• Francis P Xavier

      Articles written in Bulletin of Materials Science

    • Photoconductivity in crystalline phthalocyanines

      Francis P Xavier Géorge J Goldsmith

      More Details Abstract Fulltext PDF

      The wavelength, temperature, time and intensity dependence of photocurrent of metal-free phthalocyanine (H2Pc) and copper phthalocyanine (CuPc) single crystals were investigated. The thermal activation energies in the dark are 0·5 and 0·6 eV for H2Pc and CuPc respectively and the corresponding photo-thermal activation energies are 0·3 and 0·2 eV. An energy level scheme for single crystals of H2Pc and CuPc is proposed which consists of two trapping levels and five narrow optically active valence bands. In H2Pc (CuPc), one trapping level at 0·5 eV (0·6 eV) above the valence band edge to which the charge carriers are thermally excited in the dark; and the other trapping level is at 0·3 eV (0·2 eV) below the conduction band edge where all the optical transitions terminate. In H2Pc(CuPc), the forbidden gap is 1·44 eV (1·34 eV) wide; the five valence bands are at the band edge, and 0·09 (0·22), 0·42 (0·63), 0·69 (0·90), 1·32 (2·17) eV below the band edge.

    • Photoconductivity in thin films of phthalocyanine

      Francis P Xavier George J Goldsmith

      More Details Abstract Fulltext PDF

      The photocurrent and electrolyte electromodulation (EEM) spectra of thin films of metal-free phthalocyanine (H2Pc) and of copper phthalocyanine (CuPc) were investigated. The modulation spectra yielded three distinct features around 1·61, 2·30 and 2·93 eV for H2Pc and around 1·63, 2·04 and 3·20 eV for CuPc. The spectral dependence maxima of photoconductivity correspond to the modulation spectra. These features are interpreted to indicate transitions at critical points, i.e. the existence of transitions between three valence bands, since Pc’s are p-type, and the lowest conduction band in Pc’s.

    • Photoconductivity of iodine-doped single crystals of phthalocyanine

      Francis P Xavier George J Goldsmith

      More Details Abstract Fulltext PDF

      Single crystals of metal-free phthalocyanine (H2Pc) and of copper phthalocyanine (CuPc) were grown in the presence of iodine vapour. The presence of iodine enhances the spectral dependence of photoconductivity of H2Pc in the visible region but of CuPc in the near-IR region. The dark current is decreased but the photocurrent is increased by one order of magnitude in iodine-doped H2Pc but in the case of iodine-doped CuPc both currents are increased by nearly three orders of magnitude. Introduction of iodine results in about one order of magnitude decrease in response time for both modifications. Thus the introduction of iodine into Pc crystals decreases the energy barrier for conduction and increases the drift mobility of charge carriers thereby enhancing the conductivity of the material.

    • Single crystal growth of organic photoconductors: phthalocyanine

      Francis P Xavier George J Goldsmith

      More Details Abstract Fulltext PDF

      An effective method of growing single crystals of organic photoconductors such as phthalocyanine in the presence of doping impurity such as iodine by vacuum sublimation is discussed in this paper. This method is very useful especially when an organic material does not have a melting point but decomposes above a particular temperature. So far, doping has been done by exposing the previously grown organic single crystals to the dopant vapour, but growing the crystal in the presence of dopant vapour makes the doped-crystal more efficient and stable. The photosensitivity as well as the photoconductivity of the doped crystals by this method increases significantly.

    • Photoconductivity studies of CdSe1−xTex thin films as a function of doping concentration

      D Ravichandran Francis P Xavier S Sasikala S Moorthy Babu

      More Details Abstract Fulltext PDF

      Thin films of CdSe1−xTex were prepared by using electro deposition with varying doping concentrations of Te(x). Photoconductive studies as a function of applied field, time, wavelength, and intensity of incident light were carried out at room temperature between 1 and 3·5 eV. After establishing that the electrode contacts made using silver paint were ohmic, it was found that as the concentration of Te(x) increases in CdSe1−xTex, the band gap of the material decreases according to the empirical relationEg = (2 − 1·5x)eV.

    • Band gap study on zinc-doped cadmium sulphide

      T K Anee Francis P Xavier K Balakrishnan

      More Details Abstract Fulltext PDF

      Photoconductivity studies on cadium sulphide (CdS) crystals grown by chemical vapour transport method were carried out at room temperature (300°C) over the spectral range between the near ultraviolet and the near infrared. Three samples of CdS crystals, viz. undoped CdS crystal, 0·5 ppm zinc doped CdS crystal, and 1 ppm zinc doped CdS crystal, were used. The variation of photocurrent as a function of applied field, intensity of the incident light, response time, and incident wavelength was studied. It was observed that the band gap decreased linearly as doping concentration increased. This is interpreted as being due to doped impurity atoms acting as traps very close to the conduction band edge. The rise and decay times also decreased linearly as doping concentration increased. This has been interpreted due to more free charge carriers being created in the crystal with increase in doping concentration, thereby making the crystal more photosensitive.

    • Conduction mechanism of metal-free phthalocyanine single crystals as a function of temperature

      Ayona Simon Francis P Xavier

      More Details Abstract Fulltext PDF

      Metal-free phthalocyanine (H2Pc) single crystals grown by vacuum sublimation were investigated for their conductivity (both in dark and light). The investigations consisted of dark- and photo-current variations with (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·8 kV/cm to 6 kV/cm. The temperature range was from 300°K to around 570°K. The crystals were found to be photoconductive. Based on activation energies calculated from photoconductivity due to temperature dependence, an energy level scheme for H2Pc single crystals is proposed. The model consists of two trapping levels within the forbidden gap — one at 0·4 eV below the conduction band edge from which electrons are thermally excited into the conduction band and the other acts as recombination centre at 0·3 eV above the valence band edge. The band gap is calculated to be 1·4 eV. Comparative study of the proposed model with that of earlier investigations on the same crystals of the H2Pc is in good agreement, thereby indicating that H2Pc is thermally stable even at relatively higher temperature as semiconductor.

    • Conduction mechanism in anthracene as a function of temperature

      Jeena V Joseph Francis P Xavier

      More Details Abstract Fulltext PDF

      Conduction mechanism in anthracene single crystal grown by Bridgman method was carried out. The investigations consisted of dark- and photo-current variation with respect to (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·5 to 2·5 kV/cm and the temperature range was between 300 K and 450 K. Photo and dark current variations with temperature indicate, based on activation energy determination, that a band model can be applied to the conduction process. The band gap is calculated to be 1·6 eV. The band model consists of a recombination centre 0·37 eV above the valence band edge and a trap level 0·55 eV below the conduction band edge to which electrons are first thermo-optically excited and then they are thermally excited into the conduction band.

    • Application of electroreflectance analysis for organic semiconductor thin films

      Francis P Xavier

      More Details Abstract Fulltext PDF

      Many organic semiconductors with conjugate bond structure possess photoconductivity. Conduction mechanism of organic materials exhibits ‘dualism’ since both intramolecular as well as inter-molecular aspects are involved in the excitation, absorption and transport of charge carriers. Modulation spectroscopy promises to be the most accurate method for analysis of organic photoconductors, especially of thin films. In this technique a periodic perturbation is applied to the material under study and the effect of the perturbation is separated from reflection or absorption while scanning through a given wavelength range by use of lock-in phase sensitive detection method. In electromodulation, particularly in electrolyte electromodulation, the applied field on the material produces changes in the dielectric function which corresponds to the change in reflectance. When the applied field is low the line-shape of spectrum is third-derivative like in comparison with the unmodulated reflectance spectrum. Using Aspnes three-point method the transitions corresponding to critical points can be determined. When the field is intermediate Franz-Keldysh oscillations, which are dc bias dependent, appear on the higher energy side of the transition energy from which the role of intra-molecular as well as intermolecular aspects in conduction mechanism can be understood and the carrier concentration could be determined. Though the electroreflectance method has been developed for inorganic semiconductors, it could be effectively applied for organic/molecular semiconductors as well if the constituent molecules are assumed to be the lattices. The study of organic photoconductors is very important since they are more and more promising especially in photocopying, photovoltaic and solar cells.

    • Solid state electrical conductivity studies on manganese-molybdenum Schiff base polymer

      S Suganya Francis P Xavier K S Nagaraja

      More Details Abstract Fulltext PDF

      Heterobinuclear manganese-molybdenum complex with the Schiff base prepared by the condensation of diethylenetriamine with salicylaldehyde yielded a polymeric mixed metal coordination complex with the composition of [Mn1·5MoO3L]. The infrared spectral data indicated the presence of azomethine (HC=N) and Mo=O groups.1H NMR further confirmed the coordination of azomethine nitrogen and absence of phenolic hydrogen. The EPR studies suggested the presence of Mn(II) in the complex. The solid state conductivity studies with the variation of potential and temperature indicated a specific conductivity of 1·6×10−5 ohm−1 cm−1. The activation energy was found to be 0·019 eV. Hall measurements indicate that the material isp-type which is further confirmed by iodine doping which increases the conductivity by two orders of magnitude.

  • Bulletin of Materials Science | News

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

© 2017-2019 Indian Academy of Sciences, Bengaluru.