Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V44: Focus Session: Organic Electronics and Photonics -- Small molecule semiconductors and molecular electronics |
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Sponsoring Units: DMP DPOLY Chair: Tim Jones, University of Warwick Room: A309 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V44.00001: The Electronic Structure of a Local Charge-Transfer-Induced Spin Transition Molecular Adsorbate Xin Zhang, Ning Wu, ZhengZheng Zhang, Jean-Fran\c{c}ois L\'etard, Fran\c{c}ois Guillaume, Bernard Doudin, Peter Dowben The spin crossover phenomena has been identified in the [Fe(H$_{2}$B(pz)$_{2})$~bpy] where pz=(1-pyrazolyl)borate [Fe(H$_{2}$B(pz)$_{2})$~bpy], and but there is currently a lack of knowledge of the physical nature of this phenomena and the electronic structure of this organometallic compound has not been well characterized. We have investigated the interface electronic characteristics of molecular thin films of the metal-organic [Fe(H$_{2}$B(pz)$_{2})$~bpy] by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission (IPES). X-ray absorption spectroscopy (XAS) and Infrared spectroscopy (IR spectroscopy) were also used to study [Fe(H$_{2}$B(pz)$_{2})$~bpy]. The IPES results coincide with XAS, and the model calculations. The molecular vibrational modes have been identified from a comparison of the IR spectroscopy with model calculations. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V44.00002: Raman scattering studies of organic semiconducting charge-transfer compounds Laurie McNeil, Christian Kloc Organic semiconductors offer the possibility of devices with greater mechanical flexibility and lower production costs compared to existing materials. Reports of carrier mobilities in monomolecular organic semiconductors in the 10-50 cm$^{2}$/V-s range and success in fabricating electronic devices from organic materials has increased the interest in their properties for electronic applications. However, the range of properties displayed by the monomolecular crystals is rather narrow. Charge-transfer compounds composed of two different organic molecules in which one acts as a donor and the other as an acceptor may represent the next generation of organic semiconductors. Control of their properties by modification of the molecules or changes in stoichiometry and crystalline structure makes them particularly attractive for a wide range of applications provided that the relationship between the structure and constituents of the compounds and their physical properties can be elucidated. Raman scattering studies of single crystals of two representative charge-transfer compounds, perylene-TCNQ and anthracene-TCNQ, will be presented. Theoretical calculations suggest that these materials have the potential for ambipolar charge transport, and so intermolecular interactions in these compounds are of particular interest. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V44.00003: Observation of optically forbidden states in PC$_{60}$BM due to interfacial distortion Hemant Shah, Bruce Alphenaar PCBM is a fullerene derivative used extensively in organic solar cells. PC$_{60}$BM shows strong absorbance at wavelengths below 400 nm. A series of sub-gap transitions exist, but are symmetry forbidden in C$_{60}$, and only weakly observed in the PC$_{60}$BM absorbance. Recent theoretical calculations predict that the symmetry rules for C$_{60}$ can be lifted by the proximity of a metallic substrate due to perturbation of the electronic spatial distribution. Here we describe capacitive photocurrent measurements of PC$_{60}$BM in which the optically forbidden features are strongly observed. In agreement with the theoretical predictions, this is thought to be due to the influence of a high conductivity ITO layer in contact with the PC$_{60}$BM. The influence of the ITO is tested by introducing a thin insulator (Al$_{2}$O$_{3})$ of varying thickness between the PC$_{60}$BM and the ITO. The photocurrent due to the symmetry forbidden states drops strongly compared to the above gap photocurrent with increasing separation. Implications of these results on the polythiophene/fullerene blends will also be discussed. DOE-3048103802-08-073, NSF- DMR-0906961 [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 9:12AM |
V44.00004: Free exciton emission and vibrations in pentacene monolayers Invited Speaker: Pentacene is a benchmark organic semiconductor material because of its potential applications in electronic and optoelectronic devices. Recently we demonstrated that optical and vibrational characterizations of pentacene films can be carried out down to the sub-monolayer limit. These milestones were achieved in highly uniform pentacene films that were grown on a compliant polymeric substrate. Films with thickness ranging from sub- monolayer to tens of monolayers were studied at low temperatures. The intensity of the free exciton (FE) luminescence band increases quadratically with the number of layers N when N is small. This quadratic dependence is explained as arising from the linear dependence of the intensity of absorption and the probability of emission on the number of layers N. Large enhancements of Raman scattering intensities at the FE resonance enable the first observations of low-lying lattice modes in the monolayers. The measured low- lying modes (in the 20 to 100cm$^{-1}$ range) display characteristic changes when going from a single monolayer to two layers. The Raman intensities by high frequency intra-molecular vibrations display resonance enhancement double-peaks when incident or scattered photon energies overlap the FE optical emission. The double resonances are about the same strength which suggests that Franck-Condon overlap integrals for the respective vibronic transitions have the same magnitude. The interference between scattering amplitudes in the Raman resonance reveals quantum coherence of the symmetry-split states (Davydov doublet) of the lowest intrinsic singlet exciton. These results demonstrate novel venues for ultra-thin film characterization and studies of fundamental physics in organic semiconductor structures. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V44.00005: The interaction of charge carriers with lattice phonons in oligoacene crystals Veaceslav Coropceanu, Yuan Li, Yuanping Yi, Robert Brown, Jean-Luc Bredas We use density functional theory calculations to investigate the non-local electron-phonon interactions between charge carriers and lattice phonons (i.e., the modulation of transfer integrals by vibrations) in oligoacene crystals as a function of molecular size from naphthalene through pentacene. The results point to a significant coupling to both translational and librational intermolecular phonon modes as well as to intra-molecular vibrational modes. The impact of the interplay among these mechanisms on charge transport is investigated by treating the lattice dynamics classically. The impact of quadratic electron-phonon interaction on charge transport is studied as well. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V44.00006: Spectroscopy of organic semiconductors from first principles Sahar Sharifzadeh, Ariel Biller, Leeor Kronik, Jeffery Neaton Advances in organic optoelectronic materials rely on an accurate understanding their spectroscopy, motivating the development of predictive theoretical methods that accurately describe the excited states of organic semiconductors. In this work, we use density functional theory and many-body perturbation theory (GW/BSE) to compute the electronic and optical properties of two well-studied organic semiconductors, pentacene and PTCDA. We carefully compare our calculations of the bulk density of states with available photoemission spectra, accounting for the role of finite temperature and surface effects in experiment, and examining the influence of our main approximations -- e.g. the GW starting point and the application of the generalized plasmon-pole model -- on the predicted electronic structure. Moreover, our predictions for the nature of the exciton and its binding energy are discussed and compared against optical absorption data. We acknowledge DOE, NSF, and BASF for financial support and NERSC for computational resources. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V44.00007: Angle resolved photoemission study of rubrene single crystal Yongli Gao, Huanjun Ding, Irfan Irfan, Colin Reese Reese, Antti Makinen, Zhenan Bao We report the direct experimental observation of the band structure of a bulk organic single crystal. The electronic structure of rubrene single crystal grown by physical vapor transport method was studied with angle-resolved photoemission spectroscopy. Highly reproducible dispersive features were observed with nice symmetry about the Brillouin zone center and boundaries, representing the band structure measured for a bulk organic single crystal. The high quality of the surface was confirmed with scanning tunneling microscopy. The energy dispersion of the highest occupied molecular orbitals derived bands showed strong anisotropic behavior in the a-b plane of the unit cell. The measured band structure, however, differs unexpectedly from theoretical calculations in terms of the amount of the dispersion and the separation of the bands. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V44.00008: Charge localization and inhibition of self-assembly in tetraphenyl porphyrin on Cu(111) Geoffrey Rojas, Xumin Chen, Donna Kunkel, Jie Xiao, Peter A. Dowben, Axel Enders A study of the nature of the electronic structure and inter-molecular interaction of the adsorbed tetraphenyl porphyrin (H$_{2}$TPP)/Cu(111) system using scanning tunneling spectroscopy (STS) and inverse photoemission spectroscopy (IPES) is presented. By studying STS and IPES spectra as a function of increasing coverage, significant upshifts in the local shockley surface state near the adsorbate as a well interfacial HOMO-LUMO gap state are observed in monolayer-thick films. This, combined with observations of changes in the local workfunction and distortions of the Cu(111) surface within 1 {\AA}~of the molecules, indicates strong molecule-interface electronic interaction and stronger bonding. Such strong electron transfers and resulting charge dipoles are the origin of observed inter-molecular Coulomb repulsion, thereby preventing self-assembly of first-monolayer H$_{2}$TPP/Cu(111) systems, while allowing for self-assembly of second-monolayer and higher, where no such surface states are observed. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V44.00009: Scanning Tunneling Spectroscopy measurements of the Electronic Structure of C60 films on the Cu(100) surface D.R. Daughton, J.A. Gupta Successful implementation of organometallic electronic and photoelectronic device architectures requires understanding and engineering of molecule-electrode interfaces. Here we investigate the electronic structure of a monolayer film of C60 on a Cu(100) surface with low temperature (5 K) scanning tunneling microscopy (STM) and spectroscopy. C60 adopts four unique orientations on the Cu(100) surface, and shifts in the molecular orbital resonances for the four geometries indicate different degrees of electronic molecule-surface hybridization. At higher bias, Stark-shifted image state resonances are shown to spatially vary across the molecular film. Modulation of the image state energies are attributed to shifts in the interfacial dipole that derive from the interplay of interfacial charge transfer, surface reconstruction, and orientational ordering of the molecular film. These observations suggest the need for nanoscale interface characterization for optimizing the performance of molecular electronic devices. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V44.00010: Highly Conducting Contacts for Single Molecule Transport Measured by STM-Break Junction Jonathan R. Widawsky, Zhan-Ling Cheng, Rachid Skouta, Severin T. Schneebeli, Hector Vazquez, Mark S. Hybertsen, Ronald Breslow, Latha Venkataraman We present a novel method to directly link single alkane chains to gold electrodes using trimethyl tin (SnMe$_{3})$ linkers. We characterize electron transport through single molecule junctions using the STM-based break-junction technique, where a gold point contact is repeatedly formed and broken in a solution of the SnMe$_{3}$-alkanes while conductance is measured.~Based on analysis of more than 10,000 individual junctions, we find that we create single molecule junctions which are $\sim $100 times more conducting than those with alkanes terminated with any other linker previously studied. The contact resistance, determined by extrapolating to zero carbons, is 4k$\Omega $, two orders of magnitude lower than analogous values found using amine linkers. Strong evidence supports the hypothesis that \textit{in situ} cleaving of the SnMe$_{3}$ end groups facilitates the formation of a direct bond between the carbon backbone and gold leads, thereby enhancing conductance.~We corroborate this result by comparing the conductance of junctions formed from SnMe3- and Ph$_{3}$PAu-terminated benzenes. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V44.00011: 34 nm Charge Transport through DNA Jason Slinker, Natalie Muren, Sara Renfrew, Jacqueline Barton Long-range charge transport through DNA has broad-reaching implications due to its inherent biological recognition capabilities and unmatched capacity to be patterned into precise, nanoscale shapes. We have observed charge transport through 34 nm DNA monolayers (100 base pairs) using DNA-mediated electrochemistry. Cyclic voltammetry of multiplexed gold electrodes modified with 100mer DNAs reveal sizable peaks from distally-bound Nile Blue redox probes for well matched duplexes but highly attenuated redox peaks from 100mer monolayers containing a single base pair mismatch, demonstrating that the charge transfer is DNA-mediated. The 100mers on the gold surface are efficiently cleaved by the restriction enzyme RsaI. The 100mers in the DNA film thus adopt conformations that are readily accessible to protein binding and restriction. The ability to assemble well-characterized DNA films with these 100mers permits the demonstration of charge transport over distances surpassing most reports of molecular wires. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V44.00012: Electrical properties of metal-molecule-silicon structures with varying molecular backbones, dipoles, and atomic tethers Curt A. Richter, Nadine Gergel-Hackett, Mariona Coll, Christina A. Hacker We present the results of an extensive experimental investigation of metal-monolayer-silicon junctions. By varying the molecular dipole, the molecular backbone, the Si-molecule linkage, and the Si-doping, we indentified critical features that determine the electrical transport and injection properties of the junctions. Two basic structures were used. One is an enclosed planar structure in which an organic monolayer is directly assembled on silicon and contacted with evaporated silver. The other was made via Flip Chip Lamination, a novel approach that relies on the formation of monolayers on a gold surface first, which enables the study of a wider range of molecular layers on silicon of very high-quality. Two charge transport regimes dominate: (1) a Schottky barrier limited regime where the molecular dipole results in silicon band bending at the junction interface, and (2) a tunneling regime where the molecular dipole creates a small local electric field that screens the electrical transport. Transition Voltage spectroscopy was used to identify electrical differences between $\pi$-conjugated and alkyl backbones attributed to the extended $\pi$-delocalization and variations due to the chemical nature of Si-atom linkage. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V44.00013: Engineering controlled Au/GaAs junctions with partial molecular monolayers C. Marginean, J.P. Pelz, H. Haick, D. Cahen Advances in molecular electronics offer the possibility to use molecular-based components to enhance integrated circuits and other electronic devices. Therefore, the studies of the electronic transport properties of junctions containing molecular layers are of great interest. The local hot-electron transmittance at buried metal-dicarboxylic acid-semiconductor [1] interfaces was directly investigated with nanometer spatial resolution and meV- level energy resolution using BEEM by spatially mapping hot-electrons that were injected into the top metal thin film and passed through the diode [2]. That study found that the dominant electronic transport mechanism for some dicarboxylic ligands was though pinholes rather than direct tunneling through the molecular film, and that the effective Schottky barrier height (SBH) at the pinholes was increased by a negative electric dipole moment in the surrounding molecular film [2]. We present the results of finite element electrostatic calculations of Au/discontinuous-molecular film/GaAs structures with both positive and negative dipole films, and show that the expected decrease (or increase) of the effective SBH is consistent with BEEM measurements of these types of samples. Work supported by NSF Grant No. DMR-0805237. [1] H. Haick et al., Adv. Mater. 16, 2145 (2004). [2] H. Haick, et. al., Phys. Stat. Sol. (A) 2031, 3438 (2006). [Preview Abstract] |
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