A Reductionist Approach in Gene Screening

When a material is confined to one dimension, its electronic band structure can exhibit features termed van Hove singularities, which have been invoked to explain the sharp absorption spectra in materials such as single-walled carbon nanotubes (SWNTs). This model predicts a sealike photoexcited state of free electrons and holes. Recently, however, support has emerged for an exciton picture, in which light absorption creates excited electrons that remain strongly correlated with the positive holes left behind. Wang et al. (p. 838) present firm evidence for the exciton model in isolated SWNTs. Their experiment takes advantage of the selection rules that exciton creation imposes on oneversus two-photon absorption. The two-photon spectra are consistent with exciton-binding strengths near 0.5 electron volt, which are much higher than in bulk semiconductors.