Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W34: Focus Session: Optical Properties of Nanocrystals |
Hide Abstracts |
Sponsoring Units: DMP Chair: Alexander Govorov, Ohio University Room: C141 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W34.00001: Evolution of CdSe/ZnS Quantum Dot Ensembles Under Prolonged Excitation Georgiy Shcherbatyuk, Richard Inman, Sayantani Ghosh We study the spectral evolution of self-assembled ensembles of CdSe/ZnS core/shell quantum dots (QDs) under photo-excitation in ambient conditions. We use spatially-resolved photoluminescence (PL) scanning microscopy in conjunction with spectrally-resolved time-resolved spectroscopy to measure variations in spectral intensity, emission wavelength and recombination lifetimes. Our results indicate that the spectral intensity of the ensemble undergoes both photo-induced brightening and darkening, and these rates have a complicated dependence on the concentration of the QD samples. They initially decrease with decreasing concentration but are greatly enhanced in the dilute limit. The photo-exposure also causes a rapid spectral red-shift followed by a slow blue-shift. The recombination lifetimes increase with the red-shift for all concentrations but do not correlate to the blue-shift in a straightforward manner in different samples. We conclude the possible explanation of this behavior is the inter-play of photo-induced surface trap discharging and preferential photo-oxidation of the smaller QDs in the ensemble. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W34.00002: Photoluminescence spectroscopy and lifetime measurements from self assembled semiconducting quantum dot- metal nanoparticle hybrid arrays M. Haridas, J.K. Basu We demonstrate how the emission properties of a hybrid array consisting of semiconducting quantum dot (QD) and metal nanoparticles (NP) can be controlled by varying the density and distance between QD and NP independently. Our hybrid system consists of chemically synthesized cadmium selenide quantum dots (CdSe QDs) and polymer capped gold nanoparticles (Au NP) embedded in a block copolymer matrix having the topology of cylinders oriented perpendicular to the substrate. We have prepared hybrid arrays with two different densities of CdSe QDs ($\rho _{QD})$ each having same Au NP densities ($\rho _{Au})$. The photoluminescence measurements (PL) from such hybrid system shows enhancement in emission with increase in $\rho _{Au}$, compared to the CdSe QD film and the enhancement factor is lower for hybrid films with high $\rho _{QD}$. The lifetime measurement shows double exponent PL decay with systematic reduction in exciton lifetime for hybrid arrays with respect to $\rho _{Au}$. The film with high $\rho _{QD}$ shows larger reduction in lifetime. Similarly, the amplitudes of the two relaxations switch over with increase in $\rho _{Au}$. It is clear that the shorter time becomes the dominant relaxation mode with increasing $\rho _{Au.}$ Observed phenomena have been explained in terms of exciton plasmon interaction$_{.}$ [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W34.00003: Manipulating coupling between a single semiconductor quantum dot and single gold nanoparticle Farbod Shafiei, Daniel Ratchford, Suenne Kim, Xiaoqin Li, Stephen Gray We report the manipulation of coupling in a simple model system, a single semiconductor quantum dot (QD) near a single metallic nanoparticle, and study the resulting changes in QD photoluminescence (PL) dynamics. We used atomic force microscopy nanomanipulation to controllably push a Au NP proximal to a CdSe/ZnS QD. We observed gradual and reversible changes in the QD PL lifetime and blinking dynamics. In some cases, the total lifetime reduced from 30 ns to well below 1 ns. This decrease is accompanied by the disappearance of blinking behavior as the nonradiative energy transfer from QD to the Au NP becomes the dominant decay channel. In comparison to previous studies, our experiments report changes in the PL dynamics of the same QD, therefore, eliminating the ambiguity of variable properties of individual QDs. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W34.00004: Carbon Nanotube CdSe Nanoparticle Hybrid Materials: Synthesis and Optical Properties Austin Akey, Chenguang Lu, Irving P. Herman Carbon nanotubes present remarkable opportunities as a base for construction of advanced nanomaterials with uniques properties. We report novel heterostructures composed of single-walled carbon nanotubes and monodisperse cadmium selenide nanoparticles (3.5 to 6 nm in diameter). The optical properties of the hybrid material differ significantly from those of the unbound nanoparticles and nanotubes, pointing to the existence of strong electronic/optical interaction effects between the two. Specific differences in absorption/emission behavior and the photoluminescence Stokes shift in the nanoparticles will be presented, along with experiments exploring the underlying mechanisms of this interaction. We believe this system exhibits ``hot'' excitation-transfer behavior from the nanoparticles to the nanotubes, which makes it of great interest for photovoltaic applications. This work is primarily supported by the Nanoscale Science and Engineering Center at Columbia University, which is supported by the Nanoscale Science and Engineering Initiative of the NSF under Award Number CHE-0641523. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W34.00005: Enhanced Optical Absorption of Glancing Angle Deposited Semiconducting Nanostructures for Photovoltaic Solar Cell Applications Hilal Is, Mehmet Cansizoglu, Miria Finckenor, Tansel Karabacak Semiconducting nanostructures with controlled geometries can provide enhanced optical absorption and effective collection of photo-charges for high efficiency photovoltaic solar cells and photoconductive devices. Glancing angle deposition (GLAD) provides a unique capability of producing nanostructured arrays of various materials with controlled shapes, size, and separation. In this study, as a model system, we fabricated arrays of semiconducting indium sulfide nanostructures by GLAD in the shapes of springs, screws, rods, and zigzags. We show that GLAD nanostructures have significantly lower reflectance and higher optical absorption compared to conventional flat thin films. In addition, we observed a superior photoconductivity (PC) response of about 80{\%} for nanorod array samples, which is believed to be mainly due to their high optical absorption. On the other hand, PC response was less than 1{\%} for conventional thin films of indium sulfide. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W34.00006: In-Situ Studies of Photoluminescence Quenching in Single Crystal Quantum Dot Sensitized Solar Cells Douglas Shepherd, Yong-Qi Liang, Justin Sambur, Bruce Parkinson, Martin Gelfand, Alan Van Orden Single crystal quantum dot sensitized solar cells (QDSSC) are a promising photovoltaic system in which collection of multiple charge carriers per photon has recently been reported.\footnote{Justin B. Sambur, Thomas Novet, B.A. Parkinson, \textit{Science} \textbf{330} (6000) 63-66} Utilizing time-correlated single photon counting we have studied both the fluorescence intensity and fluorescence decay time from CdSe quantum dots coupled to both single crystal TiO$_{2}$ and ZnO substrates through short and long chain ligands. We find that for all configurations the fluorescence decay time is quenched compared to unbound quantum dots in solution, while the photovoltaic properties of the system strongly depend on the chain-length of the ligand. These results suggest there exist interactions between either the individual quantum dots or the quantum dots and substrate that may compete with the charge injection process in QDSSCs. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W34.00007: Particle Morphology and Interfacial Energy Transfer in CdSe/CdS Nanocrystals Nicholas Borys, Manfred Walter, Jing Huang, Dmitri Talapin, John Lupton CdSe/CdS core-shell nanocrystal heterostructures are unique systems to study nanoscale energy migration. We perform single-particle excitation spectroscopy at low temperatures by monitoring both the luminescence intensity and energy of the core as a function of optical excitation energy in three different heterostructure shapes: spherical particles, rods, and tetrapods. In the tetrapod and rod structures, the shapes of the PLE spectra fall into one of two classes while the spherical particles all exhibit one universal form, which we attribute to the general shape and quantum confinement symmetry of the CdS shell. We confirm this assignment by correlated single particle SEM and PLE measurements. By resolving the core emission energy as a function of excitation energy, we identify two distinct exciton species in the tetrapods indicating the presence of a barrier that prevents charge transfer across the heterostructure interface [1]. \\[4pt] [1] Borys et al., Science (in press) [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W34.00008: Copper-doped core-shell ZnSe/CdSe nanocrystals with efficient and widely tunable photoluminescence Ranjani Viswanatha, Sergio Brovelli, Victor I. Klimov We report synthesis and spectroscopic studies of Cu-doped ZnSe/CdSe nanocrystals (NCs) with a wide range of shell thicknesses. Incorporation of copper ions into the NCs introduces an atomic-like state within the NC band gap. This results in a three-level system in which emission occurs due to the transition coupling the NC lowest-energy conduction-band level to the localized hole state associated with the Cu ions. Cu is incorporated into the NC core while the shell remains nominally undoped, which allows us to manipulate the shell thickness (and thus emission color) without affecting the overall level of NC doping. We demonstrate a wide-range spectral tunability of photoluminescence (PL) (from 3.1 eV to 1.25 eV), and ``giant'' Stokes shifts ($\sim $ 0.8 eV), which reduce emission losses due to reabsorption. We show that hole trapping at the Cu sites occurs primarily from the top of the valence band (i.e., no hot-hole transfer processes are detected) and that the main nonradiative mechanism is electron trapping at surface defects, which can be suppressed at cryogenic temperatures, resulting in PL quantum yields of $\sim $40{\%}. High intrinsic emission efficiencies, wide-spectral tunability, and a large Stokes shift make these novel NCs attractive candidates for radiation detection, light-emitting diodes, and lasers. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W34.00009: Breakdown of Volume Scaling in Auger Recombination in CdSe/CdS Heteronanocrystals: The Role of the Core-Shell Interface Sergio Brovelli, Florencio Garc\'Ia-Santamar\'Ia, Ranjani Viswanatha, Han Htoon, Scott Crooker, Victor I. Klimov Spatial confinement of electronic excitations in semiconductor nanocrystals (NCs) results in a significant enhancement in nonradiative Auger recombination (AR) of multiexcitons, which is detrimental to promising NC lasing applications.~In standard NCs, AR times scale linearly with NC volume. We investigate multiexciton dynamics in NCs composed of CdSe cores and CdS shells of tunable thickness. Thicker shells dramatically reduce AR, particularly during initial shell growth, which cannot be explained by traditional volume scaling alone. Rather, low-temperature fluorescence-line-narrowing studies strongly suggest that suppressed AR derives primarily from the formation of an alloy layer at the CdSe/CdS interface, and a corresponding ``smoothing'' of the confinement potential (CP). These findings support the recent theory, which predicts that the change from abrupt to smoothly-varying CPs reduces the high-spatial-frequency Fourier components of the exciton wave function, thereby minimizing overlap with the high-energy states involved in nonradiative Auger decay. Our results highlight the importance of NC interfacial structure in the AR process in zero-dimensional NCs and provide general guidelines for the design of new nanostructures with suppressed AR for future lasing applications. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W34.00010: Complete suppression of Blinking and convergence to a single emissive state in giant nanocrystal quantum dots Anton Malko, Siddharth Sampat, Young-Shin Park, Javier Vela, Youngfen Chen, Jennifer Hollingsworth, Victor Klimov, Han Htoon We report a systematic study of photoluminescence (PL) emission intensities and lifetimes of individual core-shell CdSe/CdS ``giant'' nanocrystal quantum dots (gNQDs) as a function of the shell thickness. We observed a complete suppression of blinking for gNQDs overcoated with more than 16 monolayers (ML) of CdS shell. An analysis of the photon emission statistics reveals a highly super-Poissonian distribution for thin shell (4-12 ML) gNQDs and near perfect Poissonian distribution for non-blinking, thick-shell gNQDs. Measurements of PL decay rates as a function of PL intensity show that while PL lifetimes vary continuously with PL intensity for thin-shell gNQDs, only one PL decay constant is observed for the thicker shell gNQDs. This result clearly indicates that while the thin-shell gNQDs possess continuous distribution of emission states, PL of the non-blinking, thick-shell gNQDs originates from a single emissive state. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W34.00011: A comparative study of carrier multiplication in PbS and PbSe nanocrystals John Stewart, Lazaro Padilha, Doh Lee, Bishnu Khanal, Jeffrey Pietryga, Victor Klimov In this talk I will present our recent studies of carrier multiplication (CM) in PbS and PbSe nanocrystals (NCs). CM is a process in which absorption of a single photon produces multiple excitons. In our experiments, we evaluate its efficiency based on the amplitude of the Auger decay signature of multiexcitons in carrier dynamics recorded using both transient absorption and time-resolved photoluminescence. In the case of PbSe NCs, we have measured the CM quantum efficiencies and Auger lifetimes for a large range of NC sizes, including very large particles with a band gap around 0.5 eV. Using excitation at 3.1eV we observe that the quantum efficiency is low for small NCs and increases monotonically toward the bulk value for larger dots. Despite many apparent similarities in the band structure of PbSe and PbS NCs, our preliminary studies of PbS nanoparticles hint at a quite different spectral behavior of quantum efficiencies compared to PbSe NCs. In particular, while the quantum efficiencies are similar for smaller NCs, we see discrepancies for the larger NCs for which the energy gap approaches the bulk limit. We discuss these discrepancies in the context of our comparative studies of intraband relaxation and Auger recombination in these two types of the NCs. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W34.00012: Carrier recombination pathways in Copper Indium Sulfide (CIS) nanocrystals Anshu Pandey, Liang Li, Jeffrey M. Pietryga, Victor I. Klimov Ternary and Quaternary compounds are rapidly gaining interest because of potential applications in areas such as thin-film photovoltaics and light-emitting diodes. We will discuss carrier dynamics in Copper Indium Sulfide (CIS) nanocrystals (NCs), one of the better studied members of the ternary-quaternary family. While as-prepared CIS NCs exhibit photoluminescence (PL) quantum yields less than 10{\%}, overgrowth with a few monolayers of CdS or ZnS increases PL quantum efficiency to more than 80{\%}. We investigated the reasons for this dramatic improvement in efficiency through time-resolved PL and transient absorption measurements. Our results suggest that while electrons in CIS NCs remain delocalized, the holes are rapidly localized due to trapping at defects. PL emission arises through the radiative recombination of a delocalized electron with a hole at the interior defect site (radiative decay center). We also observe surface defects that serve primarily as centers for nonradiative recombination. Overcoating of CIS NCs with CdS or ZnS eliminates surface traps and results in a long single-exponential PL decay that appears to be unique among visible-emitting NCs. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W34.00013: Nonradiative Energy Transfer in Assemblies of Nanostructures with Mixed Dimensionality Pedro Ludwig Hernandez-Martinez, Hilmi Volkan Demir We study the exciton-exciton interaction and nonradiative energy transfer in nanostructure assemblies with mixed dimensionality. We investigate possible combinations in terms of dimensionality for these nanostructures, and analyze the resulting nonradiative energy transfer rates as a function of dimensionality. Depending on the direction of the energy transfer, arrangements of such nanostructures have potential applications in both photovoltaics [1] and light generation [2]. \\[4pt] [1] J. Sambur, et al., ``Multiple Exciton Collection in a Sensitized Photovoltaic System'', Science 330, 63 (2010). \\[0pt] [2] R. Yan, et al., ``Nanowire Photonics,'' Nature Photonics, 3, 569-576 (2009). [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W34.00014: Charge and Shape Effects on the Carrier Dynamics of 25 Atom Au Nanoclusters Matthew Sfeir, Huifeng Qian, Rongchao Jin We study a series of semiconducting gold clusters that exhibit strong quantum confinement effects on their optical properties. In contrast to larger metallic nanoparticles, the surface plasmon resonance disappears a large optical gap ($>$ 1.3 eV) is formed. Recent synthetic advances have permitted the study of truly monodisperse clusters with precise control on the atomic scale. Using femtosecond and nanosecond transient absorption spectroscopy, we have investigated the excited state relaxation dynamics of spherical and prolate 25 atom isomers. We have determined that these particles exhibit long excited state lifetimes and that the carrier dynamics that are strongly influenced by the charge state and the physical arrangement of the atoms. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W34.00015: Transparent Conducting Metallic Film for Applications in Photovoltaics and Optoelectronic Devices Trilochan Paudel, Piotr Patoka, Wen-Chen Chen, Michael Giersig, Willie Padilla, Zhifeng Ren, Kris Kempa Oxides such as Indium-Tin Oxide (ITO) known as transparent conducting oxide (TCO) have been used for a long time in most of the thin film solar cell fabrications. It simultaneously works as an electrically conducting and optically transparent electrode. However, the electrical conductivity and optical transparency are not good enough. Here, we discuss our experimental and simulated results on nanostructured metallic films as a possible alternative replacement to TCO. This structure may possibly outperform the conventional ITO for applications in photovoltaics and optoelectronic devices such as LEDs. [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