John K. Tomfohr

Making electrical contacts to molecular monolayers

Electrical contacts between a metal probe and molecular monolayers have been characterized using conducting atomic force microscopy in an inert environment and in a voltage range that yields reversible current-voltage data. The current through alkanethiol monolayers depends on the contact force in a way that is accounted for by the change of chain-to-chain tunnelling with film thickness. The electronic decay constant, βN, was obtained from measurements as a function of chain length at constant force and bias, yielding βN = 0.8±0.2 per methylene over a ±3 V range. Current-voltage curves are difficult to reconcile with this almost constant value. Very different results are obtained when a gold tip contacts a 1,8-octanedithiol film. Notably, the current-voltage curves are often independent of contact force. Thus the contact may play a critical role both in the nature of charge transport and the shape of the current-voltage curve.

Theoretical analysis of electron transport through organic molecules

We present a theoretical study of electron transport through a variety of organic molecules. The analysis uses the Landauer formalism in combination with complex bandstructure and projected densities of states calculations to reveal the main aspects of coherent electronic transport through alkanes, benzene-dithiol, and phenylene-ethynylene oligomers. We examine the dependence of the current on molecule length, the effects of molecule-molecule interactions from film packing, differences in contact geometry, and the influence of phenyl ring rotation on the conductances of phenylene-ethynylene oligomers such as 1,4-bis-phenylethynyl-benzene.

Simple Estimates of the Electron Transport Properties of Molecules

We discuss how the complex bandstructure describes tunneling within a molecule, and how it can be used to understand single molecule conduction. Simple estimates of the conduction properties of some molecules can be obtained with minimal (or no) computational effort. Examples considered include tunneling current through SiO 2 , transistor action of biphenyl-dithiol, the effect of benzene ring rotation on tunneling through polyphenyl chains, and a designed molecule which -according to the form of its complex bandstructure - may function as a p-channel transistor.

Time-dependent simulation of conduction through a molecule

We present a method for performing time-dependent simulations of electron conduction through molecules. The method is based on the time-dependent Schrodinger equation recast as a matrix equation using a basis of non-orthogonal local orbitals. We present the basic theory and show the results of a simulation of conduction through benzene 1,4-dithiolate. A comparison with experimental data and with the results of the Greens function scattering theory of the conductivity of a molecule is made.

Theoretical study of carotene as a molecular wire

Abstract Carotenoid molecules have important photo-chemical properties and may serve as molecular wires in a molecular electronic circuit. We have theoretically studied the intrinsic conducting properties of sulfur-terminated carotene between gold contacts using local orbital density functional theory. The dependence of the tunneling decay parameter “ β ” on the degree of single–double bond alternation within the polyene backbone is determined from the polyene complex band-structure. The electron tunneling current–voltage characteristics is calculated using the Landauer–Buttiker formalism. The calculations are in qualitative agreement with experiments.

A Comparison of Electronic States in Periodic and Aperiodic Poly(dA)–Poly(dT) DNA

Using an ab initio tight-binding formalism based on density-functional theory [Lewis et al., Phys. Rev. B 64, 195103-1 (2001)], we present theoretical work on the electronic states in a model periodic DNA double helix of poly(dA)-poly(dT) (10 base-pairs). Comparison of the periodic structure is made to an aperiodic DNA structure with the same sequence, but the structure is distorted as a result of thermal fluctuations from a molecular-dynamics simulation. We find that the periodic structure exhibits periodic and very extended HOMO-LUMO states; however, the equivalent states are quite localized in the aperiodic structure.

Bias-induced forces in conducting atomic force microscopy and contact charging of organic monolayers.

Contact electrification, a surface property of bulk dielectric materials, has now been observed at the molecular scale using conducting atomic force microscopy (AFM). Conducting AFM measures the electrical properties of an organic film sandwiched between a conducting probe and a conducting substrate. This paper describes physical changes in the film caused by the application of a bias. Contact of the probe leads to direct mechanical stress and the applied electric field results in both Maxwell stresses and electrostriction. Additional forces arise from charge injection (contact charging). Electrostriction and contact charging act oppositely from the normal long-range Coulomb attraction and dominate when a charged tip touches an insulating film, causing the tip to deflect away from the film at high bias. A bias-induced repulsion observed in spin-coated PMMA films may be accounted for by either mechanism. In self-assembled monolayers, however, tunnel current signals show that the repulsion is dominated by contact charging.

Making Contacts to Single Molecules: Are We There Yet?

The problem of connecting two wires to a single molecule has several practical solutions, varying from break junctions, to gaps formed by controlled evaporation and self-assembled structures. Here, we focus on gold nanoparticle self-assembled junctions, and break junctions, two techniques that allow the number of molecules in the gap to be determined. We show that the nanoparticle junctions are affected by the electronic properties of the nanoparticles, and that corrections for these effects tend to bring the data into closer agreement with both break-junction measurements and ab initio calculations.

Tunneling Through Single Molecules

Publisher Summary This chapter provides an overview of some basic aspects of single molecule transport. The system of interest is a small organic molecule connected between electrodes. The molecule serves as a bridge for electrons driven from one electrode to the other by an applied bias. While there are a variety of possible transport mechanisms and processes that can influence transport, the chapter only focuses on the simplest case of coherent transport. The theory of electron tunneling through single molecules is introduced with special focus on periodic molecules, where the problem is simply understood in terms of complex wave-vector Bloch states. The molecule length dependence of the current, the Fermi level alignment problem, barrier traversal times, and other important aspects of single molecule transport in general come in to play in this context. The Landauer formalism is a framework for more quantitative treatments of more general structures and a brief introduction to this subject is also given.