_____________________________________________________________ EE 666 April 14, 2005
Molecular quantum-dot cellular automata
Yuhui LuDepartment 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 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 response
Restoration 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)
moleculeSi-N bonds
“struts”
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FeRu Fe Ru Fe
Ru
Si
HgFe
Ru
Si
HgFe
Ru
Si
HgFe
Ru
ac
Cap
acita
nce
voltage
excited stateswitching
En
ergy
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
ene
rgy
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