Nonequilibrium dynamics of TO phonons and low-frequency interlayer shear modes are mapped in femtosecond mid-infrared pump-probe experiments. Time-resolved changes of TO phonon absorption reveal a TO phonon lifetime of 1.2 ps, while the low-frequency excitations decay with a time constant of approximately 20 ps. The coupling of intralayer TO and interlayer motions manifests in a quasi-instantaneous redshift of the TO phonon resonance by some 10 cm −1. Theoretical calculations account for the coupling scenario and underline the relevance of inter-phonon interactions for the nonlinear phonon response of few-layer materials.

Deep ultraviolet (DUV) light emitting diodes (LED) are expected to be the next generation of UV sources, offering significant advantages such as compactness, low consumption and long lifetimes. Yet, improvements of their performances are still required and the potential of AlyGa1-yN quantum dots as DUV emitters is investigated in this study. Using a stress induced growth mode transition, quantum dots (QD) are spontaneously formed on Al0.7Ga0.3N/AlN heterostructures grown on sapphire substrates by molecular beam epitaxy. By increasing the QD Al composition, a large shift of the QD photoluminescence in the UV range is observed, going from an emission in the near UV for GaN QD down to the UVC region for Al0.4Ga0.6N QD. A similar behavior is observed for electroluminescence (EL) measurements performed on LED structures and an emission ranging from the UVA (320-340 nm) down to the UVC (265-280 nm) has been obtained. The main performances of Al0.7Ga0.3N based QD LED are presented in terms of electrical and optical characteristics. In particular, the emission dependence on the input current density, including the emitted wavelength, the optical power and the external quantum efficiency are shown and discussed.

Being a flexible wide band gap semiconductor, hexagonal boron nitride (h-BN) has great potential for technological applications like efficient deep ultraviolet light sources, building block for two-dimensional heterostructures and room temperature single photon emitters in the ultraviolet and visible spectral range. To enable such applications, it is mandatory to reach a better understanding of the electronic and optical properties of h-BN and the impact of various structural defects. Despite the large efforts in the last years, aspects such as the electronic band gap value, the exciton binding energy and the effect of point defects remained elusive, particularly when considering a single monolayer. Here, we directly measured the density of states of a single monolayer of h-BN epitaxially grown on highly oriented pyrolytic graphite, by performing low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The observed h-BN electronic band gap on defect-free regions is (6.8 ± 0.2) eV. Using optical spectroscopy to obtain the h-BN optical band gap, the exciton binding energy is determined as being of (0.7 ± 0.2) eV. In addition, the locally excited cathodoluminescence and photoluminescence show complex spectra that are typically associated to intragap states related to carbon defects. Moreover, in some regions of the monolayer h-BN we identify, using STM, point defects which have intragap electronic levels around 2.0 eV below the Fermi level.

Aggregation mechanisms of amyloid β peptides depend on multiple intrinsic and extrinsic physicochemical factors (e.g., peptide chain length, truncation, peptide concentration, pH, ionic strength, temperature, metal concentration, etc.). Due to this high number of parameters, the formation of oligomers and their propensity to aggregate make the elucidation of this physiopathological mechanism a challenging task. From the analytical point of view, up to our knowledge, few techniques are able to quantify, in real time, the proportion and the size of the different soluble species during the aggregation process. This work aims at demonstrating the efficacy of the modern Taylor dispersion analysis (TDA) performed in capillaries (50 μm i.d.) to unravel the speciation of β-amyloid peptides in low-volume peptide samples (∼100 μL) with an analysis time of ∼3 min per run. TDA was applied to study the aggregation process of Aβ(1-40) and Aβ(1-42) peptides at physiological pH and temperature, where more than 140 data points were generated with a total volume of ∼1 μL over the whole aggregation study (about 0.5 μg of peptides). TDA was able to give a complete and quantitative picture of the Aβ speciation during the aggregation process, including the sizing of the oligomers and protofibrils, the consumption of the monomer, and the quantification of different early- and late-formed aggregated species.

I'll explain several lessons suggested for the issues of emergence of spacetime and background independence in quantum gravity by models of non-critical and topological strings.

Rubber-based nanocomposites prepared by solid-phase mixing with precipitated silica nanoparticles are typically strongly aggregated systems with different levels of spatial organization. The strategy that we developed several years ago to investigate such systems is based on the study of simplified industrial samples with ingredients limited to a strict minimum. On the scale of a micrometric simulation box, tens of thousands of nanoparticles are embedded in the matrix, and their dispersion strongly affects the mechanical properties of the material – and which is reflected in the scattered intensity. A statistical method based on a reverse Monte Carlo solution of this many-parameter problem will be presented, showing that some key features like percolation can be described. [1]Another key feature of rubber nanocomposites refers to the influence of the filler surfaces on the polymer dynamics, and some recent progress – including incoherent neutron spin-echo measurements – will be discussed. [2] Such results may be confronted to dielectric spectroscopy measurements which in some cases are not conclusive due the impossibility to correctly resolve the α-relaxation in presence of strong interfacial polarization processes. An original application of dielectric spectroscopy with a surprising measurement of the adsorption isotherm of coating agents onto silica buried in the polymer matrix will also be discussed. [3]References[1] Musino D, Genix A-C, Chauveau E, Bizien T, Oberdisse J, Nanoscale 2020, 12:3907.[2] Musino D, Oberdisse J, Farago B, Alegria A, Genix A-C, ACS Macro Letters 2020, 9:910.[3] Musino D, Oberdisse J, Sztucki M, Alegria A, Genix A-C, Macromolecules 2020, 53:8083.