Nano-liquid 3D printing of functional nanoprobes

Abstract

Over the past 40 years since the invention of the scanning tunneling microscope, scanning probe microscopy (SPM) has been considered as one of the most dominant characterization methods for materials and interfaces at the nanoscale. The impact of SPM on scientific disciplines such as physics, chemistry, and materials science is significant, as it enables nanoscale imaging with unprecedented sensitivity and spatial resolution. In addition to nanoscale surface topography, a number of functional imaging methodologies have been invented by modifying the physical/chemical properties of nanoprobes. For example, an ultrahigh aspect-ratio feature of the probe delivers excellent imaging fidelity for narrow and complex 3D nanostructures which are inevitable in modern semiconductor integrated circuits. The second example is scanning thermal microscopy that utilizes a multi-layered thermocouple nanoprobe to map the local temperature of a surface. However, the fabrication of these nanoprobes relies on conventional lithographic manufacturing approaches accompanying extravagant and multiple etching/deposition steps. The thesis aims to develop nano-liquid 3D printing methods for fabricating scanning functional nanoprobes. The utilization of 3D printing can offer economical, minimalist, and scalable routes to manufacture nanoprobes. First, we harness electrohydrodynamic (EHD) 3D nanoprinting for one-step, on-demand manufacturing of ultralong atomic force microscopy (AFM) nanoprobes for high-aspect-ratio nanoscale imaging. After thorough studies on the mechanisms of EHD nanodroplet ejection and evaporation, we developed an optimum protocol to print a freestanding metallic nanowire directly on a cantilever with controlled dimensions and materials, resulting in a high-aspect-ratio (over 29) AFM probe. We show that our 3D printed tip delivers a high fidelity in deep-trench imaging better than a standard pyramidal tip and comparable to commercial high-aspect-ratio probes, yet at a fraction of their cost. Second, we develop a bi-metal 3D printing method for fabricating thermocouple probes. A freestanding thermocouple junction consisting of platinum and silver is successfully fabricated by three-dimensional guiding of ink meniscus containing the respective nanoparticle inks. Furthermore, the capability of the method for freeform structuring enables the fabrication of 3D thermocouple networks for mapping microscale temperature fields in three-dimension. The proof-of-concept experiments successfully demonstrate that the 3D thermocouple network device could pave the way for exploring various microscopic thermodynamics ‘at’ and ‘near’ condensed-matter systems, e.g., Joule heating of microelectrodes or evaporative cooling in microfluidics. We expect this work to pave the new way for freely designing and manufacturing a wide range of on-chip microsensors without the restrictions of the traditional manufacturing process. The main objective of this thesis is to provide insights into the diversification of scanning probe fabrication technologies at the nanoscale powered by 3D printing.