On-chip purification of arc-discharge synthesized multiwalled carbon nanotubes via mobile liquid interface

Abstract

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