S. V. Bhoraskar

Humidity sensing with weak acid-doped polyaniline and its composites

Abstract In this paper, we report the behaviour of humidity sensors with polyaniline-based conducting polymers doped with different weak acidic dopants. Changes in surface resistivity of films were monitored as a function of relative humidity. The acidic dopants used were camphosulphonic acid (CSA), diphenyl phosphate (DPPH), and maleic acid (Mac) blends of these were formed with styrene–butyl acrylate copolymer for improving the mechanical stability. The sensitivity of responding the level of relative humidity was compared for three composites wherein, although low sensitivity the Mac-doped films were found to be repeatable and more stable compared to the others. Films prepared out of styrene–butyl acrylate copolymer with different concentrations of PANI–Mac were used for sensing humidity ranging between 20 and 95% relative humidity. The films exhibited almost linear behaviour within a chosen range of humidity.

Experimental imaging of silicon nanotubes

Transmission electron microscopy (TEM), electron energy loss near edge structures (EELNES) and scanning tunneling microscopy (STM) were used to distinguish silicon nanotubes (SiNT) among the reaction products of a gas phase condensation synthesis. TEM images exhibit the tubular nature with a well-defined wall. The EELNES spectra performed on each single nanotube show that they are constituted by nonoxidized silicon atoms. STM images show that they have diameter ranging from 2 to 35 nm, have an atomic arrangement compatible with a puckered structure and different chiralities. Moreover, the I-V curves showed that SiNT can be semiconducting as well as metallic in character.

Synthesis of nanowires and nanoparticles of cubic aluminium nitride

Nanostructures of cubic aluminium nitride were synthesized by DC arc-plasma-induced melting of aluminium in a nitrogen?argon ambient. The material flux ejected from the molten aluminium surface was found to react with nitrogen under highly non-equilibrium conditions and subsequently condense on a water-cooled surface to yield a mixture of nanowires and nanoparticles of crystalline cubic aluminium nitride. Both x-ray diffraction and electron diffraction measurements revealed that the as-synthesized nitrides adopted the cubic phase. Fourier transform infrared spectroscopy was used to understand the bonding configuration. Microstructural features of the synthesized material were best studied by transmission electron microscopy. From these analyses cubic aluminium nitride was found to be the dominating phase for both nanowires and nanoparticles synthesized at low currents. The typical particle size distribution was found to range over 15?80?nm, whereas the wires varied from 30 to 100?nm in diameter and 500 to 700?nm in length, depending upon the process parameters such as arc current and the nitrogen pressure. The reaction products inside the plasma zone were also obtained theoretically by minimization of free energy and the favourable zone temperature necessary for the formation of aluminium nitride was found to be ?K. Results are discussed in view of the highly non-equilibrium conditions that prevail during the arc-plasma synthesis.

Swift heavy ion induced growth of nanocrystalline silicon in silicon oxide

Recystallization of nanocrystalline silicon in silicon oxide has been initiated with swift heavy ion irradiation. 100 MeV Ni ions from pelletron were used for irradiating the thin films of silicon oxide (SiOx) at fluences varying from 1×1012 to 5×1013 ions/cm2. Phase separation between silicon and silicon oxide is seen to be responsible for the photoluminescence spectrum peaking around 350 and 610 nm. This spectral nature is understood on the basis of defects and interface states in SiOx matrix and silicon nanocrystals, respectively. The formation of silicon nanocrystals resulting from the phase separation has been confirmed from the complimentary evidence of change in the refractive index, Fourier transform infrared spectroscopy, and energy despersive x-ray analysis. High electronic loss associated with the 100 MeV Ni ions is thought to be responsible for the recrystallization, and rearrangement of silicon.

Simple chemical route for the quantitative precipitation of barium–strontium titanyl oxalate precursor leading to Ba1−xSrxTiO3 powders

Abstract Barium–strontium titanyl oxalate (BSTO) [Ba 1− x Sr x TiO(C 2 O 4 ) 2 ·4H 2 O with x =0.25 precursor powders] were prepared successfully by using the simple chemical exchange reaction between oxalotitanic acid (HTO) [H 2 TiO(C 2 O 4 ) 2 ·2H 2 O], barium hydroxide and strontium nitrate at room temperature. Initially, oxalotitanic acid was prepared by using the reaction between 0.1 M titanium tetrabutoxide and 0.2 M oxalic acid in isopropanol (IPA). Barium hydroxide and strontium nitrate were added directly to this solution in stoichiometric amounts. The required solubility achieved only after addition of distilled water dropwise leads to the controlled precipitation of BSTO molecular precursor. To maintain the equilibrium (pH conditions), more and more Ba(OH) 2 and Sr(NO 3 ) 2 solids get dissolved and reaction continues. The kinetics of exchange reaction is governed by the rate of addition of water. The pyrolysis of BSTO at 750 °C/4 h in air produced the Ba 1− x Sr x TiO 3 (BST) powders. The characterization studies were carried on the as-dried and calcined powders by various physicochemical techniques viz. microanalysis, chemical analysis, DTA/TGA, XRD, IR, scanning electron microscopy (SEM), etc. It revealed that the BST powders are cubic, stoichiometric, highly pure with yield>99% and agglomerated nature (size: 0.5–2 μm).

Metallophthalocyanine coated porous silicon gas sensor selective to NO2

Abstract A composite device for sensing NO2 vapors was formulated by combining metallophthalocyanine (MPC) with porous silicon (PS). Thin films of MPC were coated onto the surface of PS and the surface resistivity was observed to change with the exposure to the vapors of NO2. Three different metals namely Co, Cd and Al were used to prepare the macromolecular complex. They provided different sensitivities for detection of NO2. The paper shows the superiority of the composite sensor in comparison to single film sensor.

Nanophase alumina synthesis in thermal arc plasma and characterization: correlation to gas-phase studies

Nanophase alumina (Al2O3) was synthesized in a d.c. arc plasma reactor under isochronal oxygen flow conditions. Transmission electron microscopy revealed spherical particles (50 nm) and the corresponding electron diffraction showed a δ-Al2O3 phase. Structural morphology by X-ray diffraction (XRD) evidenced the phase changes. These measurements confirmed the existence of the nanophase structure. X-ray photoelectron spectrsocopy (XPS) of the core levels of as-deposited, and calcined powders were carried out for comparative study. Zeta potential as determined from electrophoretic mobility measurements, at a pH value of 4, indicated a high value (+42.7 mV) for the nanophase alumina as compared to that for the commercial alumina (−14.1 mV). This is a measure of the state of agglomeration, which is higher for the nanophase alumina. Fourier transform infrared spectra of the alumina powder revealed a broad band from 500 to 1000 cm−1, indicative of the complex Al–O vibration due to interactions of the octahedral and tetrahedral coordination groups. The FTIR spectra also revealed the intermediate route leading to alumina formation, as seen from the presence of gas-phase type sub-oxide bands. In addition the presence of a strained surface vibrational mode retaining itself even after complete transformation to α-Al2O3 is confirmed. Optical absorption spectroscopy was studied yielding a band gap for the nanophase alumina of >5 eV. The Infrared and the absorption spectrum is characteristic of a gas-phase type of reaction. Thus a dimensional evolution starting from a gas-phase precursor, which is molecular in nature, leads to a collisionally quenched structure resulting in very fine particulates condensing from the plasma.

A novel approach towards selective bulk synthesis of few-layer graphenes in an electric arc

The paper demonstrates the selective bulk synthesis of few-layer graphenes by optimizing an external magnetic field assisted electric arc. An ultra-high purity glassy graphite anode was sublimated in an argon atmosphere, and carbon nanotubes (CNTs), along with graphene sheets, were found inside the deposit formed on the cathode. Both the high purity CNTs and the graphene sheets, with minimal structural defects, were synthesized separately by varying the strength and orientation of the external magnetic field. The as-synthesized carbonaceous samples were characterized with the help of transmission electron microscopy, selected area electron diffraction (SAED), Raman spectroscopy and thermogravimetry with the objective of optimizing the highest selective production of 2D graphene structures. The as-synthesized graphene sheets exhibited a relatively high degree of graphitization and low structural defect density as confirmed by Raman spectroscopy. They were found to exhibit higher oxidation temperature (767 °C) than that of the carbon nanocrystalline particles (690 °C), as inferred from the thermogravimatric analysis. Moreover, they were found to roll up at their edges on account of their surface energy minimization. This was confirmed by the SAED analysis. With this new technique, we could successfully synthesize 2D graphene structures at the rate of a few g h−1.

Electroluminescence from heterojunctions of nanocrystalline CdS and ZnS with porous silicon

Electroluminescence from heterojunctions fabricated by depositing ultrathin films of nanocrystalline CdS and ZnS on porous silicon by the liquid–liquid interface reaction technique is reported. Junction current–voltage characteristics were studied for different thicknesses of the deposited films. Large forward currents on the order of 180 mA/cm2 and a rectification ratio on the order of 103 were characteristic of the diode. The reverse breakdown voltage on the order of 150 V indicated the stability of these diodes. Electroluminescence was observed to arise at around 625 nm, which was blueshifted as compared to the photoluminescence peak and showed much smaller full width at half maximum (∼40 nm).

Photoluminescence and I-V characteristics of a CdS-nanoparticles-porous-silicon heterojunction

Chemically capped CdS nanoparticles are embedded in porous silicon (PS) by a dip coating method. Atomic force microscopy measurements reveal that the PS surface is covered with CdS nanoparticles forming well-defined rectangular blocks of nearly uniform size (200×200 nm2). Photoelectron spectroscopy and energy dispersive x-ray analysis confirm the presence of CdS in PS. Optical and electrical properties of the heterojunctions so-formed are investigated. Junction characteristics show that the composite so-formed exhibits very high forward current density (145 mA cm-2) and high reverse breakdown voltage (15 V).