Photocurrent Spectroscopy of individual Carbon Nanotubes

The recent discovery of photoluminescence from single-walled carbon nanotubes has boosted the field of nanotube optical spectroscopy. The fact that sharply peaked, bright luminescence is obtained at room temperature from 1.0 to 1.6 micrometer wavelength suggests that carbon nanotube optoelectronic devices could have applications in optical telecommunications. Nevertheless, many of the elementary electronic properties of nanotubes and optical excitations in these systems have remained unexplored. The present project is concerned with the optical spectroscopy of individual single-walled carbon nanotubes. In particular, photocurrent spectroscopy is employed to study the mechanism of photocurrent generation in nanotube field-effect transistors. It will help to improve our understanding of carbon nanotube electronic and optoelectronic devices. The first part of the project is concerned with the electric-field-induced dissociation of photoexcited excitons in carbon nanotube field-effect transistors. If excitons in bare, i.e. electric-field-less, carbon nanotubes are excited in their ground state, they cannot decay into free carriers. Exciton dissociation in a strong electric field through Fowler-Nordheim tunneling into neighboring continuum states, however, can give rise to photocurrents and allows for the detection of the bound-exciton ground state. Electric fields in carbon nanotube field-effect transistors can be very large. Therefore, nanotube transistors offer a perfect opportunity for studying exciton dissociation in nanotubes. The second part of the project is devoted to the study of potential fluctuations along a nanotube. Internal electric fields in nanotube photodetectors are highly non-uniform. This implies that photocurrent generation may not occur uniformly along the tube. Photocurrent measurements performed under uniform device illumination may thus not be sufficient to establish the operational principle of the devices. Therefore, scanning photocurrent microscopy is applied to map the local current along the channel of a carbon nanotube field-effect transistor. Analysis of the spatial variation and bias dependence of the local photocurrent also provides information on structural defects along a single nanotube. The results of the investigations will shed new light on the mechanism of photocurrent generation in this exciting new class of materials.