Session D34: Focus Session: Nano I -- Clusters

Cluster Structure and Reactions: Gaining Insights into Catalytic Processes

To many researchers outside the field of cluster science it comes as a surprise that much can be learned of its relevance to catalysis, even restricting the discussion to ionized systems. The objective of this talk is to present how fundamental insights into reaction mechanisms can be gained through employing alternative approaches that complement rather than supersede more conventional methods in the field of catalysis. In view of the well acknowledged role of defect centers in effecting reactivity, and the preponderance of recent papers presenting evidence of the importance of charged sites, the desire to conduct repetitive experiments is clear. Presented herein are approaches using clusters to accomplish this in order to unravel fundamental catalytic reaction mechanisms, and to use identified superatoms and the concepts of element mimics to tailor catalysts with desired functionality.   

Photoelectron Angular Distributions of Transition Metal Dioxide Anions - a joint experimental and theoretical study

Angular-resolved photoelectron spectroscopy (PES) studies of the MO$_{2}$- (M=Ti, Zr, Hf, Co, Rh) clusters are presented for the first time along with theoretical calculations of their properties. We confirm previously reported non-angular PES results for the vertical detachment energies (VDE), vibrational energies and geometric structures of these clusters and further explore the effect of the 'lanthanide contraction' on the MO$_{2}$- clusters by comparing the electronic spectra of 4d and 5d transition metal dioxides. Angular-resolved PES provides the angular momentum contributions to the HOMO of these clusters and we use theoretical calculations to examine the HOMO and compare to our experimental results. First-principles calculations are done using both density functional theory (DFT) and the coupled-cluster, singles, doubles and triples (CCSD(T)) methods.   

Stability and Spectroscopic Properties of Singly and Doubly Charged Anions

Considerable interest currently exists in expanding the pool of hetero-atomic negative ions which have electron affinities much higher than that of halogen atoms. These molecules, called superhalogens, usually consist of a metal atom (M) at the core surrounded by halogen atoms (X) and are represented by the formula MX$_{n+1}$ where n is the maximal valence of the metal atom. We have studied the possibility if pseudohalogens such as CN, which mimic the chemistry of halogen atoms, can be used as building blocks of new superhalogens. Using density functional theory, Moller-Plessett perturbation theory and coupled clusters methods we have studied systematically the structure and spectroscopic properties of M(CN)$_{n }$systems (M=Na, Mg, Al; n=1-3 for Na, 1-4 for Mg, and 1-5 for Al) and compared the results to that of corresponding MCl$_{n}$ clusters. We find that there is a significant difference between these two systems. This is because pseudohalogens have a tendency to dimerize and hence, we find that for these clusters the values of adiabatic detachment energy and the electron affinity may not be the same. Also, we have studied the dianions of M(CN)$_{n}$ and MCl$_{n}$ complexes to determine the critical size required for their stability. We show that CN moieties stabilize a dianion better than halogen atoms do due to the increase in the phase space over which added electrons are delocalized. This could play an important role in interpreting future experimental data on M(CN)$_{n}$ complexes.   

Structure and stability of the M$_{8-n}$N$_n$C$_{12}$ (M=Ti, Zr; N=Sc, Y and n=1,2,3) Met-Cars as building blocks of cluster-assembled materials

Clusters can be used as building blocks for new materials. However, in order to form a bulk material with clusters, they should be chemically stable. This stability can be characterized by a closed-shell electronic configuration having a large HOMO-LUMO gap. Met-Cars, metal-carbon species composed of early transition metals bonded to carbon, are stable but very reactive. We propose a method to lower their reactivity by metal atom substitution with lower atomic number atoms. We report DFT results on M$_{8-n}$N$_n$C$_{12}$ (M = Ti, Zr; N = Sc, Y, and n=1,2,3) Met-Cars in the neutral, cationic and anionic charge states. Our results show that the isoelectronic M$_6$N$_2$C$_{12}$, M$_5$N$_3$C$_{12}^-$ and M$_7$N$_1$C$_{12}^+$ Met-Cars have closed-shell electronic configurations and larger HOMO-LUMO gaps (1.0-1.7 eV) than that of the M$_8$C$_{12}$. The intercluster interaction between two isolated neutral M$_6$N$_2$C$_{12}$ Met-Cars is relatively weak compared to the M$_8$C$_{12}$ dimers. Due to the weak interaction of the isolated neutral Met-Cars, their unique properties would be retained during assembly.   

Interplay of geometric and electronic structure in metalloid gallium clusters

Over the last two decades, the so-called ``renaissance of main group chemistry'' has led to significant advances in the synthesis, isolation and characterization of metalloid gallium clusters. What distinguishes these from other metalloid species (e.g. ligand-protected gold, silver, palladium, etc.) is their structural diversity, with the existence of four different Ga$_{22}$ frameworks being a particularly striking example. To gain more insight into this polymorphism, we carried out electronic structure calculations using density functional theory. Our calculations verify that two of the ligand-protected Ga$_{22}$ isomers can to some degree be viewed as superatom complexes - their respective metalloid cores are more or less close-packed, roughly spherical, and exhibit a well-defined electronic shell structure with a completely filled outer-most shell. The other two frameworks contain a slightly distorted icosahedral Ga$_{12}$ core without a central atom - an unusual arrangement for metals - and the underlying electronic structure is more complex. This talk will serve as a summary of our calculations and illustrate the interplay of geometric and electronic structure in metalloid gallium clusters.   

Cluster Structure Selection Based on High Vertical Electron Affinity: The Case of TiO$_{2}$ Clusters

We study the structure and electronic properties of (TiO$_{2})_{2-10}$ clusters using basin hopping based on density functional theory (DFT), combined with many-body perturbation theory in the GW approximation. We show that in photoemission experiments performed on anions the isomers with the high electron affinity are selectively observed rather than those with the lowest energy. These isomers possess a highly reactive Ti$^{3+}$ site. The selectivity for highly reactive clusters may be exploited for applications in catalysis.   

Growing Ag Clusters in Superfluid He Droplets

Here we report on the growth and study of Ag clusters, ranging in size from 10 to 10$^{7}$ atoms, in a beam of superfluid He droplets. The droplets were used to capture Ag atoms from a hot oven, which then recombined in the interior of the droplet at sub-Kelvin temperature. The structure of the obtained Ag clusters was studied in situ via laser spectroscopy of their plasmon resonance. Furthermore, the clusters were surface-deposited and studied via transmission electron microscopy. The images have provided for a measure of the cluster flux and size distribution, which is in good agreement with an estimate based on the energy balance of Ag cluster growth in He droplets. The images also reveal an astounding change in shape of the deposited clusters to elongated and track-shaped with increased droplet size. This is ascribed to the formation of vortices within the He droplets whose cores are traced by the Ag atoms and clusters. The possible formation mechanism of the vortices and their stability will also be discussed.   

Atomic and Electronic Structure of Ag$_{n}$ ( n $\le $ 13 ) Clusters and their Reactivity with O$_{2 }$

First principles theoretical studies on the atomic structure, stability, and electronic structure of neutral and anionic Ag$_{n}$ clusters have been carried out within a gradient corrected density functional approach. It is shown that the clusters are marked by planar or layered structures. For most clusters, the ground state of anions has lowest spin multiplicity. To examine the reactivity of clusters, containing even number of electrons, with O$_{2}$, we calculated the spin excitation energy representing the energy required to excite the cluster to the triplet configuration. It is shown that several of these even electron anionic species and in particular Ag$_{13}^{-}$ have high spin excitation energy indicating that they should be inert towards reactivity with oxygen. The theoretical predictions are shown to be in agreement with preliminary experimental data on the reactivity of anionic species.   

On the Reactivity of Al$_{13}$I$_{n}^{-}$ and Al$_{14}$I$_{n}^{-}$ Clusters with Methanol

Al$_{13}$ and Al$_{14}$ cluster anions act as halogen or alkaline earth superatoms respectively when bound by I atoms. Al$_{13}$I$_{2}^{-}$ and Al$_{14}$I$_{3}^{-}$ have enhanced resistance to oxidation by oxygen because of the clusters' closed electronic shells, however the reactivity of aluminum clusters with methanol depends on the presence of complementary active sites. We have examined the reactivity of Al$_{13}$I$_{n}^{-}$ and Al$_{14}$I$_{m}^{-}$ with methanol to identify if the presence of electronegative Iodine may induce active sites on the cluster. The presence of a single Iodine atom on Al$_{13}^{-}$ is insufficient to activate the cluster, however two adjacent ligands induce an active site and makes the cluster highly reactive. The Al$_{14}$I$_{m}^{-}$ clusters are found to be reactive with methanol highlighting the importance of geometric shell closures in ligand protected clusters.   

Bonding and Electronic Structure of Cluster Assemblies with Metal Carbonyls

Understanding the factors controlling the band gap energies of cluster-assembled materials is an important step towards creating nano-assemblies with tailored properties. To this end, we have investigated the band gap energies of cluster assemblies involving arsenic clusters bound to metal carbonyl charge-transfer complexes, [As$_{7}$M(CO)$_{3}$]$^{3-}$ M = Cr, Mo, W. The binding of a single charge-transfer complex is shown to have a small effect on the band gap energy as the arsenic lone pair orbital and metal carbonyl orbitals are closely aligned in energy, resulting in a gap similar to the original cluster. The band gap energy is also found to be insensitive to the architecture of the assembled material. In the case where two charge-transfer complexes are bound to the cluster, the bottom of the conduction band is shown to be localized on a solvent molecule bound to the metal carbonyl.   

Stability and Shell Magnetism in Transition Metal Doped Calcium clusters

Clusters of many metallic elements are known to exhibit enhanced stability at valence electron counts 2, 8, 18, 20, 34, 40{\ldots} that can be understood within a simple spherical confined nearly free electron gas model. In this work we show a magnetic species whose stability is rationalized on a modification of the above shell sequence through deformations of the spherical geometry and through enhanced exchange splitting of the electronic shells via impurity atoms with large atomic orbital exchange splitting. Through first principles theoretical studies of the electronic structure and stability of TMCa$_{8}$ (TM= Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) clusters we identify a stable magnetic FeCa$_{8}$ cluster of 24 valence electrons distributed into a closed 1S2 1P6 1D10 2S2 shell sequence of 20 paired electrons and with 4 electrons occupying the majority 2D$_{xy}$2D$_{x -y}^{2 2}$, 2D$_{xz}$ and 2D$_{yz}$ levels while the unfilled 2D$_{z}^{2}$ level is separated by a large energy gap of 0.61 eV arising from atomic deformation.   

On the stability and oxidation of Pdn (n=1-7) clusters on rutile TiO2(110)

First principles theoretical studies of the atomic and electronic structure of Pd$_n$ (n=1-7) clusters supported on a TiO$_2$(110) surface, and O$_2$ activation by such clusters, have been carried out within a gradient corrected density functional approach. It is shown that the supported Pd$_n$ cluster geometries are driven by competing effects including intra-cluster interactions favoring compact geometries and cluster support interactions that favor geometries that flatten out in the TiO$_2$(110) surface channel. When exposed to O$_2$, a single Pd atom only activates the O-O bond while all other clusters energetically favor a broken O-O bond. The differing behavior of the Pd atom is proposed to originate from the minimal amount of charge transferred from Pd to O$_2$ and its spin excitation energy. For Pd$_n$O$_2$ (n=2-7), it is shown that while the first O is adsorbed on the Pd$_n$ cluster, the second O occupies a site above a lattice Ti site at the Pd-Ti interface and is indicative of spill over O atoms. The theoretical finding are compared with recent experiments on the structure and oxidation of CO by supported clusters in the presence of O$_2$.   

Strengthening of Au-Au bonds in small gold clusters by adsorbing noble gases

In state-of-the-art experiments for the vibrational spectra of metal clusters in the gas phase, photodissociation spectroscopy is performed on clusters complexed with noble gas (RG) atoms, where a RG atom is usually expected to form a weak van der Waals bond. By employing DFT (PBE functional with selected comparisons to PBE0, and to MP2 and CCSD(T) calculations), we surprisingly find a partially covalent bond of {\em neutral} dimers with RG. For RG = Ar, Kr, Xe one or two RG atoms can bind in a linear molecule with Au$_2$. While both Hirschfeld and Mulliken analyses show a small electron transfer from the RG to Au$_2$, the Au-Au bond {\em shortens} and the Au-Au stretch frequency increases. This is inconsistent with the expected effect of electron transfer to the antibonding orbital of the dimer. Electron-density ($n$) differences between the bonded systems and the isolated fragments show an accumulation of $n$ between RG and the neighboring Au atom, and between the gold atoms. The analysis of the projected density of states reveals that, although only non-bonding orbital interactions and no charge transfer occurs between RG and Au$_2$, the $d$-electrons of Au$_2$ are redistributed due to the interaction with RG in such a way that the Au-Au $\sigma_s$ bond is strengthened.