T he main obstacle to the use of graphene on the industrial scale is the growth of a large and homogenous monolayer graphene Concerning this issue, it is worthnoting that our group has recently developed a reproducible and controlled growth process of a monolayer graphene on SiC 0001 by sublimation at low Ar pressure i e 10 mbar Still, the control of the electronic properties of the obtained graphene by this process is very challenging E g the mobility on our graphene on 4 H SiC (0001) is around 2000 cm 2 v 1 s 1 at RT which is in the range of the measured mobilities on similar substrates Yet, it is still very low when compared with the mobilities reported for suspended graphene. It is well acceptedthat the electronic properties of graphene on SiC are highly sensitive to the substrate underneath It was reported that the mobility of graphene on SiC 0001 increases with increasing SiC steps width, and its resistance increases with increasing SiC steps height. This means that the electronic properties of graphene on SiC 0001 can be tuned by controlling the height and width of the terraces that results from the surface reconstruction of SiC before the growth i e Step bunching phenomenon

We study the low frequency admittance of a quantum Hall bar of size much larger than the electronic coherence length. We find that this macroscopic conductor behaves as an ideal quantum conductor with vanishing longitudinal resistance and purely inductive behavior up to f 1 MHz. Using several measurement configurations, we study the dependence of this inductance on the length of the edge channel and on the integer quantum Hall filling fraction. The experimental data are well described by a scattering model for edge magnetoplasmons taking into account effective long range Coulomb interactions within the sample. This demonstrates that the inductance's dependence on the filling fraction arises from the effective quantum inertia of charge carriers induced by Coulomb interactions within an ungated macroscopic quantum Hall bar.

Amorphous solids lack long-range order. Therefore identifying structural defects -- akin to dislocations in crystalline solids -- that carry plastic flow in these systems remains a daunting challenge. By comparing many different structural indicators in computational models of glasses, under a variety of conditions we carefully assess which of these indicators are able to robustly identify the structural defects responsible for plastic flow in amorphous solids. We further demonstrate that the density of defects changes as a function of material preparation and strain in a manner that is highly correlated with the macroscopic material response. Our work represents an important step towards predicting how and when an amorphous solid will fail from its microscopic structure.

AlGaN based light emitting diodes (LEDs) will play a key role for the development of applications in the ultra-violet (UV). In the UVB region (280–320 nm), phototherapy and plant lighting are among the targeted uses. However, UVB LED performances still need to be improved to reach commercial markets. In particular, the design and the fabrication process of the active region are central elements that affect the LED internal quantum efficiency (IQE). We propose the use of nanometer-sized epitaxial islands (i.e., so called quantum dots (QDs)) to enhance the carrier localization and improve the IQE of molecular beam epitaxy (MBE) grown UVB LEDs using sapphire substrates with thin sub-μm AlN templates. Taking advantage of the epitaxial stress, AlGaN QDs with nanometer-sized (≤10 nm) lateral and vertical dimensions have been grown by MBE. The IQE of the QDs has been deduced from temperature dependent and time resolved photoluminescence measurements. Room temperature IQE values around 5 to 10% have been found in the 290–320 nm range. QD-based UVB LEDs were then fabricated and characterized by electrical and electroluminescence measurements. On-wafer measurements showed optical powers up to 0.25 mW with external quantum efficiency (EQE) values around 0.1% in the 305–320 nm range.

The antagonistic effect of processing and thermal annealing on both the filler structure and the polymer matrix is explored in polymer nanocomposites based on natural rubber with precipitated silica incorporated by coagulation from aqueous suspension followed by roll-milling. Their structure and linear and non-linear rheology have been studied, with a particular emphasis on the effect of high temperature thermal treatment and the number of milling passes. Small-angle X-ray scattering intensities show that the silica is organized in small, unbreakable aggregates containing ca. 50 primary nanoparticles, which are reorganized on a larger scale in filler networks percolating at the highest silica contents. As expected, the filler network structure is found to be sensitive to milling, more milling inducing rupture, as evidenced by the decreasing Payne effect. After thermal treatment, the nanocomposite structure is found to be rejuvenated, erasing the effect of the previous milling on the low-strain modulus. In parallel, the dynamics of the samples described by the rheology or the calorimetric glass-transition temperature remain unchanged, whereas the natural latex polymer network structure is modified by milling towards a more fluid-like rheology, and cannot be recovered.

We analyze the multilayer structure of sunflower leaves from Terahertz data measured in the time-domain at a ps scale. Thin film reverse engineering techniques are applied to the Fourier amplitude of the reflected and transmitted signals in the frequency range f < 1.5 Terahertz (THz). Validation is first performed with success on etalon samples. The optimal structure of the leaf is found to be a 8-layer stack, in good agreement with microscopy investigations. Results may open the door to a complementary classification of leaves.