A Novel Approach for True Work Function Determination of Electron-Emissive Materials by Combined Kelvin Probe and Photoe
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A Novel Approach for True Work Function Determination of Electron-Emissive Materials by Combined Kelvin Probe and Photoelectric Effect Measurements Bert Lägel, Iain D. Baikie, Konrad Dirscherl and Uwe Petermann, Department of Applied Physics, The Robert Gordon University, Aberdeen, UK. ABSTRACT For the development of new electron-emissive materials knowledge of the work function (φ) and changes in φ is of particular interest. Among the various methods, the ultra-high vacuum (UHV) compatible scanning Kelvin Probe has been proven to be a superior technique to measure work function changes due to e.g. UHV cleaning processes, chemical contamination, thermal processing etc. with high accuracy ( φS are collected by the Kelvin Probe tip and measured as a function of the applied backing potential Vb (see figure 2). It can be seen that, at the onset of the photocurrent collected at the Kelvin Probe tip, (fig. 2b), the work function of the tip equals the photon energy Eph plus a required bias potential supplied by Vb, i.e.,
φ tip = hν + Vb (onset).
(2)
Electronic Energy
(c)
(b)
(a) hν eVCPD
φtip εtip
hν
hν
EVac
E≠0
φS εS
φtip
E≠0
εtip
φS εS eVb
Vb = Vb(onset)
φtip
E=0
φS εS
εtip
Vb = -VCPD
Figure 2. Energy level diagrams of the Kelvin Probe in photocurrent measurement mode with φtip > hν > φS: (a) no photoelectrons are collected at the tip, (b), the onset of the photocurrent is measured where φtip = hν + Vb(onset) and in (c) the saturation current where Vb = −VCPD . With increasing bias potential, analogous to the Fowler theory of photoemisson from metals near threshold [18], the photocurrent increases with the square of Vb: Iph ∝ Vb2. When Vb equals -VCPD (fig. 2c), electrons emitted from energy states below the Fermi level with virtually zero velocity are only just able to reach the collector and the photocurrent will saturate. Experimental Setup The experiments were carried out in a UHV chamber as shown in fig. 3 with a base pressure of
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