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:
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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
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 A B X = -OCH3 x 2 Y = -CN x 2 D C In-plane Out-of-plane
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) LUMO1 1.68 eV (1.74 eV) HOMO -8.99 eV (-9.11 eV) LUMO3 3.74 eV (3.79 eV) LUMO2 2.34 eV (2.36 eV)
Results and Discussion:Out-of-plane –CH3 (B) LUMO1 1.69 eV (1.59 eV) HOMO -9.03 eV (-8.99 eV) LUMO3 3.78 eV (3.74 eV) LUMO2 2.30 eV (2.22 eV)
Results and Discussion:In-plane –OCH3 (C) LUMO1 1.65 eV (1.52 eV) HOMO -8.55 eV (-9.23 eV) LUMO3 3.90 eV (3.49 eV) LUMO2 2.31 eV (2.17 eV)
Results and Discussion:Out-of-plane –OCH3 (D) LUMO1 1.67 eV (1.50 eV) HOMO -8.58 eV (-9.24 eV) LUMO3 3.88 eV (3.74 eV) LUMO2 2.28 eV (2.12 eV)
Comparison of ΔELUMO  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