Ultrafast Spectroscopy of Excitons in Single-Walled Carbon Nanotubes

The ultrafast dynamics study of photoexcitations in semiconducting and metallic single-walled carbon nanotubes (SWNT) was presented. Various pump-probe wavelengths and intensities were used in studying the femtosecond dynamics of photoexcitations. The ultrafast response in the SWNTs was dominated by confined excitons and charge carriers with subpicosecond dynamics. The analysis of exciton excited state absorption band and multiphoton absorption resonance revealed that the transitions between the subbands were allowed

Ultrafast Spectroscopy of Excitons in Single-Walled Carbon Nanotubes

The ultrafast dynamics study of photoexcitations in semiconducting and metallic single-walled carbon nanotubes (SWNT) was presented. Various pump-probe wavelengths and intensities were used in studying the femtosecond dynamics of photoexcitations. The ultrafast response in the SWNTs was dominated by confined excitons and charge carriers with subpicosecond dynamics. The analysis of exciton excited state absorption band and multiphoton absorption resonance revealed that the transitions between the subbands were allowed

Photoexcitation dynamics in regioregular and regiorandom polythiophene films

Using a variety of optical probes techniques we studied the photoexcitation dynamics in thin films of poly-3-hexyl thiophene with regioregular order that forms lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chainlike morphology. In regiorandom films we found that intrachain excitons with correlated induced absorption and stimulated emission bands are the primary excitations; they give rise to a moderately strong photoluminescence band. In regioregular films, on the contrary, we found that the primary excitations are excitons with a much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and very weak photoluminescence band. In regioregular films we also measured photogenerated geminate polaron pairs with ultrafast dynamics that are precursor to long-lived polaron excitations

Spectroscopic studies of photoexcitations in regioregular and regiorandom polythiophene films

Optical probe techniques were used to study the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly(3-hexyl thiophene)s with regioregular order, which form self-assembled lamellae structures with increased interchain interactions and regiorandom order that preserve a chain-like morphol. In regiorandom polythiophene films intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long-lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a weak photoluminescence band. The long-lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with small relaxation energy. The DP species were thoroughly studied as a function of the alkyl side group of the polymer backbone, film deposition conditions, and solvents, at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced IR active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band were studied and explained by a Fano-type interference mechanism, using the amplitude mode mode

Size and mobility of excitons in (6,5) carbon nanotubes

reserved6Knowledge of excited-state dynamics in carbon nanotubes is determinant for their prospective use in optoelectronic applications. It is known that primary photoexcitations are quasi-one-dimensional excitons, the electron–hole correlation length ('exciton size') of which corresponds to a finite volume in the phase space. This volume can be directly measured by nonlinear spectroscopy provided the time resolution is short enough for probing before population relaxation. Here, we report on the experimental determination of exciton size and mobility in (6, 5) carbon nanotubes. The samples are sodium cholate suspensions of nanotubes (produced by the CoMoCat method) obtained by density-gradient ultracentrifugation. By using sub-15 fs near-infrared pulses to measure the nascent bleach of the lowest exciton resonance, we estimate the exciton size to be 2.00.7 nm. Exciton–exciton annihilation in our samples is found to be rather inefficient so that many excitons can coexist on a single nanotube.L. LUER; S. HOSEINKHANI; D. POLLI; J. CROCHET; T. HERTEL; G. LANZANIL., Luer; S., Hoseinkhani; Polli, Dario; J., Crochet; T., Hertel; Lanzani, Guglielm

Real time observation of non-linear coherent phonon dynamics in single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are pi-conjugated, quasi-one-dimensional structures consisting of rolled-up graphene sheets that, depending on their chirality, behave as semiconductors or metals(1); owing to their unique properties, they enable groundbreaking applications in mechanics, nanoelectronics and photonics(2,3). In semiconducting SWNTs, medium-sized excitons (3-5 nm) with large binding energy and oscillator strength are the fundamental excitations(4-8); exciton wavefunction localization and one-dimensionality give rise to a strong electron-phonon coupling(9-11), the study of which is crucial for the understanding of their electronic and optical properties. Here we report on the use of resonant sub-10-fs visible pulses(12) to generate and detect, in the time domain, coherent phonons in SWNT ensembles. We observe vibrational wavepackets for the radial breathing mode (RBM) and the G mode, and in particular their anharmonic coupling, resulting in a frequency modulation of the G mode by the RBM. Quantum-chemical modelling(13) shows that this effect is due to a corrugation of the SWNT surface on photoexcitation, leading to a coupling between longitudinal and radial vibrations