Liquid ink deposition from an atomic force microscope tip: deposition monitoring and control of feature size

The controlled deposition of attoliter volumes of liquid inks may engender novel applications such as targeted drug delivery to single cells and localized delivery of chemical reagents at nanoscale dimensions. Although the deposition of small organic molecules from an atomic force microscope tip, known as dip-pen nanolithography (DPN), has been extensively studied, the deposition of liquid inks is little understood. In this work, we have used a set of model ink-substrate systems to develop an understanding of the deposition of viscous liquids using an unmodified AFM tip. First, the growth of dot size with increasing dwell time is characterized. The dynamics of deposition are found to vary for different ink-substrate systems, and the change in deposition rate over the course of an experiment limits our ability to quantify the ink-transfer dynamics in terms of liquid properties and substrate wettability. We find that the most critical parameter affecting the deposition rate is the volume of ink on the cantilever, an effect resulting in a 10-fold decrease in deposition rate (aL/s) over 2 h of printing time. We suggest that a driving force for deposition arises from the gradient in Laplace pressure set up when the tip touches the substrate. Second, the forces acting upon the AFM cantilever during ink deposition were measured in order to gain insight into the underlying ink-transfer mechanism. The force curve data and simple geometrical arguments were used to elucidate the shape of the ink meniscus at the instant of deposition, a methodology that may be used as an accurate and real-time means of monitoring the volume of deposited dots. Taken together, our results illustrate that liquid deposition involves a very different transfer mechanism than traditionally ascribed to DPN molecular transport.