Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A10: Focus Session: Physical Chemistry of Nanoscale Systems I |
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Sponsoring Units: DCP Chair: Tim Lian, Emory University; Ken Kuno, University of Notre Dame Room: Baltimore Convention Center 302 |
Monday, March 13, 2006 8:00AM - 8:36AM |
A10.00001: “Fluorescence Kinetics in Nanoscale Systems” Invited Speaker: |
Monday, March 13, 2006 8:36AM - 8:48AM |
A10.00002: Magneto-optical spectroscopy of single CdSe nanocrystal quantum dots H. Htoon, V. I. Klimov, M. Furis, S. A. Crooker Understanding the spin structure of excitons within nanocrystal quantum dots (NCs) is important on both fundamental grounds and also for technological spintronic applications. While single-NC photoluminescence (PL) studies have become routine, there are no reports of spin-resolved magneto-PL studies with single-NC sensitivity. To this end, we measure low temperature (4 K), polarization resolved, magneto-PL of individual CdSe NCs to 5 Tesla. We clearly observe an energy splitting between left- and right-circularly polarized PL peaks and a strong degree of circular polarization in a subset ($<$10{\%}) of the studied NCs. We attribute this effect to a Zeeman splitting of spin-up and spin-down excitons in those NCs having wurzite-c axes aligned parallel to the applied magnetic field. Our data reveal that 5T Zeeman splittings vary widely from one NC to another, occasionally reaching values in excess of 2 meV. We compare the results from many single nanocrystals with magneto-PL measurements of NC ensembles. [Preview Abstract] |
Monday, March 13, 2006 8:48AM - 9:00AM |
A10.00003: Optical properties of current carrying molecular wires Michael Galperin, Abraham Nitzan We consider several fundamental optical phenomena involving single molecules in biased metal-molecule-metal junctions. The molecule is represented by its highest occupied and lowest unoccupied orbitals, and the analysis involves simultaneous consideration of three coupled fluxes: the electronic current through the molecule, energy flow between the molecule and electron-hole excitations in the leads, and the incident and/or emitted photon flux. Using a unified theoretical approach based on the non-equilibrium Green function method we derive expressions for an absorption lineshape (not an observable but a useful reference for considering yields of other optical processes) and for the current induced molecular emission in such junctions. We find that current driven molecular emission and resonant light induced electronic currents in single molecule junctions can be of observable magnitude under appropriate realizable conditions. In particular, light induced current should be observed in junctions involving molecular bridges that are characterized by strong charge transfer optical transitions. For observing current induced molecular emission we find that in addition to the familiar need to control the damping of molecular excitations into the metal substrate the phenomenon is also sensitive to the way in which the potential bias is distributed on the junction. [Preview Abstract] |
Monday, March 13, 2006 9:00AM - 9:12AM |
A10.00004: Observation of Synchronous Photoluminescence Intensity Fluctuations within Single CdSe Quantum Wires John Glennon, Rui Tang, William Buhro, Richard Loomis The intensity of the photoluminescence (PL) within single colloidal CdSe quantum wires (QWs) is observed to synchronously fluctuate along the entire length of the wire in time. Statistical analysis of `on' and `off' events within the QWs indicates a power-law temporal dependence nearly identical to that seen in colloidal CdSe quantum dots (QDs). The low PL quantum yields (0.2-2{\%}) of ensemble samples of CdSe QWs suspended in solution are placed in perspective by considering the large inhomogeneity observed in the PL intensities of different QWs. Estimates of the quantum yields for the `on' events in CdSe QWs are similar to those of CdSe QDs. We also report on the observation of coordinated PL intensity fluctuations in multiple QWs that lie in contact with each other, a surprising entity dubbed a quantum network. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:24AM |
A10.00005: Plasmons in nearly touching metallic nanoparticles: singular response in the limit of touching dimers Isabel Romero, Javier Aizpurua, Garnett W. Bryant, F. Javier Garcia de Abajo The response of gold nanoparticle dimers is studied near and beyond the limit where the particles are touching. As the particles approach each other, a dominant dipole feature is observed that is pushed into the infrared due interparticle coupling and that is associated with a large pileup of induced charge in the gap region. Lower-frequency, higher-order modes are observed in near touching dimers. After touching, singular behavior is observed through the emergence of infrared absorption peaks, accompanied by huge charge pileup at the nanoparticles’ junction region. These results explain recent experiments on metallic nanoparticle dimers and are relevant in the design of nanoparticle-based sensors and plasmon circuits. [Preview Abstract] |
Monday, March 13, 2006 9:24AM - 9:36AM |
A10.00006: Surface-Enhanced Raman Scattering Based on Novel Metal Nanostructures (Aggregates, Nanorods, and Nanoshells) Jin Zhang, Adam Schwartzberg, Leo Seballos, Tammy Oshiro, Chad Talley, Rebecca Sutphen, Yiping Zhao Rational design and study of new surface enhanced Raman scattering (SERS) substrates is key to advancing chemical and biological sensing. The next generation of biological probes will ideally be single, small, SERS active nanostructures able to penetrate the inner workings of cells. To this end, we have developed various metal nanostructures based on aggregates, nanorods, and nanoshells with the goal to optimize their SERS activities. We have very recently demonstrated SERS from single, hollow nanostructures. Exceptional sample homogeneity leads to a nearly tenfold increase in signal consistency over standard silver substrates. At 30 nm in diameter, this is the smallest confirmed single SERS active particle ever reported, representing a major step in advancing sensing technology based on SERS. In the meantime, we have applied some of the substrates developed in detection of cancer biomarkers and have achieved high sensitivity and molecular selectivity. The results have shown that SERS is extremely promising for chemical and biological sensing and imaging applications. To control the structure of the nanomaterials and thereby their optical absorption as well as SERS properties is critical for these emerging technological applications. [Preview Abstract] |
Monday, March 13, 2006 9:36AM - 10:12AM |
A10.00007: Nanoparticle Optics: New Materials, Concepts, and Characterization Methods Invited Speaker: Nanoparticle Optics is a materials driven subject. The unifying theme in this lecture will be the fabrication of size and shape-tunable, metal nanoparticles using nanosphere lithography (NSL), electron beam lithography (EBL), and chemical synthetic methods. Size and shape tunability leads to an exquisite degree of control over the magnitude and spatial extent of the surface electromagnetic fields that surround optically excited nanoparticles. In turn, this has enabled fundamental new insights into the electromagnetic (EM) field enhancement mechanism underlying both localized surface plasmon resonance (LSPR) spectroscopy and surface enhanced Raman spectroscopy (SERS). This lecture will focus on three topics: (1) LSPR spectroscopy and its application to the development of nanoscale optical biosensors for the study fundamental biological recognition events; (2) Dark-field Rayleigh scattering spectroscopy is used to show that diffractively narrowed plasmon bands can be produced in columnar arrays of Ag nanoparticles, fabricated by EBL, that are spaced by approximately the single particle plasmon wavelength; and (3) the relationship between the LSPR spectrum of Ag nanoparticles and the wavelength-scanned excitation spectra for both surface-enhanced Raman spectroscopy (WS SERES) and second harmonic generation (WS SHGES) is discussed. [Preview Abstract] |
Monday, March 13, 2006 10:12AM - 10:24AM |
A10.00008: Dielectric confinement effects on the emission lineshipe of single semiconductor nanocrystals Daniel Gomez, Joel van Embden, Paul Mulvaney We have explored the influence of different matrices on the photoluminescence line shape of individual CdSe core shell nanocrystals (NCs) at room temperature using confocal microscopy / spectroscopy. The results obtained corroborate previous observations of a correlation between blinking events and spectral diffusion but in addition, we have found that the extent of spectral diffusion is almost independent of the dielectric environment of the NC, thus suggesting that ionization and charge reorganization events in the matrix are not responsible for the observed continuous spectral shifts. We also observed that the emission line width is correlated with the emission peak position and that the correlation coefficient between these two variables is a function of the dielectric constant of the matrix surrounding the NC. These results are analyzed in terms of dynamic rearrangements of charges trapped at the surface of the nanocrystal. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 10:36AM |
A10.00009: Semi-empirical Study of a Multi-associated Rotaxane Ki-Ho Lee, Karl Sohlberg Rotaxanes are prototype molecular devices based on two components; one or more ring molecules threaded by a dumbbell-shaped, shaft molecule. In a switchable rotaxane, the shaft has two or more sites strongly attracting the ring with different binding intensity, and translation of the ring between the sites may be induced by switching the relative binding intensity. For amine binding sites, for example, this switching may be accomplished by the protonation of the amine to form a cationic ammonium site. In this study, semi-empirical (AM1) electronic structure calculations have been carried out for a multi-ring, multi-shaft rotaxane. Each of three rings is threaded by a shaft and the three shafts are chemically bonded to each other, limiting the number of degrees of freedom in co-conformations of the entire complex. Each of the three shafts contains one bipyridinium site and one amine site. The latter can be switched to an ammonium site by protonation to induce translation of the associated shaft. We investigate concerted versus stepwise protonation of the amine sites. [Preview Abstract] |
Monday, March 13, 2006 10:36AM - 10:48AM |
A10.00010: Unoccupied electronic structure of and CO Chemisorption on ultrathin Ni films Hua Yao, A.G Danese, R.A Bartynski The Ni/Cu(100) system has drawn considerable of attention in recent years because of its importance in both fundamental research and technological applications. Whereas many ultrathin metal systems are dominated by quantum size effects (QSEs), for epitaxial Ni films on Cu(100) there is an intermingling of QSEs, electronic hybridization, and surface/interface effects, making it complicated and challenging to obtain a microscopic picture of this bimetal system. We have performed a series of inverse photoemission (IPE) studies of the unoccupied electronic structure of the Ni/Cu(100) and CO/Ni/Cu(100) systems as a function of Ni thickness. IPE spectra from Ni films exhibit very rich structures. A Phase Accumulation Model calculation suggests only one of the three main features is consistent with metallic quantum well (MQW) State in Ni film. CO adsorption strongly modifies the spectrum by dramatically suppressing one of the main features indicating that this feature is a Ni surface resonance. Furthermore, by comparing spectra from Ni/Cu(100) with results from Cu/Ni/Cu(100), we suggest the third feature is a state confined to the Ni/Cu interface. Since the electronic structure of these films changes as a function of film thickness, it provides a very interesting opportunity to investigate how different electronic state can modify the chemisorption properties of Ni/Cu(100). We have used temperature programmed desorption (TPD) to investigate the bonding between CO and Ni in Ni/Cu(100) system. [Preview Abstract] |
Monday, March 13, 2006 10:48AM - 11:00AM |
A10.00011: Are mechanochemistry and thermal chemistry equal?: probing by nanonewton forces. Martin Konopka, Ivan Stich, Dominik Marx Chemical reactions can be triggered by different energies, the most common being the thermal energy. Despite the fact that mechanochemistry, where mechanical energy is used instead of the thermal energy was long known, its practical use and impact was limited. Recently advances in experimental techniques, such as atomic force microscopy, fully opened the intriguing possibility to use mechanical energy as a tool for chemical reactions driven by mechanical energy. We use technologically important systems, short--chain (ethyl) thiolated copper clusters and surfaces to investigate the differences between mechano and thermal chemistry. This is an important and opened question as mechanochemistry and thermal chemistry are rarely applied to the same system. Quantum mechanics simulations based on density functional theory indicate that the two chemical reactions are vastly different. While thermal chemistry affects selectively the thiolate-carbon bond, mechanochemistry leaves that bond intact and leads to metal-metal bond breaking processes and creation of unusual high- energy structures nonexistent in the nature. [Preview Abstract] |
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