Fast dynamics of supercoiled DNA revealed by single-molecule experiments.

The dynamics of supercoiled DNA play an important role in various cellular processes such as transcription and replication that involve DNA supercoiling. We present experiments that enhance our understanding of these dynamics by measuring the intrinsic response of single DNA molecules to sudden changes in tension or torsion. The observed dynamics can be accurately described by quasistatic models, independent of the degree of supercoiling initially present in the molecules. In particular, the dynamics are not affected by the continuous removal of the plectonemes. These results set an upper bound on the hydrodynamic drag opposing plectoneme removal, and thus provide a quantitative baseline for the dynamics of bare DNA

Spin stress contribution to the lattice dynamics of FePt

Invar-behavior occurring in many magnetic materials has long been of interest to materials science. Here, we show not only invar behavior of a continuous film of FePt but also even negative thermal expansion of FePt nanograins upon equilibrium heating. Yet, both samples exhibit pronounced transient expansion upon laser heating in femtosecond x-ray diffraction experiments. We show that the granular microstructure is essential to support the contractive out-of-plane stresses originating from in-plane expansion via the Poisson effect that add to the uniaxial contractive stress driven by spin disorder. We prove the spin contribution by saturating the magnetic excitations with a first laser pulse and then detecting the purely expansive response to a second pulse. The contractive spin stress is reestablished on the same 100-ps time scale that we observe for the recovery of the ferromagnetic order. Finite-element modeling of the mechanical response of FePt nanosystems confirms the morphology dependence of the dynamics

Mechanical Coupling in Gold Nanoparticles Supermolecules Revealed by Plasmon-Enhanced Ultralow Frequency Raman Spectroscopy

International audienceAcoustic vibrations of assemblies of gold nanoparticles were investigated using ultralow frequency micro-Raman scattering and finite element simulations. When exciting the assemblies resonantly with the surface plasmon resonance of electromagnetically coupled nano-particles, Raman spectra present an ultralow frequency band whose frequency lies below the lowest Raman active Lamb mode of single nanoparticles that was observed. This feature was ascribed to a Raman vibration mode of gold nanoparticle " supermolecules " , that is, nanoparticles mechanically coupled by surrounding polymer molecules. Its measured frequency is inversely proportional to the nanoparticle diameter and sensitive to the elastic properties of the interstitial polymer. The latter dependence as well as finite element simulations suggest that this mode corresponds to the out-of-phase semirigid translation (l = 1 Lamb mode) of each nanoparticle of a dimer inside the matrix, activated by the mechanical coupling between the nanoparticles. These observations were permitted only thanks to the resonant excitation with the coupling plasmon excitation, leading to an enhancement up to 10^4 of the scattering by these vibrations. This enhanced ultralow frequency Raman scattering thus opens a new route to probe the local elastic properties of the surrounding medium

Vibrations of spherical core-shell nanoparticles

International audienceThe acoustic vibration modes of spherical composite nanoparticles formed by a core and an arbitrary number of shells are computed using a semianalytic approach based on elastic theory. The modes observed in time-resolved pump-probe experiments are identified in the case of metal-dielectric-based spherical composite nanoparticles, and results are illustrated in the case of Ag@SiO2 core-shell nanoparticles. The presence of a light dielectric shell is shown to only weakly shift the frequency of the dominant mode observed in time-resolved experiments. Moreover, a large impact of the mechanical contact between the different materials forming the particle on the vibrational mode frequency and damping is predicted, offering the possibility of experimentally addressing this parameter

Probing Elasticity at the Nanoscale: Terahertz Acoustic Vibration of Small Metal Nanoparticles

International audienceThe acoustic response of surface-controlled metal (Pt) nanoparticles is investigated in the small size range, between 1.3 and 3 nm (i.e., 75-950 atoms), using time-resolved spectroscopy. Acoustic vibration of the nanoparticles is demonstrated, with frequencies ranging from 1.1 to 2.6 THz, opening the way to the development of THz acoustic resonators. The frequencies, measured with a noncontact optical method, are in excellent agreement with the prediction of a macroscopic approach based on the continuous elastic model, together with the bulk material elastic constants. This demonstrates the validity of this model at the nanoscale and the weak impact of size reduction on the elastic properties of a material, even for nanoparticles formed by less than 100 atoms

Ultrafast photoinduced charge separation in metal-semiconductor nanohybrids

International audienceHybrid nano-objects formed by two or more disparate materials are among the most promising and versatile nanosystems. A key parameter in their properties is interaction between their components. In this context we have investigated ultrafast charge separation in semiconductor-metal nanohybrids using a model system of gold-tipped CdS nanorods in a matchstick architecture. Experiments are performed using an optical time-resolved pump-probe technique, exciting either the semiconductor or the metal component of the particles, and probing the light-induced change of their optical response. Electron-hole pairs photoexcited in the semiconductor part of the nanohybrids are shown to undergo rapid charge separation with the electron transferred to the metal part on a sub-20 fs time scale. This ultrafast gold charging leads to a transient red-shift and broadening of the metal surface plasmon resonance, in agreement with results for free clusters but in contrast to observation for static charging of gold nanoparticles in liquid environments. Quantitative comparison with a theoretical model is in excellent agreement with the experimental results, confirming photoexcitation of one electron-hole pair per nanohybrid followed by ultrafast charge separation. The results also point to the utilization of such metal-semiconductor nanohybrids in light-harvesting applications and in photocatalysis

Inelastic Light Scattering by Multiple Vibrational Modes in Individual Gold Nanodimers

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