Effective work functions for ionic and electronic emissions from mono- and polycrystalline surfaces

Abstract

The effective work functions ( ϕ +, ϕ e and ϕ −) for positive-ionic, electronic, and negative-ionic emissions from mono- and polycrystalline surfaces are surveyed comprehensively and also investigated critically for the main purposes of (1) evaluating the most probable values of ϕ +, ϕ e and ϕ − for a variety of surface species, (2) explicating both thermionic contrasts (Δ ϕ ∗ ≡ ϕ + − ϕ e and Δ ϕ ∗∗ ≡ ϕ − − ϕ e) and their dependence on experimental conditions, and (3) demonstrating the necessity of employing ϕ + (not ϕ e) for quantitative analysis of those data on positive ion emission from polycrystalline surfaces. Careful examination of both theoretical results and experimental data on the work functions yield several conclusions. By both theory and experiment, clean monocrystalline surfaces are verified to have Δ ϕ ∗ = 0.0 eV within an error of ±0.05 eV. Next, as the density of local surface irregularities increases, the homogeneity in the work function over the whole surface area decreases and, hence, Δ ϕ ∗ increases. Also, the most probable values of ϕ + and ϕ e are recommended for many mono- and polycrystalline surfaces, mostly (∼70%) with a standard deviation of ±0.02–0.08 eV. Compared with the probable or typical values of ϕ e accepted in influential handbooks, the most probable values of ϕ e recommended here are typically (∼70%) equal to each other within a narrow gap of less than ∼0.1 eV, but some (∼20%) are different by ∼0.2 eV or more (up to ∼1 eV). Furthermore, polycrystalline surfaces of Nb, Mo, Ta, W, Re, Ir, Pt, etc. hold Δ ϕ ∗ ≈ 0.3–0.8 eV since each surface has a mean value that is different between ϕ + and ϕ e. Also, at the degree of monocrystallization ( δ m) below ∼50%, the theoretical value of Δ ϕ ∗ depends little on δ m and agrees well with experimental data on each polycrystalline surface. As δ m increases beyond ∼80%, Δ ϕ ∗ decreases rapidly to 0, showing again a good agreement between theory and experiment. In particular, those surfaces of δ m > 97% generally have Δ ϕ ∗ ≈ 0 within the uncertainty of about ±0.05 eV, which is apparently equivalent to the usually called “monocrystalline surfaces ( δ m = 100%)”. Additionally, even when both ϕ + and ϕ e are changed by up to ∼1 eV by gas adsorption, Δ ϕ ∗ itself remains little changed and, thus, the so-called “work function ( ϕ)” recommended with polycrystalline surfaces in handbooks should not be cited as ϕ + since ϕ usually coincides with ϕ e except where otherwise stated. In the case of polycrystalline surfaces, ϕ + instead of ϕ e should always be adopted to analyze accurately data on any positive ion emission, irrespective of its process or mechanism. Also, those metals covered with a two-dimensional graphitic film usually have ϕ + ≈ ϕ e ≈ 4.5 eV, which corresponds to monocrystal graphite. Finally, for any species of mono- and polycrystalline surfaces, both theory and experiment verify ϕ − = ϕ e and hence, Δ ϕ ∗∗ = 0. The features of dissociative self-surface ionization of heated ionic crystals are outlined together with typical data on ϕ +, ϕ − and ϕ e, which originate from the thermionic properties of the crystal itself. A brief description is given to typical methods and techniques to prepare clean and/or monocrystalline surfaces, to determine local work functions of real monocrystalline surfaces, and also to form graphitic carbon films on various surfaces. In 12 tables and 29 figures based on 1350 references published to date (mainly ∼1970–2006), we show data on each work function of mono- and polycrystalline surfaces and their temperature coefficient, as well as their dependence upon experimental conditions. Also, we illustrate a comparison of each work function between theory and experiment and the most probable values of ϕ + and ϕ e(= ϕ −), which are generally citable as reliable references. A comparison between the most probable values ( ϕ e) recommended here and the probable or typical ones ( ϕ) accepted elsewhere are shown, along with working conditions for keeping ϕ + as high as possible for promoting positive ionization efficiency. Also, we present relationships between ϕ + and ionic desorption energies, and typical data on negative ion emission due to thermal stimulation. Thus, we provide an extensive and up-to-date database of the effective work functions of both mono- and polycrystalline surfaces, and also summarize their peculiarities governing the emissions of positive and negative ions and electrons.