Pandiyan Murugaraj

Dielectric enhancement in polymer-nanoparticle composites through interphase polarizability

Dielectric measurements on polyimide-oxide nanoparticle composite thin films show a composite dielectric constant (ecomposite) that increased monotonically with increasing oxide content well above the value predicted by Maxwell’s rule for dielectric mixtures below the percolation threshold. Above certain volume fractions, the measured ecomposite values were found to exceed the corresponding nanoparticle e such that epolymer<eparticle<ecomposite contrasted to conventional composites where epolymer<ecomposite<eparticle. The ecomposite was independent of frequency to 10MHz with dielectric loss of <0.005 throughout this range, indicating that the observed enhancement in e does not originate from space-charge related contributions and hence should be due to dipolar contributions. The observed e enhancement (ecomposite−eMaxwell) showed a correlation with the total surface area of the nanoparticles. The dielectric model of Vo and Shi [Microelectron. J. 33, 409 (2002), and references therein] showed that the enha...

Thermistor behaviour in a semiconducting polymer–nanoparticle composite film

Carbon nanoparticle-polyimide (BTDA-ODA) composite thin films were shown to have semiconducting behaviour with a pronounced negative temperature coefficient of resistance (NTC) over a temperature range 300?425?K, unlike conventional polymer-based composite films that show predominantly positive temperature coefficients of resistance. Electrical resistivity of the nanocomposite thin films decreased progressively by six orders of magnitude as the carbon volume fraction was increased from 1% to 10% and showed a characteristic composite behaviour in the smearing region prior to the percolation threshold. The dominant conduction mechanism was shown to be fluctuation induced tunnelling in the Coulomb gap with an optimum hop energy (Wopt) and a hop distance (Ropt) of about 23.1 ? 0.05?meV and 15.8 ? 0.04?nm, respectively for a 1?vol% nanocomposite, consistent with the T?1/2 behaviour in the model of Adkins (1989 J. Phys.: Condens. Matter. 1 1253) for cermets. The NTC parameters, characteristic of thermistor performance of these thin films, indicated that polymer based nanocomposites can potentially be considered for NTC thermistor applications. The materials presently studied, when compared with commercially available ceramic thermistors, were shown to exhibit smaller thermal sensitivity parameters, which originate from their smaller activation energy for charge transport. Polymer based semiconductive composites show promise for use in flexible and light-weight electronics, particularly since they can be easily mass manufactured and imprinted on microelectronic substrates.

Enhanced electromechanical response of nonpercolating polymer-nanoparticle composite films

We have prepared semiconducting carbon nanoparticle polyimide composite thin film with spatially distributed localized energy states exhibiting three-dimensional variable range hopping electron transport. Applied strain (compression or extensional) introduces proportional variations in the spacing between these energy states, as demonstrated in the linear variation of measured electrical resistance. The electromechanical sensitivity is the same in both deformation modes due to this underlying operating mechanism, enabling quantitative measurement of torsional deformations also. Electrical responses across all deformation modes remained in phase with applied strain indicating negligible hysteresis and an ability to operate over a wide strain ranges up to 40 000 microstrains.

Electron transport properties of irradiated polyimide thin films in single track regime

We have prepared a suite of polyimide thin films containing spatially separated one-dimensional conductive-nanowires by ion-beam irradiation exhibiting temperature dependent electrical resistance consistent with thermally activated electron hopping with activation energies about 1 eV arising from localized states spatially distributed along the ion tracks. Dielectric measurements showed the formation of high dielectric constant interphase regions surrounding each ion track generated during the irradiation process, responsible for space-charge accumulation which influences electron transport within the ion tracks. This behavior suggests a role for space-charge effects and dielectric properties in this interphase region in the control of electron transport within single track nanowires.

The Nd–Mn exchange interaction in Nd0.7Sr0.3MnO3

The Nd-Mn exchange interaction in Nd0.7Sr0.3MnO3 is considered by studying its influence on the Mn-55 spin-echo NMR lineshape and spin- spin relaxation time. It is seen that the interaction is of considerable strength well above the Nd spin ordering temperature (approximate to 20 K), with a significant influence on the Mn (electron) spin dynamics.

Stable doped sp2 C-hybrid nanostructures by reactive ion beam irradiation

Boron doping of polyimide precursors yields films, that when ion irradiated produces high aspect ratio nanowires of 15–25 nm diameter of B substituted ∼4 nm sp2 C-clusters embedded within polyimide as shown by HRTEM, AFM and electrostatic force EFM. XPS confirmed B substitution within the sp2 structures and also showed that the presence of B during the ion induced thermal transformations enhanced N co-substitution, while the G and D Raman bands indicated a high degree of disorder within these C-nanoclusters which increased with atom substitution. Electron transport properties indicated the semiconducting behaviour of the C-nanowire arrays. Impedance spectroscopy separated electron hopping transport within these nanowires from electron tunnelling between neighbouring nanowires. Nanowires were clearly surrounded by an altered polymer interphase region of increased segmental chain mobility and higher dielectric polarisability allowing control of overall electron transport in device applications. This reactive ion beam irradiation route allows separation of chemical doping and synthesis from the nanostructure fabrication, as a new nanotechnology route.

The influence of interphase between nanoparticles and matrix on Young’s Modulus of nanocomposites

Thin nanocomposite films are gaining increasing significance in flexible and light weight electronic devices as well as emerging nanoelectronic structures. The present work investigates the influence of the interphase between spherical nanoparticles and a polymer matrix on the Youngpsilas modulus and shear modulus of a nanocomposite thin film. The interphase region is considered inhomogeneous, but isotropic, thus is modeled as a function of the thickness of the interphase region, which is affected by the material properties of the nanoparticles, the polymer matrix, as well as the volume fraction of the nanocomposite. The property of the interphase at the matrix boundary is assumed to be smooth and continuous, while at the particle boundary it is discontinuous. The replacement method is adopted to calculate the shear modulus and Youngpsilas modulus of the nanocomposite, based on the mathematical model of the interphase. The results are compared with the experimental data in literature.

Mechanism of Formation and Stabilization of Platinum Nanoparticles in Aqueous Solvents

The mechanism of formation of platinum nanoparticles via chemical reduction of Na2PtCl4 in aqueous solution was investigated by UV-Visible Spectroscopy and Transmission Electron Microscopy. Sodium borohydride (NaBH4) was used as the reducing agent, and tri-Sodium Citrate was used for stabilising the nanoparticles. It is possible to monitor various stages of the reduction process on an observable time scale when the Pt(II) solution is suitably aged. Under appropriate experimental conditions, the theoretically predicted plasmon resonance absorption peak from the well-dispersed Pt(s) nanoparticles is observed at 215nm for the Pt suspensions in citrate medium. It is found that an increased concentration of citrate stabilizer decreases the reaction rate, although there is only a narrow concentration range of stabilizer which produces a stable suspension with well-separated Pt(s) nanoparticles. This conclusion was also supported by the TEM observation of the nanoparticles, which had a very narrow size distribution (between 2 to 6nm).

Role of Particle Size and Crystal Microstructure on the Magnetic Behaviour of Binary Alloy Nanoparticles

The increasing research attention towards magnetic nanoparticles has been driven by interest in advanced device and biomolecular technologies. Particle size and crystal microstructure represent key structural aspects in achieving optimum performance of nanoparticle systems. To date these relationships have been studied in sputtered alloy nanoparticle thin films. In this paper, we report the relationships between the thermal annealing parameters and the physical and magnetic characteristics of CoPt alloy nanoparticles.

Use of Viscoelastic Properties of Metal Alkoxide Sols to Control the Microstructure and Quality of Spin-coated Sol-gel Thin-films

The sol-gel spin-coating technique is increasingly being used for the fabrication of thin-films with diverse applications in areas that include advanced optics, microelectronics and sensors. The performance quality of the thin film devices is affected by the thickness and homogenity of the film, which are in turn controlled by parameters such as substrate surface quality, viscoelastic properties of the sol, and the spin coating parameters. Processing high quality thin-films is simpler when the desired thickness can be achieved by a single-step coating of sol-gel film. We report on the fabrication of TiO2 thin films by single-step spin-coating of a well characterised sol on a single crystal Si substrate. By monitoring the viscoelasticity of a titanium alkoxide sol, the thickness of the coated film can be controlled and with proper calibration, the viscoelasticity of the sol can be used as a parameter to fabricate a quality film with desired thickness. The thickness of the spin coated sol-gel films and that of the processed TiO2 films was evaluated using SEM and ellipsometry. The optical interference phenomenon of Newtons colours, which are obtained for the thin transparent films on reflecting surfaces, can be used to estimate the thickness of the TiO2 thin-films. This presentation provides details on how this concept can be applied to estimate the thickness of the TiO2 thin-films in the range of 50 to 200nm. The value determined using this method was comparable to that obtained through ellipsometric and SEM measurements.