Crossover from photodarkening to photobleaching in a-GexSe100-x thin films

In this Letter, we present the interesting results of photodarkening (PD), transition toward photostability, and a slow crossover from PD to photobleaching when composition of the chalcogenide glassy thin film changes from Ge-deficient to rich. A subsequent Raman analysis on these as-prepared and irradiated samples provide the direct evidence of photoinduced structural rearrangement, i.e., photocrystallization of Se and the removal of edge-sharing GeSe4 tetrahedra. Further, our experimental results clearly demonstrate that light-induced effects can be effectively controlled by choosing the right composition and provide valuable information on synthesizing photostable/sensitive glasses.

Enhancing fluorescence signals from aluminium thin films and foils using polyelectrolyte multilayers

In this paper we investigate the application of polyelectrolyte multilayer (PEM) coated metal slides in enhancing fluorescence signal. We observed around eight-fold enhancement in fluorescence for protein incubated on PEM coated on aluminium mirror surface with respect to that of functionalized bare glass slides. The fluorescence intensities were also compared with commercially available FAST (R) slides (Whatman) offering 3D immobilization of proteins and the results were found to be comparable. We also showed that PEM coated on low-cost and commonly available aluminium foils also results in comparable fluorescence enhancement as sputtered aluminium mirrors. Immunoassay was also performed, using model proteins, on aluminium mirror as well as on aluminium foil based devices to confirm the activity of proteins. This work demonstrated the potential of PEMs in the large-scale, roll-to-roll manufacturing of fluorescence enhancements substrates for developing disposable, low-cost devices for fluorescence based diagnostic methods.

Nanostructured GdxZn1-xO thin films by nebulizer spray pyrolysis technique: Role of doping concentration on the structural and optical properties

Nanostructured GdxZn1-xO thin films with different Gd concentration from 0% to 10% deposited at 400 degrees C using the NSF technique. The films were characterized by structural, surface and optical properties, respectively. X-ray diffraction analysis shows that the Gd doped ZnO films have lattice parameters a = 3.2497 angstrom and c = 5.2018 angstrom with hexagonal structure and preferential orientation along (002) plane. The estimated values compare well with the standard values. When film thickness increases from 222 to 240 nm a high visible region transmittance (>70%) is observed. The optical band gap energy, optical constants (n and k), complex dielectric constants (epsilon(r), and epsilon(i)) and optical conductivities (sigma(r), and sigma(i)) were calculated from optical transmittance data. The optical band gap energy is 3.2 eV for pure ZnO film and 3.6 eV for Gd0.1Zn0.9-O film. The PL studies confirm the presence of a strong UV emission peak at 399 nm. Besides, the UV emission of ZnO films decreases with the increase of Gd doping concentration correspondingly the ultra-violet emission is replaced by blue and green emissions.

Tailoring the optical properties of amorphous heavily Er3+-doped Ge-Ga-S thin films

This study deals with the influence of Er-doping level and thermal annealing on the optical properties of amorphous Ge-Ga-S thin films. Nominal compositions of (GeS2)(75)(Ga2S3)(25) doped with high concentrations of 2.1 and 2.4 mol% Er2S3 (corresponding to 1.2 and 1.4 at% Er, respectively) have been chosen for this work. The results have been related to those obtained for the un-doped samples. The values of the refractive index, the absorption coefficient and optical band gap have been determined from the transmittance data. It has been found that the optical band gap of un-doped and 2.1 mol% Er2S3-doped films slightly increases with annealing temperature, whereas at 2.4 mol% Er2S3-doping level it is decreased. The dependences of the optical parameters on the erbium concentration and effect of annealing in the temperature range of 100-200 degrees C have been evaluated and discussed in relation to possible structural changes.

Growth-temperature dependent physical properties of SnS nanocrystalline thin films

In this article we have demonstrated the influence of growth-temperature on the morphology and orientation of SnS films deposited by thermal evaporation technique. While increasing the growth-temperature, the morphology of SnS films changed from flakes-like nanocrystals to regular cubes, whereas their orientation shifted from <111> to <040> direction. The chemical composition of SnS films gradually changed from sulfur-rich to tin-rich with the increase of growth-temperature. The structural analyzes reveal that the crystal structure of SnS films probably changes from orthorhombic to tetragonal at the growth-temperature of about 410 degrees C. Raman studies show that SnS films grown at all temperatures consist of purely SnS phase, whereas the optical studies reveal that the direct optical bandgap of SnS films decreased with the increase of growth-temperature. From these results it has been emphasized that the morphology and orientation along with electrical and optical properties of nearly stoichiometric SnS films strongly depend on their growth-temperature.

Structural and magnetic properties of SmCo5/Co exchange coupled nanocomposite thin films

We investigated the structural and magnetic properties of SmCo5/Co exchange coupled nanocomposite thin films grown by magnetron sputtering from Sm and Co multitargets successively. The growth of the films was carried out at elevated substrate temperature followed by in situ annealing. On Si (100) substrate, X-ray diffraction confirms the formation of textured (110) SmCo5 hard phase, whereas on MgO (110) substrate, the diffraction pattern shows the epitaxial growth of SmCo5 phase with crystalline orientation along 100] direction. Secondary Ion Mass Spectroscopy reveals the structural transformation from multilayered (Sm/Co) to SmCo5/Co nano-composite films due to high reactivity of Sm at elevated temperature. Transmission electron microscopy indicates the existence of nanocrystalline phase of SmCo5 along with unreacted Co. Observed single phase behavior in magnetic hysteresis measurements indicates well exchange coupling between the soft and the hard phases in these nano-composite films. For samples with samarium layer thickness, t(sm)=3.2 nm and cobalt layer thickness, t(Co)= 11.4 nm, the values of (BH)(max) were obtained as 20.1 MGOe and 12.38 MGOe with H-c value similar to 3.0 kOe grown on MgO and Si substrates, respectively.

Multi-resistance states in the electrical switching behavior of amorphous Si15Te75Ge10 thin films: Possibility of multi-bit storage

Electrical switching studies on amorphous Si15Te75Ge10 thin film devices reveal the existence of two distinct, stable low-resistance, SET states, achieved by varying the electrical input to the device. The multiple resistance levels can be attributed to multi-stage crystallization, as observed from temperature dependant resistance studies. The devices are tested for their ability to be RESET with minimal resistance degradation; further, they exhibit a minimal drift in the SET resistance value even after several months of switching. (c) 2013 Elsevier B.V. All rights reserved.

Investigation of Magnon Assisted Resistivity in Permalloy Thin Films

Magnon contribution to the resistance of ferromagnetic film like Permalloy is investigated by magnetotransport measurements. We are able to observe and distinguish Anisotropic-Magnetoresistance(AMR)(1) and Magnon Magnetoresistance(MMR)(2) contributions clearly in PLD grown Permalloy films. A linear non-saturating longitudinal MR observed in high field regime for permalloy films could never be explained using AMR but only MMR can account for it.

Influence of annealing on structural, morphological, compositional and surface properties of magnetron sputtered nickel-titanium thin films

Thin films of NiTi were deposited by DC magnetron sputtering from an equiatomic alloy target (Ni/Ti: 50/50 at.%). The films were deposited without intentional heating of the substrates. The thickness of the deposited films was approximately 2 mu m. The structure and morphology of NiTi films annealed at different temperatures were analyzed in order to understand the effect of annealing on physical properties of the films. The compositional investigations of fresh and annealed films were also evaluated by energy dispersive X-ray spectroscopy (EDS) and X-ray photo-electron spectroscopy (XPS) techniques. X-ray diffraction (XRD) studies showed that as-deposited films were amorphous in nature whereas annealed films were found to poly-crystalline with the presence of Austenite phase as the dominant phase. AFM investigations showed higher grain size and surface roughness values in the annealed films. In annealed films, the grain size and film roughness values were increased from 10 to 85 nm and 2-18 nm. Film composition measured by EDS were found to 52.5 atomic percent of Ni and 47.5 atomic percent of Ti. XPS investigations, demonstrated the presence of Ni content on the surface of the films, in fresh films, whereas annealed films did not show any nickel. From HR-XPS investigations, it can be concluded that annealed NiTi films have higher tendency to form metal oxide (titanium dioxide) layer on the surface of the films than fresh NiTi films. (C) 2013 Elsevier B. V. All rights reserved.

Albumin-mediated incorporation of water-insoluble therapeutics in layer-by-layer assembled thin films and microcapsules

The present study demonstrates a method to deliver hydrophobic drugs by incorporation into thin films and microcapsules fabricated via a layer-by-layer assembly approach. The hydrophobic molecule binding properties of albumin have been exploited for solubilization of a water-insoluble molecule, pyrene (model drug), by preparation of non-covalent conjugates with bovine serum albumin (BSA). Conjugation with BSA renders a highly negative zeta potential to the previously uncharged pyrene which favors the assembly formation by electrostatic interaction with a positively charged polyelectrolyte, chitosan (at acidic pH). The growth of the assembly was followed by monitoring pyrene absorbance with successive layer deposition. The thin film assembly was demonstrated to be capable of releasing its hydrophobic cargo under physiological conditions. We demonstrated the applicability of this approach by encapsulating a water-insoluble drug, curcumin. These assemblies were further loaded with the anti-cancer drug Doxorubicin. Biocompatible calcium carbonate microparticles were used for capsule preparation. The porous nature of the microparticles allows for the pre-encapsulation of therapeutic macromolecules like protein. The fabrication of protein encapsulated stable microcapsules with hydrophobic molecules incorporated into the shell of the microcapsules has been demonstrated. The microcapsules were further capable of loading hydrophilic molecules like Rhodamine B. Thus, using the approach described, a multi-agent carrier for hydrophobic and hydrophilic drugs as well as therapeutic macromolecules can be envisioned.

Temperature dependence of resistance and crystallization in amorphous Si15Te85-xGex thin films

Amorphous thin chalcogenide Si15Te85-xGex films (x: 5, 9, 10, 11, 12) are prepared by flash evaporation and the temperature dependence of resistance of these films has been studied in the temperature range 25-250 degrees C. All the compositions show a linear variation of resistance in this temperature range. Apart from the linear variation, a sharp reduction in resistance at one or at two distinct temperatures (T-TR1/T-TR2) is seen. Thin films annealed at these temperatures, when subjected to X-ray diffraction studies suggest that the dominant crystalline phase at T-TR1 and at T-TR2 is the same and the two dips are associated with varying levels of crystallization. This is also reflected in the atomic force microscopic (AFM) study. Further, the resistance of these two phases shows no drift when the films are annealed for varying lengths of time (10 min to 120 min) suggesting the stability of the phases.

Tuning the ferromagnetic transition temperature in La0.5Sr0.5CoO3 thin films

Epitaxial La0.5Sr0.5CoO3 (LSCO) thin films are grown on LaAlO3 (100) and SrTiO3 (100) substrates by pulsed laser ablation. By tuning the growth parameters, we are able to enhance the ferromagnetic transition temperature (T-C) up to 262 K as evident from ac susceptibility, dc magnetization, and resistivity measurements. The magnitude of T-C is the same as that for the bulk stoichiometric LSCO illustrating the high quality of the grown films. Detailed structural analysis clearly reveals that the induced strain in the LSCO film has no role behind this enhancement; in fact, the determining factor is the oxygen stoichiometry. The films also exhibit ageing effect as the T-C decreases with time. This is considered in terms of gradual change in the oxygen stoichiometry through diffusion process as the time progresses. (C) 2013 AIP Publishing LLC.

Near-infrared photoactive Cu2ZnSnS4 thin films by co-sputtering

The thin films of Cu2ZnSnS4 (CZTS) were grown by co-sputtering further the structural, optical and electrical properties were analyzed and confirmed the CZTS phase formation. The photo response of CZTS in near IR photodectection has been demonstrated. The detector response was measured employing both the IR lamp and IR laser illuminations. The calculated growth and decay constants were 130 m sec and 700 m sec followed by the slower components upon lamp illumination. The external quantum efficiency of 15%, responsivity of 13 AW(-1) makes CZTS a suitable candidate for the IR photodectection.

Deformation-induced grain growth and twinning in nanocrystalline palladium thin films

The microstructure and mechanical properties of nanocrystalline Pd films prepared by magnetron sputtering have been investigated as a function of strain. The films were deposited onto polyimide substrates and tested in tensile mode. In order to follow the deformation processes in the material, several samples were strained to defined straining states, up to a maximum engineering strain of 10%, and prepared for post-mortem analysis. The nanocrystalline structure was investigated by quantitative automated crystal orientation mapping (ACOM) in a transmission electron microscope (TEM), identifying grain growth and twinning/detwinning resulting from dislocation activity as two of the mechanisms contributing to the macroscopic deformation. Depending on the initial twin density, the samples behaved differently. For low initial twin densities, an increasing twin density was found during straining. On the other hand, starting from a higher twin density, the twins were depleted with increasing strain. The findings from ACOM-TEM were confirmed by results from molecular dynamics (MD) simulations and from conventional and in-situ synchrotron X-ray diffraction (CXRD, SXRD) experiments.

Photovoltaic structures using thermally evaporated SnS and CdS thin films

Polycrystalline tin sulfide thin films were prepared by thermal evaporation technique. The films grown at substrate temperature of 300 degrees C had an orthorhombic crystal structure with strong preferred orientation along (111) plane. Electrical resistivity of the deposited films was about 32.5 Omega cm with a direct optical band gap of 1.33 eV. Carrier concentration and mobility of charge carriers estimated from the Hall measurement were found to be 6.24 x 10(15) cm(-3) and 30.7 cm(2)V(-1) s(-1) respectively. Heterojunction solar cells were fabricated in superstrate configuration using thermally evaporated SnS as an absorber layer and CdS, In: CdS as window layer. The resistivity of pure CdS thin film of a thickness of 320 nm was about 1-2 Omega cm and was reduced to 40 x 10(-3) Omega cm upon indium doping. The fabricated solar cells were characterized using solar simulator. The solar cells with indium doped CdS window layer showed improved performance as compared to pure CdS window layer. The best device had a conversion efficiency of 0.4% and a fill factor of 33.5%. (C) 2013 Elsevier B.V. All rights reserved.