1. Two-Dimensional Molecular Electronics Circuits Yi Luo, C. Patrick Collier, Jan O. Jeppesen, Kent A. Nielsen, EricaDeIonno, Greg Ho,Julie Perkins, Hsian-Rong Tseng, Tohru Yamamoto, J. Fraser Stoddart, and James R. Heath ChemPhysChem2002, 3, 519-525. Tobe Lab. Keiji Nishihara 1

2. Contents ・Introduction Molecular Electronics Two-Dimensional (2D) Crossbar Circuits Molecular Switch Tunnel junctions ( MSTJs ) ・Results and Discussions ・Summary 2

3. Molecular Electronics Molecular random access memory cell Single-molecule transistor J. Chen et al. Science, 1999, 286, 1550-1552. J. R. Heath et al. Angew. Chem. Int. Ed. 2003, 42, 5706-5711. Molecular device ・Size : nanoscale ・Low cost ・Ease of fabrication For nanoscale and molecular-electronics-based computing systems Top-Down Bottom-Up Lithography Chemical Assembly http://www.nanoelectronics.jp 3

4. Two-Dimensional (2D) Crossbar Circuits 2D Crossbar Circuit “0101” the dominant architecture for nanoelectronics a) Fabrication ex. Lithography Addressing（番地付け） Imprinting Chemical Assembly b) Scalable to nanoscale 4

5. Molecular Switch Tunnel Junctions ( MSTJs ) Junction （接合） Two wires sandwich some active molecular component Active Device （能動素子） In MSTJs active molecular component ex. Rotaxane The switching is a molecular property. Catenane 5

6. Development of Molecular Devices Bistable [2]Catenane Bistable [2]Pseudo-rotaxane Bistable [2]Rotaxane 1. Molecular Design 2. Synthesis Feedback Cycle 3. Solution-Phase Electrochemical Switching 4.Solid-State Device Performance 6

7. Switching Mechanism Dioxynaphthalene (DNP) unit Tetrathiafulvalene (TTF) unit Cyclobis(paraquat-p-phenylene) ring component a) The chargedring encircles the TTF recognition site. b) Oxdation of TTF unit ( → TTF+・) Coulombic repulsion Ring position > TTF unit c) Ring’s translation up to the neutral DNP site. DNP unit d) Reduction of TTF+・radical cation ( → TTF) Thermodynamic stability Metastable state : high conductivity d) → a) : thermal relaxation or application of reversed bias 7

8. Bistable [2]Catenane MSTJs Double (x2) change in junction resistance between “0” and “1” states. At least a few hundred times’ cycles Problem 1. Low current levels : 20 pA (at 0.1 V) 2. Not sufficient switching magnitude J. F. Stoddart, J. R. Heath et al. Science, 2000, 289, 1172-1175. 8

9. Bistable [2]pseudorotaxane Design for increasing the resonant tunneling current MSTJs Much higher current levels : 100 nA (at 0.1 V) The change in junction resistance : 200 times more symmetrically between the electrodes Problem Unstable address voltages Large current amplitude fluctuations in the cycling SPM image of an LB film Domain switching J. R. Heath, J. F. Stoddart et al. J. Am. Chem. Soc. 2001, 123, 12632-12641. J. F. Stoddart et al. Acc. Chem. Res. 2001, 34, 433-444 9

10. Bistable [2]rotaxane Ⅰ Design to avoid domain switching Incorporating a dendron as the hydrophilic stopper 40 Å2 for [2]pseudorotaxane Bulky stopper In a monolayer film Much larger footprint : 140 Å2 No domain were detected with Brewster angle microscopy or various scanning probe microscopy techniques 10

11. Bistable [2]rotaxane Ⅱ Blue ring encircles Green site ? More stable or Red site ? co-conformations 1 : 1 mixture Phenolic residues one co-conformation The reason for this change a) To start off exclusively as only one of its two possible co-conformations b) To enhance oxidative stability 11

12. Fabrication Process Langmuir-Blodgett (LB) monolayer film of [2]rotaxane SiO2 / Si N-type polycrystalline silicon electrodes Ti protective layer Al wire electrode Reactive ion etching A SEM image of a 1D circuit of MSTJs remove Ti layer Junction area : 0.007 mm2 about 5000 molecules contained Y. Chen et al. Appl. Phys. Lett., 2003, 82, 1610-1612. 12

13. Device Characteristics Remnant molecular signiture hysteresis loop Micrometer-scale MSTJs “on” Dumbbell only “off” [2]rotaxane Ⅰ Electrical Breakdown Current levels : about 1 nA (at 0.1 V) Switching magnitude : 10 times “off” “on” 13

14. Device Characteristics Nanometer-scale MSTJs [2]rotaxane Eicosanoic acid C19H39CO2H Tunnel barrier Remnant molecular signiture hysteresis loop ・Lower current levels than those for microscale MSTJs ・Switching magnitude : 3 times 14

15. Device Characteristics Switching Cycle [2]rotaxane Eicosanoic acid C19H39CO2H ・Switching stopped at 260 K “off” “on” 15

16. 64-bit Random Access Memory the separation between 1s and 0s “R” character (ASCII 82) Standard ASCII character set DARPA (Defense Advanced Research Agency) “01010010” SRC (Semiconductor Research Corporation) CNSI (California NanoSystems Institute) 16

17. Summary ・ It is possible, by feeding device performance characteristics back into molecular synthesis, to improve the switching characteristics of a device. ・ The authors demonstrated point addressability of MSTJs through the operation of a 64-bit 2D crossbar random access memory circuit. 17

18. Langmuir-Blodgett Film http://www.nanoelectronics.jp

19. Hysteresis curve assoiciated with a conventional magnetic memory bit Voltage profile used in the acquisition of (C) The remnant molecular signiture