Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J44: Focus Session: Optical Properties of Nanocrystals |
Hide Abstracts |
Sponsoring Units: DMP Chair: Victor Klimov, Los Alamos National Laboratory Room: Colorado Convention Center 507 |
Tuesday, March 6, 2007 11:15AM - 11:51AM |
J44.00001: Polarization-resolved fine structure and magneto-optics of single CdSe nanocrystal quantum dots Invited Speaker: Low-temperature photoluminescence (PL) microscopy of single colloidal quantum dots has proven a very effective tool for probing the emission properties of the band-edge excitons in isolated CdSe nanocrystals (NCs). Past studies employing high spectral resolution have resolved the narrow `atomic-like' emission lines from single NCs, while separately, polarization- resolved measurements have shown that the $|+1>$ and $|-1>$ bright exciton states are nominally degenerate with transition dipoles oriented isotropically in the plane normal to the crystallographic {\it c}-axis of the NC. To date, however, these two powerful techniques have not been simultaneously employed. To this end we constructed a low-temperature (4 K) microscope to measure both polarization- and spectrally- resolved PL of individual nanocrystals. Both orthogonal polarizations (horizontal/vertical linear or right/left circular) are simultaneously recorded to minimize the effects of spectral diffusion and blinking. The data clearly show [1] that many NCs possess a clear bright exciton ``fine structure" consisting of two linearly- (and orthogonally-) polarized peaks split in energy by $\delta \sim 1-2$ meV. This splitting is attributed to a breaking of the nanocrystal's cylindrical symmetry, leading to an anisotropic electron-hole exchange that mixes the $| \pm 1>$ bright excitons. Inferred orientation of the NCs will be discussed. Finally, we study the interplay between the anisotropic exchange and magnetic Zeeman energy in single NCs by incorporating a 5 T magnet into the microscope. With increasing magnetic field, the fine structure states become elliptically polarized and eventually approach pure circular polarization in the limit where the Zeeman energy $1/2 g \mu_B B > \delta$. We extract the exciton {\it g}-factor of individual NCs from the variation of the observed energy splitting with field in this regime. \newline \newline [1] M. Furis, H. Htoon, T. Barrick, M. Petruska, V. I. Klimov, S. A. Crooker, Phys. Rev. B Rapid Comm. 73, 241313 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J44.00002: State-to-state femtosecond relaxation dynamics of excitons in semiconductor quantum dots. Patanjali Kambhampati, Samuel Sewall, Ryan Cooney, Kevin Anderson, Eva Dias Size dependent exciton relaxation dynamics are measured in colloidal CdSe quantum dots using exciton selective femtosecond spectroscopy. Preparation of the initial excitonic state allows evaluation of state-to-state exciton dynamics. These methods reveal the electron and hole relaxation dynamics from a specified initial state to a specified final state, with a precision of 10 femtoseconds. These state selective, size dependent experiments confirm previously observed confinement induced femtosecond Auger channels for electrons with increased precision. This increased precision allows for unambiguous, quantitative evaluation of size dependent transition matrix elements. These experiments furthermore show that the hole relaxation rate increases for smaller quantum dots, contradicting expected relaxation mechanisms for holes. We propose a new confinement enhanced non-adiabatic pathway for hole relaxation in colloidal quantum dots, overcoming the predicted phonon bottleneck for holes. Finally, these experiments show exciton state specific biexcitonic interactions. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J44.00003: Size Dependence of Fluorescence Blinking Statistics from CdSe Nanorods Siying Wang, Claudia Querner, Thomas Emmons, Marija Drndic, Catherine Crouch We report fluorescence blinking statistics measured from single CdSe nanorods (NRs) of seven different sizes with aspect ratios ranging from 3 to 11. The off-times follow a power-law probability distribution; on-times follow a truncated power law distribution,$ P$(\textit{$\tau $}$_{on})\sim $\textit{$\tau $}$_{on}^{-\alpha }e^{-\tau on/\tau c}$. At fixed excitation intensity, the truncation rate 1/$_{\tau c}$ increases with increasing aspect ratio. For a particular sample, 1/$_{\tau c}$ increases gradually with increasing excitation intensity. Examining 1/$_{\tau c}$ vs. single-particle photon absorption rate for all samples indicates that the shape dependence of the absorption cross-section does not fully account for the observed variation in crossover time \textit{$\tau $}$_{c}$. Surprisingly, we observe no significant difference between core and core/shell nanorods or core rods with different surface ligands. Our results suggest that NR internal structural defects or degree of quantum confinement may contribute to the shape dependence of the crossover time. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J44.00004: Size and shape dependence of CdSe nanocrystal band-edge exciton fine structure$^{\ast }$ Qingzhong Zhao, Kwiseon Kim, Peter A. Graf, Wesley B. Jones, Alberto Franceschetti, Lin-Wang Wang Advances in growth methods of nanocrystals led to controlled synthesis over size and shape, influencing their optical properties. Ground exciton states of CdSe nanocrystals are shown to be sensitive to their geometries. We investigate the exciton fine structure of CdSe nanocrystals using empirical pseudopotential and configuration interaction methods$^{1,2}$. Systematic studies of the size and shape dependency are performed on the band edge states of CdSe spherical quantum dots, elongated nanorods, flattened nanodisks, nanowires and quantum wells. Large-scale electronic calculations consisting of 100--20,000 atoms with diameters from 2 to 8 nm and lengths from 2 to 11 nm were carried out. We explore size and shape dependence of exciton fine structure over the diameter-length space and explain it by the interplay of quantum confinement, crystal field splitting, and exchange interaction. We find the experimentally observed dark-bright exciton crossing$^{3}$ and discuss its size-shape dependency. [1] L. W. Wang and A. Zunger, \textit{Phys. Rev. B} \textbf{51}, 17398 (1995). [2] A. Franceschetti, \textit{et al.}, \textit{Phys. Rev. B} \textbf{60}, 1819 (1999). [3] N. Le Thomas, \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{94}, 016803 (2005). *This work was supported by US DOE-SC-BES and ASCR~TMSN Initiative. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 1:03PM |
J44.00005: Ab initio Theory of Semiconductor Nanocrystals Invited Speaker: With blooming experimental synthesis of various nanostructures out of many semiconductor materials, there is an urgent need to calculate the electronic structures and optical properties of these nanosystems based on reliable ab initio methods. Unfortunately, due to the O(N$^{3})$ scaling of the conventional ab initio calculation methods based on the density functional theory (DFT), and the $>$1000 atom sizes of the most experimental nanosystems, the direct applications of these conventional ab intio methods are often difficult. Here we will present the calculated results using our O(N) scaling charge patching method (CPM) [1,2] to nanosystems up to 10,000 atoms. The CPM yields the charge density of a nanosystem by patching the charge motifs generated from small prototype systems. The CPM electron/hole eigen energies differ from the directly calculated results by only $\sim $10-20 meV. We will present the optical band gaps of quantum dots and wires, quantum rods, quantum dot/quantum well, and quantum dots doped with impurities. Besides good agreements with experimental measurements, we will demonstrate why it is important to perform ab initio calculations, in contrast with the continuum model k.p calculations. We will show the effects of surface polarization potentials and the internal electric fields. Finally, a linear scaling 3 dimensional fragment (LS3DF) method will be discussed. The LS3DF method can be used to calculate the total energy and atomic forces of a large nanosystem, with the results practically the same as the direct DFT method. Our work demonstrates that, with the help of supercomputers, it is now feasible to calculate the electronic structures and optical properties of $>$10,000 atom nanocrystals with ab initio accuracy. \newline \newline [1] L.W. Wang, Phys. Rev. Lett. 88, 256402 (2002). \newline [2] J. Li, L.W. Wang, Phys. Rev. B 72, 125325 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J44.00006: Size-dependent optical spectrum of CdSe nanocrystals W. Jaskolski, G.W. Bryant, J.G. Diaz An empirical $sp^3d^5$ tight-binding model has been employed to describe the optical properties of CdSe nanocrystals over a wide range of sizes. The $sp^3d^5$ model explains successfully the single-particle electron levels and excitonic effects including the evolution of both the emission and absorption peaks with confinement. We provide an interpretation of the band-edge fine structure in agreement with both the one- and two- photon spectroscopies and the PLE resonant and non-resonant Stokes shifts. Previous effective mass, pseudopotential and $sp^3s^*$ tight-binding models were unable to explain such experiments. The wurtzite lattice structure splits the lowest $S$- and $P$- hole states into two doublets that overlap, in accordance to the indistinguishability observed between the one-photon and two-photon spectroscopies. A correct description of the spin-orbit coupling allows the non-resonant Stokes shift to be reproduced. Finally, for dot radius below 2.3 nm, an optically passive $P-$ level becomes the ground hole state giving rise to the large resonant Stokes shift observed experimentally. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J44.00007: Carrier Multiplication in PbSe Quantum Dots Alberto Franceschetti, Joonhee An, Alex Zunger The efficiency of conventional solar cells is limited, because the energy of absorbed photons in excess of the band gap is converted to heat, instead of producing electron-hole pairs. Recently, efficient carrier multiplication has been observed in smiconductor quantum dots. In this process, a single, high-energy photon generates two or more electron-hole pairs, thus potentially increasing the efficiency of solar cells. Rather exotic mechanisms have been proposed to explain carrier multiplication in PbSe quantum dots. Using atomistic semi-emprical pseudopotential calculations, we show that the more conventional impact ionization mechanism - whereby a photogenerated electron-hole pair decays into a biexciton in a process driven by Coulomb interactions between the carriers - can explain both the rate ($<1$ ps) and the energy threshold ($\sim 2.2$ times the band gap) of carrier multiplication in Pbse quantum dots [1,2], without the need to invoke alternative mechanisms. The reason is that the density of biexciton states increases very rapidly with energy, thus making the rate of impact ionization faster than the rate of competing decay channels. [1] A. Franceschetti, J.M. An and A. Zunger, Nano Letters, 6, 2191 (2006). [2] J.M. An, A. Franceschetti and A. Zunger, Nano Letters, nl061684x (2006). [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J44.00008: Optical Properties of PbSe Nanocrystal Quantum Dots Under Pressure Kirill K. Zhuravlev, Jeffrey M. Pietryga, Robert K. Sander, Richard D. Schaller The optical properties of PbSe nanocrystal quantum dots (NQDs) were studied as a function of applied hydrostatic pressure over the range from ambient to 4 GPa. PbSe NQDs exhibit an energy gap that is dominated by quantum confinement energy. Despite such strong confinement, we find that the energy gaps of 3, 5, and 7 nm PbSe NQDs change monotonically with pressure with a dependence that is almost entirely determined by the deformation potential. The sizable dependence of the NQD energy gap with pressure invites applications in the areas of high speed pressure sensing and tunable IR lasers. We will also present x-ray diffraction data, including the data indicating new phase transition not observed earlier. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J44.00009: Theory of InP nanocrystals under pressure J.G. Diaz, G.W. Bryant, W. Jaskolski An empirical tight-binding theory which includes the effects of the relaxation of the lattice is employed to investigate the role of an external hydrostatic pressure on the opto-electronic properties of InP nanocrystals. For the bulk, our model describes accurately the evolution of the lowest conduction band-edges with pressure and predicts the $\Gamma_{1c}$-$X_{1c}$ crossover at the same lattice contraction as measured in the experiment. For small InP nanocrystals, the bandgap dependence on pressure predicted with this model is, for the first time, in agreement with the experimental results. Previous atomistic models, which assumed a bulk-like arrangement for the atoms in the nanocrystal under pressure, led to negligible mixing of the $\Gamma_{1c}$- and $L_{1c}$- minima and did not account for the increasing localized character of the electron and hole states as a function of pressure. The lattice-relaxed tight-binding model suggests a mechanism for the experimental red-shift different from the $\Gamma_{1c}$-$X_{1c}$ crossover predicted by bond-distance scaling models. In the lattice-relaxed model, the experimental red-shift is explained as a transition from bound states localized inside the dot to surface-like states in the dot exterior. The evolution of the near-band-edge optical spectra as a function of pressure has been analyzed for different nanocrystal sizes, geometries and degrees of surface passivation with both the bond-length scaling and lattice-relaxed tight-binding approaches. [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J44.00010: Quantum Dots Confined in Nanoporous Alumina Membranes Jun Xu, Jianfeng Xia, Jun Wang, Joseph Shinar, Zhiqun Lin Precise control over the dispersion and lateral distribution of quantum dots (QDs) within nanoscopic porous media provides a unique route to manipulate the optical and/or electronic properties of QDs in a very simple and controllable manner for applications related to light emitting, optoelectronic, and sensor devices. Here we filled nanoporous alumina membranes (PAMs) with CdSe/ZnS core/shell QDs by dip coating. The deposition of QDs induced changes in the refractive index of PAMs. The amount of absorbed QDs was quantified by fitting the reflection and transmission spectra observed experimentally with one side open and freestanding (i.e., with two sides open) PAMs employed, respectively. The fluorescence of the QDs was found to be retained within the cylindrical nanopores of PAMs. [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