Highlights from recent literature

A team from South Korea has studied the effects of alloying elements Cu and Ni on the microstructure and the thermal aging properties of Au bonding wire (Microelectronics Reliability, volume 51, issue 12, December 2011, pp. 2250– 2256). The thermal aging properties of samples bonded with an Al pad and annealed at 200°C for durations ranging from 0 to 300 h was investigated using mechanical tests. Both of the alloyed specimens showed higher thermal aging properties than the 4-N Au-bonded specimen. The Cu-alloyed Au bonding wire formed, at the Au–Al interface, a discontinuous Cu-rich layer that was considered to have delayed the growth of the Au–Al intermetallic compound (IMC). Meanwhile, at the Au–Al interface, the Ni-alloyed Au bonding wire formed secondary phase particles that were believed to have improved the bonding strength by interrupting microcrack propagation.

Each issue of Gold Bulletin contains key highlights from the research and patent literature. Authors who publish highquality work in other journals are invited to send a copy of their publication to the Editor for inclusion in the next issue.

Catalysis and chemistry
Facile removal of stabilizer ligands from supported gold nanoparticles Metal nanoparticles that comprise a few hundred to several thousand atoms have many applications in areas such as photonics, sensing, medicine, and catalysis. Colloidal methods have proven particularly suitable for producing small nanoparticles with controlled morphologies and excellent catalytic properties. Ligands are necessary to stabilize nanoparticles during synthesis, but once the particles have been deposited on a substrate the presence of the ligands is detrimental for catalytic activity. Previous methods for ligand removal have typically involved thermal and oxidative treatments, which can affect the size or morphology of the particles, in turn altering their catalytic activity. Here, Lopez-Sanchez et al. (Nature Chemistry, 3, 551-556 (2011)) report a procedure to effectively remove the ligands without affecting particle morphology, which enhances the surface exposure of the nanoparticles and their catalytic activity over a range of reactions. This may lead to developments of nanoparticles prepared by colloidal methods for applications in fields such as environmental protection and energy production.

Core@shell bimetallic nanoparticle synthesis via anion coordination
Core@shell structured bimetallic nanoparticles are currently of immense interest due to their unique electronic, optical, and catalytic properties. However, their synthesis is non-trivial. Serpell et al. (Nature Chemistry, 3, 478-483 (2011)) report a new supramolecular route for the synthesis of core@shell nanoparticles, based on an anion coordination protocol-the first to function by binding the shell metal to the surface of the pre-formed primary metal core before reduction. The resultant gold/palladium and platinum/palladium core@shell nanoparticles have been characterized by aberration-corrected scanning transmission electron microscopy (as well as other techniques), giving striking atomic-resolution images of the core@shell architecture, and the unique catalytic properties of the structured nanoparticles have been demonstrated in a remarkable improvement of the selective production of industrially valuable chloroaniline from chloronitrobenzene.

Influence of Methyl Halide Treatment on Gold Nanoparticles Supported on Activated Carbon
A multi-country team led by Chris Hardacre at Queens University, Belfast describe how methyl halides have been used to disperse large (around 20 nm) gold nanoparticles supported on carbon into dispersed gold atoms/dimers at low temperature and atmospheric pressure in an early view publication (see Angewandte Chemie International Edition, DOI: 10.1002/anie.201102066). The process occurs through the progressive removal of gold-halogen entities from the metal nanoparticles and a gradual decrease in the size of the gold nanoparticles on the minute timescale.

Patent: US 2011166010 (A1)
A shell catalyst for the preparation of vinyl acetate monomer, comprising an oxidic porous catalyst support with an outer shell, containing metallic Pd and Au, wherein the framework structure of the porous catalyst support contains hafnium oxide units. This shell catalyst is suitable for the preparation of VAM and is characterized by a relatively high activity and VAM selectivity and maintains this activity and selectivity over relatively long service lives. Also, processes for the preparation and use of the shell catalyst.

Novel materials
Anisotropic nanomaterials: structure, growth, assembly, and functions A group at IIT Madras, Chennai have made a comprehensive review (Nano Reviews 2011, 2: 5883-DOI: 10.3402/ nano.v2i0.5883) of current research in the area of anisotropic nanomaterials, with particular reference to gold. This includes an introduction to the advancements in this area followed by general aspects of the growth of anisotropic nanoparticles. They then describe several important synthetic protocols for making anisotropic nanomaterials, followed by a summary of their assemblies and conclude with major applications.

Are Nanoporous Materials Radiation Resistant?
An online Nano Letters publication (DOI: 10.1021/ nl201383u) focuses on nanoporous gold materials. The authors explain radiation damage to materials is a major issue in industries ranging from nuclear energy to aerospace. In this work experiments and computer simulations on nanoscale gold foams are reported to show the existence of a window in the parameter space where foams are radiation tolerant. The group analyzed these results in terms of a model for the irradiation response that quantitatively locates such window that appears to be the consequence of the combined effect of two length scales dependent on the irradiation conditions: (1) foams with ligament diameters below a minimum value display ligament melting and breaking, together with compaction increasing with dose (this value is typically 5 nm for primary knock on atoms (PKA) of 15 keV in Au), while (2) foams with ligament diameters above a maximum value show bulk behavior, that is, damage accumulation (few hundred nanometers for the PKA's energy and dose rate used in this study). In between these dimensions, (i.e., 100 nm in Au), defect migration to the ligament surface happens faster than the time between cascades, ensuring radiation resistance for a given dose rate.

A material with electrically tunable strength and flow stress
In a second article this quarter concerning itself with nanoporous gold, Jin and Weissmuller (Science, 3 June 2011: Vol. 332 no. 6034 pp. 1179-1182) describe how the selection of a structural material requires a compromise between strength and ductility. The material properties will then be set by the choice of alloy composition and microstructure during synthesis and processing, although the requirements may change during service life. Materials design strategies that allow for a recoverable tuning of the mechanical properties would thus be desirable, either in response to external control signals or in the form of a spontaneous adaptation, for instance in self-healing. They describe designing a material that has a hybrid nanostructure consisting of a strong metal backbone that is interpenetrated by an electrolyte as the second component. By polarizing the internal interface via an applied electric potential, fast and repeatable tuning of yield strength, flow stress, and ductility was accomplished. The concept allows the user to select, for instance, a soft and ductile state for processing and a highstrength state for service as a structural material.

Au@Hg Nanoalloy Formation Through Direct Amalgamation: Structural, Spectroscopic, and Computational Evidence for Slow Nanoscale Diffusion
This new work led by the University of Liverpool (Advanced Functional Materials. DOI: 10.1002/adfm.201100409) described unusual nanoscale alloying of two metals with a very large difference in cohesive energy; Au and Hg. A simple method to prepare Au@Hg particles with precise control over the composition up to 15 at.% mercury is introduced, based on reacting a citrate stabilized gold sol with elemental mercury. Transmission electron microscopy shows an increase of particle size with increasing mercury content and, together with X-ray powder diffraction, points towards the presence of a core-shell structure with a gold core surrounded by an Au-Hg solid solution layer. Segregation energies and metal distribution in the nanoalloys were modeled by density functional theory calculations. The results indicate slow metal interdiffusion at the nanoscale, which has important implications for synthetic methods aimed at core-shell particles.

Reduction of Au 3+ ions by activated surface atoms of platinum
In this work published in Electrochemistry Communications (Volume 13, Issue 8, August 2011, Pages 852-855) it has been shown that Pt electrodes can be activated by cathodic polarization in the hydrogen evolution region which makes it prone to oxidation at potentials below that of bulk oxide formation. When an activated Pt electrode is placed in an aqueous HAuCl4 solution the electroless deposition of Au onto the surface of the electrode was observed and confirmed by cyclic voltammetry and XPS measurements. It was demonstrated that the oxidation of active Pt surface atoms provides the driving force for the spontaneous reduction of Au 3+ ions into metallic Au to generate a Pt/Au surface which is highly active for the electro-oxidation of ethanol.

Formation and properties of Au-based nanograined metallic glasses
This Acta Materialia (Volume 59, Issue 16, September 2011, Pages 6433-6440) paper describes how, unlike crystalline materials, metallic glasses usually have a uniform structure without long-range periodicity and characteristic structural constituents. It has long been believed that no interfaces associated with separate grains can be found in metallic glasses. The authors report on the successful synthesis of a gold-based nanograined metallic glass (NGMG) exhibiting a heterogeneously granular structure but possessing a glassy nature. The smallest particle size of the NGMG can be less than 10 nm. Having the advantage of the high surface area typical for nanostructures, the Au-based NGMG shows significantly enhanced catalytic activity. In addition, this NGMG sustains the good mechanical properties of metallic glasses, showing a high hardness of 5.3 GPa. The work provides insight into glass formation, offers the opportunity for further studies of the physical and chemical properties of this new type of non-crystalline solid, and discusses the applications of metallic glasses as catalysts.

Patent: WO 2011072011 (A1)
A double-metallic deposition process is used whereby adjacent layers of different metals are deposited on a substrate. The surface plasmon frequency of a base layer of a first metal is tuned by the surface plasmon frequency of a second layer of a second metal formed thereon. The amount of tuning is dependent upon the thickness of the metallic layers, and thus tuning can be achieved by varying the thicknesses of one or both of the metallic layers. In a preferred embodiment directed to enhanced LED technology in the green spectrum regime, a double-metallic Au/Ag layer comprising a base layer of gold (Au) followed by a second layer of silver (Ag) formed thereon is deposited on top of InGaN/GaN quantum wells on a sapphire/GaN substrate.

Medical and Dental Gold Nanoparticles: A Revival in Precious Metal Administration to Patients
In an ACS "just accepted" manuscript from Sam Gambhir's group at Stanford (Nano Letters, DOI: 10.1021/nl202559p) gold's history as a therapeutic agent to treat a wide variety of rheumatic diseases including psoriatic arthritis, juvenile arthritis, and discoid lupus erythematosus is reviewed. Although the use of gold has been largely superseded by newer drugs, gold nanoparticles are being used effectively in laboratory-based clinical diagnostic methods while concurrently showing great promise in vivo either as a diagnostic imaging agent or a therapeutic agent. For these reasons, gold nanoparticles are therefore well placed to enter mainstream clinical practice in the near future. Hence, the team's review summarizes the chemistry, pharmacokinetics, biodistribution, metabolism, and toxicity of bulk gold in humans based on decades of clinical observation and experiments in which gold was used to treat patients with rheumatoid arthritis. The beneficial attributes of gold nanoparticles, such as their ease of synthesis, functionalization, and shape control are also highlighted demonstrating why gold nanoparticles are an attractive target for further development and optimization. The importance of controlling the size and shape of gold nanoparticles to minimize any potential toxic side effects is also discussed.

Self-Assembly of Gold Nanowires Along Carbon Nanotubes for Ultrahigh-Aspect-Ratio Hybrids
Yang et al. (Chem. Mater., 2011, 23 (11), pp 2760-2765 DOI: 10.1021/cm1033645) report on a novel approach for the assembly of one-dimensional hybrid nanostructures made up of gold nanowires with ultrahigh aspect ratios (L/d>500) self-assembled along the axes of multiwalled carbon nanotubes. The micrometer-long hybrid nanowires exhibit high electrical conductivity and can be easily microcontact-printed onto various substrates in a patterned form. The authors claim these materials have considerable potential as interconnects for nanoelectronic applications.

Size effect in compression of single-crystal gold microparticles
In this Acta Materialia paper (Volume 59, Issue 13, August 2011, Pages 5202-5215) single-crystal Au microparticles on a sapphire substrate were deformed under compression. Most of microparticles yielded with a large strain burst, and there was a strong dependence of the yield strength on microparticle size. Using finite-element analysis the authors concluded that the deformation is dislocation nucleation-controlled and that the stress levels reached at the onset of plasticity approach the theoretical shear strengths of Au. A significant size effect was identified in both the experimentally measured and computed strength of the microparticles.

Diffusion and Filtration Properties of Self-Assembled Gold Nanocrystal Membranes
In this work by a team led by Argonne National Laboratory (Nano Lett., 2011, 11 (6), pp 2430-2435 DOI: 10.1021/ nl200841a) the first free-standing membranes based on selfassembled from dodecanethiol-ligated gold nanocrystals are reported. For aqueous solutions the authors found filtration coefficients 2 orders of magnitude larger than those observed in polymer-based filters, sieving of large solutes, and for smaller solutes a pronounced dependence of rejection on being charged. The authors claim these results open up new possibilities for controlled delivery and separation of nano-objects.

Nanoantenna-enhanced gas sensing in a single tailored nanofocus
Metallic nanostructures possess plasmonic resonances that spatially confine light on the nanometer scale. In the ultimate limit of a single nanostructure, the electromagnetic field can be strongly concentrated in a volume of only a few hundred cubic nanometers or less. This optical nanofocus is ideal for plasmonic sensing. Any object that is brought into this single spot will influence the optical nanostructure resonance with its dielectric properties. Here, Lui et al. (Nature Materials, 10, 631-636 (2011)) demonstrate antenna-enhanced hydrogen sensing at the singleparticle level. The authors, based at institutions in Stuttgart and Berkeley, placed a single palladium nanoparticle near the tip region of a gold nanoantenna and detect the changing optical properties of the system on hydrogen exposure by dark-field microscopy. The method avoids any inhomogeneous broadening and statistical effects that would occur in sensors based on nanoparticle ensembles. This concept paves the road towards the observation of single catalytic processes in nanoreactors and biosensing on the single-molecule level.
Highly uniform and reproducible surface-enhanced Raman scattering from DNA-tailorable nanoparticles with 1-nm interior gap An ideal surface-enhanced Raman scattering (SERS) nanostructure for sensing and imaging applications should induce a high signal enhancement, generate a reproducible and uniform response, and should be easy to synthesize. Many SERS-active nanostructures have been investigated, but they suffer from poor reproducibility of the SERSactive sites, and the wide distribution of their enhancement factor values results in an unquantifiable SERS signal. Here, Lim et al. (Nature Nanotechnology, 6, 452-460 (2011)) have shown that DNA on gold nanoparticles facilitates the formation of well-defined gold nanobridged nanogap particles (Au-NNP) that generate a highly stable and reproducible SERS signal. The uniform and hollow gap (~1 nm) between the gold core and gold shell can be precisely loaded with a quantifiable amount of Raman dyes. SERS signals generated by Au-NNPs showed a linear dependence on probe concentration (R 2 >0.98) and were sensitive down to 10 fM concentrations. Single-particle nano-Raman mapping analysis revealed that >90% of Au-NNPs had enhancement factors greater than 1.0×10 8 , which is sufficient for single-molecule detection, and the values were narrowly distributed between 1.0×10 8 and 5.0×10 9 .
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