Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P35: Molecular Electronics I |
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Sponsoring Units: DCP DMP FIAP Chair: Cherie Kagan, IBM, TJ Watson Room: LACC 511B |
Wednesday, March 23, 2005 11:15AM - 11:51AM |
P35.00001: Invited Speaker: |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P35.00002: Signal and thermal noise in [2]catenenane molecular electronic switches Yong-Hoon Kim, Seung Soon Jang, Yun-Hee Jang, William A. Goddard III Due to the flexible nature of molecules, an important consideration in molecular electronics would be the influence of molecular vibrations on the device charge transport characteristics. Employing the first-principles matrix Green's function method combined with classical force-field molecular dynamics, we examine the effect of thermal vibrations on the switching in [2]catenane molecular electronic devices. Previously, we have identified frontier molecular orbitals that systematically shift within the two co-conforamations of bistable [2]catenane molecules, which results in the switching of the device when it is probed via a small bias. Here, we compute the charge transport characteristics of the molecule in the switch-on and switch-off configurations at different molecular dynamics snapshots, and find that the fluctuation of resonant transmission peaks in each conformation is smaller than the shift of the peaks between the two conformations. Thus, we confirm that the thermal noise does not mask the switching signal in the [2]catenane molecular electronic devices, which is in accordance with the experimental observation of the switching at the ambient condition. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:39PM |
P35.00003: Electronic Conduction and Switching in Metal / Molecule / Metal Structures Invited Speaker: We report both physical and electrical characterization of several metal / organic monolayer / metal device structures which display electrical switching behavior. Devices comprised a planar lower metal electrode of aluminum (Al) or platinum (Pt), a Langmuir-Blodgett or self-assembled organic alkane monolayer, and an evaporated metal upper electrode of titanium (Ti) or platinum. Single crosspoint devices of area 1600 nm$^{2}$--100 um$^{2 }$incorporated 10$^{3}$-10$^{7}$ molecules in parallel. Electrode surfaces, monolayer structure, and electrode-monolayer interactions were very sensitive to sample preparation. X-ray photoelectron spectroscopy (XPS) indicated that the thickness and stoichiometry of PtOx and TiOx species at both metal-organic interfaces were strongly affected by process conditions including deposition pressures and plasma treatments. Infra-red spectroscopy (RAIR) using ultra-flat template-stripped metal substrates showed that the physical structure of the monolayer was similarly sensitive to nanometer-scale electrode roughness. Electrical conductance hysteresis was observed in Al/monolayer/Ti and Pt/monolayer/Ti devices. Local-pressure modulated atomic force microscopy (AFM) suggested that the electrical hysteresis was dominated by one or two nano-conduction channels $<$30 nm in diameter. The asymmetric, reversible conductance switching observed remains inconsistent with a simple dielectric breakdown process. Instead, for each electrode system we suggest either an interface electrochemical process or a reversible nanoparticle growth {\&} dissolution as primarily responsible for the observed electrical switching. Technology proof-of-principle demonstrations of ultra-dense nanoscale memory and logic integrated crossbar circuits, including latch circuits showing signal restoration, have successfully utilized these organic monolayer structures. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P35.00004: Transport through thiol SAMs: Effect of monolayer order, dynamics and temperature Geetha Dholakia, W. Fan, M. Meyyappan We discuss the self assembly and charge transport of organic thiol molecules and discuss the influence of structure, order and dynamics in the monolayer [1] on the transport and also the effect of temperature by scanning tunneling microscopy/spectroscopy (STM/S). Conjugated thiol molecular wires and organometals such as terpyridine metal complexes provide a new platform for molecular electronic devices. Molecular resolution STM imaging in vacuum reveals that the molecular wires adopt an incommensurate, almost vertical SAM structure with a rectangular unit cell, while terpyridine metal thiol complexes tend to lie flat on the Au(111) substrate. STS of the molecular wires show that inherent asymmetry in the molecular structure and asymmetric coupling to contacts results in asymmetric, weakly rectifying I-Vs. STS on alkanethiols do not show a marked temperature dependence down to 150K. We also show that packing and order greatly influence the transport measurements and that the presence of molecular order in the monolayer is very important for reproducible I-Vs. Thus a good control of the molecule-substrate interface needs to be ensured for device reliability. We also point out that molecular electronic devices need to be made tolerant to fluctuations as these cannot be totally eliminated in low dimensional soft systems. [1] Geetha R. Dholakia et. al, PHYSICAL REVIEW B 69, 153402 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:27PM |
P35.00005: Molecular Engineering of Single Molecular Switches and Molecular Assemblies Invited Speaker: Paul S. Weiss We use molecular design, tailored syntheses, intermolecular interactions and selective chemistry to direct molecules into desired positions to create nanostructures, to connect functional molecules to the outside world, and to serve as test structures for measurements of single or bundled molecules. Interactions within and between molecules can be measured, understood and exploited at unprecedented scales. We look at how these interactions influence the chemistry, dynamics, structure, electronic function and other properties. Such interactions can be used to advantage to form precise molecular assemblies, nanostructures, and patterns. These nanostructures can be taken all the way down to atomic-scale precision or can be used at larger scales. We select and tailor molecules to choose the intermolecular interaction strength and the structures formed within the film. We selectively test hypothesized mechanisms for electronic switching by varying molecular design, chemical environment, and measurement conditions to enable or to disable functions and control of these molecules with predictive and testable means. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P35.00006: Ab initio study of vibrational spectrum and electron-molecular vibration interaction in molecular electronic systems Nikolai Sergueev, Dan Roubtsov, Hong Guo For the last few years it was realized that the electron-molecular vibration (e-mv) interaction has to be taken into account to predict transport properties of molecular electronic devices. For realistic calculations, however, the eigenmodes of molecular vibrations and the coupling constants in an e-mv interaction Hamiltonian have to be found from the first principles. We developed a technique to calculate eigenfrequencies of the molecular vibrations (a full spectrum) at both equilibrium and non-equilibrium conditions using the DFT within Keldysh Green's function formalism implemented into our McDCAL code. Using the obtained vibrational spectrum and the self-consistent Kohn-Sham Hamiltonian of the electrons, we can build the e-mv interaction Hamiltonian. By averaging this Hamiltonian over the scattering states of the total Kohn-Sham Hamiltonian, we obtain a dimensionless e-mv interaction strength (both elastic and inelastic ones), for each vibrational mode of the spectrum. These numbers tell us which molecular modes build strong inelastic channels in an electron tunneling through a molecule. As an example, we consider a dithiol-benzene molecule in a good covalent contact with two identical Al electrodes. In the vibrational spectrum of the dithiol-benzene in this device, we show which modes are important at different applied bias voltages. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P35.00007: Switching of the Fe Oxida\-tion State in Ferro\-cene-Capped Alkanethiols Fan Zheng, Virginia P\'erez-Dieste, J.L. McChesney, Yan-Yeung Luk, Nicholas L. Abbott, F.J. Himpsel Molecular electronics has been a rapidly-growing area, due to the simplicity of building molecular devices by self-assembly and the promise of extremely low power consumption as a result of pushing the size down to a few molecules per device. A self-assembled monolayer (SAM) of ferrocene-capped alkanethiols is produced in two stable oxidation states of Fe (Fe$^{2+}$ and Fe$^{3+})$. The oxidation states of Fe are probed with sub-monolayer sensitivity by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy at the iron L$_{2, 3}$ edges $^{[1]}$. NEXAFS provides a direct method to distinguish between the oxidation states of submonolayer by comparing with the bulk sample spectrum. The native Fe$^{2+}$ layer is converted chemically to Fe$^{3+}$, and the Fe$^{3+}$ layer can be switched back to Fe$^{2+}$ or possibly Fe$^{0}$ by irradiation with soft x-rays. The results have implications on switching mechanisms in molecular electronics. [1] Fan Zheng, V. P\'{e}rez-Dieste, J. L. McChesney, Yan-Yeung Luk, Nicholas L. Abbott, and F. J. Himpsel, Appl. Phys. Lett, to be submitted. [Preview Abstract] |
Wednesday, March 23, 2005 1:51PM - 2:03PM |
P35.00008: Theoretical Investigation on the Electronic Structure of Alq3/Al Interface Susumu Yanagisawa, Yoshitada Morikawa Alq$_{3}$ [tris-(8-hydroxyquinolinato) aluminum] is one of the most widely used electron transport and emissive material in organic light-emitting devices (OLEDs). From the experimental observation of an extra gap state at the Alq$_{3}$/Al interface, a strong chemical interaction between the Alq$_{3}$ molecule and the Al surface was suggested. Contrary to the experimental studies, previous DFT calculations concluded that the interaction was physisorptive. One possible reason for the discrepancy between the theoretical and the experimental results is the complexity of the experimentally used electrode surfaces. In the present study, we investigated the effect of the surface roughness on the electronic properties of the Alq$_{3}$/Al interface by examining various possible electrode structures. We examined three structures for the Al substrate, the flat Al(111) surface, the Al(332) stepped surface, and the Al adatom adsorbed Al(111) surface. Alq$_{3}$ molecules are bound to Al substrates through their O atoms and about 0.3-0.6 electrons are transferred from the Al substrates to Alq$_{3}$. Upward configurations, in which molecular permanent dipole moments are directed to the vacuum side, reduce the work function by 1.0-1.5 eV, in reasonable agreement with experimental results. The characteristic of the molecular orbitals of Alq$_{3}$ were kept upon adsorption, which seems inconsistent with the gap state derived from the interfacial chemical interaction observed in the UPS and MAES experiments. Further details will be presented. [Preview Abstract] |
Wednesday, March 23, 2005 2:03PM - 2:15PM |
P35.00009: Biosensor Consists of Na -- Doped Hydroxyapatite Thin Film Hiroaki Nishikawa, Masanonu Kusunoki, Shigeki Hontsu, Masami Kawashima, Tomoji Kawai Hydroxyapatite (HAp) surface has an excellent ability of adsorption for functional biomolecules such as protein, DNA and so on. We have investigated the application of the HAp as a suitable material for biosensor. Thin film of the sodium -- doped HAp (Na -- HAp) is prepared in order to decrease the electric resistivity. We have studied variation of the resistance for the Na -- HAp thin films with the adsorption of the functional biomolecules. The sample were prepared by a pulsed laser deposition technique on porous alumina substrate. After the deposition, sample was post -- annealed in O$_{2}$ / H$_{2}$O atmosphere in order to crystallize the Na -- HAp. The powder X ray diffraction pattern shows the sample has a pure HAp structure. The gold comb electrodes were evaporated on the sample for the resistance measurement. The sample was set in pure water of 100 ml in a beaker. When Fetal Bovine Serum of 100 $\mu $l was dropped in the beaker, the Na -- HAp shows the drastic change of the AC resistance (at 120 kHz). This result shows that the Na -- HAp will be one of the most effective materials for the biosensor applications. [Preview Abstract] |
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