Towards closed-loop nanopatterning: quantifying ink dynamics in dip-pen nanolithography
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Dip-pen nanolithography (DPN) is a scanning probe microscopy-based nanofabrication method that relies on a fluid-coated atomic force microscope probe for the deposition of material on a substrate with nanometer-scale resolution. The ability to tailor the structure and chemical composition of materials at the nanometer length scale is enabling in elds ranging from medical diagnostics to nano-electronics. While DPN is among the highest resolution additive manufacturing techniques to date, the conguration of ink on the probe and the process of ink transport are poorly understood. Specically, the inking and patterning procedures are susceptible to variations in the ambient environmental conditions and currently not all aspects of the processes are reliably controlled. Thus, a key challenge barring the widespread adoption of DPN beyond a research tool is reproducibility. We hypothesize that closed-loop control over the inking and patterning process could address this irreproducibility, however techniques to monitor the quantity and concentration of ink on the tip of the probe have not been yet developed. Here, we study the mechanics of atomic force microscope (AFM) probes throughout the inking and patterning process to understand if the behavior of the ink can be studied in situ. In particular, we develop an approach for conning ink to the tip of an AFM probe, which is critical for reliable patterning and modeling the mechanics of the probe. Then, we nd that the quantity of ink on an AFM probe can be determined in situ by observing the shift in the natural frequency of the probe. Finally, we show that this method allows for the observation and quantication of the ink deposited on a substrate, in real time. Collectively, these approaches lay the groundwork for a closed-loop implementation of DPN in which the inking and patterning processes are performed with drastically improved reliability. Given that these techniques are easily implemented on any commercial AFM, we expect that they could lead to new applications in the study of nanoscale soft materials.