Fabien Vialla

Hot electron cooling by acoustic phonons in graphene.

We have investigated the energy loss of hot electrons in metallic graphene by means of GHz noise thermometry at liquid helium temperature. We observe the electronic temperature T ∝ V at low bias in agreement with the heat diffusion to the leads described by the Wiedemann-Franz law. We report on T ∝ √V behavior at high bias, which corresponds to a T(4) dependence of the cooling power. This is the signature of a 2D acoustic phonon cooling mechanism. From a heat equation analysis of the two regimes we extract accurate values of the electron-acoustic phonon coupling constant Σ in monolayer graphene. Our measurements point to an important effect of lattice disorder in the reduction of Σ, not yet considered by theory. Moreover, our study provides a strong and firm support to the rising field of graphene bolometric detectors.

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.

Local Field Effects in the Energy Transfer between a Chromophore and a Carbon Nanotube: A Single-Nanocompound Investigation

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.

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.

Onset of optical-phonon cooling in multilayer graphene revealed by RF noise and black-body radiation thermometries

We report on electron cooling power measurements in few-layer graphene excited by Joule heating by means of a new setup combining electrical and optical probes of the electron and phonon baths temperatures. At low bias, noise thermometry allows us to retrieve the well known acoustic phonon cooling regimes below and above the Bloch-Grüneisen temperature, with additional control over the phonon bath temperature. At high electrical bias, we show the relevance of direct optical investigation of the electronic temperature by means of black-body radiation measurements. In this regime, the onset of new efficient relaxation pathways involving optical modes is observed.

Universal non-resonant absorption in carbon nanotubes

Photoluminescence excitation measurements in semi-conducting carbon nanotubes show a systematic non-resonant 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 non-resonant component. By comparison with the recently reported excitonic absorption cross-section on the S22 resonance, we found a universal non-resonant 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 non-resonant window is in excellent agreement with microscopic calculations based on the density matrix formalism. This non-resonant absorption of semi-conducting nanotubes is essentially frequency independent over 0.5 eV wide windows and reaches approximately the same value betweeen the S11 and S22 resonances or between the S22 and S33 resonances. In addition, the non-resonant 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 puts firm basis on the 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. In contrast to graphene, single-wall carbon nanotubes (SWNTs) show marked resonances in their optical spectrum that primarily reflect the one-dimensional quantum confinement of carriers. These resonances that combine one-dimensional and excitonic characteristics have been extensively investigated and are widely used as finger prints of the (n, m) species [1]. However, spectroscopic studies reveal that the absorption of nanotubes does not vanish between resonances and consists of a wealth of tiny structures, such as phonon side-bands, crossed exci-tons (S ij), or higher excitonic states [2–5]. In ensemble measurements, the non-resonant absorption is even more congested due to the contribution of residual catalyst or amorphous carbon and due to light scattering [6]. In total , a relatively smooth background showing an overall increase with photon energy is observed, from which it is challenging to extract any quantitative information. In this study, we show that thorough photolumines-cence excitation (PLE) measurements yield a much finer insight into the non-resonant absorption of carbon nan-otubes, that reveals the universal features of light-matter interaction in carbon nano-structures [7]. In particular, we show that the non-resonant absorption of SWNTs per unit area well above the S 11 or S 22 resonances reaches an universal value of 0.013±0.003 in good agreement with the value α √ 3 (where α is the fine structure constant) predicted by a simple band-to-band theory. Our study of non-resonant absorption is based on the global analysis of PLE maps of ensembles of carbon nan-otubes that allows us to deconvolute the contribution of each (n, m) species while keeping a high signal to noise