7.1 Introduction

From the discussion in the previous chapters, it is clear that there has been much work to establish the basic physics that governs electronic devices made with carbon nanotubes. As these electronic devices become better understood, they can be utilized as the fundamental building blocks to achieve additional functionality; an example of an emerging area of research in this direction is that of opto-electronics with carbon nanotubes. The interest in carbon nanotube opto-electronics arises because nanotubes have several properties that make them excellent opto-electronic materials. For example, an important characteristic of opto-electronic materials is the presence of a direct bandgap, which allows electronic transitions between the valence and conduction bands to proceed without the intervention of phonons. As the band structures of Fig. 1.6 indicate, nanotubes are unique materials in this aspect because all of the bands have a direct bandgap. Thus, for a single nanotube, there are multiple bands that can participate in direct opto-electronic events, spanning a wide range of energies. By combining multiple nanotubes of different bandgaps, it should be possible to obtain a nearly continuous response over a broad spectral range. In addition, a problem that affects traditional bulk materials is the presence of defects, which lead to nonradiative processes and significantly reduced device efficiency. Nanotubes, with their low defect density, should be less sensitive to this problem. A further advantage of nanotubes is related to the temperature dependence of the carrier concentration. For a three-dimensional semiconductor, the carrier density peaks slightly above the...