V.N. Bhoraskar

Refractive index and dielectric constant of the boron and fluorine diffused polyimide

Abstract The dielectric constant, e′, of polyimide (C22H10N2O5, PMDA-ODA, Kapton-H) was decreased from 3.15 to 2.57 (1 kHz) by diffusing fluorine and boron in the surface region of the polyimide using the radiation-assisted diffusion technique. Polyimide samples ( 50 μm ×12 mm ×12 mm) were immersed in a BF3 solution kept in a thin walled glass bottle, and six such bottles were used. Each bottle was exposed to Co-60 gamma rays and the dose was varied from bottle to bottle over the range of 30–180 kGy. Both the dielectric constant, e′, and the refractive index, n, of the boron and fluorine-diffused polyimide decreased with increasing radiation dose, and hence with the concentration of fluorine and boron. For these polyimide samples, the dielectric constant, e′, estimated from the measured value of refractive index, n, (632.8 nm) was 2.22, which is close to the dielectric constant, e′, (2.26) measured at 7 MHz. The results indicate that the degree of electronic polarization is lowered appreciably as compared to that of atomic and dipole polarization. The decrease in the dielectric constant, e′, and the refractive index, n, is attributed to the increase in the free volume of polyimide and to a decrease in the degree of electronic polarization.

Growth of hierarchical TiO2 nanostructures on anatase nanofibers and their application in photocatalytic activity

Three-dimensional hierarchical nanostructures of TiO2, consisting of high density nanorods on nanofibers, were synthesized by the combination of electrospinning, thermal annealing and hydrothermal methods. The morphologies, crystal structures, surface area, band gap and photocatalytic activity of the hierarchical nanostructures were characterized by Raman microscopy, X-ray diffraction, UV-vis spectroscopy, scanning electron microscopy, transmission electron microscopy and surface area analysis. The results revealed that the nanofibers, synthesized by the electrospinning method followed by the thermal annealing, were of anatase phase. Similarly, the TiO2 nanorods grown on the anatase nanofibers by hydrothermal reaction in Ti-HCl solution were of rutile phase, with tetragonal shape, and had top square facet morphologies. The diameter and the length of these rutile nanorods could be varied over the range, 10 nm to 400 nm, and 20 nm to 1 um, respectively, by changing the parameters of the hydrothermal reaction. The growth mechanisms of these rutile rods are discussed on the basis of (i) surface roughness of the anatase nanofibers, (ii) presence of the twin planes of anatase layer with structure similar to rutile phase and (iii) Cl/Ti ratio in the solution. The estimated band gap energies of the nanofibers and nanorods were close to 3.2 eV and 3.0 eV. The total surface area of the hierarchical nanostructures could reach up to ∼20.41 m2 g−1; while, the pure the anatase nanofibers surface area is ∼19.79 m2 g−1. For the first time, such TiO2 hierarchical nanostructures, rutile nanorods on anatase nanofibers, have been synthesized. The photocatalytic activity of the TiO2 hierarchical nanostructures was found to be superior to that of pure anatase nanofibers.

Dielectric properties of 1 MeV electron-irradiated polyimide

The dielectric parameters of 50-μm-thick polyimide samples irradiated in air with 1 MeV electrons and in BF3 solution with Co-60 gamma rays were studied. The dielectric constant of polyimide was found to decrease with increasing electron fluence and dose of gamma rays. At an electron fluence of ∼1015 e/cm2, the dielectric constant decreased from its virgin value of 3.15 to 2.4, measured at a frequency of ∼7 MHz. Furthermore, by irradiating polyimide samples in BF3 solution with gamma rays, boron and fluorine diffused into each polyimide sample from both sides. In these polyimide samples, the dielectric constant decreased further to ∼2.1 (7 MHz). In a plot of dielectric loss, e″, vs log (f ), two peaks were observed over a frequency range 100 Hz–7 MHz, in virgin as well as in the electron- and gamma-ray-irradiated polyimide samples. The dielectric loss increased with increasing electron fluence or dose of gamma rays. The refractive index (632.8 nm) of polyimide decreased from the virgin value of 1.74 to 1....

Damage induced by 90 MeV silicon ions in crystalline silicon

Samples of n‐type crystalline silicon were exposed to different fluences, ranging from 1.25 to 5×1014 ions/cm2, of 90 MeV silicon ions. The induced disorder in the surface region was studied by grazing‐angle x‐ray‐diffraction and optical reflection spectroscopy, and that in the bulk was studied by measuring the lifetime of the excess minority carriers generated by a 1 MeV pulsed electron beam. Both in the surface region, as well as in the bulk, the degree of induced lattice defects was found to increase with an increase in the ion fluence. The value of the damage coefficient, estimated from the lifetime of the minority carriers, gradually increased from 1.2×10−10 to 8.9×10−10 cm2 s−1 over the range of ion fluences. These results indicate that 90 MeV silicon ions create an appreciable number of defects in crystalline silicon, the concentration of which increases from the surface to their projected range of ∼31 μm, and at higher fluences secondary defects are also produced in the bulk. Even at the highest f...

Improvement in the photoluminescence efficiency of porous silicon using high-energy silicon ion irradiation

An order of magnitude enhancement in the intensity of visible room temperature photoluminescence (PL) from porous silicon (PS) was observed when irradiated with 10 MeV silicon ions from a pelletron source. The effect was associated with a blue shift of 70 nm. The stability of PL with respect to ambients was seen to be remarkably improved. The energy band gap determined from photoluminescence and photoreflectance measurements indicated a shift from 1.67 eV to 1.83 eV. Subsequently partial restructuring of Si - O - Si and Si - H type species into Si - OH was confirmed by infrared measurements recorded before and after irradiation. The effects have been correlated to a reduction in the extent of non radiative recombination centres as a consequence of chemical restructuring of the surface. By assuming that the restructured surface is sufficiently thick to reduce the crystallite size, the blue shift can be accounted for. Formation of Si - OH bonds at the surface was also observed when PS was intentionally exposed to low-energy (10 - 30 eV) ions from an electron cyclotron resonance (ECR) plasma; associated with this, the PL intensity was enhanced.

Diffusion mediated growth of (111) oriented silver nanoparticles in polyvinyl alcohol film under 6MeV electron irradiation

Silver (111) nanoparticles were synthesized by diffusing silver from a solution into polyvinyl alcohol (PVA) films under 6MeV electron irradiation at room temperature (∼25°C). The diffusion of silver in the PVA was confirmed by the Rutherford backscattering spectroscopy and scanning electron microscopy techniques. The plasmon absorption peak at ∼426nm was an evidence for the initiation of the diffusion mediated growth of silver nanoparticles. The x-ray diffraction results and the blueshift in the plasmon absorption peak reveal that the size of silver nanoparticles could be tailored in the range from 35to15nm by varying the electron fluence over the range of 1014–1015e∕cm2.

Reduction of graphene oxide by 100 MeV Au ion irradiation and its application as H2O2 sensor

Graphene oxide (GO) synthesized from a modified Hummers method was reduced (referred, rGO) by using 100 MeV Au ion species and its response to the sense H2O2 was investigated. The changes in the atomic composition and structural properties of rGO after irradiation were studied using x-ray diffraction, Fourier transform infrared spectroscopy and x-ray photo-electron spectroscopy. These results suggested that the removal of the oxygen-containing functional groups and the improvement of the electrochemical performance of reduced graphene oxide (rGO) after ion irradiation. Raman spectroscopic results revealed the increase in the disorder parameter (I D/I G) after Au ion irradiation and also the formation of a large number of small sp2 domains due to the electronic energy loss of ion beam. The resultant rGO was investigated for H2O2 sensing using electrochemical techniques and it showed a good response.

Radiation induced recrystallisation and enhancement in photoluminescence from porous silicon

Abstract Porous Silicon (PS) samples were irradiated with different ionizing radiations which included 1 and 6 MeV electrons, 60 Co gamma rays and 10 MeV silicon ions. Improvement in the efficiency of photoluminescence (PL) and its stability with time were invariably observed in all the irradiated PS samples. Improvement in the luminescent properties was best for samples irradiated with 10 MeV silicon ions. Partial restructuring of SiOSi and SiH type species into SiOH was confirmed from the infrared spectra of pre- and post- irradiated samples. Grazing angle X-ray diffraction (XRD) analysis revealed that preferential recrystallisation occurs in the irradiated region. The virgin PS sample exhibited only the (1 1 1) peak in the XRD pattern; whereas the irradiated PS sample showed a (3 1 1) peak along with the (1 1 1) peak. The average size of the microcrystallites was calculated from the diffraction peak broadening, using Scherrers formula. Depth profile studies, corresponding to the average sizes of the microcrystallites confirmed the existence of (3 1 1) planes, and revealed that the degree of recrystallisation is maximum at the end of the trajectories of silicon ions.

Some electrical measurements on gaseous ion implanted metallic and insulating thin films

Abstract Experimental results on changes in conductivity of N2+ and Ar+ bombarded thin copper, aluminium and bismuth films are given along with the preliminary observations on photoconductivity of N2+ bombarded Bi2O3 thin films. The performance of a low cost, medium resolution 200 keV ion implantation system, used in the above experiments is also discussed.

Single-cavity 8 MeV race-track microtron

This paper describes the operational details of the single-cavity race track microtron of this laboratory. The machine is capable of providing electrons of 8 MeV energy at 1 mA peak current. Important parameters of the machine and beam handling system are studied and the results are reported.