Au Diffusion Research Articles

Gold in graphene: In-plane adsorption and diffusion

We study the bonding and diffusion of Au in graphene vacancies using density-functional theory. Energetics show that Au adsorbs preferably to double vacancies, steadily in-plane with graphene. All diffusion barriers for the complex of Au in double vacancy are above 4 eV, whereas the barriers for larger vacancies are below 2 eV. Our results support the main results of a recent experiment [Y. Gan et al., Small 4, 587 (2008)] but suggest that the observed diffusion mechanism is not thermally activated but radiation enhanced.

In situ TEM study of Au–Cu alloy nanoparticle migration and coalescence

The diffusion and coalescence of Au–Cu alloy nanoparticles was studied at high magnification using in situ transmission electron microscopy. The particles prepared by physical vapor deposition onto amorphous-C support films had an average composition of Cu–43 at% Au and diameters of 15–50 nm. In the case analyzed, the larger of two nanoparticles remained stationary throughout the coalescence process while a smaller nanoparticle moved toward the larger particle at a temperature of ~573 K. The surface of the small nanoparticle was observed to fluctuate while approaching the larger particle, demonstrating that collective atom process occurs along the particle periphery. The particle also decreased in size during the process, indicating that it was losing mass as well as migrating. Direct evidence of a diffusional flux between particles was observed before the coalescence process. The small nanoparticle coalesced into the large one at a highly accelerated rate compared to its prior migration.

Diffusion of gold ions and gold particles during photoreduction processes probed by the transient grating method

The translational diffusion of Au (gold) ions and Au particles during the photoreduction process from AuCl − 4 to metallic Au particles in aqueous ethanol solutions containing poly( N-vinyl-2-pyrrolidone) was investigated by using UV–vis absorption and the laser-induced transient grating (TG) methods. The TG signal of AuCl − 4 solution before photoirradiation was composed of three contributions; the thermal grating, the species grating due to the creation of AuCl − 2, and that due to the depletion of AuCl − 4. Upon photoirradiation, the species grating signal due to AuCl − 4 diminished rapidly and the TG signal due to Au particles appeared within a few minutes and became stronger at a very short time. The subsequent reduction of AuCl − 2 was concomitant with the formation of Au metal particles. The rapid growth of Au 0 atoms into Au particles took place in the short-duration photoirradiation. With the increase of the photoirradiation time, the TG signal was composed of two kinds of Au particle fragments possessing different diffusion coefficients. This is probably due to the pulse-laser induced fragmentation of the larger Au particles. Effects of the polymer on the particle formation were investigated by the concentration dependence of the polymer in the solution. The formation of Au particles by the photoreduction was also compared with that of Pt particles.

Synthesis and optical properties of cubic gold nanoframes

This paper describes a facile method of preparing cubic Au nanoframes with open structures via the galvanic replacement reaction between Ag nanocubes and AuCl(2) (-). A mechanistic study of the reaction revealed that the formation of Au nanoframes relies on the diffusion of both Au and Ag atoms. The effect of the edge length and ridge thickness of the nanoframes on the localized surface plasmon resonance peak was explored by a combination of discrete dipole approximation calculations and single nanoparticle spectroscopy. With their hollow and open structures, the Au nanoframes represent a novel class of substrates for applications including surface plasmonics and surface-enhanced Raman scattering.

Characterization of Bimetallic Systems with UPS and PAX: Gold on Platinum and Palladium Surfaces

In the present study the thermal behavior of ultrathin Au films deposited on different noble metal single crystal surfaces was studied by means of ultraviolet photoelectron spectroscopy and photoemission of adsorbed xenon. The primary goal of this work was to demonstrate the power of photoemission of adsorbed xenon as an easy to handle and very surface sensitive technique for the characterization of multi-component systems. All investigated systems, Au–Pt(111), Au–Pd(111) and Au–Pd(110) show a particular dependence of their electronic and structural properties on the annealing temperature. Au multilayer films with a thickness of 3 monolayer prepared at 150 K undergo in all cases a smoothening by annealing up to room temperature. While on Pd this effect is mainly detectable on the topmost atomic layer the ordering process on Pt is also affecting the deeper layers. Further annealing of the Au–Pd system to 450 K and the Au–Pt system to 750 K, respectively, results in the onset of the alloying process at the interface between metal film and substrate. After annealing to 650 and 950 K, respectively, the alloying finally extends to the very surface. The complete loss of Au from the surface can be detected at 1050 K on Pd and at 1150 K on Pt. The orientation of the substrate has only a weak influence, showing up e.g. in additional electronic states of the not alloyed Au–Pd(111) and Au–Pd(111) systems. Adsorption of CO as a probe molecule illustrates the formation of different adsorption sites on the bimetallic systems during the Au diffusion process.

Control of GaP and GaAs Nanowire Morphology through Particle and Substrate Chemical Modification

We demonstrate two very different morphologies for GaP and GaAs nanowires grown by Au-assisted MOVPE on Si(111) substrates: rodlike wires and tapered wires with sharp tips. We show that the morphology is related to the stability of the particles at the wire tips during growth, and we propose that the mechanism of this effect is diffusion of Au away from the tip. Diffusion occurs, leading to tapered wires, only if there is a clean Si surface to act as a reservoir for the Au. Furthermore, the presence of indium in the particles, even at background levels from previous growth runs, inhibits the migration of Au. These results demonstrate a dramatic example of the sensitivity of wire morphology to substrate and particle chemistry, which could provide an important tool to tune nanowire morphology through particle alloying or surface treatment.

First-principles study of adsorption, diffusion, and charge stability of metal adatoms on alkali halide surfaces

In this work we have performed first-principles calculations based on the spin-polarized density-functional theory for the adsorption and diffusion of Au, Ag, and Pb atoms on NaCl(001), KCl(001), and KBr(001) surfaces. We consider also the influence of adatom charge on the adsorption and diffusion. In order to characterize the different systems we explicitly calculate charge transfer between surface and adatom and consider the relative stability of the various charge states. Our results show that in general, apart from positively charged systems, the adatoms are weakly bound to the surface via orbital polarization and ionic interactions, and relatively little charge transfer occurs. Au and Ag adatoms are highly mobile on all surfaces, although they can be pinned by removal of an electron. In contrast, Pb adatoms are fairly immobile, and their mobility increases upon charging. Analysis of the charge stability suggests that Ag offers the potential of charge controlled mobility on insulators.

Diffusion of Au, Pd, Re, and P in FeNi alloys at High Pressure

Multi-anvil and piston cylinder experiments were performed to determine the effect of both pressure and temperature on the diffusivities of several siderophile elements in a Fe 90Ni 10 core-analog alloy. Activation energies were calculated to be between 244 and 257 kJ/mol for Re, Pd, and Au at 10 GPa, and 264 kJ/mol for P at 1 GPa. It was found that pressure has a marked negative effect on the diffusivities of Au, Re, and Pd, and activation volumes for these elements were calculated to be between 3 and 6 cm 3/mol at 10 GPa. The effect of both temperature and pressure on P diffusion is noticeably less, and the absolute diffusivity of phosphorus is consistently higher than that of the other elements. It is inferred that the reason for this difference is because P is diffusing via an interstitial mechanism as opposed to Re, Pd, and Au which occupy regular lattice sites in the crystal. The effect of pressure and temperature together with depth in the Earth suggests that these elements may continue to exhibit different behavior at more extreme conditions. The significance of these new results lies in the ability to place constraints on many time-dependent processes that pertain to the formation and evolution of planetary cores, as well as the formation and cooling histories of other metal-rich bodies in the solar system, such as meteorites.

The effect of Au diffusion on some physical properties of Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy superconductors

Abstract We have investigated the effect of the gold diffusion on the crystal structure and superconducting properties of Bi-2223 superconducting samples employing X-ray diffraction (XRD), scanning electron microscopy (SEM), critical transition temperature, critical current density, and room temperature resistivity measurements. Doping of Bi-2223 was carried out by means of gold diffusion from an evaporated gold film on pellets during sintering. XRD patterns and SEM micrographs are used to obtain information about Bi-2223 phase ratio, lattice parameters and grain size, respectively. Au doping of the samples increased the critical transition temperature and the critical current density from 100 ± 0.2 to 104 ± 0.2 K and from 40 to 125 A cm−2, in comparison with those of undoped samples. The gold diffusion in Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy has been studied over the temperature range of 500–830 °C using the technique of successive removal of thin layers and measurement of the sample's conductivity at room temperature. The temperature dependence of the Au diffusion coefficient in the range of 500–830 °C was described by the relation D = 5 × 10−4 exp (−1.09 eV/kBT). The possible reasons for the observed improvements in microstructure and superconducting properties due to Au diffusion were discussed.

EDXRF measurements on gold diffusion-doped Bi 1.8Pb 0.35Sr 1.9Ca 2.1Cu 3O y

Gold (Au) diffusion in superconducting Bi 1.8Pb 0.35Sr 1.9Ca 2.1Cu 3O y was investigated over the temperature range 500–800 °C by the energy dispersive X-ray fluorescence (EDXRF) technique. It is found that the Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependences of Au diffusion coefficient in grains and over grain boundaries are described by the relations D 1=6.7×10 −5exp(−1.19 eV/k BT) and D 2=9.7×10 −4exp(−1.09 eV/k BT), respectively. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19%. For the Au-diffused samples, dc electrical resistivity and transport critical current density measurements indicated the critical transition temperature increased from 100 to 104 K and the critical current density increased from 40 to 125 A cm −2, in comparison with those of undoped samples. From scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements it is observed that Au doping of the sample also improved the surface morphology and increased the ratio of the high- T c phase to the low- T c phase. The possible reasons for the observed improvement in microstructure and superconducting properties of the samples due to Au diffusion are also discussed.

Effect of introducing gettering sites and subsequent Au diffusion on the thermal conductivity and the free carrier concentration in n-type 4H-SiC

We report on the application of introducing gettering sites by helium implantation prior to an annealing and subsequent gold diffusion as an approach to control the thermal conductivity and the charge carrier concentration in n-type 4H-SiC. Rutherford backscattering spectroscopy showed high diffusion of gold impurities to the introduced gettering sites which implied a success in introducing gettering sites. Data obtained from the measurements of the specific heat, thermal diffusivity, and room temperature density were used to deduce the thermal conductivity of the samples investigated. The thermal conductivity modeling showed that introducing gettering sites can increase the thermal conductivity in n-type 4H-SiC due to the reduction of phonon-impurity scattering. Raman measurements showed the presence of the desired defects introduted by ion implantation at 20°C. The analysis of the Fourier transform infrared reflectivity showed that the gettering sites can act as majority carrier traps and reduce the recombination process.

Characterization of Organic/Metal Interfaces Using Angle-Resolved X-ray Photoelectron Spectroscopy

Interfaces between organic films and gold substrates were investigated by angle-resolved X-ray photoelectron spectroscopy (ARXPS). We examined the emission angle dependence of the peak position of the ARXP spectra in the Au 4f region, and firstly demonstrated that the ARXPS method enable the observation of the electronic structure of the surface layer of the Au film. By comparing two types of organic adsorbed layer on the Au substrate between chemisorption (self-assembled monolayer) and physisorption [p-sexiphenyl (6P)], we found that the electronic structure of the first Au layer was changed by the chemical interaction between the Au top layer with the organic adsorbates. We discussed ARXPS results of Au deposition on the 6P film to confirm Au diffusion in the organic film. These results indicate that ARXPS can elucidate the electronic structural changes of the surface layer of a substrate and the penetration of metals in organic/metal systems.

New insights on the Cd UPD on Au(111)

AbstractThe Cd underpotential deposition (UPD) process on Au(111) was analyzed by means of combined electrochemical measurements and in situ scanning tunneling microscopy (STM). In the underpotential range 300⩽ΔE (mV) ⩽400, 2D Cd islands are formed on the fcc regions of the Au(111)‐(√3 × 22) reconstructed surface without lifting the reconstruction. At lower underpotentials, the 2D Cd islands grow and, simultaneously, new 2D islands nucleate and coalesce with the previous ones forming a complete condensed Cd monolayer (ML). STM images and long time polarization experiments performed at ΔE = 70 mV demonstrate the formation of an AuCd surface alloy. At ΔE = 10 mV, the formation of the complete Cd ML is accompanied by a significant AuCd surface alloying and the kinetic results reveal two different solid‐state diffusion processes. The first one, with a diffusion coefficient D1 = 4 × 10−17 cm2 s−1, could be ascribed to the mutual diffusion of Au and Cd atoms through a highly distorted (vacancy‐rich) AuCd alloy layer. The second and faster diffusion process (D2 = 7 × 10−16 cm2 s−1) is associated with the appearance of an additional peak in the anodic stripping curves and could be attributed to the formation of another CdzAux alloy phase. Copyright © 2008 John Wiley & Sons, Ltd.

Formation of Au–Pt bimetallic nanoparticles in a two-layer SiO2 films doped with Au and Pt, respectively, through interlayer diffusion

Bimetallic Au-Pt nanoparticles have been generated inside a relatively porous SiO2 film matrix by a two-layer (2L) coating methodology. Two overlapping coating layers were deposited on glass substrates from Au- and Pt-doped inorganic-organic hybrid silica sols and air dried at 60 degrees C. The 2L coating assembly was then UV- and followed by heat-treated at 450 and 550 degrees C in air. UV-treatment decomposes AuCl(4)(-) and PtCl(6)(2-) ions in the respective layers and the subsequent heat treatment in air influences the diffusion of Au and Pt nanometals to each other to form bimetallic Au-Pt nanoparticles inside the silica matrix. A UV-visible study showed damping of Au-plasmon after heat treatments. GIXRD and TEM analyses reveal the formation of a partial Au/Pt solid solution with a small fraction of Pt ( approximately 16%), while the major fraction of Pt remains fused with the Au(Pt) solid solution.

Influence of Au diffusion on the formation of nanodot hexagonal ordered arrays on GaSb substrates by low energy ion sputtering

We analyse the effect of Au impurities, diffused onto GaSb substrates, on the formation of nanodot hexagonal arrays created by low energy argon ion sputtering. It is concluded that oblique incidence in rotating configuration most probably delays the formation of the nanodots compared to previously reported normal incidence experiments. The presence of cracks induced by the sputtering process has been observed only in the Au-diffused sample. For the same sputtering conditions, the dimensions of the dots in the Au-diffused sample are larger than in the case of the pure sample. However, they keep the same shape in both cases. Energy dispersive X-ray analysis revealed that the sputtering induces a more pronounced Ga enrichment in the case of Au-diffused sample.

Nonalloyed Cr∕Au-based Ohmic contacts to n-GaN

Nonalloyed Cr∕Au-based metal contacts to n-GaN have been demonstrated. The deposited Au∕Cr∕n-GaN contacts exhibited a specific contact resistance (ρc) of approximately 5.6×10−5Ωcm2. Although the nonalloyed Ti∕Al-based contacts to n-GaN can also exhibit a comparable ρc value, their thermal stability is inferior to the Cr∕Au-based contacts. This could be attributed to the fact that Al tends to ball up during thermal annealing. Thus, the surface morphology of most of the annealed Ti∕Al-based contacts was quite rough, and the contacts became rectified when they were annealed at a temperature below 700°C. However, the annealed Cr∕Au-based contacts exhibited an Ohmic characteristic and had a smooth surface when annealing temperatures did not exceed 700°C. In addition, the thermal stability could be further improved by inserting a Pt layer between the Cr and Au layers. This scheme could prevent the diffusion of Au into the Cr layer, thus preventing Au from reaching the Cr∕GaN interface where it could form a possible Ga–Au phase, which would degrade the Ohmic contacts.

Calculation of the diffusion coefficient of Au in Bi-2223 superconductors

Undoped Bi-2223 samples were prepared using a conventional solid-state reactionmethod. Doping of Au in Bi-2223 was carried out by means of diffusion from anevaporated Au film on pellets. We have investigated the effect of Au diffusion anddiffusion-annealing duration on the microstructure and superconducting properties ofAu-doped samples by performing x-ray diffraction (XRD), scanning electron microscopy(SEM), dc resistivity and critical current density measurements. Gold diffusion inBi1.8Pb0.35Sr1.9Ca2.1Cu3Oy has been studied over the temperature range500–800 °C using the technique of successive removal of thin layers and the measurementsof lattice parameters from XRD patterns at room temperature. The diffusiondoping of Bi-2223 by Au causes a significant increase of the lattice parameterc by about 0.19%. This observation is used for calculation of the Au diffusion coefficient inBi-2223. The Au diffusion coefficient decreases as the diffusion-annealing temperaturedecreases. The temperature dependence of the Au diffusion coefficient in the range500–800 °C is describedby the relation D = 4.4 × 10−4exp(−1.08 eV/kBT). Au doping of the sample increased the critical transition temperature and the critical current densityfrom 100 ± 0.2 to 104 ± 0.2 K andfrom 40 to 125 A cm−2, in comparison with those of undoped samples. The critical transition temperature and criticalcurrent density of Au-doped samples increased with increasing diffusion-annealing time from10 to 50 h. Au doping of the sample also improved the surface morphology and increased thehigh-Tc phase ratio. The possible reasons for the observed improvement in superconductingproperties of the samples due to Au diffusion are discussed.

Influence of Au diffusion on structural, electrical and optical characteristics of CdTe thin films

Diffusion of Au and its effects on structural, electrical and optical properties of CdTe films fabricated by the close-spaced sublimation technique have been investigated. Diffusion of Au was studied in the range 400–550 °C using energy dispersive x-ray fluorescence analysis. Au-doped and un-doped CdTe films were characterized by x-ray diffraction (XRD), electrical and optical absorption measurements. The temperature dependence of the diffusion coefficient of Au in CdTe films is described as D = 4.4 × 10−7exp(−0.54 eV/kT). The mechanism of Au diffusion in polycrystalline CdTe films is attributed to the fast migration of Au along grain boundaries with simultaneous penetration into grains and settling on Cd-vacancies. It is supposed that the weak influence of Au diffusion on XRD patterns of CdTe films can be explained by dispersal of Au atoms preferentially on Cd-vacancies owing to proximity of the covalent radius of Au and Cd. Au atoms, placed on Cd-vacancies (AuCd) during fast cooling from diffusion temperature to room temperature, show an acceptor behaviour with an energy level about of Ev + 0.2 eV. The nature of this level is discussed.

Synthesis of Au–Ag Alloy Nanoparticles with Au/Ag Compositional Control in SiO2 Film Matrix

Au-Ag alloy nanoparticles with tunable atomic ratios have been generated in SiO2 film matrix using a new two layer (TL) approach. Two successive overlapping coating layers of similar thickness were deposited on silica glass substrates using Au- and Ag-incorporated inorganic-organic hybrid silica sols, respectively. The Au and Ag concentrations in the individual layers were varied to obtain the desired Au-Ag alloys of different compositions. Four sets of such TL coating assemblies were prepared from the following pair of sols: (i) 4 equivalent mol.% Au-96% SiO2 and 2 equivalent mol.% Ag-98% SiO2, (ii) 3 equivalent mol.% Au-97% SiO2 and 2 equivalent mol.% Ag-98% SiO2, (iii) 3 equivalent mol.% Au-97% SiO2 and 3 equivalent mol.% Ag-97% SiO2, and (iv) 2 equivalent mol.% Au-98% SiO2 and 3 equivalent mol.% Ag-97% SiO2 and subjected to UV (2.75 J/cm2) and heat-treatments (450-550 degrees C) in air and H2-N2 atmospheres for the generation of Au-Ag alloy nanoparticles of approximate compositions Au.66Ag0.33, Au0.6Ag0.4, Au0.5Ag0.5, and Au0.4Ag0.6, respectively. After UV-treatment, individual Au and Ag nanoparticles were formed in the respective layers. The heat-treatment (450-550 degrees C) induces interlayer diffusion of Au and Ag to each other with the generation of Au-Ag alloy nanoparticles, and as a result, Au-Ag alloy surface plasmon resonance (SPR) absorptions were observed in between the Ag- and Au-SPR absorption positions in the visible spectra. The expected alloy compositions are formed through several intermediate alloy nanoparticles, which can also be arrested by controlling the annealing parameters. The alloy formations were monitored by UV-VIS, FTIR, XRD, EDAX, and TEM studies.

Interfacial role in room-temperature diffusion of Au into Si substrates

Received 17 November 2006; revised manuscript received 8 February 2007; published 8 May 2007X-ray reflectivity is used in tracking the diffusion of Au into Si 001 substrates with time at room tempera-ture. It has been observed that the diffusion ofAu into Si substrates strongly depends on the initial pretreatmentconditions of Si surface. In particular, there is very little diffusion for the untreated Si surface, while the Sisurface pretreated with HF seems to be prone to strong diffusion and the surface further pretreated with Brshows diffusion in between. Such different diffusion and apparent non-Fickian-type time dependence in thediffusion can be quantitatively explained by Fickian diffusion of Au through changing unblocked interfaciallayer. The growth of the blocking oxide layer with time essentially prevents further diffusion through thoseareas, and the growth of that layer is directly related to the surface stability due to the surface pretreatmentand/or passivation conditions, which gives a control in the formation of diffusion-induced Au-Si nanolayer ofdifferent widths and compositions. The morphology and evolution of the top surface, mapped with atomicforce microscopy and scanning electron microscopy, further helped to verify and understand such differences.DOI: 10.1103/PhysRevB.75.205411 PACS number s : 66.30.Pa, 68.35.Fx, 61.10.Kw, 68.37.Ps