Kiran Jain

Faster response of NO2 sensing in graphene?WO3 nanocomposites

Graphene-based nanocomposites have proven to be very promising materials for gas sensing applications. In this paper, we present a general approach for the preparation of graphene-WO(3) nanocomposites. Graphene-WO(3) nanocomposite thin-layer sensors were prepared by drop coating the dispersed solution onto the alumina substrate. These nanocomposites were used for the detection of NO(2) for the first time. TEM micrographs revealed that WO(3) nanoparticles were well distributed on graphene nanosheets. Three different compositions (0.2, 0.5 and 0.1 wt%) of graphene with WO(3) were used for the gas sensing measurements. It was observed that the sensor response to NO(2) increased nearly three times in the case of graphene-WO(3) nanocomposite layer as compared to a pure WO(3) layer at room temperature. The best response of the graphene-WO(3) nanocomposite was obtained at 250 °C.

Growth of CdS and CdTe thin films for the fabrication of n-CdS/p-CdTe solar cell

Abstract Thin n-CdS and p-CdTe films were prepared by chemical spray pyrolysis and electrochemical deposition respectively. Excessive sulphur in the spray solution has promoted grain growth in CdS film. Microstructural features of CdS film with stoichiometric Cd:S concentration in the spray solution were more heterogeneous with grains, whereas film sprayed with excessive S show more uniformity, reduced grain boundary losses of current and improved shunt resistance through inhibition of leakage of current at narrow grain boundary or void site is expected and is indeed observed. Electrodeposition of CdTe films, beside the effect of the inherent process parameters, is also affected by crystalline and microstructural features of the underlying CdS. Nucleation of CdTe film is remarkably affected by CdS film spray deposited over glass substrate. Cell performance considerably depends upon the window layer CdS and the properties of sprayed CdS film depends considerably on the Cd:S ratio in the spray solution. A higher S content in the CdS film affects it optical transmission without changing the optical energy gap. This improves cell efficiency through reduction in CdS film resistivity. A typical increase in cell efficiency was found to increase from 8% to 10.5% using CdS film with Cd:S ratio as 1:1.1 and 1:1.3 respectively.

Porous Alumina Template based Nanodevices

Quantum confinement is an emerging technique for the development of futuristic devices. Use of porous alumina as a template for fabrication of quantum confined nanostructures of magnetic, metallic and semiconducting materials is a recent development. This process provides a simple low cost, scalable technique, suitable for economical volume production and is easily integrated with standard microelectronic technology. Lithographic steps are not involved in this self assembled nanostructure. In view of this, recent developments in this field are reviewed here. The basic alumina template is fabricated by controlled anodization of aluminium to form close packed hexagonal nanometer sized cells. The pore dimensions are easily controlled by adjusting the process parameters. A variety of materials are incorporated in these nanosize pores by suitable technique such as electrodeposition, sol-gel processing or vapour phase chemical reactions. A variety of futuristic devices can be fabricated by proper choice of the dimensions and the incorporated material. Some recent results are summarized in the present review.

Depolarization current studies of vinyl chloride-vinyl acetate copolymer thin films

Abstract Depolarization current characteristics of vinyl chloride-vinyl acetate (VC- VAc) copolymer thin films grown from solution by an isothermal immersion technique were studied as functions of the polarizing field (6.25 × 10 5 −2.25×10 6 V m − , time (7.20 × 10 3 −5.04×10 4 s), the temperature of polarization (305−425 K), the nature of the vacuum-evaporated electrode metals (gold, copper, silver and aluminium) used during polarization, the thickness of the copolymer fims (4000−10 000 A) and the heating rates of depolarization (0.033−0.20 K s −1 ). Three relaxation processes at 340, 412 and 460 K were observed with activation energies of 0.59, 1.01 and 1.37 eV and relaxation times at 300 K of 3.66×10 3 , 8.78×10 6 and 1.46×10 10 s respectively. The peak at 340 K is associated with orientation of the dipoles of the main polymer chain; that at 412 K is attributed to ionic movements in the amorphous region of the VC-VAc copolymer film; and that at 460 K is attributed to release of charge which is the resultant of the charge already present and that due to the injected charge carriers. The peaks at 340 and 412 K shift towards higher temperatures and their activation energies increase with increases in the polarizing time and temperature. This is attributed to a distribution of the relaxation times and activation energies and suggests that these two peaks are complex in nature and cannot be represented by a discrete level of activation energy having a single relaxation time.

Mechanism of enhancement in NH3 sensing for surface functionalized WO3 film

Sensors were fabricated using a screen-printing technique using WO3 produced by a sol–gel process. Surface functionalization was achieved by using normal and catalyzed SiO2 coatings. The variation of sensor response to ammonia gas was investigated for both undoped and platinum doped WO3 sensor layers. The response to ammonia exhibited significant improvement following surface modification with catalyzed silica. The increase has been attributed to an increase in Schottky barrier height.

Growth and properties of CdSe thin films by a new process of electrochemical selenization of Cd metal layers

Growth and properties of cadmium selenide semiconductor thin films prepared by a new electrochemical selenization process (ECS) are described. The as-formed CdSe thin films have large ({approximately}1 {micro}m) crystallites in hexagonal modification. The differential selenization kinetics in the intra- and intergrain regions causes the formation of stoichiometric CdSe film to be highly dependent on time. CdSe composition is independent of selenization parameters. Two direct optical band gaps at 2.09 and 1.44 eV, as opposed to a single gap at 1.7 eV, are observed in CdSe film selenized at 0.6 and 0.4 mA/cm{sup 2} current densities, respectively. A mechanism of selenization based on Cd ionization by oxygen reduction and reaction with cathodically released Se ions is proposed for the CdSe film formation.

One-step synthesis of size-controlled CZTS quantum dots

Size-controlled CZTS quantum dots (QDs) were synthesized and its application as a potential electron accepting material for polymer-based hybrid solar cell is demonstrated. The CZTS QDs with a size of 2–10 nm were synthesized in a single step by the decomposition of metal dithiocarbamate and characterized by various techniques; like, SEM, TEM, FTIR, XRD, etc. Results reveal that the CZTS QDs synthesized in oleic acid can quench the luminescence of P3HT effectively. Due to the favourable ionization potential and electron affinity values for CZTS with respect to P3HT, the CZTS QDs act as an effective electron acceptor in the hybrid solar cells based on P3HT/CZTS-QD blends which is also revealed by the charge transfer characteristics of P3HT/CZTS blend.

Green Synthesis of Nanocrystalline Cu2ZnSnS4 Powder Using Hydrothermal Route

Nanocrystalline Cu2ZnSnS4 (CZTS) powder was synthesized by a hydrothermal process, using thiourea as sulfur precursor. The powder was qualitatively analyzed using X-ray to identify the phase, and the size of the particles was determined using transmission electron microscopy (TEM). Raman peak at 337.5 cm−1 confirms the formation of pure CZTS particles. The powder was also synthesized solvothermally using ethylenediamine as solvent. The hydrothermally synthesized powder indicated the presence of the kesterite phase Cu2ZnSnS4 and particle size of about 4-5 nm. This environmentally green synthesis by hydrothermal route can produce gram scale synthesis of material with a chemical yield in excess of ~ 90%. UV Vis absorption spectra measurements indicated the band gap of as-synthesized CZTS nanoparticles to be 1.7 eV, which is near the optimum value for photovoltaic solar cell, showing its possible use in photovoltaics.

The effect of doping on the dielectric relaxations of polycarbonate thin films

Abstract The depolarization behaviour of iodine-doped polycarbonate (PC) films was studied as a function of iodine (I 2 ) concentrations varying from 0.02 to 0.3 g (100 cm 3 ) −1 of 2.5% PC solution in dichloromethane. Polarized PC:I 2 films exhibit three thermally stimulated depolarization current peaks, designated peaks I, II and III, in the temperature regions 340–350 K, 407–415 K and 455–482 K, with activation energies of 0.43–0.51, 0.69–0.75 and 1.20–1.47 eV respectively. The decrease in the magnitude of peak I and its shift towards lower temperatures has been explained on the basis of the formation of charge transfer complexes with the polymer chain by the interstitially placed iodine; this reduces the interaction between the polar molecules, and their electrostatic field may possibly act in a direction opposite to the direction of the polarizing field. These conclusions regarding charge transfer complex formation have been confirmed by visible and IR optical absorption data on pure and iodine-doped PC films. The slight decrease in the magnitude of peak II is attributed to the destruction or neutralization of some of the trapping sites on doping with iodine impurities. Peak III was associated with the asymmetrical distribution of the ionic charge carriers formed during the growth of thin films.

Electrical and dielectric properties of polyvinylbutyral II: The effect of the molecular weight on the charge storage mechanism

Abstract The effect of the degree of polymerization n on thin films of polyvinylbutyral was investigated by thermally stimulated discharge current techniques with variation of the field, time and temperature of polarization. With an increase in n , the quantity of charge associated with the β peak (at 347 K) decreases whereas that associated with the α peak (at 423 K) remains constant. These results are attributed to the lower number of end groups and to the constant number of traps per unit volume respectively. With a decrease in n the positions of the β and α peaks are shifted towards lower temperatures; this is attributed to the lowering of the glass transition temperature.