Work Function Study of Polycrystalline Metals using a UHV Scanning Kelvin Probe
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Work Function Study of Polycrystalline Metals using a UHV Scanning Kelvin Probe U. Petermann, I.D. Baikie, B. Lägel, K.M. Dirscherl Department of Applied Physics, Robert Gordon University, Aberdeen, UK ABSTRACT We have undertaken a study of high work function (φ) surfaces as part of an ongoing project searching for efficient target materials for use in Hyperthermal Surface Ionisation (HSI), a new mass spectroscopy ionisation technique. HSI relies on high work function surfaces for the production of positive ions. Polycrystalline metals as Re, W, Mo and Pt are particularly interesting materials in this respect as oxidation substantially increases their φ. We present and discuss the following experimental evidence: a) the magnitude and sign of φ changes in terms of adsorbate induced dipoles, b) the effect of molecular hydrogen exposure on the clean surface, and c) the effect of subsequent oxygen exposure. Using a novel UHV Scanning Kelvin Probe we have followed the oxidation kinetics of polycrystalline metals at different temperatures and examined the effects of oxidation, flash annealing and sputter-anneal cleaning cycles via high-resolution φ topographies. Our results indicate in particular Re as a suitable HSI target material exhibiting a φ increase of 1050 meV at 300 K increasing to 2050 meV at 900 K. Sputter-cleaned surfaces exhibit a dramatic change in the second oxidation phase. We have also examined φ changes associated with N2O and CO2 on Tungsten and Molybdenum. We observe that atomic oxygen gives similar results to O2 but has a much lower initial sticking coefficient. We report that CO2 actually lowers the φ for substrate temperatures under 650 K, the peak work function changes occurs at 850 K and is approximately 1/3 the height of the O2 or O peak. INTRODUCTION The objective of this study was to examine high work function (φ) materials, including polycrystalline Rhenium (Re), as target surfaces for Hyperthermal Surface Ionisation (HSI) [1]. In this application the sample gas is ionized by supersonic collision with a suitable target surface and the resulting ion-beam is then mass analysed by a time-of-flight (TOF) spectrometer. As this technique does not use electron impact (EI) filaments the amount of cracking products is low [2]. Analytical merits of this new HSI-TOF technique include high sensitivity, controllable selectivity, informative mass spectra and an atmospheric pressure inlet. The newly developed Ultra-High-Vacuum (UHV) compatible Scanning Kelvin Probe (SKP) offers unparalleled sensitivity to changes in surface dipole due to adsorption, contamination and structure. The Kelvin method [3,4] produces the average φ of the sample under the tip and not that biased to low φ patches as is the case for photo- and thermionic-emission. The SKP has a 1-2 meV work function resolution and employs a sample-to-tip tracking algorithm [5] without which φ measurements can be difficult to interpret [6,7]. This technique has previously been applied, together with STM, AES, LEED, TPD and EELS for research into semico
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