Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V15: Focus Session: Plasmonics, Biological, Solar Cell QDs |
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Sponsoring Units: FIAP Chair: H. Atwater, Caltech Room: LACC 405 |
Thursday, March 24, 2005 11:15AM - 11:51AM |
V15.00001: Semiconductor Quantum Dots in Metal Nanostructures Invited Speaker: We consider theory and available experimental data on optical and electronic properties of nanostructures containing semiconductor nanocrystal quantum dots (QDs) in a nano-proximity of metal surfaces and nanoparticles. Remarkable properties of such systems are due to a strong coupling between QD electrons and surface plasmons (SPs) and surface plasmon polaritons (SPPs) of the metal. Generally, the proximity to metal accelerates electronic transitions in the quantum dots. However, dependent on QD transition frequency $\omega $ and distance between QDs and the metal surface, and the size of the metal nanostructure, there may be different scenarios of such interactions with dramatically different effects. \begin{enumerate} \item For lower frequencies the QD transitions $\omega <\sim \omega _{SP} $, where $\omega _{SP} $ is a characteristic surface plasmon (SP) frequency, and $d\sim 1$ nm, the dominating interaction is that the excitation of localized SPs of the metal that undergo a strong Landau damping. This introduces a very fast Landau damping, efficiently damping the fluorescence of QDs. \item For $\omega \sim \omega _{SP} $, $d>\sim 1$ nm and nanoscale size of the metal system, coherent excitation of SPs takes place. In this case, the metal system may work as a nano-antenna for QDs enhancing the dipole oscillator strength of QD transitions. In this case, one expect \textit{enhanced} luminescence of QDs, opposite to what takes place in case ($i)$. One can also expect the surface-enhanced Raman scattering (SERS) and strongly enhanced nonlinear effects such as CARS. One unique effect that is predicted in such systems is surface plasmon amplification by stimulated emission of radiation (spaser), which is similar to laser where the coherent accumulation of SPs in a single mode takes place. \item For $\omega \sim \omega _{SP} $, $d>\sim 1$ nm, and macroscopic extension size of the metal system, besides SPs, the propagating modes of SPPs are excited. In this case, the metal system plays role of optical antenna that propagates the excitation of the quantum dots to the far zone. This leads to an increase of the fluorescence efficiency and intensity, and shortening of the fluorescence time. \end{enumerate} There are numerous potential applications of the QD/metal nanostructures that we will be discussing. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V15.00002: Plasmon-Enhanced Silicon Nanocrystal Photoluminescence Julie Biteen, Nathan Lewis, Harry Atwater We report two approaches to increase the luminescence emission intensity from Si nanocrystals (nc-Si) under optical pumping via: 1.the use of nanoporous Au (npg) as a sensitizer for nc-Si emission, and 2.near-field coupling of nc-Si emission to localized surface plasmons on npg surfaces to increase the radiative emission rate relative to conventional nc-Si emission. When nc-Si emitting at 1.7eV is coupled to npg, we have observed an order of magnitude enhancement in photoluminescence intensity while the emission energy remains unchanged. This enhancement is concurrent with a two-fold increase in the average radiative rate (which leads to faster decay rates and decreased saturation effects with increased pump powers) and a more than two-fold enhancement in the effective absorption cross section. We find the greatest enhancements when the nc-Si is 10 nm from a 100-nm thick npg film with surface roughness and voids on the order of 10 nm, and we will present a systematic study of the dependence of enhanced pump absorption cross-section and enhanced radiative emission as a function of npg-Si separation. We interpret our results as a consequence of a trade off between enhanced emission mediated by F\"{o}rster (dipole-dipole) energy transfer processes and quenching in carrier tunneling processes, each of which has a unique distance dependence. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V15.00003: Silicon Nanocrystal Internal Quantum Efficiency via Local Optical Density of States Robb Walters, Harry Atwater, Miheil de Dood, Jeroen Kalkman, Albert Polman The radiative rate of a dipole emitter is proportional to the local density of optical states (LDOS) at the physical location of the dipole in the approximation of Fermi's Golden Rule. We have measured the decay rate of silicon nanocrystals as a function of the LDOS while holding the non-radiative rate constant. The data are fit to a model to derive the internal quantum efficiency for silicon nanocrystals embedded in an oxide matrix. Nanocrystals are prepared by ion implantation into oxide at low energy (5keV) and subsequent thermal annealing (1100C). This procedure creates 2-4nm diameter silicon nanocrystals near the projected implantation range of 10nm from the oxide surface. Different oxide thicknesses are created on the same sample by etching back a thick oxide to form a ``staircase'' structure prior to implantation. This ensures that the nanocrystal populations are created through identical processing. The LDOS is calculated as a function of the distance from the nanocrystals to the high index silicon substrate. Due to the variation in the LDOS, we measure decay rates that vary from 10 to 50 kHz. The implications of our experiment on the prospects of silicon nanocrystals as an optical material will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:51PM |
V15.00004: In vivo imaging with quantum dots encapsulated in phospholipid micelles Invited Speaker: While fluorescent semiconductor nanocrystals (quantum dots) promise to revolutionize biological imaging, their use has been limited by difficulties in obtaining nanocrystals that are bio-compatible. To address this problem, we encapsulate individual nanocrystals in phospholipid block-copolymer micelles, and demonstrate both in vitro and in vivo imaging. When conjugated to DNA, the nanocrystal-micelles act as in vitro fluorescent probes to hybridize to specific complementary sequences. More importantly, when injected into Xenopus embryos, the nanocrystal-micelles are stable, non-toxic ($<$5x10$^{9}$ nanocrystals per cell), cell autonomous, and slow to photobleach. Nanocrystal fluorescence can be followed to the tadpole stage, allowing lineage tracing experiments in embryogenesis. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:27PM |
V15.00005: Exciton Relaxation Dynamics and Ultra-Efficient Exciton Multiplication in Nanocrystals: A New Model and Relevance to Solar Photon Conversion Invited Speaker: In bulk semiconductors photoinitiated electron-hole pair multiplication can occur via impact ionization (I.I.). However, this process requires photon energies of 4-5 times the bandgap because of the need to conserve crystal momentum and the competing rate of phonon emission. This limits the application of I.I. to increase the solar conversion efficiency in photovoltaic devices. For nanocrystal QDs, these constraints are relaxed, and exciton multiplication is thus expected to be greatly enhanced (1). Greatly enhanced exciton multiplication in PbSe nanocrystals was first confirmed by Schaller and Klimov (2). We report ultra-efficient multiple exciton generation (MEG) in PbSe and PbS nanocrystals, and explain our results by a new theoretical model based on the coherent superposition of excitonic states whereby multiple excitons are created instantaneously upon photon absorption (3); such a coherent process has never been reported before in semiconductors. Astonishingly high quantum yields of 300 {\%}, indicating creation of three excitons/photon for every nanocrystal, have been observed in PbSe at photon energies four times the energy gap. The new model predicts the occurrence of quantum beats between the coupled coherent states, and we observe such quantum beats in PbSe and PbS nanocrystals. Another prediction of the new model, supported by our data, is that the threshold photon energy for MEG in PbSe nanocrystals is twice its energy gap. These results have important implications for greatly improved optoelectronic devices. Refs: (1) A.J. Nozik, Physica E14, 115 (2002); (2) R. Schaller and V. Klimov, Phys. Rev. Letts. 92, 186601(2004); (3) R.J. Ellingson, M.C. Beard, P.Yu, O.I. Micic, A.J. Nozik, A. Shabaev, Al. L. Efros, submitted to Science (2004) [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V15.00006: Coherent Superposition of Multi - Exciton Complexes in Semiconductor Nanocrystals Andrew Shabaev, Alexander Efros Very efficient multi-exciton generation has been recently observed in nanocrystals where an optically excited electron-hole pair with an energy greater than the bandgap ($E_g$) produces one or more additional electron-hole pairs [1,2]. We present a theory of multiple exciton generation in nanocrystals. We have shown that very efficient and fast exciton generation in nanocrystals occurs by the optical excitation of a coherent superposition of multi-exciton states by a single photon. This model explains ultrafast dynamics of optical bleaching that arises from state filling including quantum beats between the multi-exciton states. We have also shown that although highly efficient multiple exciton generation begins at photon energy $3E_g$, the threshold of multiple exciton generation is $2E_g$ not, $3E_g$ as was suggested previously. \newline 1. R. Schaller and V. Klimov, Phys. Rev. Lett. {\bf 92}, 186601 (2004). \newline 2. R. J. Ellingson, M. C. Beard, P. Yu, O. I. Micic, A. J. Nozik, A. Shabaev, and Al. L. Efros, submitted. [Preview Abstract] |
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