Electric field and tip geometry effects on dielectrophoretic growth of carbon nanotube nanofibrils on scanning probes

TL;DR

This study investigates how electric field strength and tip geometry influence the dielectrophoretic assembly of carbon nanotube nanofibrils on scanning probes, optimizing conditions for high-quality nanofibril growth.

Contribution

It provides a detailed analysis of electric field and probe geometry effects on SWNT nanofibril assembly, identifying optimal parameters for stable and high-quality growth.

Findings

Optimal voltages for nanofibril growth are between 5 and 18 V.

Probes with narrow cone angles and long shanks produce better nanofibrils.

Rigid probes with high force constants show more stable wetting and assembly processes.

Abstract

Single-wall carbon nanotube (SWNT) nanofibrils were assembled onto a variety of conductive scanning probes including atomic force microscope (AFM) tips and scanning tunnelling microscope (STM) needles using positive dielectrophoresis (DEP). The magnitude of the applied electric field was varied in the range of 1-20 V to investigate its effect on the dimensions of the assembled SWNT nanofibrils. Both length and diameter grew asymptotically as voltage increased from 5 to 18 V. Below 4 V, stable attachment of SWNT nanofibrils could not be achieved due to the relatively weak DEP force versus Brownian motion. At voltages of 20 V and higher, low quality nanofibrils resulted from incorporating large amounts of impurities. For intermediate voltages, optimal nanofibrils were achieved, though pivotal to this assembly is the wetting behaviour upon tip immersion in the SWNT suspension drop. This process was monitored in situ to correlate wetting angle and probe geometry (cone angles and tip height), revealing that probes with narrow cone angles and long shanks are optimal. It is proposed that this results from less wetting of the probe apex, and therefore reduces capillary forces and especially force transients during the nanofibril drawing process. Relatively rigid probes (force constant >= 2 N/m) exhibited no perceivable cantilever bending upon wetting and de-wetting, resulting in the most stable process control.