Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C20: Focus Session: Metamaterials - Quantum Dots |
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Sponsoring Units: DMP Chair: Yanwen Wu, University of Texas at Austin Room: 322 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C20.00001: Phonon-induced Transparency in Quantum Dot Molecules M. Kerfoot, A. Govorov, D. Lu, R. Babaoye, A. Bracker, D. Gammon, M. Scheibner Quantum dot molecules (QDMs) formed by vertically stacked quantum dots provide a rich test ground for the investigation of elementary semiconductor excitations in a zero dimensional system. A high level of control over the mutual interactions between charges, spins and photons has been achieved with the enormous tunability of electronic states in QDMs [1]. In this work, we investigate the interaction of phonons with the QDM electronic states and demonstrate that, contrary to their usual dephasing role, phonons can actually increase control over a quantum system. This novel employment of phonons arises from the formation of a resonance-enhanced polaron. It is revealed via a Fano effect that arises from the interference of two competing optical absorption pathways, which results in a transparency of the system. One pathway involves a discrete electronic excitation with charges localized in separate quantum dots. The other pathway is for a polaron state, a hybrid of an electronic excitation with an optical phonon that results in a continuum of energy states. The pathways are coupled by the tunneling of a single hole, resulting in a Fano interference in the absorption lineshape characterized by stark dips (transparency) and peaks in the absorption. We show that the phonon-induced transparency is highly controllable by electric field, excitation energy and power. \newline [1] M. Scheibner, et. al., \textit{Essential concepts in the optical properties of quantum dot molecules}, Solid State Comm. \textbf{149}, 1427-1435 (2009). [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C20.00002: Structural characterization and magnetotransport in Ge/Si quantum dots Dongyue Yang, Chris Petz, Jerrold Floro, Jeremy Levy Artificially ordered quantum dot (QD) arrays may create unique functionalities such as cluster qubits and spintronic bandgap systems.\footnote{C. E. Pryor, M. E. Flatte, and J. Levy, Applied Physics Letters \textbf{95}, 232103 (2009)} We fabricate directed self-assembled Ge/SiC/Si arrays with fine control over QD size and spatial arrangement on the sub-35 nm length scale for this purpose. The formation, thermal stability, and structure of the QDs are studied extensively with transmission electron microscopy (TEM) and atomic force microscopy (AFM).\footnote{C. Petz, D. Yang, J. Levy and J. Floro, Journal of Material Research \ (JMR-2012-0430)} Magnetotransport measurements through the QD arrays shows a diamagnetic shift that depends on the dots' spatial configuration. We attribute this configuration dependence to the interaction of the electrons between different QDs. [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C20.00003: Tuning between Quantum-Dot and Quantum-Well-Like Behaviors in Type-II Zn-Se-Te Multilayers by Controlling Tellurium Flux during MBE Growth Haojie Ji, Bidisha Roy, Siddharth Dhomkar, Richard Moug, Maria Tamargo, Alice Wang, Igor Kuskovsky Type-II semiconductor quantum dots (QDs) characterized by spatial separation of charge carriers are good candidates for such applications as intermediate-band solar cells and IR photodetectors. Type-II QDs differ from type-I counterparts because one type of the carriers locates within the barrier material, so that their wavefunctions become to overlap early with increasing QD density. Thus, it is expected that type-II QDs coalescence into a - quantum well (QW) - like layer at much lower densities than similar type-I QDs with obvious consequences for device performance. We report here tuning between QD- and QW-like behaviors in the Zn-Se-Te multilayers with ZnTe sub-monolayer QDs. A set of samples, grown with increasing Tellurium flux, have been investigated. The critical density of QDs and the overlap of electron wavefunctions are estimated from secondary ion mass spectrometry, temperature dependent photoluminescence (PL), and magneto-PL measurements. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C20.00004: Effect of built-in electric field in stacked type-II ZnTe/ZnSe submonolayer quantum dots: enhancement and narrowing of Aharonov-Bohm oscillations Bidisha Roy, Siddharth Dhomkar, Haojie Ji, Maria Tamargo, Igor Kuskovsky Robust and narrow Aharonov-Bohm (AB) oscillations were observed in both intensity and energy of the mangeto-photoluminescence (PL) from stacked type-II ZnTe/ZnSe submonolayer quantum dots (QDs) grown via migration enhanced epitaxy. The narrowness and enhancement in the AB oscillations is a consequence of the built-in electric field in the system. Spectral analysis of cw magneto-PL and time-resolved PL suggest that the QD stacks size distribution is not large and thus broadening of the PL is possibly due to strong electron-phonon interaction as generally seen in bulk Zn-Se-Te systems. [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C20.00005: Characterizing epitaxially-grown InGaAs quantum dot chains using transmission electron microscopy Tyler Park, John Colton, Haeyeon Yang, Jeff Farrer Quantum dot chains grown by a modified Stranski-Krastanov method have been studied. The new growth technique seeks to reduce indium segregation and intermixing, compared to the conventional method, for higher quality dots. These quantum dot chains may have potential application in optoelectronics, detectors, lasers, and quantum computing. Our recent efforts have been to characterize the quantum dot chains by using transmission electron microscopy to answer morphological questions that photoluminescence spectroscopy could not. Using this method, we've been able to observe a dependence of dot flattening on temperature, a reduction in segregation and intermixing, and little influence of a capping layer on the structure of the dot-chains. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C20.00006: Electric-field control of exciton fine structure: atomic scale manipulation of exchange Garnett Bryant, Natalia Malkova, James Sims Tremendous effort has been made recently to control excitons in semiconductor quantum dots using vertical and in-plane electric fields, magnetic fields, optical fields, strain fields, annealing and crystal symmetry to manipulate exciton phase, fine structure splitting and polarization. Such control enables entangled photon generation from biexciton cascade, coherent state manipulation, and transfer between flying photonic qubits and stationary solid-state qubits needed for quantum information processing. We use atomistic tight-binding theory with a configuration interaction description of Coulomb and exchange effects to describe excitons in quantum dots in a vertical electric field. We show that field-induced manipulation of exciton orientation and phase produces a drastic reduction of fine structure splitting, an anticrossing, and a 90 degree rotation of polarization, similar to the observed anticrossing. An {\it atomistic} analysis is needed to explain how exciton reorientation by the applied field modifies anisotropic exchance and fine structure splitting without significantly altering other splittings. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C20.00007: Enhanced Luminescence in Tb/Ce co-doped Zinc- and Tin-Oxide quantum dots Christie Larochelle, JingJing Xu, Kelly McCutcheon SnO$_2$ and ZnO quantum dots doped with Tb$^{3+}$ exhibit strong luminescence from the Tb$^{3+}$ dopants due to efficient energy transfer from the semiconductor donors to the Tb$^{3+}$ acceptor ions. We report results from a study of the effect of co-doping the SnO$_2$ and ZnO dots with both Tb$^{3+}$ and Ce$^{3+}$ on the photoluminescence properties of the samples. The dots were synthesized using a sol-gel technique and the Ce$^{3+}$/Tb$^{3+}$ ratio was varied while keeping the total doping level at 1wt$\%$. X-ray diffraction and TEM results confirm the presence of nanocrystals of less than 10 nm in diameter. Photoluminescence results indicate that the Tb$^{3+}$ ions are incorporated in a crystalline environment and that co-doping with Ce$^{3+}$ enhances the energy transfer efficiency and therefore the intensity of the Tb$^{3+}$ luminescence. The effect of heat treatment on the size of the dots and the impact of size on luminescence properties was also investigated. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C20.00008: Emission in Mn-Doped Quantum Dot Que Huong Nguyen, Joseph L. Birman We theoretically investigate the magneto-PL of Mn2$+$doped semiconductor core-shell colloidal quantum dot to explain the experiment result from a recent magnetophotoluminescence study of strongly confined diluted magnetic semiconductor (DMS) in Mn2$+$-doped ZnSe/CdSe core-shell colloidal nanocrystals. The yellow emission characterized for in Mn2$+$-which is associated with the d-d internal transition 4T1-6A1, was reported not suppressed in an applied B //z magnetic field and unpolarized as usual and instead, a Mn PL circular polarization has been observed. The in Mn2$+$- photoluminescence has been found to have a large splitting between $\sigma ^{+}$ and $\sigma^{-\, }$components which depends on the applied field. We show that this behavior, which has not been found in characteristics of the Mn2$+$ PL in bulks and other conventional DMS materials, is the result of the strong confinement of the nanocrystal and its properties. Our theory and calculation show that the reasons the yellow Mn2$+$ PL band in quantum dots is not suppressed under applied magnetic field originate due to the existence of the internal piezoelectric dipole moment and the Coulomb exchange interaction of the impurity ions with the confined electrons inside the dot. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C20.00009: Modification of the conduction band edge energy via hybridization in quantum dots Robert Meulenberg, Joshua Wright X-ray absorption near edge structure spectroscopy (XANES) and theoretical modeling are used to examine effects of hybridization on the conduction band edge in doped CdSe quantum dots (QDs). Experimentally, Cd $M_3$-edge XANES provides evidence for a lowering of the CB minimum for Cu doped CdSe QDs that is dependent on Cu concentration. Theoretical modeling suggests the effects of hybridization between Cu and Cd atoms in the QD can explain our experimental results. The model can be extended for other dopant systems and provide a simple, yet effective, method to predict the effects of hybridization on the CB levels in QDs. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C20.00010: An evaluation of optical properties of solution processed colloidal chalcogenide type nanocrystals Prashant Sarswat, Michael Free Solution cast cadmium free chalcogenide type quantum dots were synthesized using environmentally friendly constituent elements. Some of the advantages of solution cast nanocrystals are easy integration with desired substrate, good control over size and shape, and their rapid processing. A range of liquid with different color luminescence was produced by changing the solvent and synthesis conditions. A green color luminescence was produced when oleylamine is used as a solvent and inert condition is maintained, whereas yellow luminescence was observed when syntheses of crystals were carried out in presence of oxygen. Detailed characterization and investigation was conducted using transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence study. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C20.00011: Ensemble brightening in size purified silicon nanocrystals Joseph B. Miller, Austin R. Vansickle, Rebecca J. Anthony, Daniel M. Kroll, Uwe R. Kortshagen, Erik K. Hobbie We report on the quantum yield, photoluminescence (PL) lifetime and ensemble photoluminescent stability of monodisperse plasma-synthesized silicon nanocrystals (SiNCs) prepared though density-gradient ultracentrifugation in mixed organic solvents. Improved size uniformity leads to a reduction in PL linewidth, band alignment, and the emergence of entropic order in dry nanocrystal films. We find a significant PL enhancement in thin solid films assembled from the fractions, and we use a combination of measurement, simulation and modeling to link this brightening to a temporally enhanced quantum yield arising from SiNC interactions in ordered ensembles of monodisperse nanocrystals. Using an appropriate excitation scheme, we exploit this enhancement to achieve photostable emission. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C20.00012: Ligand Interface Chemistry of Lead Chalcogenide Nanocrystals Kun Liu, Clive Bealing, Richard Hennig Lead chalcogenides nanocrystals (NCs) have shown promise in photovoltaic applications. Surface chemistry is one of the most important, yet least understood aspects of NC synthesis and functionalization controlling their properties. For example, an incomplete surface passivation could create trap states that enhance undesired exciton recombination. Clever choices of ligands ensure that neighboring NCs are electronically coupled while maintaining their quantum size effects. These two barriers limiting performance of NC solar cells illustrate the importance of fundamental studies for the interaction between ligands and NC surfaces. We use density functional theory to determine the binding sites and energies of ligands commonly used in NC synthesis and functionalization. Specifically we study amine-, carboxyl-, and thiol-ligands on different PbSe and PbS surfaces. For methylamine ligands we find a similar binding energy on the (100) and (111) facet while carboxylic acid ligands strongly prefer the (111) facet leading to different effective NC shapes. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C20.00013: Surfaces of nanomaterials for sustainable energy applications: thin-film 2D-ACAR and PALS studies B. Barbiellini, L. Chai, W. Al-Sawai, S.W.H. Eijt, P.E. Mijnarends, H. Schut, Y. Gao, A.J. Houtepen, L. Ravelli, W. Egger, M.A. van Huis, A. Bansil Positron (e$^{+}$) annihilation spectroscopy is one of only a few techniques to probe the surfaces of nanoparticles. We investigated thin films of PbSe colloidal semiconductor nanocrystals (NCs) in the range 2-10 nm as prospective highly efficient absorbers for solar cells. We compare and contrast our findings with previous studies on CdSe NCs. Evidence obtained from our e$^{+}$ lifetime spectroscopy study using the PLEPS spectrometer shows that 90-95\% of the implanted positrons are effectively trapped and confined at the surfaces of these NCs. The remaining 5-10\% of the e$^{+}$ annihilate in the relatively large oleic acid ligands, in fair agreement with the estimated positron stopping power of the PbSe nanoparticle ``core'' relative to the ligand ``shell.'' 2D-ACAR measurements on the same set of films using the low-energy e$^{+}$ beam POSH showed that the e$^{+}$ wavefunction at the surfaces of the PbSe NCs is more localized than for the case of CdSe NCs. Comparison with calculated e$^{+}$ - e$^{-}$ momentum densities indicates a Pb deficiency at the surfaces of the PbSe NCs, which correlates with e$^{+}$ lifetime and the NCs morphology. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C20.00014: Polymer Matrix Role in Light Absorption and Emission by Nano-CdS/PVA Composite Andrii Kovalchuk, Galyna Rudko, Volodymyr Fediv, Qijun Ren, Irina Buyanova, Weimin Chen Influence of a polymeric medium on the light absorption and emission processes of composite nano-CdS/polyvinyl alcohol is studied by activating different absorption-emission routes via changing of excitation wavelengths. The mechanisms are analyzed by employing the time-resolved photoluminescence spectroscopy. It is shown that the polymeric component of the composite contributes mainly to the excitation processes of photoluminescence via absorption of external laser excitation and its following transfer to the CdS nanoparticles that are incorporated into polymer matrix. The composite emission occurs mostly within the nanoparticles. It is also shown that time-decays of the photoluminescence emission from the CdS nanoparticles embedded in the composite depend on the excitation wavelength. Such behavior is ascribed to the interplay between the intrinsic nanoparticles excitation and extrinsic feeding of the nanoparticles via energy transfer from the excited polymer matrix. Possible mechanisms of the observed energy transfer are also discussed. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C20.00015: Water-soluble metallic cluster characterization via nanopore detection Chrisopher E. Angevine, Joseph E. Reiner Metallic quantum clusters can be ligand stabilized for aqueous environments to expand their potential as biosensors. Characterizing these clusters, while they are in solution, is an important problem because it will aid in optimizing cluster design. Nanopore-based resistive pulse sensing could be a valuable technique with which to characterize these structures because the pore is commensurate with the size of many of the clusters in use. Briefly, a single cluster enters the nanopore and creates a measurable decrease in the ionic current through the pore. These current blockades can be analyzed to deduce properties of the clusters such as size and charge. We have demonstrated this capability with a monodisperse mixture of Au$_{\mathrm{25}}$(SG)$_{\mathrm{18}}$. These clusters give rise to blockades with various mean residence times and blockade depths. We will present preliminary results and our analysis of these blockades and discuss future directions for nanopore-based cluster characterization. [Preview Abstract] |
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