By attaching light-absorbing molecules to carbon nanotube field-effect transistors, scientists in the United States have fabricated nanoscale optoelectronic detectors that are sensitive to different colours of visible light (Nano Lett. doi:10.1021/nl8032922; 2009).

Credit: © 2009 ACS

Xinjian Zhou and colleagues from Sandia National Laboratories took single-walled carbon nanotubes and functionalized them with azobenzene chromophores to create molecularly engineered photosensitive materials. In the scheme, the chromophores serve as photoabsorbers and the nanotubes act as an electronic read-out system.

The carbon nanotubes had diameters in the range of 0.8–2 nm, and were prepared in a well-dispersed solution and attached to treated silicon wafers. Three different azobenzene-based chromophores having absorption maxima at 467 nm, 381 nm and 342 nm, respectively, were anchored, through strong non-covalent interactions, to the surfaces of the nanotubes, in three separate experiments.

On illuminating the nanotubes with monochromatic light with a wavelength in the visible range of 400–700 nm, the scientists observed shifts in the threshold voltage in the electrical conduction of these nanotube–chromophore hybrid field-effect transistors. The shifts correlated with the absorption spectra of the azobenzene molecules.

Zhou et al. confirm that the process of conversion from an optical to an electrical signal is controlled by a dipole change mechanism at the molecular level. When a chromophore absorbs photons, a change in the molecular structure of the azobenzene occurs — so-called photoisomerization — and it transforms from the ground state trans-configuration to the metastable excited state cis-configuration. This photon-induced isomerization is accompanied by a large change in the electrical dipole moment of the chromophore and modifies the electrostatic potential and thus the threshold voltage of the nanotubes.

Zhou and co-workers' colour detection scheme is not only useful for making colour photodetectors with nanoscale resolution but also gives insights into the molecular interactions between single-walled carbon nanotubes and molecules. Further improvements are expected to allow them to detect single-molecule transformation activities and extend operation to other spectral regions.