Au Ge Research Articles

Epitaxial growth of Au on Ge(001) surface: Photoelectron spectroscopy measurements and first-principles calculations

A single atomic Au layer is grown epitaxially on a Ge(001) surface featured by (2×1) reconstruction. The low energy electron diffraction pattern of the Au/Ge(001) surface indicates the formation of a square structure with the length of crystalline domains of ~3nm. Ab-initio calculations show that Au growth stabilizes the Ge surface in symmetric dimers and angle-resolved photoelectron spectroscopy measurements reveal its metallic character. The modifications in the electronic properties of the Ge surface as a result of annealing are discussed and the consequences as reflected in X-ray photoelectron spectroscopy (XPS) measurements are underlined. The deformation density indicates the regions with covalent Au–Ge bonds. These bonds are identified from Au 4f and Ge 3d XPS data.

A study of the initial stages of the growth of Au-assisted epitaxial Ge nanowires on a clean Ge(100) surface

We report on the interfacial phenomena that occur at the initial stages of Ge nanowire growth using gold as the catalyst on Ge(100) substrates under ultra high vacuum (UHV) conditions using molecular beam epitaxy (MBE). Room temperature deposition of a thin Au layer using MBE showed a wetting nature while de-wetting has been observed at 500 °C and higher temperatures. The deposition of a thin layer of Ge at this condition resulted in the formation of Ge nanostructures and Au islands on Ge pedestals, corresponding to the initial growth of the Ge nanowires. Ge deposition at 600 °C yielded larger Ge nanowires below the Au/AuGe catalyst interface due to the enhancement of the lateral material transport.

Au–Ge and Ag–Ge Heterodimers with Tunable Domain Sizes: A Supersaturation-Precipitation Route to Colloidal Hybrid Nanoparticles

Colloidal hybrid nanoparticles, which contain multiple inorganic domains that are joined together through solid–solid interfaces, exhibit particle multifunctionality as well as new and enhanced properties that can emerge from the particle–particle interactions. These hybrid nanoparticles are typically synthesized using heterogeneous seeded nucleation of one nanoparticle on the surface of another as well as using phase segregation, surface dewetting of core–shell nanoparticles, and the fusion of premade nanoparticles. However, to expand the materials diversity and the potential range of applications of such systems, alternative routes to heterogeneous seeded nucleation are needed. Here, we show that solution–liquid–solid and related supersaturation-precipitation strategies, traditionally used in the synthesis of 1D structures such as nanowires and nanorods, can also be applied to the synthesis of colloidal hybrid nanoparticles. Specifically, we show that colloidal Au and Ag nanoparticles can serve as seeds...

Ferromagnetism and re-entrant spin-glass transition in quasicrystal approximants Au–SM–Gd (SM = Si, Ge)

Magnetic susceptibility and specific heat measurements on quasicrystalline approximants Au–Si–Gd and Au–Ge–Gd reveal that a ferromagnetic (FM) transition occurs at Tc = 22.5(5) K for Au–Si–Gd and at Tc = 13(1) K for Au–Ge–Gd, which are the first examples of ferromagnetism in crystalline approximants. In addition, a re-entrant spin-glass (RSG) transition is observed at TRSG = 3.3 K for Au–Ge–Gd in contrast to Au–Si–Gd. The different behaviors are understood based on the recent structural models reported by Gebresenbut et al (2013 J. Phys.: Condens. Matter 25 135402). The RSG transition in Au–Ge–Gd is attributed to a random occupation of the center of the Gd12 icosahedron by Gd atoms; a central Gd spin hinders the long-range FM order.

Electrical behavior of Au–Ge eutectic solder under aging for solder bump application in high temperature Electronics

As deep earth oil exploration transits into deeper earth, it faces challenges in producing reliable microelectronics devices and sensors that will not degrade at high temperature, especially those related to packaging materials and assembly of the devices. It has been reported that the mechanical performance of a high temperature Au–Ge eutectic solder is able to fulfill the minimum interconnection properties specified by the oil and gas exploration industry. However, the impact of thermal aging to Au–Ge eutectic solder’s electrical property is not clear. In this work, it is observed that the bulk Au–Ge electrical resistivity decreased initially with thermal aging and then saturated. This effect is mainly contributed by the grain growth of Au, which results in a reduction in grain boundaries, as well as coarsening of the Ge phases. On the other hand, study on the microstructure and electrical resistance of Au–Ge solder bump joint after aging shows that the electrical resistance increased. The increase in resistance is contributed mainly by the NiGe and Ni5Ge3 intermetallic compounds growth, which also led to a degradation in the Ni(P) barrier layer.

Formation of Au–Ge nanodots by Au− ion sputtering of Ge

Energetic Au ion sputtering of crystalline Ge surface at normal incidence and room temperature can produce nanodot morphology, which has been studied by atomic force microscopy. The cross-sectional transmission electron microscopy of the dot structure reveals that the implanted Au does not form solid solution with Ge, but preferentially segregates at the top edges of the dots as well as near the amorphous/crystalline interface. The mechanism of dot formation is discussed in the light of local transient melting of the surface due to a high rate of energy deposition by the Au ions and the subsequent cooling.

Size-selective effects in the geometry and electronic property of bimetallic Au–Ge nanoclusters

We systematically investigated the size-selective effects of geometrical structures and electronic properties on anionic AuGen- (n=1–13) nanoclusters by using density functional theory (DFT-B3LYP) calculations with double-ξ LanL2DZ basis set. The lowest-energy structures and their low-lying isomers were identified. Compared with the neutral clusters, the ground state structures of AuGe8- and AuGe9- were obviously rearranged due to the addition of an extra electron. The threshold number of endohedral cage-like AuGen- clusters was favored at n=10, whereas the most stable cluster was found at n=12, and reflected by the average binding energies and second-order difference in the total energies. The present results demonstrated that the induced effects by an additional electron to the neutral clusters can enhance their stabilities. However, the values of HOMO–LUMO gaps have a decreasing tendency along with the increasing number of Ge atoms. Adiabatic electron affinity (AEA), vertical electron affinity (VEA), and vertical detachment energy (VDE) were obtained and discussed in detail. The chemical bondings of some stable nanoclusters were also analyzed with the electron localization function (ELF).

Directed synthesis of germanium oxide nanowires by vapor–liquid–solid oxidation

We report on the directed synthesis of germanium oxide (GeOx) nanowires (NWs) by locally catalyzed thermal oxidation of aligned arrays of gold catalyst-tipped germanium NWs. During oxygen anneals conducted above the Au–Ge binary eutectic temperature (T > 361 °C), one-dimensional oxidation of as-grown Ge NWs occurs by diffusion of Ge through the Au–Ge catalyst droplet, in the presence of an oxygen containing ambient. Elongated GeOx wires grow from the liquid catalyst tip, consuming the adjoining Ge NWs as they grow. The oxide NWs’ diameter is dictated by the catalyst diameter and their alignment generally parallels that of the growth direction of the initial Ge NWs. Growth rate comparisons reveal a substantial oxidation rate enhancement in the presence of the Au catalyst. Statistical analysis of GeOx nanowire growth by ex situ transmission electron microscopy and scanning electron microscopy suggests a transition from an initial, diameter-dependent kinetic regime, to diameter-independent wire growth. This behavior suggests the existence of an incubation time for GeOx NW nucleation at the start of vapor–liquid–solid oxidation.

Experimental investigation and thermodynamic modeling of the Au–Ge–Ni system

Phase equilibria in the Au–Ge–Ni ternary system were studied by means of scanning electron microscopy, electron probe microanalysis, X-ray diffraction, and differential scanning calorimetry. The phase relations in the solid state at 600 °C as well as a vertical section at Au72Ge28–Ni were established. No ternary compound was found at 600 °C. On the basis of the experimental phase equilibria data, a thermodynamic model of the Au–Ge–Ni ternary system was developed using the CALPHAD method. Thermodynamically calculated phase diagrams are shown at 600 °C, in two vertical sections and the liquidus projection. Reasonable agreement between the calculations and the experimental results was achieved.

Micro- and nano-spheres of low melting point metals and alloys, formed by ultrasonic cavitation

Metals and alloys of low melting points (<430°C) can be melted in hot silicone oil to form two immiscible liquids. Irradiation of the system with ultrasonic energy induces acoustic cavitation in the oil, which disperses the molten metals into microspheres that solidify rapidly upon cooling. This method has been applied to seven pure metals (Ga, In, Sn, Bi, Pb, Zn, Hg) and two eutectic alloys of gold (Au–Ge and Au–Si). The morphology and composition of the resulting microspheres were examined by SEM and EDS. Eutectic Au–Si formed also crystalline Au nanoparticles, which were separated and studied by HRTEM.

Nano scale phase separation in Au-Ge system on ultra clean Si(100) surfaces

We report on the phase separation in Au–Ge system leading to the formation of lobe-lobe (bi-lobed) Au-Ge nanostructures under ultra high vacuum (UHV) conditions (≈3 × 10−10 mbar) on clean Si(100) surfaces. For this study, ≈2.0 nm thick Au samples were grown on the substrate surface by molecular beam epitaxy. Thermal annealing was carried out inside the UHV chamber at temperature ≈500 °C and following this, nearly square shaped AuxSi1-x nano structures of average length ≈48 nm were formed. A ≈2 nm Ge film was further deposited on the above surface while the substrate was kept at a temperature of ≈500 °C. Well ordered Au-Ge nanostructures where Au and Ge residing side by side (lobe-lobe structures) were formed. In our systematic studies, we show that, gold-silicide nanoalloy formation at the substrate (Si) surface is necessary for forming phase separated Au-Ge bilobed nanostructures. These results show that the Au–Ge bonding is unstable in nature. Electron microscopy (TEM, STEM–EDS, SEM) studies were carried out to determine the structure of Au–Ge nano systems. Rutherford backscattering spectrometry measurements show gold inter-diffusion into substrate while it is absent for Ge.

Formation of Nets of Corner-Shared Bicapped Gold Squares in SrAu3Ge: How a BaAl4-Type Derivative Reconciles Fewer Valence Electrons and the Origin of Its Uniaxial Negative Thermal Expansion

SrAu(3)Ge was synthesized by direct fusion of the mixed elements at high temperature followed by annealing treatments, and its structure was determined by single crystal X-ray diffraction means in space group (Pearson symbol: tP10) P4/nmm, a = 6.264(1) Å, c = 5.5082(9) Å, Z = 2 at room temperature. The structure of SrAu(3)Ge, a reapportioned √2 × √2 × 1 superstructure of CeMg(2)Si(2) (P4/mmm), exhibits checkerboard nets of corner-shared bicapped Au squares (or corner-shared Au(Au(4/2))Ge octahedra), in which the apical Au-Ge pairs in adjoining nets are strongly interbonded in the c direction. This motif contrasts with that of the common BaAl(4) (I4/mmm) prototype in which Al squares in comparable layers are alternately monocapped by Al from the top or the bottom. Typical examples show valence electron counts (vec) between 12 and 16 for the BaAl(4) type and that for CeMg(2)Si(2) is similar, 15. The special stability of SrAu3Ge, with vec = 9, derives from significant relativistic contribution of the Au 5d(10) states to the Au-Ge and Au-Au bonding. These factors are also recognized in the marked redistribution of Au and Ge site occupancies from those in CeMg(2)Si(2). SrAu(3)Ge exhibits a pronounced uniaxial negative thermal expansion along c, with a coefficient of -1.57 versus 2.16 × 10(-5) K(-1) in a and b. The reticulated Au(5)Ge octahedral layers expand in the ab plane on heating, whereas the strong, interlayer Au-Ge bonds remain fixed.

Au, Ge, and AuGe nanoparticles fabricated by laser ablation

A eutectic AuGe target immersed in distilled water was ablated by pulsed ultraviolet laser light. The structure of the ablated material was investigated by high-resolution transmission electron microscopy (HRTEM). The images show formation of nanowire structures of AuGe up to 100 nm in length, with widths of 5–10 nm. These nanostructures have Ge content significantly lower than the target material. Electron diffraction demonstrates that they crystallize in the α-AuGe structure. For comparison, laser ablation of pure Au and pure Ge targets was also performed under the same conditions. HRTEM shows that Ge forms spherical nanoparticles with a characteristic size of ~30 nm. Au forms spherical nanoparticles with diameters of ~10 nm. Similar to AuGe, it also forms chainlike structures with substantially lower aspect ratio.

Stepped-surfaced GeSe2 nanobelts with high-gain photoconductivity

Single crystalline stepped-surfaced GeSe2 nanobelts (NBs) were synthesized by vapor transport and deposition method with the presence of Au catalyst. The NBs were grown via VLS mechanism with catalytic Au–Ge–Se alloy droplet formed at the tip of the NBs. The dynamic reshaping of the catalyst particle leads to formation of steps along the NB. Photodetectors comprising individually isolated NBs were fabricated to study their photodetection properties. The photoresponsivity of the devices was investigated at four different excitation wavelengths of 405 nm, 532 nm, 808 nm and 1064 nm. High photoresponsivity of ∼1040 A W−1 and a photoconductive gain of ∼121 800% was achieved at a wavelength of 1064 nm, suggesting that the excitation to defect-related energy states near or below the mid band-gap energy plays a major role in the generation of photocurrent in these highly stepped NB devices.

Reversible phase changes in Ge–Au nanoparticles

We demonstrate a reversible phase transition in nanoparticles composed of a binary eutectic alloy, Ge–Au. The structure, 9 nm diameter nanoparticles embedded in silica, can be switched from bilobe to mixed using a 30 ns ultraviolet laser pulse. The structure can be switched back to bilobe by heating at 80 °C. The bilobe/mixed switching can be performed on the same sample at least ten times. Synchrotron X-ray diffraction studies reveal that the bilobe structure contains crystalline Ge and Au while the mixed structure consists of crystalline Ge and β Ge–Au.

Calculation of phase equilibria for an alloy nanoparticle in contact with a solid nanowire

Phase equilibria in a system constituted of an alloy nanoparticle in contact with a solid nanowire have been modelled based on the minimization of a Gibbs free energy function. The Gibbs free energy consists of a bulk, surface and interface contribution. The bulk contribution is taken from CALPHAD thermodynamic databases and the surface properties from the literature. The effect of particle size and surface and interfacial properties on the liquidus line of the Au–Ge and In–Si systems has been studied. The results are compared to the bulk phase diagram and phase equilibria calculated for nano-systems with different geometries.

Atomic Interactions in the p-Type Clathrate I Ba8Au5.3Ge40.7

Single crystals of Ba(8)Au(5.3)Ge(40.7) [space group Pm(3)n (No. 223), a = 10.79891(8) Å] were prepared by a Bridgman technique. The crystal structure refinement based on single-crystal X-ray diffraction data does not reveal any vacancies in the Au/Ge framework or in the cages. In addition to the ionic bonding between Ba and the anionic framework, a direct interaction between Ba and Au atoms was identified in Ba(8)Au(5.3)Ge(40.7) by applying the electron localizability indicator. As expected by the chemical-bonding picture, Ba(8)Au(5.3)Ge(40.7) is a diamagnet and shows p-type electrical conductivity with a hole carrier concentration of 7.14 × 10(19) cm(-3) at 300 K and very low lattice thermal conductivity of ≈0.6 W m(-1) K(-1) at 500 K. The thermoelectric figure of merit ZT of single crystals of Ba(8)Au(5.3)Ge(40.7) attains 0.3 at 511 K and reaches 0.9 at 680 K in a polycrystalline sample of closely similar composition. This opens up an opportunity for tuning of the thermoelectric properties of materials in the Ba-Au-Ge clathrate system by changing the chemical composition.

Structure and properties of the 5.5 K antiferromagnet EuAu2Ge2

Abstract A well crystallized sample of EuAu2Ge2 was synthesized by arc-melting of the elements. The structure was refined on the basis of single crystal X-ray diffracto meter data: ThCr2Si2 type, I4/mmm, a = 447.3(1), c = 1034.0(4) pm, wR2 = 0.0575, 148 F 2 values and 9 variables. The gold atoms have tetrahedral germanium coordination (261 pm Au—Ge). These AuGe4/4 tetrahedra share four common edges, leading to layers which are connected via Ge—Ge bonds (247 pm). The europium atoms fill cages of coordination number 16 within the [Au2Ge2] δ– polyanion. Temperature dependent susceptibility measurements show a magnetic moment of 7.79(1)μ B/formula unit. Together with the 151 Eu Mössbauer spectroscopic data (isomer shift of –11.43 mm/s at 77 K), the magnetic data point to stable divalent europium. EuAu2Ge2 orders antiferromagnetically at T N1 = 5.5(1) K, followed by a spin-reorientation at T N2 = 3.5 K. A metamagnetic step occurs around 20 kOe in the 4 K magnetization isotherm.

Thermodynamic description of the Au–Ag–Ge ternary system

The Ag–Ge binary system has been assessed thermodynamically using the CALPHAD method through the Thermo-calc ® software based on the available experimental information from the published literature. The solution phases including liquid, fcc_A1(Au,Ag), and diamond_A4(Ge), were modeled as substitutional solutions and their excess Gibbs energies were expressed by the Redlich–Kister polynomial. On the basis of the previous assessments of the Au–Ag and Au–Ge binary systems, the thermodynamic description of the Au–Ag–Ge ternary system has been performed. The liquidus projection and several vertical sections of this ternary system have been calculated, which are in good agreement with the reported experimental data.

Indium Flux-Growth of Eu2AuGe3: A New Germanide with an AlB2 Superstructure

The germanide Eu(2)AuGe(3) was obtained as large single crystals in high yield from a reaction of the elements in liquid indium. At room temperature Eu(2)AuGe(3) crystallizes with the Ca(2)AgSi(3) type, space group Fmmm, an ordered variant of the AlB(2) type: a = 857.7(4), b = 1485.5(10), c = 900.2(4) pm. The gold and germanium atoms build up slightly distorted graphite-like layers which consist of Ge(6) and Au(2)Ge(4) hexagons, leading to two different hexagonal-prismatic coordination environments for the europium atoms. Magnetic susceptibility data showed Curie-Weiss law behavior above 50 K and antiferromagnetic ordering at 11 K. The experimentally measured magnetic moment indicates divalent europium. The compound exhibits a distinct magnetic anisotropy based on single crystal measurements and at 5 K it shows a metamagnetic transition at ∼10 kOe. Electrical conductivity measurements show metallic behavior. The structural transition at 130 K observed in the single crystal data was very well supported by the conductivity measurements. (151)Eu Mössbauer spectroscopic data show an isomer shift of -11.24 mm/s at 77 K, supporting the divalent character of europium. In the magnetically ordered regime one observes superposition of two signals with hyperfine fields of 26.0 (89%) and 3.5 (11%) T, respectively, indicating differently ordered domains.