D.K. Aswal

A study of the CuO phase formation during thin film deposition by molecular beam epitaxy

Abstract The kinetics of CuO growth under molecular beam epitaxial (MBE) conditions has been investigated. The evaporation of Cu and its deposition onto Si(111) substrate maintained at 823 K was carried out using an electron beam heated source. For the oxidation of Cu, sources of both molecular and atomic oxygen species were employed. The films were characterized by electron spectroscopy for chemical analysis (ESCA), X-ray diffraction (XRD), infrared (IR) transmission and scanning electron microscopy (SEM). The application of a fairly high flux of molecular oxygen (3.4×1020 molecules/(m2 s)) and O2 to Cu flux ratio of ∼250 during the deposition was found to be insufficient to convert a detectable amount of Cu into Cu+/Cu2+ state. On the other hand, Cu2O films could be grown with relative ease by maintaining atomic oxygen flux of 1.6 times the stoichiometric value. In contrast, the kinetics of CuO formation has been found to be quite slow. For atomic oxygen to copper flux ratio of ∼80, only ∼95% of the copper was found to be in fully oxidized state.

Simple and low-temperature polyaniline-based flexible ammonia sensor: a step towards laboratory synthesis to economical device design

Flexible and highly sensitive polyaniline-based (PAni) ammonia (NH3) gas sensors were developed through in-situ chemical oxidative polymerization of aniline on a polyethylene terephthalate substrate at three different temperatures, viz. 35 °C, 0 °C and −5 °C. In the initial stage, they were characterized with respect to their structural, morphological, and compositional analysis studies and in the second stage, the selectivity towards oxidizing (nitrogen dioxide, NO2) and reducing (NH3, ethanol, methanol and hydrogen sulphide, H2S) gases was tested. The sensor fabricated at 0 °C showed an optimum response of 26% to 100 ppm NH3 gas, which was superior to those obtained for the sensors developed at 35 °C (19%) and −5 °C (23%). The as-developed low-temperature flexible gas sensor demonstrated fast response (19 s) as well as recovery time (36 s) periods, 99% reproducibility and good stability, revealing commercial application potential for example in industry where high temperature operation is prohibited. Impedance spectroscopy was used to investigate the plausible interaction mechanism of the NH3 gas molecules with the flexible PAni. The operation of the NH3 gas sensor device, fabricated on a laboratory scale, was tested and explored as a demo-video clip.

Core/shell, protuberance-free multiwalled carbon nanotube/polyaniline nanocomposites via interfacial chemistry of aryl diazonium salts

Highly uniform core-shell like multi-walled carbon nanotubes-polyaniline (MWCNT-PANI) nanocomposites were prepared in two steps (i) surface modification of MWCNTs with a 4-aminodiphenylamine group via in situ diazonium generation process; and (ii) polymerization of aniline onto surface modified MWCNTs. This functionalization helped to easily disperse the MWCNTs in acidic solutions; hence it is suitable for the chemical oxidative polymerization of aniline. It was found that MWCNT-PANI nano-composites with higher MWCNTs loading yield PANI chains with more quinoid units than the pure PANI, which results in significant improvement in the conductivity of the composites. This facile approach of synthesizing core-shell nanocomposites highlights the efficiency of the interfacial chemistry of aryl diazonium salts in generating conductive polymer/MWCNT nanocomposites with enhanced conductivity and high surface area.

High magnetoresistance and low coercivity in electrodeposited Co∕Cu granular multilayers

Co∕Cu multilayers were electrodeposited from a single solution electrolyte. By reducing the magnetic layer thickness from 1to0.2nm, samples with heterogeneous mixture of ferromagnetic and superparamagnetic clusters have been made. Magnetization and magnetotransport measurements revealed the structured evolution of the multilayer granular samples. With critical thickness of tCo∼0.5nm, room temperature magnetoresistance of 7% was obtained at 2kOe with hysteresis of 70Oe. The large magnetoresistance obtained in granular multilayers is attributed to the presence of a range of sizes for the Co particles.

Ultrasensitive and selective detection of dopamine using cobalt-phthalocyanine nanopillar-based surface acoustic wave sensor.

A highly selective and sensitive surface acoustic wave (SAW) sensor of dopamine (DA) was developed by depositing cobalt phthalocyanine (CoPc) nanopillars on gold-coated sensing platform of SAW sensor. The developed biosensor presents a sensitivity of 1.6°/nM, has a low limit of detection (LOD) on the order of 0.1 nM, and imparts more selectivity toward DA, since the detection limit of the interfering ascorbic acid (AA) is as high as 1 mM. To understand the selectivity mechanisms of this sensor toward DA, density functional theory-based chemical calculations were carried out. Calculations suggest two different types of interactions: dative bond with a very strong character for DA-CoPc complexes, and significant ionic character in the case of AA-CoPc ones. The interaction energies, in liquid phase, were estimated to be equal to -81 kJ mol(-1) and -38 kJ mol(-1) for DA-CoPc and AA-CoPc complexes, respectively, therefore accounting for the selective detection of DA over AA using tandem CoPc nanopillar-based SAW sensor device. This work demonstrates a simple and efficient design of SAW sensors employing thin nanostructured CoPc biomolecular recognition layers for DA detection.

Exfoliated clay/polyaniline nanocomposites through tandem diazonium cation exchange reactions and in situ oxidative polymerization of aniline

Robust, conductive clay/polyaniline nanocomposites were prepared through a simple approach which consists of in situ polymerizing aniline, in the presence of the 4-diphenylamine diazonium-modified bentonite. XPS measurements indicate that clay experiences a cation exchange of sodium by the diazonium, and the polyaniline is present in large amounts at the nanocomposite surface. As judged by XRD, the clay basal distance increased from 13.7 to 16.2 A after diazonium modification, whilst, after the in situ polymerization of aniline, the clay characteristic peak at low angle (<8°) has vanished showing the exfoliation of the resulting nanocomposites. However, the nanocomposite prepared with unmodified clay was also found to be a polyaniline-rich surface but without any sign of exfoliation. In addition, the composite morphology, imaged by electron microscopy (SEM) and (TEM), differs significantly from that of pristine clay and shows twisted layers with an inter-distance which increases with the mass loading of the diazonium salt and PANI therefore leading to the exfoliation of the clay. Furthermore, this diazonium modification resulted in a quantum jump of the conductivity of the nanocomposites compared to bentonite, ca. 6 orders of magnitude, whereas the deposition of PANI on pristine clay induced a marginal increase of conductivity from 10−9 to 2 × 10−8 S cm−1 due to an uneven coating of the conjugated polymer.

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.

Synthesis of vertically aligned polyaniline (PANI) nanofibers, nanotubes on APTMS monolayer and their field emission characteristics

Growth of aligned polyaniline (PANI) nanostructures on silicon modified with amino-silane self-assembled (3-aminopropyl)trimethoxysilane (APTMS) monolayer has been carried out by chemical polymerization in the presence of strong (HCl) and weak (CH3COOH) acids as dopant. Self assembly of aniline monomer leading to formation of aligned nanostructures on the APTMS monolayer is due to the amino (–NH2) moieties acting as growth centers. Interestingly, the nature of dopant acid (strong/weak) is found to determine the resultant form of the PANI nanostructures, either as nanofibers or nanotubes. The formation of PANI nanofibers and nanotubes has been revealed by scanning and transmission electron microscopy, whereas confirmation of the conducting phase of PANI is obtained from UV-visible and FTIR analysis. A plausible explanation illustrating the growth mechanism has been presented. Field emission studies of the PANI films have been carried out in a planar diode configuration at base pressure of ∼1 × 10−8 mbar. The turn-on field required to draw an emission current of 1 nA was found to 1.6 and 2.4 V μm−1 for aligned nanofibers and nanotubes, respectively. Interestingly, both the emitters exhibit good stability of the emission current over a duration of 4 h. The ease of the synthesis route and interesting field emission characteristics indicate the aligned PANI nanofibers and nanotubes as promising materials for field emission based applications.

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.