Measurement of the local work function of dispenser cathodes using an SPM technique

Abstract

An STM technique has been developed which makes possible direct quantitative measurements of the magnitude and distribution of the electronic property of surfaces known as the local work function (barrier height), φ. The technique was used, first, to measure the barrier height of several well defined surfaces for which values of the macroscopic work function, Φ, are available in the literature, viz. graphite, platinum and gold. The variation of measured barrier height (≡mean local work function φ ̄ of the tip and the sample) with tip-sample distance s was found to have a similar shape for each of these surfaces. Characteristic features of these φ ̄ vs. s dependencies were found to correlate well with the respective mean Φ values, Φ ̄ , derived from the published Φ values for the tip and the surface in question. The technique was used then to investigate an unprocessed CD-type and a processed B-type dispenser cathode. The results discriminated strongly between the two cathodes. In particular, the barrier height image of the B-type cathode was found to be rich in detail compared with that from the CD-type, evidence that in the former there are several different surface states discernible on the basis of their different electron emissive properties. The results demonstrate that the highly spatially resolving capabilities of SPM techniques are commensurate with length scales associated with surface variations of the local work function of cathode materials. The technique is a basis for investigations of cathode science and technology.