Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L65: Quantum Dots: Growth, Electronic, and Optical Properties |
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Sponsoring Units: DCMP Chair: Athanasios Koliogiorgos, Czech Tech Univ Room: Mile High Ballroom 4F |
Wednesday, March 4, 2020 8:00AM - 8:12AM |
L65.00001: Stabilization of Crystalline Phase in Organo-metal Halide Perovskite Quantum Dots via Surface Manipulation William Delmas, Albert Dibenedetto, Evan Vickers, Jin Zhang, Sayantani Ghosh Size tunability of semiconducting quantum dots (QDs) is their most attractive characteristic, which allows modulation of their optical and electronic properties. In the case of organo-metal halide perovskite (OMHP) QDs, surface functionalization is one of the routes that offers a unique way to vary the size in a highly precise manner. In this study we use temperature dependent static and dynamic spectroscopy to investigate the effect of four different surface modification protocols on in CH3NH3PbBr3 OMHP QDs using conductive aromatic ligands. Our results indicate that functionalization methods affect quantum yield (QY) based on how well the different ligands passivate the surface states, while simultaneously altering the contribution of those states to the energy landscape that allows us to arrest the structural phase transition of the QDs from tetragonal/cubic to orthorhombic at low temperatures. Further, these ligands allow charge delocalization around the benzoic positions, which improves inter-particle energy transfer efficiency in QD films based on the conjugation state of the ligands. The synthesis of phase stable and high QY QDs that form conducting films is a significant development that will find use in diverse applications. |
Wednesday, March 4, 2020 8:12AM - 8:24AM |
L65.00002: Advanced material system for the design of an intermediate band solar cell: type-II CdTe quantum dots in a ZnCdSe matrix Vasilios Deligiannakis, Gehan Ranepura, Milan Begliarbekov, Igor Kuskovsky, Maria C Tamargo Photovoltaics based on intermediate band (IB) absorption have the potential to overcome the Shockley-Quiesser limit and maintain a large Voc. Quantum dots (QDs) or impurities can be used to form the IB. We propose a material system of submonolayer CdTe QDs embedded a ZnCdSe matrix that is optimum for the formation of an ideal IB and has other advantages to materials previously considered. Some unique attractive features of this material are its binary composition, simplifying growth; the absence of a deleterious interfacial layer, in spite of the lack of common ion; and the presence of strain, which allows for strain engineering of the IB. A superlattice structure of alternating QD and spacer layers is analyzed by X-ray diffraction (XRD) and photoluminescent (PL) spectroscopy. Simple arguments are used, following continuum elastic theory, to deduce the size of the dots and the strain within the superlattice from XRD data. Results of structural and optoelectronic characterization of both active layer and full device structures, using XRD, PL, photocurrent and contactless electro-reflectance measurements will be presented. The results suggest that the optimized materials are very well suited for potential high-efficiency IB solar cells. |
Wednesday, March 4, 2020 8:24AM - 8:36AM |
L65.00003: Photo-induced current enhancement in core-shell type quantum dot FET Sunao Shimizu, Keiichiro Matsuki, Kazumoto Miwa, Daniele Braga, shimpei ono Low dimensional materials are promising candidates to provide unique electronic properties; among them, quantum dots (QDs) and their hybrid systems are considered highly feasible because of the tunability of physical and chemical properties by material design. Here we report the fabrication of CdSe/CdS core-shell QDs-based field effect transistors (FETs) and its application to photo FET operations under UV light irradiation. The thin films of CdSe/CdS QDs were fabricated on SiO2/Si substrates in order to perform field effect experiments. The UV light irradiation dramatically changed the transfer characteristics for the thin films, the thickness of which ranges from ~20 nm to ~200 nm. We will discuss the thickness and the gate voltage dependence of the photo-induced current enhancement in the core-shell QDs-based FET in detail. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L65.00004: Infrared Optical-Field-Driven Luminescence in Quantum Dots Ibrahim Boulares, Jiaojian Shi, Blair Connelly, Keith Adam Nelson A recent study on CdSe-CdS core-shell colloidal quantum dots (QDs) showed that extreme electric fields from ultrafast THz-frequency electromagnetic pulses alone can produce QD luminescence [1, 2]. Unlike multi-photon absorption, which occurs at much higher optical frequencies, such emission was shown to be associated with large energy shifts of the absorption edge (more than 25%) – much like electro-luminescence (EL) driven by a quasi-DC field. While details of the mechanism of THz driven-EL are still under investigation, the effects of high optical electric fields of mid- to long- wavelength infrared (IR) radiation on the optical properties of QDs remain largely unexplored. To expand our understanding of the responses of QDs to extreme optical fields, we investigated the interaction of visible bandgap QD materials with sub-picosecond infrared-frequency pulses in the 3.5 to 12 micron wavelength range, at electric field levels that exceeded 1 MV/cm. We observed significant luminescence, resulting in up-conversion of the IR light to visible wavelengths, throughout the IR range that we explored. Preliminary results suggests similar EL induced by strong THz and IR fields. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L65.00005: Exciton Fine Structure in Lead Salt Quantum Dots Serguei Goupalov, Ivan Avdeev, Mikhail Nestoklon Lead chalcogenide quantum dots (QDs) have attracted significant scientific and technological interest as their photoluminescence (PL) is tunable by the size change over a wide infrared wavelength range. Electron and hole states in bulk lead chalcogenides are highly degenerate leading to the 64-fold degenerate exciton states. In quantum dots (QDs), the 64-fold degeneracy of the excitons is lifted by the valley mixing and by the electron-hole exchange interaction. We thoroughly investigate the relative importance of the inter-valley, spin-orbit, and electron-hole exchange couplings for the splittings and calculate the resulting exciton fine structure in the framework of the empirical tight-binding method. The dependence of the exciton fine structure on QD size and shape is analyzed. The calculated exciton fine structure allows one to explain the temperature dependence of the PL lineshapes observed in recent single QD experiments on PbS/CdS core/shell colloidal nanocrystals. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L65.00006: Atomic and Electronic Structure of Epitaxial Necking in Quantum Dot Super Lattice Mahmut Sami KAVRIK, Wonhee Ko, Jordan Hachtel, Carolina Qian, Alex Abelson, Harshil Kashyap, Scott Ueda, An-Ping Li, Juan Idrobo, Matt Law, Andrew Kummel Quantum coupling between periodic nanocrystalline structures manifests novel mesoscale properties such as mini-band formation, which may induce high carrier mobility1. Colloidal PbSe quantum dots (QDs) can self-assemble into 3D superlattices and are a promising material system to realize these synergistic properties. In this work, the electronic structures of these systems are investigated with advanced metrology and the modulation of electronic band structure in QD epi-SL by the formation of epitaxial neck between the QDs were observed. Experiments revealed unexpectedly large zero conductance (band) gap formation (~1.1 eV) when the STS probe was located between the QDs, while a smaller band gap of ~0.7 eV was observed when probing the body of the QD, as expected. Monochromated STEM-EELS were performed on the monolayer epi-SL and the band gap modification as a function of the electron probe position studied which shown a high onset for the energy loss when the electron probe was positioned on the QD neck, while a smaller onset was observed when the probe was directly on the QD. It is hypothesized that the epitaxial necks between the QDs confines the electron (hole) wave functions, thereby forming large band gaps on the epitaxial necking regions. |
Wednesday, March 4, 2020 9:12AM - 9:24AM |
L65.00007: Advances in Colloidal Quantum Dot Superlattice Charge Transport Henry Travaglini
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Wednesday, March 4, 2020 9:24AM - 9:36AM |
L65.00008: Zero field splitting of heavy-hole states in Ge quantum dots Georgios Katsaros, Josip Kukucka, Lada Vukusic, Hannes Watzinger, Fei Gao, Ting Wang, Jianjun Zhang, Karsten Held Holes have gained increasing interest in the past few years as spin qubit candidates since their strong spin-orbit coupling allows full electrical control of the hole spins [1-4]. Despite the fact that a hole is simply a missing electron, their spins are behaving strikingly different than their electron counterparts. While the electron spin does not correlate with the direction of motion, for holes in semiconductors there is a strong coupling between the momentum and the hole pseudospin. |
Wednesday, March 4, 2020 9:36AM - 9:48AM |
L65.00009: Thermoelectric transport properties of coupled quantum dots Roberto Franco Pe?aloza, John Alejandro Landazabal Rodriguez, Jereson Silva Valencia, Edwin ER RAMOS, Marcos Figueira
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Wednesday, March 4, 2020 9:48AM - 10:00AM |
L65.00010: Entropy measurements in mesoscopic circuits: opportunities and limitations Tim Child, Owen Sheekey, Nikolaus George Hartman, Silvia Lüscher, Joshua Folk, Saeed Fallahi, Geoffrey C. Gardner, Michael Manfra Recently, Hartman et al. demonstrated the capability to measure the entropy of a quantum dot (QD) containing only a few electrons[1] by detecting shifts in the charge state of the dot with temperature, dN/dT. While the measurement technique in Hartman et al. achieved a high level of accuracy, it lacked versatility because it required the system to be in a weakly coupled state that is thermally broadened, and therefore that charge transitions have the standard cosh2 line-shape of classic Coulomb blockade theory. Here, we show that integrating the dN/dT signal instead of fitting to a particular line-shape enables an entropy measurement of any transition[2], independent of the transition line-shape or even whether the entropy change occurs in the QD itself or another part of the system that is directly coupled to the dot. We demonstrate an entropy measurement for QDs throughout the range from weak to strong coupling to a reservoir. The QD is also sensitive to changes in entropy of other parts of the system, illustrating the potential for this method to be used to measure the entropy of more complex and interesting systems. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L65.00011: Measured phase shift using quantum dot interferometer in Kondo regime Yujie Zhang, Rui Sakano, Mikio Eto Phase measurement was reported by the transport through an Aharonov-Bohm ring with an embedded quantum dot, so-called quantum dot interferometer, with three terminals in the Kondo regime [1]. To evaluate how precisely the phase is measured, we theoretically examine the transport through a double quantum dot (DQD) in parallel, as a tractable model for the interferometer. One of the DQD is in the Kondo regime while the other is transparent with a large line width. We report (i) the formulation of transport through the DQD in terms of Keldysh Green functions for three-terminal setup. The conductance at zero temperature is exactly given using the Bethe Ansatz solution. We find that the Kondo temperature changes with a magnetic flux penetrating the ring. (ii) For the conductance as a function of gate voltage (Coulomb peaks), we show a crossover from an asymmetric shape of Fano-Kondo resonance to a symmetric Kondo plateau with an increase in the number of conduction channels in the leads. (iii) The phase locking at π/2 can be measured around the center of Kondo valley in some conditions for the tunnel couplings between the DQD and three leads, although the measured phase is slightly deviated from the Friedel sum rule. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L65.00012: Evidence of quantum phase transition in double charge Kondo quantum dots Winston Pouse, David Goldhaber-Gordon, Andrew Mitchell, Gergely Zarand, Catalin Pascu Moca, Ulf Gennser The Kondo effect is one of the simplest many body phenomena, in which a single magnetic impurity couples to a continuum of states. Adding a second impurity coupled to the first has been suggested to provide insight into heavy fermion systems. Implementing such impurities in nanofabricated systems with tunable parameters has proven to be a powerful way to compare experiments to theoretical predictions. Recent experimental work demonstrated a new way to realize this type of physics: the charge on a hybrid metal-semiconductor quantum dot coupled to a quantum hall edge state acts as a pseudospin [1]. We build off this design to create a two impurity configuration, with a competition between a dot-dot Kondo interaction and a dot-lead Kondo interaction. We believe this yields a novel quantum critical state. In our device, we controllably tune the various interaction strengths to explore the distinct phases. We provide evidence of a phase transition via transport measurements, with an enhanced conductance when the interaction strengths are comparable. |
Wednesday, March 4, 2020 10:24AM - 10:36AM |
L65.00013: Majorana bound state in the continuum: Coupling between Majorana bound state and quantum dot mediated by continuum Juan Ramos Andrade, Pedro Orellana, Edson Vernek In this work, we consider a single-level quantum dot (QD) and a Majorana bound state (MBS) placed at the end of a topological superconducting nanowire (TSW). Both are coupled to the continuum and do not have a direct connection between them. We addressed the behavior of MBS leaking phenomena and its consequences into the QD physics in the non-interacting and Coulomb blockade regime. By employing Green's function formalism via the equation of motion procedure, we calculate the physical quantities of interest. Our results show that the leakage of the MBS into the continuum state is achieved and can alter the physics of Coulomb blockade in the system through continuum-mediated coupling between MBS and QD. As a main consequence, we found a robust and non-trivial mechanism to accomplish a bound state in the continuum in the system. |
Wednesday, March 4, 2020 10:36AM - 10:48AM |
L65.00014: Spherical topological-insulator nanoparticles: Quantum size effects and optical transitions Lei Yang, Max Goldwater Christie, Ulrich Zuelicke, Michele Governale, Alexander Sneyd We investigate the interplay between band inversion and size quantization in spherically shaped nanoparticles made from topological-insulator (TI) materials. A general theoretical framework is developed based on a continuum-model description of the TI bulk band structure subjected to a hard-wall mass confinement. Analytical results are obtained for the wave functions of single-electron energy eigenstates and the matrix elements for optical transitions between them. Quantized levels in TI nanoparticles can be labeled by angular momentum quantum numbers j and m = −j, −j+1, ... , j. Additionally TIs possess a doubling of energy-level degeneracy due to different parity eigenstates with eigenvalues (−1)j±1/2. The existence of energy eigenstates having the same j but opposite parity enables optical transitions where j is conserved, in addition to those adhering to the familiar selection rule where j changes by ±1. We treat intra- and inter-band optical transitions on the same footing and establish ways for observing unusual quantum-size effects in TI nanoparticles. Our theory also provides a unified perspective on multi-band models for charge carriers in semiconductors and Dirac fermions from elementary-particle physics. |
Wednesday, March 4, 2020 10:48AM - 11:00AM |
L65.00015: Flexible manipulation of quantum dots by single-pulse optical vortices Guillermo Federico Quinteiro, Pablo I Tamborenea, M Holtkemper, D. E. Reiter, T. Kuhn Optical vortices (OV) are light fields with surprising properties, such as orbital angular momentum (AM), strong longitudinal components, and more, that can be exploited to control matter in new ways. Here we show that a single-pulse of an OV with the right parameters --degree of focusing, polarization, orbital AM, etc-- can precisely manipulate the electronic state of semiconductor quantum dots (QD) [1, 2]. Our models demonstrate the possibility of creating: i) heavy-hole excitons with arbitrary orbital AM, and ii) light-hole excitons with zero band+spin AM, with or without orbital AM. Such states can be named "envelope-forbidden" (i) or "spin-forbidden" (ii), since they cannot be excited by Gaussian light beams. In addition, we present potential applications to quantum technology: spin-forbidden states allow sub-picosecond spin flips of an extra electron charging the QD or the encoding of information on dark excitons, while envelope-forbidden states allow for the generation of currents that can produce magnetic fields at the nanoscale. |
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