Oxidation-resistant gold-55 clusters

Enhancing the open-circuit voltage of molecular photovoltaics using oxidized Au nanocrystals

For organic photovoltaics (OPV) to realize applications effective strategies to maximize the open-circuit voltage must be developed. Herein we show that solution-processed surface-oxidized Au nanocrystals (o-AuNC) dramatically increase the open-circuit voltage (V(oc)) of OPV cells based on boron-subphthalocyanine chloride (SubPc)/C(60) and chloro-aluminum phthalocyanine (ClAlPc)/C(60) heterojunctions when incorporated at the interface between the hole-extracting electrode and the phthalocyanine donor layer. In addition, the cell-to-cell variation in V(oc) is reduced by up to 10-fold combined with a large reduction in the light intensity dependence of V(oc), both of which are important advantages for practical application. The largest increase in V(oc) is achieved for SubPc/C(60)-based cells which exhibit a 45% increase to 1.09 ± 0.01 V--an exceptionally high value for a single junction small molecule OPV. Remarkably these improvements are achieved using submonolayers of o-AuNC, which can be rationalized in terms of the exceptionally high work function of o-AuNC (∼5.9 eV) and geometric electric field enhancement effects.

Room-temperature reaction of oxygen with gold: an in situ ambient-pressure X-ray photoelectron spectroscopy investigation

The interaction of O(2) with gold foil and gold nanoparticles grown by thermal deposition on TiO(2)(110) was studied by in situ ambient pressure X-ray photoelectron spectroscopy at room temperature. No spontaneous dissociation of O(2) was observed either on Au foil or on Au nanoparticles up to 1 Torr of O(2). X-ray irradiation, however, is very effective in promoting gold oxidation on both surfaces in the presence of O(2). Our results help reconcile recent conflicting experimental observations regarding the activation of molecular oxygen, which is a crucial issue in Au catalyzed oxidation reactions.

Investigating active site of gold nanoparticle Au55(PPh3)12Cl6 in selective oxidation

We present an ab initio investigation of structural, electronic, catalytic, and selective properties of the ligand-covered gold nanoparticle Au55(PPh3)12Cl6 and associated model clusters. The catalytic activity of the Au55(PPh3)12Cl6 nanoparticle in the presence of O2 stems from a combined effect of triphenylphosphine ligands and surface structure of the "magic-number" quasi-icosahedral Au55 core, which entails numerous ligand-encompassed triangle Au6 faces as the active sites. Under the Eley-Rideal mechanism, the "triangle-socket" active site not only can accommodate one pre-adsorbed O2 (which is subsequently activated to the superoxo species) with one styrene molecule at a time but also can provide spatial confinement which favors the formation of an oxametallacycle intermediate that leads to unique selectivity in styrene oxidation.

On the application potential of gold nanoparticles in nanoelectronics and biomedicine

Ligand-stabilized gold nanoparticles (AuNPs) are of high interest to research dedicated to future technologies such as nanoelectronics or biomedical applications. This research interest arises from the unique size-dependent properties such as surface plasmon resonance or Coulomb charging effects. It is shown here how the unique properties of individual AuNPs and AuNP assemblies can be used to create new functional materials for applications in a technical or biological environment. While the term technical environment focuses on the potential use of AuNPs as subunits in nanoelectronic devices, the term biological environment addresses issues of toxicity and novel concepts of controlling biomolecular reactions on the surface of AuNPs.

Preparation and characterization of supported magnetic nanoparticles prepared by reverse micelles

Monatomic (Fe, Co) and bimetallic (FePt and CoPt) nanoparticles were prepared by exploiting the self-organization of precursor loaded reverse micelles. Achievements and limitations of the preparation approach are critically discussed. We show that self-assembled metallic nanoparticles can be prepared with diameters d = 2-12 nm and interparticle distances D = 20-140 nm on various substrates. Structural, electronic and magnetic properties of the particle arrays were characterized by several techniques to give a comprehensive view of the high quality of the method. For Co nanoparticles, it is demonstrated that magnetostatic interactions can be neglected for distances which are at least 6 times larger than the particle diameter. Focus is placed on FePt alloy nanoparticles which show a huge magnetic anisotropy in the L1(0) phase, however, this is still less by a factor of 3-4 when compared to the anisotropy of the bulk counterpart. A similar observation was also found for CoPt nanoparticles (NPs). These results are related to imperfect crystal structures as revealed by HRTEM as well as to compositional distributions of the prepared particles. Interestingly, the results demonstrate that the averaged effective magnetic anisotropy of FePt nanoparticles does not strongly depend on size. Consequently, magnetization stability should scale linearly with the volume of the NPs and give rise to a critical value for stability at ambient temperature. Indeed, for diameters above 6 nm such stability is observed for the current FePt and CoPt NPs. Finally, the long-term conservation of nanoparticles by Au photoseeding is presented.

Fabrication of micropatterns of nanoarrays on a polymeric gel surface

Micro-nano patterns of gold on the surface of poly(ethylene glycol) (PEG) hydrogels were prepared. The approach combines the technique of conventional photolithography (a top-down method for micropatterns), block copolymer micelle nanolithography (a bottom-up method for gold nanopatterns), and a linker-assistant technique to transfer a pattern on a hard surface to a polymeric surface. Hybrid micro-nano patterns on hydrogels were characterized using scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The patterned Au nanoparticles were further modified by a peptide containing arginine-glycine-aspatate (RGD). The cell-adhesion contrast of the patterned hydrogel surface was confirmed by preliminary cell experiments.

Minute synthesis of extremely stable gold nanoparticles

We describe a rapid environmentally friendly wet-chemical approach to synthesize extremely stable non-toxic, biocompatible, water-soluble monodispersed gold nanoparticles (AuNPs) in one step at room temperature. The particles have been successfully achieved in just a few minutes by merely adding sodium hydroxide (NaOH) acting as an initiator for the reduction of HAuCl(4) in aqueous solution in the presence of polyvinylpyrrolidone (PVP) without the use of any reducing agent. It is also proved to be highly efficient for the preparation of AuNPs with controllable sizes. The AuNPs show remarkable stability in water media with high concentrations of salt, various buffer solutions and physiological conditions in biotechnology and biomedicine. Moreover, the AuNPs are also non-toxic at high concentration (100 microM). Therefore, it provides great opportunities to use these AuNPs for biotechnology and biomedicine. This new approach also involved several green chemistry concepts, such as the selection of environmentally benign reagents and solvents, without energy consumption, and less reaction time.

Narrow window in nanoscale dependent activation of endothelial cell growth and differentiation on TiO2 nanotube surfaces

Critical features of biomimetic materials used for vascular grafts and stents are surface structure and chemical features of the implant material supporting adhesion, proliferation, and differentiation of endothelial cells and smooth muscle cells, the major cell types of blood vessels. Recently, experimental evidence from several laboratories have indicated a strong stimulation of cellular activities on vertically aligned TiO(2) nanotube surfaces in comparison to amorphous TiO(2) surfaces. Conflicting reports exist, however, concerning the nanoscale dimension, and the role of the chemistry and crystallinity of the nanotubes in eliciting cell responses. Here we demonstrate that 15 nm nanotubes provide a substantially stronger stimulation of differentiation of mesenchymal cells to endothelial cells and smooth muscle cells than 70-100 nm nanotubes, while high rates of apoptosis were seen on 100 nm nanotubes. Also endothelial cell adhesion, proliferation, and motility were several-fold higher on 15 nm than on 100 nm nanotubes. Furthermore, our data indicate a clear dominance of the nanoscale geometry on endothelial cell behavior over surface chemistry and crystallinity of the TiO(2) nanotube surface. These findings indicate that fine-tuning of TiO(2) surfaces at nanoscale will be an essential parameter in optimizing endothelial cell and smooth muscle cell responses to vascular implants.

Gold nanoparticles of diameter 1.4 nm trigger necrosis by oxidative stress and mitochondrial damage

Gold nanoparticles (AuNPs) are generally considered nontoxic, similar to bulk gold, which is inert and biocompatible. AuNPs of diameter 1.4 nm capped with triphenylphosphine monosulfonate (TPPMS), Au1.4MS, are much more cytotoxic than 15-nm nanoparticles (Au15MS) of similar chemical composition. Here, major cell-death pathways are studied and it is determined that the cytotoxicity is caused by oxidative stress. Indicators of oxidative stress, reactive oxygen species (ROS), mitochondrial potential and integrity, and mitochondrial substrate reduction are all compromised. Genome-wide expression profiling using DNA gene arrays indicates robust upregulation of stress-related genes after 6 and 12 h of incubation with a 2 x IC50 concentration of Au1.4MS but not with Au15MS nanoparticles. The caspase inhibitor Z-VAD-fmk does not rescue the cells, which suggests that necrosis, not apoptosis, is the predominant pathway at this concentration. Pretreatment of the nanoparticles with reducing agents/antioxidants N-acetylcysteine, glutathione, and TPPMS reduces the toxicity of Au1.4MS. AuNPs of similar size but capped with glutathione (Au1.1GSH) likewise do not induce oxidative stress. Besides the size dependency of AuNP toxicity, ligand chemistry is a critical parameter determining the degree of cytotoxicity. AuNP exposure most likely causes oxidative stress that is amplified by mitochondrial damage. Au1.4MS nanoparticle cytotoxicity is associated with oxidative stress, endogenous ROS production, and depletion of the intracellular antioxidant pool.

Ligand and solvation effects on the electronic properties of Au55 clusters: a density functional theory study

The electronic properties of the neutral, positively and negatively charged bare Au(55), passivated Au(55)(PH(3))(12), Au(55)(PH(3))(12)Cl(6), and solvated Au(55)(PH(3))(12)Cl(6) 54 H(2)O clusters are studied using density functional theory. The presence of Cl atoms in the ligand shell favors a nonmetallic behavior while a more metallic behavior is induced by explicit solvation of Au(55)(PH(3))(12)Cl(6) with water molecules. The trends observed in the electronic properties upon ligation and solvation are in agreement with experimental studies.

Fe oxidation versus Pt segregation in FePt nanoparticles and thin films

Metallic nanoparticles containing 3d elements are generally susceptible to oxidation leading to a deterioration of desired properties. Here, the oxidation behavior of differently sized FePt nanoparticles is experimentally studied by x-ray photoelectron spectroscopy (XPS) and compared to a FePt reference film. For all as-prepared metallic samples the common features are the formation of Fe(3+), becoming detectable for exposures to pure oxygen above 10(6) langmuir whereas under identical conditions the Pt(0) signal is conserved. Most notably, these features are independent of particle size. Annealing at 650 degrees C, however, affects small and large FePt particles differently. While large particles as well as the reference film show a 100-1000 times enhanced resistance against oxidation, small FePt particles (diameter 5 nm) exhibit no such enhancement due to the thermal treatment. Additional XPS intensity analysis in combination with model calculations leads to an explanation of this observation in terms of Pt segregating to the surface. In large particles and films the thickness of the resulting Pt layer is sufficient to strongly impede oxidation, while in small particles this layer is incomplete and no longer provides protection against oxidation.

Chiral thiol-stabilized silver nanoclusters with well-resolved optical transitions synthesized by a facile etching procedure in aqueous solutions

A novel approach of cyclic reduction in oxidative conditions has been developed to prepare a single dominant species of chiral thiol-stabilized silver nanoclusters (AgNCs). Such AgNCs, which are stable in solution for up to a few days, have been obtained for the first time. The generality of the established procedure is proven by using several enantiomeric water-soluble thiols, including glutathione, as protective ligands. The prepared AgNCs featured prominent optical properties including a single pattern of UV-vis absorption with well-resolved peaks. The chirality of the clusters has been investigated by circular dichroism (CD) spectroscopy. CD spectra displayed strong characteristic signatures in the visible range. Tentative identification of the cluster composition is discussed.

Core-hole screening as a probe for a metal-to-nonmetal transition in lead clusters

Metal clusters serve as model systems to study basic problems of electronic correlation. Vacuum ultraviolet light from the free-electron laser FLASH ionizes 5d electrons from mass-separated negatively charged clusters, thus transiently leading to core-ionized neutral systems. Shielding of the core hole affects the electron binding energy. From the strong deviation from expectations of the metallic droplet and jellium models we conclude on reduced electronic shielding once the cluster size falls below about 20 atoms. This suggests a metal-to-nonmetal transition, in agreement with previous local density approximation calculations.

Immunity of coated self-ordered silver nanocrystals: a new intrinsic property due to the nanocrystal ordering

New materials made of two- and three-dimensional superlattices of nanoparticles exhibit unique collective properties arising from the ordering of the nanoparticles. Here, dodecanethiol-coated silver nanocrystals self-assembled in 2D were subjected to oxygen plasma using the reactive ion etching process. The careful investigation by transmission electronic microscopy (TEM) of the same areas before and after exposure allows one to distinguish two behaviors under plasma treatment, which are determined by the level of ordering of the 2D organizations. Higher ordered self-assemblies remain unchanged, while less ordered organizations coalesce into larger nanocrystals with spheroidal shapes that could be single crystals. Electron energy-loss spectroscopy (EELS) measurements show that there is no oxidation of the silver either on the large coalesced nanocrystals or on the stable self-assemblies. A new intrinsic property, immunity of highly self-ordered nanocrystals provided by the ordering, is reported, and a mechanism of the coalescence is proposed.

Weighing supported nanoparticles: size-selected clusters as mass standards in nanometrology

We present a new approach to quantify the mass and 3D shape of nanoparticles on supports, using size-selected nanoclusters as mass standards in scanning transmission electron microscope. Through quantitative image intensity analysis, we show that the integrated high angle annular dark field intensities of size-selected gold clusters soft-landed on graphite display a monotonic dependence on the cluster size as far as approximately 6500 atoms. We applied this mass standard to study gold nanoparticles prepared by thermal vapor deposition and by colloidal wet chemistry, and from which we deduced the shapes of these two types of nanoparticles as expected.

Graphene on Ru(0001): a 25 x 25 supercell

The structure of a single layer of graphene on Ru(0001) has been studied using surface x-ray diffraction. A surprising superstructure containing 1250 carbon atoms has been determined, whereby 25 x 25 graphene unit cells lie on 23 x 23 unit cells of Ru. Each supercell contains 2 x 2 crystallographically inequivalent subcells caused by corrugation. Strong intensity oscillations in the superstructure rods demonstrate that the Ru substrate is also significantly corrugated down to several monolayers and that the bonding between graphene and Ru is strong and cannot be caused by van der Waals bonds. Charge transfer from the Ru substrate to the graphene expands and weakens the C-C bonds, which helps accommodate the in-plane tensile stress. The elucidation of this superstructure provides important information in the potential application of graphene as a template for nanocluster arrays.

Photoluminescence of atomic gold and silver particles in soda-lime silicate glasses

We report the chemistry and photophysics of atomic gold and silver particles in inorganic glasses. By synchrotron irradiation of gold-doped soda-lime silicate glasses we could create and identify unambiguously the gold dimer as a stable and bright luminescing particle embedded in the glassy matrix. The gold dimer spectra coincide perfectly with rare gas matrix spectra of Au(2). The glass matrix is, however, stable for years, and is hence perfectly suited for various applications. If the irradiated gold-doped sample is annealed at 550 degrees C a bright green luminescence can be recognized. Intense 337 nm excitation induces a decrease of the green luminescence and the reappearance of the 753 nm Au(2) emission, indicating a strong interrelationship between both luminescence centers. Time-dependent density functional theory (TD-DFT) calculations indicate that the green luminescence can be assigned to noble metal dimers bound to silanolate centers. These complexes are recognized as the first stages in the further cluster growth process, which has been investigated with small-angle x-ray scattering (SAXS). In silver-doped glasses, Ag(0) atoms can be identified with electron paramagnetic resonance (EPR) spectroscopy after synchrotron activation. Annealing at 300 degrees C decreases the concentration of Ag(1), but induces an intense white light emission with 337 nm excitation. The white luminescence can be decomposed into bands that are attributed to small silver clusters such as Ag(2), Ag(3) and Ag(4), and an additional band matching the green emission of gold-doped glasses.

One step route to the fabrication of arrays of TiO2 nanobowls via a complementary block copolymer templating and sol-gel process

Highly dense, ordered arrays of a titania (TiO2) nanomaterial with a bowl-shape morphology are generated by using poly(styrene-block-ethylene oxide) (PS-b-PEO) block copolymers as templates combined with a sol-gel process. Porous organic-inorganic hybrid films with titania nanodomains incorporated in the PEO domains can be produced by one step spin-coating. By manipulating the relative composition of the precursor ingredients, a simple protocol to fabricate hexagonally packed arrays of nanobowls is defined. Addition of a second inorganic precursor into the common solution leads to arrays of composite Au-TiO2 nanobowls. Such organic-inorganic hybrids and pure titania nanostructures with controlled shape and size exhibit unique photophysical properties.

Controlling the interparticle spacing of Au-salt loaded micelles and Au nanoparticles on flat surfaces

The self-organization of diblock copolymers into micellar structures in an appropriate solvent allows the deposition of well ordered arrays of pure metal and alloy nanoparticles on flat surfaces with narrow distributions in particle size and interparticle spacing. Here we investigated the influence of the materials (substrate and polymer) and deposition parameters (temperature and emersion velocity) on the deposition of metal salt loaded micelles by dip-coating from solution and on the order and inter-particle spacing of the micellar deposits and thus of the metal nanoparticle arrays resulting after plasma removal of the polymer shell. For identical substrate and polymer, variation of the process parameters temperature and emersion velocity enables the controlled modification of the interparticle distance within a certain length regime. Moreover, also the degree of hexagonal order of the final array depends sensitively on these parameters.

Effect of particle size on the oxidizability of platinum clusters

The catalytic properties of transition metal particles often depend crucially on their chemical environment, but so far, little is known about how the effects of the environment vary with particle size, especially for clusters consisting of only a few atoms. To gain insight into this topic, we have studied the oxygen affinity of free Pt(x) clusters as a function of cluster size (x = 1, 2, 3, 4, 5, and 10) using density functional theory (DFT) calculations (GGA-PW91). DFT-based Nosé-Hoover molecular dynamics has been used to explore the configuration space of the Pt(x)O(x) and Pt(x)O(2x) clusters, leading to the discovery of several novel Pt-oxide structures. The formation of small Pt-oxide clusters by oxidizing the corresponding Pt(x) clusters is found to be significantly more exothermic than the formation of bulk Pt-oxides from Pt metal. The exothermicity generally increases as cluster size decreases but exhibits strongly nonlinear dependence on the cluster size. The nanoclusters are also structurally distinct from the bulk oxides and prefer one- and two-dimensional chain and ringlike shapes. These findings help elucidate the oxidation behavior of Pt nanoclusters and lay the foundation for understanding the reactivity of Pt nanoclusters in oxidizing chemical environments.