Emergence and Evolution of Crystallization in TiO2 Thin Films: A Structural and Morphological Study.

Among all transition metal oxides, titanium dioxide (TiO2) is one of the most intensively investigated materials due to its large range of applications, both in the amorphous and crystalline forms. We have produced amorphous TiO2 thin films by means of room temperature ion-plasma assisted e-beam deposition, and we have heat-treated the samples to study the onset of crystallization. Herein, we have detailed the earliest stage and the evolution of crystallization, as a function of both the annealing temperature, in the range 250-1000 °C, and the TiO2 thickness, varying between 5 and 200 nm. We have explored the structural and morphological properties of the as grown and heat-treated samples with Atomic Force Microscopy, Scanning Electron Microscopy, X-ray Diffractometry, and Raman spectroscopy. We have observed an increasing crystallization onset temperature as the film thickness is reduced, as well as remarkable differences in the crystallization evolution, depending on the film thickness. Moreover, we have shown a strong cross-talking among the complementary techniques used displaying that also surface imaging can provide distinctive information on material crystallization. Finally, we have also explored the phonon lifetime as a function of the TiO2 thickness and annealing temperature, both ultimately affecting the degree of crystallinity.

Left/right asymmetry of the dipole field due to reflection from a periodic multilayer of a topological insulator and a columnar thin film.

The problem of a vertical electric dipole radiating above a periodic multilayer whose unit cell comprises a layer of a topological insulator (TI) and a columnar thin film (CTF) was solved in order to investigate the left/right asymmetry of the total electric field in the far zone in the half-space containing the dipole. Occurring in a wide range of the polar observation angle, the left/right asymmetry of Eϕ is due to both the CTFs and the TI layers. Occurring in a narrow range of the polar observation angle, the left/right asymmetry of Eθ is entirely due to the TI layers. For presently available values of the magnitude of the surface admittance γTI of TIs, significant left/right asymmetry occurs if the number of unit cells in the periodic TI/CTF multilayer is high enough.

Enhanced left/right asymmetry in reflection and transmission due to a periodic multilayer of a topological insulator and an anisotropic dielectric material.

Very weak left/right asymmetry in reflection and transmission is offered by a layer of a topological insulator on top of a layer of an anisotropic dielectric material, but it can be enhanced very significantly by using a periodic multilayer of both types of materials. This is an attractive prospect for realizing one-way terahertz devices, because both types of materials can be grown using standard physical-vapor-deposition techniques.

Bicontrollable terahertz metasurface with subwavelength scattering elements of two different materials.

Transmission of a normally incident plane wave through a metasurface with bicontrollable subwavelength scattering elements was simulated using a commercial software. Some pixels composing the H-shaped scattering elements were made of a magnetostatically controllable material whereas the remaining pixels were made of a thermally controllable material, the metasurface designed to operate in the terahertz spectral regime. The copolarized transmission coefficients were found to exhibit stopbands that shift when either a magnetostatic field is applied or the temperature is increased or both. Depending on spectral location of the stopband, either the magnetostatic field gives coarse control and temperature gives fine control or vice versa. The level of magnetostatic control depends on the magnetostatic-field configuration.

Effect of low frequency (LF) electric fields on gene expression of a bone human cell line.

We evaluated the effects, on cultured human SaOS-2 cells, of exposures to the low frequency (LF) electric signal (60 kHz sinusoidal wave, 24.5 V peak-to-peak voltage, amplitude modulated by a 12.5 Hz square wave, 50% duty cycle) from an apparatus of current clinical use in bone diseases requiring regenerating processes. Cells in flasks were exposed to a capacitively coupled electric field giving electric current density in the sample of 4 µA/cm(2). The whole expressed cellular mRNAs were systematically analyzed by "DNA microchips" technology to identify all individual species quantitatively affected by field exposure. Comparisons were made between RNA samples from exposed and control sham-exposed cells. Results indicated that immediately and 4 h after exposure there were almost no differentially modulated mRNA species. However, samples obtained at 24 h after exposure showed a small number of limitedly differential signals (7 down-regulated and 3 up-regulated with a cut-off value of ±1.5; 38 and 11, respectively, with a cut-off value of ±1.3), which included mostly mRNA encoding transcription factors and DNA binding proteins. Nevertheless, in identical experimental conditions, we previously demonstrated enzymatic changes of alkaline phosphatase occurring immediately after exposure and declining in a few hours. Therefore, since enzymatic changes occur before those observed at gene regulation level, it is conceivable that only earlier effects are directly due the treatment and then these effects are later able to affect gene expression only indirectly.

Induction of alkaline phosphatase activity by exposure of human cell lines to a low-frequency electric field from apparatuses used in clinical therapies.

Low-frequency (LF) electric fields (EFs) are currently used in clinical therapies of several bone diseases to increase bone regenerative processes. To identify possible molecular mechanisms involved in these processes, we evaluated the effects on cell cultures of 1 h exposures to the signal generated by an apparatus of current clinical use (frequency 60 kHz, frequency of the modulating signal 12.5 Hz, 50% duty cycle, peak-to-peak voltage 24.5 V). Two different human cell lines, bone SaOS-2 and liver HepG2, were used. Exposures significantly increased alkaline phosphatase (ALP) enzymatic activity in both cell lines. The increase was about 35% in SaOS-2 cells and about 80% in HepG2 cells and occurred in the first 4 h after exposure and decreased to almost no change by 24 h. Since ALP represents a typical marker of bone regeneration, these results represent a first molecular evidence of biological effects from 60 kHz EF exposures. The finding of similar effects in cells derived from two different tissues more likely indicates the effective operation of the mechanism in living organisms.

Simulation and analysis of prismatic bioinspired compound lenses for solar cells: II. Multifrequency analysis.

Multifrequency numerical simulations of the light-coupling efficiency of a prismatic bioinspired compound lens (BCL) of silicon atop a thick silicon substrate were carried out within the framework of geometrical optics. Comparison was made with untextured and groove-textured silicon substrates as well as with untextured silicon substrates with a double-layer anti-reflection (DLAR) coating. Taking into account the broadband nature and the sea-level spectral irradiance of the insolation flux, and averaging over all admissible directions and both linear polarization states of the incident light, we found that the light-coupling efficiency can be almost doubled with respect to the untextured silicon substrate and enhanced by about a third with respect to a DLAR-coated untextured silicon substrate, by adopting a DLAR-coated silicon BCL.

Simulation and analysis of prismatic bioinspired compound lenses for solar cells.

Inspired by the apposition compound eyes of many dipterans, we formulated a fractal scheme to design prismatic lenses to improve the performance of silicon solar cells. We simulated the absorption of light, both directly illuminating and diffuse, using the geometrical-optics approximation. We found that properly designed bioinspired compound lenses (BCLs) can significantly improve the light-harvesting capabilities of silicon solar cells. The degree of improvement will depend on the material chosen to make the BCLs as well as the operating conditions.