TABLE OF CONTENTS As we have shown in the previous sections, it is possible to generate photocurrents in carbon nanotubes using monochromatic illumination at laser intensities. However, for optical detection and switching, much lower light intensities need to be converted into electrical signals; one approach to achieve this goal is to functionalize carbon nanotubes with molecules that are optically active, and use the response of the molecules to modulate the current in a carbon nanotube field-effect transistor [32 34]. This approach relies on three mechanisms in optically-active molecules: (1) chromophores that undergo a change of dipole moment when exposed to light [32], (2) molecules that transfer charge when exposed to light [34], and (3) molecules that cause an increase in scattering in the nanotube when irradiated [33]. These approaches can be used to modulate the current in a field-effect transistor either by effectively changing the gate voltage felt by the nanotube, by effectively changing the nanotube doping or by causing a reduction of the current due to increased scattering.
Fig. 7.43 shows a sketch of a carbon nanotube functionalized with Disperse Red 1 (DR1), an azobenzene molecule that is used to impart red color to acrylics. This molecule is known to isomerize under UV illumination, accompanied by a significant change in the dipole moment. Experiments have shown that at equilibrium, the DR1 chromophore is in the trans conformational state (Fig. 7.43) in which there is significant orbital overlap between the phenyl...
