RESUMO
In this work, we show a correlation between the composition and the microstructural and optical properties of bright and uniform luminescent porous silicon (PSi) films. PSi films were synthesized by electrochemical etching using nitric acid in an electrolyte solution. PSi samples synthesized with nitric acid emit stronger (up to six-fold greater) photoluminescence (PL) as compared to those obtained without it. The PL peak is shifted from 630 to 570 nm by changing the concentration ratio of the HF:HNO3:(EtOH-H2O) electrolyte solution, but also shifts with the excitation energy, indicating quantum confinement effects in the silicon nanocrystals (Si-NCs). X-ray photoelectron spectroscopy analysis shows a uniform silicon content in the PSi samples that emit the strongest PL. High-resolution transmission electron microscopy reveals that the Si-NCs in these PSi samples are about ~2.9 ± 0.76 nm in size and are embedded in a dense and stoichiometric SiO2 matrix, as indicated by the Fourier transform infrared analysis. On the other hand, the PSi films that show PL of low intensity present an abrupt change in the silicon content depth and the formation of non-bridging oxygen hole center defects.
RESUMO
Heat transfer phenomena in multilayer structures have gained interest due to their promising use in thermal insulation and thermoelectricity applications. In such systems, nanostructuring has been used to introduce moderate interfacial density, and it has been demonstrated that interfacial thermal resistance plays a crucial role in reducing thermal conductivity κ. Nevertheless, the main constraint for actual applications is related to their tiny size because they are extremely thin to establish appreciable temperature gradients. In this work, by severe plastic deformation process of accumulative roll bonding (ARB), a 250 µm thick Cu-Nb multilayer containing more than 8000 interfaces with periods below 40 nm was obtained, enabling the production of bulk metallic nanolaminates with ultralow κ. Multilayers present an ultralow κ of â¼0.81 W/mK at 300 K, which is 100 times smaller than its Cu-Nb bulk counterpart, and even lower than the amorphous lattice limit for the Cu-Nb thin film system. By using electron diffusive mismatch model (EDMM), we argue that both electrons diffusively scattering at interface and those ballistically crossing the constituents are responsible for heat conduction in the Cu-Nb multilayers at nanoscale. Hence, ARB Cu-Nb multilayers are intriguing candidate materials which can prove avenues to achieve stable ultralow κ thermal barriers for robust applications.