Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H11: Focus Session: Transport Properties of Nanostructures II: Molecular Junctions I |
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Sponsoring Units: DMP Chair: Latha Venkataraman, Columbia University Room: 305 |
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H11.00001: Conductance of Conjugated Organic Compounds in Controlled Environments Invited Speaker: We use the mechanical and the electromigration break junction technique, as well as nanoparticle arrays, to measure the electrical conductance of a range of conjugated organic molecules with different end functionalities at room temperature in a liquid cell. We first report on a comparison between oligo(phenylene vinylene) (OPV) oligo(phenylene ethynylene) (OPE). We find that OPV conducts slightly better than OPE. Solubilizing side groups do not prevent the molecules from being anchored within a break junction. With the aim to realize a functional switch, we show preliminary electrical conductance studies of a newly synthesized cruciform molecule. Using the nanoparticle platform we further demon-strate light and electrochemical-induced conductance switching of photochromic and redox molecules. We further discuss OPV and OPE molecules with different end groups, including asymmetric ones. To our surprise, molecules having an anchor group only on one side also gave rise to a pronounced mo-lecular signal. We attribute this effect to the interaction between neighboring molecules in the junction likely induced by $\pi$-$\pi$ stacking. This remarkable property highlights the importance of intermolecu-lar interaction in molecular junctions, an often overlooked aspect. If time permits, a recent study on low- frequency fluctuations in molecular junctions will be mentioned as well. Collaborators are (alphabetic order): J. Agustsson, J. Brunner, M. Calame, T. Gonzalez, S. Grunder, V. Horhoiu, R. Huber, J. Liao, M. Mayor, M. Mangold, S. Oberholzer, M. Steinacher, S. Wu, Z.M. Wu, (all at the Swiss Nanoscience Institute at the Univ. of Basel) and M. R. Bryce (Durham University, UK). [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H11.00002: Externally controlled spin state switching in metal-organic complexes. Alexei Bagrets, Velimir Meded, Mario Ruben, Ferdinand Evers Recent transport experiments have demonstrated that a manipulation of the charge of individual molecules is feasible using electromigrated metal junctions [1] or electrochemical gates in conjunction with the STM [2]. Using elaborated density functional theory calculations, we will discuss a possibility to induce -- by means of charging or applied stress -- a switching between low and high spin states in certain metal-organic systems, [Fe(bpp)$_2$]$^{2+}$ (bpp:\ bispyrazolyl pyridine) and [Mn(tpy)$_2$]$^{2+}$ (tpy:\ terpyridine). Based upon a recent success of the single molecular conduction experiment through Ru(II) complex [3], we anticipate the transport properties of Fe(II) and Mn(II) complexes to be gate controlled via exploiting their spin degree of freedom. \medskip \newline [1] E.\ A.\ Osorio {\it et al.}, J.\ Phys.:\ Condens.\ Matter{\bf 20}, 374121 (2008); [2] F.\ Chen {\it el al.}, Ann.\ Rev.\ Phys.\ Chem.\ {\bf 58}, 535 (2007); Li {\it et al.}, Nanotechnology {\bf 18}, 044018 (2007). [3] M.\ Ruben, A.\ Landa, E.\ L{\"o}rtscher, H. Riel, M. Mayor, H.\ G{\"o}rls, H.\ Weber, A.\ Arnold, and F.\ Evers, Small (online), {\small DOI}: 10.1002/smll.200800390 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H11.00003: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H11.00004: Quantum-Interference-Controlled Molecular Electronics San-Huang Ke, Weitao Yang, H. U. Baranger Quantum interference in coherent transport through single molecular rings may provide a mechanism to control the current in molecular electronics. We investigate its applicability, using a single-particle Green function method combined with \textit{ab initio} electronic structure calculations. We find that the quantum interference effect (QIE) is strongly dependent on the interaction between molecular $\pi $-states and contact $\sigma $-states. It is masked by $\sigma $ tunneling in small molecular rings with Au leads, such as benzene, due to strong $\pi -\sigma $ hybridization, while it is preserved in large rings, such as [18]annulene, which then could be used to realize quantum interference effect transistors. [Nano Letters 8, 3257 (2008)] [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H11.00005: Simultaneous electronic transport and Raman spectroscopy in single-molecule devices Invited Speaker: Over the last decade several techniques have been developed to examine electronic transport through individual small molecules. These include scanned probe methods, mechanical break junctions, and electromigrated junctions. One recurring challenge is the need to confirm that current flow is, indeed, through the molecule of interest rather than a contaminant. We recently discovered (D. R. Ward \textit{et al}., Nano Lett. \textbf{7}, 1396 (2007)) that the same electromigrated Au source and drain electrodes used for transport are tremendously effective optical antennas in the near infrared. Surface plasmon modes localized to the nm-scale interelectrode gap lead to large enhancements of the local electric field relative to that from incident radiation. The result is that these nanoscale gaps are tremendous ``hot spots'' for surface-enhanced Raman scattering (SERS). We perform simultaneous measurements of electronic transport and SERS in junctions incorporating molecules of interest, and find in 10-15\% of devices that the conductance and SERS emission are strongly correlated in time (D. R. Ward \textit{et al}., Nano Lett. \textbf{8}, 919 (2008)). Since the conductance mechanism is tunneling and therefore dominated by a volume comparable to that of a single molecule, this strongly implies that the SERS emission comes from that same molecule. The distinctive SERS spectra allow us to confirm that conduction in these devices is through the molecule of interest. Furthermore, these devices open up many opportunities, including studies of electron-vibrational couplings and dissipation at the single-molecule level. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H11.00006: Simultaneous Measurements of Force and Conductance through Single Molecular Junctions Michael Frei, Maria Kamenetska, Max Koentopp, Mark S. Hybertsen, Latha Venkataraman Simultaneous conductance and force measurements of single molecule junctions are performed by repeatedly forming and breaking junctions between a molecule coated gold substrate and a gold-coated cantilever in a simplified atomic force microscope (AFM). We show that the forces required to break the molecular junctions in the case of 1,4 diaminobutane and 1,4 bis (methyl thiol) butane are significantly smaller than the breaking force of a single atom gold contact. This indicates a breaking of the Au-N and Au-SMe bond respectively. Data for 1,2-bis(dimethyl phosphino) ethane differs significantly. We find that the force required to break the molecular junction is comparable to that required to break a gold-gold bond. We find further that for a significant fraction of the traces measured, the molecular junction conductance often drops by an order of magnitude, while the forces do not change significantly. We will discuss the implications of these findings and show how they relate to detailed simulation of the junction elongation process for these links. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H11.00007: Simulation and Measurement of Single Molecule Junction Evolution Under Stress: Comparison of Amine and Phosphine Link Groups Max Koentopp, Mark Hybertsen, Maria Kamenetska, Adam Whalley, Young Park, Michael Steigerwald, Colin Nuckolls, Latha Venkataraman Reliable measurements of single molecule conductance are performed by repeated breaking of Au point contacts in a solution containing target molecules with link groups that readily form donor-acceptor bonds to specific Au atoms on the electrodes. Measured traces present the junction conductance as a continuous function of junction elongation under applied stress. Our extensive experimental database shows strong correlation between measured conductance step length and molecular backbone length, with phosphine link groups showing longer steps than amine link groups. We model adiabatic junction evolution by discrete steps, with structure determined by energy minimization in a DFT approach and the low bias junction conductance at each step computed using a Green's function approach. We identify different mechanisms whereby the attachment point to the electrode can shift while maintaining similar conductance, explaining why conductance steps can extend over distances of several angstroms. Phosphine and amine link groups sustain different maximum forces, accounting for key differences in junction evolution. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H11.00008: Kondo effect in the electronic transport of magnetic atomic-size contacts M. Reyes Calvo, Joaquin Fernandez-Rossier, Juan Jose Palacios, David Jacob, Douglas Natelson, Carlos Untiedt Low coordination alters dramatically the magnetic properties of materials at the nanoscale. Our results indicate that, in the case of atomic contacts of certain materials, a localized magnetic moment appears at the contact and is screened at low temperatures by means of the Kondo effect. We observe characteristic Fano-Kondo lineshapes in the spectroscopy of atomic contacts of typical ferromagnetic materials (Fe, Co and Ni). The parameters obtained from the fitting of these curves to the Fano equation show statistical distributions that agree with the Kondo theory. The Kondo origin of the measured resonances is confirmed by their temperature dependence and supported by our theoretical calculations. These results are surprising since ferromagnetism and Kondo effect are expected to compete. We have also observed a similar spectroscopy in atomic contacts of palladium and platinum monoatomic chains. In this case, the Kondo resonances would be the signature of an emergent magnetism that is ultimately screened. Partially founded by Spanish MEC (grant nr. MAT2007-65487 and CONSOLIDER CSD2007-0010) and EU (project nr. 211284) [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H11.00009: Magnetic phenomena, ferro and antiferro Kondo, and transport in transition metal break junction nanocontacts Erio Tosatti Transport in transition metal break junctions is of particular interest in connection with magnetism, which may be present either in the leads, and/or in bridging magnetic impurities, or else which may emerge spontaneously at the nanocontact, as expected in pristine Pt or Pd.[1] The standard Landauer ballistic conductance across the locally magnetic atomic contact is modified by Kondo phenomena that are predicted using ab initio electronic structure as a starting point, and the numerical renormalization group as a tool. The case of a magnetic impurity in a nonmagnetic contact[2] is shown to illustrate the competition between antiferro and ferro Kondo screening[3] and its effects on conductance. *) Work in collaboration with A. Smogunov, A. Dal Corso, L. De Leo, P. Gentile, M. Fabrizio, P. Lucignano, and R. Mazzarello. [1] A. Delin, et al., Phys. Rev. Lett. 92, 057201 (2004); A. Smogunov et al., Phys. Rev. B78, 014423 (2008). [2] R. Mazzarello, P. Lucignano, A. Smogunov, M. Fabrizio, and E. Tosatti, in preparation [3] P. Gentile, L. De Leo, M. Fabrizio, and E. Tosatti, in preparation [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H11.00010: Spectra of discrete electrons-in-a-box energy levels in chemically-formed Au and Pt nanoparticles. Su-Fei Shi, F. Kuemmeth, K.I. Bolotin, W. Li, D.C. Ralph We report tunneling spectroscopy measurements of the discrete electron spectra of individual metal nanoparticles formed by chemical synthesis, so that they are well-defined in their composition, size, and shape. The spectra of 5-15 nm diameter gold particles exhibit as many as 40 resolvable electronic excited states for a fixed value of gate voltage. We find excellent agreement between the measured level statistics and random matrix predictions for the regime of strong spin-orbit coupling and ballistic transport. As a function of changing gate voltage, the energy-level spectra in the Au particles are not scrambled by the addition of electrons, indicating that in these particles the variation in the strength of electron-electron interactions between states is negligible. We have also succeeded in fabricating single-electron transistors from individual Pt nanoparticles and hope to present measurements of their discrete spectra. Pt, unlike Au, has a sufficiently strong exchange interaction that it is expected to exhibit non-zero values of the ground state spin in the form of ``mesoscopic magnetism''. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H11.00011: Ballistic transport in gold [110] nanowire Yoshihiko Kurui, Yoshifumi Oshima, Masakuni Okamoto, Kunio Takayanagi Conductance of gold nanowire elongated along the [110] direction (gold [110] nanowire) was measured during many breaking procedures, while simultaneously acquiring transmission electron microscope images. The conductance histogram exhibits a series of peaks whose conductance values increased nearly in steps of the conductance quantum, G0 =2e$^{2}$/h. However thick nanowires above 10G$_{0}$ showed dequantization, where the increment was only 0.9G$_{0}$. The structure for each peak was determined to be either an atomic sheet or a hexagonal prism. The number of conductance channels calculated for each atomic structure by first principles theory, coincided well with the peak index in the conductance histogram. The present study shows that the [110] nanowire behave as ballistic conductors, and a conductance peak appears whenever a conductance channel is opened. [Preview Abstract] |
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