Molecular quantumdot cellular automata Yuhui Lu Department of

Molecular quantum-dot cellular automata Yuhui Lu Department of Electrical Engineering University of Notre Dame _______________________________ EE 666 April 14, 2005

Outline of presentation • • • QCA overview Metal-dot QCA devices Molecular QCA Clocking molecular QCA Summary _______________________________ EE 666 April 14, 2005

Quantum-dot Cellular Automata Represent binary information by charge configuration Cell-cell response function A cell with 4 dots cell 1 cell 2 2 extra electrons Tunneling between dots Polarization PP == +1 -1 Bit value “ 1” “ 0” Neighboring cells tend to align. Coulombic coupling cell 1 cell 2 Bistable, nonlinear cell-cell response Restoration of signal levels _______________________________ EE 666 April 14, 2005

QCA devices Binary wire 10 Majority gate A B C M Inverter 0 1 1 0 A B Out C Programmable 2 -input AND or OR gate. _______________________________ EE 666 April 14, 2005

Metal-dot QCA cells and devices Metal-dot QCA implementation Al/Al. Ox on Si. O 2 electrometers 70 m. K “dot” = metal island Greg Snider, Alexei Orlov, and Gary Bernstein _______________________________ EE 666 April 14, 2005

Metal-dot QCA cells and devices • Demonstrated 4 -dot cell 1 3 2 4 A. O. Orlov, I. Amlani, G. H. Bernstein, C. S. Lent, and G. L. Snider, Science, 277, pp. 928 -930, (1997). _______________________________ EE 666 April 14, 2005

Metal-dot QCA cells and devices • Majority Gate A B C M Amlani, A. Orlov, G. Toth, G. H. Bernstein, C. S. Lent, G. L. Snider, Science 284, pp. 289 -291 (1999). _______________________________ EE 666 April 14, 2005

From metal-dot to molecular QCA “dot” = redox center Mixed valence compounds 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. _______________________________ EE 666 April 14, 2005

Binary information encoded in the molecular charge configuration “ 0” “ 1” Mobile charges are created by chemical oxidation or reduction. _______________________________ EE 666 April 14, 2005

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 “ 0” “ 1” trans-Ru-(dppm)2(C≡CFc)(NCCH 2 NH 2) dication Fe group and Ru group act as two unequal quantum dots. _______________________________ EE 666 April 14, 2005

Surface attachment and orientation molecule Si-N bonds Si(111) PHENYL GROUPS “TOUCHING” SILICON “struts” Molecule is covalent bonded to Si and oriented vertically by “struts. ” _______________________________ EE 666 April 14, 2005

Measurement of molecular bistability Applied field equalizes the energy of the two dots layer of molecules applied potential Fe Fe Ru Ru When equalized, capacitance peaks. 2 counterion charge configurations on surface _______________________________ EE 666 April 14, 2005

Charge configurations Fe Fe Ru Ru “ 0” “ 1” HOMO orbitals from quantum chemistry calculation show the localization of mobile electron. Bistable charge configuration. _______________________________ EE 666 April 14, 2005

Switching by an applied field Mobile electron driven by electric field, the effect of counterions shift the response function. Click-clack correspond to: _______________________________ EE 666 April 14, 2005

4 -dot molecule 6Å 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. _______________________________ EE 666 April 14, 2005

Bistable configurations “ 0” “ 1” HOMO orbital show the localization of mobile electron. Guassian-98 UHF/STO-3 G/LANL 2 DZ _______________________________ EE 666 April 14, 2005

Can one molecule switch the other ? _______________________________ EE 666 April 14, 2005

Switching molecule by a neighboring molecule driver response Coulomb interaction is sufficient to couple molecular states. _______________________________ EE 666 April 14, 2005

Intermolecular Interaction Energy Excited State “ 1” Ekink=0. 25 e. V “ 0” Ground State “ 1” Kink energy is greater than k. BT, thus room temperature operation is possible. _______________________________ EE 666 April 14, 2005

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. _______________________________ EE 666 April 14, 2005

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. _______________________________ EE 666 April 14, 2005

The ground and first excited energy levels 1, 4 -diallyl butane radical cation • Ground state • First excited state “ 1” “ 0” _______________________________ EE 666 April 14, 2005

Discrete energy levels under the switching field + • Ground state • First excited state _______________________________ EE 666 April 14, 2005

Discrete energy levels under the switching field + • Ground state • First excited state _______________________________ EE 666 April 14, 2005

Discrete energy levels under the switching field + • Ground state • First excited state _______________________________ EE 666 April 14, 2005

Clocked QCA input How to control the information flow? Clocking: 1. Control of information flow around the circuit. 2. Restore the dissipative energy. Cells fully polarized to be “ 0” or “ 1”. _______________________________ EE 666 April 14, 2005

Clocking field E “ 1” E or “null” E “ 0” active null Use local electric field to switch molecule between active and null states. _______________________________ EE 666 April 14, 2005

energy Adiabatic switching 0 1 x 0 1 null _______________________________ EE 666 April 14, 2005

Clocked molecular QCA _______________________________ EE 666 April 14, 2005

Model clock QCA Switching field 1, 5, 9 decatriene Using ethene as quantum dot. Clocking field “null” “ 1” “ 0” _______________________________ EE 666 April 14, 2005

Molecular energy • ground state • first excited state • second excited state “ 0” “ 1” “ 0” Gaussian 03 CASSCF(5, 6) 6 -31 G* “null” “ 1” The molecule is locked in “null” state, thus carries no information. _______________________________ EE 666 April 14, 2005

Molecular energy • ground state • first excited state • second excited state “null” “ 0” “null” “ 1” A clock voltage “turns on” the devices. _______________________________ EE 666 April 14, 2005

Molecular energy “null” “ 0” • ground state • first excited state • second excited state “ 1” Large enough clock voltage “pins” the mobile charge. _______________________________ EE 666 April 14, 2005

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. _______________________________ EE 666 April 14, 2005