A simple method for measuring the size of metal nanoclusters in solution

The connection between quantum size effects and the surface plasmon resonance of metal nanoclusters is introduced and the pros and cons of in situ and ex situ cluster analysis methods are outlined. A new method for estimating the size of nanoclusters is presented. This method combines core/shell cluster synthesis, UV-visible spectroscopy, and Mie theory. The core/shell approach enables the estimation of metal cluster sizes directly from the UV-visible spectra, even for transition metal nanoclusters such as Pd that have no distinct surface-plasmon peak in UV-visible region. Pd/Au and Au/Pd core/shell clusters as well as Au-Pd alloy clusters are synthesized and used as test cases for simulations and spectroscopic measurements. The results of the simulations and UV-visible spectroscopy experiments are validated with transmission electron microscopy.     

The size dependence of thermal EMF for Au,Pd, and Pt nanoclusters deposited onto highly oriented pyrolitic graphite surface

Data are presented on the tunnel current-voltage characteristics of gold, palladium, and platinum nanoclusters formed by pulsed laser deposition on the surface of highly oriented pyrolytic graphite. Differential tunnel current-voltage characteristics measured by scanning tunnel spectroscopy have been used to restore the size dependences of the thermal emf values of the studied nanoclusters. The behavior of the thermal emf of the nanoclusters as depending on their sizes has been found to depend on the nature of a metal. The data obtained have been analyzed.     

Calculation of the size dependence of the ionization energy and autoionization energy of Hg-clusters

To study the transition from van der Waals to metallic bonding we calculate the size dependence of the ionization energy and 5d→6p autoionization energy of Hg _n -clusters using a parametrized LCAO model. Our results are in good qualitative agreement with experiment. Comparison with experimental results suggests that electron correlations play an important role for the transition from localized (van der Waals-like) to delocalized (covalent or metallic) electronic states occuring in Hg _n atn?13–19.     

Low energy structures of gold nanoclusters in the size range 3–38 atoms

Using a combination of first principles calculations and empirical potentials we have undertaken a systematic study of the low energy structures of gold nanoclusters containing from 3 to 38 atoms. A Lennard-Jones and many-body potential have been used in the empirical calculations, while the first principles calculations employ an atomic orbital, density functional technique. For the smaller clusters (n=3–5) the potential energy surface has been mapped at the ab initio level and for larger clusters an empirical potential was first used to identify low energy candidates which were then optimised with full ab initio calculations. At the DFT-LDA level, planar structures persist up to six atoms and are considerably more stable than the cage structures by more than 0.1 eV/atom. The difference in ab initio energy between the most stable planar and cage structures for seven atoms is only 0.04 eV/atom. For larger clusters there are generally a number of minima in the potential energy surface lying very close in energy. Furthermore our calculations do not predict ordered structures for the magic numbers n=13 and 38. They do predict the ordered tetrahedral structure for n=20. The results of the calculations show that gold nanoclusters in this size range are mainly disordered and will likely exist in a range of structures at room temperature.     

Temperature dependence of the surface free energy of a crystal-liquid interface

A dimensionless complex containing the surface free energy of a crystal-liquid interface γ, and the entropy jump, temperature, and density of a crystal phase is described using the phenomenology of thermodynamic similarity; this complex remains constant at the melting line. It is demonstrated that the complex refines the result obtained by Skripov and Faizullin in [6] and enables us to estimate the temperature dependence of γ. Our calculations show that the surface free energy of the crystal-liquid interface of normally melting compounds is a monotonically increasing function of temperature.     

Chemiluminescence and electrochemiluminescence applications of metal nanoclusters

Due to strong photoluminescence, extraordinary photostability, excellent biocompatibility, and good water-solubility, metal nanoclusters have attracted enormous attention since discovered. They are found to be novel fluorescence labels for biological applications and environmental monitoring. Recently the chemiluminescence (CL) or electrochemiluminescence (ECL) of metal nanoclusters has received increasing attention. This review covers recent vibrant developments in this field of the past 5 years, and highlights different functions of metal nanoclusters in various CL and ECL systems, such as luminophores, catalysts, and quenchers. Latest synthetic methods of metal nanoclusters used in CL or ECL are also summarized. Furthermore, we discuss some perspectives and critical challenges of this field in the near future.     

Photoreductive synthesis of water-soluble fluorescent metal nanoclusters

Water-soluble fluorescent copper, silver and gold nanoclusters with quantum yields of 2.2, 6.8 and 5.3%, respectively, are prepared by a robust photoreduction of their inorganic precursors in the presence of poly (methacrylic acid) functionalized with pentaerythritol tetrakis 3-mercaptopropionate.     

Synthesis, structure and properties of metal nanoclusters

Metal nanoclusters have physical properties differing significantly from their bulk counterparts. Metallic properties such as delocalization of electrons in bulk metals which imbue them with high electrical and thermal conductivity, light reflectivity and mechanical ductility may be wholly or partially absent in metal nanoclusters, while new properties develop. We review modern synthetic methods used to form metal nanoclusters. The focus of this critical review is solution based chemical synthesis methods which produce fully dispersed clusters. Control of cluster size and surface chemistry using inverse micelles is emphasized. Two classes of metals are discussed, transition metals such as Au and Pt, and base metals such as Co, Fe and Ni. The optical and catalytic properties of the former are discussed and the magnetic properties of the latter are given as examples of unexpected new size-dependent properties of nanoclusters. We show how classical surface science methods of characterization augmented by chemical analysis methods such as liquid chromatography can be used to provide feedback for improvements in synthetic protocols. Characterization of metal clusters by their optical, catalytic, or magnetic behavior also provides insights leading to improvements in synthetic methods. The collective physical properties of closely interacting clusters are reviewed followed by speculation on future technical applications of clusters. (125 references).     

An entanglement model for the dependence of fracture surface energy on molecular weight

Over the last decade, empirical evidence has indicated that the effective surface energy γ associated with the fracture of noncrystalline is a linear function of the reciprocal of the viscosity–average molecular weight: \documentclass{article}\pagestyle{empty}\begin{document}$ \gamma = \gamma _\infty - b\bar M_v ^{ - 1} $\end{document}. For poly(methyl methacrylate), data of J. P. Berry, G. C. Berry and Fox show that gamma; ～ 0 at about the same value of M?_v that corresponds to the polymer chain-entanglement length. From this fact, we have developed an entanglement network model for fracture, that bears a resemblance to F. Bueche's entanglement model for the melt viscosity of bulk polymers. Our model allows for the expression of the previously empirical constants, γ_∞ and b, in terms of molecular parameters: \documentclass{article}\pagestyle{empty}\begin{document}$ {{\gamma _\infty = \gamma _{\rm s} A_{\rm s} Z_{\rm c} \rho _{\rm c} N_A } \mathord{\left/ {\vphantom {{\gamma _\infty = \gamma _{\rm s} A_{\rm s} Z_{\rm c} \rho _{\rm c} N_A } {\bar M_{\rm s} }}} \right. \kern-\nulldelimiterspace} {\bar M_{\rm s} }} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ b = 2({{\bar M_v } \mathord{\left/ {\vphantom {{\bar M_v } {\bar M_n }}} \right. \kern-\nulldelimiterspace} {\bar M_n }})\gamma _\infty M_{\rm f} $\end{document} where M?_n and M?_f are the number-average molecular weights of the polymer and of the free chain ends, M?_v is the viscosity-average molecular weight, γ_s is the average fracture-energy per entanglement in the craze volume, A_s is the average cross-sectional area of the polymer chain, Z_c and ρ_c are the thickness and density of crazed material on the fracture surface, respectively; M?_s is the average strand molecular weight between entanglements, and N_A is AvogadrO's number.     

On the OCS2 Singlet potential energy surface

The reaction between atomic oxygen and carbon disulfide is predicted to lead to at least two primary products, which are the dithiiranone ( 1 ) and the oxathiirane-thione ( 2 ) and/or the carbon disulfide S-oxide ( 4 ). The possible intramolecular equilibria 1 ? 2, 1 ? 3, 2 ? 4 , and 2 ? 5 as well as the fragmentations of the possible intermediates 1 – 5 have been studied theoretically within the semiempirical CNDO /B framework as conceivable ground-state reactions. On the basis of MO correlations and potential energy changes along the reaction paths, supplementary with previously reported experimental data, the single molecular transformations and the eventual product formations are discussed.     

Classical dependence of polarization on potential energy surface in rotationally inelastic collisions

Quasiclassicol trajectory calculations have been performed for model potential energy surfaces to investigate polarization in (j,m_j) → (j',m_j) integral cross sections For j = 0, it was found that the occurrence of polarization requires an attractive well, and that it be in the collinear configuration Results for j≠ 0 are also presented.     

Modeling size effects on the surface free energy of metallic nanoparticles and nanocavities

Based on the rigorous consideration of the bond broken rule and surface relaxation, a model for the size-dependent surface free energy of face-centered-cubic nanoparticles and nanocavities is presented, where the surface relaxation is calculated by the BOLS relationship. It is found that the surface free energy of nanoparticles and nanocavities represents a reverse size effect-the surface free energy of nanoparticles decreases with the decrease of particle size while it rises with the shrinkage of cavities. The size effect on the surface free energy of nanoparticles and nanocavities is not evident in large size ranges, while it becomes more and more distinct with decreasing size, especially for sizes smaller than 10 nm. The present predictions are in good agreement with the available literature data.     

On the interaction of ions with a platinum metal surface

Ab initio calculations have been performed to obtain analytical potential functions for describing the interactions between a platinum surface and both lithium and iodide ions. The accuracy of the results is tested by model calculations with large basis sets and the inclusion of correlation energy. The potentials obtained are to be used in computer simulations of electrochemical phenomena near solid–liquid interfaces.     

Stabilization of surface reaction intermediates by added metal atoms on metal surfaces of low free energy

Dynamic restructuring of the Ag(111) surface occurs during the reaction of sulfur dioxide with Ag(111)-p(4 x 4)-O at 300 K, resulting in the incorporation of added silver atoms into the unit cells of both adsorbed sulfite and sulfate. This result clearly demonstrates that incorporation of metal atoms into the structures of adsorbates and reaction intermediates is not restricted to more open, higher free energy single crystal planes. These observations indicate that the participation of added metal atoms must be considered in the theoretical treatment of metal catalyzed reactions.     

Non-ideality in Born-free energy of solvation in alcohol-water and dimethylsulfoxide-acetonitrile mixtures: Solvent size ratio and ion size dependence

Recent extension of mean spherical approximation (MSA) for electrolyte solution has been employed to investigate the non-ideality in Born-free energy of solvation of a rigid, mono-positive ion in binary dipolar mixtures of associating (ethanol-water) and non-associating (dimethylsulfoxide-acetonitrile) solvents. In addition to the dipole moments, the solvent size ratio and ion size have been treated in a consistent manner in this extended MSA theory for the first time. The solvent-solvent size ratio is found to play an important role in determining the non-ideality in these binary mixtures. Smaller ions such as Li⁺ and Na⁺ show stronger non-ideality in such mixtures compared to bigger ions (for example, Cs⁺ and Bu₄N⁺). The partial solvent polarization densities around smaller ions in tertiary butanol (TBA)-water mixture is found to be very different from that in other alcohol-water mixtures as well as to that for larger ions in aqueous solutions of TBA. Non-ideality is weaker in mixtures consisting of solvent species possessing nearly equal diameters and dipole moments and is reflected in the mole fraction dependent partial solvent polarization densities.     

The size dependent shift of the surface plasmon absorption band of small spherical metal particles

It has been previously established that the surface plasmon of small spherical silver particles, which are embedded in a noble gas matrix, shifts to higher energies (blue shift) as the mean diameterD of the particles decreases (100 Å>D>20 Å). This blue shift has also been found for supported silver particles, and quite recently we observed it by elastic light scattering in the gas phase. This latter experiment proves unambiguously that the blue shift in small silver particles is not induced by interactions with the environment, the presence of which is clearly recognized in less inert matrices such as O₂ or CO. From self-consistent calculations of the surface response of planar jellium surfaces one would expect a red shift, which is also directly confirmed by a few calculations for selected jellium spheres. The contradiction between the observed blue shift for small particles and the predicted red shift for jellium spheres disappears, if one accounts for thed-electrons of silver in a very simple approximation.     

The surface temperature dependence of the inelastic scattering and dissociation of hydrogen molecules from metal surfaces

High-dimensional, wave packet calculations have been carried out to model the surface temperature dependence of rovibrationally inelastic scattering and dissociation of hydrogen molecules from the Cu(111) surface. Both the molecule and the vibrating surface are treated fully quantum-mechanically. It is found, in agreement with experimental data, that the surface temperature dependence of a variety of dynamical processes has an Arrhenius form with an activation energy dependent on molecular translational energy and on the initial and final molecular states. The activation energy increases linearly with decreasing translational energy below the threshold energy. Above threshold the behavior is more complex. A quasianalytical model is proposed that faithfully reproduces the Arrhenius law and the translational energy dependence of the activation energy. In this model, it is essential to include quantized energy transfer between the surface and the molecule. It further predicts that for any process characterized by a large energy barrier and multiphonon excitation, the linear change in activation energy up to threshold has slope-1. This explains successfully the universal nature of the unit slope found experimentally for H2 and D2 dissociation on Cu.     

Synthesis of metal nanoclusters within microphase-separated diblock copolymers: ICP-AES analysis of metal ion uptake

The rate and extent of aqueous metal ion transport (Ag⁺, Au⁺³, Cu⁺², Eu⁺², Eu⁺³, Fe⁺², Fe⁺³, Gd⁺³, Ni⁺², Pd⁺², Pt⁺², and Pt⁺⁴) into microphase-separated block copolymers of methyl-tetracyclododecene (MTD) and 2-norbornene-5,6,-dicar☐ylic acid (NORCOOH) was monitored using inductively coupled plasma atomic emission spectroscopy. Continuous interconnected polyNORCOOH domains were found to be necessary for metal ion uptake. The metal-bound car☐ylate ions in the polyNORCOOH domains can be re-protonated by hydrogen reduction, rendering them active for further participation in metal binding. Following reduction, metal nanocluster-containing films were characterized using wide-angle X-ray scattering, infrared spectroscopy, and ultraviolet-visible spectroscopy.     

On the temperature dependence of electronically non-adiabatic vibrational energy transfer in molecule-surface collisions

Here we extend a recently introduced state-to-state kinetic model describing single- and multi-quantum vibrational excitation of molecular beams of NO scattering from a Au(111) metal surface. We derive an analytical expression for the rate of electronically non-adiabatic vibrational energy transfer, which is then employed in the analysis of the temperature dependence of the kinetics of direct overtone and two-step sequential energy transfer mechanisms. We show that the Arrhenius surface temperature dependence for vibrational excitation probability reported in many previous studies emerges as a low temperature limit of a more general solution that describes the approach to thermal equilibrium in the limit of infinite interaction time and that the pre-exponential term of the Arrhenius expression can be used not only to distinguish between the direct overtone and sequential mechanisms, but also to deduce their relative contributions. We also apply the analytical expression for the vibrational energy transfer rates introduced in this work to the full kinetic model and obtain an excellent fit to experimental data, the results of which show how to extract numerical values of the molecule-surface coupling strength and its fundamental properties.     

Efficient statistical mapping of energy surfaces of nanoclusters and molecules

A statistical technique to efficiently map out the energy surfaces of nanoclusters and molecules is described. Global energy minimizations are performed to reach of the catchment basins of the lowest energy stationary points. Saddle points are located by using a large value of the iterative energy change as the stopping criterion of a final local relaxation. Minima are derived from saddle points by simply tightening the stopping criterion and continuing the relaxation. A statistical approximation to the widths of the paths in phase space between saddle points and minima is obtained. Application is made to argon clusters of 7 and 38 atoms.