molecular quantum-dot cellular automata
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_____________________________________________________________ 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 responseRestoration of signal levels
cell1 cell2
cell1 cell2
Cell-cell response function
Neighboring cells tend to align.Coulombic coupling
_____________________________________________________________ EE 666 April 14, 2005
0 01 1
01 10
Binary wire
Inverter
A
B
C
Out
Majority gate
MABC
Programmable 2-input AND or OR gate.
QCA devices
_____________________________________________________________ EE 666 April 14, 2005
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
_____________________________________________________________ EE 666 April 14, 2005
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
_____________________________________________________________ EE 666 April 14, 2005
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).
_____________________________________________________________ EE 666 April 14, 2005
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.
_____________________________________________________________ EE 666 April 14, 2005
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.
_____________________________________________________________ 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
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
_____________________________________________________________ EE 666 April 14, 2005
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”
_____________________________________________________________ EE 666 April 14, 2005
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
_____________________________________________________________ EE 666 April 14, 2005
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.
_____________________________________________________________ EE 666 April 14, 2005
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
_____________________________________________________________ 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
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.
_____________________________________________________________ EE 666 April 14, 2005
• 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
+
_____________________________________________________________ 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
“1”
“0”
null
E
E
E
or
Use local electric field to switch molecule between active and null states.
active
“null”
_____________________________________________________________ EE 666 April 14, 2005
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.
_____________________________________________________________ EE 666 April 14, 2005
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
_____________________________________________________________ EE 666 April 14, 2005
Molecular energy
“0” “1”
“null”
A clock voltage “turns on” the devices.
“0” “null” “1”
• ground state• first excited state• second excited state
_____________________________________________________________ EE 666 April 14, 2005
Molecular energy
“0” “1”
“null”
“0” “1”
“null”
Large enough clock voltage “pins” the mobile charge.
• ground state• first excited state• second excited state
_____________________________________________________________ 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.
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