• Volume 42, Issue 6

December 2019

• A cellulosic paper-based sensor for detection of starch contamination in milk

Chemical and biomolecular analysis form an integral part of food contamination testing and healthcare diagnostics.The majority of the currently established detection methods are expensive, sophisticated and time consuming.Microfluidic-based lab-on-a-chip systems offer a promising alternative in this regard by enabling the development of simpler and relatively cost-effective platforms for analytical applications. In this work, we report the fabrication of a portable sensor platform using selective wax impregnation of cellulosic paper. The application of the fabricated paper-based lab-on-achip device is demonstrated for the qualitative and quantitative detection of starch contamination in milk using colorimetry. Integration of a detection procedure with smartphone imaging allows for on-the-spot data collection, thus overcoming thechallenges of sample storage, handling and transportation to lab facilities. This approach offers a versatile method for diagnostic and sensing applications in resource-limited areas, especially in developing and underdeveloped countries.

• Ligand exchange in Cu$_2$ZnSnS$_4$ nanoparticles and its effect on counter electrode performance in dye-sensitized solar cells

Ligand-exchanged Cu$_2$ZnSnS$_4$ (CZTS) nanoparticles (NPs) were successfully synthesized from colloidal NPs by replacing the long chain organic ligand from the surface of NPs via a bi-phasic method. It was found that ammonium sulphide salt ((NH$_4$)$_2$S) plays a key role in changing the surface of the NPs from hydrophobic to hydrophilic. The efficacy of the ligand exchange process over the surface of the CZTS NPs was analysed using X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy and scanning electron microscopy with energy dispersive X-ray. The ligand-exchanged CZTS NP-based counter electrodes (CEs) were fabricated by drop casting the inorganic ligand (ammonium sulphide)-capped CZTS nanoink onto the conducting substrate. Our result indicates that dye-sensitized solar cells (DSSCs) with inexpensive CZTS NP-based CEs show 2.42% efficiency. The present result indicates that CZTS CEs will be helpful as an alternative CE to a Pt CE in DSSC application.

• Enhancement of quantum capacitance by chemical modification of graphene supercapacitor electrodes: a study by first principles

In this paper, we specify a powerful way to boost quantum capacitance of graphene-based electrode materials by density functional theory calculations. We performed functionalization of graphene to manifest high-quantum capacitance. A marked quantum capacitance of above 420 $\mu$F cm$^{−2}$ has been observed. Our calculations show that quantum capacitanceof graphene enhances with nitrogen concentration. We have also scrutinized effect on the increase of graphene quantum capacitance due to the variation of doping concentration, configuration change as well as co-doping with nitrogen and oxygen ad-atoms in pristine graphene sheets. A significant increase in quantum capacitance was theoretically detected in functionalized graphene, mainly because of the generation of new electronic states near the Dirac point and the shift of Fermi level caused by ad-atom adsorption.

• Development of a silicon photodiode-based compact gamma spectrometer using a Gd$_3$Ga$_3$Al$_2$O$_{12}$:Ce,B single crystal scintillator

A compact gamma spectrometer was developed by employing an in-house grown single crystal of Gd$_3$Ga$_3$Al$_2$O$_{12}$:Ce,B scintillator optically coupled with a silicon photodiode. The performance of the detector was characterized in detail. The detector setup works with a low bias voltage of 9 V, drawn from a single battery. Power to the electronic components of the entire system is derived from a single universal serial bus port by employing required DC–DC converters. In addition to the low voltage operation, this developed spectrometer is very compact in size compared to the one developedby employing photo-multiplier tubes. The system offers excellent linearity over the gamma energy range of 344–1408 keV and an optimum energy resolution of about 13% at 662 keV.

• Analysis on the influence of ZnO addition on microwave dielectric properties of Li$_2$MgSiO$_4$ ceramics

Dielectric ceramics have gained wide attention owing to their applicability in a broad range of domains including a microwave communication system. Silicates are considered as the prospective substrate materials for microwave circuitsdue to their properties complementing with the requisites of a microwave substrate. Lightweight lithium-based silicate ceramic namely lithium magnesium silicate (Li$_2$MgSiO$_4$, LMS) was prepared by a solid-state reaction method. Zinc oxide (ZnO) was used as an additive to LMS in different weight percentages (0.5, 1.0, 1.5 and 2.0) and LMS-ZnO ceramics were also prepared. The microwave dielectric properties of pure LMS and ZnO-added LMS were measured and the influence of ZnO on LMS was investigated. A dielectric resonance technique was utilized to study the dielectric properties of ceramic samples at 12 GHz. A comparatively higher quality factor of 24,000 GHz was attained by the LMS-ZnO (2wt%) system with a relative density of 90.2% and <10 dielectric constant of 5.76 making it suitable for substrate applications.

• Joining of tubular steel–steel by unconventional magnetic pulse force: environmentally friendly technology

Electromagnetic welding uses environment-friendly, unconventional non-contacting magnetic pulse force for joining of two metals. The present paper focuses on the welding of tubular mild steel to two different steel bars, ferrite-pearlite 1018 carbon steel and austenite 304 stainless steel using a 40 kJ electromagnetic instrument. A qualitative metallurgical bonding was obtained for a selected set of optimum process parameters. The bonded region did not show localized melting for a mild steel–carbon steel joint and was found to be homogeneous liquid state bonding for a mild steel–stainless steel joint within a restricted distance of 3 $\mu$m from the interface. Both the joints indicated good peel strength and leak tightness. Simulation studies were validated using experimental parameters such as voltage, current, impact velocity, magnetic fluxand displacement.

• Band structure and thermoelectric properties of Cu$_2$O from GGA and GGA$+$U approaches

Electronic band structures and thermoelectric (TE) properties of cuprous oxide crystallizing in the $Pn3m$ space group are investigated using the linearized augmented plane wave method. The generalized gradient approximation (GGA) and GGA$+$U approaches are adopted for calculations at the level of the density functional theory. After achieving the groundstate of the crystal, the electronic band structures are calculated. The ab initio calculations are interfaced with the Boltzmann transport equations to unveil TE properties.We have found the Seebeck coefficient, power factor and electrical conductivity to compute the electronic fitness function (EFF) further. The effect of temperature is also studied. The EFF suggests that thematerial may become a useful TE material after p-type doping.

• Evaluation and conceptual design of triphenylphosphonium bromide-based deep eutectic solvent as novel thermal nanofluid for concentrated solar power

In a concentrated solar power (CSP) plant, an increase of heat transfer effect of the working fluid is a key deliverable which is usually obtained by enhancing its thermo-physical properties. The current work reports the synthesis of heat transfer fluids (HTF) based on deep eutectic solvents (DESs) consisting of a hydrogen bond donor (HBD), namely, triphenylphosphonium bromide, and a hydrogen bond acceptor (HBA), namely, ethylene glycol. Initially, the thermophysical properties, namely, density, viscosity, thermal conductivity (TC) and specific heat capacity were measured and compared with the conventional solvents. The properties were further enhanced by the dispersion of spherical Al$_2$O$_3$ nanoparticles in DESs. The alumina nanoparticles were found to have a negligible effect on the physical properties (density and viscosity) of the base fluid, thereby limiting the pressure drop and also the coefficient of friction. For their potential application as thermal fluids for CSP plants, the thermal properties of DESs and nanoparticle dispersed deep eutectic solvents (NDDESs)were measured within a temperature range of 25–60$^{\circ}$C. The TC of 1 wt% Al$_2$O$_3$ with the base fluid was around eight times higher than the base DES. It was found that the TCs of DES and NDDES were higher when compared to the commercial HTF, namely, Therminol VP-1. Eventually, the Aspen plus flowsheet was conceptualized to ascertain the steam generation rate and the overall heat transfer coefficient of these novel solvents. A combination of U-shaped for latent heat and shell and tube heat for sensible heat was employed in the flowsheet. The CSP scheme gave a steam generation rate of 1.7 kg h$^{−1}$ at 180$^{\circ}$C with a corresponding DES flow rate of 1 m$^3$ h$^{−1}$.

• A mesoporous nickel oxide nanosheet as an electrode material for supercapacitor application using the 1-(2$^{\prime}$,3$^{\prime}$-dihydroxypropyl)-3- methylimidazolium hydroxide ionic liquid electrolyte

NiO nanosheets were deposited on the surface of a stainless steel substrate by using a facile, environmentally friendly, reflux deposition approach for supercapacitor (SC) applications. X-ray diffraction patterns and field emissionscanning electron microscopy images revealed the formation of a face centred cubic crystal structure with a uniform, compact, smoothly ordered nanosheet like structure. This study focuses on the electrochemical supercapacitive propertiesof NiO nanosheets with respect to cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy techniques in a 1-(2$^{\prime}$ ,3$^{\prime}$-dihydroxypropyl)-3-methylimidazolium hydroxide [DHPMIM][OH] ionic liquid (IL) as an electrolyte. The electrochemical study revealed that NiO (0.3 M) showed a high-specific capacitance of 205.5 F g$^{−1}$ andan excellent cycling stability (80% specific capacitance retention after 5000 cycles) in the [DHPMIM][OH] IL electrolyte. Thus, the result showed that NiO nanosheets act as an active electrode material hold for SCs.

• Impurity concentration dependent electrical conduction in germanium crystals at low temperatures

A germanium single crystal of 7 N$+$ purity with a diameter of 45 mm and a length of 100 mm has been grown by the Czochralski method. The structural quality of the crystal has been characterized by Laue diffraction. Electrical conduction and Hall measurements are carried out on samples retrieved from different parts of the crystal along the growth axis. The top part of the crystal exhibits the lowest impurity concentration ($\sim$10$^{12}$ cm$^{−3}$) that gradually increases towards the bottom (10$^{13}$ cm$^{−3}$). The crystal is n-type at room temperature and the resistivity shows a non-monotonic temperature dependence. There is a transition from n-type to p-type conductivity below room temperature at which bulk resistivity shows a maximum and dip in carrier mobility. The intrinsic to extrinsic transition region shifts towards room temperature as the impurity concentration increases and it reflects the purity level of the crystal. A similar trend is observed in the boron-implanted high purity germanium (HPGe) crystal at different doping levels. The phenomena can be understood as a result of interplay between a temperature dependent conduction mechanism driven by an impurity band and an intrinsic carrier in Ge crystalshaving fairly low acceptor concentrations (<10$^{12}$ cm$^{−3}$).

• Morphology-dependent electrochemical performances of nickel hydroxide nanostructures

Electrochemical capacitors form part of the developing technologies in the field of alternative energy sources. In the present work, nickel hydroxide (Ni(OH)$_2$) nanosheets and microflowers are hydrothermally prepared employing different chemical precursors. Structure, morphology and chemical analysis are conducted using powder X-ray diffraction, field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy measurements. Electrochemical performances as supercapacitor electrodes of the synthesized nanostructures are evaluated through cyclic voltammetry and galvanostatic charge–discharge measurements with three-electrode configurations. The results indicated the specific capacitance of 180 and 417 F g$^{−1}$ at a scan rate of 5 mV s$^{−1}$ for Ni(OH)$_2$ nanosheets and microflowers, respectively.The higher specific capacitances for Ni(OH)$_2$ microflowers could be attributed to the higher specific surface area, morphology, electronic conductivity and porosity. Both Ni(OH)$_2$ nanostructures exhibited good capacitance retention for 1500 cycles.

• Elucidation of structural, morphological, optical and photoluminescence properties of single and (In, Ga) co-doped ZnO nanocrystalline thin films

Single and co-doped ZnO thin films are currently under intense investigation and development for optoelectronic applications. Here in this study, pristine, indium-doped (IZO), gallium-doped (GZO) and co-doped (IGZO) ZnO thin filmswere deposited on a glass substrate using radio frequency magnetron sputtering. A comparative study of all the films was carried out on the basis of their various properties. The effect of single and co-doping on the structural (X-ray diffraction(XRD) studies and Raman studies), morphological (field emission scanning electron microscopy and energy dispersiveX-ray spectroscopy studies) and optical properties (ultraviolet–visible (UV–Vis) and photoluminescence (PL)) of the deposited films was investigated. X-ray photoelectron spectroscopy (XPS) characterization was employed to analyse the surface chemical composition and bonding of the deposited film. From the XRD patterns, it was found that the films were highly crystalline in nature and preferentially oriented along the (002) direction with a hexagonal wurtzite structure, consistent with Raman analysis. IGZO films displayed a dramatic improvement in the surface morphology as compared with the single dopant films due to the compensation effect of gallium and indium doping which reduced the lattice strain. The XPS analysisconfirmed the presence of the oxidized dopants in each film. All thin films have shown excellent optical properties with more than 90% transmission in the visible range of light. The blue-shift of the absorption edge accompanied by the increase of the optical band gap confirmed the Burstein–Moss effect. The UV PL peak originated from the near band edge emission of crystalline ZnO, while the visible PL was associated with the radiative transition related to oxygen interstitial (Oi) defects in the ZnO structure.

• A comparative study on phase co-existence of La$_{1/4}$Pr$_{3/8}$Ca$_3$/8MnO$_3$ in a polycrystalline bulk and thin film

We present a comparative study on phase co-existence in La$_{1/4}$Pr$_{3/8}$Ca$_{3/8}$MnO$_3$ (LPCMO) in a polycrystalline bulk and thin film form. The X-ray diffraction results confirm the formation of single-phase polycrystalline bulk and c-axisoriented strained epitaxial thin films on a LaAlO$_3$ substrate. Scanning electron microscopy images show a homogeneous microstructure growth for both the bulk as well as thin film samples. The temperature dependent magnetization curves for bulk and thin film show remarkably different features indicating the presence of very different phase separation scenarios.A two-step rise and a large thermal hysteresis are observed between zero-field cooled and field-cooled magnetization plots in the bulk sample. However, the magnetization of the film shows a very small hysteresis and a negligible signature of the CO-AFM state for the same composition. The field dependent magnetization plots indicate that in a bulk form LPCMO is a soft magnet and shows a metamagnetic behaviour while in a thin film form it acts as a hard magnet and the metamagnetic behaviour is completely lost.

• Two-step fabrication of MAPbI$_3$ perovskite thin films with improved stability

Perovskite solar cells (PSCs) are a part of an emerging technology in photovoltaics (PV), which uses perovskite structured hybrid compounds as light absorbers such as methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$ or MAPbI$_3$). PSCs with a record certified efficiency of 22.1% are among the fastest advancing PV technology; but PSCs suffer from fast degradationin the presence of moisture. PSCs prepared by a solution process have additional limitation of small device area. In order to improve the stability of perovskite layer, we have used a two-step method for depositing a perovskite layer. First step is the thermal evaporation of lead iodide (PbI$_2$) and second is dip coating (DC) in methylammonium iodide (CH$_3$NH$_3$I or MAI) solution to prepare a MAPbI$_3$ perovskite. At the same time, we have also used conventional spin coating (SC) for PbI$_2$ films and DC in MAI to obtain MAPbI$_3$ for a comparative study. The gradual formation of MAPbI$_3$ after dipping a PbI$_2$ film into MAI solution can be seen from X-ray diffraction patterns. Structural, optical and electrical properties of the MAPbI$_3$ films are also studied. Better stability of perovskite films was observed using vapour-deposited PbI$_2$ films as compared with spin-coated PbI$_2$ films.

• Investigations on vacuum sintered ytterbium-doped YAG ceramic: a laser–host material

Laser–host ceramics are a new class of materials and have proved to be a good alternative to the single-crystals. They can be produced in large size with higher concentration of activator ions having more homogeneity which is difficultto achieve with the conventional single-crystal growth technology. Ytterbium (Yb)-doped yttrium aluminium garnet (YAG) is a very attractive laser–host material because of its broad emission band, low quantum defect, no excited state absorption or up-conversion loss and wide range of tunability. In the present work, 10 atm% Yb-doped YAG transparent ceramic was fabricated using nano-powder technology and vacuum sintering. Single-phase nano-powders of Yb:YAG were synthesized by a co-precipitation method and thereafter sintered under vacuum at 1750$^{\circ}$C for 5 h. The as-sintered ceramic samples are ofdark greenish colour which is attributed to the reduction of Yb$^{3+}$ to Yb$^{2+}$ during vacuum sintering. However, after annealing in air at 1350$^{\circ}$C for 8 h they retain their natural colour attributed to the transformations from Yb$^{2+}$ to Yb$^{3+}$. The transmission spectra of the as-sintered polished ceramics show the presence of broad absorption bands near 250, 380, 480 and 630 nm due to the electronic transitions in the Yb$^{2+}$ ion. These bands disappear after annealing in air. The well-established bandsdue to transitions in Yb3+ were observed between 900 and 1000 nm.

• Discussion on growth, emission and piezoelectric properties of zinc guanidinium phosphate single crystal: a potential candidate for transducer and LED applications

Zinc guanidinium phosphate, a semi-organic single crystal, was grown successfully by a slow evaporation solution growth technique. The cell parameters of the grown crystal were confirmed using a powder X-ray diffractionstudy. The electrical property of the sample was analysed using dielectric and piezoelectric studies. The dielectric constant, dielectric loss and alternating-current conductivity were calculated for various frequencies and temperatures. The solid state parameters such as plasma energy, Penn gap energy, Fermi energy and electronic polarizability were calculated. The piezoelectric property of the title crystal was ascertained by determining its piezoelectric charge coefficient ($d_{33}$). The optical transmittance and energy gap of the grown crystal were examined using ultraviolet–visible–near-infrared spectral analysis.The title molecule was optimized and the frontier molecular orbital was performed using B3LYP/6-31 $+$ G(2d,2p). The emission property of the titular compound was analysed using a photoluminescence study. The dominant colour emission ofthe grown crystal was found using CIE colour chromaticity coordinates. The nonlinear property of the titular compound was confirmed using a Kurtz–Perry powder technique. The above results illustrate that the title crystal is a potential candidate for light emitting and nonlinear applications. This work explains the dominant colour emitting behaviour and electrical properties such as dielectric and piezoelectric properties of the title crystal, which are reported for the first time.

• A comparative study of structural, electronic and optical properties of cubic CsPbI$_3$: bulk and surface

In order to acquire a reasonable description of the structural, electronic and optical properties of the perovskite compound CsPbI$_3$, first principle calculations have been computed by density functional theory implemented in the WIEN2k code. The calculations are presented within PBE-sol for exchange correlation functions coupled with modified-Becke–Johnson (mBJ) exchange potential. The (001) surfaces of CsPbI$_3$ for varying thicknesses have been constructed using the Structeditor program implemented in the WIEN2k code. The lattice constant, band gap and DOS have been computed. The CsPbI$_3$ bulk and surface exhibit a direct band gap located at the $R$ symmetry point of the Brillouin zone. The band gap approaches experimental values when the exchange correlation function is coupled with mBJ. The optical propertiesof CsPbI$_3$ were computed in terms of dielectric properties, refractive index, extinction coefficient, absorption coefficient, conductivity, reflectivity and energy loss. The direct band gap nature and high-absorption power of the surfaces of CsPbI$_3$ in the (001) direction in the infrared, visible and ultraviolet energy range make it suitable for use in optical and optoelectronic devices.

• A modified high-temperature vapour deposition technique for fabricating CH$_3$NH$_3$PbI$_3$ thin films under an ambient atmosphere

A modified two-step deposition technique was performed by vapour depositing methylammonium iodide (MAI) on lead iodide (PbI$_2$) films to fabricate good quality methylammonium lead iodide (MAPI) thin films under anambient atmosphere, and their properties were compared with conventional two-step solution processed MAPI thin films. Scanning electron microscopy results depicted that the MAI vapour-processed films have a uniform coverage and planar surface compared with the non-uniformly distributed granule-like morphology of the solution-processed MAPI film. X-ray diffraction results confirm that the vapour-processed films have better crystallinity compared to the conventional solutionprocessed MAPI thin films. An enhancement in the optical absorption and emission was observed for the vapour-processed films. The higher processing temperature of the modified-vapour deposition eliminates the effect of moisture during the ambient atmosphere processing of the MAPI films.

• Junction edge passivation study of silicon surface barrier detectors using organic films deposited by L–B technique

A new technique to passivate silicon surfaces using SnO$_2$ films, produced by decomposing organic films of the octadecylamine–stannate complex, deposited by the Langmuir–Blodgett (L–B) technique, has been attempted to fabricate silicon surface barrier detectors. This method of passivation is relatively simpler and can be carried out at a much lower temperature as compared to the usual passivation method of the silicon surface by growing the SiO$_2$ layer on it. Also, the passivating layer of SnO$_2$ produced in this new method has a good shelf-life. The detectors fabricated with a passivating layer of SnO$_2$ were subsequently tested for $I$–$V$, alpha spectrum and long-term performance.

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 6
December 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019