Electrical conduction of succinic acid doped glycine pellet

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 4, July 004, pp lectrical conduction of succinic acid doped glycine pellet D K Burghate, S H Deshmukh, Laxmi Joshi, V S Deogaonkar & P T Deshmukh Postgraduate Department of Physics, Shri Shivaji Science College, Amravati *Dr Panjabrao Deshmukh Polytechnic, Amravati Received 6 August 003; revised 0 February 004; accepted 5 April 004 The electrical conduction of succinic acid doped glycine pellet has been measured by studying the I-V characteristics at various temperatures in the range K. The results are presented in the form of I-V characteristics, and analysis has been made in the light of Poole-Frenkel, Fowler-Nordheim, Schottky, log(j) versus T plots, Richardson and Arrehenius plots. Results analysed suggest that, Schottky-Richardson mechanism is primarily responsible for observed conduction. [Keywords: Succinic acid glycine pellet, Conductivity] IPC Code: G 0 R Introduction The molecules found in living organisms not only conform to all the known physical and chemical principles governing the behaviour of molecules but in addition, interact with each other in accordance with the principles which is called the molecular logic of the living state. This molecular logic is maintained by biomolecules. The bio-electret state plays an important role in many biological phenomenon, and is a universal property of biomolecules, The bio-electret state manifests itself in a number of biological materials. Anagnostopoulou et al. and Bridlli et al. 3 have worked in biopolymers like polypeptides and proteins. Mishra et al. 4 have reported study of bio-electret state in glycine, which serves as a precursor in the biosynthesis of a number of biomolecules and also acts as part of active sites in many enzymes. The dcconductivity of glycine was measured to study the mechanism of electrical conduction by Vinay Mishra et al. 5 Belsare et al. 6 have reported the increase in electrical conductivity of polystyrene (PS) and polymethyl methacrylate (PMMA) with the increase in iodine doping concentration. In recent years a great deal of interest has been centered on the study of physical properties 7,8 of amino acids, because it may be helpful in understanding both physical and chemical properties of peptides, proteins and enzymes, as amino acids are the basic building blocks of these biological macromolecules 9. The interest in organic and polymeric semiconductors has arisen, particularly because of their electro-photographic and solar cell applications. Many synthetic polymers 0, like polyacetylene, polypyrole, poly-carbazol etc. have been studied. In the case of organic solids, where the conductivity due to electrons excited from valence band to conduction band,3 is negligible, a complex conduction behaviour 3,4 has been explained usually in terms of electron emission from cathode i.e. Schottky-Richardson mechanim 5, or by electron liberation from the traps in the bulk of the material i.e. Poole-Frenkel mechanism 6. However, possibility of tunneling 7, space charge limited conduction 8 etc have also been investigated in the literature. The electrical conduction in iodine doped polystyrene (PS) and polymethyl methacrylate (PMMA) has already been reported 9,0. Japanese scientists have been particularly active in early research and development of these devices with work on natural and synthetic polymers. Most of the early polymer electret work in the US has been focussed on using the polyelectric response for electromagnetic radiation detection,3. In the present study, dc conductivity of succcinic acid doped glycine was measured to identify the mechanism of electrical conduction. It is shown how the I-V data of the sample can be used to arrive at possible conclusions. Results have been discussed by plotting different mechanisms, such as Poole-Frenkle, Fowler-Nordheim, log( j) verses T Plots, Schottky

2 534 INDIAN J PUR & APPL PHYS, VOL 4, JULY 004 plots, Richardson plots and Arrehenious plots. In case of Schottky-Richardson mechanism, the current shows strong temperature dependence but not in case of the Poole-Frenkel mechanism. The study of temperature dependence of current density is, therefore, of great importance. xperimental Details. Preparation of Samples Glycine and succinic acid used in the present investigation were of standard grade product supplied by HI-MDIA laboratories Pvt Ltd, Mumbai, in the powder form. The mixture of 9% glycine, and 8% succinic acid, was prepared by using a shell and mortar, and successively rubbing together for hours, so that a homogenous mixture is obtained. This intimate mixture was poured in a pellet die, which was then subjected to a uniform pressure of 3 tons/cm using a KBR press. The pressure was applied for three minutes. A fine, uniform, shining pellet of the sample mixture was obtained with perfectly smooth parallel faces. Both the surfaces of the pellet were coated with quick drying silver paint supplied by lteck Pvt Ltd, Bangalore to ensure good electrical contacts. The coated sample pellet was subjected to uniform heating at a temperature of 00 o C in the furnace. The X-ray powder diffractograph of pure glycine +8% succinic acid has been recorded on Philips (Holland) automated diffractograph at RSIC, Nagpur. Fig. shows the two diffractograms (DG-pure glycine and DG3-glycine+8% succinic acid), Fig. X-ray powder diffractograph

3 BURGHAT et al.: LCTRICAL CONDUCTION OF GLYCIN PLLT 535 Table Indexing of diffractograms d (Å) Values Standard Observed γ g γ _ glycine γ s γ succinic acid h k l 0 0 γ s 0 γ g 0 γ g 0 γ s 0 0 γ s 0 γ s 0 γ g 0 γ g γ g γ g 3 0 γ g 3 0 γ g 3 γ g γ g respectively. As the strongest line of α-glycine (d =.990 Å [h k l]=[0 4 0] and β-glycine (d=4.9 Å [h k l]=[0 0 ] are not found in the diffractogram, it is inferred that α and β glycine are absent in the sample. Almost all peaks indicate to be those of γ-glycine and/or γ-succinic acid. Thus the sample is inferred as γ-glycine doped with 8% succinic acid. Table shows the indexing of the diffractogram.. Measurements For the measurement of current and voltages, the thermostatically controlled furnace supplied by Tempo Industrial Corporation, Mumbai, was used for heating purpose. The mercury thermometer with an accuracy ± o C was used to record the temperatures. The regulated power supply supplied by Nupur India was used as the voltage source, while the current was recorded by using highly sensitive Pico-Ammeter (Model DPA with accuracy ±0.% supplied by scientific equipments, Roorkee). The sample pellet coated with silver electrodes was sandwiched between two brass-electrodes of the sample holder specially fabricated in the laboratory having electrode diameter of.4 cm each. This formed the metal-insulator-metal (M-I-M) system, which was placed in a furnace. The current (I)-voltage (V) measurements have been done at various constant temperatures from 33 to 353 K. 3 Results and Discussion The average molecular mass of glycine and succinic acid as a first approximation, from the atomic masses can be taken as 7.5 amu and 8.43 amu which are not very much different. Also the densities 4 are.670 and.57, respectively, which also do not differ much. As such, their masses and densities being in the same range, the impurity dopants should have the substitutional occupancy giving rise to Schottky defect. The logi-logv plots of succinic acid doped glycine pellet at various temperatures 33, 33, 333, 343 and 353K are shown in Fig.. The current increases non-linearly with the applied voltage and does not follow a power law, I = kv m, where k and m are constants. The current in the beginning at low values of voltages increases at a slower rate while it increases at a faster rate, at higher values of voltages. Figure indicates that (i) the current at a constant temperature increases with applied voltage (ii) the current at constant applied voltage increases with temperature. The mechanism operative in present case is discussed in the light of the Poole-Frenkel, Fowler-Nordheim, Schottky, log(j) versus T plots, Richardson and Arrehnius plots. 3. Poole-Frenkel Mechanism The current-voltage relationship for Poole- Frenkel mechanism 6 is expressed as, J = Be φ +β PF Fig. Current-voltage characteristics

4 536 INDIAN J PUR & APPL PHYS, VOL 4, JULY 004 where PF = e e β = ( ) πεε d o Constant and predicts a field dependent conductivity as:...() 3.3 Schottky plots Thermal activation of electron, may occur over the metal insulator interface barrier, which is further helped by the applied electric field effect, which reduces the height of the barrier. The Schottky- Richardson current voltage relationship is expressed as: o βpf σ = σ e...() or log βpf σ = logσ o + (3) So that the Poole-Frenkel mechanism is characterized by the linearity of log σ versus / plots i.e. Poole-Frenkel plots predicted by q. (3) are linear with a +ve slope (Fig. 3). In the present case of glycine doped with succinic acid the logσ versus / plots are linear but with a ve slope indicating the absence of PF mechanism. 3. Fowler-Nordheim Mechanism The Fowler-Nordheim relation 7 for current density is: φ V J = AV e So that, J φ log loga V = V And the log J/V versus /V plot is expected to be a linear relation with a ve slope. In this case the log J/V versus /V plot for the sample is presented in Fig. 4 excepting one or two points which have strayed away, the graphs are very nearly straight line with a +ve slope, indicative of the absence of tunneling current as is suggested by F-N relation. J = AT e -φs + β β being the field lowering constant given by, e e ( )( ) β = and hence logj = logat 4ππε d 0 φ s + β and that log J versus plot should be a straight line with a +ve slope. Schottky-plots for the present case are shown in Fig. 5. The relation expects a +ve slope, which is observed in the present case, and as such indicates the Fig. 4 Fowler-Nordheim plots Fig. 3 Poole-Frenkal plots Fig. 5 Schottky plots

5 BURGHAT et al.: LCTRICAL CONDUCTION OF GLYCIN PLLT 537 applicability of the mechanism. Further, in the case of Schottky-Richardson mechanism the current shows strong-temperature dependence but not in case of the Poole-Frenkel mechanism. The study of temperature dependence of current density is, therefore, of great importance. 3.4 Current density versus temperature plots The temperature dependence of current density is presented in the form of log(j) versus temperature plots in Fig. 6, which shows that log(j) increases linearly with temperature. The strong temperature dependence is in agreement with the Schottky- Richardson mechanism. Further, the straight lines with constant slope are observed for all the fields indicating the absence of any thermodynamic transition in the temperature range studied. 3.5 Richardson mechanism The Richardson current voltage relationship is expressed as, J = AT e -φs + β J φs β = Ae + T J φ = + S log log A + β T J log log A + β T = φs The graph between log (J/T ) versus (/) from this relation should be a straight line with a negative slope. Fig. 6 Current density versus temperature plots Fig. 7 Richardson plots Fig. 8 Arrehenious plots In the present case such straight line graphs have been obtained with a ve slope. The linearity of the plots support Schottky-Richardson mechanism (Fig. 7). 3.6 Arrehenius plots The logσ versus /T plots (Fig. 8) at all values of applied voltages show parallel straight line with a ve slope. From the slope of straight line, the activation energy is calculated and is found to be in the neighbourhood of 0. ev. This is in good agreement with the reported order of magnitudes. 4 Conclusion mechanism of conduction predominates over other mechanisms in the doped sample. The applied field value seems to be insufficient to liberate electron from traps, showing absence of PF mechanism. ven though the electric field value and the temperature range is lower to activate contribution from other mechanism, yet the activation energy can safely be calculated and is found to be well within range for similar samples. Acknowledgement The authors are extremely thankful to Prof V G Bhamburkar, Principal, Shri Shivaji Science College,

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