Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials II |
Hide Abstracts |
Sponsoring Units: DCP Chair: Ulises Reveles, Virginia Commonwealth University Room: Morial Convention Center 213 |
Monday, March 10, 2008 2:30PM - 3:06PM |
D21.00001: From Superatoms to Cluster Assembled Materials Invited Speaker: A collaborative effort with the theoretical group of S.N. Khanna at VCU has led to the concept of superatoms comprised of clusters which mimic elements of the periodic table. The latest advances will be presented which support the contention that there should be no limitation in developing a 3-D periodic table based on this idea. As the behavior of clusters can be controlled by size and composition, the superatoms offer the potential to create unique compounds with tailored properties. One of the prime objectives of current research is to lay the foundation for forming new nanoscale materials utilizing these ``elements'' as the building blocks. This is viewed as one of the most promising frontiers in materials research. The current status of success in this endeavor will be discussed. AWC, Jr. gratefully acknowledges the United States Air Force Office of Scientific Research, Grant {\#}FA9550-07-1-0151, the U. S. Department of Energy, Grant No. DE-FG02-02ER46009, and the U.S. Department of the Army through a MURI Grant {\#}W911NF-06-1-0280, for financial support of the experimental work reported herein. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D21.00002: From Designer Clusters to Synthetic Crystalline Nano-Assemblies Meichun Qian, S. Khanna, A. Reber, A. Castleman, A. Sen, A. Ugrinov, K. Davis, S. Peppernick, M. Merritt Clusters have the potential to serve as building blocks of materials, enabling the tailoring of materials with novel properties. We have recently proposed a new protocol that combines gas phase investigations to examine feasible units, theoretical investigations of energy landscapes to identify potential motifs, and synthetic chemical approaches to synthesize cluster assemblies. We had earlier applied the protocol to As$_{7}^{3-}$ based cluster assemblies. In this work, we extend our investigations to cluster assembled materials based on As$_{11}^{3-}$ units as building blocks. By varying the alkali cation and introducing crypts, it is possible to form materials with arsenic clusters arranged to form one dimensional chain, two dimensional layers or three dimensional lattices and X-ray studies provide information on bond lengths etc. Theoretical studies have been carried out to examine their microscopic structure and electronic properties. It will be shown that these new compounds have the tunable electronic and optical properties. The theoretical predictions on the As$_{11}^{3-}$Crypt(K)$_{3}$ and [As$_{11}$Cs$_{2}$]$^{1-}$Crypt(K) are in good agreement with the experimental observations. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D21.00003: Geometries and stabilities of Ag-doped Si$_{n}$ (n =1 - 13) clusters: a first-principles study Feng-chuan Chuang, Yun-Yi Hsieh, Chih-Chiang Hsu, Marvin Albao The structures of AgSi$_{n}$ (n = 1 - 13) clusters are investigated using first-principles calculations. Our studies suggest that AgSi$_{n}$ clusters with n = 7, and 10 are relatively stable isomers and that these clusters prefer to be exohedral rather than endohedral. Moreover, doping leaves the inner core structure of the clusters largely intact. Additionally, the plot of fragmentation energies as a function of silicon atoms shows that the AgSi$_{n} $ are favored to dissociate into one Ag atom and Si$_{n}$ clusters. Alternative pathways exist for n $>$ 7 (except n = 11) in which the Ag-Si cluster dissociate into a stable Si$_{7}$ and a smaller fragment AgSi$_{n-7}$. The AgSi$_{11}$ cluster dissociates into a stable Si$_{10}$ and a small fragment AgSi. Lastly, our analysis indicate that doping of Ag atom significantly decreases the gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for n $>$ 7. Reference: J. Chem. Phys. 127, 144313 (2007). [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D21.00004: Transition Metal Doped and Encapsulated ZnO Cages Marcela R. Beltran, Arthur Reber, Shiv N. Khanna ZnO is a wide band gap semiconductor with potentail for applications. We had earlier shown that Zn$_{12}$O$_{12}$, Zn$_{15}$O$_{15}$, Zn$_{18}$O$_{18}$, and Zn$_{21}$O$_{21}$ clusters are particularly stable and exhibit cage structures. In this work, we examine the possibility of making magnetic materials by either substituting Zn atoms by transition metal atoms or by encapsulating transition metal clusters inside the cages. Our studies are carried out within a density functional framework employing gradient corrected functionals. The studies cover all the 3d elements and examine the strength of the magnetic moment as well as the nature of the coupling between the local moments. We also present results on the coupling between the endohedral transition metal cluster and the transition metal atoms substituting for Zn atoms within the cage. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D21.00005: DFT study of a carotenoid-porphyrin-C$_{60}$ light-harvesting molecular triad Invited Speaker: The abundance of solar energy reaching the earth presents an attractive alternative energy source. Nearly 75\% of the solar energy striking the upper atmosphere reaches the earth in the form of photons of energies typically higher than 1 eV. Biological light-harvesting systems are highly efficient in utilizing the solar radiation. Bio-mimetic molecules are investigated to mimic the photosynthesis process efficiently in laboratory. We present a computational study of the process in a bio-mimetic carotenoid-porphyrin-C$_{60}$ molecular triad which is about 5 nm long. The description of the photo-induced charge separation process requires accurate excited state energies and coupling between electrons and the phonons of the system. Since charge-transfer excitations create large changes in a molecular dipole moment, changes in excited-state energies due to coupling between a ~5 nm molecular photovoltaic and the surroundings (solvent and spectator molecules) also has to be taken into account. A density functional theory based method including all these effects to describe the photo-induced charge separation process will be presented. ~ [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D21.00006: Spin Accommodation and Reactivity of Superatoms. Invited Speaker: We have recently discovered novel effects that may allow tuning of the reactivity of small clusters by controlling their spin excitation, electronic structure, and local geometry. These findings offer the prospect of designing novel catalysts through cluster assemblies where chosen clusters, called superatoms, serve as elemental building blocks. Taking aluminum as an example, I will present our recent findings that illustrate how reactive clusters can be made non-reactive while inert species can be made reactive by adding hydrogen atoms. These findings offer a microscopic understanding of the recent experimental reactivity studies on aluminum and aluminum-hydrogen clusters that show variable reactivity in even electron systems and rapid etching in odd electron systems. It is shown that the reactivity of even electron clusters is governed by a spin transfer, from the triplet oxygen to the cluster, that fills the spin down antibonding orbitals on oxygen. Theoretical investigations show that when the spin transfer cannot occur, the species is unreactive, and when spin accommodation is possible, more subtle effects appear. Secondly, I will examine the reactivity of aluminum clusters with simple nucleophiles such as water. The reactivity and nature of the ensuing products is wildly variable with the size and shape of the cluster. Again, the electronic structure and local coordination of the active sites allow for an understanding of changing barrier heights and resulting reactivity. This work provides a framework with which new catalysts may be designed. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D21.00007: Peierls distortion of endohedral atoms in clathrate I Hidekazu Tomono, Kazuo Tsumuraya The guest atom displacements in type II clathrates have been reported on experimental and theoretical points of view. The displacements are reported to be 0.6 {\AA} from the cage center of the Si$_{28}$ cage to the hexagonal in the hydrogen terminated double caged Si$_{28}$ cluster [1]. The distortion can be expected to occur in the type I clathrate which forms with bamboo structures in the x, y, and z directions. The guest atoms show Peiels distortion when we calculate the equilibrium distances between the two Na atoms which locate at the neighboring Si$_{24}$ cages in the bamboo structure using periodic density functional calculation. The binding energy between the guest atoms is $-0.10$ eV/Na$_{2}$. We also confirm the tendency of the Peierls distotion from the force directions of guest atoms in the double unit cells that contain four Na atoms in one dimension; We will propose the cohesion mechanism of the clathrates that the clathrates are precipitated states of the connecting endohedral atoms in the 14 group atoms. So are the hydroclathrates in which the guest molecules bind each other with chains. [1] H. Takenaka and K. Tsumuraya, Mater. Trans. 47, 63 (2006). [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D21.00008: [Te$_{2}$As$_{2}$]$^{2-}$: A Planar Motif with Potential for Ferromagnetism Shiv Khanna, Arthur Reber, Meichun Qian, Angel Ugrinov, Ayusman Sen Here we report the synthesis and crystal structure of [K(18-crown-6)]$_{2}$[Te$_{2}$As$_{2}$], the first four-membered ring Zintl anion of elements from groups XV and XVI, isolated from an ethylenediamine solution of As, K, and As$_{2}$Te$_{3}$ at room temperature. X-ray analysis indicates that the [Te$_{2}$As$_{2}$]$^{2- }$anion has an unexpected planar rhombic structure with alternating bonds. First-principles electronic structure investigations within the density functional framework, indicate that the Te$_{2}$As$_{2}$K$_{2}$ motif possesses a triplet ground state where the spin configuration leads to a distortion of the square geometry into rhombus structure marked by two Te-As shorter bond length pairs joined by longer bond lengths. A NICS analysis reveals that the triplet motif has a net aromatic character. Supercell calculations on the periodic solid show that the spin moments on the individual motifs order ferromagnetically thus offering the potential of an aromatic ferromagnet made of traditionally non-magnetic elements. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700