Nanometer-sized Clusters Research Articles

Synthesis and characterization of homo- and heteronuclear molecular Al3+ and Th4+ species chelated by the ethylenediaminetetraacetate (edta) ligand

Four molecular species chelated by edta have been synthesized from aqueous solutions and characterized by single-crystal X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis. [Th(H2O)4(edta)] (Th1) crystallized in monoclinic space group P2(1)/c with unit cell parameters of a = 8.5275(5) Å, b = 12.0635(7) Å, c = 15.8825(9) Å, and β = 105.340(2)°. Monoclinic space group C2 was identified for [Al2(edta)(OH)2(H2O)2] (Al2) with a = 11.1089(10) Å, b = 9.4830(8) Å, c = 7.6116(7) Å, and β = 112.026(3)°. [Th4(H2O)4Al10(H2O)8(OH)28(edta)4]·Cl2(H2O)29 (Th4Al10) formed the triclinic space group P1 with a = 9.3172(8) Å, b = 16.6099(14) Å, c = 19.5080(14) Å, α = 102.314(3)°, β = 95.615(3)°, and γ = 92.473(3)°. [Th2(H2O)2Al6(H2O)10(OH)14(edta)2]·(NO3)4(H2O)18 (Th2Al6) also crystallized in triclinic space group P1[combining macron] with unit cell parameters a = 10.9899(12) Å, b = 11.6107(12) Å, c = 14.3350(17) Å, α = 73.012(4)°, β = 87.411(4)°, and γ = 75.717(4)°. Small molecular species are observed in the Th1 and Al2 compounds, while the metal cations hydrolyze to create nanometer-sized heterometallic clusters in the Th4Al10 and Th2Al6 materials. The topological characteristics of each species are described and related to aqueous speciation in Th(4+), Al(3+)-edta systems.

Aggregation Behavior of Bovine κ- and β-Casein Studied with Small Angle Neutron Scattering, Light Scattering, and Cryogenic Transmission Electron Microscopy

In the native bovine casein micelle the calcium sensitive caseins (α(S1)-, α(S2)- and β-casein) sequester amorphous calcium phosphate in nanometer-sized clusters, whereas the calcium-insensitive κ-casein limits the growth of the micelle. In this paper, we further investigate the self-association of κ- and β-casein, which are two of the key proteins that control the substructure of the milk casein micelle, using neutron and light scattering techniques and cryogenic transmission electron microscopy. Results demonstrate that κ-casein can, apart from the known self-assembly, form amyloid-like fibrils already at temperatures of 25 °C when subject to agitation. This extended aggregation behavior of κ-casein is inhibited by β-casein, as reported by others. These findings have implications for the structure and stability of casein micelles. The neutron scattering data was used to gain information on the self-assembly structure of κ-casein. β-Casein shows similar self-association behavior as κ-casein, but unlike κ-casein, the self-association exhibits temperature dependence within the studied temperatures (6 and 25 °C). Here, we will discuss our extended study of the known self-assembly of casein in the context of the fibrillation of κ-casein.

Bond Flexibility and Low Valence Promote Finite Clusters of Self-Aggregating Particles

Systems of complex particles such as proteins or colloidal particles have a widely observed tendency to form nonconnected nanometer-size clusters at steady state, but the underlying mechanisms remain poorly understood. We report here a numerical study on the self-aggregation of low-valence particles with flexible bonds (i.e., free bond orientations) in two dimensions and predict the formation of a stable cluster phase for average valences ranging from 2 to 3.6. For the intermediate case of trivalent particles, we show that a cluster phase is present over a wide range of concentrations and interaction energies. The clusters are polydisperse in size, have a fractal dimension of 1.5, and tend to fully saturate their bonds at high interaction energies. The number of unformed bonds scales linearly with the number of particles in a cluster, which implies the absence of phase transition in the explored region of interaction energies and concentrations. We discuss possible implications of our model for membrane protein clustering.

Reversible precipitation/dissolution of precious-metal clusters in perovskite-based catalyst materials: Bulk versus surface re-dispersion

High-angle annular dark-field imaging with aberration-corrected scanning transmission electron microscopy unambiguously establishes that nanometer-size Pt-rich clusters reversibly precipitate and dissolve under reducing and oxidizing conditions, respectively, within the oxide matrix of Pt-doped CaTiO3, previously proposed as a novel self-regenerating perovskite-based three-way catalyst. In fact, most of the Pt-rich clusters formed upon reduction are within the oxide matrix and thus unavailable for gas-phase catalysis. Those clusters of Pt that do form on the surface tend to coarsen rather than dissolve upon oxidation.

Thickness dependency of field emission in amorphous and nanostructured carbon thin films

Thickness dependency of the field emission of amorphous and nanostructured carbon thin films has been studied. It is found that in amorphous and carbon films with nanometer-sized sp2 clusters, the emission does not depend on the film thickness. This further proves that the emission happens from the surface sp2 sites due to large enhancement of electric field on these sites. However, in the case of carbon films with nanocrystals of preferred orientation, the emission strongly depends on the film thickness. sp2-bonded nanocrystals have higher aspect ratio in thicker films which in turn results in higher field enhancement and hence easier electron emission.

Platinum Nanoclusters Stabilized on γ-Alumina by Chlorine Used As a Capping Surface Ligand: A Density Functional Theory Study

Controlling the size of metallic nanoclusters supported on an oxide support such as γ-alumina represents a challenging but important task in the case of noble metals such as platinum. By using density functional theory (DFT), we investigate the thermodynamic, structural and electronic properties of small nanometer-sized Ptn clusters (n ≤ 13) interacting with four relevant γ-alumina surfaces exhibiting various hydroxylation and chlorination states. The presence of chlorine on the (110) surface of γ-alumina implies a thermodynamic stabilization of small platinum clusters. This stabilization originates from the simultaneous migrations of chlorine atoms and protons from the support toward the Pt clusters. The migration of H and Cl from the support induces a stronger interaction of the Ptn cluster with the available AlIII site, associated with strong H–Ptn–Cl interaction. In particular, this trend leads to a local energy minimum, as a function of cluster size, for the Pt3 cluster. This atomic-scale stabilization of subnanometer clusters is thus proposed to be at the origin of the formation of highly dispersed platinum particles and to prevent their sintering into supranano ones. A detailed energetic and electronic analysis is provided to rationalize this effect of chlorine. A rational interpretation of experimental data is finally given.

Effects of self-assembled monolayers on amperometric glucose biosensors based on an organic–inorganic hybrid system

Ultrathin-form amperometric glucose biosensors have been fabricated on gold substrates by using self-assembled monolayers (SAMs) of carboxylic acid groups (COOH). The glucose oxidase (GOx) was covalently immobilized on the SAMs and covered with Langmuir–Blodgett films including nanometer-sized clusters of Prussian Blue (PB) playing a role of mediator for glucose detection, which makes the present sensor, Au/SAM/GOx/PB, work at very low potentials of approximately 0.0V vs. Ag/AgCl. An amperometric biosensor comprising a SAM of 4-mercaptobenzoic acid (MBA) exhibited a fast response current of 3s, a detection limit of 12.5μM, a high sensitivity of 50nA/(cm2mM), and a long-term stable linearity ranging from12.5μM to 70mM. The sensitivity is significantly affected by the conductivity of the SAM layer. The analysis of the electron transfer confirmed a high conductive nature of MBA among carboxylic acid groups. The obtained linearity in a high-concentration region is attributed to both the ultrathin nature and high electron transfer of the MBA SAM.

Visualizing Local Doping Effects of Individual Water Clusters on Gold(111)-Supported Graphene

The local charge carrier density of graphene can exhibit significant and highly localized variations that arise from the interaction between graphene and the local environment, such as adsorbed water, or a supporting substrate. However, it has been difficult to correlate such spatial variations with individual impurity sites. By trapping (under graphene) nanometer-sized water clusters on the atomically well-defined Au(111) substrate, we utilize scanning tunneling microscopy and spectroscopy to characterize the local doping influence of individual water clusters on graphene. We find that water clusters, predominantly nucleated at the atomic steps of Au(111), induce strong and highly localized electron doping in graphene. A positive correlation is observed between the water cluster size and the local doping level, in support of the recently proposed electrostatic-field-mediated doping mechanism. Our findings quantitatively demonstrate the importance of substrate-adsorbed water on the electronic properties of graphene.

Formation of Nanometer-Sized Surface Platinum Oxide Clusters on a Stepped Pt(557) Single Crystal Surface Induced by Oxygen: A High-Pressure STM and Ambient-Pressure XPS Study

We studied the oxygen-induced restructuring process on a stepped Pt(557) single crystal surface using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at O(2) pressures up to 1 Torr. HP-STM has revealed that nanometer-sized clusters are created on Pt(557) at 1 Torr of O(2) and at room temperature. These clusters are identified as surface Pt oxide by AP-XPS. The appearance of clusters is preceded by the formation of 1D chain structures at the step edges. By using a Pt(111) surface as a reference, it was found that the step sites are the nucleation centers for the formation of surface oxide clusters. These surface oxide clusters disappear and the stepped structure is restored on Pt(557) after evacuating O(2) to 10(-8) Torr. Changes in the surface oxide concentration in response to variations in the O(2) gas pressure are repeatable for several cycles. Our results that small clusters are initiated at step sites at high pressures demonstrate the importance of performing in situ characterization of stepped Pt catalysts under reaction conditions.

Structure and Energetics of Nanometer Size Clusters of Sulfuric Acid with Ammonia and Dimethylamine

The structures of positively and negatively charged clusters of sulfuric acid with ammonia and/or dimethylamine ((CH(3))(2)NH or DMA) are investigated using a combination of Monte Carlo configuration sampling, semiempirical calculations, and density functional theory (DFT) calculations. Positively charged clusters of the formula [(NH(4)(+))(x)(HSO(4)(-))(y)](+), where x = y + 1, are studied for 1 ≤ y ≤ 10. These clusters exhibit strong cation-anion interactions, with no contribution to the hydrogen-bonding network from the bisulfate ion protons. A similar hydrogen-bonding network is found for the [(DMAH(+))(5)(HSO(4)(-))(4)](-) cluster. Negatively charged clusters derived from the reaction of DMA with [(H(2)SO(4))(3)(NH(4)(+))(HSO(4)(-))(2)](-) are also studied, up to the fully reacted cluster [(DMAH(+))(4)(HSO(4)(-))(5)](-). These clusters exhibit anion-anion and ion-molecule interactions in addition to cation-anion interactions. While the hydrogen-bonding network is extensive for both positively and negatively charged clusters, the binding energies of ions and molecules in these clusters are determined mostly by electrostatic interactions. The thermodynamics of amine substitution is explored and compared to experimental thermodynamic and kinetic data. Ammonia binds more strongly than DMA to sulfuric acid due to its greater participation in hydrogen bonding and its ability to form a more compact structure that increases electrostatic attraction between oppositely charged ions. However, the greater gas-phase basicity of DMA is sufficient to overcome the stronger binding of ammonia, making substitution of DMA for ammonia thermodynamically favorable. For small clusters of both polarities, substitutions of surface ammonium ions are facile. As the cluster size increases, an ammonium ion becomes encapsulated in the center of the cluster, making it inaccessible to substitution.

Nucleation and growth of intragranular defect and insoluble atom clusters in nuclear oxide fuels

Uranium and plutonium oxides are subjected to high levels of radiation damage due to the slowing of fission fragments. In addition the composition of the material evolves over time as a result of fission events. Rare gases which constitute an abundant class of fission products are particularly insoluble and therefore tend either to be released from the fuel or form small nanometre size clusters. Bubbles are liable to grow and become trapping sites for migrating defects or other insoluble atoms. Interactions between migrating atoms, defects and existing clusters will determine the rate and extent to which clusters grow. Because the transfer of gas from within the grain to the grain boundaries is thought of as being the rate limiting process for fission gas release, a review of phenomena occurring on the sub-grain scale is carried out. The microstructural modifications induced by neutron irradiations of UO2 fuels are discussed with an emphasis on their relation to fission gas release. Based mainly on TEM studies, the phenomena which are usually taken into account in fission gas behaviour models are looked at and the limitations of these models outlined. More recent experimental and modelling approaches involving ion-irradiation experiments and atomic scale modelling are presented. It is shown that combining these approaches may lead, despite the complexity inherent to the system, to a better understanding of basic radiation induced microstructural changes, clustering events, and rare gas behaviour.

Epitaxial Electrodeposition of Cobalt on a Pt(111) Electrode Covered with a Cu(111) Film

Electrodeposition is an inexpensive alternative to the conventional molecular beam epitaxy technique used to fabricate artificial magnetic materials, such as cobalt thin film. Reported here is a scanning tunneling microscopy (STM) study on the electrodeposition of Co on a Pt(111) single-crystal electrode precoated with a Cu thin film in 0.1 M KClO4 + 1 mM HCl + 0.04 M CoCl2 (pH 3). Deposition of Co started with the nucleation of nanometer-sized clusters preferentially at pits on the Cu support, followed by lateral expansion and coalescence of Co nuclei to form a uniform Co layer. Normally a few Co layers would grow simultaneously to produce a smooth Co deposit until the 12th layer. Cobalt grew in three dimensions afterward. Atomic-resolution STM imaging showed that the first Co layer assumed a double-lined pattern, which was lifted by the deposition of another layer of Co. The second Co layer exhibited a hollow-ring pattern, which transformed into a moiré pattern and triangular pits at the third Co layer. The moiré pattern gained prominence at the expense of the triangular pits as the Co deposit thickened. The amplitude of the intensity modulation of the moiré pattern decreased with the thickness of the Co deposit and eventually became indiscernible at the 12th layer. These restructuring events resulted from a gradual release of the stress at the Co/Cu interface. Since the morphology of the copper substrate was hardly changed by the deposition of cobalt, mixing at the Co/Cu interface seems to be negligible. Similar to the deposition process, dissolution of Co deposit proceeded in layers also.

Magnetic dynamics of dilute iron nano-clusters in silver films from Mössbauer spectroscopy and muon spin rotation

A silver film containing nanometer size clusters of iron (nominal conc. 1 at%) has been studied by Mossbauer spectroscopy and Low-Energy Muon Spin Rotation. Below about 20 K spin glass freezing due to interparticle interactions is found from both methods. Whereas Mossbauer spectra are insensitive to the fast fluctuations of cluster moments above spin glass freezing temperature, muon spin rotation in magnetic fields applied perpendicular to the polarized muon spins allows tracing the fluctuations of superparamagnetic moments. The temperature dependence of the damping of the muon spin rotation signal shows Arrhenius behavior between 10 to 100 K. Depending on the assumed shape of damping the activation energy of superparamagnetic fluctuations of cluster moments ranges between about 20 K ·kB and 40 K ·kB. Above about 120 K muon spin depolarization indicates diffusion and trapping of muons.

Small gold clusters on graphene, their mobility and clustering: a DFT study

Motivated by the experimentally observed high mobility of gold atoms on graphene andtheir tendency to form nanometer-sized clusters, we present a density functional theorystudy of the ground state structures of small gold clusters on graphene, their mobilityand clustering. Our detailed analysis of the electronic structures identifies theopportunity to form strong gold–gold bonds and the graphene-mediated interactionof the pre-adsorbed fragments as the driving forces behind gold’s tendency toaggregate on graphene. While clusters containing up to three gold atoms have oneunambiguous ground state structure, both gas phase isomers of a cluster with fourgold atoms can be found on graphene. In the gas phase the diamond-shapedAu4D cluster is the ground state structure, whereas the Y-shapedAu4Y becomes the actual ground state when adsorbed on graphene. As we show, both clusters can beproduced on graphene by two distinct clustering processes. We also studied in detail the stepwiseformation of a gold dimer out of two pre-adsorbed adatoms, as well as the formation ofAu3. All reactions are exothermic and no further activation barriers, apart from thediffusion barriers, were found. The diffusion barriers of all studied clusters rangefrom 4 to 36 meV only, and are substantially exceeded by the adsorption energies of − 0.1 to − 0.59 eV. This explains thehigh mobility of Au1 − 4 on graphene along the C–C bonds.

Enhancement of Er3+ luminescence by metal aggregates

We present recent results on the occurrence of an Er fluorescence sensitization induced by ultra-small Au clusters (NC) incorporated in Er-doped silica by a low fluence ion implantation; it is shown that the energy transfer from the metal cluster to the Er ions critically depends on the evolution of the Au metal clustering process, starting from dispersed metal atoms up to the formation of nanometer-sized clusters. To investigate sub-nanometric Au structures and dopant ions (Au, Er) dispersed into the matrix, extended X-ray absorption fine structure spectroscopy has been used because, by measuring the local site around the dopant (Er, Au) independently of any long-range order considerations, it can detect both few-atom clusters and the possible presence of dopant dispersed into the matrix. This investigation allowed us to directly follow the early stages of Au nucleation, where clusters nearly have a the critical radius (i.e. aggregates formed by two to three atoms), and to relate the Au NC size with the Er3+photoluminescence (PL) enhancement. The defect distribution and radiation damage recovery have been investigated using positron annihilation spectroscopy; the effect of the radiation damage recovery on the Er site and PL is also reported.

Thermally Activated Processes at the Co/ZnO Interface Elucidated Using High Energy X-rays

A detailed picture of the thermally activated processes occurring at the Co/ZnO interface is obtained by a combination of high energy X-ray based techniques: X-ray photoelectron and absorption spectroscopies and the kinematical X-ray standing wave method. At room temperature, the growth of a few monolayers of cobalt proceeds by the nucleation of nanometer-sized clusters on the polar oxygen-terminated (0001) surface of a ZnO single crystal. Progressive annealing from 600 to 970 K allows separating the various interfacial reactions. At the lowest annealing temperature, Co clusters coalesce while keeping their metallic character. Above 700 K Co0 is gradually oxidized to Co2+ and a thin Co rich (Zn,Co)O layer is formed. It is observed that rock salt CoO phases may form at the surface when the initial Co thickness exceeds 1 nm. At the highest annealing temperature (970 K), Co diffuses deeper into ZnO and Zn vacancies are created at subsurface sites that were previously occupied by Co.

Synthesis and Characterization of Single-Layer Silver−Decanethiolate Lamellar Crystals

We report the synthesis of silver-decanethiolate (AgSC10) lamellar crystals. Nanometer-sized Ag clusters grown on inert substrates react with decanethiol vapor to form multilayer AgSC10 lamellar crystals with both layer-by-layer and in-plane ordering. The crystals have strong (010) texture with the layers parallel to the substrates. The synthesis method allows for a precise control of the number of layers. The thickness of the lamellae can be manipulated and systematically reduced to a single layer by decreasing the amount of Ag and lowering the annealing temperature. The single-layer AgSC10 lamellae are two-dimensional crystals and have uniform thickness and in-plane ordering. These samples were characterized with nanocalorimetry, atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray reflectivity (XRR), Fourier transform infrared spectroscopy (FTIR), and Rutherford backscattering spectroscopy (RBS).

ChemInform Abstract: Polymer‐Incarcerated Metal(0) Cluster Catalysts

AbstractReview: 201 refs.

Particulate matter (PM) air pollution and health: regulatory and policy implications

Particulate matter (PM), an ambient air criteria pollutant, is a complex mixture of chemical agents in particles ranging from nanometer-sized molecular clusters to dust particles too large to be aspirated into the lung airways (>10 μm in aerodynamic diameter). Although particle composition is known to affect health risks, our current health-based PM standards are limited to the mass concentrations within two specified size ranges (those below 2.5 μm, which are largely attributed to combustion products, and those below 10 μm, which includes mechanically generated dusts). Both size ranges have been associated with excess mortality and morbidity in numerous epidemiological studies. There is an urgent need for routine air monitoring data on components of the PM mass. Such data are needed to enable epidemiological studies that can define the roles of PM chemical components in causing adverse health effects in order to guide more targeted emission controls for the most hazardous components, thereby gaining public health benefits with the least impact on societal costs of emission controls.

Sugar‐Decorated Dendritic Nanocarriers: Encapsulation and Release of the Octahedral Rhenium Cluster Complex [Re6S8(OH)6]4−

The encapsulation of a nanometer-sized octahedral anionic rhenium cluster complex with six terminal hydroxo ligands [Re(6)S(8)(OH)(6)](4-) in maltose-decorated poly(propylene amine) dendrimers (POPAM, generation 4 and 5) has been investigated. Ultrafiltration experiments showed that maximal loading capacity of the dendrimers with the cluster complex is achieved after about ten hours in aqueous solution. To study the inclusion phenomena, three different methods have been applied: UV/Vis, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and laser-induced liquid bead ion desorption mass spectrometry (LILBID-MS). From the results obtained, it could be concluded that: a) the hydrolytic stability of the rhenium cluster complex is significantly enhanced in the presence of dendritic hosts; b) the cluster anions are preferentially bound inside the dendrimers; c) the number of cluster complexes encapsulated in the dendrimers increases with rising dendrimer generation. On average, four to five cluster anions can preferentially be captured in the interior of sugar-coated dendritic carriers. An asymptotic progression of the release of cluster complexes from the loaded dendrimers was observed under physiologically relevant conditions (isotonic sodium chloride solution: approximately 93 % within 4 days for loaded POPAM-G4-maltose; approximately 86 % within 4 days for loaded POPAM-G5-maltose). These encapsulation and release properties of maltose-decorated nanocarriers imply the possibility for the development of the next generation of dendritic nanocarriers with specific targeting of destined tissue for therapeutic treatments.