Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W27: Charge Transport and Plasmonics |
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Sponsoring Units: DCP Chair: David Nesbitt, University of Colorado at Boulder Room: 204B |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W27.00001: Lifetimes for fast charge transfer of core excited molecules on gold and graphene Gian Paolo Brivio, Guido Fratesi, He Lin, Abhilash Ravikumar, Olgun Adak, Latha Venkataraman, Gregor Kladnik, Dean Cvetko, Alberto Morgante The charge transfer time from an excited organic molecule both adsorbed on gold and graphene is studied in terms of the resonant linewidth of the molecular orbital energy levels interacting with the valence band of the substrate. The calculations are performed by density functional theory including the van der Waals contribution. Experiments are carried out by the core level resonant spectroscopies with fs resolution. The core valence exciton is described by a static perturbation of the atomic potential. The calculated widths are consistent with the experimental transfer times. They display a dependence on the molecular adsorption angle both in theory and experiments, and this effect is predicted to be function of the excited orbital. [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W27.00002: Rhodium Nanoparticles for Ultraviolet Plasmonics Anne Watson, Xiao Zhang, Rodrigo Alcaraz de la Osa, Juan Sanz, Francisco Fernandez, Fernando Moreno, Gleb Finkelstein, Jie Liu, Henry Everitt We introduce the non-oxidizing catalytic noble metal rhodium for ultraviolet (UV) plasmonics. 8 nm tripod-shaped planar Rh nanoparticles (NPs) were synthesized by a modified polyol reduction chemistry. They have a calculated local surface plasmon resonance (LSPR) near 330 nm. To illustrate the UV plasmonic performance of Rh, p-aminothiophenol (PATP) was attached to the Rh NPs and enhanced Raman and fluorescence were observed upon UV illumination. The PATP Raman spectra produced by UV and visible excitation were respectively in and out of resonance with the Rh NP LSPR. This clearly revealed resonant spectral enhancement in the UV and accelerated photo-damage produced by intense local fields concentrated near the plasmonic Rh NPs. Simultaneously, surface enhanced fluorescence increased during 13 minutes of resonant UV illumination, providing direct evidence of charge transfer from the Rh NPs. The combined local field enhancement and charge transfer demonstrate essential steps toward plasmonically-enhanced ultraviolet photocatalysis. Due to its high chemical stability and strong plasmonic effect, Rh nanoparticles could find wide applications in UV plasmonics. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W27.00003: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W27.00004: Analyzing biphasic surface plasmon resonance data Purushottam Tiwari, Xuewen Wang, Jin He, Yesim Darici Surface plasmon resonance (SPR) is a widely used label-free biophysical technique to quantitatively study biochemical processes. Analysis of monophasic SPR profiles by fitting using a single exponential function is straightforward. However, there is no simple procedure for SPR data fitting with double exponential functions. An existing approach is to fit the biphasic SPR profiles with numerical solutions of the rate equations. This procedure requires some prior knowledge of the underlying interaction mechanism, and the extracted rate constants often have large uncertainties. We propose a new method of analyzing the biphasic SPR data using the three commonly employed biphasic models. Our method is based on a general analytical solution of the biphasic rate equations. Our method can be used to determine the underlying biphasic interaction mechanism from the analysis of the SPR data, and to extract the rate constants with high confidence levels. [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W27.00005: Two-Photon Optical Properties of AIE-active D-TPE-A Molecules: Aggregation Enhancement and Structure-Property Relationships Yilin Zhang, Jie Li, Ben Zhong Tang, Kam Sing Wong We present an aggregation enhancement in two-photon-excited fluorescence (TPEF) of about two orders of magnitude in a series of novel non-centrosymmetric D-$\pi$-A molecules. Aggregation-induced emission characteristics are introduced into these D-$\pi$-A molecules via tetraphenylethylene (TPE), which is used as their $\pi$-bridge. Detailed analysis shows that the TPEF of these molecules are enhanced in aggregation environment with both fluorescence quantum efficiency and two-photon absorptivity concomitantly. The two-photon absorption (TPA) transition bands of these branched- or butterfly-configured molecules are similar to those in their linear absorption. The molecular TPA cross sections in aggregation environment reach around 50-130 GM, and peak within the available wavelength ranges of a Ti:Sapphire femtosecond oscillator. We also observe that two-photon absorptivity increases progressively with the addition of donor/acceptor moieties on the TPE backbone. This phenomenon is presumably attributed to the improved conjugation length and enhanced intramolecular charge transfer, hence better delocalization of $\pi$-electrons. For each compound, the aggregation enhancement in TPA may also offers clues of aggregation effect on the molecular electronic structure. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W27.00006: Simple and accurate method for time-dependent transport along nanoscale junctions Ignacio Franco, Thorsten Hansen, Liping Chen A simple method that accurately captures the dynamics of metal-molecule-metal junctions under the influence of time-dependent driving forces is presented. In it, the metallic contacts are modeled explicitly as a discrete set of levels that are dynamically broadened via an artificial damping term in the equations of motion. The approximations that underlie the method are revealed via a derivation of the effective equations of motion within the framework of non-equilibrium Green's functions (NEGF) theory. As shown, the method applies to junctions that can be described by an effective independent-fermion Hamiltonian, admits arbitrary time dependence in the molecular Hamiltonian and is restricted to time-dependent voltages that are adiabatically slow. The method is trivial to computationally implement, has a well defined range where the results are independent of artificial model parameters, and is numerically shown to quantitatively reproduce the time-dependent transport characteristics of a model molecular junction driven by laser fields as described by an exact NEGF method. As such it constitutes an intuitive and technically accessible method to model time-dependent transport phenomena in molecular junctions that are driven by electric fields or fluctuating environments. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W27.00007: Revealing intra-molecular transient dynamics in junctions by mean of Noise spectroscopy Maicol Ochoa, Yoram Selzer, Uri Peskin, Michael Galperin Recent theoretical investigations have shown that Ultrafast Laser Pulse Pair Sequences applied to molecular junctions and {\sl dc} current measurements can be used to detect and quantify intra-molecular processes that occur in the picosecond timescale during the transient regime. In this work, we have continued these investigations and found that while averaged current measurements can capture dynamics directly related to electron transport, one needs to go beyond and consider averaged Noise measurements in order to detect intra-molecular processes not directly participating in the electron transport. In this talk, we will introduce the formalism for this Noise spectroscopy and illustrate our findings within simple numerical models, displaying time-dependent and average responses in junctions that include destructive interference and circular currents. [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W27.00008: Beyond the Lorentzian Model in Quantum Transport: Energy-Dependent Resonance Broadening in Molecular Junctions Zhenfei Liu, Jeffrey B. Neaton In quantum transport calculations, transmission functions of molecular junctions, as well as spectral functions of metal-organic interfaces, often feature peaks originating from molecular resonances. These resonance peaks are often assumed to be Lorentzian, with an energy-independent broadening function $\Gamma$. However, in the general case, the wide-band-limit breaks down, and the Lorentzian approximation is no longer valid. Here, we develop a new energy-dependent broadening function $\Gamma(E)$, based on diagonalization of non-Hermitian matrices within a non-equilbrium Green's function (NEGF) formalism. As defined, $\Gamma(E)$ can describe resonances of non-Lorentzian nature and can be decomposed into components associated with the left and right leads, respectively; and it is particularly useful in understanding transport properties in terms of molecular orbitals in asymmetric junctions. We compute this quantity via an ab initio NEGF approach based on density functional theory and illustrate its utility with several junctions of experimental relevance, including recent work on rectification in Au-graphite junctions. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W27.00009: Quantum model of capacitance of nanostructures Junqiang Lu Modeling high-frequency electronic properties of nanostructures in nanocircuits presents particular challenge because of contribution from electrodes. In this talk, I present the difference in modeling steady and dynamic electronic transport properties of nano-gap structures and a quantum model to measure capacitance of nanostructures. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W27.00010: Finding Destructive Interference Features in Molecular Transport Junctions Matthew Reuter, Thorsten Hansen Associating molecular structure with quantum interference features in electrode-molecule-electrode transport junctions has been difficult because existing guidelines for understanding interferences only apply to conjugated hydrocarbons. Herein we use linear algebra and the Landauer-B\"uttiker theory for electron transport to derive a general rule for predicting the existence and locations of interference features. Our analysis illustrates that interferences can be directly determined from the molecular Hamiltonian and the molecule-electrode couplings, and we demonstrate its utility with several examples. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W27.00011: The role of dimensionality on the molecule-lead coupling in molecular electronic junctions Tamar Zelovich, Leeor Kronik, Oded Hod We present new insights into the role dimensionality plays in the lead-molecule coupling scheme at molecular electronic junctions. A key ingredient of our approach is a transformation of the Hamiltonian matrix from an atomistic to a state representation of the molecular junction. This provides direct access to the different couplings between the molecular states and the energy manifold of the leads, which underlie the transport properties of molecular junctions. We explore several tight-binding junction models and predict the appearance of coupling bands that depend on the dimensionality and shape of the leads. We believe that a similar analysis may contribute to the understanding of many phenomena characteristic to the fields of nano- and molecular-electronics. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W27.00012: `Soft' amplifier circuits based on field-effect ionic transistors Niels Boon, Monica Olvera de la Cruz Soft materials can be used as building blocks of electronic devices with extraordinary properties. We demonstrate that an ionic analogue of the semiconductor field-effect transistor (FET) could be used for voltage and current amplifiers. Our theoretical model incorporates readily-available soft materials, such as conductive porous membranes and polymer electrolytes to represent a current-gating device that can be integrated in electronic circuits. By means of Nernst-Planck numerical simulations as well as an analytical approach towards expressions that describe steady-state currents, we find that the behavior in response to various input voltages can be categorized into ohmic, sub-threshold, and active modes. This is fully analogous to what is known for semiconductor FETs. Pivotal FET properties such as threshold voltage and transconductance must be related to half-cell redox potentials as well as polyelectrolyte and gate material properties. We further extend the analogy with semiconductor FETs through numerical simulations of elementary amplifier circuits in which we successfully substitute the semiconductor transistor by an ionic FET. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W27.00013: Curl flux, coherence, and population landscape of molecular systems: Nonequilibrium quantum steady state, energy (charge) transport, and thermodynamics Zhedong Zhang, Jin Wang We established a theoretical framework in terms of the curl flux, population landscape, and coherence for non-equilibrium quantum systems at steady state, through exploring the energy and charge transport in molecular processes. The curl quantum flux plays the key role in determining transport properties and the system reaches equilibrium when flux vanishes. The novel curl quantum flux reflects the degree of non-equilibriumness and the time-irreversibility. We found an analytical expression for the quantum flux and its relationship to the environmental pumping (non-equilibriumness quantified by the voltage away from the equilibrium) and the quantum tunneling. Furthermore, we investigated another quantum signature, the coherence, quantitatively measured by the non-zero off diagonal element of the density matrix.quantum flux is promoted by the coherence in the regime of small tunneling while reduced by the coherence in the regime of large tunneling, due to the non-monotonic relationship between the coherence and tunneling. For the systems coupled to bosonic (photonic and phononic) reservoirs the flux is significantly promoted at large voltage while for fermionic (electronic) reservoirs the flux reaches a saturation after a significant enhancement at large voltage. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W27.00014: EPR Studies of orthorhombic Jahn-Teller effect in single crystal of ferroelectric Cu(II):Cd$_{2}$(NH$_{4})_{2}$(SO$_{4})_{3}$ Yerima Benson, Dilip De In this paper we report the first EPR observation and theoretical explanation of orthorhombic Jahn-Teller effect in Cu(II) doped single crystal of ferroelectric cadmium ammonium sulphate: Cu(II):Cd$_{2}$(NH$_{4})_{2}$(SO$_{4})_{3}$. The isotropic EPR spectra of the $^{2}$D ion (in regular octahedral symmetry) at higher temperature becomes anisotropic at low temperature with clear manifestation of orthorhombic g and hyperfine tensors at 15 K. The static Jahn-Teller(JT) effect can only be explained theoretically by assuming the three JT potential wells energetically inequivalent, unlike the potential wells in most of the Cu(II) doped crystalline materials where JT effect manifests. The measured splitting of the JT potential wells in this ferroelectric crystal fall in the sub millimeter wave region pointing to possible application of the material. [Preview Abstract] |
Thursday, March 5, 2015 5:18PM - 5:30PM |
W27.00015: Probing molecular dynamics at the nanoscale via an individual paramagnetic center Tobias Staudacher, Nicole Raatz, Sebastien Pezzagna, Jan Meijer, Friedemann Reinhard, Carlos Meriles, Joerg Wrachtrup We use shallow NVs to probe mesoscale proton ensembles from different substances deposited on the diamond surface. We resort to a form of correlation spectroscopy to reconstruct the equivalent of a nuclear ``free-induction-decay'' (FID), which, unlike the NMR counterpart, does not require nuclear spin pre-polarization. This pseudo FID has a limit decay time governed by the NV spin-lattice relaxation time T1 (typically longer than the NV coherence lifetime T2), which allows us to attain spectral resolution superior to that possible with standard magnetometry techniques. Upon applying this scheme to solid- and liquid-state substances we find substantial differences in the correlation signal lifetime, which we associate with the presumably different molecular dynamics governing these systems. In particular, we observe long-lived 1H signals from oil molecules, likely a consequence of the interplay between fast molecular tumbling and slow self-diffusion. [Preview Abstract] |
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