• Volume 37, Issue 7

December 2014,   pages  1563-1782

• ALD TiO2 thin film as dielectric for Al/p-Si Schottky diode

Electrical analysis of Al/p-Si Schottky diode with titanium dioxide (TiO2) thin film was performed at room temperature. The forward and reverse bias current–voltage (𝐼–𝑉) characteristics of diode were studied. Using thermionic emission (TE) theory, the main electrical parameters of the Al/TiO2/p-Si Schottky diode such as ideality factor (𝑛), zero bias barrier height (𝜙Bo) and series resistance (𝑅s) were estimated from forward bias 𝐼–𝑉 plots. At the same time, values of 𝑛, 𝜙Bo and 𝑅s were obtained from Cheung’s method. It was shown that electrical parameters obtained from TE theory and Cheung’s method exhibit close agreement with each other. The reverse-bias leakage current mechanism of Al/TiO2/p-Si Schottky barrier diodes was investigated. The 𝐼–𝑉 curves in the reverse direction are taken and interpreted via both Schottky and Poole–Frenkel effects. Schottky effect was found to be dominant in the reverse direction. In addition, the capacitance–voltage (𝐶–𝑉) and conductance–voltage (𝐺/𝑤–𝑉) characteristics of diode were investigated at different frequencies (50–500 kHz). The frequency dependence of interface states density was obtained from the Hill–Coleman method and the voltage dependence of interface states density was obtained from the high–low frequency capacitance method.

• Low energy ion beam modification of Cu/Ni/Si(100) surface

Cu/Ni bilayer has been prepared by thermal evaporation of pure Cu and Ni metals onto Si(100) surface in high vacuum; it was sputtered using argon ion beam in ultra-high vacuum. The ion beam-induced surface and interface modification was investigated using X-ray photoelectron spectroscopy and atomic force microscopy techniques. The deposited sample exhibits the formation of CuO nano-structures of size 40 nm on Cu surface and after sputtering with argon ion beam at a fluence of 5 × 1015 ions/cm2, the surface exhibits a mound structure with an average size of about 100 nm. Interestingly, with sputtering at higher fluence of 2.4 × 1016 ions/cm2, the surface exhibits broad pits of sizes ranging from 100 to 300 nm with an average depth of 10 nm. Bottom surface of these pits contains Ni atoms. The Cu 2𝑝3/2 peak exhibits a shift of 0.3 eV towards high binding energy and also a large asymmetry of 0.11 after sputtering at high fluence compared with pure copper. These changes are attributed to Cu–Ni interactions at the interface.

• Effect of temperature on the AC impedance of protein and carbohydrate biopolymers

The influence of temperature on the electrical behaviour of protein biopolymer papain and carbohydrate biopolymers like gum acacia, gum tragacanth and guar gum has been investigated using AC impedance technique. The observed semi-circles represent the material’s bulk electrical property that indicate the single relaxation process in the biopolymers. An increase in bulk electrical conductivity in the biopolymers with temperature is due to the hopping of charge carriers between the trapped sites. The depression parameter reveals the electrical equivalent circuit for the biopolymers. The AC electrical conductivity in the biopolymers follows the universal power law. From this, it is observed that the AC conductivity is frequency dependent and the biopolymer papain obeys large polaron tunnelling model, gum acacia and gum guar obey ion or electron tunnelling model, and gum tragacanth obeys the correlated barrier hopping model of conduction mechanisms.

• Optimization of nanocrystalline 𝛾-alumina coating for direct spray water-cooling of optical devices

In this study, aluminium oxide films were deposited on BK7 glass substrates using radio frequencymagnetron sputtering. The purposes of this study are to clarify the influence of O2 flow as reactive partial gas, which is necessary to form Al2O3 films, and then the influence of substrate temperature on structure and rigidity of coatings towards water injection. The fabricated metal oxide films were characterized using techniques such as atomic force microscopy (AFM), X-ray diffraction (XRD), spectrophotometry, ellipsometry and Rutherford backscattering (RBS) analysis. Modifications of the partial gas percentage influences the optical properties and composition of the deposited aluminium oxide, the best samples being those deposited with 5% and 8% oxygen. The substrate temperature affects the structure and crystallization of the films. Nanocrystalline 𝛾-Al2O3 has been observed at temperatures above 300 °C with the grain size of 25 nm. After water injection, there was a large diversity in the surface roughness of samples with different substrate temperature. Experiments have shown that the best resistance against water injection occurs for the sample deposited at 350 °C with 5%partial gas. We conclude that the rigidity of nanocrystalline 𝛾-Al2O3 coatings can be explained by both Hall–Petch and Coble creep mechanism. In this case, there is an optimum grain size of around 42 nm against water spray.

• Growth, optical, thermal and mechanical characterization of an organic crystal: Brucinium 5-sulfosalicylate trihydrate

Single crystals of Brucinium 5-sulfosalicylate trihydrate (B5ST) were grown from ethanol–water (1:1) mixed solvent by the slow solvent evaporation method. X-ray powder diffraction analysis reveals that the crystal belongs to orthorhombic system with space group 𝑃212121. The various reflections were indexed and the lattice parameters were calculated. Photoluminescence (PL) shows peaks corresponding to protonation of the amino group. The optical absorption spectrum shows that the crystal has 90% transmittance in the visible region with a lower cut-off wavelength of 312 nm. Thermal analysis performed on the grown crystal indicates the thermal stability of the crystal and various thermodynamical parameters were calculated from the thermogravimetry (TG) data. The mechanical properties like Vickers microhardness number (𝐻v), stiffness constant (𝐶11) and yield strength (𝜎v) of the crystal were estimated by Vickers hardness test.

• In𝑥Ga1−𝑥N fibres grown on Au/SiO2 by chemical vapour deposition

The growth of In𝑥Ga1−𝑥N films (𝑥 = 0.1 and 𝑥 = 0.2) on a thin gold layer (Au/SiO2) by chemical vapour deposition (CVD) at 650 °C is reported. As a novelty, the use of a Ga–In metallic alloy to improve the indium incorporation in the In𝑥Ga1−𝑥N is proposed. The results of high quality In𝑥Ga1−𝑥N films with a thickness of three micrometres and the formation of microfibres on the surface are presented. A morphological comparison between the In𝑥Ga1−𝑥N and GaN films is shown as a function of the indium incorporation. The highest crystalline In𝑥Ga1−𝑥N films structure was obtained with an indium composition of 𝑥 = 0.20. Also, the preferential growth on the (002) plane over In0.2Ga0.8N was observed by means of X-ray diffraction. The thermoluminescence (TL) of the In𝑥Ga1−𝑥N films after beta radiation exposure was measured indicating the presence of charge trapping levels responsible for a broad TL glow curve with a maximum intensity around 150 °C. The TL intensity was found to depend on composition being higher for 𝑥 = 0.1 and increases as radiation dose increases.

• Electric field dependence of the electron mobility in bulk wurtzite ZnO

The electric field dependence of the electron mobility in bulk wurtzite zinc oxide (ZnO) material is studied. The low-field electron mobility is calculated as a function of doping concentration and lattice temperature. The results show that above nearly 50 K the electrical conduction is governed by activation through the bulk material and the conduction is then influenced by both lattice and impurity scattering mechanisms. The high-field characteristics are also considered. The transition between the low-field and high-field regions is specified. The negative differential mobility for bulk ZnO at room temperature is observed at electric fields above 280 kV/cm.

• Microwave synthesis and mechanical characterization of functionally graded material for applications in fusion devices

Functionally graded tungsten–copper bimetallic compact with fine microstructure and good mechanical property has been synthesized by employing microwave heating method at a temperature of 800 °C and in a short processing time of 30 min. Scanning electron microscopy and energy dispersive X-ray analysis revealed the graded structure of synthesized sample. The fine microstructure of tungsten in each layer is caused by arrested grain growth because of the short sintering time. The overall relative density of the W/Cu functionally graded sample has reached 87% of the theoretical density. Vickers microhardness measurements, across the length of a compact, show increase in hardness value of the sample with the increase in tungsten content. The experimental hardness values match well with the theoretically calculated hardness values.

• Enhancement of proton conductivity of sulfonated polystyrene membrane prepared by plasma polymerization process

This work reports the achievement of higher proton conductivity of polystyrene based proton exchange membrane synthesized in a continuous RF plasma polymerization process using two precursors, styrene (C8H8) and trifluoromethane sulfonic acid (CF3SO3H). The chemical composition of the developed membranes is investigated using Fourier transform infrared spectroscopy and energy dispersive spectroscopy. Scanning electron microscopy has been used for the study of surface morphology and thickness measurement of the membrane. The membranes deposited in the power range from 0.114 to 0.318 Wcm-2 exhibit a lot of variation in the properties like proton transport, water uptake, sulfonation rate, ion exchange capacity and thermal behaviour. The proton conductivity of the membranes is achieved up to 0.6 Scm-1, measured with the help of potentiostat/galvanostat. The thermogravimetric study of the plasma polymerized membrane shows the thermal stability up to 140 °C temperature.

• Effect of Au/SiO2 substrate on the structural and optical properties of gallium nitride grown by CVD

The improvement of the growth of thick GaN films using a fused silica wafer covered with a thin gold layer by chemical vapour deposition at 800 °C is reported. In order to compare the surface properties, crystalline quality, micromilling performance and luminescence, the characterization of a GaN film grown on a silicon wafer is presented as well. The different morphologies of the surface observed on the GaN films are compared on each substrate and the resulting microstructures are presented in detail. High resolution TEM images of the GaN films show the main crystallographic planes characterizing these structures. The wurtzite structure was determined for each sample using the substrates of Au/SiO2 and Si (100) from the XRD patterns. Also, the re-deposition effect after ion milling of the GaN films is reported. The performance of ionic beam on the surface of the GaN thick films for the geometries patterning of rectangular, circular and annular with two different ion doses was compared. Cathodoluminescence spectra showed that the top surfaces of the samples emit strong UV emissions peaked at 3.35 and 3.32 eV which are related to the Y4 and Y6 transitions.

• Synthesis of selenium nanorods with assistance of biomolecule

Nanorods of one-dimensional (1D) trigonal selenium (t-Se) are synthesized using biomolecule substances for five different aging times (1 h, 2 h, 3 h, 1 day and 4 days) by precipitation method. XRD analysis indicates a shift of the (1 0 1) plane towards higher diffraction angle for 1 day aging time. It is observed that the crystallite size decreases with increase in aging time except for an aging period of 4 days. FTIR analysis confirmed that the presence of stretching and bending vibrations of Se–O in both synthesized and commercial selenium samples at 465, 668 and 1118 cm-1. The FESEM micrographs are evident for the changes of rod size as a function of aging time. It is observed that the optical band gap energy is increased with aging time up to 1 day, whereas it decreases in 4 days aging time.

• Novel composite sorbent AAm/MA hydrogels containing starch and kaolin for water sorption and dye uptake

A novel polymer/clay composite sorbent based on acrylamide/maleic acid, starch and clay such as kaolin was synthesized with free radical solution polymerization by using ammonium persulfate/𝑁,𝑁,𝑁',𝑁'-tetramethylethylenediamine as redox initiating pair in the presence of poly(ethylene glycol)diacrylate as a crosslinker. Surface morphology was characterized by the scanning electron microscopy technique. FT-IR analysis was used to identify the presence of different repeating units in the semi-interpenetrating polymer networks (semi-IPNs). Some swelling and diffusion characteristics were calculated for different semi-IPNs and hydrogels prepared under various formulations. The hydrogels were used in experiments on sorption of water-soluble cationic dye such as toluidine blue. Sorption of toluidine blue into the polymeric systems was studied by the batch sorption technique at 25 °C. Water uptake and dye sorption properties of the crosslinked polymeric systems were investigated as a function of chemical composition of the hydrogels.

• Optical, mechanical and TEM assessment of titania-doped Bi2V1−𝑥Ti𝑥O$_{5.5−\delta}$ bismuth vanadate oxides

Optical, mechanical and structural behaviors have been studied for titania-doped Bi2V1−𝑥Ti𝑥O$_{5.5−\delta}$ which are used as electrolytes for intermediate temperature fuel cells. Parameters like band gap ($E_{g}$), Urbach energy ($E_{u}$), refractive index, hardness (𝐻) and fracture strength (𝐾) have been calculated as a function of dopant concentration, i.e. 0.05 ≤ 𝑥 ≤ 0.2. Furthermore, analysis of transmission electron microscopy (TEM) images for all the oxides was conducted along with line spectra of planes. Results are discussed in light of correlation of these optical and mechanical parameters to their structural properties. Band gap has also been correlated to the conductivity of these oxides. Good correlation has been obtained between them.

• Bipolar resistive switching behaviours in ZnMn2O4 film deposited on p+-Si substrate by chemical solution deposition

ZnMn2O4 active layer for resistance random access memory (RRAM) was deposited on p+-Si substrate by chemical solution deposition. The bipolar resistive switching behaviours of the Ag/ZnMn2O4/p+-Si capacitor are investigated. The bipolar resistive switching is reproducible and shows high ON/OFF ratio of &gt; 102 and long retention times of &gt; 105 s. The conduction mechanism of the Ag/ZnMn2O4/p+-Si capacitor in the low-resistance state (LRS) is ohmic conduction, whereas that of the device in high-resistance state (HRS) successively undergoes Ohm’s law, trap-filled-limited and Child’s law conduction procedure at room temperature.

• Influence of growth temperature on morphological, structural and photoluminescence properties of ZnO nanostructure thin layers and powders deposited by thermal evaporation

Zinc oxide (ZnO) nanostructures were grown as thin films on the p-silicon (100) wafer and also in the form of powder inside the boat by heating (550–950 °C) zinc powder in the presence of oxygen without any catalyst or additives, using the thermal evaporation method. The field-emission scanning electron microscopy images, as well as energy-dispersive X-ray spectroscopy and X-ray diffraction spectra, indicate that although the grown samples are covered with various nanostructure shapes, such as nanowires, nanorods, flower-like nanostructures andmicrocages, all have a reasonable stoichiometric composition in the polycrystalline wurtzite phase along (002) in the thin layer samples and along (101) in the powder samples within the boat. The room-temperature photoluminescence spectra of the thin layer samples revealed not only the ultraviolet (UV) emission blue shift of the samples with an increase in the growth temperature, but also found that the emission intensity ratio of UV/visible (∼510 nm) has a maximum and minimum, corresponding to that grown at 750 and 950 °C, respectively.

• Synthesis of Ag-doped hydrogenated carbon thin films by a hybrid PVD–PECVD deposition process

Silver-doped hydrogenated amorphous carbon (Ag-DLC) films were deposited on Si substrates using a hybrid plasma vapour deposition–plasma enhanced chemical vapour deposition (PVD–PECVD) process combining Ag target magnetron sputtering and PECVD in an Ar–CH4 plasma. Processing parameters (working pressure, CH4/Ar ratio and magnetron current) were varied to obtain good deposition rate and a wide variety of Ag films. Structure and bonding environment of the films were obtained from transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and Fourier transform infrared (FTIR) spectroscopy studies. Variation of processing parameters was found to produce Ag-doped amorphous carbon or diamond-like carbon (DLC) films with a range of characteristics with CH4/Ar ratio exercising a dominant effect. It was pointed out that Ag concentration and deposition rate of the film increased with the increase in d.c. magnetron current. At higher Ar concentration in plasma, Ag content increased whereas deposition rate of the film decreased. FTIR study showed that the films contained a significant amount of hydrogen and, as a result of an increase in the Ag content in the hydrogenated DLC film, $sp^{2}$ bond content also increased. The TEM cross sectional studies revealed that crystalline Ag particles were formed with a size in the range of 2–4 nm throughout an amorphous DLC matrix.

• A study on role of nanosized SiO2 on deformation mechanism of vinyl ester

Vinyl ester (VE) resins have good resistance against corrosive and hostile environments, and formation of micro cracks. They also have good thermal resistivity as well as mechanical properties which result in their usage in many applications such as sewer pipes, solvent storage tanks, and mining and other industrial equipments. In the present work, nanosized silica was employed as reinforcement to improve VE mechanical properties. Hence, to study the influence of nanosilica particles on the tensile strength, Young’s modulus and toughness of mentioned nanocomposites, nanosilica-vinyl ester nanocomposites with different silica weight percentages (0.3, 0.75, 1) were fabricated. Moreover, the effects of nanosilica particles on the tensile fracture surfaces and VE deformation mechanisms were studied by scanning electron microscopy (SEM). It was observed that increasing the nanosilica fillers results in tensile strength deterioration as well as Young’s Modulus increasing. Adding nanosilica reinforcements up to 0.3 (%wt) improves the fracture toughness while decreasing the fracture energy.

• Processing of water-based LiNi1/3Mn1/3Co1/3O2 pastes for manufacturing lithium ion battery cathodes

In order to meet the demand for more ecological and economic fabrication of lithium ion (Li-ion) batteries, water is considered as an alternative solvent for electrode paste preparation. In this study, we report on the feasibility of water-based processing of LiNi1/3Mn1/3Co1/3O2-based pastes for manufacturing cathode electrodes. The influence of the total solid content, the amount of conductive agent and binder materials on paste rheology and the final electrode properties was investigated. Suitable paste formulations which enable favourable paste flow behaviour, appropriate electrode properties and good electrochemical performance have been found. Results show that a substitution of the conventional organic solvent-based manufacturing route for LiNi1/3Mn1/3Co1/3O2 cathodes by water-based processing exhibits a promising way to realise Li-ion batteries with comparable electrochemical behaviour, while avoiding toxic processing aids and reducing overall manufacturing costs.

• Substrate bias voltage and deposition temperature dependence on properties of rf-magnetron sputtered titanium films on silicon (100)

Thin films or a coating of any sort prior to its application into real world has to be studied for the dependence of process variables on their structural and functional properties. One such study based on the influence of substrate conditions viz. substrate-bias voltage and substrate temperature on the structural and morphological properties, could be of great interest as far as Ti thin films are concerned. From X-ray texture pole figure and electron microscopy analysis, it was found that substrate bias voltage strongly influence preferential orientation and morphology of Ti films grown on Si (100) substrate. Deposition at higher substrate temperature causes the film to react with Si forming silicides at the film/Si substrate interface. Ti film undergoes a microstructural transition from hexagonal plate-like to round-shaped grains as the substrate temperature was raised from 300 to 50 °C during film deposition.

• Improving electrochemical performance of tin-based anodes formed via oblique angle deposition method

An oblique angle electron beam co-deposition technique was used to fabricate nanostructured Sn-based thin films: Sn, Cu–Sn and Cu–Sn–C. The morphological and structural properties of the films were observed via scanning electron microscopy (SEM) and thin film X-ray diffraction (XRD) methods. The electrochemical (CV and EIS) and the galvanostatic test results demonstrated that the addition of Cu with or without C affected the electrochemical performance of the thin film positively since Cu and C improved both the mechanical and the electrical properties of the nanostructured Sn thin film electrode. The high cycleability and capacity retention were achieved when the nanostructured Cu–Sn–C thin film was used as an anode material since C increased the mechanical tolerance of the thin film to the volume expansion due to its grain refiner effect. Cu not only improved the electrical conductivity and the adhesion of the film to substrate but also the mechanical tolerance of the film with its ductile property.

• Effect of fluoride additives on production and characterization of nano-𝛼-Al2O3 particles

Nano-𝛼-Al2O3 particles were synthesized by a sol–gel method using aqueous solutions of aluminum isopropoxide and 0.5 Maluminum nitrate. Sodium dodecylbenzenesulfonate (SDBS) was used as surfactant stabilizing agents. The solution was stirred for 48 h at 60 °C. The microstructural observation showed that surfactant with 48 h stirring time nanoscale 𝛼-Al2O3 powder was produced at 1200 °C in the range of 20–30 nm in the presence of SDBS as a surfactant with 48 h stirring time. Temperature reduction of 𝛼-Al2O3 formation has been achieved by introducing fluoride in the alumina precursor. The effects of ZnF2 additive and milling on the phase transformation as well as micrograph of the prepared 𝛼-Al2O3 particles were investigated. The samples were characterized by different techniques such as X-ray diffraction, thermogravimetry analysis, differential scanning calorimetry, Fourier transform infrared spectra, scanning electron microscopy and transmission electron microscopy. Results indicated that using zinc fluoride (ZnF2) additive accompanied by milling led to retardation of the transformation temperature and modification of the alumina particle shape. The finest size for nonagglomerated nano-plate-like 𝛼-Al2O3 particles (15–20 nm) was achieved at 950 °C.

• Investigation of diode parameters using 𝐼–𝑉 and 𝐶–𝑉 characteristics of Al/maleic anhydride (MA)/p-Si structure

Al/maleic anhydride (MA)/p-Si metal–polymer–semiconductor (MPS) structures were prepared on p-Si substrate by spin coating. Device parameters of Al/MA/p-Si structure have been determined by means of capacitance–voltage (𝐶–𝑉) and conductance–voltage (𝐺–𝑉) measurements between 700 kHz and 1.5 MHz and current–voltage (𝐼–𝑉) measurements at 300 K. The parameters of diode such as the ideality factor, series resistance, barrier height (BH) and flat band barrier height were calculated from the forward bias 𝐼–𝑉 characteristics. The investigation of interface states that density and series resistance from 𝐶–𝑉 and 𝐺–𝑉 characteristics in Al/MA/p-Si device has been reported. The frequency dependence of the capacitance could be attributed to trapping states. Several important device parameters such as the BH $\phi_{b}$, fermi energy ($E_{F}$), diffusion voltage ($V_{D}$), donor carrier concentration ($N_{D}$) and space charge layer width ($W_{D}$) for the device have been obtained between 700 kHz and 1.5 MHz. The 𝐼–𝑉, 𝐶–𝑉-𝑓 and 𝐺–𝑉-𝑓 characteristics confirm that the parameters like the BH, interface state density (𝐷it) and series resistance ($R_{s}$) of the diode are strongly dependent on the electrical parameters in the MPS structures.

• Pr and Gd co-doped bismuth ferrite thin films with enhanced multiferroic properties

Pr and Gd co-modified Bi0.95−𝑥Pr𝑥Gd0.05FeO3 (𝑥 = 0.00, 0.05, 0.10) (BPGFO) thin films on Pt(111)/Ti/SiO2/Si(100) substrates were prepared by a sol-gel together with spin coating technique. A detailed study of electrical and magnetic properties of these thin films is reported. X-ray diffraction analysis shows that, with an increase in Pr content, the crystal structures of BPGFO thin films retain rhombohedral (R3c) symmetry accompanied by structure distortion. Polarization-electric field hysteresis loops of these thin films demonstrate that the incorporation of Pr and Gd into the Bi site of BiFeO3 thin film could enhance the ferroelectric performance. Compared to other thin films, the optimal ferroelectric behaviours in Bi0.85Pr0.1Gd0.05FeO3 thin film are ascribed to its large dielectric constant, low dissipation factor and low leakage current density. Room temperature magnetization-magnetic field curves of these thin films indicate that all the samples are of paramagnetic behaviours and the enhanced saturation magnetic properties can be found.

• First principle calculations for improving desorption temperature in Mg16H32 doped with Ca, Sr and Ba elements

Using ab initio calculations, we predict the improvement of the desorption temperature and the hydrogen storage properties of doped Mg-based hydrides such as,Mg15AMH32 (AM = Ca, Sr and Ba) as a super cell 2 × 2 × 2 of MgH2. In particular, the electronic structure has been obtained numerically using the all-electron full-potential local-orbital minimum-basis scheme FPLO9.00-34. Then, we discuss the formation energy calculations in terms of the material stabilities and the hydrogen storage thermodynamic properties improvements. Among others, we find that the stability and the temperature of desorption decrease without reducing significantly the high storage capacity of hydrogen. Moreover, it has been observed that such a doping procedure does not affect the electronic behavior as seen in MgH2, including the insulator state in contrast with the transition metal hydrides, which modify the electronic structure of pure MgH2.

• Subject index

• Author index

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 6
December 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019

Click here for Editorial Note on CAP Mode

© 2017-2019 Indian Academy of Sciences, Bengaluru.