Doctor of Philosophy
School of Chemistry
O'Connell, Cathal D., Novel strategies for depositing nanoelectrode materials using dip-pen nanolithography of liquid inks, Doctor of Philosophy thesis, School of Chemistry, University of Wollongong, 2013. http://ro.uow.edu.au/theses/4024
Medical bionic devices restore human function by interfacing electrical technology with the body. The emerging field of nanobionics is borne from advances in our ability to control the structure of materials on finer and finer length-scales, coupled with an increased appreciation of the sensitivity of living cells to nanoscale topographical, chemical and mechanical cues. As we envisage and prototype nanostructured bionic devices there is a crucial need to understand how cells feel and respond to nanoscale materials, particularly as material properties (surface energy, conductivity etc.) can be very different at the nanoscale than at bulk. However, the patterning of bionic materials of interest is often not achievable using conventional fabrication techniques, especially on soft, biocompatible substrates. Nonconventional nanofabrication strategies are required.
Dip-pen nanolithography (DPN) is a nanofabrication technique which uses the nanoscale tip of an atomic force microscope to direct-write functional materials. This thesis contributes to the development of the DPN technique in two main aspects. The first aspect is the development of two novel methods of patterning electro-materials at submicron- to nano resolution. The second is a contribution to the understanding of ink transport in liquid ink DPN.
A novel oxidant ink was developed for in situ synthesis of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) via vapour phase polymerisation. DPN patterning of the oxidant ink was facilitated by the incorporation of an amphiphilic block copolymer thickener, an additive that also assisted with stabilization of the oxidant. When exposed to EDOT monomer in a VPP chamber, each deposited feature localized the synthesis of conducting PEDOT structures of several micro-meters down to 250 nm in width, at a scale (picogram) which is much smaller than any previously reported.
A strategy was developed to DPN print a platinum precursor (H2PtCl6) based liquid ink onto insulating substrates with nanoscale resolution. The ink formulation was printable on Si, glass, ITO, Ge, PDMS, Parylene C and even a human hair. A mild plasma treatment effected reduction of the precursor patterns in situ without subjugating the substrate to destructively high temperatures. Feature size was controlled via dwell time and degree of ink loading, and platinum features with 50 nm dimensions could be routinely achieved on silanized Si. We confirmed the electrical conductivity of printed platinum by two point probe measurements and we characterized electrochemical activity using Scanning Electrochemical Microscopy (SECM). A modified method enabled deposition of micron scale Pt snowflakes. By tuning the substrate hydrophobicity using functionalization with a long chain alkane group the spreading of the precursor ink was tempered, and growth of a fractal-like crystal proceeded via a diffusion limited aggregation mechanism. Reduction of the precursor crystal by plasma treatment resulted in a 2D dendritic structure composed of Pt nanoparticles. This combined top-down/bottom-up approach enabled the arbitrary placement of < 20 nm Pt fractal-like structures on Si or glass.
Model ink-substrate systems, which exhibiting a range of viscosities and wettabilities, were used to explore various methods of controlling feature size in liquid ink DPN. The ink-transfer mechanism was investigated using AFM force measurements acquired during ink deposition. This data was used to elucidate the shape of the meniscus during deposition and illustrate a method to monitor the volume of deposition in-situ. However, the deposition rate was found to change dramatically over the course of an experiment due to a dependence of deposition rate on the changing volume of ink on the pen.
The effect of depleting ink volume on deposition rate over a long term experiment was investigated. A hierarchy of phenomena were uncovered which were related to ink movement and reorganisation along the cantilever. These ‘ink-on-tip hydrodynamics’ were suggested to arise from (I) changes in ink volume on cantilever, (II) ‘rest-time’ between grids and (III) travel time between individual dots. In light of our conclusions, we posed critical questions of reservoir-on-tip liquid ink DPN as a nanofabrication technique and discuss the various parameters which need to be controlled in order to achieve uniformity of feature size.
The novel electro-material printing strategies developed in this thesis may have applications in the fabrication of nanoelectronic and nanobionic platforms, particularly on flexible, polymeric substrates. The insights gained into the dynamics of liquid ink transport may have implications in the rational design of DPN inks and probes.