The functional form of the photoluminescence ͑PL͒ line shape from individual single-walled carbon nanotube ͑SWNT͒ species is found to contain a significant Lorentzian component and the Stokes shift is observed to be very small ͑Ͻ8 meV͒, which suggests an excitonic dephasing mechanism that is largely decoupled from surrounding solvent and surfactant molecules. The PL quantum yield ͑PLQY͒ of two SWNT species is determined to be ϳ5 ϫ 10 −4 , and it is suggested that this is lower than the "true" value due to quenching of the PL in bundles by metallic tubes. Time-resolved PL measurements reveal a dominant, luminescence lifetime component of 130 ps that, when combined with a predicted natural radiative lifetime of ϳ20 ns, suggests that the true PLQY is ϳ6.5ϫ 10 −3 . Finally, deconvoluted PL excitation spectra are produced for eight SWNT species, and the appearance of a higher-energy excitonic subband is discussed.

The kinetics of single-walled carbon nanotube rebundling have been investigated by photoluminescence (PL) spectroscopy. The rate of loss of PL intensity was measured for 12 different nanotubes in three common aqueous surfactants (sodium dodecyl sulfate, SDS; sodium dodecylbenzene sulfonate, SDBS; and sodium cholate, SC) as the surfactant suspensions were diluted to promote nanotube rebundling, quenching of semiconductor nanotube PL, and precipitation. The rate of PL decay was first-order in the concentration of isolated nanotubes, as expected if surfactant desorption is rate-limiting in the rebundling process. Temperature-dependent measurements permitted an Arrhenius analysis from which diameter-dependent activation energies were determined. SDS was found to have very strong diameter dependence for activation energy, with stronger binding to smaller-diameter nanotubes, whereas SDBS displayed a weaker diameter dependence. SC was found to bind strongly to certain nanotubes and weakly to the (10,2) nanotube. The PL emission red shifted with time after dilution as surfactant desorption proceeded. This effect is attributed to an increase in the micropolarity at the nanotube surface.

Self-assembled monolayers (SAMs) of 4-[4′-(phenylethynyl)-phenylethynyl]-benzenethiols on Au(111) surfaces are investigated by scanning tunneling microscopy (STM) under ultrahigh vacuum. STM images reveal longrange order in these SAMs with a rectangular unit cell containing two molecules. In high-resolution images, two new structural features are resolved, which cannot be explained by the previously proposed ( 3 × 2 3)-R30°commensurate structure which consists of three equivalent domains. First, six equivalent domains are present, and the orientations of these domains with respect to the three 〈121〉 directions of Au( 111) are (5°. Second, superstructures are observed. Periodical ridges are observed as a modulation of the STM imaging contrast within the ordered domains. A new model is proposed, which is very similar to the crystalline structure of p-terphenyl. The closest-packed row is aligned along the next-nearest neighbor or 〈121〉 direction of Au-( 111) with phenyl planes arranged in a herringbone fashion. The lack of simple commensurate structure of arenethiol SAMs is mainly attributed to intermolecular interactions.