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Design of Molecular Rectifiers
Shriram Shivaraman
School of ECE, Cornell University
Molecular Electronics or “Moletronics”
• Computation using molecules• Replacement devices and interconnects• Key feature : Few molecules per device
Why do we care?
Main issues with conventional scaling:
• Rising costs of conventional fabrication
~ $200 billion by year 2015
• Physical limitations - Leakage currents, Doping non-uniformity
Advantages of Molecules
• Small and identical units
• Bottom up fabrication: Self-assembly by functionalization
• Discrete energy levels – A design handle
• Special properties e.g. flexible substrates and low-cost printing, sensors etc.
Some outstanding issues
• Lack of suitable production methods: Interfacing techniques
• Inherent disorder because of self-assembly: Defect-tolerant architectures
• Speed, Stability, Reproducibility
About this work
Design of molecular rectifiers
Molecular Rectifier
• Aviram and Ratner in 1974
• Donor-spacer-acceptor configuration
• X = e- donating e.g.
-NH2, -OH, -CH3 etc.
• Y = e- accepting e.g.
-NO2, -CN, -CHO etc.
• R = insulating aliphatic group (barrier)
J.C. Ellenbogen et al, Proc. IEEE, Vol. 88, No. 3, March 2000
( ) ( )LUMO LUMO LUMOE E Donor E Acceptor
Working of the Rectifier
J.C. Ellenbogen et al, Proc. IEEE, Vol. 88, No. 3, March 2000
Design of a Rectifier
• Promote charge localization on either side of the barrier : high ΔELUMO
• Shortest aliphatic chain allowing planarity: dimethylene group –CH2CH2-
• Optimal geometries have parallel rings: assumed to be enforced by embedding medium
Candidate Rectifiers
X = -CH3 x 2
Y = -CN x 2
X = -OCH3 x 2
Y = -CN x 2
In-plane Out-of-plane
A B
C D
Method
• Geometries optimized with Gaussian 03
• Ab-initio HF/STO 3-21G basis set calculation
• HOMO/LUMO calculated using Koopman’s Theroem
• Orbitals plotted using Molekel to visualize localization
Results and Discussion:In-plane –CH3 (A)
HOMO -8.99 eV (-9.11 eV)
LUMO2 2.34 eV (2.36 eV)
LUMO1 1.68 eV (1.74 eV)
LUMO3 3.74 eV (3.79 eV)
Results and Discussion:Out-of-plane –CH3 (B)
HOMO -9.03 eV (-8.99 eV)
LUMO2 2.30 eV (2.22 eV)
LUMO1 1.69 eV (1.59 eV)
LUMO3 3.78 eV (3.74 eV)
Results and Discussion:In-plane –OCH3 (C)
HOMO -8.55 eV (-9.23 eV)
LUMO2 2.31 eV (2.17 eV)
LUMO1 1.65 eV (1.52 eV)
LUMO3 3.90 eV (3.49 eV)
Results and Discussion:Out-of-plane –OCH3 (D)
HOMO -8.58 eV (-9.24 eV)
LUMO2 2.28 eV (2.12 eV)
LUMO1 1.67 eV (1.50 eV)
LUMO3 3.88 eV (3.74 eV)
Comparison of ΔELUMO
Molecule Calculated ΔELUMO ΔELUMO [1]
A 2.06 eV 2.05 eV
B 2.09 eV 2.15 eV
C 2.25 eV 1.97 eV
D 2.21 eV 1.99 eV
[1] J.C. Ellenbogen et al, Proc. IEEE, Vol. 88, No. 3, March 2000
Conclusions
• Both molecules A and C have significant intrinsic potential drops (> 2 V)
• They show robustness to out-of-plane rotation
• C seems to have higher built-in voltage from the simulations
Final thoughts
• Koopman’s theorem doesn’t take into account relaxation energies.
• Though that maybe overcome, HF method doesn’t take into account electron correlation.
• DFT and other semi-empirical methods like OVGF(AM1) maybe used. But, they might not always give better results.
Experiments are the only means of knowledge at our disposal. The
rest is poetry, imagination.
-Max Planck