Electronic entropy, shell structure, and size-evolutionary patterns of metal clusters

We show that electronic-entropy effects in the size-evolutionary patterns of relatively small (as small as 20 atoms), simple-metal clusters become prominent already at moderate temperatures. Detailed agreement between our finite-temperature-shell-correction-method calculations and experimental results is obtained for certain temperatures. This agreement includes a size-dependent smearing out of fine-structure features, accompanied by a measurable reduction of the heights of the steps marking major-shell and subshell closings, thus allowing for a quantitative analysis of cluster temperatures.Comment: Latex/Revtex, 4 pages with 3 Postscript figure

Density functional study of Aun_n (n=2-20) clusters: lowest-energy structures and electronic properties

We have investigated the lowest-energy structures and electronic properties of the Au$_n$(n=2-20) clusters based on density functional theory (DFT) with local density approximation. The small Au$_n$ clusters adopt planar structures up to n=6. Tabular cage structures are preferred in the range of n=10-14 and a structural transition from tabular cage-like structure to compact near-spherical structure is found around n=15. The most stable configurations obtained for Au$_{13}$ and Au$_{19}$ clusters are amorphous instead of icosahedral or fcc-like, while the electronic density of states sensitively depend on the cluster geometry. Dramatic odd-even alternative behaviors are obtained in the relative stability, HOMO-LUMO gaps and ionization potentials of gold clusters. The size evolution of electronic properties is discussed and the theoretical ionization potentials of Au$_n$ clusters compare well with experiments.Comment: 6 pages, 7 figure

Work functions, ionization potentials, and in-between: Scaling relations based on the image charge model

We revisit a model in which the ionization energy of a metal particle is associated with the work done by the image charge force in moving the electron from infinity to a small cut-off distance just outside the surface. We show that this model can be compactly, and productively, employed to study the size dependence of electron removal energies over the range encompassing bulk surfaces, finite clusters, and individual atoms. It accounts in a straightforward manner for the empirically known correlation between the atomic ionization potential (IP) and the metal work function (WF), IP/WF$\sim$2. We formulate simple expressions for the model parameters, requiring only a single property (the atomic polarizability or the nearest neighbor distance) as input. Without any additional adjustable parameters, the model yields both the IP and the WF within $\sim$10% for all metallic elements, as well as matches the size evolution of the ionization potentials of finite metal clusters for a large fraction of the experimental data. The parametrization takes advantage of a remarkably constant numerical correlation between the nearest-neighbor distance in a crystal, the cube root of the atomic polarizability, and the image force cutoff length. The paper also includes an analytical derivation of the relation of the outer radius of a cluster of close-packed spheres to its geometric structure.Comment: Original submission: 8 pages with 7 figures incorporated in the text. Revised submission (added one more paragraph about alloy work functions): 18 double spaced pages + 8 separate figures. Accepted for publication in PR

Tuning the energetics and tailoring the optical properties of silver clusters confined in zeolites

The integration of metal atoms and clusters in well-defined dielectric cavities is a powerful strategy to impart new properties to them that depend on the size and geometry of the confined space as well as on metal-host electrostatic interactions. Here, we unravel the dependence of the electronic properties of metal clusters on space confinement by studying the ionization potential of silver clusters embedded in four different zeolite environments over a range of silver concentrations. Extensive characterization reveals a strong influence of silver loading and host environment on the cluster ionization potential, which is also correlated to the cluster's optical and structural properties. Through fine-tuning of the zeolite host environment, we demonstrate photoluminescence quantum yields approaching unity. This work extends our understanding of structure property relationships of small metal clusters and applies this understanding to develop highly photoluminescent materials with potential applications in optoelectronics and bioimaging

Electronic Effects in Homonuclear Singly and Multiply Charged Noble Metal Clusters

The mass spectra of silver- and gold-clusters, generated by a gas aggregation technique and ionized by electron impact, reveal anomalies in the relative abundance of both singly and multiply charged clusters. Concentration maxima for singly charged species Agn+ (n = 3, 9, (19), 21) and Aun+ (n = 3, 9) are in agreement with experimental data of Katakuse and with predictions from the electronic shell model. Minima in the abundance distri-bution for doubly charged clusterions at Me232+ (Me = Ag, Au), and maxima in the concentration distribution for triply charged ions, Agn3+ (n = 37, 43, 61, 71), are explained in terms of electronic shell closing with shell ordering within a spherically symmetric potential.In den Massenspektren von einfach und zweifach geladenen Silber- und Goldclustern zeigen sich Intensitätsanomalien. Die Cluster werden mittels Gasaggregationstechnik erzeugt und durch Elektronenstoß ionisiert. Die Konzentrationsmaxima für einfach gela-dene Cluster Agn+ (n = 3, 9, (19), 21) und Aun+ (n = 3, 9) stimmen mit experimentellen Ergebnissen von Katakuse und Vorhersagen des Elektronenschalenmodells überein. Mi-nima der Intensitäten doppelt geladener Clusterionen für Me232+ (Me = Ag, Au), sowie Maxima für dreifach geladene Silberclusterionen Agn3+ (n = 37, 43, 61, 71) lassen sich ebenfalls im Rahmen des Elektronenschalenmodells unter Annahme einer durch ein sphärisch symmetrisches Potential gegebenen Schalenordnung erklären

Observation of size-dependent coalescence of Ag clusters

Clusters generated by the gas aggregation technique with Diameters are condensed at room temperature on thin carbon films and investigated by means of electron microscopy. Spontaneous coalescence of clusters occurs during deposition. Larger clusters, on the other hand, do not tend to coalesce and mainly preserve their shape, exhibiting only small alterations due to sintering in the boundary regions. Surface area determination by the BET method using N2 as the adsorbate gives a specific surface area of 3.6 m2/g in the case of powder resulting from small clusters