Delivered by Ingenta to: McMaster University IP: 188.72.126.65 On: Mon, 13 Jun 2016 03:33:56 Copyright: American Scientific Publishers RESEARCH ARTICLE Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 11, 6400–6403, 2011 Control of Nanogap Separation by Surface-Catalyzed Chemical Deposition Hyung Ju Park 1 , Cho Yeon Lee 1 , Yong-ho Chung 1 , Young Shik Chi 1 , Insung S. Choi 2 , and Wan Soo Yun 1 ∗ 1 Center for Nano and Quantum Science, Korea Research Institute of Standards and Science (KRISS), Daejeon 305-340, Korea 2 Department of Chemistry, KAIST, Daejeon 305-701, Korea The nanogap devices, which comprise multiple electrodes separated by a few to a few tens of nanometers, have opened up new possibilities in biomolecular sensing as well as various frontier electronics. One of the key aspects of the nanogap device research is how to control the gap distance following each specific needs of the gap structure. Here, we report the extensive study on the fine control of the gap distance between electrodes within the range of 1–80 nm via surface- catalyzed chemical deposition. The initial gap electrodes were prepared via conventional e-beam lithography, and the gap distance was narrowed to a designed value through the surface-catalyzed reduction of gold ion on the predefined electrode surfaces, by simple dipping of the electrodes into the aqueous solution of gold chloride and hydroxylamine. The final gap distance was controlled by adjusting the repetition number, reductant concentration, reaction time, and reaction temperature. The dependence of the gap-narrowing reaction on these parameters was systematically examined based on the results of field emission scanning electron microscopy and atomic-force microscopy. Keywords: Nanogap, Surface-Catalyzed Chemical Deposition, Size Control. 1. INTRODUCTION Nanogap electrodes are of great interest due to their use- fulness in various fields such as molecular electronics, 1 2 nanoparticle-based biosensor, 3 4 optical grating, 5 and elec- trochemical sensors. 6 A number of the fabrication meth- ods of the nanogap electrodes, therefore, have been reported so far such as optical lithography, 3 4 scanning probe lithography, 7 8 carbon nanotube template method, 9 plating, 10 11 and break junction. 1 12 Nevertheless, these methods still have difficulties in controlling the gap dis- tance in the range of a few to a few tens of nanome- ters, particularly when the electrode shape is complex and arbitrary. Recently, we have reported a simple and highly reproducible method of fabricating the nanogap, where a surface-catalyzed growth on the surface of pre-defined electrodes leads to the narrowing of the gap distance to a designed value of a few nm. 13 Although the success of the gap narrowing process was clearly demonstrated in the previous paper, the fabrication of the nanogap in a wide range of the gap separation and detailed investigation of the reaction parameters affecting the final gap distance are still demanding. ∗ Author to whom correspondence should be addressed. In this paper, the effect of the reaction parameters in the surface-catalyzed chemical deposition was extensively investigated and, by the elaborate control of those param- eters, the gap distance was finely tuned in the range of 1–80 nm. Moreover, it was shown that the process was applicable to various geometries of the electrodes and the electrode morphology could be improved by adopting a simple heat treatment step. 2. EXPERIMENTAL DETAILS The nanogap electrode fabrication process consisted of the following steps: construction of connection pads, fabrica- tion of initial gap electrodes, and the gap-narrowing process. Connection pads were formed using photolithogra- phic techniques, while initial gap electrodes were formed by e-beam lithographic techniques using conventional polymer resist and metal lift-off. Gap distance between electrodes was controlled by the gap narrowing process. 2.1. Construction of Connection Pads A 1.2-m-thick photoresist (AZ-5214E) layer was spin- coated on a silicon oxide wafer at 5000 rpm for 30 sec, 6400 J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 7 1533-4880/2011/11/6400/004 doi:10.1166/jnn.2011.4358