SURFACTANT REMOVAL FROM CARBON NANOTUBES
by Larry Nagahara, Timothy Burgin, Justin Lewenstein, Thomas Harvey III, Chan-Long Shieh, and Raymond Tsui
Carbon nanotubes (CNTs) are a new class of carbon material with unique electrical and mechanical properties. CNTs are formed using one of three major growth techniques, namely laser ablation, arc discharge, or chemical vapor deposition (CVD).
The former two growth techniques are the most common and require the CNTs to be collected and purified using wet chemistry. CNTs are considered as possible candidates for novel molecular devices (e.g., SETs, molecular FETs). For these device structures, a single CNT needs to be selectively placed across metal contacts.
The large mass of CNTs (up to Cl0,000,000) precludes them from being soluble in aqueous or nonaqueous solvents. As such, surfactants are needed to aid in suspending the CNTs to practical concentration levels by surrounding the CNTs with a coating of surfactant molecules.
However, the inherent properties of the CNTs can be hindered with the presence of the surfactant and thus removal of surfactant from the CNTs is needed to better utilize their properties.
For solution-based CNT dispersion, removal of the surfactant is critical for improved device performance. However, removal of the surfactant has not been actively investigated in part on the belief that a surfactant residual surrounding the CNT is minimal after simple cleaning.
It happens that excessive surfactant residual remains on the CNTs even after repeatedly rinsing with methanol and water (a common cleaning procedure described in the literature). Thus, alternative methods are needed to remove the surfactant in a manner that can be used in the manufacturing of CNT-based components.
Once the CNT is selectively placed on a surface, removal of the surfactant from the CNT is desirable without degrading the CNT and surrounding material (e.g., metal contacts). Thermal, optical, and/or chemical treatment can be used to remove the surfactant without damaging the underlying material as will be outlined below. The procedure described below removes the surfactant (Triton X-100) from the CNTs via a thermal treatment (example 1) or a chemical treatment (example 2).
In example 1, the CNTs are dispersed from a solution containing Triton X-100 onto a pre-patterned electrode surface. Figure 1 shows a characteristic I-V plot for the as deposited CNTs on the electrode before and after heat treatment.
After heat treatment for 10 min at 320 °C under argon atmosphere, the I-V characteristics significantly improved. This heat treatment desorbs and/or decomposes the Triton X-100 in such a manner as to improve the CNT contact with the underlying metal electrode (i.e., reduce contact resistance) as depicted schematically in Figure 2.
In example 2, repeated rinsing with distilled water followed by methanol is desc...
