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THE UNIMOLECULAR RECTIFIER AND BEYOND

THE UNIMOLECULAR RECTIFIER AND BEYOND. Robert M. Metzger. Laboratory for Molecular Electronics Department of Chemistry The University of Alabama Tuscaloosa, AL 35487, USA Tel = 1-205-348-5952, fax = 1-205-348-9104 Email = rmetzger@bama.ua.edu.

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THE UNIMOLECULAR RECTIFIER AND BEYOND

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  1. THE UNIMOLECULAR RECTIFIER AND BEYOND Robert M. Metzger Laboratory for Molecular Electronics Department of Chemistry The University of Alabama Tuscaloosa, AL 35487, USA Tel = 1-205-348-5952, fax = 1-205-348-9104 Email = rmetzger@bama.ua.edu International Workshop on Advances in Molecular Electronics: from Molecular Materials to Single-Molecule Devices Max-Planck Institute for the Physics of Complex Systems, Dresden, Germany 25 February 2004 NSF DMR-00-95215 (R.M.Metzger) NSF-DMR-00-99674 (D.L.Mattern) NSF-DMR-01-20967 (L. R. Dalton)

  2. GEOGRAPHY In 1539, Chief Tuscaloosa (“Black Warrior” in Choctaw) fought and died in the only full battle between the Indians and the Spaniards (led by Hernando de Soto) at the village of Mauvilia (now lost) on one of the rivers of Alabama. [The city of Mobile, AL is named after Mauvilia]. Hernando de Soto, in his exploration from Florida seeking gold, went on to the banks of the Mississippi river, where he died of syphilis in 1541.

  3. UNIVERSITY OF ALABAMA QUAD, CAMPANILE, & CHERRY BLOSSOMS

  4. ABSTRACT • UNIMOLECULAR RECTIFICATIONr (asymmetric DC conductivity) for 1- molecule thick Langmuir-Blodgett (LB) (vertical transfer) or Langmuir-Schaefer (LS) (horizontal pickup) monolayers was: • (a) confirmed in cells “Al | LB of 1 | Al” [J. Am. Chem. Soc. 119: 10455 (1997)] and found in cells “Au | LB of 1 | Au” [Angew. Chem. Intl. Ed. 40: 1749 (2001); J. Phys. Chem. B105: 7280 (2001)] • (b) found in cells “Au | LB of 2 | Au” [J. Phys. Chem. B106: 12158 (2002)] (ion-pair rectifier?) • (c) found in cells “Au | LB of 3 | Au”: weak rectification; extremely high forward currents in some cells are due to Au stalagmites [J. Phys. Chem. B107: 1021 (2003)] • ================== Acc. Chem. Res. 32: 950 (1999); Chem. Reviews 103: 3803 (2003) • (d) found in cells “Au | LS of 4 | Au”:vert sturdy film; the asymmetry persists [unpublished] • Code : VIOLET: ONE-ELECTRON DONOR; BLUE: ONE-ELECTRON ACCEPTOR

  5. WHY ? • INORGANIC ELECTRONICS: Gordon E. Moore’s “Law”: • At present, speed of computation doubles every 18 months • Design rule of components (their distance) halves every 18 months. Now at 100 nm. What is the limit? • Cost of fabrication laboratory increases exponentially with time Field-effect transistors can be scaled down, until semiconductor or oxide fail (? 15 nm ?) Junction transistors can be scaled down, but not so far (? 50 nm?) ….. (from David L. Allara) R. M. Metzger, Chem. Reviews 103: 3803 (2003).

  6. RESULTS IN MOLECULAR ELECTRONICS • in large sense, (sensu lato) or in narrow sense (sensu stricto) • MOLECULAR ELECTRONICS (sensu lato) or MOLECULE-BASED ELECTRONICS • Organic metals [TTF TCNQ, 1973] • Organic superconductors [TMTSF2PF6, 1979: Tc= 1 K, BEDT-TTF)2Cu(NCS)2, Tc = 13 K] • Charge-transfer light-emitting diodes [Tang, 1987] • Charge-transfer polymers for electrostatic copiers [1967] • Alkali fullerides [Cs2HC60, 1993, Tc ≈ 40 K] • Conducting polymers [doped polyacetylene, 1977; polypyrrole, poly-p-phenylenevinylene, polythiophene]] • Organic polymeric light-emitting diodes [Friend, 1991] • (UNI)MOLECULAR ELECTRONICS (sensu stricto), or MOLECULAR-SCALE ELECTRONICS • Molecular lines, spacers, alligator clips, tinkertoys, meccano components, resistors • Molecular wires, antennas, conductors [conducting polymers, carotenes] • Unimolecular rectifiers (Aviram-Ratner), switches, .negative differential resistance devices; diode logic • Single-electron transistors & single-atom transistor (Coulomb blockade): no gain . • Must reach out and touch molecules… STM, break junctions, macroscopic pads • Molecules with gain? Unimolecular “transistor” (molecular amplifier) with gain? • When and if all components exist, we can start to plan organic interconnects, instead of metal wires… R. M. Metzger, Chem. Reviews 103: 3803 (2003).

  7. EIGHT SIGNIFICANT MILESTONES IN UNIMOLECULAR ELECTRONICS • STS: currents across alkanethiols on Au (111) << STS currents through aromatic thiols on Au(111) [L. A. Bumm, et al., Science271: 1705 (1996)]. • Break junction: resistance between two Au shards with a single 1,4-benzenedithiol bonded to them is several M. [M. A. Reed, et al., Science278: 252 (1997)] • Molecules of 2’-amino-4-ethynylphenyl-4’-ethynylphenyl-2’-nitro-benzene-1-thiolate, attached to Au on one side and topped by a Ti electrode on the other, exhibit negative differential resistance [J. Chen, et al., Science 286: 1550 (1999)]. • The Landauer quantum of resistance, 12.9 k was measured at 300 K between a single-walled carbon nanotube, glued to a conducting atomic force microscope (AFM) tip, and a pool of liquid Hg [S. Frank, et al., Science 280: 1744 (1999)]. • FET behavior was observed for a single-walled carbon nanotube curled over parallel Au lines, with the STM acting as a gate electrode; the power gain was 0.33. [S. J. Tans, et al., Nature 386: 474 (1997)]. • For an LB monolayer of a bistable [2]catenane closed-loop molecule, with a naphthalene group as one ”station”, and TTF as the second “station”, and a tetracationic catenane hexafluorophospate salt traveling on the catenane, like a “train” on a closed track, deposited on poly-silicon, and topped by a 5 nm Ti layer, then Al, the current-voltage plot is asymmetric as a function of bias (which moves the train on the track),as the train stops in different stations [C. P. Collier et al., Science289: 1172 (2000)]. (filaments??????) • The organometallic equivalent of a single-electron transistor has been realized at 0.1 K with a Co(II) tris-bipyridyl: this structure has no power gain, but can be ascribed to the redox behavior (Co(II) <--> Co(III)) [J. Park, et al., Nature417: 722 (2002)]. • Unimolecular rectification [R. M. Metzger et al., J. Am. Chem. Soc.119: 10455 (1997)] R. M. Metzger, Chem. Reviews 103: 3803 (2003).

  8. 0 0 + - 0 0 D - -A s D - -A s D - -A s D – – A s D- -A molecule s AVIRAM & RATNER PROPOSAL OF UNIMOLECULAR RECTIFICATION (1973) ET ET ELECTRON FLOW IVT + + - - + - Step 1 Step 2 + + + - - - + + + - - - + - - + + - Forward bias: preferred direction of electron flow ET ET NC CN Fermi level (metal 2) IVT LUMO(A) S S Fermi level (metal 1) S S HOMO(D) NC CN • Down-hill inelastic electron tunneling from A– to D+ strongly favored. • Molecule never synthesized. Aviram & Ratner, Chem. Phys. Lett. 29: 277 (1974).

  9. MOLECULAR ENERGY LEVELS & METAL WORK FUNCTIONS • Aviram-Ratner rectifier requires strong donors D and strong acceptors A for conventional metal electrodes. • Graphite or nanotube electrodes tolerate weaker D and weaker A , but D/A asymmetry is needed. R. M. Metzger, Chem. Reviews 103: 3803 (2003).

  10. ASSEMBLY STRATEGIES • B. CHEMISORBED MOLECULES: • Thermodynamically stable (single layer) • Often not well ordered • 1. Thiolates on Au, Ag, Cu • [Dewar, Allara, Nuzzo, Sagiv, Whitesides] • Partially polar RS- Au+ bond • 2. Organosilicon on oxides of Si, Al • [Sagiv, Allara, Ulman, Rondelez] • can order into a perfect monolayer at the right temperature • covalent bond (not polar) • 3. Alcohols on Pt [Nuzzo, Allara, Whitesides] • 4. Amines on Pt [Nuzzo, Allara, Whitesides] • 5. Carboxylic acids on oxide of Al and Ag[Nuzzo, Allara] • Polar carboxylate RCOO-Ag+ • 6. Sequential bonding of bifunctional monomers on surfaces • [Marks, Ratner] • To bond bifunctional molecules • X-A-Y, Z-B-W, etc., onto a surface S, • forming S-A-B-etc; used to make light- • emitting diode structures • A. PHYSISORBED MOLECULES: • 1. Langmuir-Blodgett films • [Langmuir, Blodgett, Gaines, Kuhn]: • Thermodynamically often metastable • Kinetically very ordered • (for 10-100 layers) • 2. Polymerizable Langmuir-Blodgett films • [Wegner, Tripathy] • Thermodynamically stable • polydiacetylenes, other polymers • kinetically ordered monomers • Disordered polymers (except for topotactic polymers) R. M. Metzger, Chem. Reviews 103: 3803 (2003)

  11. LANGMUIR-BLODGETT (LB) VERSUS LANGMUIR-SCHAEFER (LS) TRANSFER LB TRANSFER ONTO HYDROPHILIC SUBSTRATE (e.g. Al, “fresh” Au) X MULTILAYER Y MULTILAYER Z MULTILAYER LS TRANSFER ONTO HYDROPHOBIC SUBSTRATE LB MULTLAYER TYPES (from dissertation of Xiang-Li Wu)

  12. METZGER & PANETTA SYNTHESES AND MEASUREMENTS (UNIV. OF MISSISSIPPI, 1981-91) • LB films chosen for monolayer assembly; Carbamate link between TCNQ and donors successful. • Rectification measurements were primitive, and failed. Metzger & Panetta, New J. Chem. 15: 209 (1991). Metzger, Matrl. Sci. and Engrg. C3: 277 (1995).

  13. I - V PLOTS FOR Ag | Mg | C16H33Q-3CNQ LB | Pt monolayer 3 layers 4 layers Molecule is a zwitterion; it forms Z-type Langmuir-Blodgett (LB) multilayers (dipoles all oriented in the same direction) and has a high (resonance-enhanced) second-order non-linear optical susceptibility To see whether Schottky barrier was responsible for rectification, LB layers of fatty acids were later put between C16H33Q-3CNQ and metal electrodes: rectification persisted [A. S. Martin, J. R. Sambles, and G. J. Ashwell, Phys. Rev. Lett 70, 218 (1993)]. Ashwell, Sambles, Martin, Parker, Szablewski, Chem. Comm., 1374 (1990)

  14. IS C16H33Q-3CNQ ZWITTERIONIC? If the molecule were flat, then the two states (zwitterionic and undissociated) would be in resonance, as shown at right . No crystal structure is available for C16H33Q-3CNQ (reflection from micro-crystals with overlapping unit cells could be indexed), but there is a steric hindrance due to peri hydrogen (see arrow), sothe torsion angle  is probablynon-zero. But: a crystal structure was solved for the related compound, or -picolinium tricyano-quinodimethanide 2: the torsion angle  = 30.13˚ Thus: in C16H33Q-3CNQ the torsion angle must be non-zero, and the zwitterionic and undissociated states are not in resonance. Which is lower? The zwitterionic one, as we’ll see… R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997).

  15. CYCLIC VOLTAMMOGRAM OF C16H33Q-3CNQ: MOLECULE IS REVERSIBLE WEAK ELECTRON ACCEPTOR (E1/2 IS SIMILAR TO THAT OF p-BENZOQUINONE) Hiromi Sakurai R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997).

  16. DIPOLE MOMENT OF C16H33Q-3CNQ: MOLECULE IS A ZWITTERION! m = 43 ± 8 Debyes Measured in dichloromethane solution; Kirkwood-Froehlich equation was used for calculation. Calculated from the temperature dependence of the dielectric constant. (-10°C < T < 30°C) For +1 charge on N and -1 charge were on bridgehead C, 50 Debyes would be expected…. Dominique Vuillaume R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997).

  17. VIS ABSORBANCE AND NIR-FLUORESCENCE OF C16H33Q-3CNQ IVT Absorbance in visible region (A) is strongly hypsochromic (gnd > exc) Near IR fluorescence (F) emission is solvatochromic; exc= 3 to 5 Debyes if gnd= 43 Debyes; Kirkwood-Westheimer calculation (1938 paper) yields exc = 8.7 Debyes Ground state ≈ D+-π-A- gnd=43 Debyes Excited state ≈ D0-π-A0 exc = 3 to 9 Debyes Intervalence transfer band (565 nm in LB films) Jeffrey Baldwin, Camino Simpson, & RMM J. Baldwin et al., J. Phys. Chem. B103: 4269 (1999)

  18. MOLECULAR ORBITAL CALCULATION FOR C16H33Q-3CNQ (A TWIST ANGLE 30˚ IS ASSUMED) HOMO-LUMO gap (5.6 eV) is probably too large. Dipole moment is maximum if twist angle  becomes 90˚ In LUMO, charge density is localized on 3CNQ moiety Other calculations: O. Kwon, M. L. McKee, and R. M. Metzger, Chem. Phys. Letters 313, 321 (1999). C. Krzeminski, C. Delerue, G. Allan, D. Vuillaume, and R. M. Metzger, Phys. Rev. B64, 085405 (2001). Hiromi Sakurai R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)

  19. VISIBLE SPECTRUM OF 11-LAYER Z-TYPE LB FILM OF C16H33Q-3CNQ The high (2) = 180 pm / V measured by Ashwell for Z-type multilayers of C16H33Q-3CNQ with a Nd-YAG laser at 1064 nm is due to resonance at 532 nm…. Hiroaki Tachibana R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)

  20. GRAZING-ANGLE REFLECTION ABSORPTION IR SPECTRUM OFC16H33Q+-3CNQ-LB MONOLAYER ON Au There is also a 2217 cm-1 CN stretch in C16H33Q-3CNQ that is Raman-active but is almost IR-silent. There are three CN stretches…. ==> The molecule cannot be lying flat on the Au surface T. Xu, T. Morris, G. Szulczewski, R. R. Amaresh, Y. Gao, S. Street, L. D. Kispert, R. M. Metzger, and F. Terenziani, J. Phys. Chem. B106: 10374 (2002)

  21. N(1s) CORE-LEVEL XPS OF THE D+--A- MOLECULE HEXADECYLQUINOLINIUM TRICYANOQUINODIMETHANIDE, C16H33Q+-3CNQ: TWO PEAKS Tao Xu T. Xu, T. Morris, G. Szulczewski, R. R. Amaresh, Y. Gao, S. Street, L. D. Kispert, R. M. Metzger, and F. Terenziani, J. Phys. Chem. B106: 10374 (2002)

  22. ORIENTATION OF THE LB FILMS OF C16H33Q+-3CNQ- ON Al Monolayer thickness is 2.3 nm; Molecular length is 3.0 nm if extended; Hence angle of tilt on substrate is 45˚ Z-type LB multilayers Symmetrical electrodes. Two-probe measurements: all resistances add. Oxides exist on Ga/In, and defect oxides on Al electrodes limit the current to where the oxide is not…. Bo Chen & Dominique Vuillaume R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)

  23. RECTIFICATION IN “Al | 1LB C16H33Q-3CNQ | Al” Electron flow for V > 0 The best result. Rectification ratio at 1.5 Volts: RR = [ I(V=1.5 Volts) ] / [ - I(V=-1.5 Volts] = 26 RR decreases upon cycling…… Current= 0.33 electrons per molecule per second Bo Chen & Dominique Vuillaume Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)

  24. RECTIFICATION IN “Al | 1 LB OF C16H33Q-3CNQ | Al” AT 105 K Rectification was observed for a monolayer of C16H33Q-3CNQ between Al electrodes, between 370 K and 105 K, with no temperature dependence for rectification ratio. Current increases as T increases. B. Chen & R. M. Metzger, J. Phys. Chem. B103: 4447 (1999)

  25. SCANNING TUNNELING SPECTROSCOPY OF 15 LB MONOLAYERS OF C16H33Q-3CNQ ON HOPG First monolayer adheres 50% to HOPG and is X-type; the other 14 layers transfer 100% as X-type. Electron flow is much higher for direction of intervalence transfer (IVT) within layers 2 to 15. Electron flow for V < 0 Ulf Höpfner R. M. Metzger, H. Tachibana, X. Wu, U. Höpfner, B. Chen, M. V. Lakshmikantham, and M. P. Cava, Synth. Metals 85, 1359 (1997)

  26. STM IMAGE OF C16H33Q-3CNQ MONOLAYER ON HOPG Pattern agrees with image of dicyanomethylene tails standing closest to Pt/Ir tip HOPG Jeffrey Baldwin Pt Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)

  27. ENERGY LEVELS FOR METALS AND FOR C16H33Q-3CNQ The LUMO energy is estimated from the appearance of enhanced rectification current from Al or HOPG The HOMO-LUMO gap is taken from the energy of the IVT band (570 nm = 2.17 eV). Theoretical HOMO-LUMO gap is too big; theory and experiment must be brought into better agreement R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)

  28. AVIRAM-RATNER MODEL MODIFIED FOR DONOR(+)-π-ACCEPTOR(-) ZWITTERION R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)

  29. Filled with liquid nitrogen Silicon coated mask LB film bottom gold layer(50nm) Cr layer (20nm) Glass substrate Thermally conductive gel Thermoinsulator cryocooling copper plate Aluminium foil as radiation reflector Chamber filled with ca. 110-3 mbar Ar to scatter Au atoms thickness sensor Au source DEPOSITING THE TOP AU ELECTRODE, TO FORM THE “Au | LB MONOLAYER | Au” SANDWICH • Mean free path of gold atoms in 110-3 mbar Ar at 300 K is 7 cm; • Therefore: about 7 Au-Ar collisions should occur before Au atom deposits onto the LB monolayer 50 cm T. Xu, I. Peterson, M. Lakshmikantham and R. M. Metzger, Angew. Chem. Int. Ed. Engl. 40, 1749 (2001)

  30. SETUP FOR RECTIFICATION MEASUREMENTS BETWEEN Au ELECTRODES

  31. ELECTRICAL RECTIFICATION IN “Au | LB MONOLAYER OF C16H33Q+-3CNQ- | Au” SANDWICH Resistance R = 2.47 k, Current I = 0.891 mA/ pad = 9,830 electrons molecule-1 s-1, Rectification ratio (RR) = 27.53 at 2.2 Volts. R. M. Metzger, T. Xu, and I. R. Peterson, J. Phys. Chem. B, 105, 7280 (2001); T. Xu, I. Peterson, M. Lakshmikantham and R. M. Metzger, Angew. Chem. Int. Ed. Engl. 40, 1749 (2001)

  32. RECTIFICATION RATIOS DECREASING IN LATER CYCLES : MOLECULES PROBABLY RE-ORIENT (OR ARE DESTROYED) IN THE EXTERNAL ELECTRIC FIELD (UP TO 1 GV / m) The subsequent scans show systematic decreases in the forward current; RR (@2.2 V) decreases from 27.2 (first scan) to 10.1, 4.76, and 2.44 in cycles 2-4, respectively. Metzger, Xu, and Peterson, J. Phys. Chem. B 105: 7280 (2001)

  33. SATURATION OF THE FORWARD CURRENT IN“Au | LB MONOLAYER OF C16H33Q+-3CNQ- | Au” SANDWICH Saturation of current seen, fits Aviram-Ratner prediction This cell shows a saturation current at Imax = 20.0 mA at 3.2 V; other pads show similar behavior (but not all of them) Metzger, Xu, and Peterson, J. Phys. Chem. B 105: 7280 (2001)

  34. MECHANISMS FOR RECTIFICATION IN “Metal | MONOLAYER | Metal” SANDWICHES • Schottky barriers at metal | molecule interface(s) (“S” for Schottky) [ W. Schottky, Z. f. Phys.118: 539 (1942) ] • Asymmetrical placement of chromophore between metal electrodes (“A” for asymmetric) [ C. Krzeminski, C. Delerue, G. Allan, D. Vuillaume, and R. M. Metzger, Phys. Rev. B64: # 085405 (2001); M. L. Chabinyc et al., J. Am. Chem. Soc. 124: 11730 (2002)] • Asymmetry within the molecular chromophore (“U” for unimolecular) [A. Aviram and M. A. Ratner, Chem. Phys. Lett.29: 277 (1974)]

  35. POSSIBLE MECHANISMS FOR UNIMOLECULAR (“U”) RECTIFICATION Does it involve ONLY ONE molecular orbital or energy level in the gap? L. E. Hall, J. R. Reimers, N. S. Hush, and K. Silverbrook, J. Chem. Phys.112: 1510 (2000) I. R. Peterson, D. Vuillaume, and R. M. Metzger, J. Phys. Chem. A105: 4702 (2001). Does it involve TWO molecular orbitals or energy levels in the gap? A. Aviram and M. A. Ratner, Chem. Phys. Lett.29: 277 (1974) C. Krzeminski, C. Delerue, G. Allan, D. Vuillaume, and R. M. Metzger, Phys. Rev. B64: # 085405 (2001) Data are not conclusive either way, but stay tuned…..

  36. ELECTRICAL INTER-IONIC RECTIFICATION IN “Au | LB MONOLAYER OF (Bu2NV)2BuPy+I- | Au” SANDWICH Au Electron flow for V > 0 High forward current is in the direction of electron flow fromiodide counterion(oramines) to pyridinium ring, and decreases with every successive cycle. Rectification ratio is as high as 60. Probably back- charge transfer fromiodide to pyridinium Au J. W. Baldwin, R. R. Amaresh, I. R. Peterson, W. J. Shumate, M. P. Cava, M. A. Amiri, R. Hamilton, G. J. Ashwell, and R. M. Metzger, J. Phys. Chem., B106: 12158 (2002)

  37. ELECTRICAL RECTIFICATION IN “Au | LB MONOLAYER OF DMAn-NC60 | Au” SANDWICH Strong film; small area; C60 staggered Large ohmic current due to gold stalagmites! Dimethylanilino-azaC60 DMAn-N C60 Au Electron flow under forward bias Au R. M. Metzger, J. W. Baldwin, W. J. Shumate, I. R. Peterson, P. Mani, G. J. Mankey, T. Morris, G. Szulczewski, S. Bosi, M. Prato, A. Comitoand Y. Rubin, J. Phys.Chem. B107: 102 (2003) The molecule rectifies (RR=2)

  38. SYNTHESIS AND PRESSURE-AREA ISOTHERMOF CH3C(O)S-C11H22Q+-3CNQ: COMBINE LB AND SELF-ASSEMBLY A. Jaiswal, R. R. Amaresh, M. V.Lakshmikantham, A. Honciuc, M. P. Cava, and R. M. Metzger, Langmuir 19: 9043 (2003).

  39. STM IMAGE OF LB MONOLAYER OF -S-C11H22Q+-3CNQ- ON Au (111) A. Jaiswal, R. R. Amaresh, M. V.Lakshmikantham, A. Honciuc, M. P. Cava, and R. M. Metzger, Langmuir 19: 9043 (2003).

  40. BONDING OF S-C11H22Q+-3CNQ- ON Au: THIOLATE AND C(CN)2 GROUPS COMPETE !!! Case A: Above Left and Right (Top and Middle) Case B: Above Right and Right (Bottom) A. Jaiswal, R. R. Amaresh, M. V.Lakshmikantham, A. Honciuc, M. P. Cava, and R. M. Metzger, Langmuir 19: 9043 (2003)..

  41. NEW RECTIFIER: “Au | LS MONOLAYER OF 4 | Au” Au Electron flow under “forward bias” (V < -2 Volts) Pressure-Area isotherm: very rigid solid monolayer (LS transfer is good) Au Zoomed STM: LS monolayer on Au has good hexagonal local order (balls are 10 Å apart) Rectification onset at -2 Volt; it does NOT decay with cycles. Some hysteresis. No ohmic regions. LS monolayer is very robust. Andrei Honciuc, Archana Jaiswal, Charles W. Spangler, RMM, et al. (unpublished)

  42. NEW RECTIFIERS IN “Au | LB MONOLAYER | Au” SANDWICHES (UNDER STUDY)

  43. WHAT NEXT? Unimolecular amplifier? • Rectifiers, bonded covalently to gold - in progress - • Other rectifiers - two more found 3. Can a single molecule exhibit power gain? (unimolecular junction transistor, not an FET or a single-electron transistor). To test this one needs: A. Make at least 3 metal electrodes meet to within 1 to 2 nm of each other (emitter, base,collector). Better break junctions (M. A. Reed)? Burn ultra-thin wires (P. McEuen)? B. How will power gain be possible? (different electron mobilities within a molecule? Back-to-back rectifiers plus a middle region?)

  44. WHAT AFTER THAT ? HOW DO WE BUILD CIRCUITS? Need “orthogonal synthesis”: attach molecule A covalently to surface, then molecule B covalently to A, then molecule C covalently to B, etc. (Li, Ratner, & Marks). At first, “surface” will be thin inorganic wires. Ultimately, “surface” will be a conducting organic wire. NEEDED: GOOD WORKING TEAMS OF SYNTHETIC CHEMISTS, PHYSICAL CHEMISTS, AND DEVICE PHYSICISTS

  45. ACKNOWLEDGEMENTS • Colleagues: (Prof. Geoffrey J. Ashwell) Cranfield University, UK • Dr. M. V. Lakshmikantham University of Alabama • Prof. Michael P. Cava University of Alabama • Dr. Dominique Vuillaume Institut de Microéléctronique du Nord, Lille, France • (Prof. Maurizio Prato) University of Trieste, Italy • Prof. Gary J. Mankey University of Alabama • Post-docs: Prof. Ian R. Peterson Coventry University, UK • Dr. Hiroaki Tachibana AIST, Tsukuba, Japan • Dr. Tsuyoshi Kawai University of Kyushu, Japan • Dr. Hiromi Sakurai Tokyo, Japan • Dr. Ramiya R. Amaresh University of Virginia • Dr. Rajugopal University of Alabama • Dr. Archana Jaiswal University of Alabama • Dr. Akihiko OtsukaKyoto University • Grad. students:Dr. Xiangli Wu Etek Dynamics, San Jose, CA • Dr. Jeffrey W. Baldwin Naval Research Laboratory, Washington, DC • Dr. Terry V. Hughes Howard Hughes Res. Ctr., La Jolla, CA • Dr. Bo Chen Xeotron Corp., Houston, TX • Dr. Tao XuTexas A&M University • Mr. Petie Shumate University of Alabama • Mr. Andrei Honciuc University of Alabama • Undergrads: Miss Christina Hosch Hatcher (Clarke College) University of Wisconsin at Madison • Miss Camino Simpson (Talladega College) United States Navy • University of Mississippi colleagues: Profs. Charles A. Panetta, Norman E. Heimer, Daniell L. Mattern • Yale University colleague: Prof. Mark A. Reed • Funding: NSF • Past funding: ONR, DOE-EPSCoR, JSPS

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