D'Arcy, R J (1971) The work function of titanium and selected metals in vacuum. Doctoral thesis, Keele University.

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Abstract

The work functions of titanium films have been measured at room temperature with a scanning probe version of the Kelvin-Zisman dynamic capacitor. The values for the clean surfaces of films deposited onto a gold substrate at 2.10-9 torr lay between 4.5 and 6.0 eV. The surface potentials of hydrogen were positive apparently due to penetration of the adsorbed hydrogen atom with a partial negative ionicity below the surface electronic plane. The maximum surface potential increased approximately linearly with the initial work function and varied between 0.26 and 1.30 eV. A surface potential model was proposed which attempted to account for dissociative adsorption of molecular hydrogen and solution of adsorbed atomic hydrogen. The time dependant changes of the surface potential were consistent with the theoretical predictions and the activation energy for solution of atomic hydrogen was derived to be between 0.7 and 1.5 Kcal gr-atom-1. The work function and surface potential of rapidly deposited films at 1.10-7 torr onto a silver substrate behaved similarly to those above and mass spectrometric analysis of the residual gas provided evidence for a predominant adsorption of hydrogen. Slowly deposited films at this pressure onto a silver substrate, however, possessed a low work function between 2.5 and 3.8 eV which broadly encompassed the range of values recently published by others for thin titanium films deposited apparently under similar conditions. The low values apparently arise from impurities. The surface potential of the residual gas adsorbate was negative on contaminated films and the time dependant changes were of a similar form to those of purer films. These results were discussed in relation to the hydrogen surface potential model and the adsorption of other gases. The work function of two polished stainless steel electrodes were 4.65 + 0.10 eV at 1.10-7 torr and 5.60 + 0.10 eV (after baking) at 2.10-9 torr. The work function was reversibly decreased by illumination, prolonged exposure to hydrogen and by applied electrostatic fields; it was reversibly increased by an incident electron current. These effects are interpreted as the result of an oxide layer on the steel surface. Theoretical models are proposed which attempt to account for these as due to changes in the density of surface states at a semiconducting oxide surface. The results are generally consistent with the theoretical predictions with the exception of the field effect. The effect of stray capacitive coupling on contact potential measurements was experimentally investigated. The results were partially consistent with the predictions of a model which is proposed. These effects are shown to be a major source of error if simple precautionary measures are not taken to avoid them.

Item Type: Thesis (Doctoral)
Subjects: Q Science > QC Physics
Divisions: Faculty of Natural Sciences > School of Chemical and Physical Sciences
Depositing User: Lisa Bailey
Date Deposited: 19 Feb 2019 09:29
Last Modified: 19 Feb 2019 09:29
URI: http://eprints.keele.ac.uk/id/eprint/5888

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