Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G10: Focus Sessions: Physical Chemistry of Nanoscale System IV |
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Sponsoring Units: DCP Chair: Jin Zhang, University of California, Santa Cruz Room: Baltimore Convention Center 302 |
Tuesday, March 14, 2006 8:00AM - 8:36AM |
G10.00001: Nanowire-based solar cell Invited Speaker: Excitonic solar cells -- including organic, hybrid organic-inorganic and dye-sensitised cells (DSCs) -- are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient and stable excitonic photocell. Central to this device is a thick nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. However, nanoparticle DSCs rely on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths. Here we introduce a new version of the dye-sensitised cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode is synthesized via mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 2.5{\%} is demonstrated, limited primarily by the surface area of the nanowire array. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G10.00002: Structural and Conductance Change in Hydridosilsesquioxane Based Silicon Oxide Clusters Induced by an Electric Field Saroj Nayak, Philip Shemella Interesting results arise from the addition of a hydrogen atom to a hydridosilsesquioxane (HSQ) Si$_{8}$O$_{12}$H$_{8}$ cluster, and have been studied with density functional theory techniques. Three stable minima have been found (open, cluster, center), and energy barriers between each have been computed. Each state has different values in the HOMO-LUMO\footnote{Highest Occupied Molecular Orbital, Lowest Unoccupied Molecular Orbital} energy gap, which is usually a metric for conductance. Simulating the electronic interaction with an applied electric field, we model the reactions and compute the energy barriers with and without an applied electric field. Asymmetric states with strong dipole moments will have the most energetic effects in an applied field, based on the electric field-dipole energy relation, $U= - \textbf{p} \bullet \textbf{E}$. \\ \\ We propose that the field forces the open structure to become energetically unstable, resulting in an overall loss in structures found in the open configuration and loss of the highly conducting properties. We have also modeled the critical energies points in implicit solvent, with results similar to gas phase calculations. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G10.00003: Designing Self-assembled Nanostructures: Metal-Organic Coordination Networks at Surfaces Steven Tait, Nian Lin, Sebastian Stepanow, Klaus Kern Networks consisting of organic molecules and isolated metal atoms have been demonstrated to self-assemble at surfaces. The components of the networks are designed to assemble in a desired pattern, forming periodic arrays of isolated metal atoms (or dimers) separated by organic molecule ligands. This `bottom-up' formation of a nanometer-scale structure opens a wide range of questions related to the properties of the components of the networks: metal atom nodes, molecule ligands, and nanopores within the networks. Our group explores these properties, including the magnetic and catalytic properties of the isolated metal atoms and adsorptive properties of the nanopores within the networks. Some properties can be `tuned' by rational design of the organic molecule size, structure, and functionality. The ability to tailor the size and functionality of nanometer-scale arrays produced by self assembly represents a fantastic opportunity for molecular recognition, heterogeneous catalysis, and other fields. We present here recent results from our group, including the use of ligands containg pyridyl groups and recent adsorption measurements on the networks. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G10.00004: Physical Chemistry of TiO$_2$ functionalized with oxo-manganese catalysts Sabas G. Abuabara, Clyde W. Cady, Jim M. Schleicher, Jason Baxter, Gary W. Brudvig, Robert H. Crabtree, Charles A. Schmuttenmaer, Victor S. Batista We describe the development and application of dye-sensitized semiconductors to heterogeneous photocatalysis. \textit{Ab initio}-DFT electronic structure calculations and molecular dynamics simulations combined with quantum dynamics propagation of transient electronic excitations indicate that a surface complex consisting of a catalytic Mn oxo complex adsorbed onto a TiO$_2$ substrate via a catechol-substituted terpyridine ligand can be activated by photoinduced subpicosecond interfacial electron transfer. Experimental realization of the Mn oxo surface complex, achieved by a novel sequential synthesis technique, is briefly described and computational results supporting the spectroscopic characterization of the nanoscale assembly are presented. Studying the photocatalytic reaction dynamics of these uniquely functionalized semiconductor materials offers the prospect of gaining unprecedented control over a wide range of contrathermodynamic reactions. Furthermore, such biomimetic materials capable of splitting water or fixating CO$_2$ could provide viable solutions to problems ranging from current energy concerns to reducing atmospheric greenhouse gases. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:48AM |
G10.00005: Silver and Gold Nanostructures: Engineering their Optical Properties for Biomedical Applications. Invited Speaker: We have focused on shape-controlled synthesis of silver and gold nanostructures. While the synthetic methodology mainly involves solution-phase redox chemistry, we have been working diligently to understand the complex physics behind the simple chemistry -- that is, the nucleation and growth mechanisms leading to the formation of nanostructures with a specific shape. Polyol synthesis of silver nanostructures provides a good example to illustrate this concept. We discovered that the shape of silver nanostructures were dictated by both the crystallinity and shape of nanocrystallite seeds, which were, in turn, controlled by factors such as reduction rate, oxidative etching, and surface capping. We also exploited the galvanic replacement reaction between silver and chloroauric acid to transform silver nanocubes into gold nanocages with controlled void size, wall thickness, and wall porosity. We were able to engineer the optical properties of resulting gold nanocages with optical resonance peaks ranging from the blue (400 nm) to the near infrared (1200 nm) simply by controlling the molar ratio of silver to chloroauric acid. Thanks to their exceptionally large scattering and absorption coefficients in the transparent window for soft tissues, this novel class of gold nanostructures has great potential emerging as both a contrast agent for optical imaging in early-stage tumor detection, and a therapeutic agent for photothermal cancer treatment. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G10.00006: Aligned Silver Nanorod Array as SERS Substrates for Viral Sensing Yiping Zhao, Saratchandra Shanmukh, Stephen B. Chaney, Les Jones, Richard A. Dluhy, Ralph A. Tripp The aligned silver nanorod array substrates prepared by the oblique angle deposition method are capable of providing extremely high enhancement factors ($\sim $10$^{9})$ at near-infrared wavelengths (785 nm) for a standard reporter molecule 1,2 trans-(bis)pyridyl-ethene (BPE). The enhancement factor depends strongly on the length of the Ag nanorods, the substrate coating, as well as the polarization of the excitation laser beam. With the current optimum structure, we demonstrate that the detection limit for BPE can be lower than 0.1 fM. The applicability of this substrate to the detection of bioagents has been investigated by looking several viruses, such as Adenovirus, HIV, Rhinovirus and Respiratory Syncytial Virus (RSV), at low quantities ($\sim $0.5uL). Different viruses have different fingerprint Raman spectrum. The detection of virus presented in infected cells has also been demonstrated. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G10.00007: Colloidal Platinum Nanoparticles: Synthesis, Structure and Properties. Daniel Katz, Vikas Murali, Rebecca Isseroff, Yuan Sun, Yimei Zhu, Vladimir Samuilov, Nadine Pernodet, Miriam Rafailovich, Jonathan Sokolov Interest in nanoparticles has risen in recent years as unique properties are being obtained from otherwise ordinary materials by reducing their sizes to molecular dimensions. We synthesized colloidal platinum nanoparticles with sizes of 1$\sim $8 nm through four novel chemical methods and investigated their properties. These particles had an electrical resistivity of 1$\sim $4 $\Omega \cdot $m and also exhibited a ferromagnetic moment. When the particles were exposed to hydrogen we measured a 15{\%} increase in their mass, indicating a high level of hydrogen absorption. The lattice constant was measured using HRTEM and was found to be the same as bulk Pt, even after H$_{2}$ exposure, indicating no lattice distortion occurred. When dermal fibroblasts were exposed to the particles, the particles disrupted cellular actin, structure, and function. Practical applications include the use of Pt particles in semiconductor chips, hydrogen storage in fuel cells and particle chemotherapy targeted against cancer cells. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G10.00008: Self organized growth of regular arrays of Pd-Au bimetallic nanoclusters on nanostructured alumina films Claude R. Henry, Guido Hamm, Conrad Becker Pd-Au clusters (1-4 nm)were grown under UHV on nanostructured alumina films in a two-step process. The alumina films, prepared by high temperature oxidation of a Ni3Al (111) surface, present an hexagonal array of defects, separated by 4.1 nm, which is a template for the growth of the bimetallic clusters. Pd and Au atoms are condensed sequentially on the alumina film, under UHV. The maximum density of metal clusters is fixed by the density of defects. The mean size of the clusters is controlled by the total amount of the deposited metals, while their composition is controlled by the ratio of the deposited amounts of the two metals. The nucleation and growth of the bimetallic clusters are followed in situ by STM. These arrays of bimetallic clusters are used as model catalysts for CO oxidation studied by molecular beam methods. The regular spatial organisation of the clusters allows the study and the correction of the reverse spillover effect of the CO during the reaction. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G10.00009: Reduction of nickel upon annealing of nickel zinc ferrite nanoparticles S. Calvin, S.A. Morrison, M.D. Shultz, E.E. Carpenter, R. Swaminathan, M.E. McHenry Nickel zinc ferrite (NZFO) nanoparticles were synthesized by two methods: radio-frequency plasma torch and reverse micellar syntheses. Upon annealing, x-ray absorption spectra of both sets of samples provided definitive evidence of a progressive reduction of nickel to an fcc metallic form. X-ray diffraction, however, does not show clear peaks corresponding to fcc nickel, suggesting the metallic crystallites are very small. In the case of the plasma torch samples, annealing under air for one hour at 900\r{ }C is sufficient to reduce nearly all of the nickel metal. Testing is still underway for the reverse micellar samples, but in at least one case approximately 80{\%} nickel reduction was achieved by annealing under nitrogen for one hour at 500\r{ }C. To elucidate the mechanism of this reduction, a comparative study of annealing under air, nitrogen, and argon is underway, as well as thermogravimetric analysis. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G10.00010: Insulator -- polaron conductor -- metal transitions in a complex oxide 12CaO$\cdot $7Al$_{2}$O$_{3}$. P. Sushko, A. Shluger, A. Stoneham, K. Hayashi, S. Matsuishi, M. Hirano, H. Hosono Recent experiments have demonstrated that a complex nano-porous oxide 12CaO$\cdot $7Al$_{2}$O$_{3}$ (C12A7) built of positively charged sub-nanometer cages can be converted from a transparent insulator to a transparent conductor by H$_{2}$ doping followed by UV-light irradiation with 4--4.5 eV photons [1]. This irradiation induces optical absorption bands with maxima at 0.4 eV and 2.8 eV and high concentrations of unpaired electrons. We use ab initio calculations to reveal the mechanism of photo-induced insulator--conductor transition and the role of H atoms in this process and to elucidate the transport properties of the electrons in this system as a function of electron concentration. Our theoretical modeling suggests that at concentration below 10$^{20}$ cm$^{-3}$ electrons are responsible for the polaron type electrical conductivity with the activation energy close to 0.1 eV as well as for the optical absorption at 0.4 eV and 2.8 eV [2]. We demonstrate that, as the electron concentration exceeds 10$^{20}$ cm$^{-3}$, the character of electronic conductivity changes from polaron type to metallic. [1] K. Hayashi et al., Nature \textbf{419}, 462 (2002). [2] P.V. Sushko, et al., Phys. Rev. Lett. \textbf{91}, 126401 (2003); P.V. Sushko et al., Appl. Phys. Lett. \textbf{86}, 092101 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G10.00011: Electronic control inside a molecule : towards single molecule devices Mathieu Lastapis, Yurie Fukuma, John Boland The chimerical single molecule engineering has been proven to be accessible through the use of scanning tunnelling microscopy (STM) [1]. In this field, one particularly attractive area is the study of single molecules adsorbed on semiconductor surfaces. It has been recently demonstrated that a spatial fine control of the molecular dynamics is possible through the use of tunnelling current [2]. In order to improve the electronic control of a single molecule, we are currently investigating a promising system: CaF2 on Si(111). This system has been extensively studied as a model system to deposit insulator on silicon. Here we are using this system to electronically decouple the molecule from the substrate. I will present LT STM experiments on atomically thick CaF islands on Si(111). The measured electronic properties of these islands demonstrate their potential as ideal templates to study single molecules. Finally I will present some preliminary results on N-HBC [3] adsorbed on a CaF island. \newline \newline [1] G. Binnig and H. Rohrer, ``In touch with atoms'', Rev. Mod. Phys. 71, S324-S330 (1999) \newline [2] M. Lastapis et al, Science, 308, 1000 (2005) \newline [3] S.Draper et al, JACS, 126, 8694 (2004) [Preview Abstract] |
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