  Spectroscopic, time-resolved scanning photocurrent microscopy is shown to
distinguish the intrinsic photoresponse of monodisperse semiconducting (99%)
single-walled carbon nanotubes (SWCNTs) from the extrinsic photoresponse of the
substrate. A persistent positive photocurrent induced by near-IR excitation is
attributed to the generation of free carriers by inter-band excitation in
SWCNTs. For shorter excitation wavelengths, absorption by the Si substrate
generates two types of photocurrent: a transient positive photoresponse,
identified as a displacement current, and a persistent negative photocurrent
that arises from photogating of the SWCNT thin film.
