Session B21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials I

Physical and chemical properties of supported, suspended and trapped clusters

Investigations of physical systems of small sizes and reduced dimensionalities, exhibiting discrete quantized energy level spectra and specific structures and morphologies, open avenues for systematic explorations of the emerging unique behavior of materials at the nanoscale, as well as of the transition from the atomic and molecular domain to the condensed phase regime. We will discuss computer simulations and accompanying experiments exhibiting emergent physical and chemical behavior at the nanoscale, focusing on several confinement configurations -- free, supported and trapped clusters. These include investigations of metallic, semiconducting, superconducting [1] and mixed metal/molecular [2] suspended nanowires, recent advances in understanding the structure and catalytic activity of supported gold nanoclusters and methods for controlling their dimensionality and properties [3], strongly correlated states and formation of electron (Wigner) clusters in 2D quantum dots, and crystallization of trapped finite boson clusters [4]. \newline \newline [1] A. Marchenkov, Z. Dai, C. Zhang, R. N. Barnett, U. Landman, Phys. Rev. Lett. 98, 046802 (2007); A. Marchenkov, Z. Dai, B. Donehoo, R. N. Barnett, U. Landman, Nature Nanotech. 2, 481 (2007). \newline [2] C. Zhang, R.N. Barnett, U. Landman, Phys. Rev. Lett., in press (2007). \newline [3] U. Landman, B. Yoon, C. Zhang, U. Heiz, M. Arenz, Topics in Catal. 44, 145 (2007). \newline [4] C. Yannouleas and U. Landman, Reports on Progress in Physics 70, 2067 (2007).   

Electric Dipole Moments of Neutral Sodium Clusters

Recent ab initio studies have predicted large electric dipole moments in neutral Na clusters. We performed an electric deflection experiment on these clusters at 20 K and have found upper bounds on their dipole moments that are orders of magnitude smaller than predicted, and consistent with zero.   

Enhanced magnetic moments of Sc, Ti and V doped Na$_{n }$(n =4, 5, 6) clusters

Theoretical studies on the geometry, electronic structure and spin multiplicity of Sc, Ti and V doped Na$_{n}$ (n = 4, 5, 6) clusters have been carried out within a gradient corrected density functional approach. Two complementary approaches including all-electron calculations on free clusters, and supercell calculations using planewave pseudopotential and projector augmented wave formalisms have been carried out. It is shown that spin magnetic moments of the transition metal atoms, the magnitude of host polarization, and the sign of the host polarization all change with the number of alkali atoms. In particular the transition metal atoms are shown to attain spin moments that are higher than their atomic values. The role of hybridization between the transition atom $d$-states and the alkali \textit{sp}-states is highlighted to account for the evolutions in the spin moments and host polarization.   

Structure, Growth and Optical Properties of Si$_{n}$O$_{m}$ Nanoparticles: From SiO molecules to Silicates in Circumstellar Space

Synergistic effort combining formation of clusters in molecular beams and first principles electronic structure studies within a gradient corrected density functional scheme are employed to examine the geometries, stability, vibrational frequencies and optical properties of Si$_{n}$O$_{m}$ clusters. It is proposed that the oxygen enrichment needed to form silicates in interstellar space, starting from SiO molecules can occur via two processes. (1) Chemically driven compositional separation in (SiO)$_{n}$ motifs resulting in oxygen rich and oxygen poor regions, and (2) reaction between Si$_{n}$O$_{m}$ clusters leading to oxygen richer and poorer fragments. Theoretically calculated optical and infrared spectra of Si$_{n}$O$_{m}$ clusters exhibit features observed in the extended red emissions and blue luminescence from interstellar medium indicating that the Si$_{n}$O$_{m}$ fragments could be contributing to these spectra.   

Cage Clusters of Gold and Tin: Golden Buckyballs and Stannaspherene

Photoelectron spectroscopy (PES) yields direct electronic structure information for size-selected clusters. Combining PES with theoretical calculations has become an effective approach to obtain structural information for small and medium-sized clusters. We present recent discoveries of two classes of cage clusters in gold and tin. Negatively charged gold clusters (Au$_{n}^{-})$ have been shown to exhibit a remarkable structural diversity from 2D structures for $n$ = 4-12 and the pyramidal structure for $n$ = 20. Using PES and DFT calculations, we have found that gold clusters with $n$ = 16-18 possess unprecedented hollow cage structures. We have been able to successfully dope a variety of transition-metal atoms into the empty spaces in the golden cages, confirming their structural robustness, as well as demonstrating chemical tuning of their electronic, magnetic, and catalytic properties. Unlike carbon, the heavier congeners of the group 14 elements are not known to form hollow cage structures similar to the fullerenes. In PES studies of tin clusters, we noted that the spectrum of Sn$_{12}^{-}$ is distinctly different from that of its neighbors or its Si/Ge counterpart. This observation led to our discovery of a highly symmetric and stable icosahedral Sn$_{12}^{2-}$ cage, for which we coined a name ``stannaspherene'' to describe its high symmetry and spherical pi bonding. We have also shown that all transition metals including the f-block elements can be doped inside Sn$_{12}^{2-}$ to form a whole class of endohedral stannaspherenes, which may be used as potential building blocks for new cluster-assembled materials. In a preliminary experiment to synthesize stannaspherene in the bulk, a new cluster, Pd$_{2}$@Sn$_{18}^{4-}$, was crystallized and characterized, suggesting all stannaspherene and endohedral stannasphernes may be fabricated in the bulk under suitable conditions.   

Cooperative effect between electronic and geometric structures in binary clusters of superatoms

The fabrication of cluster-assembled materials is dependent upon finding a suitable building block for a cluster that is chemically stable and that interacts weakly with other clusters of the same material. For designing the characteristics of clusters, binary systems are very important to create functionality of materials, and application of hetero-atom doping has been undertaken in the present study to two prototypes: metallic aluminum (Al) clusters and covalent silicon (Si) clusters. In particular, efforts to examine the superatom behavior involved in electronic and geometric shell closings have focused on substituting the central atom in Al$_{12}$X and MSi$_{16}$. Binary aluminum and silicon clusters of Al$_{12}$X and MSi$_{16}$ were generated by a double laser vaporization method, and the electronic and geometric stabilities for the resulting hetero-atom encapsulated clusters were examined experimentally. For aluminum-based binary superatoms of Al$_{12}$X, both experimental and theoretical results show that Al$_{12}$Si has high ionization energy and low electron affinity, and Al$_{12}$P has low ionization energy, both with the icosahedral structure having a central Si or P atom, revealing that Al$_{12}$Si and Al$_{12}$P exhibit rare-gas-like and alkali superatoms, respectively. Experiments confirmed the possibility that the change in the total number of valence electrons on substitution could produce ionically bound binary superatom complexes; the binary cluster salts (Al$_{12}$P)$^{+}$F$^{-}$ and (Al$_{12}$B)$^{-}$Cs$^{+}$. For silicon-based binary superatoms of MSi$_{16}$, on the other hand, results obtained by mass spectrometry, anion photoelectron spectroscopy, and adsorption reactivity towards H$_{2}$O show that the neutral cluster doped with a group-4 atom features an electronic and a geometric closing at n=16. The MSi$_{16}$ cluster with a group-4 atom undergoes an electronic change in (i) the number of valence electrons when the metal atom is substituted by the neighboring metals with a group-3 or -5 atom, and in (ii) atomic radii with the substitution of the same group elements of Zr and Hf. The reactivity of a halogen atom with the MSi$_{16}$ clusters reveals that VSi$_{16}$F forms a superatom complex with ionic bonding.   

Stability and Magnetic Coupling between Mn doped Stannaspherenes

The discovery of carbon fullerenes has stimulated considerable interest in the search of cage clusters involving not only group IV elements but also metallic systems. The recent discovery of stannaspherene, a highly stable Sn$_{12}^{2-}$ hollow cage cluster with a reported diameter of 6.1 {\AA} has triggered a renewed interest in the search for stable endohedral cage complexes with 3$d$ transition metal atoms as dopants. It is anticipated that these complexes may carry large magnetic moments and can be used as building blocks for novel magnetic materials. We report the first density functional theory based study of the interaction between two Mn doped stannaspherenes (Mn@Sn$_{12})$ and show that Mn@Sn$_{12}$ is not only highly stable and carry a magnetic moment of 5 $\mu _{B}$, but the clusters retain their structural identity even when they interact with each other. Equally important, the magnetic coupling depends strongly on the orientation of the clusters. We believe that these new results would encourage the scientific community to explore the possibility of synthesizing novel magnetic materials with magnetic element doped Sn$_{12}$ clusters as building blocks.   

Structural and electronic properties of chiral gold nanoclusters

Chiral structures had been found as the most stable isomers of bare and thiolate-passivated gold nanoparticles of several sizes, from density functional calculations (DFT). These results provided theoretical support for the existence of chirality in metal clusters, suggested by the intense optical activity measured from the metal-based electronic transitions of size-separated glutathione-passivated gold nanoclusters, and more recently, of penicillamine-passivated gold clusters with metal core mean diameters of 0.57, 1.1, and 1.75 nm. A further structural analysis using the Hausdorff chirality measure, as well as, a semiclassical calculation of the circular dichroism spectrum, has confirmed the existence of chirality in Au nanoparticles. In this work, the structural and electronic properties of chiral Au nanoclusters are studied by using global optimization methods combined with semiempirical many body potentials and first principles density functional calculations. In particular, we study the Au$_{34}$ cluster. For this system there exists experimental evidence on the energetic stability of a chiral structure with $C_{3}$ symmetry. Our calculations theoretically confirm these results, providing further evidence on the existence of chiral gold nanoclusters.