Elsevier

Surface Science

Volume 514, Issues 1–3, 10 August 2002, Pages 41-47
Surface Science

Nanoscale wiring by controlled chain polymerization

https://doi.org/10.1016/S0039-6028(02)01605-9Get rights and content

Abstract

Chain polymerization of amphiphilic diacetylene compounds in a monomolecular layer on a graphite substrate can be observed and controlled at a nanometer scale using a scanning tunneling microscope (STM). The wavelength dependence of photoreactivity suggests that the graphite substrate sensitizes the photoreaction. The chain polymerization can be initiated not only by irradiating light, but also by applying stimulation using an STM tip. We can also control the termination of chain polymerization by an artificial defect, so that we can create a polydiacetylene nanowire of designated length at any designated position.

Introduction

The size of silicon-based devices has become smaller and smaller, and the performance of computers has become better and better. However, when the size of each device becomes smaller than a few tens of nanometers, the laws of quantum mechanics and the limitations of fabrication techniques may soon prevent further reduction. Thus a novel device concept different from that of the current silicon-based device has been widely explored, and its discovery will make a profound impact on society in this century. One promising possibility is the construction of novel electronic devices with organic molecules, as supported by various recent investigations [1], [2], [3], [4], [5], [6]. However, in order to fabricate such molecular electronic devices on the nanometer scale, it is necessary to develop conductive wires of nanometer width (nanowires) which can interconnect them. Recently, we have demonstrated that the initiation and termination of chain polymerization of diacetylene compounds can be controlled by stimulation using a probe tip of scanning tunneling microscope (STM), so that a conjugated polymer nanowire of designated length can be created at designated positions [7], [8]. In this paper, we will review the results of controlled chain polymerization, and we will also show a preliminary result indicating that the graphite substrate sensitizes the photoreaction of diacetylenes.

Although atom-by-atom crafting of structures with an STM [9], [10], [11], [12] will also be able to create nanoscale wires, the chain polymerization technique used in our work has several advantages. Namely, the nanowires obtained are guaranteed to have perfect structures without any defects, and they are stable at room temperature. Furthermore, the entire process is accomplished in a short time span as a single event and no additional energy is needed for the progression of the chain reaction, other than the first stimulation.

Solid-state topochemical polymerization of diacetylene compounds (general formula R–CC–CC–R, where CC–CC is the diacetylene moiety and R and R are substituent groups) has been widely investigated [13], [14]. The polymerization is initiated by appropriate stimulation, such as heating or ultraviolet irradiation, and produces polydiacetylene compounds which are conjugated linear polymers represented by the general formula (RC–CC–CR)n. Although numerous studies have been performed on the polydiacetylene system, only a few studies have dealt with STM observations of physisorbed layers of diacetylene compounds on graphite [15], [16], [17]. In the present work, we have used 10,12-pentacosadiynoic acid [CH3(CH2)11–CC–CC–(CH2)8COOH] and 10,12-nonacosadiynoic acid [CH3(CH2)15–CC–CC–(CH2)8COOH] monomolecular layers on a graphite substrate. These two species gave essentially the same results.

Section snippets

Experimental

10,12-Pentacosadiynoic acid and 10,12-nonacosadiynoic acid (Tokyo Chemical Industry Co., Ltd.) were used as received. In order to prepare their thin films, the monomer molecules were dissolved in chloroform and the solution was applied onto the surface of purified water. After the evaporation of chloroform, the thin film of the monomer molecules on the water surface was transferred to a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG, Advanced Ceramics Co., grade ZYH) by the

Self-ordering of monomer molecules

A typical STM image of the 10,12-pentacosadiynoic acid layer on a graphite substrate is shown in Fig. 1a. The image consists of parallel bright lines separated by two different alternate spacings of about 3.0 and 3.8 nm, indicating that the 10,12-pentacosadiynoic acid molecules on the graphite surface are self-ordered at room temperature in air. The bright lines form an angle of ±27° with the main crystal axis in the basal plane of graphite, 〈21̄1̄0〉. In this image, individual molecules are

Conclusion

We have demonstrated that we can initiate linearly propagating chain polymerization of organic molecules at any predetermined point and terminate it at another predetermined point with a spatial precision of the order of one nanometer, using the probe tip of an STM. We have used a graphite surface to adsorb a monomolecular layer of the diacetylene compounds, 10,12-pentacosadiynoic acid and 10,12-nonacosadiynoic acid. The molecules arrange themselves into parallel lines on the substrate. Since

Acknowledgements

The authors thank Yuji Kuwahara (Osaka University) for helpful discussion. This work was partially supported by the Program of Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation. This work was also supported by the Special Coordination Funds for Promoting Science and Technology, the Science and Technology Agency, Japan.

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