Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y10: Focus Session: Optical Properties of Nanostructures VI: Artificially Structured Materials |
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Sponsoring Units: DMP Chair: Stefano Cabrini, Molecular Foundry, Lawrence Berkeley National Laboratory Room: 304 |
Friday, March 20, 2009 8:00AM - 8:36AM |
Y10.00001: Green's Functions in Nanoscience Invited Speaker: Theoretical nanoscience is a fast growing area in physics. It gains momentum with the recent advances in related areas such as nanodevice design, synthesis and characterization of novel nanostructures, nanoscale imaging and spectroscopy. Several techniques common to quantum chemistry and condensed matter physics have been applied successfully to the modeling of nanoscale structures. In particular, methods based on many-body Green's functions (MBGF) are becoming more and more popular. One of the reasons for this success is that these methods have predicted several phenomena at nanoscale, for example the peculiar dimensional confinement of excitons in nanostructures. Further advances in quantum transport and exciton dynamics can be foreseen. Moreover, algorithms tailored to confined systems have made calculations of Green's functions in nanostructures much more manageable [1]. With those algorithms, we were able to investigate the properties of correlated excitations in clusters of semiconductors (CdSe and silicon)[1,2]. We were also able to explain the properties of electronic excitations in fullerenes and other organic compounds [1,3]. This talk will present an overview of the current stage of MBGF techniques, discuss the various approximations that have been proposed, and review recent advances. \\[4pt] References: \\[0pt] [1] M. L. Tiago and J. R. Chelikowsky, Phys. Rev. B 73, 205334 (2006) \\[0pt] [2] M. L. Del Puerto, M. L. Tiago and J. R. Chelikowsky, Phys. Rev. Lett.97, 096401 (2006). \\[0pt] [3] M. L. Tiago, P. R. C. Kent, R. Q. Hood, and F. A. Reboredo, J. Chem. Phys. 129, 084311 (2008). [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y10.00002: First Principles Absorption Spectra of Cu$_n$ ($n=1-10$) Clusters Kopinjol Baishya, Juan C. Idrobo, Serdar Ogut, Mingli Yang, Koblar A. Jackson, Julius Jellinek First principles optical absorption spectra, obtained within time-dependent density functional theory, for the ground state and low-energy isomers of Cu$_n$ ($n = 1- 10$) are presented. Overall our theoretical results exhibit good agreement with available experimental data. We analyze the orbital character of the optical excitations as a function of size and energy. Compared to noble metal clusters of Ag and Au in the same size range,\footnote {J. C. Idrobo {\em et al.} Phys. Rev. B {\bf 76}, 205422 (2007); K. Baishya {\em et al.} {\em ibid.} {\bf 78}, 075439 (2008).} we find that Cu clusters have much higher d-electron contribution to low-energy optical excitations. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y10.00003: ABSTRACT HAS BEEN MOVED TO S1.00259 |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y10.00004: Calculations of second harmonic generation by periodically-structured surfaces William Schaich We have done finite-difference time-domain (fdtd) calculations of second harmonic generation (shg) from various surface structures. The periodic structures include pairs of metal strips, split-ring resonators, and fishnets. The nonlinear response is described by a set of parameters that multiply products of linear field components. These fields and the resultant second harmonic radiated fields are found in separate fdtd calculations. The shg spectra do not always correlate well with that seen in linear response. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y10.00005: Optical properties of the Folic Acid/APTMS/TiO$_{2}$ nanosystems Volodymyr Turkowski, Michael Leuenberger, Talat Rahman, Duy Le, Sudipta Seal, Sanku Mallik, Andre Gesquiere Our photoluminescence experiments on folic acid (FA) conjugated nanoparticles of ${\rm TiO_{2}}$, ${\rm CeO_{2}}$ and ${\rm SiO_{2}}$ show great promise for a variety of optoelectronic applications for these materials, in particular in the field of modern molecular photoelectronic devices, since they demonstrate a dramatic increase of the photoemission intensity at wavelengths between 500 to 700 nm when the nanoparticles are coated with the 3-aminopropyltrimethoxylane (APTMS) linker/spacer molecule. We report here results of accompanying time-dependent density- functional theory studies of the FA/APTMS/${\rm TiO_{2}}$ nanosystems by using the B3LYP exchange-correlation potential. We demonstrate that the large increase of the photoemission is due to enhanced optical transitions which involve the intermediate energy levels related to the APTMS states. We present details of the geometric and electronic structure and excited states of our nanosystems and their dependence on the characteristics of the nanoparticle. We discuss possible optoelectronic applications for this effect. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y10.00006: Optical Properties and Aging of Gasochromic WO$_{3}$ Rudresh Ghosh, Matthew B. Baker, Rene Lopez WO$_{3}$ as a possible optical gas sensor has gained increasing importance with H$_{2}$ becoming a major fuel of the future. This has led to efforts to understand the theoretical and practical aspects of the gasochromic behavior of WO$_{3}$. WO$_{3}$ films were fabricated using pulsed laser deposition (PLD). Morphological and stoichiometric ratios of films obtained were observed as functions of deposition parameters. We present the optical constants induced by 2{\%} H$_{2}$:Ar in WO$_{3}$ films. This allows us to obtain the limits of the gasochromic change in comparison to ion injection. It was found using Langmuir's adsorption equation that at low H$_{2}$ concentrations a high sensitivity is predicted but the coloration could saturate at 57.9 {\%} of the material's maximum ion adsorption. Poisoning of the films was also addressed by coating with a permeable polydimethylsiloxane layer. It is shown that gasochromic degradation is prevented thus eliminating common atmospheric gases as possible contaminants. Our studies suggest WO$_{3}$ thin films as highly sensitive and stable optical hydrogen sensors. . [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y10.00007: {\it Ab initio} Bethe-Salpeter Equation approach for aperiodic materials and core-excitations H.M. Lawler, J. Vinson, J.J. Rehr, E.L. Shirley We have recently developed an interface dubbed AI2NBSE between the Bethe-Salpeter optical spectroscopy code NBSE developed at NIST and the {\it ab initio} electronic structure code ABINIT [1]. This interface facilitates first-principles calculations of valence-band dielectric response including excitonic effects in insulating crystals. Here we report on the extension of this interface for calculations of 1) dielectric response in complex nano-scale, disordered, and molecular systems, and 2) core-level UV and x-ray response. For the treatment of complex systems, we discuss issues of cell selection, basis size, and the treatment of the screened electron- hole interaction. For the core level response, we address various strategies including explicit treatments of core and semi-core states with plane waves and the PAW representation. *Supported by DOE Grant DE-FG03-97ER45623. [1] H. M. Lawler, J. J. Rehr, F. Vila, S. D. Dalosto, E. L. Shirley and Z. H. Levine, Phys. Rev. B {\bf 78}, 205108 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y10.00008: Franz-Keldysh effect in the interband optical absorption of quantum wires Harold Spector, Congxin Xia We present a theoretical calculation of the effect of an electric field applied either parallel or perpendicular to the axis of a rectangular quantum wire on the interband optical absorption. We find that the application of the electric field decreases the optical absorption coefficient for both the parallel and perpendicular to the axis electric field configurations. The absorption is greater when the electric field is along the direction of carrier confinement than when it is along the axis of the wire. This difference is due to the effect of the field on the overlap of the electron and hole wave functions. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y10.00009: Surface Enhanced Raman Spectroscopy (SERS) of pyridine on Pt Qingzhen Hao, Lasse Jensen, Peter Eklund SERS studies were carried out on vertically oriented Pt cylinders patterned on quartz substrates via e-beam lithography. Optical absorption indicates that Localized Surface Plasmon Resonance (LSPR) of the Pt cylinders in the UV region, around 300nm. Discrete Dipole Approximation (DDA) simulation was performed to confirm the position of the substrate's LSPR and also to map the electric field distribution inside and at the surface of the Pt, which allows us to estimate the Electromagnetic enhancement factor (EF). Experimentally, we demonstrate that the total SERS EF is about 5$\times $10$^{4}$ using 514.5nm excitation (far away from the LSPR resonance). Using time-dependent density functional theory we have calculated the off-resonance chemical SERS enhancements of pyridine interacting with small Pt clusters. Our results show that the enhancements are much larger than results obtained for small Ag clusters. We are currently exploring the enhancements for different Pt cluster sizes as well as the importance of charge-transfer excitations. These results will provide detailed insights into the mechanism responsible for the chemical enhancement in SERS. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y10.00010: The Effect of Roughened Metallic Films on Colloidal Quantum Dot Energy Transfer Christopher Ferri, Somnath Ghosh, Brent Rich, Michelle Khine, Sayantani Ghosh We investigate self-organized, roughened metallic surfaces as a platform for enhanced energy transfer between colloidal Cadmium Selenide (CdSe) quantum dots (QD). Pre-stressed thermoplastic substrates are sputter coated with gold palladium (AuPd) to create thin films. When heated, due to differing coefficients of thermal expansion of the plastic and metal, the AuPd film buckles to form micro- to nano-meter sized structures. QDs deposited on these self-organized metallic structures exhibit changes in their static and dynamic optical characteristics, which include spectral red-shift and multiple recombination decay rates. These observations can be attributed to a combination of enhanced electronic coupling between close-packed QDs and plasmonic coupling between the QD and metallic structures. We then leverage these properties to fabricate controlled, directional structures using this self-organized method which can be utilized as biochemical sensors. [Preview Abstract] |
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