Carbon nanotubes (CNTs) continue to attract interest and are considered as potentially promising materials for the development of devices based on new operating principles. Devices based on a single nanotube may be a good example paving a way towards a nanotechnology-based electronics. Due to strong and spectrally tunable absorption bands in visible and IR spectral regions and related photoconductivity, as well as high carrier mobility, CNTs are also promising for the development of materials with enhanced useful electronic properties for more conventional photoelectronic devices. Photoconductivity is one of the most important processes, however still not sufficiently clear.
We combined several experimental techniques - time resolved electric field-induced second harmonic (TREFISH) generation, conventional transient photocurrent and time-delayed collection field – to directly track the carrier generation and motion processes in individual single wall CNTs (SWNTs) and their bundles. The investigated samples were prepared on combs of interdigitated electrodes as thin films of 6.5-SWNTs with different concentrations of fullerene derivative PCBM serving as electron acceptors. The samples were excited by IR light pulses to the first excitonic band and voltage applied to the interdigitated electrodes facilitated charge carrier generation.
Four photoinduced processes taking place on an ultrafast (ps to several ns) time scale in individual SWNTs were identified: first, electron transfer from photoexcited SWNT to PCBM or impurity creating CT state during less than 1 ps; second, CT state dissociation during 200-300 ps; third, a fast hole drift and fourth, recombination of a fraction of photohgenerated charge carriers on a ns time scale. Intertube hole motion in SWNT bundles is much slower taking several microsceonds.
Direct tracking of electronic processes in photoexcited SWNTs give reference data necessary for clear understanding of the photoinduced processes in CNTs and development of CNT-based electronic devices.