The charge transport in liquid sulphur and fluorescence of the solid.
thesisposted on 19.11.2015, 09:19 by P. K. Ghosh
Specimens of liquid sulphur were obtained by melting orthorhombic sulphur crystals or ultrapure granules of sulphur. Between the melting point (119°C) and 160°C, liquid sulphur is composed of puckered S8 ring molecules bound by weak van der Waal forces as in the solid. Above 160°C, polymerisation of ring molecules takes place, Drift mobility techniques have been used to study the charge transport during melting and in the liquid up to about 200°C. The specimen cell consisted of two parallel quartz discs separated by glass spacers, 50 to 700 microns thick. Transparent electrodes were applied to the inner surfaces of the discs. Charge carriers were generated close to one of the electrodes by a fast ultraviolet light pulse (~ 10 microsecond duration), and carriers of one polarity were drawn across the specimen in the applied field. Their transit time was measured giving a value for the drift mobility. The results showed that the charge transport is due to both negative and positive charges which move in the liquid by means of phonon-assisted random hopping between neighbouring molecules. From a detailed discussion of the results it is concluded that we are dealing here with an electronic, rather than an ionic charge transport. The electron and hole mobilities at the melting point are 1.0 x 10-4 and 5.5 x 10-5 cm 2 / volt sec respectively. The Electron mobility which is a hopping transport in the solid, shows a drop of a factor of 30 during change of state. This has been attributed to a decrease of overlap J by a factor of 5.5. Between 119°C and 160°C the electron mobility increases in the liquid with increasing temperature. with an activation energy of 0.15 + 0.03 ev. Holes have the same activation energy within a larger experimental error. Above 160°C both electron hole mobilities decrease slowly with increasing temperature. This has been attributed to a decrease in the hopping probability resulting from a loss of ring molecules by polymerisation. Dark current measurements on liquid sulphur suggested that in less pure specimens impurity ions set up a space charge near each electrode of opposite polarity to that of the electrode which modifies the current in accordance with Thomson and model of conduction in gases. The measurement of the efficiency of carrier generation in liquid sulphur has been carried out at an applied field of 33 K volt/cm, considerably less than the saturation value. The results show that the ultraviolet photoconductivity reaches a peak at about 2600 A with a generation efficiency of 10-2. The recombination lifetime of generated carriers has been estimated to be about 10-7 sec. The visible emission of solid sulphur under x-ray excitation has been studied. The observed emission has an estimated efficiency between 10-4--10 -06. Three emission peaks at 4900, 5300 and 5700 A have been detected. The emission is thought to be due and transitions from a molecular excited state, 2.7 ev above the ground state, to the singlet ground state of the S8 molecule.