The goal of this program was to reliably prepare extremely miniaturized samples of polymers. The technique for fabricating these samples was based on an atomic force microscope, which consists of a sharp tip at the end of a compliant cantilever. Specifically, we utilize dip-pen nanolithography, a process for using an atomic force microscope probe as a pen that transfers materials from the tip to a substrate at precise locations by programmably bringing the two into contact. While this approach was already capable of patterning very small samples, it was only able to do so without control over the amount of material to be deposited, limiting its use as a manufacturing tool. This program was developed to overcome this limitation and transform dip-pen nanolithography into a closed-loop patterning system in which the size of patterned features could be predicted, specified, and measured. As a foundational discovery, we invented a process for measuring the quantity of fluid on an atomic force microscope probe by examining changes in the vibrational resonance frequency of the cantilever. This process required innovating in the type of atomic force microscope probe, how it is treated, and the type of fluid used for patterning. By measuring the fluid mass before and after patterning actions, it is possible to measure the amount of fluid written. Using this foundation, we were able to study the fluid transport physics and discover that the amount of fluid transferred can be controlled by adjusting the speed at which the probe is withdrawn from the surface. Collectively, these innovations allowed us to realize patterning of extremely small quantities of fluid with control over the amount and location of this fluid.

Having invented a process for patterning extremely small quantities of fluid, we sought both to further expand the manufacturing capabilities of this process and to use it to produce material innovations. For example, we found that the fluid transfer process allowed us to load controlled amounts of epoxy onto an atomic force microscope probe. As a second step, this epoxy-tipped probe can then be brought into contact with samples of interest to adhere microscopic materials to the end of this probe. We found that this process works for both attaching microscopic spherical beads as well as metal-organic framework microcrystals.

While controlling the amount of material patterned is an important facet of nanomanufacturing, it is also important to control the composition of the material that is patterned. Thus, we sought to overcome the long-standing limitation of dip-pen nanolithography that the composition of the material to be patterned has to be dictated in advance of the experiment by the operator. We hypothesized that control over fluid transfer would allow us to realize mixtures of fluids. In particular, we found that when a probe loaded with a fluid sample is brought into contact with a fluid sample on a surface, these fluids mix and the resulting mixture can be controllably patterned. This is a critical innovation as it means that preparing a set of reservoir samples can allow a user to build a compositional gradient of materials. 

Despite this program predominantly focusing on advancing the manufacturing aspects of dip-pen nanolithography, it also resulted in a major advance that moves this approach closer to being a platform for materials discovery. Atomic force microscopy is noteworthy as a tool for nanoscience due to its versatility. In addition to allowing one to pattern features using dip-pen nanolithography, it also allows one to functionally characterize and topographically image samples with exceptionally high spatial resolution. While this capability has already been extensively leveraged for nanoscience, advancing the manufacturing capabilities of atomic force microscopy has the important implication that interesting and useful structures can now be made and interrogated using the same instrument. Crucially, this would allow one to select a desired material composition, realize a structure with this composition, and then interrogate it using a single system. As an initial proof of this concept, we used the atomic force microscope to both pattern photocurable resin and determine the mechanical properties of the cured resin. In addition to allowing us to compare bulk and nanoscale mechanical properties, this experiment provided an important proof-of-concept showing that it is possible to perform closed-loop materials discovery.

Last Modified: 12/22/2022
Modified by: Keith A Brown