Electrosharpening of Tungsten Probes for Arc Discharge Assembly of Carbon Nanotubes

Abstract

Ultra-sharp tungsten probes fill a key role in science for allowing measurements and interactions at the nanoscale. However, their current method of fabrication is outdated, fundamentally limited in length, sharpness and consistency, and often referred to as an ‘art’.
A new process of fabricating ultra-sharp tungsten probes known as ‘Tungstate Sharpening’ was invented. This electrochemical process utilises solely the WO42- by-product to create a gradient of etching which results in a sharpening effect. It was shown to electrochemically etch probes controllably over lengths from 0.5 – 4.5 mm with tip radii of 10 nm via a fully automated process. Tungstate sharpening overcomes many of the limitations of the previous methods as well as creating new opportunities for further research into electrosharpening.
Tungstate sharpening was improved to use bulk coulometry analysis which allows users to select specific probe lengths. The process was also modified to allow etching of five probes simultaneously, which is fundamentally impossible with conventional techniques. Furthermore, this batch process was improved with the application of a magnetic field that reduced fabrication time and inconsistencies. Flow simulations were conducted to confirm experimental observations of the electrode separation influence on turbulence within the electrochemical system, supporting the underlying theory and observations of the tungstate layer. Finally, this processing technique was expanded with various materials and shapes to demonstrate versatility. Razor blades with edge radii of 40 nm were produced demonstrating that electrosharpening is no longer limited to ‘1D’ objects.
Another process was developed to fabricate carbon nanotubes (CNTs) as a macroscopic material. ‘Arc Assembly’ was invented to explore the possibility of forming long chains of CNTs whilst maintaining the sp2 crystalline bonding within and between individual CNTs.
For the first time, dielectrophoresis was combined with arc discharges to form ‘threads’ of CNTs using the tungsten probes produced. The tungsten probes were applied as electrodes for dielectrophoresis of CNT chains and simultaneous arcing between them. Multiple high voltage circuits with outputs ranging from beneath the breakdown threshold and negative K(ω) up to 1000 V and 8 MHz were constructed and tested. Dispersed carbon nanotube mixtures for a variety of dielectric organic fluids, solvents and polymers were placed between the electrodes. The resulting phenomenon was the assembly of dielectrophoretic chains with arc-induced adhesion between nanotubes: termed as “Arc Assembly”.
As Arc Assembly was developed, the CNT threads were analysed using scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy and Raman analysis. Observations were made of both amorphous and crystalline links between the CNTs, as well as embedded CNT chains within in-situ formed polymer composites. The process produced threads of carbon nanotubes up to 5 mm length, indicating that this may be a viable means of exploiting CNTs in every-day life.