Competing length scales and 2D versus 3D dimensionality in relatively thick superconducting NbN films.

Magneto-transport characteristics of 2D and 3D superconducting layers, in particular, temperature and angular dependences of the upper critical field Hc2, are usually considered to be fundamentally different. In the work, using non-local resistance measurements at temperatures near the normal-to-superconducting transition, we probed an effective dimensionality of nm-thick NbN films. It was found that in relatively thick NbN layers, the thicknesses of which varied from 50 to 100 nm, the temperature effect on Hc2 certainly pointed to the three-dimensionality of the samples, while the angular dependence of Hc2 revealed behavior typical for 2D samples. The seeming contradiction is explained by an intriguing interplay of three length scales in the dimensionally confined superconducting films: the thickness, the Ginzburg-Landau coherence length, and the magnetic-field penetration depth. Our results provide new insights into the physics of superconducting films with an extremely large ratio of the London penetration depth to the Ginzburg-Landau coherence length exhibiting simultaneously 3D isotropic superconducting properties and the 2D transport regime.

Fluoride-free synthesis of anodic TiO2 nanotube layers: a promising environmentally friendly method for efficient photocatalysts.

TiO2 nanotube (TNT) layers are generally prepared in fluoride-based electrolytes via electrochemical anodization that relies on the field-assisted dissolution of Ti metal forming nanoporous/nanotubular structures. However, the usage of fluoride ions is considered hazardous to the environment. Therefore, we present an environmentally friendly synthesis and application of TNT layers prepared in fluoride-free nitrate-based electrolytes. A well-defined nanotubular structure with thickness up to 1.5 µm and an inner tube diameter of ∼55 nm was obtained within 5 min using aqueous X(NO3)Y electrolytes (X = Na+, K+, Sr2+, Ag+). For the first time, we show the photocatalytic performance (using a model organic pollutant), HO˙ radical production, and thorough characterization of TNT layers prepared in such electrolytes. The highest degradation efficiency (k = 0.0113 min-1) and HO˙ radical production rate were obtained using TNT layers prepared in AgNO3 (Ag-NT). The intrinsic properties of Ag-NT such as the valence band maximum of ∼2.9 eV, surface roughness of ∼6 nm, and suitable morphological features and crystal structure were obtained. These results have the potential to pave the way for a more environmentally friendly synthesis of anodic TNT layers in the future using the next generation of fluoride-free nitrate-based electrolytes.

Remarkable Improvement in Hydrogen Sensing Characteristics with Pt/TiO2 Interface Control.

Owing to their excellent hydrogen surface susceptibility, TiO2 thin films have been proven worthy of sensing hydrogen. However, these sensors work best at temperatures of 150-400 °C, with poor selectivity and a low response at room temperature. In this context, the novelty of this paper includes an investigation of the critical role of electrode fabrication that is found to significantly define the surface as well as the performance of a sensor. Sensors prepared with optimized conditions showed the best sensor response (SR) of ∼1.58 × 107 toward 10 000 ppm H2 with excellent linearity (R-square ∼ 0.98 for 300-10 000 ppm) at room temperature (∼20 °C). In addition, the said sensor showed a response time of ∼125 s with full baseline recovery and a selectivity factors (SF) of ∼1754, 2456, and 4723 to 1000 ppm of interfering reducing gases CH4, CO, and NH3, respectively, at 100 °C. At room temperature, the selectivity factor (for 300 ppm H2) of the sensor is ∼3.41 to 90% RH and ∼37.35 to 250 ppm oxygen, 200 ppm CO, and 1600 ppm CO2. Last but not least, our X-ray diffraction, X-ray photoelectron spectroscopy, and electrical transport characteristics enabled us to explain the high sensing mechanism on the basis of the estimated grain size, the quantitative atomic composition, the barrier at the Pt/TiO2 interface, and the thermal activation energy (also known as the intergranular barrier height) of the thin films.

Surface charge and carbon contamination on an electron-beam-irradiated hydroxyapatite thin film investigated by photoluminescence and phase imaging in atomic force microscopy.

The surface properties of hydroxyapatite, including electric charge, can influence the biological response, tissue compatibility, and adhesion of biological cells and biomolecules. Results reported here help in understanding this influence by creating charged domains on hydroxyapatite thin films deposited on silicon using electron beam irradiation and investigating their shape, properties, and carbon contamination for different doses of incident injected charge by two methods. Photoluminescence laser scanning microscopy was used to image electrostatic charge trapped at pre-existing and irradiation-induced defects within these domains, while phase imaging in atomic force microscopy was used to image the carbon contamination. Scanning Auger electron spectroscopy and Kelvin probe force microscopy were used as a reference for the atomic force microscopy phase contrast and photoluminescence laser scanning microscopy measurements. Our experiment shows that by combining the two imaging techniques the effects of trapped charge and carbon contamination can be separated. Such separation yields new possibilities for advancing the current understanding of how surface charge influences mediation of cellular and protein interactions in biomaterials.

Directly created electrostatic micro-domains on hydroxyapatite: probing with a Kelvin Force probe and a protein.

Micro-domains of modified surface potential (SP) were created on hydroxyapatite films by direct patterning by mid-energy focused electron beam, typically available as a microprobe of Scanning Electron Microscopes. The SP distribution of these patterns has been studied on sub-micrometer scale by the Kelvin Probe Force Microscopy method as well as lysozyme adsorption. Since the lysozyme is positively charged at physiological pH, it allows us to track positively and negatively charged areas of the SP patterns. Distribution of the adsorbed proteins over the domains was in good agreement with the observed SP patterns.

Charge specific protein placement at submicrometer and nanometer scale by direct modification of surface potential by electron beam.

The understanding and the precise control of protein adsorption is extremely important for the development and optimization of biomaterials. The challenge resides in controlling the different surface properties, such as surface chemistry, roughness, wettability, or surface charge, independently, as modification of one property generally affects the other. We demonstrate the creation of electrically modified patterns on hydroxyapatite by using scanning electron beam to tailor the spatial regulation of protein adsorption via electrostatic interactions without affecting other surface properties of the material. We show that domains, presenting modulated surface potential, can be created to precisely promote or reduce protein adsorption.