Physical foundations of the oxide cathodes § Stanislaw Halas a, * , Tomasz Durakiewicz b a Institute of Physics, M. Curie-Sklodowska University, 20-031 Lublin, Poland b Los Alamos National Laboratory, MST-10 Group, Los Alamos, NM 87544, USA Available online 21 June 2006 Abstract A novel explanation of the low values of work function in case of activated (partly deoxidized) polycrystalline oxides of alkali and alkaline earth metals is offered. Use of the metallic plasma model to the conducting oxides leads to the following values (in eV): 1.00, 1.67, 1.50, 1.44, 1.46 and 1.59 for activated Cs 2 O, CaO, SrO, BaO, Y 2 O 3 and La 2 O 3 , respectively. The main reason of low work function of the oxide cathodes is very low density of free electrons in the emitting surface layer. # 2006 Elsevier B.V. All rights reserved. Keywords: Work function; Metal oxides; Oxide cathodes; Cs 2 O; CaO; SrO; BaO; Y 2 O 3 ; La 2 O 3 ; LaB 6 ; Thermionic emission 1. Introduction Work function (WF) has been defined by Einstein [1] in 1905 as the minimum quantity of energy sufficient to remove an electron from a surface to the field-free vacuum. This fundamental property of solid surfaces was somewhat earlier defined by Richardson [2] as the height of an electric barrier, which keeps ‘‘negative particles’’ (electrons) within the metal volume. That barrier can be passed by the highest energy electrons at high temperatures, hence the ‘‘thermionic emission’’ term was coined to describe the phenomenon. Thermoionic emission may be also considered quantitatively as evaporation of atoms from a monoatomic solid [3]. I. Langmuir used this idea in his Nobel Lecture on surface chemistry (delivered December 14, 1932) and therein described WF as the heat of evaporation of electrons. In 1904 Wehnelt [4] has shown that not only pure metals, but also a series of metal oxides readily emit electrons in vacuum at high temperatures. In 1908 his PhD student, Jentzsch [5] published experimental WF values for 17 oxides. Results fell within the range of 2.0 and 4.4 eV, with 2.0 eV being a surprisingly low value. Though the oxide cathodes were widely applied in vacuum tubes over the entire 20th century, no satisfactory explanation of their low value of work function was reported by far. The lowering of WF due to presence of alkaline earth oxides has a great significance nowadays in production of negative ions in thermal ionization sources in mass spectro- metry [6], in catalysis and microelectronics [7] and generally in production of high intensity electron beams. In this short note the mechanism of lowering the WF in the case of activated polycrystalline oxides of alkali, alkaline earth and rare earth metals is explained. Presented considerations are also relevant to clean surfaces of the metals when they are subjected to doses of oxygen. The metallic plasma model, which was already exploited in WF calculation for pure and hydrogenated metals [8,9], is allied here. 2. WF of metal oxides—an overview Ever since the early experimental work by Langmuir [10], as well as from more recently published work, it is well known that metals subjected to doses of oxygen either increase or decrease their WF. W, Re, Ta and other transition metals belong to the first class, and their oxidized surfaces are widely used in thermal emission of positive ions in mass spectrometry. In the case of metals like Cs, Ca, Sr, Ba, Y, La and lanthanides, their oxidized surfaces have significantly lower WF (by 1 eVor even more) than respective pure metals. These oxides are widely used in electronic vacuum tubes as ‘‘composite emitters’’, being extremely useful nowadays in mass spectrometry applications as high efficiency www.elsevier.com/locate/apsusc Applied Surface Science 252 (2006) 6119–6121 § This paper is published as part of the proceedings of the International Workshop on Surface Physics 2005. The rest of the proceedings can be found in Surface Science Volume 600 issue 8. * Corresponding author. Tel.: +48 81 5376275; fax: +48 81 537 6191. E-mail address: halas@tytan.umcs.lublin.pl (S. Halas). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.05.012