Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V22: Focus Session: Organic Electronics: Molecular Junctions |
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Sponsoring Units: DPOLY DMP Chair: James Kushmerick, National Institute of Standards and Technology Room: Morial Convention Center 214 |
Thursday, March 13, 2008 11:15AM - 11:51AM |
V22.00001: Molecular Thermoelectrics Invited Speaker: Thermoelectric materials for energy generation have several advantages over conventional power cycles including lack of moving parts, silent operation, miniaturizability, and CO2 free conversion of heat to electricity. Historically, these materials suffered from low efficiency and have involved very expensive inorganic compounds. We demonstrate that some metal-molecule-metal junctions produce voltage when exposed to heat. This exciting initial demonstration of thermopower from molecular junctions is reinforced by the fact that relatively minor changes to chemical structure result in an unprecedented simultaneous increases in both thermopower and electrical conductance, suggesting that highly efficient devices may be \textit{designed} in a manner impossible in traditional thermoelectric materials. Furthermore, thermopower measurements offer an alternative transport measurement that can characterize the dominant transport orbital and is independent of the number of molecules in the junction. In particular, the effect of substituents and endgroups on the electronic behavior of gold-1,4-benzenedithiol -gold junctions (BDT) is explored. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V22.00002: Chemical Structure and Molecular Switches. Amy Blum, David Long, Martin Moore, James Kushmerick, James Tour, Banahalli Ratna The future of molecular electronics depends on designing molecules to exhibit specific nonlinear properties such as rectification or bistable switching. In pursuit of this goal, two distinct types of switching were observed in matrix isolated and complete monolayers of bipyridyl-dinitro-oligophenylene-ethynylene (BPDN). Several groups have observed conductance state switching in this molecule. However, the mechanism of switching between the two conductance states is still not understood. Using BPDN as a starting point, chemically related structures such as bipyridyl- oligophenylene-ethynylene, dinitro-oligophenylene-ethynylene, and biphenyl- oligophenylene-ethynylene were measured in matrix isolated monolayers. By means of such comparisons to related molecules, we determine the key functional groups leading to switching in BPDN. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V22.00003: Inelastic electron spectroscopy of single alkanedithiol molecules Nicolas Agrait, Carlos R. Arroyo We measure charge charge transport through single molecules between gold electrodes at low temperature using STM. The derivative of the conductance vs voltage (IES spectrum) shows clear peaks corresponding to the vibrational modes of the molecule. The frequency dependence on stretching and comparison with other vibrational spectroscopy studies makes possible to assign these peaks to the longitudinal modes of the molecule. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V22.00004: A Molecular Switch Made of Charge Transfer Complexes on Au (111) U.G.E. Perera, F. J\"ackel, V. Iancu, K.-F. Braun, N. Koch, J.P. Rabe, S.-W. Hla A low temperature scanning tunneling microscope (STM) and spectroscopy study of organic charge transfer complexes is presented. The complexes are formed by self assembly of the electron donor $\alpha $- sexithiophene (6T) and the electron acceptor tetrafluro-tetracyano-quinodimethane (F$_{4}$TCNQ) on Au (111) surface. The formation of new hybrid molecular orbitals in CTCs is evident by STM imaging at different bias voltages as well as by differential conductance spectra. The charge transfer lead to the shift of HOMO and LUMO orbitals of the molecules in complexes with respect to the pure molecular orbitals. Finally, we use a voltage dependent resonance-tunneling scheme to demonstrate a molecular switch made of F4TCNQ/6T charge transfer complexes. This work is financially supported by the US-DOE grant DE-FG02-02ER46012. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V22.00005: Correlating Structure and Conductivity of Pentathiopene Monolayers Bas Hendriksen, Yabing Qi, Florent Martin, Frank Ogletree, Miquel Salmeron Understanding the electrical conduction mechanisms in organic materials is important for the development of plastic and molecular electronics. The charge transport properties of conducting molecular layers are expected to strongly depend on the order of the layer and the conformation of the molecules. We used atomic force microscopy (AFM) to study and correlate the structural, mechanical and electrical properties to molecular monolayers of pentathiophene based molecules on solid substrates prepared by the Langmuir-Blodgett technique. The molecular monolayers consist of two phases: one phase of compact micrometer size flower-shaped islands and a phase with less order and a high density of holes. We found that the perpendicular conductivity, i.e. through the monolayer sandwiched between the conductive AFM probe and the conductive substrate, is more than 5 times higher on the well-ordered island phase. This shows that the molecular lattice order has a significant effect on the electronic properties. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V22.00006: Direct measurement of photomechanical switching cross-sections of single-molecules on a surface Jongweon Cho, Matthew J. Comstock, Niv Levy, Luis Berbil-Bautista, Frank Lauterwasser, Jean M. J. Frechet, Michael F. Crommie The photomechanical switching of photoactive molecules in solution strongly depends on the wavelength of light. This dependence is crucial to reliably control the photomechanical state of target molecules. Recently, reversible photomechanical switching of individual azobenzene molecular derivatives on the Au(111) surface has been reported for one particular wavelength of UV illumination [1]. To further understand this process and its possible applications in future nanotechnologies, we have investigated photomechanical switching rates and saturation behavior for azobenzene molecular derivatives at a surface under optical stimulation at different wavelengths. Using single-molecule-resolved scanning tunneling microscopy, we have determined both the forward and reverse photomechanical molecular switching cross-sections at different wavelengths. In a dramatic departure from solution-based environments, visible light does not efficiently reverse the photoreaction. [1] Matthew J. Comstock, Niv Levy, Armen Kirakosian, Jongweon Cho, Frank Lauterwasser, Jessica H. Harvey, David A. Strubbe, Jean M. J. Fr\'echet, Dirk Trauner, Steven G. Louie, and Michael F. Crommie, Phys. Rev. Lett. 99, 038301 (2007) [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V22.00007: Many-body treatment of quantum transport through single molecules Justin Bergfield, Charles Stafford We investigate multi-terminal quantum transport through single molecules including intramolecular correlations exactly by using the nonequilibrium Green function approach, but treating the lead-molecule coupling perturbatively via a Dyson expansion \footnote{Cardamone D. et al. Nano Lett. Vol 6 2422, 2006}, with an extended Pariser--Parr--Pople molecular model. As a validation of the theory we calculate the linear and non-linear transport properties of 1,4-Benzenedithiol(BDT) and compare these results to experiment\footnote{Xiao X. et al. Nano Lett. Vol 4 267, 2004}. We find many transport features which are not accessible via meanfield approaches such as Coulomb Blockade steps and an incipient Hubbard-Mott insulator gap. We also calculated the thermopower exactly and find, in accordance with recent experimental\footnote{Reddy P. et al. Science. Vol 315 1568, 2007} and theoretical reports, that the transport in this junction is dominated by holes (p-type). This result allowed us to then extract the remaining free parameter, the lead-molecule coupling $\Gamma$. The resulting nonlinear I-V curve was found to be in good quantitative agreement with experiment. Finally, we calculated the differential conductance as a function of gating and bias potential to construct a full molecular `Coulomb diamond'. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V22.00008: Single molecule characterization with well-defined contacts Alex Neuhausen, Frank Jaeckel, Jeremy Hiatt, Joseph Sulpizio, David Goldhaber-Gordon, Chris Chidsey, W. E. Moerner, Zhenan Bao We demonstrate a novel method to reliably achieve ohmic contact to single molecules in a geometry that allows for simultaneous transport measurements and Raman spectroscopy. We achieve this by lithographically defining gold contacts to a structure composed of a single molecule bridging a pair of gold nanoparticles. The transport measurements indicate negligible resistance from the contacts as compared to the single molecule behavior, and the Raman spectroscopy benefits from strong field enhancement between the two nanoparticles. We prove the presence of single molecules with both stoichiometric and spectroscopic analyses. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V22.00009: Transport Fluctuations in Metal-Molecule Junctions Jonathan Malen, Kanhayalal Baheti, Peter Doak, Rachel Segalman, Arun Majumdar Thermopower of metal-molecule junctions is an alternative transport characteristic to conductance that can be experimentally measured. A scanning tunneling microscope break junction was used to measure the thermopower of such molecular junctions. Temperature bias applied between gold contacts across the bridging molecules generates a thermoelectric voltage. Hitherto, the statistical analysis of the data from both thermopower and conductance measurements has focused on the histogram peaks rather than the spread of the data. We find that the full width half maximums (FWHM) of the voltage histograms are finite at zero temperature bias and increase in proportion to the temperature bias. Johnson Noise is the most likely cause of the zero bias FWHM, and its magnitude is thereby related to the junction conductance. For 1,4,Benzenedithiol (BDT) the junction conductance associated with the zero bias FWHM is 0.02G$_{0}$, in close agreement with prior conductance measurements of BDT. The dependence of FWHM on temperature bias may provide further insight to the origin of stochastic fluctuations in metal molecule junctions. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V22.00010: Electronic transport through single-molecule- and monolayer-based molecular junctions Luis Agapito, Hai-Ping Cheng We report our development for calculating tunneling electronic transport through molecular junctions, which are composed of two contact leads and the active device in between. The surface Green's function of the contact leads is obtained following a non-iterative, exact procedure using ab initio data computed with the same level of theory and localized basis set than those used for the active device$^{\ast }$. In a 1-dimensional in-wire setting, we describe the electrical switching performance of a single oligo-phenylene-ethynylene molecule connected to graphene-nanoribbons leads. Moreover, in a more realistic 2-dimensional setting, such as the case of self-assembled molecular monolayers, the method is extended to include intermolecular and packing-density effects. * Agapito, L. A.; Cheng, H. P. \textit{Journal of Physical Chemistry C} \textbf{2007}, $111$, 14266. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V22.00011: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V22.00012: Substrate-Dependent Electronic Behavior of Polydiacetylene Nanowires Rajiv Giridharagopal, K. F. Kelly Scanning tunneling microscopy (STM) has been used to study individual polydiacetylene (PDA) nanowires. STM analysis of PDA nanowires on different substrate electrode materials at varying sample bias voltage conditions reveals interesting substrate-dependent effects. PDA nanowires were formed on both graphite and molybdenum disulfide (MoS$_{2})$ substrates. Interestingly, the nanowires on graphite appear with different topographic heights depending on the substrate bias voltage, and the height varies substantially with respect to voltage polarity. A similar effect is observed on MoS$_{2}$ at negative sample bias voltages, except that the nanowires are almost twice as tall on MoS$_{2}$. Even more intriguing is that at positive sample bias voltage conditions, the nanowires on MoS$_{2}$ are invisible in all STM images. A comparison of these voltage-dependent effects points to a strong influence of the substrate electrode material on the electronic behavior of these polymer nanostructures. The results reported here have implications for recently-demonstrated technologies such as monolayer PDA transistors and PDA-based organic solar cell devices as well as potential molecular electronic systems. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V22.00013: Organic memory devices using the negative differential resistance effect R. Osterbacka, J.K. Baral, H.S. Majumdar, F. Jansson, A. Laiho, R.H.A. Ras, J. Ruokolainen, O. Ikkala, H. Jiang, E. Kauppinen Of all the organic memory devices reported so far the ones having the negative differential resistance (NDR) is the best in terms of yield, reproducibility and repeatability. We have observed two different kinds of NDR in nanoparticle based organic memory devices. One is the memory-NDR which follows the observation in SiO2 devices [Simmons et. al. \textit{Proc. R Soc. Lond. Ser. A} \textbf{301}(1967)77]. Here the I-V characteristics trace different paths based on device history. The second type is the tunneling-NDR, where the I-V curves always trace the same path, irrespective of the history. This behavior is similar to the one observed in resonant tunnel diodes and multiple tunneling is the explanation. We will discuss these two phenomena in light of our experimental results in a polystyrene:fullerene nanocomposite system and present a physical model for the same. We have performed a multitude of optical and electrical experiments and clarified the influences of morphology on the complex and interesting device performance observed in this new class of organic electronic devices. [Preview Abstract] |
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