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. -NH 2, -OH, -CH 3 etc. Y = e - accepting e.g. -NO 2, -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 ΔE LUMO Shortest aliphatic chain allowing planarity: dimethylene group –CH 2 CH 2 - Optimal geometries have parallel rings: assumed to be enforced by embedding medium

Candidate Rectifiers X = -CH 3 x 2 Y = -CN x 2 X = -OCH 3 x 2 Y = -CN x 2 In-planeOut-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 –CH 3 (A) HOMO eV (-9.11 eV) LUMO eV (2.36 eV) LUMO eV (1.74 eV) LUMO eV (3.79 eV)

Results and Discussion: Out-of-plane –CH 3 (B) HOMO eV (-8.99 eV) LUMO eV (2.22 eV) LUMO eV (1.59 eV) LUMO eV (3.74 eV)

Results and Discussion: In-plane –OCH 3 (C) HOMO eV (-9.23 eV) LUMO eV (2.17 eV) LUMO eV (1.52 eV) LUMO eV (3.49 eV)

Results and Discussion: Out-of-plane –OCH 3 (D) HOMO eV (-9.24 eV) LUMO eV (2.12 eV) LUMO eV (1.50 eV) LUMO eV (3.74 eV)

Comparison of ΔE LUMO MoleculeCalculated ΔE LUMO ΔE LUMO [1] A 2.06 eV2.05 eV B 2.09 eV2.15 eV C 2.25 eV1.97 eV D 2.21 eV1.99 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