Shiv K. Gupta

One step synthesis of highly ordered free standing flexible polypyrrole-silver nanocomposite films at air–water interface by photopolymerization

Free standing polypyrrole-silver nanocomposite films were prepared by interfacial photopolymerization of pyrrole (in DCM) using AgNO3 (aqueous) as photosensitizer. During the photopolymerization process, film formation starts first at the DCM–water interface and later at the air–water interface. The films prepared at the air–water interface are thin (<1 μm), flexible, having a very low content of uniformly distributed metallic Ag nanoparticles and exhibiting high electrical conductivity ∼14.8 S cm−1. The thick films (∼200 μm) prepared at the DCM–water interface are porous, mechanically weak, contain a very high amount of Ag micro and nanoparticles and exhibit two orders of magnitude lower conductivity ∼0.1 S cm−1. High conductivity of PPy-Ag films formed at the air–water interface is attributed to controlled polymerization due to the limited availability of pyrrole and Ag+ ions at this interface.

Flexible organic semiconductor thin films

Research on organic semiconductor thin films has been accelerated due to their potential for low cost and large area flexible devices. Already there are various products based on organic semiconductor thin films such as displays which have been commercialized. Further studies are needed for the development of flexible devices. In this paper, investigation of various processes for organic semiconductor thin film deposition on flexible substrates and their characterization carried out by us will be reviewed. Two different strategies have been adopted for the fabrication of flexible thin films using conducting polymers as well as molecular semiconductors and they are: (i) synthesis of freestanding films where there is no need for substrates, and (ii) preparation of thin films on flexible substrates. Devices such as organic field effect transistors, memory devices and gas sensors have been demonstrated using various flexible films. The effect of bending on characteristics of films and devices has also been investigated.

Fast Response and High Sensitivity of ZnO Nanowires-Cobalt Phthalocyanine Heterojunction Based H2S Sensor.

The room temperature chemiresistive response of n-type ZnO nanowire (ZnO NWs) films modified with different thicknesses of p-type cobalt phthalocyanine (CoPc) has been studied. With increasing thickness of CoPc (>15 nm), heterojunction films exhibit a transition from n- to p-type conduction due to uniform coating of CoPc on ZnO. The heterojunction films prepared with a 25 nm thick CoPc layer exhibit the highest response (268% at 10 ppm of H2S) and the fastest response (26 s) among all samples. The X-ray photoelectron spectroscopy and work function measurements reveal that electron transfer takes place from ZnO to CoPc, resulting in formation of a p-n junction with a barrier height of 0.4 eV and a depletion layer width of ∼8.9 nm. The detailed XPS analysis suggests that these heterojunction films with 25 nm thick CoPc exhibit the least content of chemisorbed oxygen, enabling the direct interaction of H2S with the CoPc molecule, and therefore exhibit the fastest response. The improved response is attributed to the high susceptibility of the p-n junctions to the H2S gas, which manipulates the depletion layer width and controls the charge transport.

Thermally assisted OSL: A potent tool for improvement in minimum detectable dose and extension of dose range of Al2O3:C

The influence of electron-phonon interaction on the shape of the optically stimulated luminescence decay curve of Al2O3:C has been studied using thermally assisted optically stimulated luminescence (TA-OSL). The minimum detectable dose (MDD) of a phosphor depends on the standard deviation of the background signal which affects the signal-to-noise ratio. The standard deviation of the background signal reduces at lower stimulation light intensity while the readout time increases. Further, measurement at higher temperature enhances the OSL signal with faster decay due to the temperature dependence of photo-ionization cross-section. To achieve the same decay constant and more signal, the temperature of measurement was raised. As a result of lowering the stimulation in-tensity at higher temperature (85°C) the overall MDD of α-Al2O3:C was found to improve by 1.8 times. For extension of dose linearity in higher range, deeper traps were studied by simultaneous application of CW-OSL and thermal stimulation up to 400°C, using a linear heating rate of 4K/s. By using this method, two well defined peaks at 121°C and 232°C were observed. These TA-OSL peaks have been correlated with two deeper defects which can be thermally bleached at 650°C and 900°C respectively. These deeper defects are stable up to 500°C, so they can store absorbed dose information even if the sample is inadvertently exposed to light or heat. The dose vs. TA-OSL response from deep traps of α-Al2O3:C was found to be linear up to 10 kGy, thus extending its application for high dose dosimetry.

Flexible cobalt-phthalocyanine thin films with high charge carrier mobility

The structural and charge transport characteristics of cobalt phthalocyanine (CoPc) films deposited on flexible bi-axially oriented polyethylene terephthalate (BOPET) substrates are investigated. CoPc films exhibited a preferential (200) orientation with charge carrier mobility of ∼118 cm2 V−1 s−1 (at 300 K). These films exhibited a reversible resistance changes upon bending them to different radius of curvature. The charge transport in CoPc films is governed by a bias dependent crossover from ohmic (J–V) to trap-free space-charge limited conduction (J–V2). These results demonstrate that CoPc films on flexible BOPET having high mobility and high mechanical flexibility are a potential candidate for flexible electronic devices.

Low temperature thermopower and electrical transport in misfit Ca(3)Co(4)O(9) with elongated c-axis

The temperature dependence of the electrical resistivity (ρ) and thermopower (S) of misfit cobaltite Ca3Co4O9 has been studied. The compound is a promising thermoelectric material, exhibiting very low ρ(~1.3 × 10−4 Ω cm) and reasonably high S (~56 µV K−1) at room temperature, which is desirable for thermoelectric applications. At T < 80 K, S exhibits T3/2 dependence, suggesting that electron–magnon scattering has a pronounced influence on S at low temperatures. Ca3Co4O9 exhibits a temperature independent power factor in the temperature range 300–80 K. A comparison of the temperature dependence of the power factor of Ca3Co4O9 with various materials projected or used as a thermoelectric cooler (e.g. single crystal of short c-axis Ca3Co4O9, polycrystalline La0.95Sr0.05CoO3, Bi0.85 Sb0.15, Bi2Te3, Bi-SiO2 composite) indicates that title compound Ca3Co4O9 is a much superior candidate for thermoelectric applications.

Effect of interface pinning on dissipation, volume pinning force and measurement of upper critical magnetic field in MgB2 thin films

Abstract Angular dependences of resistivity ( ρ ) and critical current density ( J c ) have been measured to determine the effect of pinning at film–substrate and film–air interfaces on dissipation and volume pinning force in c -axis oriented MgB 2 thin films prepared by the ex situ laser ablation technique. It is seen that for the field applied parallel to the ab -plane, interface pinning may reduce resistivity by two orders of magnitude while making nearly same contribution to total pinning force as pinning in bulk of the film. Angular dependence of the upper critical magnetic field ( H c2 ) has been measured and it shows deviations from the anisotropic Ginzburg–Landau model. These deviations have also been understood in terms of the interface pinning effect. The results show that controlled addition of impurities and defects may significantly improve critical current densities in MgB 2 superconductor.

Resistivity behaviour of ultra thin Pd films with respect to temperature for sensing applications

Ultra thin Pd films (5-30 nm) deposited by RF sputtering have shown fast and detectable responses for low concentrations of H/sub 2/ gas (0.3-4%) and compounds containing hydrogen such as ammonia, methanol (/spl sim/100 ppm mixtures in air) at different temperatures. These films have been characterized in dynamic temperature programmed and static conditions for 100-300/spl deg/C range, in air with respect to their resistivity. The electrical resistivity of Pd films gets modified on exposure to such gas mixtures and measured changes are reported. The observed changes in resistivity on exposure to above gas mixtures are much larger (more then by an order) then earlier reported results in literature.

Influence of adsorbed oxygen on charge transport and chlorine gas-sensing characteristics of thin cobalt phthalocyanine films

We have investigated the morphology, charge transport, and gas-sensing characteristics of thin films of cobalt phthalocyanine (CoPc) deposited on glass and sapphire substrates, using molecular beam epitaxy (MBE). CoPc films deposited on glass were found to be highly disordered. The ambient oxygen was found to be chemisorbed and created deep trap states, which led not only to hysteretic current-voltage (J-V) characteristics but also reduce the charge mobility. These properties render them unsuitable for gas-sensing. On the other hand, films deposited on sapphire were polycrystalline, which was attributed to an improved molecule-substrate interaction. The physically sorbed oxygen only created shallow traps, and the J-V characteristics were non-hysteretic, rendering them suitable for gas-sensing applications. It was demonstrated that the ultrathin (20 nm) CoPc films deposited on sapphire acted as highly sensitive and selective sensors for chlorine present in the wCl concentration range of 5 × 10−9−2 × 10−6 (5–2000 ppb).

Stable negative differential resistance in porphyrin based σ–π–σ monolayers grafted on silicon

Two Si-based hybrid self-assembled monolayers were synthesized by electro-grafting two di-O-alkylated porphyrins as the σ–π–σ systems. The monolayers showed a stable and reversible negative differential resistance (NDR) property at room temperature. The monolayer, fabricated using the porphyrin with fluorophenyl groups was more compact and showed a tenfold peak-to-valley ratio (PVR) relative to the other similar system devoid of the fluorine atoms in the porphyrin moiety. This suggested better pre-organization of the former, possibly by hydrogen bonding through the electro-negative fluorine atoms.