Ink‐on‐Probe Hydrodynamics in Atomic Force Microscope Deposition of Liquid Inks

Abstract

The controlled deposition of attolitre volumes of liquids may engender novel applications such as soft, nano-tailored cell-material interfaces, multi-plexed nano-arrays for high throughput screening of biomolecular interactions, and localized delivery of reagents to reactions confined at the nano-scale. Although the deposition of small organic molecules from an AFM tip, known as dip-pen nanolithography (DPN), is being continually refined, AFM deposition of liquid inks is not well understood, and is often fraught with inconsistent deposition rates. In this work, the variation in feature-size over long term printing experiments for four model inks of varying viscosity is examined. A hierarchy of recurring phenomena is uncovered and there are attributed to ink movement and reorganisation along the cantilever itself. Simple analytical approaches to model these effects, as well as a method to gauge the degree of ink loading using the cantilever resonance frequency, are described. In light of the conclusions, the various parameters which need to be controlled in order to achieve uniform printing are dicussed. This work has implications for the nanopatterning of viscous liquids and hydrogels, encompassing ink development, the design of probes and printing protocols.

On‐Tip Photo‐Modulated Molecular Printing

“…This is enabled since ar eservoir of ink surrounding the base of each pyramid is protected from photopolymerization via an opaque gold layer ( Figure 3A and B). [23] This is clearly seen in the experiment where NOAink features were generated via the on-tip photo-modulated molecular printing technique ( Figure 3D,t op 3r ows) followed by the subsequent generation of similar arrays of features in the dark ( Figure 3D,b ottom 5r ows). SEM images of the rinsed array clearly show solid polymerized NOAink on the exposed tip,b ut not in the protected reservoir regions ( Figure 3C).…”

Nanoscale platinum printing on insulating substrates

“…More particularly, the deposition rate depends on the local volume of ink on the tip apex, and ink distribution along the cantilever is subject to dynamic reorganisation during printing [40]. Thus, feature size can be influenced by factors such as the order of dots printed [40]. In our case, the system is rendered more complex as the precursor (H 2 PtCl 6 ) is also soluble in water.…”

“…Several phenomena unique to liquid deposition may contribute to the transition in deposition rate observed in Figure 5 C. For liquid inks, the volume of ink on the cantilever affects ink flow from tip to substrate [37]. More particularly, the deposition rate depends on the local volume of ink on the tip apex, and ink distribution along the cantilever is subject to dynamic reorganisation during printing [40]. Thus, feature size can be influenced by factors such as the order of dots printed [40].…”

“…Our group has recently shown how deposition dynamics in liquid ink deposition from an AFM tip can change as a result of decreased volume of ink on the cantilever [37]. The ink available at the tip apex is also liable to reorganise during printing, which may explain why some dots print and some do not [40]. Future AFM based deposition methods could measure cantilever deflection for in situ monitoring and correction of deposition errors [37].…”

Development of Dip‐Pen Nanolithography (DPN) and Its Derivatives

“…Liquid inks are not only good at directly constructing desired nanostructures, but also can be used as carriers to transport or synthesize other nanostructures. Therefore, liquid inks, especially viscous polymer inks, have become a new research hotspot in the development of DPN . Up to now, only a few complete models have been set up to study this transport process, and a detailed understanding is not yet available.…”