E. Prokhorov

Determination of the glass transition and nucleation temperatures in Ge2Sb2Te5 sputtered films

Amorphous Ge2Sb2Te5 films were prepared by rf sputtering from the bulk alloy. Their electrical and optical properties were analyzed using impedance and optical transmission measurements as a function of temperature. From this analysis, it is found that the glass transition temperature of the as-prepared amorphous films is of about 100 °C. This result is confirmed by calorimetric measurements. Impedance and optical measurements in films measured at temperatures between the glass transition and the crystallization temperatures show the appearance of the nucleation centers. Using models for two-phase materials, the electrical parameters of the crystalline and amorphous phases were estimated and related with structural parameters of the samples.

Impedance spectroscopy studies on SnO2 films prepared by the sol–gel process

Abstract The stannic chloride was used as the starting material to prepare undoped SnO 2 films. Films with different thickness were made by multiple applications of the precursor solutions using a dip coating apparatus. The as-prepared and heat treated, in air, films were analyzed using impedance spectroscopy as a function of the thickness or number of layers constituting the films. The impedance spectra of the films represented in the complex plane showed two well-defined but partially convoluted semicircles. The impedance spectra were analyzed using the brick layer model, generally employed for polycrystalline materials. X-ray diffraction, optical transmission and reflection as well as atomic force microscopy measurements were also used in the characterization of the films. The results show that the as-deposited SnO 2 films are amorphous and that the annealing treatments produce their crystallization and densification. The impedance spectra provided quantitative information about the densification process, which is in agreement with the qualitative information obtained by the atomic force microscopy measurements.

Surface functionalized halloysite nanotubes decorated with silver nanoparticles for enzyme immobilization and biosensing

Improving enzyme immobilization with high loading capacity and achieving direct electron transfer (DET) between the enzyme and the electrode surface is key to designing highly sensitive enzymatic electrochemical biosensors. Herein, we report a novel approach based on the selective modification of the outer surface of halloysite nanotubes (HNTs) that supports silver nanoparticles (AgNPs) to obtain a hybrid nanocomposite. AgNPs of about 10 nm average size could be uniformly supported on silane-modified HNTs through in situ reduction of Ag+ ions. The resultant nanocomposite shows an excellent support capability for the effective immobilization and electrical wiring of redox enzyme glucose oxidase (GOx). The GOx immobilized HNT/AgNPs were deposited on the glassy carbon electrode (GCE) and utilized for the bioelectrocatalyzed electrochemical detection of glucose. The GOx modified composite electrodes show glucose sensitivity as high as 5.1 μA mM-1 cm-2, which is higher than for the electrodes prepared without surface functionalization.

Molecular dynamics analysis of PVA- AgnP composites by dielectric spectroscopy

The molecular dynamics of PVA/AgnP composites were studied by dielectric spectroscopy (DS) in the 20-300°C temperature range. Improper water elimination leads to misinterpretation of thermal relaxations in PVA composites in agreement with the previous report for pristine PVA. The evaporation of water and its plasticizing effect are more evident in pure PVA confirming the existence of strong interaction between OH groups of PVA chains and AgnP. Dry films show a single nonlinear VFT dependence (from 45°C until melting) associated to the α-relaxation and, therefore, to the glass transition phenomenon and from dielectric measurements, the Tg of composites vary from 88*deg;C for pristine PVA to 125°C for PVA/AgnP (5wt%). Below 45°C, dry films exhibit a single Arrhenius behavior showing a 3D hopping conductivity as explained based on the variable range hopping model. PVA/AgnP composites have higher conductivity compared to pristine PVA, and it increases as AgnP weight percent increases. Finally, DMA measurements support the statement that a secondary relaxation was erroneously assigned as the glass transition of PVA and composites in previous reports.

Temperature dependence of structure and electrical properties of germanium–antimony–tellurium thin film

The interest in the study of Ge:Sb:Te thin films is due to their use as optical and electrical memory materials. Both of these applications are based on the structural change from the amorphous to the crystalline state. Thus, understanding of the mechanism of crystallization in this material is important from basic and technological points of view. In this work we have studied the kinetics of the crystallization of Ge:Sb:Te films prepared by thermal evaporation. For that, in situ resistance and capacitance measurements during heating were used. The transformation kinetics from the amorphous to the crystalline state were analyzed using the Kissinger model, from which the activation energy of the crystallization process is obtained. Using x-ray diffraction, Raman spectroscopy and optical microscope measurements, we have observed that during heating at different heating rates, crystallization of the film is accompanied by the segregation of micrometric inclusions formed by amorphous tissue, perhaps some segregated impurities and crystalline tellurium particles. The number and size of these inclusions depend on the heating rate. From our measurements we found that the capacitance measurements is a sensitive method by which to analyze the crystallization process in thin films. It provides additional information not obtained using other methods.

Mechanism of crystallization of oxygen-doped amorphous Ge1Sb2Te4 thin films

The aim of this article is to study the mechanism of the amorphous-to-crystalline phase transformation in Ge1Sb2Te4 alloys doped by oxygen (in the range of 4–28 at. %) using the electrical, optical, and x-ray measurements. Experimental results have shown that samples with oxygen in the range of about 4–10 at. % first crystallize into the Ge1Sb2Te4 fcc phase at temperatures in the range 130–145 °C, at higher temperature (around 275 °C) the fcc phase is transformed into the Ge1Sb2Te4 hexagonal phase. In samples with oxygen in the range of 10–15 at. % the crystallization occurs in two stages producing samples where Ge1Sb2Te4 and the Sb2Te3 coexist. Samples with higher oxygen content, in the range of 15–28 at. % the films crystallized into the rhombohedral Sb2Te3 phase due to the formation of amorphous germanium oxide. The experimental results have been interpreted employing the Johnson–Avrami–Mehl–Kolmogorov theory. Using isothermal measurements and the time dependence of the Avrami exponent, three different...

Convergence of a Hydraulic Solver with Pressure-Dependent Demands

This paper analyzes the convergence of a pressure-driven analysis (PDA) model of a water distribution network solver based on Todini’s global gradient algorithm. The PDA model is constructed by embedding a pressure−demand relationship in the EPANET simulator code. To avoid spurious convergence, a residual-based convergence error was used. The introduction of pressure-dependent demands is shown to result in a far poorer convergence. The study of solver convergence as a function of the smoothness of the pressure−demand curve has demonstrated that, statistically, a smooth pressure−demand relationship gives a somewhat better convergence. To improve convergence, use was made of a quadratic approximation of the Hazen–Williams head loss−flow relationship in the vicinity of zero and the correct implementation of the Darcy−Weisbach formula in the solver. To further improve convergence, an iteration step control technique called the line search was used. The analysis of solver convergence for different line search variants has shown that the line search in its usual form is not efficient enough and may result in poorer convergence. A necessary error decrease algorithm, whose use in the line search improves solver convergence, is proposed. It is shown that due to the convergence improvement methods the convergence of the PDA solver is somewhat better than that of the demand-driven analysis solver and sufficient for direct problems such as design, for example.

Structural and electrical properties of Ge1Sb2Te4 face centered cubic phase

The aim of this article is to study the properties of the crystalline face centered cubic Ge1Sb2Te4 phase as a function of annealing and measuring temperature. This material is one of the stoichiometric members of the Ge:Sb:Te family, which is widely used in the phase change data storage and one candidate for multistate recording. The electrical properties of this material have been investigated using two independent methods, the four probe impedance and the Hall measurements, and the results are interpreted in terms of structural parameters obtained from x-ray diffraction and transmission electron microscopy results. Experimental results have shown the polycrystalline nature of the film and that the transport properties have a strong dependence on the annealing temperature. An increase in the annealing temperature leads to an increase in the grain size, carrier concentration, mobility, and to a decrease in the volume fraction of grain boundaries. The mobility is limited by scattering of charge carries at...

Chitosan supported silver nanowires as a platform for direct electrochemistry and highly sensitive electrochemical glucose biosensing

The development of low-cost and sensitive glucose biosensors has been the focus of substantial research interest due to their diverse applications in medical diagnosis, healthcare, and environmental monitoring. Herein, we report the successful use of chitosan (CS) supported silver nanowires (AgNWs) based enzyme electrodes for highly sensitive electrochemical glucose biosensing. The glucose oxidase (GOx) enzyme is electrically contacted using highly conductive AgNWs and thereby significant enhancement in the direct electron transfer (DET) between the redox enzymes and the electrode surface. In addition, the CS polymer matrix enables distinct self-assembly of GOx enzymes adjacent to the electrode surface, which further favours DET by increasing the charge transfer. Characterization by Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM) measurements were used to evidence the intermolecular interaction and self-assembly of the GOx on CS polymers. AFM results clearly revealed the self-assembly of the GOx on CS surfaces. The immobilized GOx exhibits a well-defined quasi-reversible redox peak with an electron rate constant (ks) of 6.52 s−1 compared to the bare glassy carbon electrode (GCE). The resultant biosensor demonstrates a high sensitivity of 16.72 μA mM−1 cm−2 with a wide linear range (1–15 mM), good selectivity and long-term stability for glucose detection. Our current approach represents a promising platform for the immobilization and electrical wiring of biomolecules with higher loading efficiency for designing low-cost, high sensitive enzymatic biosensors.

Temperature-induced Au nanostructure synthesis in a nonaqueous deep-eutectic solvent for high performance electrocatalysis

Structure-controlled synthesis of gold nanostructures (AuNSs) induced by temperature in a nonaqueous urea–choline chloride deep eutectic solvent (DES) is reported. Modulation of nanostructures with well-defined structures and shapes is obtained by simply varying the reaction temperature. The supramolecular soft template provided by the DES structure and its viscosity at different temperatures drives directed growth of crystalline gold and self-assembly producing star-shaped AuNSs. Additionally, the effect of AuNS shape and surface area on their catalytic activity towards the reduction of hydrogen peroxide (H2O2) has been tested. With the advantage of their high surface area and presence of high-index facets in the edge of the star arms, the star-shaped nanostructures showed superior electrocatalytic activity than other morphologies. The use of DES as a green chemistry platform for the synthesis of shape-controlled Au nanostructures with high catalytic properties may offer new avenues for fuel cell and biosensor applications.