Dip-Pen Nanolithography Using MHA and Optical Inks
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Many capillarity-related phenomena originate from the well-documented formation of water films on surfaces. Thin water films have been studied by a variety of techniques including ellipsometry, adsorption isotherms, and atomic force microscopy (AFM). These investigations have shown that the films are often just a few monolayers in thickness. When a water meniscus forms between an AFM tip and a surface, it has a profound effect on tip–sample interactions, both degrading image quality and interfering with force measurements. However, the meniscus formed between the tip and surface has been proposed as the crucial feature that enables the AFM tip to be used as a tool for controlled deposition of molecules on surfaces in dip-pen nanolithography (DPN), while other groups have suggested that the meniscus plays no or little role in the DPN process.
We investigated the effects of humidity, tip speed, and dwell time on feature size during DPN using alkanethiol “ink” on gold “paper.” Our results indicate two distinct deposition regimes. The dwell time at which the transition between the two regimes occurs is independent of ambient humidity. In addition, while the feature size increases with humidity, the relative increase is independent of dwell time. The observed behavior is described using a deposition model that takes into account the kinetics of alkanethiol detachment and attachment at the tip. The model suggests that, at short dwell times and high tip speeds, the most important parameter controlling the feature size is the activation energy for detachment of the ink molecules. Additional results will be presented for luminescent molecules on a combined atomic force/scanning confocal microscope. We show that this method can be used successfully to push the limits of DPN down to controlled deposition of single molecules. We also demonstrate that this method is able to create and visualize protein patterns on surfaces. Finally, we show that our method can be used to fabricate polymer nanowires of controlled size using conductive polymers.