Photocurrent generation in semiconducting and metallic carbon nanotubes

The fundamental mechanism underlying photocurrent generation in carbon nanotubes(1-3) has long been an open question. In photocurrent generation, the temperature of the photo-excited charge carriers determines the transport regime by which the electrons and holes are conducted through the nanotube. Here, we identify two different photocurrent mechanisms for metallic and semiconducting carbon nanotube devices with induced p-n junctions(4-7). Our photocurrent measurements as a function of charge carrier doping demonstrate a thermal origin(8,9) for metallic nanotubes, where photo-excited hot carriers give rise to a current. For semiconducting nanotubes we demonstrate a photovoltaic mechanism(10-12), where a built-in electric field results in electron-hole separation. Our results provide an understanding of the photoresponse in carbon nanotubes, which is not only of fundamental interest but also of importance for designing carbon-based, high-efficiency photodetectors and energy-harvesting devices.