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.     

纳米镍团簇表面性质及能量的分子动力学研究

在单分散金属纳米粒子制备过程中,金属烧结现象需要尽量避免。烧结与诸多因素有关,其中金属纳米粒子的表面性质和能量对烧结作用有着重要影响。本工作利用分子动力学,以4种不同粒径的金属Ni纳米团簇为研究对象,在COMPASS力场下对不同温度下其表面积、扩散性质、表面能以及比表面能等进行了计算。结果显示,随着温度从300K升至1000K,纳米团簇的表面积稍微增加了约5%,表面层扩散系数显著增加了约3个数量级,表面能量升高了约15%,同时表面层与体相的能量差明显增加了近3倍。比表面能定义为增加单位表面积所引起的表面能的增量。计算结果表明,700K时团簇的比表面能比镍熔点处的表面张力高出约3个数量级,预示着团簇烧结具有强大的推动力。比表面能随温度升高以及粒径增大而下降,与热力学原理相一致。     

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.     

The density of surface ligands regulates the luminescence of thiolated gold nanoclusters and their metal ion response

The fascinating luminescence properties of gold nanoclusters(AuNCs) have drawn considerable research interests,and been widely harnessed for a wide range of applications.However,a fundamental understanding towards ligand density's role in the luminescence properties of these ultrasmall AuNCs remains unclear yet.In this communication,through systematic investigation of surface chemistries of glutathione-protected Au NCs(GSH-Au NCs) with diffe rent density of GSH as well as other thiolates,it is discovered that the density of surface ligands can significantly regulate the luminescence properties of AuNCs.Fluorescence lifetime spectroscopy and X-ray photoelectron spectroscopy showed that AuNCs with a higher density of electron-rich ligands facilitate their luminescence generation.Moreover,differences in the surface coverage of AuNCs can also affect their interactions with foreign species,as illustrated by significantly different fluorescence quenching capability of GSH-AuNCs with different ligand density towards Hg~(2+).This study provides new insight into the intriguing luminescence properties of metal NCs,which is hoped to stimulate further research on the design of metal NCs with strong luminescence and sensitive/specific responses for promising optoelectronic,sensing and imaging applications.     

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.     

Recent progress in applications of ligand-stabilized metal nanoclusters

Due to a nanotechnology boom in science and technology, the metal nanoclusters or nanoparticles stabilized by polymers and organic ligands have achieved much attention recently all over the world. We have studied on the preparation of polymer-stabilized metal nanoclusters by chemical methods, and applied them mainly to catalyses. Here the recent progresses in our group are presented in the structure control of bimetallic and trimetallic nanoclusters and in the applications of metal nanoclusters not only to the catalyses but also to the sensing responsive to pH and molecular recognition, and to the electro-optic properties of liquid-crystalline display rapidly responsive to frequency modulation. Preparation of trimetallic nanoclusters with a triple core/shell structure is especially emphasized to serve as a very active catalyst at a special atomic ratio of three elements.     

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.     

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.     

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.     

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.