The Genesis of Molecular Electronics

1. The Genesis of Molecular Electronics

2. Conventional Electronics: Transistor development • In 1915 AT&T opened their transcontinental telephone system; required signal amplification. • 1945: AT&T and Bell Labs set up the Solid State Physics group. • First transistor invented in 1947 at Bell Labs. • Junction transistors used to develop first integrated circuit in 1958; Jack Kilby at Texas Instruments (2000 Nobel Prize in Physics). • FET’s in 1961. • 1965 Moore’s Law.

3. Moore’s Law

4. Boundaries of conventional techniques • Miniaturization achieved by “top down” approach using improvements in lithography technique. • Even with the development of ever-improving lithographic tools, silicon is approaching fundamental physical limitations of operation. As gate widths decrease below 100 nm, bulk properties yield to quantum phenomena and leakage currents from electron tunneling prevent proper device operation. • Chemistry operates at the nanometer scale by controlling the placement of individual atoms and functional groups on molecules through synthetic chemistry, allowing macroscopic properties from rigidity to optical and electronic behavior to be engineered. • “Bottom up” approach is promising instead of carving lithographically bigger blocks into smaller and smaller chunks.

5. Molecular Electronics • First coined by Mark Ratner, in 1974. • Molecular electronics involves the replacement of a wire, transistor or other basic solid-state (usually silicon) electronic element with one or a few molecules. • Molecular electronic device must exchange information, or transfer states or must be able to interface with components at the macroscopic level. • Simple molecular electronic devices usually consist of organic molecules sandwiched between conducting electrodes.

6. Molecules displaying functional behavior

7. Molecular Rectifiers

8. Early Work In the 1970’s Aviram and Ratner surmised that an organic analogue of a p-n junction would act as a molecular rectifier. • Computational results from study of one such D-σ-A molecule composed of a donor moiety tetrathiafulvalene connected by a methylene bridge to an acceptor moiety, tetracyanoquinodimethane, showed a rectification of current should be possible. D-σ-A Rectifier A. Aviram, M. A. Ratner, Chem. Phys. Lett. 1974, 29, 277.

9. Difference in threshold voltage for the mechanisms under positive and negative bias gives rise to rectifying behavior. Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed.2002, 41, 4378-4440.

10. Molecular Rectifiers D-π-A Rectifier • Metzger and co-workers have studied Langmuir Blodgett (LB) films of (nhexadecyl)quinolinium tricyanoquinodimethanide between metal electrodes and observed strong rectification behavior. The donor is the quinolinium moiety, connected to the acceptor, tricyanoquinodimethanideby a bridge. Metzger, R. M. Chem. Rev.2003, 103, 3803-3834.

11. I/V curves from two different LB-film configurations. a) 1 LB monolayer b) 4 LB monolayers. Metzger, R. M. Chem. Rev.2003, 103, 3803-3834.

12. Comparison of Mechanisms Aviram-Ratner model for neutral D-σ-A species Model for zwitterionic D+-π-A- species Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed.2002, 41, 4378-4440.

13. Langmuir-Blodgett Monolayer Photodiode Electrochemical photodiode: D-S-A Under positive bias, e- moves from to D to S (G.S). Photochemical excitation promotes e- to first E.S. of S, to A and finally to Au. No current when light is off. Sakomura, M.; Lin, S.; Moore, T. A.; Moore, A. L.; Gust, D.; Fujihira, M. J. Phys. Chem. A2002, 106, 2218.

14. Molecular Wires Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed.2002, 41, 4378-4440.

15. Tour Wires James M.Tour’s group over 15 years have been synthesizing molecules with aromatic, alkene, and alkyne bridges, terminating in thiols at one or both ends. These are known as Tour Wires. A wire is defined as a two-terminal entity that possesses a reasonably linear I(V) curve prior to the breakdown limit. Precise molecular wires bearing protected alligator clips (SAc) at one and two ends.

16. Tour Wire: Molecular Devices Molecular devices could be systems having two or more termini with current-voltage responses that would be expected to be nonlinear due to intermediate barriers or heterofunctionalities in the molecular framework. Two terminal wire with tunnel barrier; wire with a quantum well: RTD; three terminal system: switch; four terminal system: logic gate P(m,n) refers to the molecular electrostatic potential impedance of a system with m 1,4-phenylene moieties and n ethynylene moieties. Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486-8493.

17. Resonant Tunnelling Diode • RTD allows voltage bias to switch “on” and “off” the current. • Current passes equally well in both directions. • Aliphatic groups with high P.E. establish aromatic ring between them as narrow “island” of lower P.E. through which electrons must pass to traverse the entire length of the wire.

18. Resonant Tunnelling Diode; Operation • Smaller the region in which the electrons are confined, farther apart are the allowed quantized energy levels, eg. “island” and regions to left and right of barrier. • Electrons injected under bias into LUMO on LHS. • If the K.E. is’nt enough, no tunneling occurs; switched “off”. • If bias is high enough, incoming electron’s energy resonate with energy levels inside well, tunneling ocuurs, etc.; switched “on”. “Peak” to “valley” ratio ~1.3:1

19. Negative Differential Resistance • A negative differential resistance (NDR) is characterized by a discontinuity in the monotonic increase of current as the voltage is increased. • Several of these devices can be combined to give I/V curves with multiple peaks–this behavior has been proposed to lead to multi-state memory and logic devices. • Reed and Tour et al. reported the clearest example of molecule-based NDR to date.

20. At 60 K, assembly was found to display a very strong NDR with a peak-to-valley ratio (PVR) of 1030:1. Control molecules (having no nitro or amine moieties) showed no NDR. In the singly reduced state, the LUMO becomes fully delocalized, allowing enhanced conduction, thus creating the onset of the NDR peak. As the bias voltage is increased the molecule becomes doubly reduced, the LUMO becomes localized across the molecule and decreases the conductivity of the molecule, reducing the current passed through the molecule.

21. Three terminal devices Molecular three-terminal junction that could be used as a molecular interconnect. Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486-8493.

22. Low input potential: impedance = 2P (in series) High input potential: impedance = 3P/2 Switch like properties. Can behave like a NOT logic gate. Molecular-sized switch with corr. equivalent of source, drain, and gate terminals of a bulk solid-state FET. Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486-8493.

23. Rotaxane: Molecular Switch • Docking stations: Benzidine and Benzophenol. • Bulky stopper groups. • Bead: tetracationic cyclophane. • Protonation/Oxidation: bead shifts to benzophenol • Molecular shuttle switched electrostatically Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed.2002, 41, 4378-4440.

24. Rotaxane: Logic Device • “Ring” and “Thread” fluoresce separately. • Upon threading (CT complex), fluorescence extinguished. • Addition of protons or base recovers the fluorescence. • Neutralization removes fluorescence again. • If the fluorescence is taken as an indicator of truth, and B and H+are taken as inputs, then the system has the same behavior as an XOR gate. Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed.2002, 41, 4378-4440.

25. Rotaxane: Logic Device • Tetracationic cyclophane with two bipyridinium units interlocked with a crown ether containing a TTF and a NP unit on opposite sides. • TTF inside : A0 • On oxidation, TTF outside: B+ • At 0 V , goes to B0 • Bistability is the basis of the device. Stoddart et al. Science 2000, 289, 1172-1175.

26. Probing and Interconnecting Molecules:Self Assembly and Directed Self Assembly • How to attach probe electrodes to either side of molecule? Self assembly to adsorb molecules on an electrode. • Alligator clips: R-NC, R-S-S-R, R-COOH etc. • Alkanethiolates used as insulating host matrix; electronic properties of embedded molecules can be explored. • Directed self assembly: grow rigid substrate molecules normal to the adsorbate by selective insertion into host at defect sites, or at step edges. B. A. Mantooth, P. S.Weiss, “Fabrication, Assembly And Characterization Of Molecular Electronic Components,” Proc. IEEE, vol. 91, pp. 1785-1802, Nov. 2003.

27. Molecular Conductivity • Electron Transfer: • Coherent nonresonant tunneling : Electronic states of the molecule are far from the energy of the tunneling electrons; rate of electron transport exponentially dependent on the length of the molecule. • Coherent resonant tunneling Energy of tunneling electrons resonant with the energy of the molecular orbitals’ rate of electron transport is essentially independent of length .

28. Measuring Molecular Electronic Components • Mercury Drop Junction: • Mercury can form thiol-based SAMs. • The junction is created by forming a mechanical contact of a SAM supported on a solid • substrate and a SAM supported on a suspended mercury drop. • The resulting metal–SAM–SAM–metal junction allows for the ensemble measurement of pure and mixed monolayers.

29. Break Junctions Some of the first single-molecule conductivity measurements executed using mechanically controllable break (MCB) junctions. Benzene-1,4-dithiol, one of the simplest molecules used in characterizing molecular conductance.

30. Nanopore The nanopore consists of a SAM of conjugated molecules sandwiched between two electrodes. Fabrication of the heterostructure. (a) Cross section of a silicon wafer showing the bowl-shaped pore etched in suspended SiN membrane with a diameter about 300 A. (b) Au-Ti top electrode/self-assembled monolayer/Au bottom electrode sandwich structure in the nanopore. (c) 4-thioacetylbiphenyl and detail diagram of the sandwich heterostructure. Reed, et al. “The electrical measurement of molecular unctions,” in Molecular Electronics: Science and Technology, 1998, vol. 852, Annals of the New York Academy of Sciences, pp. 133–144.

31. Fabrication of the nanopore • The devices are fabricated using a combination of electron-beam lithography, plasma etching, and use of an anisotropic etchant to create a suspended silicon nitride membrane with a 30–50 nm aperture . • An Au contact is evaporated on the top of the aperture and the device is immersed in a solution of the molecule of interest to form a SAM. • After deposition, the bottom electrode is formed by evaporating 200 nm of Au onto the sample, which is held at 77 K to minimize damage to the SAM. • Using the nanopore device, Reed and coworkers have measured the properties of biphenyl-4-thiol. • This molecule exhibited strong rectifying behavior arising from the asymmetry of the molecule.

32. Other methods STM Contact Conductive Probe AFM Nanoparticle Coupled CP-AFM

33. Conclusion • Molecular electronics will mature into a powerful technology only if its development is based on sound scientific conclusions that have been tried and tested at every step. • Detailed understanding of the molecule/electrode interface, as well as developing methods for manufacturing reliable devices needed.