Géraud Delport

Chirality Dependence of the Absorption Cross Section of Carbon Nanotubes

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.

Strong reduction of exciton-phonon coupling in high crystalline quality single-wall carbon nanotubes: a new insight into broadening mechanisms and exciton localization

Carbon nanotubes are quantum sources whose emission can be tuned at telecommunication wavelengths by choosing the diameter appropriately. Most applications require the smallest possible linewidth. Therefore, the study of the underlying dephasing mechanisms is of utmost interest. Here, we report on the low-temperature photoluminescence of high crystalline quality individual single-wall carbon nanotubes synthesized by laser ablation (L-SWNTs) and emitting at telecom-munication wavelengths. A thorough statistical analysis of their emission spectra reveals a typical linewidth one order of magnitude narrower than that of most samples reported in the literature. The narrowing of the PL line of L-SWNTs is due to a weaker effective exciton-phonon coupling subsequent to a weaker localization of the exciton. These results suggest that exciton localization in SWNTs not only arises from interfacial effects, but that the intrinsic crystalline quality of the SWNT plays an important role. Photoluminescence (PL) emission in semiconducting carbon nanotubes arises from exciton recombination [1–3] and has been extensively studied in view of possible applications in opto-electronics, bio-imaging or photovoltaics [4–7]. Observation of photon antibunching in the near infrared [8, 9] suggests that SWNTs are also promising single-photon sources for the implementation of quantum information protocols. Interestingly, the PL emission energy (i.e. the excitonic recombination energy) strongly depends on the tube diameter and can be easily tuned in the telecommunication bands at 0.83 eV (1.5µm) by choosing SWNTs with a diameter of about 1-1.2 nm [10]. SWNTs could therefore make up a very versatile light source for quantum optics. Several studies suggested that the optical properties of SWNTs at low temperature are best described in terms of localized excitons (zero-dimensional confinement), leading to a quantum dot like behavior [11, 12]. Nevertheless, the nature of the traps responsible for this exciton localization is not elucidated yet. In order to address the issue of exciton localization, we studied carbon nanotubes produced by high-temperature synthesis methods such as electric arc or laser ablation methods, which are known for their higher crystalline quality, with a lower density of defects [13–17].

Davydov Splitting and Self-Organization in a Porphyrin Layer Noncovalently Attached to Single Wall Carbon Nanotubes

We study the ability of porphyrin molecules to cooperate upon adsorption on the sp2 curved surface of carbon nanotube. We discuss the role of the phenyl substituents in the cooperativity of the functionalization reaction. Moreover, a specific spatial organization of the molecules around the nanotube is unveiled through polarization sensitive experiments. Furthermore, we observe an increase of the energy splitting of the porphyrin main transition upon the adsorption on the nanotube. This effect, interpreted as a Davydov splitting, is analyzed quantitatively using a dipole-dipole coupling model. This study demonstrates the ability of porphyrin molecules to create an organized self-assembled layer at the surface of the nanotubes where molecules are electronically coupled together.