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We present a comprehensive study of the effects of doping and temperature on the conductivity of single-walled carbon nanotube (SWNT) networks. We investigated nearly type-pure networks as well as networks comprising precisely tuned mixtures of metallic and semiconducting tubes. Networks were studied in their as-produced state and after treatments with nitric acid, thionyl chloride, and hydrazine to explore the effects of both intentional and adventitious doping. For intentionally and adventitiously doped networks, the sheet resistance ( Rs) exhibits an irreversible increase with temperature above ∼350 K. Dopant desorption is shown to be the main cause of this increase and the observed hysteresis in the temperature-dependent resistivity. Both thermal and chemical dedoping produced networks free of hysteresis. Temperature-programmed desorption data showed that dopants are most strongly bound to the metallic tubes and that networks consisting of metallic tubes exhibit the best thermal stability. At temperatures below the dopant desorption threshold, conductivity in the networks is primarily controlled by thermally assisted tunneling through barriers at the intertube or interbundle junctions.

We present a comprehensive study of the optical and electrical properties of transparent conductive films made from precisely tuned ratios of metallic and semiconducting single-wall carbon nanotubes. The conductivity and transparency of the SWNT films are controlled by an interplay between localized and delocalized carriers, as determined by the SWNT electronic structure, tube−tube junctions, and intentional and unintentional redox dopants. The results suggest that the main resistance in the SWNT thin films is the resistance associated with tube−tube junctions. Redox dopants are found to increase the delocalized carrier density and transmission probability through intertube junctions more effectively for semiconductor-enriched films than for metal-enriched films. As a result, redox-doped semiconductor-enriched films are more conductive than either intrinsic or redox-doped metal-enriched films.

We study the effects of adding H2O2to acid-purified and unpurified single-walled carbon nanotubes (SWNTs) in aqueous suspensions using photoluminescence (PL) and optical absorption spectroscopies. The addition of H2O2to suspensions of unpurified SWNTs results in a rapid (1−2 h) quenching of the photoluminescence from all tubes, whereas H2O2addition to acid-purified SWNTs causes the nanotube PL to grow in intensity over a period of several days before decaying in a tube-specific manner that depends on the binding strength of the surfactant sheath. With the appropriate choice of surfactants, the PL for specific acid-purified SWNTs can be protected such that novel mid-gap and phonon-assisted absorption and emission transitions can be observed without the obscuring effects associated with emission from other nanotubes. The H2O2treatment also results in a reduction of the high-energy absorption background that has been associated with either carbonaceous impurities or the SWNT -plasmon oscillation. An understanding of the related mechanisms leads to a new method for separating nanotubes by type based on selective oxidation followed by selective precipitation. These findings offer the possibility of efficiently separating large quantities of nanotubes by chirality.

We have performed steady-state photoluminescence, time-correlated single photon counting, and Raman spectroscopy measurements on single-wall carbon nanotubes from 4 to 293 K. We observe novel photoluminescence spectra that cannot be attributed to vibronic transitions and verify the existence and energy levels of weakly emissive excitonic states. By combining photoluminescence intensity and lifetime data, we determine how nonradiative and radiative excitonic decay rates change as a function of temperature and contrast this with theoretical predictions. The results suggest that recombination kinetics are influenced by multiple excitonic states, including a dark lower state.