Electrically Biased Nanolithography with KOH-Coated AFM Tips Jae-Won Jang, Raymond G. Sanedrin, Daniel Maspoch, † Seongpil Hwang, Tsuyohiko Fujigaya, ‡ You-Moon Jeon, Rafael A. Vega, Xiaodong Chen, and Chad A. Mirkin* Department of Chemistry and International Institute for Nanotechnology, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113 Received February 12, 2008; Revised Manuscript Received February 27, 2008 ABSTRACT This letter provides the first study aimed at characterizing the desorption and nanolithographic processes for SAM-coated, gold-coated silicon substrates oxidatively patterned with an AFM with a tip under potential control. The process either results in recessed patterns where the monolayer has been removed or raised structures where the monolayer has been removed and silicon oxidation has taken place. Eleven different SAMs have been studied, and the type of pattern formed depends markedly upon SAM chain length, end functional group, and applied bias. We show how local pH and choice of monolayer can be used to very effectively control the type of pattern that is ultimately formed. Interestingly, we show that hydroxide anion accessibility to the substrate surface is one of the most significant factors in determining the pattern topography. Moreover, control over the pattern topography can be achieved by controlling the concentration of the KOH in the water meniscus formed at the point of contact between tip and surface in the context of a bias-controlled DPN experiment with a KOH-coated tip. The work provides important insight into the factors that control SAM desorption and also ways of controlling the topography of features made in a potential-controlled scanning probe nanolithographic process. A variety of scanning probe microscopy-based methods, such as dip-pen nanolithography (DPN), 1,2 nanografting, 3–5 and local oxidation or reduction nanolithography, 6–11 have been developed to nanopattern inorganic substrates. These methods often utilize self-assembled monolayers (SAMs) either as the pattern product 12–18 or resist material 19–21 to create the surface architectures of interest. When alkanethiol SAMs are used as a resist in local oxidation or reduction lithography on gold thin-film substrates, an interesting monolayer-dependent phenomenon has been observed. 22 When a negative bias is applied to the tip (and therefore positive to the substrate), a 16-mercaptohexadecanoic acid (HS(CH 2 ) 15 COOH, MHA) monolayer can be oxidatively desorbed, leaving a recessed area on the substrate. In contrast, in the case of 1-octa- decanethiol (HS(CH 2 ) 17 CH 3 , ODT), the monolayer is de- sorbed but there is also an oxidation of the underlying silicon, which results in the appearance of a raised structure, Scheme 1. Herein, we describe a study aimed at definitively characterizing the process, and we show how local pH and choice of monolayer can be used to effectively control the type of pattern that is ultimately formed. Interestingly, we show that hydroxide anion accessibility to the substrate surface is one of the most significant factors in determining the pattern topography (recessed or raised features). More- over, control over the pattern topography can be achieved by controlling the concentration of the aqueous KOH ink in the context of the bias-controlled DPN experiment on MHA functionalized gold films. In a typical experiment, a series of alkanethiol SAMs on Au thin films were prepared by immersing a substrate in a 1 mM ethanolic solution of the corresponding alkanethiol for 1 h, followed by copious rinsing with ethanol and drying with N 2 (Table 1). The alkanethiol molecules used contained different terminal groups (-CH 3 , -COOH, -PO 3 H 2 , -OH, * Corresponding author. E-mail: chadnano@northwestern.edu. Fax: (+1)847-467-5123. † Present address: Institut Català de Nanotecnologia, Campus Universitari de Bellaterra, 08193 Bellaterra, Spain. ‡ Present address: Department of Applied Chemistry, Kyushu University, Japan. Scheme 1. Schematic Diagram of (A) Recessed and (B) Protruded Pattern Formation NANO LETTERS 2008 Vol. 8, No. 5 1451-1455 10.1021/nl080418b CCC: $40.75  2008 American Chemical Society Published on Web 04/01/2008