LETTERS Theoretical Analysis of Complementary Molecular Memory Devices Jorge M. Seminario,* Angelica G. Zacarias, and Pedro A. Derosa Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: September 14, 2000; In Final Form: NoVember 13, 2000 The electrical behavior of π-conjugated oligo(phenyleneethynylene) systems functioning as memory devices is studied using quantum chemistry methods, including density functional and Green function formalisms combined in a fully self-consistent manner. Electron charge alters a molecule impedance characteristic providing in some cases distinguishable “impedance states” that can serve to determine experimentally the state of charge of the molecule. Conducting and nonconducting states can be strategically engineered by arranging substituents in a molecule. The NH 2 group localizes the highest energy occupied electronic states whereas the NO 2 group localizes the lowest energy unoccupied orbitals of the oligomer systems. These effects yield two complementary molecular memories, each occupying a volume smaller than 1 nm 3 . Introduction Single molecules assembled by chemical and other precise nanoscopic techniques can be arranged in such a way that their electronic behavior can be tested and used as random access memory devices. 1 However, the knowledge of single-molecule characteristics is needed in order to design fully operating memories. Presently, addressing single molecules for electrical measurements is highly challenging. One of the ways around this problem is to perform experimental measurements on small nanosystems (nanopores) having about 1000 molecules in parallel, chemically independent from each other. It is expected that the characteristics of the whole nanopore would correspond to the additive effect of the individual molecular characteristics. The electrical characteristics of a single molecule can be calculated with acceptable accuracy using present quantum chemistry techniques, which become an important complement to experiments and an essential tool for the design of molecular electronic devices. Recent experiments on nanopores of few π-conjugated oligo- (phenyleneethynylene)s yielded conclusive experimental evi- dence of molecular memory behavior. 1 The molecules in a nanopore were self-assembled on a gold surface, and sulfur atoms served as alligator clips 2 to make the connection of the molecules to the gold terminal. The other terminal is built by gold vapor deposition to the H-terminated molecules. In earlier theoretical 3 and experimental 4 reports, it was shown that one group of these molecules could act as molecular resonant tunneling diodes, presenting negative differential impedance (NDZ). For a thorough and excellent review with a complete reference list on molecular electronics, including molecules related to those presented in this Letter, the reader is forwarded to the introduction and background sections of a recent article by Ellenbogen. 5 We report in this Letter a theoretical interpretation of the memory experiments 1 using ab initio methods. Methods We use quantum density functional theory (DFT) techniques at the B3PW91/6-31G* and B3PW91/LANL2DZ levels of theory to investigate the characteristics of the neutral and charged species of the molecules tested in the memory experi- ments. 1 The computational techniques were explained in our earlier work. 6 Four minimal systems representing the basic * Corresponding author. E-mail: jorge@mail.chem.sc.edu. Fax: 803- 777-9521. URL: www.cosm.sc.edu/∼jorgemgr. © Copyright 2001 by the American Chemical Society VOLUME 105, NUMBER 5, FEBRUARY 8, 2001 10.1021/jp003283q CCC: $20.00 © 2001 American Chemical Society Published on Web 12/22/2000