Absorption spectroscopy of large ensembles of carbon nanotubes show marked resonances, attributed to the different exitonic levels of each chiral species, as well as a relatively smooth background. This signal consists of a large number of different intrinsic and extrinsic contributions, such as phonon sidebands, amorphous carbon absorption, light scattering... It is therefore challenging to extract any quantitative information from these direct absorbance measurements. In contrast, photoluminescence excitation measurements in semi-conducting nanotubes give access to the intrinsic non-resonant absorption contribution between the excitonic resonances. Using a global analysis method, we were able to delineate the contribution of each chiral species including its tiny non-resonant component that shows up as 0.5 eV wide plateaus between the excitonic resonances. By comparison with the recently reported excitonic absorption cross-section on the S22 resonance, we found a universal non-resonant absorption which turns out to be of the order of one half of that of an equivalent graphene sheet. This value as well as the absorption line-shape in the non-resonant window is in excellent agreement with microscopic calculations. This non-resonant absorption cross-section is the same (within our resolution) for all the chiral species we measured in this study.

We report on the nonlinear optical response of a monochiral sample of (6,5) single-wall carbon nanotubes by means of broadband two-color pump-probe spectroscopy with selective excitation of the S-11 excitons. By using a moment analysis of the transient spectra, we show that all the nonlinear features can be accurately accounted for by elementary deformations of the linear absorption spectrum. The photogeneration of S-11 excitons induces a broadening and a blueshift of both the S-11 and S-22 excitonic transitions. In contrast, only the S-11 transition shows a reduction of oscillator strength, ruling out population up-conversion. These nonlinear signatures result from many-body effects, including phase-space filling, wave-function renormalization, and exciton collisions. This framework is sufficient to interpret the magnitude of the observed nonlinearities and stress the importance of intersubband exciton interactions. Remarkably, we show that these intersubband interactions have the same magnitude as the intraband ones and bring the major contribution to the photobleaching of the S-22 excitonic transition upon S-11 excitation through energy shift and broadening.

The delocalized π-electronic system of carbon nanotubes allows them to link non-covalently to a lot of different organic molecules. In contrast to covalent functionalization, this weak interaction preserves most of the nanotubes intrinsic properties (photoluminescence or mobility for instance) but still leads to a strong enough coupling so as to give stable compounds and to induce new and efficient functionalities [1, 2]. We focus on nanotubes / chromophores compounds where the latter acts as an optical nano antenna that absorbs light and transfers its energy to the nanotube, allowing a uniform excitation of all the chiral species in the sample [3, 4, 5, 6, 7]. In this presentation, we will focus on the latest developments of our research in this topic. References: [1] C. Roquelet, et al, ChemPhysChem 2010, 11, 1667 [2] G. Clave et al, Chem. Mat. 2013, 25, 2700 [3] G. Magadur et al. ChemPhysChem 2008, 9, 1250 [4] C. Roquelet, et al, Appl. Phys. Lett. 2010, 97,141918 [5] D. Garrot et al, J. Phys. Chem C 2011, 115, 23283 [6] C. Roquelet et al, ACS Nano 2012, 6, 8796) [7] F. Vialla et al, Phys. Rev. Lett. 2013, 111, 137402

Photoluminescence excitation measurements in semiconducting carbon nanotubes show a systematic nonresonant contribution between the well-known excitonic resonances. Using a global analysis method, we were able to delineate the contribution of each chiral species, including its tiny nonresonant component. By comparison with the recently reported excitonic absorption cross section on the S-22 resonance, we found a universal nonresonant absorbance which turns out to be of the order of one-half of that of an equivalent graphene sheet. This value, as well as the absorption line shape in the nonresonant window, is in excellent agreement with microscopic calculations based on the density-matrix formalism. This nonresonant absorption of semiconducting nanotubes is essentially frequency independent over 0.5-eV-wide windows and reaches approximately the same value between the S-11 and S-22 resonances and between the S-22 and S-33 resonances. In addition, the nonresonant absorption cross section turns out to be the same for all the chiral species we measured in this study. From a practical point of view, this study provides a solid framework for sample content analysis based on photoluminescence studies by targeting specific excitation wavelengths that lead to almost uniform excitation of all the chiral species of a sample within a given diameter range.

The fabrication of tailor-made functional hybrid materials that preserve and combine the properties of their building blocks is a central issue of nanosciences. In particular, the development of efficient techniques for the functionalization of carbon-based nanomaterials preserving their exceptional quality, while robustly enriching their. functionalities (in particular their optical properties), is highly desirable for demanding applications. In this work, we describe a new method of functionalization of carbon nanotubes which combines most advantages of both covalent and noncovalent methods without their principal drawbacks. Our method is based on the controlled polymerization of hydrophobic molecules on nanotubes dispersed in micelles. This approach permits us to obtain carbon nanotube hybrids exhibiting high stability while preserving their pi-conjugated system responsible for their outstanding optical and electrical properties. The nanotube hybrids can be purified, manipulated, and dispersed in various solvents without loss of their functionality. Extensive characterizations based on optical and microscopic measurements demonstrate the strength of this method for designing new functional materials.

We present recent developments in the synthesis and in the functional study of non covalently bound porphyrin/carbon nanotube compounds. The issue of the chemical stability of non covalent compounds is tackled by means of micelle assisted chemistry. The non covalent functionalization allows to preserve the electronic integrity of the nanotubes that display bright NIR luminescence. In the same time, the coupling between the subunits is very strong and leads to efficient energy transfer and PL quenching of the chromophore. This transfer occurs on a subpicosecond time-scale and leads to a near 100% efficiency. It allows to uniformly excite a whole set of chiral species with a single wavelength excitation. Insight into the transfer mechanism is gained by means of transient absorption spectroscopy.

The variation of the optical absorption of carbon nanotubes with their geometry has been a long-standing question at the heart of both metrological and applicative issues, in particular because optical spectroscopy is one of the primary tools for the assessment of the chiral species abundance of samples. Here, we tackle the chirality dependence of the optical absorption with an original method involving ultraefficient energy transfer in porphyrin-nanotube compounds that allows uniform photoexcitation of all chiral species. We measure the absolute absorption cross section of a wide range of semiconducting nanotubes at their S22 transition and show that it varies by up to a factor of 2.2 with the chiral angle, with type I nanotubes showing a larger absorption. In contrast, the luminescence quantum yield remains almost constant.

Single-wall carbon nanotubes noncovalently functionalized with porphyrin molecules have proven to be a very promising light harvesting system either for energy or charge transfer. In this work, we investigate the dynamics of this coupling at a subpicosecond time-scale, by means of transient absorption spectroscopy. We show that the ground state recovery time of the porphyrin is reduced by several orders of magnitude in the compound compared to the case of pristine porphyrin. Concomitantly, a strong bleaching signal is observed on the optical resonances of the nanotubes showing an ultrafast population buildup upon excitation of the porphyrin. We conclude that the energy transfer occurs on a time-scale shorter than 100 fs. Two-color measurements show that higher excited states of the nanotubes are populated on the same time-scale raising the point of the transfer mechanism. We briefly discuss two possible mechanisms.