Absorption spectroscopy in the ultraviolet-visible-near infrared (UV-Vis-NIR) wavelength region has been used to quantify the aggregation factor of single-walled carbon nanotubes (SWCNTs) in liquid media through a series of controlled experiments. SWCNT bundles are dispersed in selected solvents using a calibrated ultrasonicator, which helps in determining the true amount of energy used in the exfoliation process. We also establish the selectivity of the centrifugation process, under the conditions used, in removing the nanotube aggregates as a function of the sonication time and the dispersion solvent. This study, along with the calibration of the sonication process, is shown to be very important for measuring the true aggregation factor of SWCNTs through a modified approach. We also show that the systematic characterization of SWCNT dispersions by optical spectroscopy significantly contributes to the success of dielectrophoresis (DEP) of nanotubes at predefined on-chip positions. The presence of individually dispersed SWCNTs in the dispersions is substantiated by dielectrophoretic assembly and post-DEP electromechanical measurements. 1. Introduction Single-walled carbon nanotubes (SWCNTs) have attracted significant interest in basic and applied nanomaterials research [1, 2] due to their exceptional electrical [3], mechanical [4], optical [5], and thermal properties [6]. In order to exploit these attractive properties [7], SWCNTs have been proposed as components in a variety of applications like sensors [8, 9], field effect transistors [10], interconnects in CMOS technology [11], electromechanical springs [12], and field emission sources [13], as additives in composite materials for enhanced mechanical properties [14], and as medical therapeutic agents [15]. In spite of their huge promise, the success of SWCNT devices still remains uncertain at a commercial level. This is because SWCNTs exist in a wide range of diameters, lengths, chiralities (the rollup axis), structural purity, and states of aggregation [16]. Therefore, fabrication schemes need good selectivity in order to control the physical properties of SWCNTs and thus their device properties. This selectivity can be obtained either through controlled growth to limit the variability among the as-grown nanotubes [17–19] or postgrowth purification and sorting techniques [20–22]. SWCNTs in the native form exist in a bundled state. For a number of the postgrowth SWCNT sorting techniques, it is desired to have the nanotubes dispersed uniformly in a liquid medium [20–22]. It is, however, difficult to
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