Precise control of surface wettability of mixed monolayers using a simple wiping method Jung Ah Lim, Jeong Ho Cho, Yunseok Jang, Joong Tark Han, Kilwon Cho ⁎ Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea Received 29 September 2005; received in revised form 19 April 2006; accepted 4 July 2006 Available online 17 August 2006 Abstract To control the wettability of a silicon surface, mixed monolayers of 11-cyanoundecyltrichlorosilane (CN(CH 2 ) 11 SiCl 3 ) and dodecyltrichlorosilane (CH 3 (CH 2 ) 11 SiCl 3 ) in different ratios were fabricated using a simple wiping method. The surface of the wiped mixed monolayers was reproducibly homogeneous and smooth and the water wettability of wiped surfaces was controlled successfully with linear dependency, where the composition of the mixed monolayers matched the composition of the deposition solution in a direct 1:1 fashion. From these results, the spread of the inkjet printed poly(3,4- ethylenedioxythiophene) doped with poly(styrene sulfonic acid) dots was controlled finely by the surface wettability of the wiped mixed monolayers. © 2006 Elsevier B.V. All rights reserved. Keywords: Self-assembled monolayer; Inkjet printing; Wettability; Soft lithography; X-ray photoelectron spectroscopy 1. Introduction In recent years, soft lithography techniques, such as micro- contact printing and inkjet printing, have been the subject of increasing scientific interest in various research fields [1–3]. The geometry and pattern resolution of soft lithography is deter- mined by the wettability of the depositing liquid with respect to the substrate, since most soft lithography processes are based on liquid sources. Therefore, controlling the surface wettability of a substrate is a key issue for soft lithography techniques. Alkylsilane are frequently used for surface engineering at the molecular level in order to control the wettability of a surface [4– 6]. Specifically, mixed monolayers of alkylsilanes provide an excellent route for surface engineering because the surface free energy and chemical reactivity can be finely controlled by coadsorption of alkylsilane molecules with different terminal functionalities. However, there are few reports on mixed alkylsilane monolayers, and direct control of the surface prop- erties of mixed monolayers is difficult because the relation between the composition of a mixed monolayer and that of the immersion solution is complicated [7–10]. This is primarily due to the difference in the reaction kinetics and sensitivity of each alkylsilane molecule in the mixed alkylsilane solution, which in turn are highly dependent upon humidity and reaction tem- perature [11–13]. Besides, the formation of alkylsilane mono- layers by immersing a substrate into the reaction solution for long periods of time is not appropriate for practical surface engineering applications such as roll-to-roll processes. In this paper, we propose a simple wiping treatment to control the wettability of a silicon surface as suitable method for the deposition of mixed monolayers. A mixed monolayer of 11- cyanoundecyltrichlorosilane and dodecyltrichlorosilane was introduced to control the surface wettability of the silicon substrate. The surface characteristics of the mixed monolayers deposited by both mechanical wiping and general solution dipping methods are compared. In addition, we demonstrate the feasibility of our method by inkjet printing a conductive poly- mer, poly(3,4-ethylenedioxythiophene) doped with poly(sty- rene sulfonic acid) (PEDOT/PSS), onto a wiped substrate by monitoring the spread of the resulting droplet. 2. Experimental details 2.1. Materials Dodecyltrichlorosilane (CH 3 (CH 2 ) 11 SiCl 3 , Gelest Inc.), 11- cyanoundecyltrichlorosilane (CN(CH 2 ) 11 SiCl 3 , Gelest Inc.), Thin Solid Films 515 (2006) 2079 – 2084 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +82 54 279 2270; fax: +82 54 279 8269. E-mail address: kwcho@postech.ac.kr (K. Cho). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.07.003