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Molecular quantum-dot cellular automata

Yuhui LuDepartment of Electrical Engineering

University of Notre Dame

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Outline of presentation

• QCA overview• Metal-dot QCA devices• Molecular QCA• Clocking molecular QCA• Summary

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Quantum-dot Cellular AutomataRepresent binary information by

charge configuration

A cell with 4 dots

Tunneling between dots

Polarization P = +1Bit value “1”

2 extra electrons

Polarization P = -1Bit value “0”

Bistable, nonlinear cell-cell responseRestoration of signal levels

cell1 cell2

cell1 cell2

Cell-cell response function

Neighboring cells tend to align.Coulombic coupling

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0 01 1

01 10

Binary wire

Inverter

A

B

C

Out

Majority gate

MABC

Programmable 2-input AND or OR gate.

QCA devices

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Metal-dot QCA cells and devices

“dot” = metal island

electrometers

70 mK

Al/AlOx on SiO2

Metal-dot QCA implementation

Greg Snider, Alexei Orlov, and Gary Bernstein

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Metal-dot QCA cells and devices

• Demonstrated 4-dot cell

A.O. Orlov, I. Amlani, G.H. Bernstein, C.S. Lent, and G.L. Snider, Science, 277, pp. 928-930, (1997).

1

2

3

4

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Metal-dot QCA cells and devices

• Majority Gate

MABC

Amlani, A. Orlov, G. Toth, G. H. Bernstein, C. S. Lent, G. L. Snider, Science 284, pp. 289-291 (1999).

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From metal-dot to molecular QCA

Key strategy: use nonbonding orbitals ( or d) to act as dots.

“dot” = redox centerMixed valence compounds

Why molecule?

1. Natural, uniform quantum dots. 2. Small. High density. 3. Room temperature operation.

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Binary information encoded in the molecular charge configuration

“0” “1” “0” “1”

“0” “0”“1” “1”

Mobile charges are created by chemical oxidation or reduction.

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Experiments on molecular double-dot

Fehlner, Snider, et al. (Notre Dame QCA group)Journal of American Chemical Society,125:15250, 2003

Ru Ru

Fe Fe

“0” “1”

Fe group and Ru group act as two unequal quantum dots.

trans-Ru-(dppm)2(C≡CFc)(NCCH2CH2NH2) dication

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Surface attachment and orientation

N

Si Si3.8

2.4 106o

PHENYL GROUPS“TOUCHING” SILICON

Molecule is covalent bonded to Si and oriented vertically by “struts.”

Si(111)

molecule Si-N bonds

“struts”

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FeRu Fe Ru Fe

Ru

Si

HgFe

Ru

Si

HgFe

Ru

Si

HgFe

Ru

ac C

apac

itanc

e

voltage

excited stateswitching

Ene

rgy

ground state

Applied field equalizes the energy of the two dots

When equalized, capacitance peaks.

appliedpotential

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

C(oxidized) C(reduced) C

VHg

(V)C

(nF)

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

C

(nF)

Measurement of molecular bistabilitylayer of molecules

Ru

Fe

Ru

Fe

2 counterion charge configurations on surface

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Charge configurations

HOMO orbitals from quantum chemistry calculation show the localization of mobile electron.

“1”“0”

Bistable charge configuration.

Ru

Fe

Ru

Fe

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Switching by an applied field

FeRu

FeRu

Fe Ru

Mobile electron driven by electric field, the effect of counterions shift the response function.

Click-clack correspond to:

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4-dot molecule

Each ferrocene acts as a quantum dot, the Co group connects 4 dots.

Fehlner et al(Notre Dame chemistry group)Journal of American Chemical Society125:7522, 2003

6 Å

Advantage:neighboring molecules have the samecharge configurations. No need to keeptrack on the numbers in the array.

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Bistable configurations

“0” “1”

Guassian-98 UHF/STO-3G/LANL2DZ

HOMO orbital show the localization of mobile electron.

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Can one molecule switch the other ?

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Switching molecule by a neighboring molecule

Coulomb interaction is sufficient to couple molecular states.

driver response

driver response

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Intermolecular Interaction

Ekink=0.25 eV

Kink energy is greater than kBT, thus room temperature operation is possible.

“1” “1”

Ground State

“1” “0”

Excited State

Ene

rgy

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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.

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Origin of energy band

Bonding orbital

Anti-bonding orbital

…. ….

Atomicorbital The interaction between two atomic orbitals

form a bonding orbital and an anti-bondingorbital.

band

band

Band originated from theinteraction of large numberof atomic orbitals in the periodic potential.

In single molecules, energy levels are discrete.

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• Ground state• First excited state

The ground and first excited energy levels

“1” “0”

1,4-diallyl butaneradical cation

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Discrete energy levels under the switching field

• Ground state• First excited state

+

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Discrete energy levels under the switching field

• Ground state• First excited state

+

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Discrete energy levels under the switching field

• Ground state• First excited state

+

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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”.

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Clocking field

“1”

“0”

null

E

E

E

or

Use local electric field to switch molecule between active and null states.

active

“null”

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Adiabatic switching

0 1

0 1

null

ener

gy

x

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Clocked molecular QCA

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Model clock QCA

Clocking field

Switching field

“null” “1”“0”

1,5,9 decatrieneUsing ethene asquantum dot.

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Molecular energy

“0” “1”“null”

Gaussian 03CASSCF(5,6)6-31G*

“0” “1”

“null”

The molecule is locked in “null” state, thus carries no information.

• ground state• first excited state• second excited state

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Molecular energy

“0” “1”

“null”

A clock voltage “turns on” the devices.

“0” “null” “1”

• ground state• first excited state• second excited state

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Molecular energy

“0” “1”

“null”

“0” “1”

“null”

Large enough clock voltage “pins” the mobile charge.

• ground state• first excited state• second excited state

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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.