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Title: Rapid, electrostatic self-assembly of nanoparticles with Kelvin probe characterisation
Author: Porter, Benjamin Francis
ISNI:       0000 0004 6062 3293
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2015
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The pick-and-place of components to build up complex working machines, such as the robotic arms on automotive assembly lines, has been integral to the enormous success and complexity of modern heavy industry. Modern nanomanufacturing stands in stark contrast to this strategy, reliant instead on the two-dimensional patterning of radiation-sensitive resists to build high-density machines of relative simplicity. The driving goal of our research has thus been to develop a new method of manipulation that would allow the pick-and-place of nanoscale components to mirror that of the assembly lines. A manipulator that could reliably perform this process would open the door to a new age of complex nanomachines. After analysing the current state-of-the-art in nano-object manipulation, we decided to utilise a combination of voltage-driven electric fields with chemically functional monolayers to achieve this goal. The mechanism was intended to be highly compatible with a Scanning Probe Microscope-style tip that could act as the manipulator tool on such an assembly line. This led to the design of several devices that were intended to capture single nanoparticles from a colloidal suspension, which had a negative surface charge in solution. We used numerical simulations using the Guoy-Chapman model of ionic solutions to determine what electrical and geometrical parameters would create the best devices single particle selectivity. We then describe the fabrication of devices using nanolithography techniques including electron-beam lithography, thin-film deposition and etching. Scanning electron microscopy of these devices after voltage-driven assembly showed that the mechanism had very limited success with few incidences of nanoparticle funnelling. This led through several different troubleshooting analyses of the mechanism that identified a myriad of issues affecting these devices. Ultimately we successfully created a radical new approach that incorporated hydrophobic monolayers with polar surfaces. This method exploited the hydrophobic interaction to overcome the hydrostatic barrier and resulted in repeatable single-particle assembly onto devices in sub 5-minute timeframes. Another aspect identified in this work was that the electronic state of nano-objects and monolayers on device surfaces is very important, but is difficult to pinpoint even when data sheets are available. We developed an emerging method of open-loop Dual-Harmonic Kelvin Probe Microscopy to identify these materials by their electronic surface potentials, successfully performing measurements in electrolytes where nanomanipulation would take place. This line of research led to novel considerations of how the size of the nanoparticles fundamentally distorts KPM measurements. We identify this as an important and often ignored effect that must be considered when using probes to differentiate between different materials at the nanoscale.
Supervisor: Bhaskaran, Harish Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available