Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X37: Spectroscopy and Dynamics of Single Molecules and Nanoparticles |
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Sponsoring Units: DCP Chair: Will Castleman, Pennsylvania State University Room: 409 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X37.00001: Reactivity of Aluminum and Doped Aluminum Clusters Arthur Reber, Shiv Khanna, Patrick Roach, William Woodward, A. Welford Castleman Jr. We examine the reactivity of aluminum cluster anions with water, and other nucleophiles. The clusters reveal size dependent reactivity which primarily results in either the chemisorption of one or more water molecules, or no observable reactivity. The reactivity of the clusters is found to be dependent on surface sites which facilitate the splitting of the water on the surface of the cluster. Clusters with two sets of active sites are found to selectively release molecular Hydrogen. We also investigated the reactivity of aluminum clusters doped with another metal and their reactivity with molecular Oxygen. As the reactivity of aluminum clusters with oxygen depends on the electronic shell closing, this serves as a probe of the effects of doping on the electronic shell structure. The results reveal variable effects of doping on the electronic structure depending on the precise geometry and electron count. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X37.00002: Multiple Isomers in the Photoelectron Spectra of NbC$_{n}$ clusters Ivan Iordanov, Jorge Sofo We calculate the photoelectron spectrum of small NbC$_{n}$ clusters (2$<$n$<$7), to identify the atomic structure that best matches experimental photoelectron spectra. We use Density Functional Theory calculations to find all stable isomers. In order to obtain more accurate spectra, we use the Symmetry Adapted Cluster Configuration Interaction method for the smaller clusters where the highly correlated niobium d-orbitals dominate the electronic structure. The most stable isomer configurations are linear and cyclic structures, with the cyclic being the ground state for all but NbC$_{6}$. To fully explain all experimental observations we are required to use the combined spectra of both ring and linear structures. This means that both structures must be present in the cluster beam, even in cases where the higher energy isomer is up to 0.5 eV higher than the ground state. This surprising result is confirmed by both DFT and Configuration Interaction. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X37.00003: Photoelectron Velocity Map Imaging and Density-Functional Investigation of Bismuth and Lead Anions Solvated in Ammonia Kristen Casalenuovo, Mohamed A. Sobhy, J. Ulises Reveles, Ujjwal Gupta, Shiv N. Khanna, A.W. Castleman, Jr. We present the results of photoelectron velocity map imaging experiments for the photodetachment of small negatively charged Bi$_{n}$ and Pb$_{n}$ (n = 1-2) clusters solvated in ammonia using a Nd:YLF 527 nm laser. We report the vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters derived from the photoelectron images. Density-functional theory calculations with generalized gradient approximation for the exchange-correlation potential were performed on these clusters to determine their molecular and electronic structures. Calculated ammonia binding energies and electronic charge transfers are used to rationalize the observed mass spectra distributions. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X37.00004: Dielectrophoresis and Dissociation of Micelles in AC-Electric Fields Victoria Froude, Yingxi Elaine Zhu Dielectrophoresis (DEP) of natural and synthetic colloids has been explored as a new route to rapidly manipulate and assemble colloidal particles in suspensions. Most work has been done with micron to submicron sized particles, yet AC-polarization and dielectrophoretic effects on molecules and nanocolloids remain little understood. In this work, we examine the dynamic responses of micelles to applied AC-electric fields by using fluorescence correlation spectroscopy (FCS) at a single-molecule resolution. We focus on the AC-field induced transport of sodium dodecyl sulfate (SDS) micelles tagged with various fluorescent probes and molecular encapsulates. Micelle concentration and DEP mobility of SDS micelles are examined over a broad range of AC-field frequency (1 KHz -10 MHz) and amplitude (100mV -20V). We observe a strong AC-frequency dependence of micelle concentration, from which the DEP crossover frequency switching between the positive and negative DEP response is determined. Surprisingly, we also observe the AC-field induced dissociation of the micelle structure and the resultant release of fluorescent encapsulates at a characteristic low AC-field frequency of 1-10 kHz; the dissociation frequency can be tuned by encapsulated molecules with a strong dependence of their surface conductivity, which could have a potential application for controlled drug release by AC-electric fields. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X37.00005: Raman Correlation Spectroscopy Maki Nishida, Edward Van Keuren We have developed a simple method for measurement of diffusion coefficients of specific components in complex mixtures of nanoparticles in a suspension. As a variation of Dynamic Light Scattering (DLS), this method analyzes temporal fluctuations of Raman scattered light from particles caused by Brownian motion. Due to the coherent nature of Raman scattering, the time autocorrelation functions of Raman emission lines will yield similar information as that obtained by DLS. Because each Raman emission line arises from a specific type of a molecular bond, only the diffusion coefficient of the particles containing that specific chemical species is measured. We demonstrate that this method can isolate diffusion coefficients from individual components in mutlicomponent nanoparticle dispersions. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X37.00006: Nanosphere Templating Through Controlled Evaporation: A High Throughput Method For Building SERS Substrates Kristen Alexander, Meredith Hampton, Rene Lopez, Joseph DeSimone When a pair of noble metal nanoparticles are brought close together, the plasmonic properties of the pair (known as a ``dimer'') give rise to intense electric field enhancements in the interstitial gap. These fields present a simple yet exquisitely sensitive system for performing single molecule surface-enhanced Raman spectroscopy (SM-SERS). Problems associated with current fabrication methods of SERS-active substrates include reproducibility issues, high cost of production and low throughput. In this study, we present a novel method for the high throughput fabrication of high quality SERS substrates. Using a polymer templating technique followed by the placement of thiolated nanoparticles through meniscus force deposition, we are able to fabricate large arrays of identical, uniformly spaced dimers in a quick, reproducible manner. Subsequent theoretical and experimental studies have confirmed the strong dependence of the SERS enhancement on both substrate geometry (e.g. dimer size, shape and gap size) and the polarization of the excitation source. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X37.00007: Resolving Single Molecule Dynamics with a Point-Functionalized Single-Walled Carbon Nanotube Danny W. Wan, Issa S. Moody, Brett R. Goldsmith, John G. Coroneus, Gregory A. Weiss, Philip G. Collins Outside of fluorescence measurements, there are currently few means of observing characteristic time constants of individual molecules. We describe the development of a single molecule technique utilizing a point-functionalized SWCNT electronic circuit [1]. Time-dependent components of the SWCNT conductance reveal real-time interactions between a covalently attached protein and the immediate electrolytic environment. We will demonstrate electronic transduction of protein-substrate interactions with single molecule resolution. On-line analysis based on normalization of the power spectrum helps to enhance the resulting signals, even to the extent of providing the user with real time feedback regarding the experiment status. [1] B. Goldsmith et al, Science 315 77 (2007) [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X37.00008: Auger Rate Quenching in Nanocrystals George E. Cragg, Xiaoyong Wang, Megan A. Hahn, Sara Maccagnano-Zacher, John Silcox, Todd D. Krauss, Alexander L. Efros Single nanocrystal (NC) photoluminescence (PL) blinking is believed to arise from a photoionization event, resulting in a charged NC state made dark by the dominance of the non-radiative Auger rate. Suppression of the Auger rate has been suggested as the underlying mechanism for the non-blinking PL observed in soft-confinement, single CdZnSe/ZnSe NCs. To probe the interplay between the confinement geometry and the PL, we employ a coupled, two-band NC model which is analyzed with numerical routines based on the propagation matrix formalism. The results obtained will verify whether smooth confining potentials mitigate the Auger process, thereby eliminating blinking by allowing NCs to photoluminesce even in their charged state. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X37.00009: Dynamical charge and structural strain in MoS2 and MnO nanoparticles Qi Sun, Xiaoshan Xu, Janice Musfeldt, Reshef Tenne, Alla Zak, Sheila Baker, Andrew Christianson We measured the far infrared vibrational properties of bulk and nanoscale MoS$_2$ and MnO in order to investigate finite length scale effects and chemical bonding in these materials. From an analysis of frequencies, oscillator strengths, and the high frequency dielectric constants, we extract Born and local effective charges for both materials. In the intralayer direction of MoS$_2$, we find that the Born effective charge of the nanoparticles is decreased significantly compared to the layered bulk, a result that we attribute to structural strain (and resulting change in polarizability). Preliminary results on the 7 nm MnO nanoparticles suggest that structural strain impacts both polarizability and depolarization field. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X37.00010: Ultrafast electronic energy redistribution in hollow gold nanoparticles. Kenneth Knappenberger, Adam Schwartzberg Nanostructured materials offer great potential for novel ways to generate, utilize, store and transport energy. These unique opportunities arise because nanoclusters often portray strikingly different chemical and physical properties than their bulk counterparts, and, perhaps more intriguingly, these vary widely with cluster size and shape. Here we report on the redistribution of electronic energy to thermal phonons in a series of hollow gold nanoparticles using femtosecond transient absorption. Qualitatively, the relaxation processes are similar to those of solid nanoparticles, however distinct differences are observed, likely owing to the unique properties of the hollow structures. In particular, a larger excitation power density is required to observe coherent vibrational oscillations in hollow gold nanoparticles than is needed for solid particles following electronic excitation. This effect is systematically studied over a range of hollow and solid particles, including multiple diameters and wall thicknesses. Models will be presented to account for the different relaxation mechanism observed for hollow and solid gold nanoparticles. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X37.00011: Optical spectra with spin-orbit effects on gold nanostructures Aldo Romero, Alberto Castro, Miguel Marques, Micael Oliveira, Angel Rubio The quest for more efficient optoelectronic devices requires a thorough understanding of the intrinsic properties of the metallic nanostructures such as the optical spectra. Many optoelectronic devices are based upon gold nanostructures but even though, there is a large set of experimental studies, little is known theoretically. Between the concerns, it is important to identify where the spin-orbit effect has influences on the optical spectra on those nanostructural materials. We report here the analysis of the effect of the spin-orbit interaction on the shape of the photoabsorption cross section of small gold clusters (Au$_n$ $n \le 8$ and $n=20$) and small nanowires ($n \le 7$). As it is shown, the spin-orbit coupling has a strong effect on the absortion spectra mainly for nanowires and much less effect on static properties such as the dipole static polarizability. This has strong implications on transport calculations where no spin-orbit effects are considered. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X37.00012: First-principles studies of surface-enhanced Raman scattering: Benzene thiol on Au Alexey Zayak, Jeffrey Neaton First-principles calculations based on density functional theory are used to investigate how chemisorption of organic molecules on metal surfaces affects their Raman spectra. Experiments have long reported Raman intensity enhancements of many orders of magnitude for molecules on rough metal surfaces or near nanofabricated metallic tips. The goal of this work is to explore ``chemical'' effects that may contribute to this enhancement, specifically hybridization and charge transfer between the molecule and its metallic substrate. We consider benzene thiol chemisorbed on extended Au(111) surfaces and finite Au and Ag clusters. Using a finite-difference scheme, we compute the absorption site, molecular orientation, and coverage dependence of Raman-active phonon modes and their intensities. We also examine how the electronic structure of the molecule is modified in each case, and discuss implications for the strength of Raman processes. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X37.00013: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X37.00014: Photoluminescence anti-blinking of single CdZnSe/ZnSe nanocrystals Xiaoyong Wang, Xiaofan Ren, Keith Kahen, Megan Hahn, Manju Rajeswaran, Sara Maccagnano-Zacher, John Silcox, George Cragg, Alexander Efros, Todd Krauss We have synthesized soft-confinement nanocrystals (NCs) of CdZnSe/ZnSe that on the single particle level exhibit complete elimination of photoluminescence (PL) blinking. These continuously emitting NCs have a very short PL decay lifetime of 5 ns. Moreover, single CdZnSe/ZnSe PL spectra are highly unusual with three distinct peaks. These unique and remarkable optical properties are collectively explained by the radiative recombination of a trion due to suppressed Auger processes in a NC. Upon deformation of the soft-confinement potential, the PL intensity of a single CdZnSe/ZnSe NC switches between two bright states, but still never turns off. Possible mechanisms for this PL anti-blinking behavior will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X37.00015: Single-Electron Spectroscopy of Quantum Dots using Vertically Self-aligned Electrode Structure. Ramkumar Subramanian, Pradeep Bhadrachalam, Vishva Ray, Seong Jin Koh We demonstrate single-electron tunneling spectroscopy of individual quantum dots using new vertical electrode structure, where the source and drain electrodes are vertically self-aligned and separated by a thin dielectric spacer. A quantum dot placed on the periphery between the source and the drain electrodes forms a double barrier tunnel junction, allowing for single-electron spectroscopy measurements. CMOS compatible fabrication allows many quantum dot units to be fabricated in parallel processing. This technique not only provides an accurate electronic structure of a ``single'' quantum dot, but such measurement can be made for many of individual quantum dot units fabricated in a single batch process. Thus, this simple procedure provides accurate energy level measurement of ``single'' quantum dots over the entire quantum dot population. The band gap (E$_{g})$, charging energy (E$_{c})$ and energy level spacing ($\Delta $E) were measured directly from the current-voltage and differential conductance spectra for colloidal CdSe quantum dots ($\sim $6.5nm). The band gap was measured to be E$_{g} \quad \sim $1.75-1.85eV, charging energy E$_{c} \quad \sim $60meV and the `s' to `p' level separation ($\Delta $E ) was measured to be $\sim $60-100meV. (Supported by NSF CAREER (ECS-0449958), ONR (N00014-05-1-0030), and THECB ARP (003656-0014-2006)). [Preview Abstract] |
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