We have used optical spectroscopy to observe spectral broadening of WS2 exciton reflectance peaks in heterostructures of monolayer WS2 capped with mono- to few-layer graphene. The broadening is found to be similar for the A and B excitons and on the order of 5-10 meV. No strong dependence on the number of graphene layers was observed within experimental uncertainty. The broadening can be attributed to charge- and energy-transfer processes between the two materials, providing an observed lower bound for the corresponding time scales of 65 fs.

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.

We report efficient nonradiative energy transfer (NRET) from core shell, semiconducting quantum dots to adjacent two-dimensional sheets of graphene and MoS2 of single- and few-layer thickness. We observe quenching of the photoluminescence (PL) from individual quantum dots and enhanced PL decay rates in time-resolved PL, corresponding to energy transfer rates of 1-10 ns(-1). Our measurements reveal contrasting trends in the NRET rate from the quantum dot to the van der Waals material as a function of thickness. The rate increases significantly with increasing layer thickness of graphene, but decreases with increasing thickness of MoS2 layers. A classical electromagnetic theory accounts for both the trends and absolute rates observed for the NRET. The countervailing trends arise from the competition between screening and absorption of the electric field of the quantum dot dipole inside the acceptor layers. We extend our analysis to predict the type of NRET behavior for the near-field coupling of a chromophore to a range of semiconducting and metallic thin film materials.

We have identified excited exciton states in monolayers of MoS2 and WS2 supported on fused silica by means of photoluminescence excitation spectroscopy. In monolayer WS2, the positions of the excited A exciton states imply an exciton binding energy of 0.32 eV. In monolayer MoS2, excited exciton transitions are observed at energies of 2.24 and 2.34 eV. Assigning these states to the B exciton Rydberg series yields an exciton binding energy of 0.44 eV.

At low temperature the photoluminescence of single-wall carbon nanotubes show a large variety of spectral profiles ranging from ultranarrow lines in suspended nanotubes to broad and asymmetrical line shapes that puzzle the current interpretation in terms of exciton-phonon coupling. Here, we present a complete set of photoluminescence profiles in matrix embedded nanotubes including unprecedented narrow emission lines. We demonstrate that the diversity of the low-temperature luminescence profiles in nanotubes originates in tiny modifications of their low-energy acoustic phonon modes. When low-energy modes are locally suppressed, a sharp photoluminescence line as narrow as 0.7 meV is restored. Furthermore, multipeak luminescence profiles with specific temperature dependence show the presence of confined phonon modes.

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 report on the realization of monolithic optical microcavities using a single wall carbon nanotubes doped polymer as active material. Thanks to the control of the polymer thickness, a fine control of the cavity mode energy is achieved, which allows to tune it in exact resonance with a specific chiral species emission line. The quality factor of the filled cavity mode (Q = 40) allows to selectively extract the luminescence of the (7,5) chiral species. Finally, angle resolved experiments show the tunability of the emission energy within a 150 meV range. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4801984]

The coupling between a localized emitter and acoustic phonons gives rise to characteristic signatures in the PL spectra of nanostructures, known as Huang-Rhys side bands. Such spectra consist of a narrow central line - the so called zero-phonon line (ZPL) - and of a weak temperature dependent red shifted (resp. blue shifted) side bands corresponding to the simultaneous emission of a photon and a phonon (resp. emission of a photon and absorption of a phonon). In the case of carbon nanotubes the one dimensional nature of the phonon bath leads to a drastic enhancement of this coupling for low energy phonons, which results in a full merging of the ZPL line into the phonon wing [1]. Therefore, typical low temperature PL spectra show a broad (5-10 meV) and asymmetric line-shape. This property may be seen as particularly detrimental for photonic applications since it intrinsically limits the ultimate coherence length of the PL line. We show that this limitation can be bypassed when the vibrational properties of the nanotube are slightly modified by achieving a weak coupling to the substrate. In this case, a bright and narrow (down to ~500 µeV) ZPL is retrieved. We present low temperature PL spectra of a large set of CoMoCat nanotubes and show that many of the exotic PL spectra, including multi-peak spectra, are perfectly reproduced by this model when considering the different possible relative positions of the exciton and the mechanical contact zone(s).

We present a review on the excitonic homogeneous linewidth, with an emphasis on original experiments by means of nonlinear spectral hole-burning. We address three specific aspects of the excitonic homogeneous broadening: (i) the variation of the homogeneous linewidth with the sonication energy, revealing the influence of extrinsic processes such as exciton-defect interactions, (ii) the collision-induced broadening due to exciton–exciton interactions, investigated by power-dependent measurements, and (iii) the phonon-induced dephasing due to exciton–phonon interactions, deduced from temperature-dependent experiments. We discuss the specific properties of one-dimensional excitons in carbon nanotubes in the context of the homogeneous broadening.

The delocalized pi-electronic system of carbon nanotubes allows them to link non-covalently to a large variety of organic molecules. In contrast to covalent functionalization, this mild interaction preserves most of the intrinsic nanotubes properties (photoluminescence, mobility...) but still leads to a strong enough coupling so to give stable supramolecular assemblies and to induce new and efficient functionalities [1]. We focus on nanotubes / chromophores compounds where the latter acts as a nano optical antenna that absorbs light and then transfer almost 100% of the energy to the nanotube [2, 3, 4, 5]. Compounds made of different chromophores such as metal- or free base porphyrins, porphyrin polymers, and cyanines were synthesized by means of the micelle swelling method [1, 7, 8]. Energy transfer was analysed both on ensembles [5, 6] and at the single compound level [9]. We will first discuss the energy transfer mechanisms in these kinds of supramolecular assemblies [5-9]. In a second part, we will show that the new functionalities created by the molecules allow to gain new insight into the intrinsic electronic properties of nanotubes [10] . References [1] C. Roquelet, et al, ChemPhysChem, 11 (2010), 1667 [2] G. Magadur et al. ChemPhysChem, 9 (2008),pp 1250 [3] J. P. Casey et al, Journal of Materials Chemistry, 18, 1510 (2008) [4] S. D. Stranks et al, ACS Nano, 5, (2011), 2307 [5] C. Roquelet, et al, Appl. Phys. Lett. 97 (2010), 141918 [6] F. Ernst et al, Advanced functional materials, 22 (2012), 3921. [6] D. Garrot et al, J. Phys. Chem C 115, (2011) 23283 [7] C. Roquelet, et al ChemPhys in press (http://dx.doi.org/10.1016/j.chemphys.2012.09.004) [8] G. Clave, et al submitted [9] C. Roquelet et al, ACS Nano 6, (2012), 8796. [10] C. Roquelet et al, in preparation

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.

Energy transfer in noncovalently bound porphyrin/carbon nanotube compounds is investigated at the single-nanocompound scale. Excitation spectroscopy of the luminescence of the nanotube shows two resonances arising from intrinsic excitation of the nanotube and from energy transfer from the porphyrin. Polarization diagrams show that both resonances are highly anisotropic, with a preferred direction along the tube axis. The energy transfer is thus strongly anisotropic despite the almost isotropic absorption of porphyrins. We account for this result by local field effects induced by the large optical polarizability of nanotubes. We show that the local field correction extends over several nanometers outside the nanotubes and drives the overall optical response of functionalized nanotubes.

Excitons are composite bosons that allow a fair description of the optical properties in solid state systems. The quantum confinement in nanostructures enhances the excitonic effects and impacts the excitonexciton interactions, which tailor the performances of classical and quantum optoelectronic devices, such as lasers or single-photon emitters. The excitonic nonlinearities exhibit significant differences between organic and inorganic compounds. Tightly bound Frenkel excitons in molecular crystals are for instance affected by an efficient excitonexciton annihilation (EEA). This Auger process also governs the population relaxation dynamics in carbon nanotubes that share many physical properties with organic materials. Here, we show that this similarity breaks down for the excitonic decoherence in carbon nanotubes. Original nonlinear spectral-hole burning experiments bring evidence of pure dephasing induced by excitonexciton scattering (EES) in the k-space. This mechanism controls the exciton collision-induced broadening, as for Wannier excitons in inorganic semiconductors. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems.

Dans le domaine du photovoltaique, les cellules hybrides organiques sont une des voies les plus prometteuses, notamment grâce aux proprietes de collection de lumiere des molecules de type chromophore. Les nanotubes de carbone, quant a eux, sont des nano-objets quasi unidimensionnels qui presentent des proprietes de transport exceptionnelles. La realisation d’un couplage important entre une molecule collectrice de lumiere et un nanotube de carbone represente donc une voie importante a explorer. Ce travail de recherche est consacre a l’etude du transfert d’energie dans les composes nanotubes de carbone/chromophore. Une nouvelle methode de fonctionnalisation non covalente des nanotubes de carbone est presentee. Basee sur une suspension micellaire de nanotubes, cette methode permet d’obtenir un fort taux de fonctionnalisation tout en preservant les proprietes intrinseques des nanotubes. Le transfert d’energie est mis en evidence sur les composes nanotube/porphyrine par des mesures d’excitation de la photoluminescence sur ensemble de nanotubes ainsi que sur objets uniques. L’evaluation du rendement quantique de transfert par trois methodes independantes montre un couplage de l’ordre de 100% entre la molecule et le nanotube- et ce malgre la faiblesse des interactions entre orbitales «Pi» mises en jeu dans la fonctionnalisation non covalente. Le dernier volet de ce travail est consacre a des mesures d’anisotropie a l’echelle de l’objet unique permettant d’obtenir des informations quant a l’arrangement des molecules a la surface des nanotubes.