Subhas Chandra

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

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Publications (42)65.53 Total impact

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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 01/2006; 21(7). · 1.92 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. · 0.85 Impact Factor
  • N. Kumar, Bansi D. Malhotra, Subhas Chandra
    Journal of Polymer Science Part C Polymer Letters 04/2003; 23(2):57 - 61.
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    ABSTRACT: The optical and electrical properties of BF−4 deposed polypyrrole (PPY) have been studied in detail. The effect of annealing on the optical and electrical properties of BF−4 doped PPY is reported. Interestingly, these properties are modified with annealing and it is observed that PPY is similar to other inorganic amorphous semiconductors. It is also seen that the electrical and optical properties of BF−4 doped PPY are stabilized after 70 h of annealing at 70°C. © 1993 John Wiley & Sons, Inc.
    Journal of Applied Polymer Science 03/2003; 50(3):411 - 417. · 1.40 Impact Factor
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    ABSTRACT: An attempt has been made to estimate various electronic parameters such as work function, barrier height, and ideality factor through I-V measurements carried out on metal/polyaniline Schottky junctions. The results of optical studies conducted on chemically synthesized polyaniline used in the fabrication of such Schottky devices have been presented.
    Journal of Applied Polymer Science 03/2003; 44(5):911 - 915. · 1.40 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 01/2003; 441:243-247. · 1.87 Impact Factor
  • Indian Journal of Pure and Applied Physics 01/2003; 41(8):641-645. · 0.85 Impact Factor
  • Analytical Chemistry - ANAL CHEM. 04/2002; 66(19).
  • Chemistry of Materials - CHEM MATER. 04/2002; 6(10).
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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.
    Proc SPIE 11/1999;
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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.
    Proc SPIE 11/1999;
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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 01/1999; 104(2):137-144. · 2.11 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. · 2.21 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; 304:65-69. · 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; · 2.21 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.
    Philosophical Magazine B. 03/1997; 75(3):419-430.
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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 01/1997; · 3.77 Impact Factor
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    ABSTRACT: The alternating current (ac) conductivity [σ m (ω)], dielectric constant (ϵ′) and loss (ϵ″) of polypyrrole (PPY), poly(N‐methyl pyrrole) [P(NMPY)] and their copolymers; poly(N‐methyl pyrrole–pyrrole [P(NMPY–PY)] have been measured in the frequency range 10<sup>2</sup>–10<sup>6</sup> Hz and in the temperature range 77–350 K. At 77 K, the ac conductivity can be expressed by the relation; σ ac =Aω<sup>s</sup> where the slope s lies in the range 0.72–0.81 for these three polymers and decreases with increase in temperature. The well‐defined loss peaks, whose magnitude decreases with the increase in frequency, have been observed in the temperature region where the measured ac conductivity approaches the direct current (dc) conductivity. These loss peaks have been associated with the movement of charge carriers in these polymeric films. The dc conductivity has also been measured in the temperature range 77–350 K and an attempt has been made to correlate it with dielectric data. © 1996 American Institute of Physics.
    Journal of Applied Physics 08/1996; · 2.21 Impact Factor
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    ABSTRACT: The temperature dependence of the dc conductivity of electrochemically polymerized films of polypyrrole, poly(N‐methyl pyrrole) and their copolymers, poly(N‐methyl pyrrole‐pyrrole), having different percentage of BF<sup>-</sup> 4 ions has been investigated in the temperature range 77–350 K. The observed behavior could be explained in terms of Mott’s variable range hopping model involving a single phonon process. The estimated values of polaron radius yield the realistic values of density of states at the Fermi level which are in good agreement with the values reported earlier for other polyconjugated systems. © 1996 American Institute of Physics.
    Journal of Applied Physics 03/1996; · 2.21 Impact Factor
  • Proc SPIE 01/1996;

Publication Stats

543 Citations
65.53 Total Impact Points

Institutions

  • 1991–2003
    • National Physical Laboratory - India
      Old Delhi, NCT, India
    • University of Delhi
      • Department of Physics & Astrophysics
      Delhi, NCT, India