Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L14: Focus Session: Transport Properties of Nanostructures IV: Charge Dynamics and Imaging of Photoactive Molecules |
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Sponsoring Units: DMP Chair: Douglas Natelson, Rice University Room: B113 |
Tuesday, March 16, 2010 2:30PM - 3:06PM |
L14.00001: Imaging Photoinduced Charge Transport in Single Molecules Invited Speaker: The ability to perform sub-Angstrom imaging of photoinduced charge transport in single molecules in ultrahigh vacuum with a low temperature scanning tunneling microscope (STM) indicates the defeat of diffraction limited resolution and the opportunity to understand in new directions nanostructures and their functions. The experiments are enabled by the enhanced field due to coupling of the light from cw and femtosecond lasers to the plasmons in the STM nano-junction and the enhanced field to the molecule that can be monitored by tunneling electrons. By imaging the tunneling electrons as a function of energy and time, it is then possible to record the spatial and temporal evolution of the topography, and the electronic, vibrational, and magnetic states of single molecules. The observation of photoinduced phenomena in nanostructures, including molecules, is relevant to a number of technologies, such as photocatalysis, solar energy harvesting, and optical communication. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L14.00002: Single-molecule conductance studies of photo-active and photochromic molecules E. S. Tam, J. J. Parks, M. B. Santiago-Berrios, Y.-W. Zhong, H. D. Abruna, D. C. Ralph We perform statistical measurements of single molecule conductance in repeatedly-formed metal-molecule-metal junctions at room temperature. Our results on diaminoalkanes are consistent with those reported by the Venkataraman group. We focus on photo-active and photochromic molecules, including a series of transition-metal complexes with different metal centers and endgroups. We compare the trend in conductance across the family of complexes with that expected from electrochemical measurements. We will also report initial results on the voltage dependence of single-molecule conductances and the effects of optical excitations. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L14.00003: First-Principles Studies of Charge Separation in Single-Molecule Heterojunctions Pierre Darancet, Peter Doak, Jeffrey Neaton Single-molecule heterojunctions, consisting of donor and acceptor moieties linked by covalent bonds and coupled to metal electrodes, provide an interesting model system for understanding processes fundamental to organic solar cells, such as light absorption and charge separation. However, how the covalent contact with metallic leads influence these processes -- and metal-molecule interface electronic structure -- remains largely unknown. Using density functional theory and many-body perturbation theory, we discuss the influence of the metal contacts and binding groups on junction electronic level alignment for small asymmetric molecules containing covalently-linked moieties based on thiophene, durene and tetrafluoro-, dinitrile-, and metoxy-benzene. Implications for photocurrent and rectification are discussed. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L14.00004: Photo-excited semiconductor superlattices as constrained excitable media: Motion of dipole domains and current self-oscillations Luis L. Bonilla, J. Ignacio Arana, Holger T. Grahn A model for charge transport in undoped, photo-excited semiconductor superlattices, which includes the dependence of the electron-hole recombination on the electric field and on the photo-excitation intensity through the field-dependent recombination coefficient, is proposed and analyzed. Under dc voltage bias and high photo-excitation intensities, there appear self-sustained oscillations of the current due to a repeated homogeneous nucleation of a number of charge dipole waves inside the superlattice. In contrast to the case of a constant recombination coefficient, nucleated dipole waves can split for a field-dependent recombination coefficient in two oppositely moving dipoles. The key for understanding these unusual properties is that these superlattices have a unique static electric-field domain. At the same time, their dynamical behavior is akin to the one of an extended excitable system: an appropriate finite disturbance of the unique stable fixed point may cause a large excursion in phase space before returning to the stable state and trigger pulses and wave trains. The voltage bias constraint causes new waves to be nucleated when old ones reach the contact. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L14.00005: Conductance measurement of a single molecular chain as a continuous function of its length Leif Lafferentz, Francisco Ample, Christian Joachim, Hao Yu, Stefan Hecht, Leonhard Grill One prerequisite for the realization of molecular electronics is a fundamental understanding of charge transport through single molecules. We have conducted experiments with a low temperature scanning tunneling microscope (STM) that allow to measure currents going through a single long molecular wire as a continuous function of the distance between the two contacts on one and the same molecule (Science 323, 1193 (2009)). Conjugated polyfluorene chains, grown on a Au(111) surface, exhibit two important properties for a conductance measurement as they are \textit{flexible} (they can change their curvature) and \textit{mobile} on the surface. We show that it is possible to contact one end of a molecular chain with the STM tip and pull it gradually off the surface, up to more than 20 nm. Since the rest of the molecule remains attached to the sample, this results in a metal-molecule-metal junction with a distance between the two electrodes that can be adjusted. The resulting conductance curves reveal not only insight into the charge transport through the junction, but also information on the mechanical properties of the polymer as one molecular unit after another is detached from the surface. [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L14.00006: Ultraflat Nanoelectrodes: A Novel Platform for Fundamental Research on the Electronic Properties of Organic Monolayers Allard Katan, Florent Martin, Bas Hendriksen, Bruce Harteneck, Miquel Salmeron We have developed a novel platform to allow the study of electronic properties of ultrathin films. This platform, dubbed Ultraflat Nano-Electrodes, is similar to a bottom-contact geometry for transistors, but has the metal source and drain electrodes embedded in the gate oxide, and the entire device has sub-nm roughness. Onto the nearly atomically flat surface of the device molecular layers can be deposited without breaking the continuity of the film. Thus an organic transistor is formed with a nano-sized channel, where all of the active material is accessible to an AFM tip, so that structural details of the entire active region can be imaged or manipulated with molecular resolution. We will show the fabrication method of our device and present the results of in-situ AFM investigations, combined with transport measurements on monolayers of several organic semiconductors. These measurements illuminate the role of structural defects and grain structure in charge transport through molecular films. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L14.00007: Many-body electronic structure and Kondo properties of Co-porphyrin molecules on metallic surfaces Murilo Tiago, Luis Dias da Silva, Sergio E. Ulloa, Fernando Reboredo, Elbio Dagotto Scanning tunneling spectroscopy (STS) studies of the Kondo effect in metallo-organic complexes adsorbed in metallic surfaces has been a thriving area. Despite increasing experimental interest, quantitative theoretical studies of the Kondo properties are rare, largely due to the complex nature of the problem, which requires the accurate treatment of both the molecular electronic structure and the low-energy many-body correlations. In this work, we use a combination of first principles many-body methods (GW) and the numerical renormalization-group (NRG) technique to study the Kondo regime of cobalt-porphyrin compounds adsorbed on a Cu(111) surface [1]. We find the Kondo temperature ($T_K$) to be highly sensitive to both molecule charging and distance to the surface, which can explain the variations observed in recent STS measurements. We discuss the importance of many-body effects in the molecular electronic structure controlling this phenomenon and suggest scenarios where larger $T_K$ values can be achieved in experiments. \\[4pt] [1] L. Dias da Silva, M.~L. Tiago et al. PRB {\bf 80} 155443 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L14.00008: ABSTRACT WITHDRAWN |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L14.00009: Conductance in Vertically Aligned Zn Porphyrin Molecular Junctions Kim Lewis, Guoguang Qian, Swatilekha Saha Porphyrin molecules are a key platform for understanding molecular conductance. These molecules play an important role in biological processes, such as oxygen storage and transport, which researchers want to exploit for exotic applications, including multibit storage, memory elements, and photovoltaics. Recently, experiments investigated conductance in porphyrin molecules ligating a metal atom of zinc. In the current-voltage curves two steplike changes were identified for this molecule, which adopted a configuration parallel to the substrate. Here, we observe the existence of a two state conductance in vertically aligned tunnel junctions formed by porphyrin molecules ligating a zinc atom. Measurements are performed by forming single molecule junctions between a scanning tunneling microscope tip and a gold substrate. Peaks in the conductance histograms show molecules change from a high conducting state to a low conducting state. However, a similar effect is not observed for porphyrin molecules without a ligating atom. Possible origins for the observed phenomenon are discussed. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L14.00010: First-Principle Study of the Charge Transport in PTCDA/Ag(111) Interfaces Tatsuhiko Ohto, Hisao Nakamura, Koich Yamashita We focused on the transport property of organic / metal interfaces, which is one of the central issues in organic based devices as well as the prototype of charge injection at interface. We performed first principle calculations for the interface between 3, 4, 9, 10 -- perylenetetracarboxylic dianhydride (PTCDA) molecules and Ag (111) surface based on non-equilibrium Green's function method combined with the density functional theory. In the present report, the range of the coverage of PTCDA was taken from a single molecule region ($<$ 1 ML) to 3 ML. We found hybrid interfacial states between the large aromatic part of PTCDA and Ag (111) substrate, and they have characteristic effective masses along the stacking direction of PTCDA molecules. Energy level alignments and effective masses relating to these hybrid interfacial states are quite sensitive to the coverage. We will report detail transport properties such as transmission coefficients and show the comparison with the properties of molecular crystals of PTCDA. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L14.00011: Surface Reconstruction Transition Induced by Adsorption of Molecules: Perylene and FePc on Au(111) Surface Jiatao Sun, L. Gao, S. X. Du, F. Liu, H.-J. Gao Gold is the most commonly used electrode material in molecular electronics. It is well known that real Au(111) surface will reconstructs into a 22 $\times \sqrt{3}$ herringbone structure at ambient conditions. The stability of the Au(111) surface reconstruction is subject to changing under many conditions. Thus fabrication of high-quality organic thin films on metal surfaces is one crucial issue in molecular electronics. New Au(111) surface reconstruction patterns have been observed by STM upon the adsorption of monolayer perylene (C$_{20}$H$_{12})$ and monolayer iron phthalocyanine (C$_{32}$H$_{16}$N$_{8}$Fe) respectively. First-principles calculations and Frenkel-Kontorova model have been used to investigate the physical mechanism. Stress anisotropy has been found to be the main factor driving the transition of surface reconstruction. The difference in strength of the interaction between molecules and the gold substrate, may lead to different stress anisotropy for both systems. These results will be very useful to transport properties in electronic devices. [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L14.00012: Nano-Schottky Contacts Realized by Bottom-up Technique Hakan Pettersson, Dmitry Suyatin, Johanna Tragardh, Maria Messing, Jakob Wagner, Lars Montelius, Lars Samuelson Here we present a comprehensive study of a rectifying nano-Schottky contact formed at the interface between a gold catalytic particle and an epitaxially grown GaInAs/InAs nanowire. Selective electrical connections formed by electron beam lithography to the catalytic particle on one side, and to the InAs segment on the other side allowed electrical and optical characterization of the formed Schottky junction. From IV measurements taken at different temperatures we have deduced the Schottky barrier height and the height of the barrier formed in the graded GaInAs nanowire segment. The IV characteristics measured under laser stimulation showed that the device can be used as a unipolar photodetector with extremely small detection volume and potentially ultra fast response. [Preview Abstract] |
Tuesday, March 16, 2010 5:18PM - 5:30PM |
L14.00013: Charge distribution on semiconductor nanorods Philip Avraam, Peter Haynes, Nicholas Hine, Paul Tangney Semiconductor wurtzite nanostructures such as those of ZnO have been observed to exhibit a large dipole moment directed along the wurtzite polar-axis. Its presence can have implications for the optical properties of such nanostructures as well as for self-assembly. The surfaces on these nanostructures, particularly the polar (0001) surfaces, are thought to play a critical role in controlling the charge distribution. Reconstructions of polar surfaces in bulk crystals are known to be driven by the electrostatic requirement that the surfaces have neutral charge (resulting in a phenomenon called the ``polar instability''), but those on wurtzite nanostructures do not have neutral charge, and little is known about the role played by surface polarity. We use the ONETEP code to perform linear-scaling plane-wave pseudopotential density-functional calculations on entire isolated GaAs nanorods consisting of thousands of atoms. We find novel relationships between the surface termination on both the lateral and polar surfaces and the charge distribution along the nanorod. [Preview Abstract] |
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