  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.
