Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T2: Electron, Exciton and Phonon Interactions in Nanoparticles |
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Sponsoring Units: DCMP DMP Chair: Phillippe Guyot-Sionnest, University of Chicago Room: Spirit of Pittsburgh Ballrom BC |
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T2.00001: Mapping surface plasmons on a single metallic nanoparticle Invited Speaker: |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T2.00002: Controlled Crystallinity and Fundamental Coupling Interactions in Nanocrystals Invited Speaker: Metal and semiconductor nanocrystals show many unusual properties and functionalities, and can serve as model system to explore fundamental quantum and classical coupling interactions as well as building blocks of many practical applications. However, because of their small size, these nanoparticles typically exhibit different crystalline properties as compared with their bulk counterpart, and controlling crystallinity (and structural defects) within nanoparticles has posed significant technical challenges. In this talk, I will firstly apply silver metal nanoparticles as an example and present a novel chemical synthetic technique to achieve unprecedented crystallinity control at the nanoscale. This engineering of nanocrystallinity enables manipulation of intrinsic chemical functionalities, physical properties as well as nano-device performance [1]. For example, I will highlight that electron- phonon coupling constant can be significantly reduced by about four times and elastic modulus is increased $\sim $40{\%} in perfect single crystalline silver nanoparticles as compared with those in disordered twinned nanoparticles. One important application of metal nanoparticles is nanoscale sensors. I will thus demonstrate that performance of nanoparticles based molecular sensing devices can be optimized with three times improvement of \textit{figure}-\textit{of}-\textit{merit} if perfect single crystalline nanoparticles are applied. Lastly, I will present our related studies on semiconductor nanocrystals as well as their hybrid heterostructures. These discussions should offer important implications for our understanding of the fundamental properties at nanoscale and potential applications of metal nanoparticles. \\[4pt] [1] Yun Tang and Min Ouyang, Nature Materials, 6, 754, 2007. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T2.00003: New Developments in Nanocrystal Lasing: Type-II Nanostructures and ``Giant'' Quantum Dots Invited Speaker: Nanocrystal (NC) quantum dots show high photoluminescence quantum yields and size-dependent emission colors tunable through the quantum-confinement effect. Despite their favorable light-emitting properties, NCs are difficult to use in optical amplification. Because of almost exact balance between absorption and stimulated emission in nanoparticles excited with single excitons, optical gain can only occur due to NCs that contain at least two excitons. A resulting complication is fast optical-gain decay induced by nonradiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. In this talk, I will discuss two approaches for resolving the problem of ultrafast Auger recombination in NCs. In one approach, we utilize core/shell hetero-NCs engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a large, $\sim$100 meV transient Stark shift of the absorption spectrum with respect to the luminescence band. This effect breaks the exact balance between absorption and stimulated emission and allows us to demonstrate optical amplification in the single-exciton regime when Auger recombination is simply inactive. In another approach, we use recently developed ``giant'' dots that comprise a small emitting CdSe core overcoated with a thick shell (up to 20 monolayers) of a wider-gap CdS. These nanostructures produce a peculiar quasi-type-II localization regime, which develops as a result of a significant difference in effective volumes of the electron and hole wave functions. These structures show greatly suppressed Auger recombination, which allows us to realize broadband optical gain (extends over $>400$ meV) due to multiexcitons of various orders with an excitation threshold, which is at least a thousand times lower than in regular CdSe NCs. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T2.00004: Plasmon-enhanced Absorption, Modulation and Spontaneous Emission in Semiconductor Quantum Dots and Films Invited Speaker: Recent advances in plasmon dispersion and localization in quantum dot layers and semiconductor thin films have enabled study of several phenomena in coupled metal/semiconductor nanophotonic structures, including i) enhanced spontaneous emission in quantum dots, ii) all-optical modulation of plasmon propagation in quantum dot active media, and iii) enhanced absorption in plasmonic solar cells. Metal-dielectric plasmon waveguides with quantum dot active layers can serve as switching elements when the complex refractive index is actively modulated. We demonstrate all-plasmonic modulation in which the complex refractive index seen by a surface plasmon polariton at an infrared free-space wavelength of 1420 nm is modulated via interband excitation of the quantum dots at a visible wavelength of 514 nm. Metallic nanostructures can excite surface plasmons which can dramatically increase the optical path length in thin active photovoltaic layers to enhance overall photoabsorption, with potential for increased photovoltaic conversion efficiency, and new solar cell device designs. The strong mode localization of surface plasmon polaritons at metal-dielectric interfaces leads to strong absorption in very thin semiconductor films, enabling a dramatic (10-100X) reduction in the semiconductor absorber physical thickness needed to achieve optical thickness. Modal analysis in full wave simulation allows us to determine the fraction of power absorbed for both dielectric waveguide and plasmonic modes in a thin solar cell. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T2.00005: Exciton-plasmon interactions and energy transfer in nanoparticles Invited Speaker: Energy transfer between optically-excited nanocrystals coupled by the Coulomb interaction can be very efficient. The interaction of excitons and plasmons in nanocrystals leads to several effects: energy transfer between nanoparticles (NPs), electromagnetic enhancement, reduced exciton diffusion in nanowires (NWs), exciton energy shifts, and interference and non-linear phenomena [1-3]. Using kinetic equations for excitons, we model exciton transport in a NW and explain the origin of the blue shift of exciton emission observed in the recent experiments on hybrid NW-NP assemblies [2]. We also model artificial light-harvesting complexes composed of chlorophylls, bacterial reaction centers, and NPs [3]. Using superior optical properties of metal and semiconductor NPs, one can strongly enhance the efficiency of light harvesting [3]. An interaction between a discrete state of exciton and a continuum of plasmonic states can give rise to interference effects (Fano-like asymmetric resonances). These interference effects greatly enhance visibility of relatively weak exciton signals and can be used for spectroscopy of single nanoparticle and molecules. In the nonlinear regime, the Fano effect becomes strongly amplified [4]. In conclusion, our theory explains present experimental results and also provides motivation for future experiments and applications. Potential applications of dynamical exciton-plasmon systems include sensors and light-harvesting. The above theoretical studies were performed in collaboration with several groups [1-4]. \\[4pt] [1] A. O. Govorov, G. W. Bryant, W. Zhang, T. Skeini, J. Lee, N. A. Kotov, J. M. Slocik, and R. R. Naik, Nano Letters \textbf{6}, 984 (2006).\\[0pt] [2] J. Lee, P. Hernandez, J. Lee, A. Govorov, and N. Kotov, Nature Materials \textbf{6}, 291 (2007).\\[0pt] [3] A. O. Govorov and I. Carmeli, Nano Lett. \textbf{7}, 620 (2007); S. Mackowski, S. W\"{o}rmke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Br\"{a}uchle, Nano Lett.~\textbf{8}, 558 (2008). \\[0pt] [4] M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R.Barbour, B. D. Gerardot, R. J. Warburton, and K. Karrai, Nature \textbf{451}, 311 (2008). [Preview Abstract] |
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