This chapter discusses the recent observations of the impact of surface effects on the optical properties of GaN-based NWs. It focuses on three aspects: (1) the existence of specific exciton and donor quantum states related to surfaces; (2) the intrinsic non-radiative activity of surfaces; (3) the impact on electron-hole pair recombination dynamics of the physical and chemical environment of the NWs. The non-radiative recombination at the surface of nitride NWs is generally believed to occur and that the band bending resulting from the pinning of Fermi level on surface states is generally believed to control access of carriers to those surface states, whereas it would also efficiently separate photo-generated electrons and holes. In particular, holes are believed to be driven toward the surface. The photochemical activity of this surface, most certainly related to photocarrier transport from and toward surface states, has strong effects on the optical properties of the NWs.

AlGaN-based ultra-violet (UV) light emitting diodes (LEDs) are seen as the best solution for the replacement of traditional mercury lamp technology. By adjusting the Al concentration, a large emission spectrum range from 360 nm (GaN) down to 200 nm (AlN) can be covered. Owing to the large density of defects typically present in AlGaN materials usually grown on sapphire substrates, LED efficiencies still need to be improved. Taking advantage of the 3D carrier confinement, quantum dots (QDs) are among the solutions currently under investigation to improve the performances of UV LEDs. The objectives of this work are to present and discuss the morphological and optical properties of GaN nanostructures grown by molecular beam epitaxy on the (0 0 0 1) and the (11-22) orientations of Al0.5Ga0.5N. In particular, the dependence of the morphological properties of the nanostructures on the growth conditions and the surface orientation will be presented. The optical characteristics as a function of the nanostructure design (size, shape and dimensionality) will also be shown and discussed. The electroluminescence characteristics of a first series of QD-based GaNAl0.5Ga0.5N LEDs grown on the polar (0 0 0 1) plane will be investigated

The optical properties of GaN/Al0.15Ga0.85N multiple quantum wells grown on m-plane oriented substrate are studied in 8K–300K temperature range. The optical spectra reveal strong in-plane optical anisotropies as predicted by group theory. Polarized time resolved temperature-dependent photoluminescence experiments are performed providing access to the relative contributions of the non-radiative and radiative recombination processes. We deduce the variation of the radiative decay time with temperature in the two polarizations. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

The understanding of carbon nanotubes (CNT) growth is crucial for the control of their production. In particular, the identification of structural changes of carbon possibly occurring near the catalyst particle in the very early stages of their formation is of high interest. In this study, samples of nascent CNT obtained during nucleation step and samples of vertically aligned CNT obtained during growth step are analysed by combined spatially resolved Raman spectroscopy and X-Ray diffraction measurements. Spatially resolved Raman spectroscopy reveals that iron-based phases and carbon phases are co-localised at the same position, and indicates that sp2 carbon nucleates preferentially on iron-based particles during this nucleation step. Depth scan Raman spectroscopy analysis, performed on nascent CNT, highlights that carbon structural organisation is significantly changing from defective graphene layers surrounding the iron-based particles at their base up to multi-walled nanotube structures in the upper part of iron-based particles.

Microporous AlPO4-54·xH2O, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction and Raman spectroscopy in diamond anvil cells. The material was found to begin to amorphize near 2 GPa using either a nonpenetrating pressure transmitting medium (PTM) silicone oil or no PTM. When H2O is used as a PTM, amorphization begins at a lower pressure of 0.9 GPa. In this case, superhydration effects are observed and higher relative unit cell volumes are observed prior to the beginning of pressure-induced amorphization (PIA) as compared to the experiment in silicone oil due to insertion of the H2O molecules in the pores. In all cases, in these experiments at room temperature, amorphization was irreversible. Ex situ experiments were used to investigate the local structure of pressure-amorphized AlPO4-54·xH2O by nuclear magnetic resonance and by X-ray absorption spectroscopy, which show that, upon increasing pressure, two water molecules enter in the coordination sphere of IVAl, thereby increasing the coordination number from 4 to 6, which destabilizes the structure. The present results show that the insertion of and/or reaction with guest species can be used to strongly modify the stability of microporous materials with respect to PIA.

We propose the notion of integrable boundary in the context of discrete quad-graph systems. The equation characterizing the boundary must satisfy a compatibility equation with the one characterizing the bulk that we called the three-dimensional (3D) boundary consistency. In comparison to the usual 3D consistency condition which is linked to a cube, our 3D boundary consistency condition lives on half of a rhombic dodecahedron. We provide a list of integrable boundaries associated to each quad-graph bulk equation of the classification obtained by Adler, Bobenko and Suris. Then, the use of the term 'integrable boundary' is justified by the facts that there are Bäcklund transformations and a zero curvature representation for systems with a boundary satisfying our condition. We discuss the three-leg form of boundary equations and hence obtain associated discrete Toda models with a boundary. Finally, the connection between the 3D boundary consistency and the set-theoretical reflection equation is established.

We collect and review works which treat two-dimensional electron gases (2DEGs) in quantum wells (QWs, mostly GaAs/ GaAlAs heterostructures) in the presence of quantizing magnetic fields as open systems in contact with outside reservoirs. If a reservoir is sufficiently large, it pins the Fermi level to a certain energy. As a result, in a varying external magnetic field, the thermodynamic equilibrium will force oscillations of the electron density in and out of the QW. This leads to a number of physical phenomena in magneto-transport, interband and intraband magneto-optics, magnetization, magneto-plasma dispersion, etc. In particular, as first proposed by Baraff and Tsui, the density oscillations in and out of QW lead to plateaus in the integer quantum Hall effect (IQHE) at values observed in experiments. The gathered evidence, especially from magneto-optical investigations, allows us to conclude that, indeed, in most GaAs/GaAlAs hetrostructures one deals with open systems in which the electron density in Qws oscillates as the magnetic field varies. Relation of the density oscillations to other factors, such as electron localization, and their combined influence on the quantum transport in 2DEGs, is discussed. In particular, a validity of the classical formula for the Hall resistivity rxy.B/Nec is considered. It is concluded that the density oscillations are not sufficient to be regarded as the only source of plateaus in IQHE, although such claims have been sometimes made in the past and present. Still, our general conclusion is that the reservoir approach should be included in various descriptions of 2DEGs in the presence of a magnetic field. An attempt has been made to quote all the relevant literature on the subject.