Delivered by Ingenta to: Dental Library Seoul Natl Univ IP : 147.46.233.69 Tue, 19 Jun 2012 10:58:41 RESEARCH ARTICLE Copyright © 2012 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 12, 3496–3500, 2012 Highly Ordered Large-Area Colloid Templates for Nanostructured TiO 2 Thin Film Gas Sensors Hi Gyu Moon 1 2 , Young-Seok Shim 1 , Do Hong Kim 1 , Ho Won Jang 1 ∗ , Seung-Hee Han 3 , Hyung-Ho Park 2 , and Soek-Jin Yoon 1 1 Electronic Materials Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea 2 Department of Material Science and Engineering, Yonsei University, Seoul 120-749, Korea 3 Solar Cells Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea We report a novel process that uses highly ordered colloidal templating to fabricate nanostructured TiO 2 thin film gas sensors. An O 2 plasma treatment is used to decrease the contact angle of a water droplet on a SiO 2 /Si substrate from 46  to 3  . The formation of this hydrophilic surface enhances the adhesion of polystyrene microspheres to the substrate during the spin coating of the colloidal solution, leading to a large-area colloid template of closely packed monolayer microspheres on the substrates. Embossed TiO 2 thin film gas sensors fabricated through highly ordered colloidal templating using O 2 plasma exhibit substantially enhanced gas sensing performance as compared to those without surface treatments prior to colloidal templating. Keywords: Gas Sensor, TiO 2 , Colloid Template, O 2 Plasma, Hollow Hemispheres. 1. INTRODUCTION Nanostructured metal oxide thin films offer a high surface-to-volume ratio and good comparability with well- established semiconductor processes, making them great candidates for applications as gas sensors with high sensitivity, fast response time, and small size. 1 Various methods such as sol–gel processing, 2–4 photolithographic patterning, 5 laser irradiation, 6 and anodized aluminum oxide templating 7 have been demonstrated for the fabrica- tion of nanostructured metal oxide thin film gas sensors. There have recently been multiple demonstrations of the enhanced gas responses of nanostructured metal oxide thin films obtained using colloidal templating, 8 which is an effective method for fabricating quasi-ordered submicron structures of various materials. 9–11 However, exploiting this method for large-area uniformity and throughput remains a challenge because the spreading and coating of polymer spheres from the original colloidal solution onto a sub- strate is very sensitive to the surface homogeneity and wet- tability of the substrate. 12–15 The formation of sphere-free regions, agglomerates, 14 and nonuniform multilayers 12 is a major obstacle to achieving close-packed monolayer col- loidal templates that are more than a square millimeter in size, which is essential in developing reliable wafer-scale fabrication processes for metal oxide thin film gas sensors. ∗ Author to whom correspondence should be addressed. Thus, offering an effective method of large-area colloidal templating is a critical step for real device applications of colloid-templated metal oxide thin films. In this work, we investigate how surface treatments influence the surface wettability of SiO 2 /Si substrates and how the colloidal templating of monolayer polymer microspheres influences the substrates. O 2 plasma treat- ment creates a hydrophilic surface on the substrates so that well-distributed large-area monolayer microspheres are obtained subsequent to colloid deposition. Colloid- templated TiO 2 thin film gas sensors treated with the O 2 plasma exhibit enhanced CO sensing properties as com- pared to colloid-templated TiO 2 thin film gas sensors that have not received surface treatments prior to colloidal templating. The experimental results suggest that highly ordered large-area colloidal templating via O 2 plasma treatments is a very promising method for fabricating novel metal oxide thin film gas sensors. 2. EXPERIMENTAL DETAILS An aqueous suspension of 1 m polystyrene spheres (2.6 wt%, Polysciences) was used in this work to fab- ricate colloid templates. The suspension was ultrasoni- cated for 3 hours to disperse the microspheres uniformly without agglomeration. Three types of SiO 2 (1 m)/Si substrates were prepared. The first is as-cleaned by 3496 J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 4 1533-4880/2012/12/3496/005 doi:10.1166/jnn.2012.5589