CHAPTER M Molecular Electronic Devices J. Chen, T. Lee, J. Su, W. Wang, M. A. Reed Departments of Electrical Engineering, Applied Physics, and Physics, Yale University, New Haven, Connecticut, USA Adam M. Rawlett, Masatoshi Kozaki, Yuxing Yao, Raymond C. Jagessar, Shawn M. Dirk, David W. Price, J. M. Tour Department of Chemistry and Center for Nanoscale Science and Technology, Rice University, Houston, Texas, USA D. S. Grubisha, D. W. Bennett Department of Chemistry, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin, USA CONTENTS 1. Introduction ................................................2 2. Basic Concepts of Molecular Electronics .............................2 2.1. Self-Assembled Monolayers ...................................2 2.2. Conjugated Oligomeric Systems ................................3 2.3. Basic Charge Transport Mechanisms .............................4 2.4. Initial Studies in Molecular Electronics ...........................7 3. Synthesis of Molecular Wires and Devices ............................7 3.1. Synthesis of One-Terminal Oligo(phenylene ethynylene) Molecular Wires ......................................... 8 3.2. Synthesis of Two-Terminal Oligo(phenylene ethynylene) Molecular Wires ......................................... 8 3.3. Synthesis of Three-Terminal Molecular Scale Wires ...................9 3.4. Molecular Wires with Internal Methylene and Ethylene Transport Barriers ............................... 9 3.5. Synthesis of Molecular Scale Devices with Heteroatomic Functionalities ..............................11 3.6. Porphyrin Containing Molecular Scale Wires ......................14 3.7. Synthesis of Dipole-Possessing Molecular Wire SAMs to Control Schottky Barriers in Organic Electronic Devices ...............15 Molecular Nanoelectronics ISBN: 1-58883-006-3 / $35.00 Edited by M. A. Reed and T. Lee All rights of reproduction in any form reserved. 1 Copyright © 2003 by American Scientific Publishers 2 Chen et al. 3.8. Experimental ...........................................16 4. Fabrication of Molecular Transport Devices ..........................43 5. Simple SAM MIM Tunneling ....................................45 6. Basic Transport Measurement in Molecular Layers ......................50 6.1. Conduction Mechanisms ....................................50 6.2. Isocyanide SAM .........................................52 6.3. Au–Isocyanide–Au Junctions .................................52 6.4. Pd–Isocyanide–Pd Junctions .................................55 6.5. Localized States in Metal–SAM–Metal Junctions ....................56 7. Device Applications of Molecular Layers ............................60 7.1. Conductor–Insulator Transition Caused by Molecular Conformation ..................................60 7.2. Negative Differential Resistance in Molecular Junctions .....................................60 7.3. Temperature Dependence ...................................63 7.4. Molecular Memory Effects ..................................66 7.5. Demonstration of a Molecular Memory Storage Cell ......................................70 8. Conclusions and Outlook ......................................72 References ................................................74 1. INTRODUCTION of the art lithography and etching. Chemical synthesis makes it possible to make large quantities of nanometer- The ability to utilize single molecules that function as self- size molecules with the same uniformity but at signif- contained electronic devices has motivated researchers icantly less cost, compared to other batch-fabrication around the world for years, concurrent with the con- processes such as microlithography. One can envision tinuous drive to minimize electronic circuit elements in that in assembling molecular circuits, instead of build- semiconductor industry. The microelectronics industry is ing individual components on a chip one will synthesize presently close to the limit of this minimization trend molecules with structures possessing desired electronic dictated by both laws of physics and the cost of produc- configurations and attach/interconnect them into an elec- tion. It is possible that electronically functional molecular tronic circuit using surface attachment techniques like components can not only address the ultimate limits of self-assembly. Self-assembly is a phenomenon in which possible miniaturization but also provide promising new atoms, molecules, or groups of molecules arrange them- methodologies for novel architectures, as well as nonlin- selves spontaneously into regular patterns and even rela- ear devices and memories. tively complex systems without outside intervention [2]. Molecular electronics [1–3] is conceptually different from conventional solid state semiconductor electron- ics. It allows chemical engineering of organic molecules 2. BASIC CONCEPTS with their physical and electronic properties tailored OF MOLECULAR ELECTRONICS by synthetic methods, bringing a new dimension in 2.1. Self-Assembled Monolayers design flexibility that does not exist in typical inor- ganic electronic materials. It is well known that semi- Self-assembled monolayers (SAMs) are ordered molec- conductor devices are fabricated from the “top-down” ular structures formed by the adsorption of an active approach that employs a variety of sophisticated litho- surfactant on a solid surface (Fig. 1). A SAM film can graphic and etch techniques to pattern a substrate. This be deposited on a substrate surface simply by exposing approach has become increasingly challenging as feature the surface to an environment containing surface active size decreases. In particular, at nanometer scale, the elec- molecular species for a certain period of time (solu- tronic properties of semiconductor structures fabricated tion or vapor phase deposition). The molecules will be via conventional lithographic processes are increasingly spontaneously oriented toward the substrate surface and difficult to control. In contrast, molecules are synthe- form an energetically favorable ordered layer. During this sized from the “bottom-up” approach that builds small process, the surface active head group of the molecule structures from the atomic, molecular, or single device chemically reacts with and chemisorbs onto the substrate. level. It in principle allows a very precise position- Because a self-assembling system attempts to reach a ing of collections of atoms or molecules with specific thermodynamically stable state driven by the global min- functionalities. For example, one can selectively add an imization of free energy, it tends to eliminate grow- oxygen atom to a molecule with a precision far greater ing foreign or faulty structures of molecules during the than an oxidation step in microfabrication using state assembly process. This simple process with its intrinsic Molecular Electronic Devices 3 electronic states across the entire molecule is necessary for electronic conduction. A brief review of the basic con- Substrate cepts underlying the physics and chemistry of conjugated oligomers as follows should help one to understand elec- tronic transport in molecular systems. SAM on Solution Immersion 2.2.1. Bonding in Molecular Orbitals substrate The molecular orbitals (MO) of a molecule are created R-F + M R-FM by the overlap of the atomic orbitals of its constituents. R: Backbone ; F: Functional end group; M: Metal. For instance, a bonding MO– bond is formed when two sp3 hybridized atomic orbitals form head-on over- lap, with electron density localized between two bonded backbone intermolecular nuclei. It is a single bond and acts essentially as structure interactions glue (a bond with no node). A bonding bond is formed functional 2 end group when remaining parallel p orbitals on two sp hybridized chemisorption at the surface atomic orbitals combine with each other. Compared with a bond, a bond is often weaker and less localized Figure 1. Self-assembled monolayers are formed by immersing a sub- (two bonding regions above and below a nodal plane, strate (e.g., a piece of metal) into a solution of the surface-active mate- because the bond—the ground state—has zero nodes rial (consisting of backbone R and functional end group F). The func- tional end group chemically reacts with the metal and the material while the bond—the first excited state—has one node). spontaneously, forming a two-dimensional assembly. Its driving force Together the bond and bond make a double bond includes chemical bond formation of functional end group in molecules (e.g., C2H4), whereas linear triple bonds [e.g., in alkynes with the substrate surface and intermolecular interactions between the (C2H2) and nitriles] are the results of the formation of backbones. two bonds using two mutually perpendicular p orbitals on each of the triple bonded atoms, with a bond in the error-correction advantage makes SAMs inherently man- middle. ufacturable and thus technically attractive and cost effec- tive. 2.2.2. Delocalized Bonds and Benzene In addition, SAMs can be designed and engineered to More than two adjacent p orbitals can combine to form provide extremely high functional density. For example, a set of molecular orbitals where the electron pairs are one can realize a switch or a memory out of a single shared by more than two atoms. These form “delocal- monolayer. On the other hand, in order to perform highly ized” bonds. In the case of benzene (C6H6), it is a complex functions as those of current integrated circuits, planar molecule with the shape of a regular hexagon.