Rectifying Behavior of Electrically Aligned ZnO Nanorods Oliver Harnack,* ,† Claudia Pacholski, ‡ Horst Weller, ‡ Akio Yasuda, † and Jurina M. Wessels † Materials Science Laboratories, Sony International (Europe) GmbH, Sony Corporate Laboratories Europe (SCLE), Hedelfinger Strasse 61, 70327 Stuttgart, Germany, and Institute of Physical Chemistry, UniVersity of Hamburg, Bundesstrasse 45, 20146 Hamburg, Germany Received April 18, 2003; Revised Manuscript Received June 2, 2003 ABSTRACT We report on the electrical alignment of ZnO nanorods and their electrical properties. The ZnO nanorods were wet-chemically synthesized, and their length and diameter were adjusted to about 200-300 nm and 15-30 nm, respectively. The nanorods were deposited onto electrode structures and directed into 200- to 800-nm-wide electrode gaps by using alternating electric fields at frequencies between 1 and 10 kHz and field strengths between 10 6 and 10 7 V/m. The nanorods align parallel to the electric field lines and make electrical contact with the gold electrodes. Clear photoresponse to 366-nm ultraviolet light irradiation was demonstrated. The current-voltage characteristics of the aligned rods are strongly nonlinear and asymmetrical, showing rectifying, diode-like behavior and asymmetry factors up to 25 at 3-V bias voltage. The continuous downscaling of the feature sizes of micro- electronic devices well below 100 nm 1 has initiated an enormous amount of research activity on alternatives to tra- ditional electronic components such as complimentary metal oxide semiconductor (CMOS)-based field-effect transistors and lithographically defined interconnects. The demand for alternative device concepts is related to the expectation that downscaling of conventional CMOS devices will reach physical limits within the next few years. 2 For the patterning technology, optical lithography will still be the method of choice for mass production because of its high throughput capabilities. 3 However, the development of optical lithography tools for continuously decreasing feature sizes will become increasingly expensive. Thus, radical new device and fabrication concepts that offer high performance and low price are required. A current trend is to invent device concepts based on unique nanostructures of materials such as insulators, semi- conductors, metals, or combinations of these materials. Nanorods and tube-shaped nanostructures have been fabri- cated recently using cheap and simple wet-chemical and physical synthesis methods. Semiconducting tubes and nanorods have been made from CdSe, CdTe, ZnO, and others, and they have shown a large variety of properties such as chemical sensing, 4 luminescence, 5 field effects, 6 lasing, 7 and photoresponse. 8 Synthesis concepts are well established; however, there is less experience to date on the alignment of nanostructures to build working electronic devices or to integrate them into complex architectures. 9 Un- conventional lithography approaches such as microcontract printing 10 or dip-pen lithography 11 have the potential to con- trol nanostructure assembly. The funtionalization of carbon nanotubes toward the directed self-assembly and alignment of more complex nanostructures has already been demon- strated. 12 Here, we describe the successful alignment of wet- chemically synthesized, semiconducting ZnO nanorods in the suspension phase using an electric field to place them onto predefined electrodes. We also demonstrate their unique properties such as photoresponse and current rectification. Synthesis. ZnO nanorods were prepared according to a method described earlier. 13 Briefly, 29.5 g (0.13 mol) of zinc acetate dihydrate was dissolved in 125 mL of methanol at 60 °C. Then, a solution of 14.8 g (0.23 mol) of potassium hydroxide in 65 mL of methanol was added. The reaction mixture was stirred for several days at 60 °C. During this time, the nanoparticles precipitate. The length and the width of the resulting nanorods depend on the reaction time. The length of the nanorods increased considerably with longer reaction times, but the width of the nanorods grew only slightly. The precipitate was washed with methanol and centrifuged (5500 rpm, 30 min), and the resulting gel was redispersed in chloroform. The nanoparticles were diluted with a mixture of ethyleneglycol/water (2:1). Figure 1 shows a TEM image of ZnO nanorods after a reaction time of 3 days. The nanorods have a diameter of 15 to 30 nm and a length of 200 to 300 nm. * Corresponding author. E-mail: harnack@sony.de. Phone: +49 711 5858 212. Fax: +49 711 5858 484. ² Sony Corporate Laboratories Europe. ‡ University of Hamburg. NANO LETTERS 2003 Vol. 3, No. 8 1097-1101 10.1021/nl034240z CCC: $25.00 © 2003 American Chemical Society Published on Web 06/24/2003