This thesis reports a novel approach for purification of carbon nanotube (CNT)
samples deposited on smooth silicon substrates through a mobile liquid interface
interacting with carbonaceous debris particles that contaminate the deposition. The
method is based on physical interaction of the particles and the three-phase contact line via capillary interface forces, i.e. the surface tension, which results in the
detachment of chemically indispersible colloids from the sample surface.
In the experiments reported in this work, we focus primarily on arc-discharge
grown multi-walled carbon nanotubes, whose synthesis is particularly plagued by
carbonaceous debris that is difficult to remove without harsh chemical treatments.
The irregular debris particles are preferentially captured by the mobile contact line,
while tubular CNTs are retained in large numbers due to their appreciably stronger
adhesion. Our cleaning method has the added benefit that the intrinsic chemical
properties of
the CNTs are fully preserved, since no chemical treatments are needed;
only prerequisite is the hydrophilic treatment of the underlying substrate, which on
silicon wafers can be accomplished through oxygen plasma.
Various aspects of the cleaning process, including the effects of contact line
velocity, chemical composition of the immersion liquid, and the CNT orientation
relative to the liquid front are investigated in this thesis. Furthermore, atomic force
microscopy (AFM) imaging and the image analysis methodology is discussed. Ad-
ditionally, we demonstrate some research applications that directly benefit from
our technique, e.g. microraman spectroscopy of individual carbon nanotubes, and
nanomechanical investigation of suspended tubes via AFM-deflection measurements.
Theoretical treatment of the detachment process is presented in the framework
developed for spherical microcolloids in existing works. In this model, the particle
detachment is primarily attributed to the surface tension force. Other forces affecting
the process include adhesion, which in liquid electrolyte medium can be estimated
via the DLVO adhesion theory, and hydrophobic interactions.
We conclude that the particle detachment is qualitatively well-explained by
the physical interactions at the contact line, while the selectivity of the process for
the irregular debris particles in favor of the tubular CNTs probably stems from
differences in the magnitude of the adhesion force. Chemical factors associated with
the composition of the immersion liquid on the other hand seem to only have very
limited effect on the quantitative cleaning result. Regarding to detachment of CNTs,
orientation-dependent behavior is implied, with species oriented perpendicular to
the advancing contact line reflecting the greatest probability to be retained. Overall,
the detachment of all particles is found to increase with decreasing velocity of the
liquid interface. These results are generally in qualitative agreement with existing
works dealing with spherical microcolloids
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