Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H17: Focus Session: Transport Through Molecules: Single Molecule Junctions |
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Sponsoring Units: DMP DCP Chair: Robert L. Willett, Bell Laboratories Room: LACC 404 B |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H17.00001: Electronic States and Electronic Coupling in Organic Molecular Systems Invited Speaker: The understanding of electronic transport in organic molecular systems is at an early stage. The systems of interest range from crystals and thin films with well defined bulk transport to the limit of single molecule devices where the source and drain electrodes are integral to the transport processes. Nonetheless there are several important factors that are shared among these different systems, including frontier electronic energy level alignment, electronic coupling and structural reorganization or vibrational coupling. In this talk, the electronic coupling and energy level alignment for several prototypical systems will be presented, based on self consistent first principles calculations. The goal is to understand the relationship between molecular structure, the binding of the molecules to each other or to a metal contact and the electronic coupling that is developed among the frontier states that support transport. For intermolecular coupling in organic molecular crystals, the impact of structure on electronic coupling has been analyzed by comparing the energy bands in derivatized acenes that form crystals with different packing motifs. In the case of single aromatic molecules on metal contacts, several link chemistries have been studied. One of the themes that emerges from these studies is that the electronic coupling pertinent for transport plays a minor role in the binding of the system. This poses a significant challenge in the search for molecular systems with improved transport characteristics. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H17.00002: Near-Perfect Conduction through a Single Molecular Orbital in a Ferrocene-Based Molecular Wire Michael S. Fuhrer, Stephanie A. Getty , Chaiwat Engtrakul, Lixin Wang, Lawrence R. Sita, Rui Liu, San-Huang Ke, Harold U. Baranger, Weitao Yang We have studied the electron transport through metal-molecule-metal junctions formed by electromigration of a gold wire in the presence of two phenylethynyl-based dithiol molecules: one containing a central ferrocene moiety (Fc-OPE), and the other a similar-length phenylethynyl analog (OPE). We find that the bias-dependent differential conductance of Fc-OPE shows Lorentzian peaks at small (less than 100 mV) positive and negative bias, with magnitude exceeding 70\% of the conductance quantum G$_0$. The results demonstrate the expected resonant conduction through an extended orbital network long-predicted (but not previously observed) for a conjugated organic system. In contrast, experiments on OPE show much lower conductance, and a gapped region of several hundred millivolts, consistent with previous experimental results on similar all-organic molecular wires. Density functional theory (DFT) and Green function techniques confirm the existence of a low-lying molecular orbital with high transmission in Fc-OPE. Calculations also predict high conductance in OPE, reproducing the long-standing disagreement between experiment and DFT for all-organic molecular wires. While the results do not resolve this dilemma, they place important constraints on future theoretical explanations. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H17.00003: The Kondo Effect in Transition Metal Ion Based Single-Molecule Transistor Lam Yu, Zachary Keane, Jacob Ciszek, Long Cheng, Michael Stewart, James Tour, Douglas Natelson We have used an electromigration technique to fabricate single-molecule transistors (SMTs). The molecule used is C$_{32}$H$_{16}$XN$_{10}$S$_{4, }$where X is a single transition metal ion (e.g. Co, Ni, Cu, Zn), coordinated by conjugated ligands. Upon assembly on gold in tetrahydrofuran (THF), the molecule undergoes loss of the (CN) moieties, and the remaining molecule is covalently bonded to surface Au atoms on the electrodes of the transistors. In several devices we observed conductance features characteristic of the Kondo effect, a coherent many-body state comprising an unpaired spin on the molecule coupled by exchange to the conduction electrons of the leads. We will discuss the temperature dependence of the peak conductance and the low temperature width of the Kondo resonance of the various transition metal ion based SMTs. We have also observed asymmetric Fano-like resonances in some SMTs, which we believe result from interference between a resonant and a nonresonant conduction path through the SMTs. We will report the temperature and gate voltage dependences of the lineshape of these Fano-like resonances, and discuss possible origins of the nonresonant conduction path. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H17.00004: Mechanically-Adjustable and Electrically-Gated Single-Molecule Transistors Alexandre Champagne, Abhay Pasupathy, Joshua Parks, Daniel Ralph We describe the experimental characterization of molecular transistors that can be both mechanically adjusted and electrostatically gated. We control the source-drain spacing with better than 1 pm stability by making use of a mechanically-controlled breakjunction geometry. In addition, by using silicon substrates, we can employ standard lithographic techniques to suspend our breakjunctions only 40 nm above the substrate surface, enabling the use of the substrate as an electrostatic back-gate. We present data for single-electron-transistor C$_{60}$ devices which show that we can simultaneously tune the source-drain electrodes spacing by 5 $\AA$ mechanically and shift the molecule’s energy levels by 160 meV electrostatically. With the independent in-situ variations provided by these two experimental ``knobs'', we are able to achieve a much more detailed characterization of electron transport through the molecule than is possible with either technique separately. We also present initial results for gated devices in which a single carbon nanotube is stretched axially. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H17.00005: Prevalence of Coulomb blockade in electro-migrated junctions with conjugated and non-conjugated molecules Anat de Picciotto, Jennifer Klare, Kenji Sugo, Colin Nuckolls, Artur Erbe, Kirk Baldwin, Robert Willett The conduction properties of electro-migration gap junctions with various organic molecules incorporated in the gaps are studied in order to expose the full range of possible transport processes, their prevalence, and the model morphologies: metal to molecule to metal, metal to spurious gold clusters to metal, and their permutations. Comparisons are made between molecules with an electron delocalized vs. electron-localized backbone, between molecules with one vs. two thiol end groups, and between molecules with and without large side chains. Coulomb blockade can be observed in all molecular species tested, including bare junctions and those coated by molecules with no electron-accepting properties, but at significantly lower prevalence than molecules with delocalized electron backbones. Importantly, Coulomb blockade with high charging energy values is seen almost exclusively on junctions with molecules possessing the delocalized electrons. These results indicate the scope of variation in transport possible for molecules on electro-migrated junctions. This work is supported by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number CHE-0117752. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H17.00006: Organometallic Spintronics: Dicobaltocene Switch Rui Lui, San-Huang Ke, Weitao Yang, Harold Baranger We propose a spintronic switch formed from a single molecule containing two cobaltocene complexes. Spin-dependent transport has been investigated, using first-principles density functional theory and the non-equilibrium Green function method. By switching the dicobaltocene system between singlet and triplet states, our calculations reveal that the spin antiparallel configuration blocks electron transport in the vicinity of the Fermi energy, while the spin parallel configuration enables much higher current through the lead-molecule-lead junction. We find that the energy difference between the ground state (antiparallel) and the excited state (parallel) is a function of the length of insulating spacer separating the two spins residing in the cobaltocenes; this implies that the switching functionality can be realized by applying a moderate magnetic field. This work was supported in part by the NSF (DMR-0103003). [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H17.00007: Vibrational Excitations in Single-Trimetal-Molecule Transistors Dong-Hun Chae, Zhen Yao, John F. Berry, Carlos A. Murillo, F. Albert Cotton Transistors have been fabricated by incorporating single molecules of Cu$_{3}$(dpa)$_{4}$Cl$_{2}$ and Ni$_{3}$(dpa)$_{4}$Cl$_{2}$ (dpa = 2,2'-dipyridylamide) in a nanometer-sized gap between a pair of gold electrodes created by the electromigration technique on an oxidized aluminum electrode which serves as a gate. Conductance is measured as a function of the bias and gate voltages at 4.2 K, showing single-electron tunneling behavior through the inserted complexes. Additional structures corresponding to the excitations in the molecules have been observed, which are attributed to the intramolecular vibrational excitations of the molecules coupled to the single-electron tunneling processes. The energies of the vibrational states are dependent on the redox states of the included molecules. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H17.00008: Room-temperature single-electron transistors with individual alkanedithiol molecules Kang Luo, Dong-Hun Chae, Suyong Jung, Zhen Yao We present the fabrication and characterizations of single-molecule transistors consisting of individual alkanedithiol molecules. The devices are fabricated by assembling molecules between nanometer-spaced Au electrodes which are first created by the electromigration technique on top of the oxidized Al gate electrode. The measurements at 4.2 K show clear Coulomb blockade behavior with addition energies on the order of several hundred meV. There also exist additional excitations outside the blockaded region. Because of the extremely large addition energies, single-electron tunneling behavior persists even up to room temperature. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H17.00009: Phonon-Rabi-Resonance and Assisted Tunneling in Molecules Edson Vernek, Enrique Anda, Nancy Sandler, Sergio Ulloa Transport through nanoscopic systems with strong electron interactions, such as atoms, molecules, and quantum dots, has received increasing attention. It is known that electron-electron (EE) and electron-phonon (EP) interactions have non-trivial effects on the transport properties of these systems. However, relatively little is known about the {\em joint} effect of EE and EP interactions, and one expects new transport properties in systems within this regime. We study a model for a system of two quantum dots connected to two independent leads. Electron hopping between the dots is accompanied by phonon emission or absorption. We use the equations of motion to calculate electronic Green's functions and relevant transport properties. We find that Coulomb repulsion modifies the usual resonant condition, and phonon energy and electron level spacing in the dots are no longer matched. The new resonant condition involves an energy scale arising from EE interactions for partially or fully occupied sites, {\em irrespective of inter-site correlations}. We also find that Coulomb blockade peaks in resonance exhibit Rabi-like splittings in the conductance. These effects are strongly affected by temperature, which would be a clear experimental signature of the effect. \newline \newline Supported by CAPES-Brazil and NSF-IMC and NSF-NIRT. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H17.00010: Effective Tunneling of Torsional Polarons in Short Molecules Efta Yudiarsah, Sergio Ulloa We study the properties of polaron states in short molecules modeled by electron hopping among few sites and interacting {\em anharmonically} with rotational vibrational modes [1]. Our study includes numerical solution of the polaron models as well as analytical limiting expressions for different coupling regimes. We consider the interference of two types of hopping terms in the Hamiltonian, one that is angle dependent while the other one is not. Our model considers how dynamical coupling of torsional modes to orbital overlaps affects the transport properties of the electrons in molecules. We use different models of angle-dependent hopping with built-in asymmetries and find that they give opposite behavior in the polaron effective hopping constant: while polaron ground states are found to be typically more ``massive'' and have smaller effective hopping, asymmetries result in {\em larger} hopping with increasing restoring force of the vibrational modes. A more complex behavior is found for excited states. This behavior may have consequences for molecular transport experiments in flexible short molecules. Supported by NSF-NIRT. \\[4pt] [1] W. Zhang, A. O. Govorov and S. E. Ulloa, Phys. Rev. B 66, 060303(R) (2002) [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H17.00011: Theory of tunneling and electron transport through single molecules of polyaniline Otto Sankey, Myeong Lee, Gil Speyer Polyaniline is a linear organic polymer used often in battery devices, films, and organic electrodes, and it exhibits a dramatic increase in conductivity due to electrochemical oxidation or acidic treatment. The polymer chain has different bonding configurations depending on the oxidation state -- they include leucoemeraldine, emeraldine, and pernigraniline bases and their salts. Most forms are semiconducting and electron transport through these forms as single molecules is expected to occur via a tunneling mechanism. We have computed the complex bandstructure (which include imaginary k-vectors to allow for tunneling) of the semiconducting forms to gain insight into the expected length dependence of the electron tunneling current ($\sim $e$^{-\beta L})$ and the energy dependence of the decay parameter $\beta $. Model calculations of the I-V curves for metal/single-molecule/metal molecular electronics geometries will be discussed (the metal is gold). The electron transport calculations use DFT Green's function scattering methods. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H17.00012: Electrical Response of Molecular Chains from Density Functional Theory Stephan Kuemmel, Leeor Kronik, John P. Perdew The electrical response of molecular chains is dramatically overestimated by local and semilocal density functionals. We show that Kohn-Sham density-functional theory yields accurate linear and nonlinear polarizabilities when the exact exchange energy is employed together with the corresponding exact Kohn-Sham potential. We further show that approximations to the exact-exchange Kohn-Sham potential that are very accurate for the ground-state energy can nevertheless fail badly for the response because of potential barriers that have little effect on the ground-state energy but strongly affect the electron mobility. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H17.00013: Molecular Spintronics: Theory of Spin-Dependent Electron Transport Between Iron Nano-Contacts Bridged by Organic Molecules and Fe Atomic Chains* Hugh Dalgleish, George Kirczenow Recent experiments [1] have lent support to theoretical predictions [2] that organic molecules connecting nickel nano-contacts may exhibit magneto-resistance and spin-valve effects. Here we present predictions of spintronic phenomena in another class of ferromagnetic nano-systems: Fe nano-contacts bridged by single conducting or insulating molecules or chains of Fe atoms. Models are constructed based on semi-empirical considerations, the known electronic structure of bulk Fe and ab initio density functional calculations. Using Lippmann-Schwinger and Green's function techniques, and Landauer theory, significant magneto-resistance is predicted in these systems. Under appropriate conditions, novel device characteristics such as negative magneto-resistance are also predicted to emerge. \newline \newline * Supported by NSERC and the Canadian Institute for Advanced Research. \newline 1 J. R. Petta et al., Phys. Rev. Lett. 93, 136601 (2004). \newline 2 E. G. Emberly and G. Kirczenow, Chem. Phys. 281, 311 (2002); R. Pati, et al., Phys. Rev. B 68, 100407 (2003). [Preview Abstract] |
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H17.00014: Rectifying effects in di-block oligomer molecular diodes Ivan Oleynik, Mortko Kozhushner, Vladimir Posvyanskii Very recently, diode-type I-V characteristics were measured in di-block oligomer molecular diodes. These molecules consist of electron-rich and electron-deficient $\pi $-congugated oligomers that exhibit built-in electronic asymmetry. We will discuss fundamental transport mechanisms in di-block oligomer molecular diodes based on the features of the energy spectrum of the tunneling electron. Green's function theory sub-barrier scattering is applied to predict current-voltage characteristics of these complicated molecules. In particular, the rectifying effects are explained by different energy dependence of scattering operators of thiophene and thiazole structural units. [Preview Abstract] |
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H17.00015: Temperature-dependent molecular conduction measured by the electrochemical deposition of a platinum electrode in a lateral configuration Y.W. Park, B. Kim, S.J. Ahn, J.G. Park, S.H. Lee, Eleanor E.B. Campbell Temperature-dependent current--voltage ($I$--$V)$ characteristics
of a molecule,
1,4-benzenedimethanethiol, was measured for 30 K$ |
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