Molecular quantum-dot cellular automata

1. Molecular quantum-dot cellular automata Yuhui Lu Department of Electrical Engineering University of Notre Dame

2. Outline of presentation • QCA overview • Metal-dot QCA devices • Molecular QCA • Clocking molecular QCA • Summary

3. Cell-cell response function A cell with 4 dots 2 extra electrons cell1 cell1 cell2 cell2 Polarization P = -1 Bit value “0” Quantum-dot Cellular Automata Represent binary information by charge configuration Tunneling between dots Polarization P = +1 Bit value “1” Bistable, nonlinear cell-cell response Restoration of signal levels Neighboring cells tend to align. Coulombic coupling

4. A M B Majority gate C A 1 0 1 1 1 0 0 0 B Out C Programmable 2-input AND or OR gate. QCA devices Binary wire Inverter

5. electrometers Metal-dot QCA implementation Al/AlOx on SiO2 Metal-dot QCA cells and devices 70 mK “dot” = metal island Greg Snider, Alexei Orlov, and Gary Bernstein

6. Metal-dot QCA cells and devices • Demonstrated 4-dot cell 1 3 4 2 • A.O. Orlov, I. Amlani, G.H. Bernstein, C.S. Lent, and G.L. Snider, Science, 277, pp. 928-930, (1997).

7. A M B C Metal-dot QCA cells and devices • Majority Gate Amlani, A. Orlov, G. Toth, G. H. Bernstein, C. S. Lent, G. L. Snider, Science284, pp. 289-291 (1999).

8. “dot” = redox center Mixed valence compounds From metal-dot to molecular QCA Why molecule? 1. Natural, uniform quantum dots. 2. Small. High density. 3. Room temperature operation. Key strategy: use nonbonding orbitals (p or d) to act as dots.

9. Binary information encoded in the molecular charge configuration “1” “0” “1” “0” “0” “1” “0” “1” Mobile charges are created by chemical oxidation or reduction.

10. Experiments on molecular double-dot Fehlner, Snider, et al. (Notre Dame QCA group) Journal of American Chemical Society, 125:15250, 2003 Fe Fe Ru Ru “1” “0” trans-Ru-(dppm)2(C≡CFc)(NCCH2CH2NH2) dication Fe group and Ru group act as two unequal quantum dots.

11. “struts” PHENYL GROUPS “TOUCHING” SILICON Surface attachment and orientation molecule Si-N bonds Si(111) Molecule is covalent bonded to Si and oriented vertically by “struts.”

12. applied potential Fe Fe Ru Ru 2 counterion charge configurations on surface Measurement of molecular bistability layer of molecules Applied field equalizes the energy of the two dots When equalized, capacitance peaks.

13. Fe Fe Ru Ru Charge configurations “0” “1” HOMO orbitals from quantum chemistry calculation show the localization of mobile electron. Bistable charge configuration.

14. Switching by an applied field Mobile electron driven by electric field, the effect of counterions shift the response function. Click-clack correspond to:

15. 4-dot molecule 6 Å Fehlner et al (Notre Dame chemistry group) Journal of American Chemical Society 125:7522, 2003 Advantage: neighboring molecules have the same charge configurations. No need to keep track on the numbers in the array. Each ferrocene acts as a quantum dot, the Co group connects 4 dots.

16. “0” “1” Bistable configurations HOMO orbital show the localization of mobile electron. Guassian-98 UHF/STO-3G/LANL2DZ

17. Can one molecule switch the other ?

18. Switching molecule by a neighboring molecule driver response driver response Coulomb interaction is sufficient to couple molecular states.

19. Energy Intermolecular Interaction Excited State Ekink=0.25 eV “1” “0” Ground State “1” “1” Kink energy is greater than kBT, thus room temperature operation is possible.

20. Kroemer’s lemma • If, in discussing a semiconductor problem, you cannot draw an Energy-Band-Diagram, this shows that you don't know what you are talking about. • If you can draw one, but don't, then your audience won't know what you are talking about. • There is no energy band for single molecule. Single molecule only has discrete energy levels.

21. …. …. Origin of energy band Anti-bonding orbital Atomic orbital The interaction between two atomic orbitals form a bonding orbital and an anti-bonding orbital. Bonding orbital band Band originated from the interaction of large number of atomic orbitals in the periodic potential. band In single molecules, energy levels are discrete.

22. The ground and first excited energy levels 1,4-diallyl butane radical cation • Ground state • First excited state “0” “1”

23. + Discrete energy levels under the switching field • Ground state • First excited state

24. + Discrete energy levels under the switching field • Ground state • First excited state

25. + Discrete energy levels under the switching field • Ground state • First excited state

26. input Clocked QCA How to control the information flow? • Clocking: • Control of information flow around the circuit. • 2. Restore the dissipative energy. Cells fully polarized to be “0” or “1”.

27. E E E Clocking field “1” “null” or “0” null active Use local electric field to switch molecule between active and null states.

28. 0 1 null Adiabatic switching energy 0 1 x

29. Clocked molecular QCA

30. Switching field Clocking field “null” “1” “0” Model clock QCA 1,5,9 decatriene Using ethene as quantum dot.

31. “1” “0” “null” “null” “1” “0” Molecular energy • ground state • first excited state • second excited state Gaussian 03 CASSCF(5,6) 6-31G* The molecule is locked in “null” state, thus carries no information.

32. “null” “0” “null” “1” “0” “1” Molecular energy • ground state • first excited state • second excited state A clock voltage “turns on” the devices.

33. “null” “1” “0” Molecular energy “null” • ground state • first excited state • second excited state “0” “1” Large enough clock voltage “pins” the mobile charge.

34. Summary • The binary information is encoded in the molecular charge configuration. • Coulomb interaction couples the information transport. • Room temperature operation. • Clocking controls the information flow.