1. Two-Dimensional Molecular Electronics Circuits Yi Luo, C. Patrick Collier, Jan O. Jeppesen, Kent A. Nielsen, Erica DeIonno, Greg Ho, Julie Perkins, Hsian-Rong Tseng, Tohru Yamamoto, J. Fraser Stoddart, and James R. Heath ChemPhysChem 2002, 3, 519-525. Tobe Lab. Keiji Nishihara 1

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

3. Molecular Electronics Top-Down Bottom-Up LithographyChemical Assembly Molecular device For nanoscale and molecular-electronics-based computing systems http://www.nanoelectronics.jp 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. ・ Size : nanoscale ・ Low cost ・ Ease of fabrication 3

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

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

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

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

8. Bistable [2]Catenane J. F. Stoddart, J. R. Heath et al. Science, 2000, 289, 1172-1175. 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 MSTJs 8

9. Bistable [2]pseudorotaxane Design more symmetrically between the electrodes for increasing the resonant tunneling current MSTJs Much higher current levels : 100 nA (at 0.1 V) The change in junction resistance : 200 times Problem Domain switching Large current amplitude fluctuations in the cycling Unstable address voltages 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 SPM image of an LB film 9

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

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

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

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

14. Device Characteristics Nanometer-scale MSTJs Remnant molecular signiture hysteresis loop [2]rotaxaneEicosanoic acid C 19 H 39 CO 2 H Tunnel barrier ・ Switching magnitude : 3 times ・ Lower current levels than those for microscale MSTJs 14

15. Device Characteristics Switching Cycle [2]rotaxaneEicosanoic acid C 19 H 39 CO 2 H ・ Switching stopped at 260 K 15 “on”“off”

16. 64-bit Random Access Memory DARPA (Defense Advanced Research Agency ) SRC (Semiconductor Research Corporation ) CNSI (California NanoSystems Institute) “R” character (ASCII 82) “01010010” Standard ASCII character set the separation between 1s and 0s 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. Voltage profile used in the acquisition of (C) The remnant molecular signiture Hysteresis curve assoiciated with a conventional magnetic memory bit