Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L9: Optical Properties of Nanocrystals |
Hide Abstracts |
Sponsoring Units: DCOMP Chair: Danny Kim, Naval Research Laboratory Room: A105 |
Tuesday, March 16, 2010 2:30PM - 2:42PM |
L9.00001: Effect of Air Exposure on Carrier Relaxation Dynamics in Colloidal Quantum Dots Milan Sykora, Alexey Koposov, John McGuire, Roland Schulze, Jeff Pietryga, Victor Klimov The development of wet-chemistry synthetic routes to high-quality lead chalcogenide (PbS, PbSe, and PbTe) nanocrystals (NCs) created an opportunity for exploitation of these materials in practical applications where the tunability of electronic properties in the near-infrared region of the optical spectrum is of particular importance. One potential obstacle to broad application of lead chalcogenide NCs is their limited chemical and photochemical stability. Several recent studies of air exposed PbS and PbSe NC films provided evidence that under aerobic conditions the dominant degradation mechanism is surface oxidation of the NCs by atmospheric O$_{2}$. In the present work, we show that exposure of solutions of QDs to air leads to rapid oxidation of QDs that has pronounced effect on their chemical composition, electronic structure and carrier relaxation dynamics. Dramatic variations in PL quantum yield, observed following air exposure, are explained in terms of changes in the efficiencies of surface carrier trapping and nonradiative interband relaxation. After accounting for enhanced carrier trapping and oxidation-induced reduction in NC core size, we demonstrate that the dramatic changes in the surface properties of oxidized NCs do not significantly affect the dynamics of the Auger relaxation or the efficiency of Carrier Multiplication process. [Preview Abstract] |
Tuesday, March 16, 2010 2:42PM - 2:54PM |
L9.00002: Atomistic calculations of the biexciton fine structure in CdSe nanocrystals O. Voznyy, M. Korkusinski, E. Kadantsev, P. Hawrylak We present an atomistic tight-binding (TB) theory of exciton (X) and bi-exciton (XX) confined in CdSe spherical nanocrystals, of fundamental interest for multi-exciton generation applications. The single-particle electron (e) and hole (h) states are computed using an atomistic 20-band TB model accounting for the crystal field splitting and with a model surface passivation. The optically excited states are expanded in e-h configurations and the many-body spectrum is computed in the configuration-interaction (CI) approach accounting for the Coulomb mixing between X and XX configurations. The emission and absorption spectra are obtained using the Fermi's Golden Rule. We find that the electronic and optical properties of X and XX are determined by the hole spectrum being composed of four quasi-degenerate low-lying states well separated from the rest of spectrum. As a consequence, the X and XX present manifolds of states, whose structure is determined by electron-hole exchange for X, and by correlation effects for XX. This results in a fine structure of XX optical spectra. We also treat the mixing between X and XX in an exact manner in CI approach. We find that in CdSe nanocrystals it is weak, resulting only in minor shifts of intensity and a small line broadening of emission spectra. [Preview Abstract] |
Tuesday, March 16, 2010 2:54PM - 3:06PM |
L9.00003: Computational Analysis of Core/Shell-like Structure Formation through Equilibrium Segregation in Ternary Compound Semiconductor Nanocrystals Sumeet C. Pandey, Tejinder Singh, Triantafillos J. Mountziaris, Dimitrios Maroudas We present a computational analysis of equilibrium surface segregation in nanocrystals of In$_{x}$Ga$_{1-x}$As, ZnSe$_{1-x}$Te$_{x}$, and ZnSe$_{1-x}$S$_{x}$. The analysis is based on coupled compositional, structural, and strain relaxation employing Monte Carlo and conjugate-gradient methods according to proper parameterizations within the valence-force-field (VFF) description. The VFF parameterizations are validated by comparisons of their segregation energy predictions with first-principles density functional theory (DFT) calculations. We report results for the equilibrium concentration distributions in the nanocrystals as a function of the compositional parameter x and nanocrystal size; the nanocrystal morphologies are polyhedral with distinct facets of low-index surface orientation as determined from DFT calculations of equilibrium crystal shapes. The results identify the particle-size and composition ranges that allow for assembly of core/shell-like nanocrystal structures with increased band-gap tunability. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L9.00004: ABSTRACT WITHDRAWN |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L9.00005: The Effect of Surface Ligands on Charge Carrier Dynamics in Semiconductor Nanoclusters Svetlana Kilina, Sergei Ivanov, Oleg Prezhdo, Sergei Tretiak Using Density Functional Theory we investigate the impact of passivating ligands on morphology, electronic structure, and radiative and nonradiative dynamics in CdSe quantum dots (QDs). We consider the Cd$_{33}$Se$_{33}$ cluster -- within a range of the smallest synthesized CdSe QDs, passivated by either amine ore phosphine oxide, modeling the common ligands used for the QD surface passivation. Our calculations show surface reorganization of both the bare and ligated clusters. The surface-ligand interactions leads to charge redistribution and polarization on the surface and result in the development of hybridized states, with the electronic density distributed over the CdSe cluster and the ligands. Neither the ligand nor hybridized molecular orbitals appear as trap states near the band gap of the QD. Instead, being optically dark, hybridized states open new relaxation channels for high-energy photoexcitations. Hybridized states are denser at the edge of the QD conduction band of the cluster ligated with phosphine oxide than that with amines. Such difference in electronic structure impacts electron-phonon interactions and non-adiabatic couplings and leads to faster electron relaxation rate in QDs passivated by phosphine oxide than by amine. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L9.00006: Fiber-based spectroscopy of 1.55 $\mu $m PbS Quantum Dots coupled to Si Microcavities Matthew Rakher, Ranojoy Bose, Chee Wei Wong, Kartik Srinivasan The development of an emitter with atom-like properties in the telecommunications band that can be integrated with Si photonics has many implications for optical communication and quantum information science. In these areas, efficient collection of the optical signal is of the utmost importance, nowhere more so than for low-light levels in the near-infrared. Towards that end, we use an optical fiber-taper waveguide to probe near-infrared PbS quantum dots (QDs) solution deposited onto Si photonic crystal cavities, Si microdisks, and the taper itself. We show that the tapered fiber can be used to excite the QDs and efficiently collect the subsequent photoluminescence (PL). Furthermore, we show how the many modes of a microdisk can be exploited to provide both efficient pump and collection channels enabling time-resolved PL measurements of a few QDs. Finally, we show how the tapered fiber can be used in and of itself as a convenient means to pump and collect PL at 30 K and room temperature. This work should enable single particle spectroscopy with near-infrared colloidal QDs. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L9.00007: Unconventional gap state in lead sulfide quantum dots probed by photoinduced absorption Jason Lewis, Jian Zhang, Xiaomei Jiang Infrared quantum dots such as PbSe or PbS have several attractive properties for use as photoactive material in optoelectronic devices. However, the performance of devices containing such quantum dots is yet to be improved significantly before viable commercial applications. One of the fundamental problems is the existence of trap states in these quantum dots, usually associated with imperfection in surface passivation during the colloidal synthesis process. Trap state usually has lies within the quantum dots bandgap, and continuous wave (cw) photoinduced absorption (PA) spectroscopy has proven to be a convenient and successful technique to study any below gap long-lived photoexcitations. We have recently measured the cw photoinduced absorption of four different sizes PbS quantum dots films on sapphire. In this work, we will present a complete study of size-dependent gap state PA spectra features, including spatial and temporal information of this gap state. We then attempted to interpret this confinement-dependent gap state is from trapped exciton. [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L9.00008: Ge Nanocluster Enhanced Er Photoluminescence Julian Guzman, Daryl C. Chrzan, Eugene E. Haller We investigated the enhancement of the Er$^{3+}$ photoluminescence (PL) at 1540 nm by the incorporation of Ge nanoclusters into Er-doped silica using ion beams. We found that the Er$^{3+}$ PL enhancement is due to the presence of Ge and not to the radiation damage from the ion-implantation process. We determined that the Er$^{3+}$ PL depends on the Ge content, postgrowth annealing, and crystallinity of the Ge nanoclusters. Furthermore, we observed that the Er$^{3+}$ PL signal is maximized after annealing at 685 \r{ }C for 1 h. This is the temperature at which Ge nanoclusters begin to crystallize. Transmission electron microscopy studies were conducted to determine the size distribution of the Ge nanoclusters. Moreover, extended X-ray absorption fine structure measurements performed at the Ge-K and Er-L$_{III}$ edges revealed that there is negligible Ge-Er bonding. This suggests that Er is either fully oxidized or that it is not located in the Ge nanoclusters. Therefore, we believe that the energy transfer process from the Ge nanoclusters to the Er ions occurs through a non-optical resonant dipole transfer (F\"{o}rster Process\footnote{T. F\"{o}rster, \textit{Discuss. Faraday Soc.} \textbf{27}, 7 (1959).} similar to what has been proposed for the Si nanocrystal case.\footnote{M. Fujii, M. Yoshida, S. Hayashi, and K. Yamamoto, \textit{J. Appl. Phys.} \textbf{84}, 4525~(1998).} [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L9.00009: Photoluminescence properties of terbium-doped tin-oxide quantum dots Christie Larochelle, Rebecca Sobel Tb$^{3+}$-doped SnO$_2$ quantum dots embedded in an SiO$_2$ glass matrix have been synthesized using a sol-gel technique. The optical properties of a series of these samples with constant Tb$^{3+}$ concentrations, but increasing SnO$_2$ concentrations were studied to determine the effect of concentration on the size of the nanocrystals and the dynamics of energy transfer between the SnO$_2$ donor and the Tb$^{3+}$ impurity ions. X-ray diffraction and TEM results confirm the presence of nanocrystals of less than 10 nm in diameter while photoluminescence results indicate that the Tb$^{3+}$ ions are indeed incorporated into a crystalline environment. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L9.00010: Temperature dependent luminescence study of quantum-confined EuS nanocrystals Suseela somarajan, Melissa Harrison, Dmitry Koktysh, He Weidong, James Dickerson Europium sulfide (EuS) has been investigated extensively in its bulk forms for its interesting magnetic and magneto-optical properties. Enhanced physical properties of EuS nanocrystals resulted from the combined contributions of surface strain phenomena, high surface to volume ratio, and strong quantum confinement. Here we present the optical properties of EuS nanocrystals, synthesized using a novel colloidal synthetic procedure. To our knowledge, this would be the first temperature dependent luminescence study of EuS nanocrystals. Monodisperse single crystalline EuS nanoparticles were synthesized by solvothermolysis in a size-controlled manner from respective precursors. Structural properties were characterized by transmission electron microscopy (TEM), selected area electron diffraction (SAED), and X-ray diffraction (XRD). Optical absorption and temperature dependent photoluminescence (PL) techniques were used to investigate the energy band gaps of EuS nanocrystals of different sizes. Luminescence characteristics of EuS are due to the 5d $\to $ 4f7 relaxation (4f6 $\to $ 5d transitions) of Eu2+. Based on the temperature dependent photoluminescence studies, the characteristics and origins of the band-edge luminescence of EuS nanocrystals with different sizes will be discussed. [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L9.00011: Change in Stokes Shift of Embedded Quantum Dots in Response to Pressure Tyson Olheiser, Thomas Carlson, Jonathan Trovillion, Ghassan Al-Chaar, Robert Lozar-McDonald, Charles Marsh, Munir Nayfeh Although the fluorescence of nanoparticles (NP) has been known for years, there has been renewed interest in their possible applications. One innovative idea is to exploit the Stokes shift in such a way that the absorbed UV and emitted visible light can be used as a probe and output data source, respectively. The focus of our research has been to characterize the pressure dependence of the emission wavelength of NPs embedded in a transparent, amorphous material. A wavelength shift associated with embedding the NPs (Effect 1) has been experimentally measured by our group as well as a further shift due to an external pressure (Effect2). These results are explained by a theoretical model that considers a quantum confinement picture coupled with a deformation potential contribution. In addition, the analysis addresses surface energy terms as well as effects caused by a NP-encapsulating shell. In short, the theory predicts the emission wavelength for a given external pressure exerted on a continuum containing NPs. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L9.00012: Formation of Patterned Colloidal Nanoparticle Superlattices in a Two Solvent System Chenguang Lu, Austin Akey, Irving Herman A two solvent system consisting of a high boiling point solvent and a low boiling point solvent was found to greatly aid the self-assembly ofapproximatel -layer thicknanoparticle superlattices. Nanoparticle mixtures were prepared under multiple solvent evaporation conditions in a system of capillary channels patterned on Si substrates. The resulting films were to be highly ordered, and analyzed by SEM Grazing Incidence Small Angle X-ray Scattering(GISAXS).. capillary effect, introduced by patterned susbtrate the evaporation rateof solvents the channels are believed to be the driving factor the self-assemblyof the superlattices. Three-dimensional icrometer-scale superlattices of CdSe and Fe$_{2}$O$_{3}$ nanoparticles were fabricated this technique. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L9.00013: Enhanced Raman Scattering from InSb Nanodots; Temperature and Laser-Power Dependent Studies Noboru Wada, Haruki Takayama, Satoshi Morohashi InSb nanodots were uniquely fabricated by vapor-transport on a Si substrate which had previously been bombarded by FBI Ga ions. The InSb nanodots were then examined by spatially-resolved Raman scattering using an Ar-ion laser ($\lambda $= 514.5 and 488 nm with P=1$\sim $15 mW) with an optical microscope and CCD detector. In addition to the TO and LO peaks of InSb observed at $\sim $180 and 191 cm$^{-1}$ respectively, two peaks were observed at $\sim $110 and 150 cm$^{-1}$. Those Raman peaks were tentatively attributed to the 2TA and TO-TA second-order Raman processes. Those two peak intensities appeared to grow at the expense of the TO and LO Raman peak intensities with increasing the sample temperature from 10 K to 450 K. Also, the two-phonon peak intensities increased non-linearly with the probing laser power used. Hot carriers and their interactions with phonons in the restricted regions will be discussed together with Raman scattering results obtained from single-crystal InSb. [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L9.00014: Spectroscopy of Nanoparticles Frank Li, Robert Schafer, Carol Tanner, Steven Ruggiero We present results for the analysis of particle size, geometry, and density based on laser spectroscopy. The range of applicability of the technique is comparable to dynamic light scattering, but with approximately six orders of magnitude higher sensitivity (down to ~ 1000 particles/mL). We discuss results for a variety of particle types including metal, polystyrene, and metal-oxide particles, and organisms including viruses and bacteria. [Preview Abstract] |
Tuesday, March 16, 2010 5:18PM - 5:30PM |
L9.00015: Excitonic Effect on the Nonlinear Optical Properties of 3D Quantum Dots Jefferson Florez-Gutierrez, Angela Camacho-Beltran The excitonic effect on the optical nonlinearities of quantum dots is studied considering both the confinement potential of the electron-hole pair and the Coulomb interaction between them. The exciton is confined in a 3D quantum dot by means of a spherical harmonic potential. First, we obtain numerically the exciton states and then calculate the nonlinear optical coefficients such as the optical rectification and the second harmonic generation susceptibilities and the nonlinear absorption coefficient. We observe that the nonlinear optical properties are modified, compared with the results obtained just taken into account the non-interacting electron-hole pair. We also study the effect of an asymmetric confinement potential on the nonlinear optical properties of quantum dots. The asymmetric harmonic potential is set by restricting the azimuthal angle to values between 0 and $\pi/2$. Our results show that this asymmetry increases the nonlinear optical coefficients. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700