PL and XPS depth profiling of Si/Al2O3 co-sputtered films and evidence of the formation of silicon nanocrystals

We have studied the correlation between light emission and structural properties of silicon nanocrystals formed in Al2O3 matrix by co-sputtering followed by an annealing in the temperature range of 900-1100 degrees C. X-ray diffraction (XRD) experiments revealed the nanocrystal formation which is a function of Si sputtering power and annealing temperature. The chemical structure and atomic compositions of Si, Al, O and their variations with depth from the sample surface have been investigated by X-ray photoelectron spectroscopy (XPS). Formation of Si nanocrystals is identified from the presence of Si-Si bonds whose strength increases with the annealing temperature. However, Si atoms resting in the region close to the surface of the sample is found to be oxidized. Photoluminescence (PL) spectroscopy is employed as a function of Si power, annealing temperature and sample depth. The origin of various PL peaks is discussed in terms of defect formation in Al2O3, emission from nanocrystals and interfaces. Defect related peaks are attributed to Cr3+, Ti3+ and F centers which are commonly observed in Al2O3. The emission from the Si nanocrystals is clearly identified in the wavelength range of 700-900 nm. The variation of this emission with the depth is in good correlation with the depth profile of Si-Si bonds as determined by the XPS analysis

Analysis of temporal evolution of quantum dot surface chemistry by surface-enhanced Raman scattering

\u3cp\u3eTemporal evolution of surface chemistry during oxidation of silicon quantum dot (Si-QD) surfaces were probed using surface-enhanced Raman scattering (SERS). A monolayer of hydrogen and chlorine terminated plasma-synthesized Si-QDs were spin-coated on silver oxide thin films. A clearly enhanced signal of surface modes, including Si-Cl x and Si-H x modes were observed from as-synthesized Si-QDs as a result of the plasmonic enhancement of the Raman signal at Si-QD/silver oxide interface. Upon oxidation, a gradual decrease of Si-Cl x and Si-H x modes, and an emergence of Si-O x and Si-O-H x modes have been observed. In addition, first, second and third transverse optical modes of Si-QDs were also observed in the SERS spectra, revealing information on the crystalline morphology of Si-QDs. An absence of any of the abovementioned spectral features, but only the first transverse optical mode of Si-QDs from thick Si-QD films validated that the spectral features observed from Si-QDs on silver oxide thin films are originated from the SERS effect. These results indicate that real-time SERS is a powerful diagnostic tool and a novel approach to probe the dynamic surface/interface chemistry of quantum dots, especially when they involve in oxidative, catalytic, and electrochemical surface/interface reactions.\u3c/p\u3

Nucleation of silicon nanocrystals in a remote plasma without subsequent coagulation

We report on the growth mechanism of spherical silicon nanocrystals in a remote expanding Ar plasma using a time-modulated SiH4 gas injection in the microsecond time range. Under identical time-modulation parameters, we varied the local density of the SiH4 gas by changing its stagnation pressure on the injection line over the range of 0.1–2.0¿bar. We observed that nanocrystals were synthesized in a size range from ~2 to ~50¿nm with monocrystalline morphology. Smaller nanocrystals (~2–6¿nm) with narrower size distributions and with higher number densities were synthesized with an increase of the SiH4 gas-phase density. We related this observation to the rapid depletion of the number density of the molecules, ions, and radicals in the plasma during nanocrystal growth, which can primarily occur via nucleation with no significant subsequent coagulation. In addition, in our remote plasma environment, rapid cooling of the gas in the particle growth zone from ~1500 to ~400¿K significantly reduces the coalescence rate of the nanoparticles, which makes the coagulation process highly unlikely. Our observations on nanocrystal formation via nucleation indicated that subsequent coagulation for further growth is not always an essential step on nanoparticle formation

Porous titania photoelectrodes built on a Ti-web of microfibers for polymeric electrolyte membrane photoelectrochemical (PEM-PEC) cell applications

\u3cp\u3eConventional photoelectrochemical (PEC) cells are based on planar photoelectrodes supported on glass substrates and liquid electrolytes. Only few recent studies have examined an alternative PEC design which is robust and scalable, where the key elements are polymeric electrolyte membranes and porous photoelectrodes. This work aims to give further insights on the operation of such cells utilizing titania photoelectrodes and proton and hydroxide ion conducting membranes. Two families of photoelectrodes were developed on Ti porous substrate; TiO\u3csub\u3e2\u3c/sub\u3e nanotubes grown by anodization and subsequent oxygen annealing, and TiO\u3csub\u3e2\u3c/sub\u3e layers developed under oxygen annealing. Initial screening of the photoanodes for water splitting and (poly)alcohol photo-oxidation took place in conventional PEC cells. We found that the annealing temperature affects the performance of the photoanodes, evidenced by a monotonic increase in the activity for water photo-oxidation with increasing annealing temperature. Moreover it was demonstrated that anatase phase is predominantly active for the (poly)alcohol electro-oxidation, while there is a synergy between rutile and anatase which is beneficial for water splitting. In addition, the most promising photoanodes for water splitting were evaluated in our polymeric electrolyte membrane photoelectrochemical(PEM-PEC) cell during gas phase operation. It was found that PEM-PEC operation is more efficient when OH\u3csup\u3e-\u3c/sup\u3e conducting membranes are used, while the nature of the carrier gas does not significantly influence the activity. Overall, PEM-PEC operation is more promising than conventional PEC in both acidic and alkaline media, since comparable (or even at some cases higher) photocurrents were obtained while liquid pumping systems are not required for PEM-PEC devices.\u3c/p\u3

An analytical force balance model for dust particles with size up to several Debye lengths

\u3cp\u3eIn this study, we developed a revised stationary force balance model for particles in the regime a/λD<10. In contrast to other analytical models, the pressure and dipole force were included too, and for anisotropic plasmas, a novel contribution to the dipole moment was derived. Moreover, the Coulomb logarithm and collection cross-section were modified. The model was applied on a case study where carbon dust is formed near the plasma sheath in the linear plasma device Pilot-PSI. The pressure force and dipole force were found to be significant. By tracing the equilibrium position, the particle radius was determined at which the particle deposits. The obtained particle radius agrees well with the experimentally obtained size and suggests better agreement as compared to the unrevised model.\u3c/p\u3

A quantitative investigation on recycling attitudes of gifted/talented students

Uncontrolled industrialization process since nineteenth century and the trend towards consumer society have resulted in an enormous amount of waste materials. As a result of this problem, management and safe removal of urban waste has become one of the most important environmental issues of today. Recycling projects that have been developed to solve this problem will reduce waste materials, recycle discarded materials and save energy, money and time. Therefore, recycling applications and related researches are significantly growing due to their enormous potential to solve many environmental problems of society. In this perspective, the aim of this study was to determine the recycling attitude levels of gifted/talented students. Also, gifted/talented students' recycling attitude levels was compared according to some independent variables, such as age, gender and grade. For these purposes, single scan model was performed in the present research. As a means of data collection, Recycling Attitude Scale (RAS) and the personal information form were used. The RAS was applied to 122 gifted students enrolling in Science and Art Center in Manisa Province of Turkey in 2012-2013 academic years. Gifted/talented students' ages ranged from 10 to 13 and, also, the 11-year-old candidates, who participated in the study, were majority. Statistical package for the social sciences (SPSS 17.00) programme was utilized to analyse the obtained data. The analysis results showed that gifted and talented students had fairly well recycling attitude level and there were no significant difference between gifted and talented students in terms of gender, age and class

Visible-light-promoted gas-phase water splitting using porous WO\u3csub\u3e3\u3c/sub\u3e/BiVO\u3csub\u3e4\u3c/sub\u3e photoanodes

\u3cp\u3eWe recently described the use of Ti(0) microfibers as an anodization substrate for the preparation of TiO\u3csub\u3e2\u3c/sub\u3e nanotubes arrays as porous photoanodes. Here, we report the use of these fibers as a scaffold to build porous photoanodes based on a WO\u3csub\u3e3\u3c/sub\u3e/BiVO\u3csub\u3e4\u3c/sub\u3e heterojunction. The obtained photoelectrodes show promising results under visible light irradiation for water oxidation both in a typical liquid-phase photoelectrochemical setup and in a gas phase reactor (developed in-house) based on a polymeric electrolyte membrane.\u3c/p\u3

Nostalgia de Grecia en Schiller y Hölderlin

Ponencias de los tres talleres del Congreso "El fondo de la historia: Idealismo, Romanticismo y sus Repercusiones" celebrado del 22 al 24 de noviembre de 2010 en la Universidad Carlos III de Madri