Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A15: Charge Transport at the NanoscaleFocus
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Sponsoring Units: DMP Chair: Colin Van Dyck, National Institute for Nanotechnology, National Research Council Room: LACC 304C |
Monday, March 5, 2018 8:00AM - 8:36AM |
A15.00001: Diodes and switches from single molecules Invited Speaker: Yonatan Dubi The ultimate goal of molecular electronics is to create technologies that will complement—and eventually supersede—Si-based microelectronics technologies. To reach this goal, the field of single-molecule electronics is aiming at recognizing and characterizing single-molecule devices that mimic at least some of the behaviors of today's semiconductor components. In this talk I will review several such single-molecule devices, focusing on DNA-based molecular rectifiers and electro-optical (photo-conductance) switches. I will describe both the theoretical aspects and experimental demonstrations of these devices (coming from the lab of Prof. B.-Q. Xu at the university of Georgia). I will discuss the basic physical processes which are responsible for the devices’ behavior, and design principles for improving them. I will conclude with some thoughts about the future of molecular electronics. |
Monday, March 5, 2018 8:36AM - 8:48AM |
A15.00002: Investigating Single-Molecule Near-Resonant Charge Transport in Electrochemical Environments E-Dean Fung, Jianlong Xia, Brian Capozzi, Luis Campos, Latha Venkataraman Electrochemical methods have been used to realize a number of functions in single-molecule devices such as rectification and gating. Here, we perform scanning tunneling microscope-based break-junction measurements near the resonant tunneling regime of a family of methyl-sulfide terminated oligomers bridged with n units of thiophene-1,1-dioxide (TDOn). We produce current-voltage (IV) curves measured in an electrochemical environment and fit them using a single-level model of transport within the Landauer formalism. Unlike previous works, we do not make the low-temperature approximation, which we find introduces significant errors at room-temperature near the resonant-tunneling regime. We show that one can model the effect of the polar solvent by introducing a bias-dependent parameter into the level alignment. We demonstrate that junction rupture is strongly correlated with the level alignment, suggesting rupture mechanisms stimulated by resonant tunneling. Finally, we compare two-electrode and three-electrode gating measurements to characterize the voltage profile of the electrochemical environment around a single-molecule junction. These advances show that creative use of junction-by-junction IV curve-fitting can uncover a variety of interesting physics in nanoscale devices. |
Monday, March 5, 2018 8:48AM - 9:00AM |
A15.00003: Towards Understanding Energy Level Alignment in Single Molecule Charge Transport with Break Junctions Jeffrey Ivie, Nathan Bamberger, Roland Himmelhuber, Oliver Monti Single molecule based devices represent the ultimate limit in device design, but uncovering the major factors that determine energy level alignment in single molecule junctions and their effect on the quantum transport through single molecules is still a major challenge. We have developed a robust experimental platform, using lithographically-fabricated mechanically controlled break-junctions (MCBJ) in conjunction with high-speed custom instrumentation to address this issue. By introducing electrostatic moments in the molecular design, we are able to tune the energy level alignment in the molecular junction and reveal its influence on electronic transport in a set of closely related molecules. Analysis of the resulting data benefits from advanced data clustering methods capable of revealing the deep structure of the highly stochastic data. |
Monday, March 5, 2018 9:00AM - 9:12AM |
A15.00004: Single Molecule Conductance of Sequence-Defined Oligomers Songsong Li, Kenneth Schwieter, Hao Yu, Bo Li, Jeffrey Moore, Charles Schroeder Understanding electron transport through sequence-defined oligomers is a crucial step for designing new functional materials for energy storage. Recent advances in molecular electronics have brought us closer achieving the ultimate limits in miniaturization and spatial and functional control over electronic performance. Despite recent progress, our knowledge of molecular-scale electron transport is limited by the inability to explore the vast chemical sequence space using existing synthetic methods. In this work, we investigate the conductance properties of sequence-defined oligomers using a scanning tunneling microscope-break junction technique (STM-BJ). Starting from organic small molecules, we find that oxazole can serve as an effective anchor group to gold electrode. Using this approach, we systematically study the conductance pathways through pi-conjugated oxazole-containing molecules and oligomers. In particular, we characterize the electron transport properties of new classes of conjugated oligomers by varying the chemical identity of constituent monomers and the primary monomer sequence. In this way, our work provides the fundamental electron and charge transport information to inform future molecular electronics design. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A15.00005: Conductance of a Freestanding Molecular Wire Aran Garcia-Lekue, T. Jasper-Toennies, Thomas Frederiksen, Sandra Ulrich, Rainer Herges, Richard Berndt Measuring the electrical conductance along a molecule in a reproducible way is an extremely difficult task, as it requires an atomic scale control of the contact formation between the molecule and the electrodes. In this work, we have succeed in controllably forming a contact between the tip of a STM and a freestanding molecular wire, which is placed vertically on the substrate using a platform molecule. The measured complex variation of the conductance with electrode separation is rationalized using DFT transport calculations.[1] At large electrode separation, the transport properties are controlled by the deformation of the molecule, which gives rise to a symmetry mismatch between the tip and molecule orbitals and, hence, to a decrease in the conductance. At closer distances, a covalent bond is formed between the Au tip and a triple CC bond, inducing a significant change of the electronic spectrum and an order of magnitude increase in the conductance. Therefore, by a tip-controlled reversible bond formation or rupture we are able to switch the current on or off. [1]T. Jasper-Tönnies, A. Garcia-Lekue, et al. Phys. Rev. Lett. 119, 066801 (2017). |
Monday, March 5, 2018 9:24AM - 9:36AM |
A15.00006: Electronic Structure and Charge Transport Properties of N-Heterocyclic Carbenes on Au Giacomo Lovat, Evan Doud, Deyu Lu, Michael Inkpen, Gregor Kladnik, Dean Cvetko, Alberto Morgante, Mark Hybertsen, Xavier Roy, Latha Venkataraman N-heterocyclic carbenes (NHCs) enable the fabrication of robust self-assembled monolayers on gold substrates that can outperform thiol-based counterparts. Here, we study the metal-molecule interactions and electronic structure of NHCs on an Au (111) surface using X-ray photoelectron spectroscopy, single-molecule transport and density functional theory. Through X-ray measurements and density functional theory, we reveal how steric constraints introduced by different ancillary substitutions shape the interaction of NHCs with Au and steer the molecular adsorption geometry. By means of the scanning tunneling microscope break-junction method, we investigate the formation and charge transport properties of NHC-terminated molecular conductors, demonstrating that these carbenes can function as novel and robust linker groups for single-molecule electronics. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A15.00007: Electron Transport Through Peptides and Blue-Copper Azurins Linda Angela Zotti, Marta P. Ruiz, Albert C. Aragonés, Nuria Camarero, Jose Guilherme Vilhena, Maria Ortega, Ruben Perez, Juan Carlos Cuevas, Pau Gorostiza, Ismael Diéz-Pérez Inspired by recent experiments [1], we present a theoretical study of the electron transport through heptapeptides based on alanine, glutammic acid, lysine and tryptophan. For them all, we found very low conductance values and we ascribed them to the high localization of their frontier orbitals [2]. We also show a combined experimental and theoretical work on the transport through a bluecopper azurin and a mutant; we found that the conductance channels in the single-protein electrical contact can be finely tuned by performing point-site mutations in the outer protein structure [3]. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A15.00008: Carrier scattering with sources and absorbers in Nanoscale Systems Sathwik Bharadwaj, L Ram-Mohan We present a novel method based on sources and absorbers to examine scattering in finite, nanoscale systems. The Cauchy (mixed) boundary conditions (BCs) needed in an action integral formulation of scattering are reduced to simpler Dirichlet BCs by introducing totally absorbing elements, or “stealth elements,” whose material properties are optimized to give decaying solutions that vanish at the boundaries. The method retains all the physical aspects of the usual theory while providing new insights into scattering effects and predicts highly accurate numerical “near-field” solutions. The action integral is discretized and evaluated to derive the wavefunction everywhere. In 1D, we provide concrete examples and demonstrate the accuracy of this method. In 2D confined waveguides, we obtain scattered wavefunctions for geometrically complex scattering centers, and multiple scattering that go beyond the traditional perturbative and far field approximations. The modal analysis of reflected and transmitted waves allows us to obtain transmission coefficients for both propagating and evanescent modes. The Landauer conductance and thermopower for the ballistic transport as well as in the presence of scatterers are calculated. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A15.00009: Shot Noise in Benzenedithiol Single-molecule Junctions from the Perspective of Coherent Wave Function Yu-Chang Chen, Bin OuYang, Bailey C. Hsu Electron transport is typically incoherent in mesoscopic systems. However, electron transport in single-molecule junctions is coherent. Discrete channels may not be reformed in coherent wave functions due to the lack of wave guides. Therefore, shot noise theory identifies fundamental differences between mesoscopic systems and single-molecule junctions. To understand shot noise in single-molecule junctions from the perspective of coherent transport, we calculate shot noise in a benzenedithiol single-molecule junction in terms of effective single-particle wave functions that are obtained self-consistently within density functional theory in scattering approaches. The theoretical value of shot noise is approximately S≈4.03×10-26 A2/Hz at VB=0.01 V at a high conductance state (σ≈0.23 G0), which is in good agreement with the value obtained from a recent experiment (S≈4.37×10-26 A2/Hz). Our calculations show that the S-Au bonds form the bottlenecks for the current, where the Px-y orbital dominates the density of states in the energy window between EFL and EFR. This finding implies that the shot noise is roughly carried by a single conduction channel owing to the π-bond formed between the sulfur and gold atom, as suggested by the experiment results. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A15.00010: Theory of current fluctuation for single molecule junction with intra-molecule Coulomb interaction and multimode vibronic interactions Kuniyuki Miwa, Hiroshi Imada, Feng Chen, Kensuke Kimura, Miyabi Imada, Yousoo Kim, Michael Galperin With recent progresses in experimental techniques at nanoscale, it becomes possible to observe the transport properties of a single molecule in contact with metals beyond average flux measurement. Fluctuations in the electrical current originating from the discreteness of charges are known as shot noise that provides complementary information not accessible through conductance measurements. In this study, we develop the theoretical technique to calculate the current noise for single molecule junction with intra-molecule Coulomb interaction and multimode vibronic interactions within the framework of quantum many-body theory. The calculations are performed employing nonequilibrium Green’s function method formulated via Hubbard X-operators to unveil and predict the effects of intra-molecular interactions on noise characteristics of the system. The verification of the results and details of the noise evaluations will be discussed. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A15.00011: Third Moment of Current Fluctuations in a Diffusive Conductor Edouard Pinsolle, Christian Lupien, Bertrand Reulet There has been only a few attempts to push deeper the study of current fluctuations in mesoscopic conductors by tackling the measurement of higher order moments such as the third one <δI3>. Such measurements have been performed only in tunnel junctions and quantum dots, revealing physics hidden in the study of the second moment <δI2> such as the coupling with the environment or the ordering of operators in a quantum measurement. Even the statistics of current fluctuations in the simplest system, an electrical wire, has never been probed experimentally. |
Monday, March 5, 2018 10:36AM - 10:48AM |
A15.00012: Quantum current fluctuations in a tunnel junction at optical frequency Pierre FEVRIER, Julien Gabelli We report the measurement of emission noise by a planar metallic tunnel junction driven far from its equilibrium state. To investigate this regime, we bias the junction at voltage V>1Volt, and measure the emitted photons at optical frequency (f < eV/h ∼ 1014Hz). This emission results from the scattering of surface-plasmon-polaritons, generated by high frequency current fluctuations inside the junction due to tunneling electrons (shot-noise). This allows noise measurement at timescales smaller than the RC time of the junction. We show that the emitted photon power Pf(V) at frequency f, which is proportionnal to the current spectral density SII(f,V), isn't given by the usual fluctuation-relation which have been previously demonstrated in the GHz range for metallic and supraconductive linear tunnel junction. Our results are in agreement with a prediction based on the Landauer-Büttiker scattering approach accounting for the energy/voltage dependence of the transmission of the tunnel barrier. With a quantitative calculation of the emission efficiency, we demonstrate that the photon emission results from current fluctuations inside the barrier. |
Monday, March 5, 2018 10:48AM - 11:00AM |
A15.00013: When can Time-Dependent Currents be Reproduced by the Landauer Steady-State Approximation? Rachel Carey, Liping Chen, Bing Gu, Ignacio Franco We establish well-defined limits in which the time-dependent electronic currents across a molecular junction subject to a fluctuating environment can be quantitatively captured via the Landauer steady state approximation. For this, we calculate the exact time-dependent non-equilibrium Green’s function (TD-NEGF) current along a model two-site molecular junction, in which the site energies are subject to correlated noise, and contrast it with that obtained from the Landauer approach. The Landauer steady-state approach is found to be a useful approximation when (i) the fluctuations do not disrupt the degree of delocalization of the molecular eigenstates responsible for transport and (ii) the characteristic time for charge exchange between the molecule and leads is fast with respect to the molecular correlation time. These criteria can be employed to adopt effective modeling strategies for transport through molecular junctions in interaction with a fluctuating environment, as is necessary to describe experiments. |
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