Quantitative characterization of the average structure of dense nanoparticle assemblies and aggregates is a common problem in nanoscience. Small-angle scattering is a suitable technique, but it is usually limited to not too big assemblies due to the limited experimental range, low concentrations to avoid interactions, and monodispersity to keep calculations tractable. In the present paper, a straightforward analysis of the generally available scattered intensity – even for large assemblies, at high concentrations – is detailed, providing information on the local volume fraction of polydisperse particles with hard sphere interactions. It is based on the identical local structure of infinite homogeneous nanoparticle assemblies and their subsets forming finite-sized clusters. The approach is extended to polydispersity, using Monte-Carlo simulations of hard and moderately sticky hard spheres. As a result, a simple relationship between the observed structure factor minimum – termed the correlation hole – and the average local volume fraction  on the scale of neighboring particles is proposed and validated through independent aggregate simulations. The relationship shall be useful as an efficient tool for the structural analysis of arbitrary aggregated colloidal systems.

We report direct and unambiguous evidence of the existence of inner semiconducting tube (ISCT) photoluminescence (PL) from measurements performed on four individual freestanding index-identified double-walled carbon nanotubes (DWNTs). Based on thorough Rayleigh scattering, Raman scattering, and PL experiments, we are able to demonstrate that the ISCT PL is observed with a quantum yield estimated to be a few 10−6 independent of the semiconducting or metallic nature of the outer tube. This result is mainly attributed to ultrafast exciton transfer from the inner to outer tube. Furthermore, by carrying out PL excitation experiments on the (14,1)@(15,12) DWNT, we show that the ISCT PL can be detected through the optical excitation of the outer tube, indicating that the exciton transfer can also occur in the opposite way.

We report picosecond acoustic measurements of longitudinal sound dispersion and attenuation in an amorphous SiO2 layer at temperatures from 20 to 300 K over frequencies ranging from about 40 to 200 GHz. The sample is a radio frequency cathodic sputtered silica layer grown on a sapphire substrate with an aluminum filmtransducer deposited on top. Acoustic attenuation is evaluated from the simultaneous analysis of three successive echoes using transfer matrix calculation. Results are found to follow rather well a model combining coupling to thermally activated relaxations of structural defects and interactions with thermal vibrations. This leads to a nontrivial variation of the attenuation coefficient with frequency and temperature. The number density of relaxing defects in the SiO2 layer is found to be slightly higher than that in bulk v-SiO2. In contrast, similar anharmonic contribution to acoustic absorption is observed in both systems. The velocity variations are also measured and are compared to the dynamical velocity changes deduced from the sound attenuation.

I will discuss here our recent results in isotopically-purified hBN crystals. We combined Raman scattering, photoluminescence, and X-ray diffraction for an in-depth characterization. On the one hand, as with standard semiconductors, both the phonon energy and electronic bandgap varied with the boron isotope mass, the latter due to the quantum effect of zero-point renormalization. On the other hand, temperature-dependent experiments focusing on the shear and breathing motions of adjacent layers revealed the specificity of isotope engineering in a layered material, with a modification of the Van der Waals interactions upon isotope purification.

I will discuss our recent studies showing that the optical response in hBN displays prominent evidence for phonon-assisted optical transitions, with a very unusual phenomenology. By two-photon spectroscopy, we demonstrated that the intrinsic optical properties at the band edge are characteristic of an indirect bandgap material. Polarization-resolved experiments with a detection from the sample edge allowed us to show that the phonon symmetries can be traced back in the optical response. I will further highlight the unique properties of this material where the optical response is tailored by the phonon group velocities in the middle of the Brillouin zone. I will finally present optical characterization results of the promising high-temperature MBE growth of hBN, in collaboration with Nottingham University.

I will discuss our recent studies showing that the optical response in hBN displays prominent evidence for phonon-assisted optical transitions, with a very unusual phenomenology. By two-photon spectroscopy, we demonstrated that the intrinsic optical properties at the band edge are characteristic of an indirect bandgap material. Polarization-resolved experiments with a detection from the sample edge allowed us to showthat the phonon symmetries can be traced back in the optical response. I will further highlight the unique properties of this material where the optical response is tailored by the phonon group velocities in the middle of the Brillouin zone. I will finally present optical characterization results of the promising high-temperature MBE growth of hBN, in collaboration with Nottingham University.