Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M15: Focus Session: Electronic Properties and Energy Conversion in Semiconductor Nanostructures |
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Sponsoring Units: DMP Chair: Alexander Govorov, Ohio University Room: 008B |
Wednesday, March 4, 2015 11:15AM - 11:51AM |
M15.00001: Photocatalytic Solar Fuel Generation on Semiconductor Nanocrystals Invited Speaker: Jochen Feldmann I will review our scientific work on photocatalytic solar fuel generation utilizing colloidal semiconductor nanocrystals decorated with catalytic metal clusters. In particular, nanocrystals made of CdS, TiO$_{2}$ and organo-metal halide perovskites will be discussed. Key issues are the role of hole scavangers (M. Berr et al., Appl. Phys. Lett. 100, 223903 (2012)), the size and density of catalytic clusters (M. Berr et al.: Appl. Phys. Lett. 97, 093108 (2010) and Nano Letters 12, 5903 (2012) , and dependencies on external parameters such as pH (T. Simon et al., Nature Mat. 13, 1013 (2014)). [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M15.00002: Gate-Induced Carrier Delocalization in Quantum Dot Field Effect Transistors M.E. Turk, J.-H. Choi, S.J. Oh, A.T. Fafarman, B.T. Diroll, C.B. Murray, C.R. Kagan, J.M. Kikkawa We study the low temperature resistance and magnetotransport of high-mobility indium-doped CdSe quantum dot (QD) field effect transistors [1]. Low temperature resistance measurements show a characteristic dependence of $R(T)=R_0 \exp{(T_0/T)^p}$ with $p=2/3$, consistent with a recent model based on Coulomb gap variable range hopping plus thermal broadening. We show that using the gate bias $V_G$ to accumulate electrons in the QD channel increases the ``localization product'' $\kappa a$ (localization length $a$, dielectric constant $\kappa$), as expected for Fermi level changes near an Anderson mobility edge. Under any reasonable assumptions, $a$ increases significantly beyond the QD diameter as gate bias is applied. Magnetoresistance (MR) measurements display both positive and negative MR contributions that vary with $V_G$ and $T$. For each $V_G$, we observe a universal negative MR lineshape for higher temperatures ($T > 20$K) that scales as $T^{-4/3}$, consistent with Zeeman MR for $p=2/3$ with a gate bias-modulated mobility gap ($\Delta \varepsilon$).\\[4pt] [1] Turk, \textit{et al.}, Nano Lett., 14, 5948 (2014) [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M15.00003: Charge trapping and de-trapping in isolated CdSe/ZnS nanocrystals under an external electric field: indirect evidence of a permanent dipole moment Huidong Zang, Mingzhao Liu, Fernando Camino, Mircea Cotlet The charge trapping and de-trapping processes in single CdSe/ZnS nanocrystals under external electric field were systematically studied. The results clearly demonstrated that the external electric field can reversibly modulate the exciton dynamics and photoluminescence blinking, which provide further evidence for the existence of a permanent ground state dipole moment in isolated nanocrystals. A model which assumes energetically deep charge traps is proposed to explain on/off blinking in isolated CdSe/ZnS nanocrystals with the presence of a permanent dipole moment. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M15.00004: Probing Energy Levels of Large Array Quantum Dot Superlattice by Electronic Transport Measurement S.Z. Bisri, E. Degoli, N. Spallanzani, G. Krishnan, B. Kooi, C. Ghica, M. Yarema, L. Protesescu, W. Heiss, M. Kovalenko, O. Pulci, S. Ossicini, Y. Iwasa, M.A. Loi Colloidal quantum dot superlattice (CQDS) emerges as new type of hybrid solids allowing easy fabrication of devices that exploits the quantum confinement properties of individual QD. This materials displays peculiar characters, making investigation of their transport properties nontrivial. Besides the bandgap variations, 0D nature of QD lead to the formation of discrete energy subbands. These subbands are crucial for multiple exciton generation (for efficient solar cell), thermoelectric material and multistate transistor. Full understanding of the CQDS energy level structure is vital to use them in complex devices. Here we show a powerful method to determine the CQDS electronic energy levels from their intrinsic charge transport characteristics. Via the use of ambipolar transistors with CQDS as active materials and gated using highly capacitive ionic liquid gating, Fermi energy can be largely tuned. It can access energy levels beyond QD's HOMO \& LUMO. Ability to probe not only the bandgap, but also the discrete energy level from large assembly of QD at room temperature suggests the formation of energy minibands in this system. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M15.00005: Singlet and Triplet Exciton Harvesting in the Thin Films of Colloidal Quantum Dots Interfacing Phosphorescent Small Organic Molecules Burak Guzelturk, Pedro Ludwig Hernandez-Martinez, Dewei Zhao, Xiao Wei Sun, Hilmi Volkan Demir Efficient nonradiative energy transfer is reported in an inorganic/organic thin film consists of a CdSe/ZnS colloidal quantum dot (QD) layer interfaced with a phosphorescent small organic molecule (FIrpic) codoped fluorescent host (TCTA) layer. The nonradiative energy transfer in these films has a cascaded energy transfer behaviour: first from the fluorescent host TCTA to phosphorescent FIrpic and then to QDs. The nonradiative energy transfer in these films enables very efficient singlet and triplet state harvesting by the QDs with a fluorescence enhancement factor of 2.5-fold, while overall nonradiative energy transfer efficiency is over 90{\%}. The experimental results are nicely supported by the theoretical model which includes exciton diffusion assisted F\"orster-type near-field dipole$-$dipole coupling within the films. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M15.00006: Electron-Electron and Electron-Phonon interactions effects on the tunnel electronic spectrum of PbS quantum dots Hongyue Wang, Emmanuel Lhuillier, Qian Yu, Alireza Mottaghizadeh, Christian Ulysse, Alexandre Zimmers, Benoit Dubertret, Herve Aubin We present a tunnel spectroscopy study of the electronic spectrum of single PbS Quantum Dots (QDs) trapped between nanometer-spaced electrodes, measured at low temperature T=5 K. The carrier filling of the QD can be controlled either by the drain voltage in the shell filling regime or by a gate voltage. In the empty QD, the tunnel spectrum presents the expected signature of the 8x degenerated excited levels. In the drain controlled shell filling regime, the levels degeneracies are lifted by the global electrostatic Coulomb energy of the QD; in the gate controlled shell filling regime, the levels degeneracies are lifted by the intra-Coulomb interactions. In the charged quantum dot, electron-phonons interactions lead to the apparition of Franck-Condon side bands on the single excited levels and possibly Franck Condon blockade at low energy. The sharpening of excited levels at higher gate voltage suggests that the magnitude of electron-phonon interactions is decreased upon increasing the electron filling in the quantum dot. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M15.00007: Impact of particle interactions on the photoluminescent stability of silicon nanocrystal clusters Joseph B. Miller, Naveen Dandu, Rebecca J. Anthony, Uwe R. Kortshagen, Daniel M. Kroll, Svetlana Kilina, Erik K. Hobbie We combine experiments, Monte Carlo simulations and ab initio calculations to explore the influence of inter-particle interactions on the photoluminescent stability of silicon nanocrystal (SiNC) clusters. The time-dependent photoluminescence (PL) emitted by structures ranging in size from a single nanocrystal to collections of several thousand SiNCs is compared with Monte Carlo simulations of non-interacting nanocrystal ensembles. The discrepancy is modeled using calculations of the energy transfer rate between neighboring SiNCs as a function nanocrystal size, separation, and size disparity, and the influence of surface defects and excitation power on PL stability is explained. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M15.00008: Plasmonic Cavity Transparency Induced by a Single Quantum Dot Thomas Hartsfield, Wei-Shun Chang, Sung-Cheol Yang, Tzuhsuan Ma, Jinwei Shi, Liuyang Sun, Gennady Shvets, Stephan Link, Xiaoqin Li There are a large number of studies devoted to designing and characterizing plasmonic cavities. However, few experiments investigate interaction of individual quantum absorbers and emitters with a plasmonic cavity, which is essential for exploring cavity quantum electrodynamic (QED) effects. The main experimental challenge lies in the difficulty of placing an absorber and emitter at the desired positions. The very virtue of the small mode volume of plasmonic cavities demands precise spatial placement of emitters. Here, we study the simplest plasmonic cavity: a spherical metallic nanoparticle (MNP). By placing a semiconductor quantum dot (QD) controllably in the close proximity of the MNP cavity, its scattering spectrum is modified drastically. A Fano resonance is observed due to interference between the plasmonic resonance of the MNP and the exciton resonance in the QD. Our experiment demonstrates that transparency of the MNP cavity can be effectively induced by a single quantum dot, achieving an important step toward realizing plasmonic quantum devices. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M15.00009: Ultrafast Excitonic and Plasmonic Processes at the Nanoscale: Understanding Energy Flow in Hybrid Nanostructures Gary Wiederrecht Nanoscale plasmonic and excitonic structures frequently possess ultrafast processes that can be initiated and monitored by light. Nanoscale structures lend themselves to strong light-matter interactions for a variety of reasons, including a tendency towards large optical extinction and polarizability. Many times these nanostructures have strong resonances due to collective excitations with coherence, a property that lends itself very well to optical control opportunities. These types of collective excitations can also couple strongly to excitations of other nanostructures with different composition and with disparate properties in order to realize hybrid excitations. Hybridization presents unique opportunities for inducing directional energy and charge flow initiated by light. Thus, using ultrafast pulses of appropriate photon energy, combined with considerations of material composition and shape, brings the possibility to control energy flow in excitonic and plasmonic hybrid nanostructures. In this talk, I discuss our recent efforts to create and characterize electronically coupled nanostructures and the impact this has on ultrafast photoresponse. These processes have strong impact on applications such as light harvesting and nonlinear optical responses in nanoscale structures. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M15.00010: Directed synthesis of Mn$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{2}$ tunnel structured materials for energy applications Tim Droubay, Anne Chaka, Sebastien Kerisit, Eugene Ilton Mn oxides with tunnel structures are crucial in technological applications such as Li batteries, catalysis, fuel cells, electrochemical capacitors, sensors, and groundwater remediation. However, the complexity and poor quality of natural Mn oxide has hindered efforts to understand their fundamental structure-property relationships. To address this issue, we used PLD to make high-quality Mn$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{2-\delta}$ single-crystal films. Attempts to synthesize pure $\beta $-MnO$_{2}$ thin films on TiO$_{2}$ substrates resulted in Mn$_{2}$O$_{3}$ dominant films. Results of ab initio thermodynamics to explain film stability as a function of growth conditions and Mn/Ti composition suggest that ``protecting'' the Mn in a TiO$_{2}$ matrix by co-deposition would be beneficial. This approach has met with initial success even though the resultant films have oxygen vacancies. XPS indicates that (110) oriented films are Mn-rich near the surface while (001)-oriented films are Mn-rich near the interface. XRD shows that the films are coherently strained to the substrate which may influence the oxygen non-stoichiometry. Aberration corrected TEM results corroborate the XPS and XRD results and indicate a potential Mn-dependent defect. These films will be discussed along with multilayered (MnO$_{2})_{\mathrm{m}}$-(TiO$_{2})_{\mathrm{n}}$ films. Atomistic modeling shows that alternating cation rows that only contained Ti or Mn (m,n $=$ 1) greatly lower the activation energy for Li diffusion relative to films where Mn and Ti were homogeneously mixed. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M15.00011: Energy filtering in nanowires Maarten Thewissen, Bart Sor\'{e}e, Wim Magnus Nanowires present a viable geometry to allow for future semiconductor device scaling. When the dimensions of these nanowires become comparable to the wavelength of the carriers, quantum effects may have a profound impact on the transport properties of the wire. An example of such effect, theoretically investigated here, is the introduction of a periodic potential profile along the transport direction. This could be achieved by repeatedly varying the diameter of the wire, by including a superlattice perpendicular to the wire, by applying a periodic electric field etc. The consequent resonances will effectively block electrons at some energies, while allowing others to pass, and hence function as an energy filter. Such property might be beneficial for its use in a transistor. The transmission of electrons through the wire is examined here, by solving Schr\"{o}dinger's equation in the effective mass approximation and Poisson's equation self-consistently. As for the contacts, quantum transmitting boundary conditions are used as suggested by Lent. The result shows that energy filtering as described can indeed occur in realistic device structures. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M15.00012: Influence of non-gaussian statistics and dynamic non-locality in temporal evolution of open quantum systems Carlos Florez, Leonardo Pach\'on The study of quantum dissipation and non-local dynamics in phase space demands an extension of the Ullersma-Caldeira-Leggett framework to include non-linearities either in the system or the bath or even in the couplings between them. In this work, the special case of a linear open system interacting with a harmonic thermal bath by means of non-linear couplings is considered. This framework is constructed by extending the path integral formulation into phase space and applying the Feynam-Vernon influence functional theory to study the perturbative regime at different orders in the couplings. In doing so, the formal correspondence between the perturbative contributions and the Feynman diagrams that arise from the $n$-point correlation functions in the canonical variables are used. The effect of the non-local behavior induced by the non-linear contributions on the dissipative and decohering mechanisms are analyzed. The main features are the presence of non-Gaussian statistics and multiplicative, instead of additive, noises. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M15.00013: First-Principles Approach to Transient Heat Flow in Quantum Systems Kamil Walczak, Kirk Yerkes We examine heat transfer via quantum advection modes (coherently correlated quantum states) between two thermal baths of different temperatures mediated by quantum system with discrete spectrum of accessible energy levels. Nanoscale transport is treated within the first-principles method by including the superposed wave functions into the quantum expression for heat flux. Our results show the specific modifications of heat transport characteristics due to the dynamics of quantum systems under consideration. Such dynamics is captured by non-steady-state solutions to time-dependent Schr\"{o}dinger wave equation or by specific solutions of interrelated Pauli rate equations. Since the applicability of Fourier's law is questionable at nanoscale and in the case of transient heat conduction, we pay particular attention to the new physics of post-Fourier heat transport and its further consequences. For instance, the non-equilibrium conditions may establish and maintain certain degree of coherence between correlated quantum states which are involved into the energy conduction process. Understanding and gaining control of coherent manipulations of qubits (two-level quantum systems) is crucial for further development of quantum informatics. [Preview Abstract] |
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