarified and with that aim many experiments need to be performed. Copper and iron are miscible in the liquid state, while silver and iron are not. However, both systems are thermally immiscible in the solid state. In order to have an insight into the importance of mixing within thermal spikes during ion beam irradiation, we deposited Cu/Fe and Ag/Fe bilayers onto Si substrates and irradiated them at room temperature with 2 MeV Cu and 2.5 MeV Au ions. A combination of Rutherford backscattering spectrometry (RBS) and atomic force microscopy (AFM) was used to analyze the atomic transport at the interface and the morphology changes of the samples. From the element profiles at the interface we conclude a mixing efficiency, which is indeed larger than the prediction of the ballistic model in the Cu/Fe system and smaller in the Ag/Fe system. Since ballistic mixing is expected in any case, we argue that demixing and phase separation in the Ag/Fe system occur in the thermal spike phase of the cascade as a consequence of the positive heat of mixing. Further mixing does occur in the thermal spike in the Cu/Fe system and they remain mixed even at the solid state because of the high cooling rate. In addition, ion irradiation induces a large surface roughening of the Ag and Cu top layers as proven by AFM. This effect is important for the correct interpretation of the results. Furthermore, this recrystallization affects also the interface, producing a rough interface, that appears in the RBS spectra as an atomic ‘diffusion’ at the interface.     

A glow discharge mass spectrometry study of reactive sputtering

Reactive sputtering of different metallic targets (Ti, Fe, Co, Ni, Cu, Zn, Mo, Pd, Ag, Sn, Ta, Pt and Au) was studied by glow discharge mass spectrometry. The amounts of atomic and molecular species were measured as functions of the oxygen total pressure. The relative oxide molecular contribution decreases with increasing pressure. A correlation was found between the ratio I_MO⁺/I_M⁺ of the molecular current to the atomic current and the formation enthalpy of MO(g). The mechanism of reactive sputtering could be interpreted in most cases by: (1) a dissociative ejection of metals and oxygen from the cathode; (2) molecular formation close to the surface; (3) molecular dissociation during the transfer from the surface to the substrate.     

In-situ X-ray Diffraction study of Metal Induced Crystallization of amorphous silicon

By covering amorphous silicon (a-Si) with a thin metal film, it is possible to lower the crystallization temperature of the a-Si (typically around 800 °C when using ramp anneals) to levels which can be used in a manufacturing process. This phenomenon of Metal Induced Crystallization (MIC) has been reported previously for Ni, Au and Al. In this work, in-situ X-ray Diffraction was used to study the MIC process for 20 different metals (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Al). The 7 metals which lower the crystallization temperature the most are Ni, Pt, Pd, Cu, Au, Al and Ag. The crystallization kinetics were studied in detail for these 7 materials. In order to explain the MIC process, two models where used depending on the interaction of the metal with Si (eutectic or compound forming).     

Effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu and SnPb solders with Au/Ni/Cu substrate during aging

The effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu (SAC) and SnPb solders with Au/Ni/Cu substrate during aging was investigated in this study. AuSn₄ needle-like intermetallic compound (IMC) formed at the interfaces of both SAC and SnPb solder joints after laser reflow. In SnPb solder joints, a layer-type ternary IMC, (Au,Ni)Sn₄, formed over a thin layer quaternary IMC (Au,Ni,Cu)₃Sn₄ after aging at 150 °C for 4 days, in which Cu came from the Cu pad below the Ni layer through diffusion. This (Au,Ni,Cu)₃Sn₄ IMC evolved into a new kind of quaternary IMC (Au,Ni,Cu)₆Sn₅ with further aging at 150 °C for 8 days because more Cu diffused through Ni and was involved into the IMC formation. For SAC solder joints, needle-like binary AuSn₄ IMC in as-soldered state changed into a layer-type quaternary IMC (Au,Ni,Cu)Sn₄. The quaternary IMC (Au,Ni,Cu)₆Sn₅, which has the same composition as those found in SnPb solder joints, was developed below the (Au,Ni,Cu)Sn₄ layer after aging at 150 °C for 8 days. It was found that Cu diffusion played a key role in the interfacial reaction and compound formation.     

Ultramicrohardness experiments on vapour-deposited films of pure metals and alloys

The ultramicrohardness of thin alloy films is determined by indentation experiments at very low loads. The indentation apparatus is briefly described. We measured the hardness of a number of pure metal films, either vapour deposited or magnetron sputtered. In addition, a detailed study was made of the composition dependence of the hardness of the six binary alloy systems of AgAuCu and AuCoNi. These films were prepared by high vacuum vapour deposition. The thickness of all the films was 1 μm.The hardness data were interpreted with the help of the metastable alloy phase diagram, e.g. as obtained by transmission electron microscopy. The results are discussed in terms of the blocking of the motion of dislocations, solid solution hardening, compound formation, grain size reduction due to alloying, and differences in atomic radii.     

Synthesis of Ni nanoparticles by hydrolysis of Mg<Subscript>2</Subscript>Ni

A simple, new method utilizing the hydrolysis of Mg₂Ni has been successfully developed for the synthesis of Ni nanoparticles. The possible growth mechanisms of Ni nanoparticles are discussed. Compared with conventional preparation methods for Ni nanoparticles, this method has the potential to inexpensively produce Ni nanoparticles on a large scale. In addition, the principle of the method could be applied to the synthesis of other transition metal nanoparticles such as Co, Cu, Ag, Au, Pt, and Pd.     

Nano-scaled Pt/Ag/Ni/Au contacts on p-type GaN for low contact resistance and high reflectivity

We synthesized the vertical-structured LED (VLED) using nano-scaled Pt between p-type GaN and Ag-based reflector. The metallization scheme on p-type GaN for high reflectance and low was the nano-scaled Pt/Ag/Ni/Au. Nano-scaled Pt (5 A) on Ag/Ni/Au exhibited reasonably high reflectance of 86.2% at the wavelength of 460 nm due to high transmittance of light through nano-scaled Pt (5 A) onto Ag layer. Ohmic behavior of contact metal, Pt/Ag/Ni/Au, to p-type GaN was achieved using surface treatments of p-type GaN prior to the deposition of contact metals and the specific contact resistance was observed with decreasing Pt thickness of 5 A, resulting in 1.5 x 10(-4) ohms cm2. Forward voltages of Pt (5 A)/Ag/Ni contact to p-type GaN showed 4.19 V with the current injection of 350 mA. Output voltages with various thickness of Pt showed the highest value at the smallest thickness of Pt due to its high transmittance of light onto Ag, leading to high reflectance. Our results propose that nano-scaled Pt/Ag/Ni could act as a promising contact metal to p-type GaN for improving the performance of VLEDs.     

Optical radiation efficiencies of metal nanoparticles for optoelectronic applications

The optical radiation efficiencies, defined by the ratio of the scattering cross-section to the extinction cross-section, of spherical nanoparticles of 11 kinds of metal, Ag, Al, Au, Co, Cr, Cu, Ni, Pd, Pt, Sn and Ti, in the air were calculated based on the classical electromagnetic theory. This optical radiation efficiency represents the energy fraction of the incident light reradiated from the particle, not wasted as heat, and the obtained data is an effective guide for the selection of metal elements for nanoparticle-enhanced optoelectronic devices. Ag, Al, Au and Cu were found to have much higher optical radiation efficiencies than the other metals for most range of wavelengths. Strikingly, Ag and Al nanoparticles with diameters around 150 nm were found to exhibit over − 90% optical radiation efficiencies at most optical frequencies.     

XRF测定铂制品中Pt、Au、Pd、Ag、Cu、Ni等6种元素含量

本文利用XRF金标样的多元素回归方程,对Pt、Pd的荧光强度进行修正,修正后的荧光强度当作Au、Ag的荧光强度代入回归方程中,利用计算机编程计算铂制品中的Pt、Au、Pd、Ag、Cu、Ni等6种元素的含量。该法测定的准确度优于0.5％,适用于Pt的测定范围为85％～99.92％.具有无损、快速、准确、可靠等优点。     

Metal nanoparticle doped coloured coatings on glasses and plastics through tuning of surface plasmon band position

Several noble metal nanoparticles doped sol-gel derived thin coloured films have been synthesized and characterized. These are pure (Ag, Au, Cu and Pt), mixed/alloy (Ag-Cu, Au-Cu, Au-Ag and Au-Pt) nanoparticles in SiO₂, Au in mixed SiO₂-TiO₂ and SiO₂-ZrO₂, Au and Ag nanoparticles in inorganic-organic hybrid film matrices etc. This investigation leads to the development of tailor-made coloured coatings by tuning the surface plasmon resonance (SPR) band positions originating from the embedded nanometals by controlling mainly (i) refractive index of the film matrices and (ii) nanoalloy composition. In the later case a new layer-by-layer (two-layer) synthetic protocol has been developed to prepare binary nanoalloy particles with controlled atomic ratios.     

A General Strategy Assisted with Dual Reductants and Dual Protecting Agents for Preparing Pt‐Based Alloys with High‐Index Facets and Excellent Electrocatalytic Performance

Synthesizing noble metallic nanoparticles (NPs) enclosed by high‐index facets (HIFs) is challenged as it involves the tuning of growth kinetics, the selective adsorption of certain chemical species, and the epitaxial growth from HIF enclosed seeds. Herein, a simple and general strategy is reported by using dual reduction agents and dual capping agents to prepare Pt‐based alloy NPs with HIFs, in which both glycine and poly(vinylpyrrolidone) serve as the reductants and capping agents. Due to the facilely tunable growth/nucleation rates and protecting abilities of the reductants and capping agents, Pt concave nanocube (CNC), binary Pt–Ni CNC, ternary Pt–Mn–Cu CNC, and Pt–Mn–Cu ramiform polyhedron alloy NPs terminated by HIFs as well as other NPs with well‐defined morphologies such as Pt–Mn–Cu nanocube and Pt–Mn–Cu nanoflower are obtained with this approach. Owing to the high density of low‐coordinated Pt sites (HIF structure) and the unique electronic effect of Pt–Mn–Cu ternary alloys, the as‐prepared Pt–Mn–Cu NPs show enhanced catalytic activity toward methanol and formic acid electro‐oxidation reactions with excellent stability. This work provides a promising methodology for designing and fabricating Pt‐based alloy NPs as efficient fuel cell catalyst.     

Evaluation of glass forming ability in binary and ternary metallic alloy systems – an application of thermodynamic phase diagram calculations

Abstract

The glass forming ability (GFA) of a wide range of binary and ternary alloy systems (Au–Si, Pd–Si, Ti–Be, Zr–Be, Hf–Be, Cu–Ti, Co–Zr, Ni–Zr, Cu–Zr, Ni–P, Pd–P, Ni–Pd–P, Cu–Pd–P, Co–Ti–Zr, Zr–Be–Hf, Ti–Be–Hf, Ti–Be–Zr) was calculated using a combined thermodynamic and kinetic approach. There is good agreement between the predicted glass forming ranges and those experimentally observed. By using this combined approach it has also proved possible to estimate critical cooling rates for phases not observed in the equilibrium phase diagram. A significant advantage of the approach is that, for multicomponent alloys, the melting temperatures and thermodynamic input parameters for the kinetic equations are derived using the constituent binary thermodynamic phase diagram calculations and, therefore, it has the potential to predict GFA in multicomponent systems using information from mainly binary systems.

MST/788     

Effects of Au/Cu ratio and gallium content on the low-temperature age-hardening in Au-Cu-Ga alloys

The effects of Au/Cu ratio and gallium content on the low-temperature age-hardening were examined in Au-Cu binary and Au-Cu-Ga ternary alloys by hardness testing, X-ray diffraction and differential thermal analysis. In alloys in which the Au/Cu ratios greatly deviated from the equiatomic one, age-hardenability at 37 °C significantly decreased. Gallium addition lowered the liquidus temperature and increased the age-hardening rate. It is considered that the diffusion rate of the constituents increased with lowering of the liquidus temperature, and that consequently, gallium addition improved the age-hardenability.     

Correlation of microstructure and magnetic properties in Cu–Co–Ni alloys

The present study concerns correlation of microstructure and magnetic properties of nanocrystalline binary 50Cu–50Co and ternary 50Cu–25Co–25Ni (wt%) alloys prepared by ball milling and subsequent isothermal annealing of the ball milled alloys. High resolution transmission electron microscopic (HR-TEM) investigation has shown deformation-induced microstructural features. Field emission scanning electron microscopy (FE-SEM) has revealed a distinct change in morphology of as-milled CuCoNi alloys after annealing. Differential scanning calorimetric (DSC) and X-ray diffraction (XRD) analysis have revealed that annealing of the CuCoNi alloy above 350 °C results into precipitation of nanocrystalline Co (fcc) in the CuNi matrix by spinodal decomposition. It is also demonstrated that isothermal annealing of the ball milled alloys in the temperature range between 350 and 650 °C significantly influence the magnetic properties, e.g. coercivity (H_c), remanence (M_r) and magnetic saturation (M_s) due to annihilation of defects such as stacking and twin fault along with dissolution and/or precipitation of magnetic phases in the Cu-rich matrix.     

Roles of alloying additions on local structure and glass-forming ability of Cu–Zr metallic glasses

To identify the structural role of alloying element M (M = Ti, Ga, Co, Fe) on the glass-forming ability (GFA) of Cu₅₀Zr₅₀ base alloy, the atomic structures of the binary and ternary metallic glasses were examined by extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS curve-fitting analysis indicates that the main structural difference among the metallic glasses is in the atomic packing density of Cu-centered clusters. The relative shortening of the Cu–M distance is closely related to the heat of mixing between Cu and M: the more negative the heat of mixing, the larger is the shortening of the Cu–M distance. Based on a systematic analysis of the component properties and GFA data for Cu–Zr based alloys, it is suggested that alloying elements that bring a more uniform distribution of atomic size and possess strong chemical interactions with the main components should be selected in developing large-size bulk metallic glasses.     

Nucleation of small metal particles on ultrathin SiO2 films on Si

Metal particles of Pt, Ni, Au and Ag, typically less than 50 Å in diameter, were deposited by vacuum evaporation onto substrates of ultrathin (～25 Å) SiO₂ on Si. The particle size and charge were determined by transmission electron microscopy and MOS type capacitance-voltage measurements, respectively. It was observed that the Pt and Ni particles were smaller and more closely spaced than Au and Ag particles under comparable deposition conditions. The particle charge for Pt and Ni appeared to exceed that for Au and Ag for the same substrate coverage, suggesting possible charge contributions to heterogeneous nucleation. Since Pt and Ni particles were observed to carry a positive charge, they are presumably stabilized at negative substrate sites; the opposite appears to be true for Au and Ag.