Subhas Chandra

National Physical Laboratory - India, Old Delhi, NCT, India

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Publications (60)140.17 Total impact

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    ABSTRACT: The ac conductivity and dielectric constant of poly(N-methyl pyrrole) thin films have been investigated in the temperature range 77–350 K and in the frequency range 10 2-102 Hz. The well defined loss peaks have been observed in the temperature region where measured ac conductivity approaches dc conductivity. These loss peaks are associated with the hopping of the charge carriers. The frequency and temperature dependence of ac conductivity have been qualitatively explained by considering the contribution from two mechanisms; one giving a linear dependence of conductivity on frequency and other having distribution of relaxation times giving rise to broad dielectric loss peak.
    Bulletin of Materials Science 04/2012; 23(3):227-232. DOI:10.1007/BF02719915 · 0.87 Impact Factor
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    ABSTRACT: White organic light emitting devices (WOLEDs) are being considered as substitutes for conventional white light sources. They are efficient solid-state lighting sources and their power efficiencies have surpassed those of the incandescent light sources, especially due to recent improvement in device architectures, molecular engineering in synthesis of new materials and the incorporation of electrophosphorescent emitters. This paper reviews the various approaches to achieve white light emission from organic light emitting diodes (OLEDs), their advantages, disadvantages and recent progress. The device architecture and problems related to various device designs have been discussed.
    Semiconductor Science and Technology 07/2006; 21(7). DOI:10.1088/0268-1242/21/7/R01 · 2.21 Impact Factor
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    ABSTRACT: Light emitting diodes based on organic materials are of considerable interest owing to their attractive characteristics and potential applications in flat panel displays. This paper focuses on some important blue light emitting organic materials. Particularly the attention has been paid to the absorption, photoluminescence (PL), electroluminescence (EL) spectra, current-voltage (I-V) and luminescence-voltage (L-V) characteristics of blue light emitting organic semiconductors, as well as those of the para-hexaphenyl (PHP). Syntheses of some important blue light emitting materials have also been discussed. Para hexaphenyl (PHP), which was synthesized by the conventional Kovacic method, is one of the important blue light emitting p-conjugated oligomers. OLEDs fabricated, using PHP as an emitter, TPD and PBD as the hole transport layer and electron transport layer respectively, gave efficient blue electroluminescence. The photoluminescence (PL) and electroluminescence (EL) of PHP occur at 423 nm, indicating PHP as an excellent blue emitter. This oligomer showed strong absorption peaks at 297 and 318 nm. The devices showed non-linear electrical properties and bright electroluminescence with low turn on voltages (9V). Some of our most recent work in the development of a variety of blue light emitting devices has been included and discussed in detail.
    Indian Journal of Pure and Applied Physics 09/2004; 42:793-805. · 0.71 Impact Factor
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    ABSTRACT: Poly-p-phenylene is a very attractive conducting polymer due to its resemblance with poly-p-phenylene vinylene. Since it is very rigid, intractable and difficult to process, it has not been very popular in the fabrication of organic light emitting diodes (OLEDs). We have co-polymerized benzene and naphthalene using Kovacic's method, which uses the mechanism of propagation of oxidative coupling occurring through a dehydrogenation step via radical cations. The resulting co-polymer is soluble in common organic solvents like toluene and has interesting electrical and optical properties and shows strong photoluminescence. The co-polymer has low melting point and can be purified using vacuum sublimation. The new polymer has an optical absorption band in the visible region of the optical spectrum and a photoluminescence maximum in the orange region. The polymer has been characterized using spectroscopic and thermogravimetric techniques. OLED structure has been fabricated using indium-tin oxide as the bottom electrode, evaporated polymer as the luminescent layer and aluminum as the cathode. The devices give bright electroluminescence on application of approximately 5-V d.c. The photoluminescence of the polymer has also been reported.
    Thin Solid Films 09/2003; 441(1-2):243-247. DOI:10.1016/S0040-6090(03)00969-6 · 1.87 Impact Factor
  • Indian Journal of Pure and Applied Physics 01/2003; 41(8):641-645. · 0.71 Impact Factor
  • Chemistry of Materials 04/2002; 6(10). DOI:10.1021/cm00046a004 · 8.54 Impact Factor
  • Analytical Chemistry 04/2002; 66(19). DOI:10.1021/ac00091a023 · 5.83 Impact Factor
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    ABSTRACT: The direct current (DC) conductivity of poly(3-methyl thiophene) was measured in the temperature range of 77-300 K. The observed DC conductivity data were analyzed in the light of Mott's variable range hopping model. Different Mott's parameters such as characteristic temperature (T0), average hopping distance (R), average hopping energy (W), and density of states at the Fermi level (N [E(F)]) were evaluated. By taking the inverse of the coefficient of exponential decay of the localized states involved in the hopping process as 0.5 nm, a realistic value of density of states at the Fermi level (N [E(F)]) was obtained that agrees well with the values reported earlier for other conjugated polymers.
    Applied Biochemistry and Biotechnology 09/2001; 96(1-3):119-24. DOI:10.1385/ABAB:96:1-3:119 · 1.69 Impact Factor
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    ABSTRACT: Poly(3-methyl thiophene) was synthesized by oxidative chemical polymerization technique using ferric chloride as the dopant in an inert atmosphere. Samples of different doping levels were prepared and analyzed by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy, and direct current (DC) conductivity measurement at room temperature (300 K). Synthesis of the polymer was confirmed by FTIR studies. FTIR spectra showed a shift in the heterocyclic bands in the region of 700-1200 cm(-1) with a decrease in synthesis temperature. It was evident from the scanning electron micrographs that the surface structure of the polymer became denser with an increase in doping level. The measured DC conductivity increased initially up to the doping level of 0.8 M and then this increase tended to slow down. Samples having a doping level of 0.4 M were synthesized at 300, 280, and 270 K while maintaining the other synthesis parameters. The conductivity and yield were found to increase as the temperature of the polymerization decreased.
    Applied Biochemistry and Biotechnology 09/2001; 96(1-3):109-17. DOI:10.1385/ABAB:96:1-3:109 · 1.69 Impact Factor
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    ABSTRACT: Polymer thin film devices are fast emerging as efficient active elements for processing of optical signals for using civil, space and military applications. The requirements for a thin film material to be used in optical processing and communication are the high second and third order non-linear susceptibilities, low propagation loss and variable depth, propagation to allow the same device operate at different wavelength.
    Proceedings of SPIE - The International Society for Optical Engineering 11/1999; DOI:10.1117/12.369459 · 0.20 Impact Factor
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    ABSTRACT: Polymers are the new and fast emerging materials with a growing scientific and technological interest. The material, though of recent origin, offers unique possibility of tailoring their properties to suit a large variety of applications. The properties and characteristics of intrinsically conducting polymers make them suitable materials for several applications. The detectors made with such organic polymers can help in quick and easy identification of the microbes.
    Proceedings of SPIE - The International Society for Optical Engineering 11/1999; DOI:10.1117/12.369460 · 0.20 Impact Factor
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    ABSTRACT: Dielectric constant and AC conductivity measurements on lightly doped polyaniline (PAN) have been made over the frequency range of 100 Hz–1 MHz and in the temperature range of 77–410 K. At temperatures below 100 K, the AC conductivity data could be described by the relation; σ(ω)=Aωs, where the parameter `s' lies close to unity and decreases with the increase in temperature. The measured AC conductivity is substantially higher than the DC conductivity in the low temperature region and is mainly controlled by the process of dipolar origin. The observed dielectric behavior does not exhibit well defined loss peaks. In the absence of well defined loss peaks, another approach of dielectric modulus has been utilized. Analysis in terms of the dielectric modulus explains the conductivity behavior of the system although it fails to explain the temperature variation of dielectric constant. The activation energy calculated from DC, dipolar and modulus analysis have approximately the same value which support the existence of a thermally activated behavior in the system.
    Synthetic Metals 07/1999; 104(2):137-144. DOI:10.1016/S0379-6779(99)00043-0 · 2.22 Impact Factor
  • Suresh Chand, G D Sharma, Subhas Chandra, R Chandra
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    ABSTRACT: The thermally stimulated discharge behaviour of vacuum-deposited polyvinyl fluoride films (about thick) studied as a function of polarization temperature (313 - 453 K) and polarization field (1 - ) shows a single relaxation peak whose peak temperature, charge, activation energy and so on depend strongly on the polarization parameters. The peak has been attributed to space-charge distributed polarization due to the migration of the charge carriers, injected from the electrodes or present inherently, over macroscopic distances and their subsequent trapping; the trapping levels being distributed in the range of energies 0.55 - 0.69 eV.
    Journal of Physics D Applied Physics 12/1998; 30(8):1243. DOI:10.1088/0022-3727/30/8/014 · 2.52 Impact Factor
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    ABSTRACT: Strontium titanate sol was prepared using strontium ethyl haxanoate and titanium isopropoxide. The sol was then spin coated on fused silica, p-type single-crystal silicon wafers (100) and stainless-steel substrates and annealed to give polycrystalline, transparent, and crack-free films. The surface morphology and structural properties of the films were studied using scanning electron microscopy and x-ray diffraction, respectively, and differential thermal analysis was used to observe structural transition. The dielectric measurements were conducted on films with metal–insulator–metal and metal–insulator–semiconductor configurations. Capacitance–voltage (C–V) measurements were carried out and the effect of the annealing temperature was studied. The dielectric constant and loss tangent at 1 MHz at room temperature were found to be 105 and 0.02, respectively, for 1.1 μm thick films. These measurements were also carried out at low temperatures down to 20 K. There are indications for a phase transition from a cubic perovskite to tetragonal perovskite structure at about 100 K where the tan δ shows some fluctuation, a characteristic of such transitions. The absence of a peak in the dielectric constant and the absence of hysteresis below the transition temperature have been explained on the basis of the low value of the tetragonal distortion (c/a = 1.003) reported on bulk material. © 1997 American Institute of Physics.
    Journal of Applied Physics 10/1997; 82(9):4484-4488. DOI:10.1063/1.366181 · 2.19 Impact Factor
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    ABSTRACT: Sharp changes in the transport and structural properties were noticed when a pH-dependent study of polyaniline (PAN) with monovalent (Cl−) and multivalent (PO4)3− ions was carried out. Scanning electron micrographs (SEM) of these samples reveal that the sharp changes induced by doping are more prominent at the pH where the transformation of the PAN lattice takes place. A channel formation has been observed at a pH of 3.0 in the case of HCl and 2.5 in the case of H3PO4 doping, with flattening of the structure. The d.c. conductivity of the samples in this pH range has been measured in the temperature range 77–300 K. The d.c. conductivity data have been analysed in the light of existing theoretical models. Mott's 3D variable range hopping (VRH) is the mechanism responsible for charge transport at low temperatures for HCl as well as H3PO4 doped samples, whereas a deviation is observed at higher temperatures for all the HCl doped samples. However, lightly doped H3PO4 samples give a dependence. At a moderate doping level (pH 2.5), a deviation from 3D VRH is observed at higher temperatures, indicating thereby a temperature and composition dependent charge transport. A lesser conductivity is obtained in the case of H3PO4 doping in comparison to HCl for the same pH value, which may be due to cross-linking induced by the multivalent ion.
    Polymer 09/1997; DOI:10.1016/S0032-3861(97)00013-X · 3.77 Impact Factor
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    ABSTRACT: Conducting poly(o-toluidine) (POT) films have been prepared on indium-tin-oxide (ITO) glass plate by electrochemical, dip-coating and Langmuir-Blodgett (LB) film deposition techniques. The electrochemical and optical properties of poly(o-toluidine) films prepared by different methods have been compared using cyclic voltammetry, UV-visible and Fourier transform infrared spectrophotometry techniques.
    Thin Solid Films 07/1997; 304(1-2):65-69. DOI:10.1016/S0040-6090(97)00186-7 · 1.87 Impact Factor
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    ABSTRACT: Electron spin resonance and dc conductivity investigations in the polypyrrole family of polymers support the existence of Mott’s variable range hopping mechanism. © 1997 American Institute of Physics.
    Journal of Applied Physics 05/1997; DOI:10.1063/1.365495 · 2.19 Impact Factor
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    ABSTRACT: The measured ac conductivity σm(ω) and dielectric constant ε′(ω) of lightly doped poly(N-methyl pyrrole-pyrrole) copolymer were investigated in the temperature range 77-350K and in the frequency range 100 Hz-100kHz. At low temperatures, the measured ac conductivity shows a strong dependence on frequency and is almost independent of temperature and can be described by the relation [sgrave](ω)=Aω, where the exponent s is found to be less than unity. The observed temperature and frequency dependence of the ac conductivity [sgrave](ω) can be qualitatively explained by considering contributions from two mechanisms, one giving a linear dependence of conductivity on frequency and the other giving rise to a broad dielectric loss peak having a distribution of relaxation times. At high temperatures the dielectric constant shows a frequency dispersion consistent with Debye-type relaxation. The origin of this relaxation was critically analysed. The estimated value pf site spacing d calculated from the dielectric strength and spin density is ≈ 19Å which suggests one charge carrier per 4-6 pyrrole rings.
    03/1997; 75(3):419-430. DOI:10.1080/13642819708202328
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    M. N. Kamalasanan, Subhas Chandra
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    ABSTRACT: Transparent and crackfree thin films of ZnO have been deposited on fused silica, soda glass, silicon wafers and KBr single crystals using the sol-gel technique. A sol has been prepared by reacting zinc acetate and ethylene glycol and dissolving the resultant transparent brittle solid in dry n-propanol. A proton acceptor like triethyl amine was added to assist hydrolysis of zinc acetate. The resulting solution was readily gelled on addition of water. Films were spin cast on polished substrates, gelled in humid air and pyrolysed at 450 °C to get polycrystalline ZnO thin films. The films were characterized using X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and UV-Vis absorption spectroscopy.
    Thin Solid Films 11/1996; 288(1):112-115. DOI:10.1016/S0040-6090(96)08864-5 · 1.87 Impact Factor
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    ABSTRACT: Electrical properties of poly(2,6- dimethyl-1,4-phenylene oxide) (PPO) have been studied by measuring thermally stimulated discharge currents. Depolarization behavior of poled PPO film exhibits temperature- and time-dependent single relaxation around 468 K which is attributed to dipole reorientation. The peak maxima, however, shifts from 468 K to 484 K with increasing poling temperature. This indicates that the peak has a distribution of relaxation times. The polarization in semicrystalline PPO is found to decay slowly, indicating that the overall dipoles are more stable in comparison to amorphous PPO; hence, crystalline PPO forms comparatively stable electrets. © 1996 John Wiley & Sons, Inc.
    Journal of Applied Polymer Science 08/1996; 61(9):1431-1437. DOI:10.1002/(SICI)1097-4628(19960829)61:93.0.CO;2-K · 1.64 Impact Factor