Sized Gold Clusters Research Articles

Exchange-correlation generalized gradient approximation for gold nanostructures

We compare the performance of different exchange-correlation functionals, based on the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation, for the structural and electronic properties of gold nanostructures. In particular we consider PBEsol (constructed to correctly describe solid-state systems) and PBEint [Phys. Rev. B 82, 113104 (2010)] which was recently introduced for hybrid interfaces and preserves the correct second-order gradient expansion of exchange energy (as in PBEsol) providing as well a significant nonlocality for higher density variation (as in PBE). We find that the PBEint functional gives a well balanced description of atomization energies, structural properties, energy differences between isomers, and bulk properties. Results indicate that PBEint is expected to be the most accurate functional for medium and large size gold clusters of different shapes.

Facile, hetero-sized nanocluster array fabrication for investigating the nanostructure-dependenceof nonvolatile memory characteristics

A facile fabrication of a hetero-sized nanocluster array has been demonstrated byusing inert-gas condensation at room temperature. The array consisted of 1.5 and3.2 nm sized gold clusters. The nonvolatile memory characteristics of a memory cellthat had a hetero-sized nanocluster array to be used as a charge-trapping layerwere compared to those of two cells that had 1.5 and 3.2 nm sized cluster arrays,respectively. While the average cluster size or the average number of electronstrapped in a cluster was reflected in the programming/erasing characteristics, thenanostructure effect was revealed in the retention characteristics, i.e. the proposed cellwas found to have the gentlest degradation of the memory window. This can beexplained by a preferential pathway for charge-carrier redistribution, caused by theionization potential difference between the two nanoclusters of different sizes.

Derivatives of the thiolate-protected gold cluster Au25(SR)18 -1

Loss of small fragments (like AuL, Au2L3, Au4L4) have been found systematically in several MALDI and FAB experiments on thiolate-protected gold clusters of different sizes. When using the cluster Au25L18-1 as parent cluster, the fragmented cluster Au21L14-1 has been reported to be obtained in high proportion (L = SCH2CH2Ph). Here we analyse a few possible fragmentation patterns of the well-known parent cluster Au25L18-1 (L = SCH3). Using DFT calculations we study the different atomic configurations obtained after a AuL fragment is lost from Au25L18-1. We found energetically favourable configurations that can be written as Au13 [Au2L3]6-z[AuL2]z-1, where the modification can be described as a replacement of the long protecting unit by a short one (Au2L3 → AuL2). A full replacement (z = 6) gives rise to a protected Au19L12-1 cluster. This mechanism does not modify the super-atomic electronic structure of the gold core, i.e., all these fragments remain an 8 electron super-atom clusters exactly like the parent Au25L18-1. We suggest that the Au19L12-1 cluster could be realized by using a bulky thiolate, such as the tert-butyl thiolate SC(CH3)3 .

First-Principles Investigation of the Atomic and Electronic Structure and Magnetic Moments in Gold Nanoclusters

We determine the structure, energetics, and emerging magnetic propertiesof Aun clusters using first-principles plane-wave density functional theory. We compare and contrast the findings obtained with and without spin−orbit interaction in the gold clusters of sizes down to 0.5−1 nm. The shapes are chosen to be representative of spherical gold nanoparticles: (a) Au13 and Au12, with cuboctahedral, icosahedral, and decahedral structures, and (b) Au25 and Au24, with truncated octahedral structures. We find that the trends in the binding energies are unaltered with the choice of the exchange correlation (LDA or GGA) or the inclusion of relativistic effects. The cuboctahedral Au13 is found to have the lowest binding energy compared with others at a given level of theory. Within the scalar relativistic description, these gold clusters exhibit a wide variety of magnetic moments: the stability and magnetic properties can be readily understood in terms of degeneracies of the HOMO and LUMO levels. Further, there is evidence of Jahn−Teller activity in the cases of cuboctahedral Au12 and Au13 that leads to structural distortion, inducing magnetism in the 12-atom cluster. Analysis of electronic states with projection on atomic orbitals for the scalar relativistic case shows that the magnetism in these gold clusters has an sp rather than the otherwise believed s character. By employing a fully relativistic description with inclusion of spin−orbit interaction and noncollinear magnetization, the magnetic moment in the icosahedral-shaped clusters is found to reduce substantially and that in the 12- and 13-atom clusters with a cuboctahedral structure becomes vanishingly small. The loss of magnetism in Au12 and Au13 appears to originate from the splitting of degenerate HOMO states in these clusters and an overlap of the d and sp states, whereas the magnetic moment of around 1 μB in Au25 is mainly caused by the s states of the central atom.

Theory of gold on ceria

The great promise of ceria-supported gold clusters as catalysts of the future for important industrial processes, such as the water gas shift reaction, has prompted a flurry of activity aimed at understanding the molecular-level details of their operation. Much of this activity has focused on experimental and theoretical studies of the structure of perfect and defective ceria surfaces, with and without gold clusters of various sizes. The complicated electronic structure of ceria, particularly in its reduced form, means that at present it is highly challenging to carry out accurate electronic structure simulations of such systems. To overcome the challenges, the majority of recent theoretical studies have adopted a pragmatic and often controversial approach, applying the so-called DFT + U technique. Here we will briefly discuss some recent studies of Au on CeO(2){111} that mainly use this methodology. We will show that considerable insight has been obtained into these systems, particularly with regard to Au adsorbates and Au cluster reactivity. We will also briefly discuss the need for improved electronic structure methods, which would enable more rigorous and robust studies in the future.

O2 adsorption and dissociation on neutral, positively and negatively charged Aun (n = 5–79) clusters

The adsorption and dissociation of an O(2) molecule on gas-phase gold clusters of size varying from 5 to 79 atoms have been investigated by means of first principles density functional theory calculations. The adsorption energies and dissociation barriers have been determined for neutral, positively and negatively charged gold clusters in order to analyze in a systematic way the role of the charge on the cluster reactivity. While there is beneficial effect on O(2) activation of an extra electron on the small gold clusters (Au(5) and Au(13)), the effect is absent for positively charged clusters. The effect of the charge vanishes rapidly by increasing the cluster size and is not visible for clusters containing about 40 atoms or more. Au(38) appears to be the most reactive among the clusters considered and strong oscillations in adsorption energies and dissociation barriers are found even for clusters containing several tens of atoms like Au(38), Au(55), and Au(79).

The gold-ammonia bonding patterns of neutral and charged complexes Aum±1–(NH3)n. I. Bonding and charge alternation

The gold-ammonia bonding patterns of the complexes which are formed between the ammonia clusters (NH(3))(1< or =n< or =3) and gold clusters of different sizes that range from one gold atom to the tri-, tetra-, and 20-nanogold clusters are governed by two basic and fundamentally different ingredients: the anchoring Au-N bond and the nonconventional N-H...Au hydrogen bond. The latter resembles, by all features, a conventional hydrogen bond and is formed between a typical conventional proton donor N-H group and the gold cluster that behaves as a nonconventional proton acceptor. We provide strong computational evidence that the gold-ammonia bonding patterns exhibit distinct characteristics as the Z charge state of the gold cluster varies within Z=0,+/-1. The analysis of these bonding patterns and their effects on the N-H...N H-bonded ammonia clusters are the subject of this paper.

Single-electron tunneling phenomena on preformed gold clusters deposited on dithiol self-assembled monolayers

We report on detailed low temperature scanning tunneling spectroscopy measurements performed on preformed nanometer sized gold clusters. The clusters are deposited on Au(1 1 1) films that are coated with a self-assembled monolayer of 1,4-benzenedimethanethiol molecules, thereby creating a double barrier tunnel junction. In this configuration electrons can only move between the scanning tunneling microscope tip and the substrate through tunneling via the cluster. Due to the very small cluster size this results in pronounced single-electron tunneling effects. Anomalies in the I( V) curves measured above a single cluster can be consistently described in terms of charging of the double barrier junction for the majority of the particles. Both Coulomb blockade and Coulomb staircase with step widths in the 200–600 meV range were observed on various individual clusters. The results agree with the commonly used semiclassical theory of charging effects in a double barrier tunnel junction. Furthermore, for a limited number of clusters, additional conductance peaks are observed which are attributed to strong electron confinement inside the nanoscale clusters.

Density-functional global optimization of gold nanoclusters

The structure of gas-phase gold clusters of size $\ensuremath{\sim}20$ is studied by density-functional global optimization in the full configuration space. The putative global minimum of ${\mathrm{Au}}_{20}$ is confirmed to be a tetrahedron $({T}_{d})$ independently of the choice of the exchange-correlation functional, whereas the structure of the low-lying excited states depends on the theoretical approach. The peculiar stability of ${T}_{d}$ is rationalized in terms of the synergic effects of $s\text{\ensuremath{-}}d$ hybridization and electronic shell closure. Calculations on ${\mathrm{Au}}_{16}$ and ${\mathrm{Au}}_{18}$ show that ${T}_{d}$ ${\mathrm{Au}}_{20}$ possibly represents a ``unicum'' in the sequence of gold clusters.

Structure and oxidation state of gold on different supports under various CO oxidation conditions

The structure of gold clusters of different sizes supported on various metal oxides (Al 2O 3, TiO 2, SiO 2) exposed to different CO oxidation conditions was investigated in situ using X-ray absorption spectroscopy at the Au L 3 edge. In all catalysts, the only phase detected during catalysis was Au 0. In the most active sample with small gold particles (Au/Al 2O 3), variation in the electronic structure of the gold clusters with changing reaction conditions was observed by XANES spectroscopy and ascribed to the adsorption of CO on the metallic gold clusters. FEFF8 calculations proved that the changes in the XANES signature of Au/Al 2O 3 can be explained by backdonation of d-electrons into the π ∗ orbitals of CO. For Au/Al 2O 3, the presence of Au O backscattering in the EXAFS suggested weak cluster–support interactions. For Au/SiO 2 and Au/TiO 2, the structure of the gold clusters remained unchanged throughout all experiments.

Molecular-dynamics study of possible packing sequence of medium size gold clusters: Au 2–Au 43

Growing pattern, structural stability, energetics and magic sizes of gold clusters, Au n ( n = 2 –43), have been investigated by using molecular-dynamics simulations. Starting from the dimer configuration, following rearrangement collision of the system in fusion process, and absorbing its energy step by step up to 0 K, possible stable structures of the clusters have been identified via an empirical model potential energy function. It has been found that gold clusters prefer to form three-dimensional compact structures and five-fold symmetry appears on the spherical medium clusters. This approach serves an efficient alternative to the growing path determination and the global optimization techniques.

Growth of Uncapped, Subnanometer Size Gold Clusters Prepared via Electroporation of Vesicles

Electric-field-induced transient pore formation (electroporation) in synthetic unilamellar vesicles is utilized for the preparation of subnanometer size uncapped gold quantum dots. With the precursor AuCl4(-) placed in the aqueous bulk solution and the reducing agent BH4(-) originally entrapped in the vesicles' compartments, the redox reaction--that occurs in the bulk--is initiated by the opening of transient pores in the vesicles' bilayers. The absence of caps permits (i) continued growth of the Au clusters formed, (ii) the assessment of their true absorption spectra unaltered by stabilizing ligands, and (iii) the previously inaccessible live observation of the growth of the clusters in the molecular size regime. The normally rapid self-aggregation of Au atoms is slowed to the time scales of hour and week by their adsorption at the exterior surface of the vesicles. The UV spectra exhibit novel, time-dependent, oscillating red and blue shifts of the characteristic absorption band, which can be attributed to the evolution of cluster size transiently halting at magic aggregation numbers corresponding to Au2, Au8, Au20, and Au34. Subsequent growth is associated with a monotonic red shift of the absorption band up to the characteristic surface plasmon absorption at 520 nm.

Polyaniline-Gold Nanocomposite System

Nanostructured composite materials of polyaniline (PANI) with gold clusters of different sizes and amounts have been produced. PANI films of the same thickness were grown electrochemically on platinum substrates. Then nucleation of gold clusters in the PANI film was initiated by chemical reduction of tetrabromoaurate ion in acidic solution. The spontaneous process of gold cluster formation was evaluated using UV-vis spectroscopy, scanning electron microscopy, and the Kelvin probe. It was found that the amount and size distribution of gold clusters vary with immersion time. The measured work function of the PANI/Au composite changes its magnitude according to the changes in composition of the materials. Despite the fact that the preparation method makes the gold cluster growth difficult to control in size and in distribution, it is of interest to applications for solid-state potentiometric sensors, batteries, supercapacitors, or catalysis since it modifies the electron affinity of the synthesized conducting polymer/gold composite. © 2003 The Electrochemical Society. All rights reserved.

STUDY ON GOLD NANOCLUSTERS AND GOLD NANOWIRES ON THE SURFACE OF HIGHLY ORIENTED PYROLYTIC GRAPHITE

We have investigated the growth of gold nanoclusters on the surface of highly oriented pyrolytic graphite in ultrahigh vacuum.Studies by ultrahigh vacuum scanning tunneling microscopy indicated that the size distribution of gold clusters was very narrow and quasi-one-dimensional chains of gold nanoclusters of approximately 2 nm diameter were produced after annealed at 74℃.And gold nanowires made of different sized gold clusters could also be formed after annealed at 122℃.These results suggest the viability of a new route to the creation of ordered nanoscale structures.

Phonon properties of transition-metal clusters

Abstract This paper is aimed at studying the phonon distribution for gold and copper clusters of different sizes and shapes. Icosahedral and fcc structures are considered. The clusters are regarded as classical systems interacting via a Finnis-Sinclair potential, thus neglecting the effects of the electronic degrees of freedom. The phonon spectrum influences several physical properties such as the specific heat and the thermodynamic stability at low temperatures. A careful positional analysis of the harmonic oscillations demonstrates the existence of phonons localized at the surface of the clusters. Those phonons prove to be responsible for an enhanced specific heat at low temperatures. In order to outline the effects related to the finite dimension of the aggregates, a spectral analysis of the dynamic matrix was performed in real space, thus avoiding the usual continuum approximations in Fourier space.

Collision induced dissociation of doubly charged stored metal cluster ions

Singly charged noble metal cluster ions from an external laser vaporization source are accumulated in a Penning trap. They are subjected to electron bombardment for further ionization. The doubly charged clusters are size selected and their low energy fragmentation pathways are studied by multiple-collision-induced dissociation. The reaction products are analysed by time-of-flight detection. In general, large clusters tend to evaporate neutral atoms while small clusters fission into two singly charged fragments, one of which is a trimer. The details, however, vary and depend on the specific elements and cluster sizes as exemplified by the fragmentation of doubly charged copper, silver, gold and platinum clusters of size around n = 16. © 1997 John Wiley & Sons, Ltd.

Preparation of Functional Silane-Stabilized Gold Colloids in the (Sub)nanometer Size Range

A synthesis method is introduced for very small uniform gold particles (diameter less than 5 nm), based on the reduction of hydrogen tetrachloroaurate(III) in ethanol in the presence of (γ-mercaptopropyl)trimethoxysilane (MPS). The surface layer of MPS molecules gives the gold particles a high colloidal stability and allows in principle further reaction with any silane coupling agent. Decrease of the HAuCl4:MPS ratio allows a controlled reduction of gold particle size, resulting in remarkably uniform gold clusters of (sub)nanometer size, observed with high-angle-annular-dark-field scanning transmission electron microscopy. After attachment of (γ-aminopropyl)triethoxysilane (APS) to the MPS surface layer, other molecules may be covalently bound to the gold colloid via the amine group of APS. As an illustrative example we prepared in this manner gold particles labeled with a fluorescent dye. The chemical structure of the surface silanes was studied with Fourier transform infrared spectroscopy.

Optical constants of gold and silver clusters in the spectral range between 1.5 eV and 4.5 eV

Optical constants of gold and silver clusters of various sizes are determined from measurements of their optical extinction in the range from 1.5 eV to 4.5 eV photon energy. For this purpose, it is shown that the optical extinction by spherical clusters satisfies a Kramers-Kronig relation, yielding the second quantity needed for the determination of the complex dielectric constant e of the cluster material. The dielectric constant is then obtained applying a generalized Newton-Raphson iteration method on the measured extinction data and the Kramers-Kronig data. The results show a clear dependence on cluster size, and also deviations from bulk dielectric constants in the range of the interband transitions. From the various e-data, a “bulk” dielectric constant of gold and silver is obtained, which fits the positions of the cluster plasmons more precisely than bulk dielectric constants from the literature.

Molecular Dynamics Simulation of the Elastic Deformation of Nanometer Diameter Metal Clusters

ABSTRACTIndentation using the AFM is a powerful method for determining elastic properties of small supported clusters. However, a theoretical framework has yet to be developed to interpret such measurements. The elastic deformation of nanometer sized gold clusters are modeled using the Embedded Atom Method (EAM) potential of Foiles et al. [1]. Force versus deformation curves are obtained for a series of truncated octahedral clusters having FCC symmetry (N=38, 201, 586, 1289, 2406). It is found that the MD results both for static compression and for harmonic vibration can be analytically estimated by using an elastic constant for the clusters analogous to the elastic modulus of a bulk material. However MD predictions for static compression are not in agreement with the deformation results of Schaefer et al. [2].

Simulated gold clusters and relative extended x-ray-absorption fine-structure spectra.

Using a tight-binding many-body potential we estimated classical ground states for gold clusters of various structures and sizes. Cluster contractions are observed as expected. We obtained cluster vibrational properties at very low temperature by evaluating phonon spectra in the pure harmonic approximation. Finally we simulated extended x-ray-absorption fine structure for both the icosahedral and the fcc structures. At sizes corresponding to the closed shells in the icosahedral and cuboctahedral packages, the icosahedral clusters turn out to be more cohesive than the corresponding fcc ones. Critical comparison with available experimental data suggests the presence of such structures in samples obtained by evaporation.