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 $T\propto\sqrt{V}$ behavior at high bias, which corresponds to a T4 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 $\Sigma$ in monolayer graphene. Our measurements point to an important effect of lattice disorder in the reduction of $\Sigma$, not yet considered by theory. Moreover, our study provides a strong and firm support to the rising field of graphene bolometric detectors.Comment: 5 figure

Synthesis, antitubercular activity and mechanism of resistance of highly effective thiacetazone analogues

Defining the pharmacological target(s) of currently used drugs and developing new analogues with greater potency are both important aspects of the search for agents that are effective against drug-sensitive and drug-resistant Mycobacterium tuberculosis. Thiacetazone (TAC) is an anti-tubercular drug that was formerly used in conjunction with isoniazid, but removed from the antitubercular chemotherapeutic arsenal due to toxic side effects. However, several recent studies have linked the mechanisms of action of TAC to mycolic acid metabolism and TAC-derived analogues have shown increased potency against M. tuberculosis. To obtain new insights into the molecular mechanisms of TAC resistance, we isolated and analyzed 10 mutants of M. tuberculosis that were highly resistant to TAC. One strain was found to be mutated in the methyltransferase MmaA4 at Gly101, consistent with its lack of oxygenated mycolic acids. All remaining strains harbored missense mutations in either HadA (at Cys61) or HadC (at Val85, Lys157 or Thr123), which are components of the bhydroxyacyl-ACP dehydratase complex that participates in the mycolic acid elongation step. Separately, a library of 31 new TAC analogues was synthesized and evaluated against M. tuberculosis. Two of these compounds, 15 and 16, exhibited minimal inhibitory concentrations 10-fold lower than the parental molecule, and inhibited mycolic acid biosynthesis in a dose-dependent manner. Moreover, overexpression of HadAB HadBC or HadABC in M. tuberculosis led to high level resistance to these compounds, demonstrating that their mode of action is similar to that of TAC. In summary, this study uncovered new mutations associated with TAC resistance and also demonstrated that simple structural optimization of the TAC scaffold was possible and may lead to a new generation of TAC-derived drug candidates for the potential treatment of tuberculosis as mycolic acid inhibitors

Projet e-DENT : téléconsultation bucco-dentaire en EHPAD

International audienc

Electron and Lattice Heating Contributions to the Transient Optical Response of a Single Plasmonic Nano-Object

The sudden absorption of light by a metal nanoparticle launches a series of relaxation processes (internal thermalization, acoustic vibrations, and cooling) which induce a transient modification of its optical response. In this work, the transient optical response associated with the internal thermalization of a single gold nanodisk (occurring on a few picoseconds time scale) was quantitatively investigated by time-resolved spectroscopy experiments, and the measured signals were compared with a model accounting for the effects of both electron and ionic lattice heating. We show that experimental time-resolved signals at delays posterior to nanodisk excitation and electron gas thermalization can be simply interpreted as a combination of electron and lattice temperature evolutions, with probe wavelength-dependent weights. This demonstrates the possibility to selectively probe the electronic or lattice dynamics, through the choice of specific probe wavelengths. Additionally, the time-dependent spectral shape of transient extinction cross-section changes is shown to be successively dominated by the effects of electron and lattice heating, which present distinct spectral signatures

Biexcitons in semiconducting carbon nanotubes

International audienceSingle-walled carbon nanotubes (SWNT) are one-dimensional nanostructures where the Coulomb interactions between charge carriers are strongly enhanced compared to systems of higher dimensionality. This results in an electron-hole bound state -the so-called exciton- with a binding energy of the order of one third of the bandgap, which controls the SWNT's optical properties. As a matter of fact, the exciton-exciton interactions are particularly efficient and drive the exciton recombination and dephasing dynamics. The investigation of excitonic complexes in semiconducting SWNTs is currently a topic of intense debate. The biexciton and the trion are expected to have a binding energy of about a hundred meVs. Whereas the trion has been recently observed, first in doped nanotubes and then by means of all-optical generation, there is no experimental evidence for the biexciton. Here we present the first observation of the biexciton in semiconducting single-wall carbon nanotubes using nonlinear optical spectroscopy. Our experiments consist in a spectrally resolved pump-probe technique in SWNTs embedded in a gelatine at cryogenic temperature. Our measurements of the differential transmission spectrum reveal the universal asymmetric line shape of the Fano resonance intrinsic to the biexciton transition

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

International audienc

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

International audienceCarbon 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]