Photoluminescence and optical absorption properties of silicon quantum dots embedded in Si-rich silicon nitride matrices

Silicon nitride (SiN_x) films were prepared with a gas mixture of SiH₄ and NH₃ on Si wafers using the plasma-enhanced chemical vapor deposition (PECVD) method. High-resolution transmission electron microscopy and infrared absorption have been used to reveal the existence of the Si quantum dots (Si QDs) and to determine the chemical composition of the silicon nitride layers. The optical properties of these structures were studied by photoluminescence (PL) spectroscopy and indicate that emission mechanisms are dominated by confined excitons within Si QDs. The peak position of PL could be controlled in the wavelength range from 1.5 to 2.2 eV by adjusting the flow rates of ammonia and silane gases. Absorbance spectra obtained in the transmission mode reveal optical absorption from Si QDs, which is in good correlation with PL properties. These results have implications for future nanomaterial deposition controlling and device applications.     

Macrotricyclic quaternary tetraammonium receptors: halide anion recognition and interfacial activity at an aqueous interface. A molecular dynamics investigation

We report a molecular dynamics study of the halide inclusion complexes X(-) subset L(4+) of a macrotricyclic tetrahedral receptor L(4+) built from four quaternary ammonium sites connected by six (CH(2))(n) chains. The hydrophilic/hydrophobic character of the complexes is investigated at a water/chloroform interface, represented explicitly and, despite their +3 charge and "spherical" shape, they are found to display amphiphilic behavior and to concentrate at the interface. The more lipophilic N-substituted CH(2)phi derivative, as well as less charged models are more surface active than the N-Me substituted host. In relation with the Hofmeister series, I(-) exo neutralizing counterions are compared with Cl(-) anions and are found to sit closer to the interface, which becomes more neutral. The "macrocyclic interfacial effect" is investigated by a comparison of L(4+) complexes with their acyclic counterparts. Finally, we address the question of anion binding selectivity by L(4+) and its L(1) (4+) and L(2) (4+) topological isomers. F(-) is too small for these three hosts, while I(-) is too big. According to free energy perturbation calculations, Cl(-) is preferred to Br(-), but somewhat more by L(1) (4+) than by L(4+).     

Retrosynthetic analysis of fullerene C(60): structure, stereochemistry, and calculated stability of C(30) fragments.

The total number of possible retrosynthetic bisections of C(60) leads to nine different isometric C(30) fragments. These molecules include five chiral units, four of which derive from partitions corresponding to four distinct "Coupes du Roi". The energies, curvatures, and homodesmotic stabilization energies of the C(30) fragments are evaluated at the ab initio 6-31G level.     

Stabilité élastique des structures unidimensionnelles

In this paper, we consider the problem of stability of one-dimensional structures and give a general formulation by means of the energy method. We propose then a numerical method of deriving the critical rate requiring eigenvalues of a sequence of small matrices rather than of a big one.

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Cet article est le résumé d'une thèse présentée à l'Ecole Polytechnique de l'Université de Lausanne [17]. Je tiens à exprimer ma vive gratitude au ProfesseurCh. Blanc, directeur de l'Institut de Mathématiques Appliquées, dont les conseils et encouragements m'ont été très précieux.     

Z-scan measurements of the nonlinear refractive indices of novel Yb-doped laser crystal hosts

We report on the z-scan characterization of the nonlinear refractive indices of three borate crystals (GdCOB, YCOB, and BOYS), which are new promising Yb-doped laser hosts. The results indicate the possibility of substantial Kerr-lensing and self-phase modulation effects in femtosecond oscillators and amplifiers. High values of the nonlinear refractive indices suggest possible KLM operation of such lasers. PACS 42.65.An; 42.65.Hw; 42.55.Rz; 42.65.Re     

Structural and zeolitic features of a series of heterometallic supramolecular porous architectures based on tetrahedral {M(C2O4)4}4- primary building units

The utilization of tetrahedral pre-formed coordination compounds {M(C2O4)4}4- (M = Zr(IV), U(IV); C2O4(2-) = oxalate) permitted the efficient construction of rare examples of heteronuclear supramolecular nano-porous architectures. A series of metal-organic coordination frameworks prepared by association of these building units with either Mn2+, Cd2+, or Mg2+ have been structurally characterized and are described. Their 3-D chemical scaffold is based on the primary tetrahedral building unit but their pore sizes and topologies could be varied through the M2+ metal ion involved in the assembling process, and the anionic tetrahedral moiety. These structures display channels with apertures up to 12 Angstrom x 8 Angstrom which are emptied of solvates at mild temperatures without affecting the chemical scaffold, the integrity of which is maintained up to 250-300 degrees C. A certain degree of flexibility of the coordination polymers upon guest release is suggested by the temperature dependence of the powder X-ray patterns and N2 sorption experiments, but reversible and selective sorption of small molecules has been observed substantiating that these open-frameworks behave like sponges.