ABSTRACT
Nanostructured ZnO nanoarrays deposited on silicon oriented substrates is a very promising area in the study of the control of physicochemical properties, in which photoluminescence plays a crucial role. This optical property inherent to ZnO, can be favorably modified through the inclusion of doping elements, with the purpose of appropriately modifying their optical absorption and luminescence. Following this objective, in the present work we present the development of Zn(1-x-y)Ce(x)Eu(y)O nanostructured thin films. The samples were produced in two steps process by atomic layer deposition technique followed by a solvothermal synthesis. The purpose of Cerium and Europium incorporation into the ZnO compound is to enhance the photoluminescence in ZnO thin films. In a first stage textured thin films were obtained from diethylzinc at a temperature of 190 °C and a pressure of 3.29 × 10-4 atm, on silicon substrates (111). Subsequently, the perpendicular growth of nanostructures was induced under a solvothermal process, where Zn(NO3)2 was used as Zn precursor and hexamethylene-tetramine operating as a dual-ligand to promote the linking of Zn2+ ions. The growth of cerium-europium ZnO nanostructures was promoted with Ce(C2H3O2)3·H2O and Eu(NO3)3·5H2O. The obtained Zn(1-x-y)Ce(x)Eu(y)O nanostructured thin films, were examined through SEM-microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and photoluminescence studies. The attained results show that it is feasible to produce Ce-Eu-doped ZnO nanostructures with tailored photoluminescence and crystal size. Interestingly the Ce-Eu doping induces a strong shift in comparison to the typical UV emission of ZnO; an effect that can be related with the increase of lattice defects in ZnO.
ABSTRACT
Hysteresis loops exhibited by the thermal properties of undoped and 0.8 at.% W-doped nanocrystalline powders of VO2 synthesized by means of the solution combustion method and compacted in pellets, are experimentally measured by photothermal radiometry. It is shown that: (i) the W doping reduces both the hysteresis loops of VO2 and its transition temperature up to 15 °C. (ii) The thermal diffusivity decreases (increases) until (after) the metallic domains become dominant in the VO2 insulating matrix, such that its variation across the metal-insulation transition is enhanced by 23.5% with W-0.8 at.% doping. By contrast, thermal conductivity (thermal effusivity) increases up to 45% (40%) as the metallic phase emerges in the VO2 structure due to the insulator-to-metal transition, and it enhances up to 11% (25%) in the insulator state when the local rutile phase is induced by the tungsten doping. (iii) The characteristic peak of the VO2 specific heat capacity is observed in both heating and cooling processes, such that the phase transition of the 0.8 at.% W-doped sample requires about 24% less thermal energy than the undoped one. (iv) The impact of the W doping on the four above-mentioned thermal properties of VO2 mainly shows up in its insulator phase, as a result of the distortion of the local lattice induced by the electrons of tungsten. W doping at 0.8 at.% thus enhances the VO2 capability to transport heat but diminishes its thermal switching efficiency.