We report a detailed Raman study of single layergraphene dispersed in degassed water without additives (SLGiw), so-called “‘eau de graphene”’ (EdG).23 The most characteristic signature of SLGiw is a narrow (28 ± 2 cm−1) and symmetric 2D band. The intensity of the D band is dominated by the contribution of sp3 defects due to slight functionalization of the basal-planes. The density of defects is estimated in the range 200-650 ppm by studying thin films prepared from EdG. These defects can be fully and easily cured by annealing the films at 800 °C. The position of the G and 2D bands, blue-shifted with respect to pristine SLG, are assigned to moderate biaxial compressive strain (≈0.09 ± 0.02%) and likely weak n doping (<4 × 1012 electrons/cm2) of SLGiw.

Dispersing graphite in water to obtain true (single-layer) graphene in bulk quantity in a liquid has been an unreachablegoal for materials scientists in the past decade. Similarly, a diagnostic tool to identify solubilized graphene in situ has beenlong awaited. Here we show that homogeneous stable dispersions of single-layer graphene (SLG) in water can be obtainedby mixing graphenide (negatively charged graphene) solutions in tetrahydrofuran with degassed water and evaporatingthe organic solvent. In situ Raman spectroscopy of these aqueous dispersions shows all the expected characteristics ofSLG. Transmission electron and atomic force microscopies on deposits confirm the single-layer character. The resultingadditive-free stable water dispersions contain 400 m2 l–1 of developed graphene surface. Films prepared from thesedispersions exhibit a conductivity of up to 32 kS m–1.

Excitonic recombination dynamics in (11-22) -oriented semipolar In0.2Ga0.8N/In0.06Ga0.94N multiquantum wells (MQWs) grown on GaN/m-sapphire templates have been investigated by temperature-dependent time-resolved photoluminescence (TRPL). The radiative and nonradiative recombination contributions to the PL intensity at different temperatures were evaluated by analysing temperature dependences of PL peak intensity and decay times. The obtained data indicate the existence of exciton localization with a localization energy of Eloc(15K) =7meV and delocalization temperature of Tdeloc = 200K in the semipolar InGaN MQWs. Presence of such exciton localization in semipolar (11-22) -oriented structures could lead to improvement of excitonic emission and internal quantum efficiency.

The magnetoabsorption spectra in double HgTe/CdHgTe quantum wells (QWs) with normal and inverted band structures are investigated. The Landau levels in symmetric QWs with a rectangular potential profile are calculated based on the Kane 8 × 8 model. The presence of a tunnel-transparent barrier is shown to lead to the splitting of states and “doubling” of the main magnetoabsorption lines. At a QW width close to the critical one the presence of band inversion and the emergence of a gapless band structure, similar to bilayer graphene, are shown for a structure with a single QW. The shift of magnetoabsorption lines as the carrier concentration changes due to the persistent photoconductivity effect associated with a change in the potential profile because of trap charge exchange is detected. This opens up the possibility for controlling topological phase transitions in such structures.

We present the concept, the fabrication processes and the experimental results for materials and optics that can be used for terahertz field-effect transistor detector focal plane arrays. More specifically, we propose 3D printed arrays of a new type - diffractive multi-zone lenses of which the performance is superior to that of previously used mono-zone diffractive or refractive elements and evaluate them with GaN/AlGaN field-effect transistor terahertz detectors. Experiments performed in the 300-GHz atmospheric window show that the lens arrays offer both a good efficiency and good uniformity, and may improve the signal-to-noise ratio of the terahertz field-effect transistor detectors by more than one order of magnitude. In practice, we tested 3 × 12 lens linear arrays with printed circuit board THz detector arrays used in postal security scanners and observed significant signal-to-noise improvements. Our results clearly show that the proposed technology provides a way to produce cost-effective, reproducible, flat optics for large-size field-effect transistor THz-detector focal plane arrays.

Electromagnetic coils are ubiquitously used in the modern world in motors, antennas, etc. In many applications (magnetic field coil calibration and nuclear magnetic resonance spectroscopy and imaging) there is a strong need for a homogeneous magnetic field. In this paper, we propose a simple modelling based on serial Fourier decomposition allowing the determination of the electrical conductor distribution to make the magnetic field homogeneous. The method is valid both for plane and axisymmetric geometries. The method allows the retrieval of the classical configurations of saddle coil for the plane geometry and Helmholtz coil for the axisymmetric one. The method is generalized for any number of electrical conductors and brings the perspective of new homogeneous magnetic resonance imaging (MRI) coil configuration.

In this paper, we discuss our recent experiments by two-photon spectroscopy demonstrating that hBN is an indirect bandgap material. We will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions involving either virtual or real excitonic states. We will present our measurements of the exciton binding energy by two-photon excitation, leading to the estimation of 6.08 eV for the single-particle bandgap in hBN. We will also discuss our experimental evidence that transverse optical phonons at the K point of the Brillouin zone assist inter-K valley scattering in hexagonal boron nitride.