Organic Molecules in an Electrical Circuit: An AFM Study of a Negative-Differential-Resistance Molecule
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ORGANIC MOLECULES IN AN ELECTRICAL CIRCUIT: AN AFM STUDY OF A NEGATIVE-DIFFERENTIAL-RESISTANCE MOLECULE GANESH K. RAMACHANDRAN1,*, ADAM M. RAWLETT2, THERESA J. HOPSON2, LARRY A. NAGAHARA2, RAYMOND K. TSUI2, & STUART M. LINDSAY1 1
Department of Physics and Astronomy, Arizona State University, Tempe AZ 85287 Physical Sciences Research Laboratories, Motorola Labs, 7700 S. River Pkwy, Tempe AZ 85284 *e-mail- [email protected] 2
ABSTRACT 2,5-di(phenylethynyl)-4′-4′′-dithiolate-1-nitrobenzene has been shown to exhibit negative differential resistance (NDR) and spontaneous switching when inserted into inert molecular monolayers between metal contacts. We have used conducting atomic force microscopy to measure the electronic properties of individual dithiolated molecules 2,5-di(phenylethynyl)-4′4′′-dithiolate-1-nitrobenzene and 2,5-di(phenylethynyl)-4′-4′′-thioacetyl-benzene inserted into an alkanethiol monolayer and chemically bonded to gold nano-contacts to form a covalentlyconnected molecular circuit (bonded contacts). The data show qualitative agreement with previously published results for similar molecules deposited in a nanopore containing several hundred molecules, allowing us to make the important conclusion that the measured negative differential resistance (NDR) is native to the molecule and not an intermolecular phenomenon. INTRODUCTION In recent years, electron transport studies in molecular systems have revealed a wide range of interesting properties that mimic those in traditional solid-state devices including transistor action, diode characteristics, switching and gain [1-4]. Such observations, and the ease of sample preparation (thermodynamically driven self-assembly) have sparked optimism that devices incorporating organic molecules would in the future overcome the problem of miniaturization limits faced by current silicon-based technology. Despite these advances, there are inconsistencies in the reported properties of the same or similar chemical systems: problems attributed in part to the contacts, and the techniques used to address these systems. It was recently shown that reproducible current-voltage characteristics could be obtained by using gold nanoclusters and dithiolated molecules to make a covalently wired molecular circuit using a conducting atomic force microscope (c-AFM) [5]. In this paper we report our progress in the application of this technique to study the conjugated 2,5-di(phenylethynyl)-4′-4′′-dithiolate-1nitrobenzene (1) and 2,5-di(phenylethynyl)-4′-4′′-thioacetyl-benzene (2) systems. Our choice of molecules 1 and 2 for this study stemmed largely from the observation of negative-differential-resistance (NDR) and the observation of conformational-switching in chemical systems similar to 1 and 2 in independent studies [6-8]. It has been suggested [6] that the observation of NDR is due to the presence of a conducting species upon a one-electron reduction, whereas a second reduction leads to an insulating state resulting in the I-V spectra resembling that of a tunnel-diode. The effect
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