Surface Adsorption¶
Changes on the surface of a carbon nanotube form trap states that lead to a non-radiative decay of the excitonic state and changes in the dielectric surrounding influence the exciton energy.¶
Interactions with the surface of a carbon nanotube are predominantly dispersion (London) interactions.
These strong intermolecular forces are responsible for stable suspensions of carbon nanotubes in various solvents where non-covalently bound surfactants are wrapped around the nanotube. Such wrappings prevent bundling and preserve the excitonic emission features of carbon nanotubes in solution and films over months. The exact mechanism and the kinetics of surfactant sorption on the surface of carbon nanotubes is rather complex and not fully understood [7] as it usually includes concomitant effects. The separate study of such effects is important and will be delved deeply for the Adsorption of Solid.
A downside for the use of surfactants is that a significant percentage of photoluminescence quantum yield is lost [160] to exciton quenching at the sites of adsorbed solvent and surfactant molecules. Placing carbon nanotubes in solvents is, however, inevidable for most wet processing methods or for contacting nanotubes to electrolytes. The effect of the Adsorption of Liquid is therefore handled in more detail in its own chapter.
The use of surfactants reduces and alters the available free nanotube surface. A clean surface, however, is required for sensing molecular interactions [161]. A clean surface serves as a clear reference point for studies on the adsorption process. The search for this initially clean surface and its definition as a reference is discussed in the Adsorption of Gas.