Dependence of Transport Rate on Area of Lithography and Pretreatment of Tip in Dip-Pen Nanolithography Tzu-Heng Wu, † Hui-Hsin Lu, † and Chii-Wann Lin* ,†,‡ † Institute of Biomedical Engineering and ‡ Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei 106, Taiwan * S Supporting Information ABSTRACT: This study examines the lithographic capacity of tips in dip-pen nanolithography (DPN). The dependence of the transport rate (R) decay on the area of lithography (A lith ), the dependence of A lith on the lithographic time (t), and the effect of piranha cleaning on the lithographic capacity are considered herein. The dependencies in the line-drawing lithography process are studied using 16-mercaptohexadeca- noic acid (MHA) ink. On the basis of the linear decay dependence discovered in the R-A lith dependence, piranha treatment can increase the lithographic capacity by up to 35.5- fold, an improvement that may originate from a change in the tip’s surface chemistry. Moreover, a theoretical model is derived to describe the A lith -t dependence accurately and to predict the tips’ lifetime. Furthermore, an experiment involving DPN-based nanostructure fabrication demonstrates the importance of monitoring the tips’ transport rate and lifetime. In addition to shedding light on the physical and chemical principles behind DPN, this study provides a comprehensive model for a quantitative analysis of the tips’ behavior. ■ INTRODUCTION Ever since the development of dip-pen nanolithography (DPN), 1,2 it has been applied in numerous applications 3-5 including the fabrication of nanostructures, gas sensors, bioessay chips, and electronic devices. Owing to the recent development of parallel pen arrays, 3,6 DPN has become a high- throughput process for rapid fabrication. Although pen arrays represent the potential use of DPN beyond laboratory-scale fabrication into industrial manufactur- ing, more challenges lie ahead. As noted by Saha et al., 7 the efficacy of applying DPN in large-scale fabrication is closely related to the lithography capacity of the tips. A tip’s lifetime is limited mainly, but not exclusively, by the fact that the tip is not an unlimited source of ink in practice. Some studies 8,9 have noted that the transport rate of a tip (denoted hereinafter as R) decreases gradually in a manner determined by factors such as the coating method, writing conditions, and humidity. However, the interaction among the transport-rate decay, lithographic capacity, and lifetime of tips has seldom been studied thoroughly. Without monitoring, the R decay results in an inaccurate output pattern. Such errors in the lithography pattern can then hinder subsequent applications such as nanofabrication, thereby limiting the efficacy of DPN. Hence, thoroughly understanding the decay process is necessary to achieve the DPN’s full potential for large-scale synthesis. This study elucidates how R and A lith , A lith and the lithographic time (t) are related. These dependencies are examined by using MHA as an ink to carry out line-patterning lithography on a gold substrate. Two types of tips are investigated: one treated with piranha solution before use and the other one used as purchased. Experimental results indicate that the transport rate decays linearly with an increasing area of lithography for both tip types. Closely examining the cutoff area reveals that piranha treatment increases the tip lithographic capacity by up to 35.5-fold. Exactly why this increase occurs is discussed in detail. Moreover, on the basis of a combination of the linear R-A lith decay equation with the nonlinear A lith -t dependence described herein, a theoretical model is derived to address issues involving the patterning precision and the tip’s lifetime. Furthermore, an experiment involving nanostructure fabrication demonstrates the effect of the R decay and the importance of monitoring the tips’ lifetime in DPN-based applications. ■ MATERIALS AND METHODS All gold substrates used in this study were prepared by the template stripping method. 10 The tips were coated exclusively by the dip- coating method. Some tips were immersed in piranha solution for 30 min before use to study the effect of pretreatment on the lithographic capacity, and the others were used as purchased. DPN was then performed using the Nscriptor system purchased from Nanoink with type-A probes (side A-1, spring constant = 0.041 N/m). The transport rate of the tips was calibrated by using the built-in Ink-Cal function of Received: July 4, 2012 Revised: September 28, 2012 Published: September 28, 2012 Letter pubs.acs.org/Langmuir © 2012 American Chemical Society 14509 dx.doi.org/10.1021/la302680k | Langmuir 2012, 28, 14509-14513