Work functions of ceramics/insulators

Question

Is anyone aware of or know of a good source or means of estimating the work function of a ceramic material? Typically, work functions are given for pure elemental metals, rather than for compounds, such as oxides or nitrides.

The definition of work function that I am using is: the minimum thermodynamic work (i.e. energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface.

This could be viewed in the context of the photoelectric effect, the minimum energy photon required, incident to a surface, to liberate an electron from the material with zero kinetic energy.

One example that I have found successfully, is for lanthanum hexaboride $(\mathrm{LaB_6})$, often used as a cathode because of its low work function (2.5 eV) and high melting point. But aside from this, I haven't been able to find work functions of other materials. Perhaps because $\mathrm{LaB_6}$ is ubiquitous in charged particle optics as a cathode that its work function is readily available.

For instance, I'd be interesting in knowing the work function for silica $(\mathrm{SiO_2})$, alumina $(\mathrm{Al_2O_3})$, magnesia $(\mathrm{MgO})$, and potassium oxide $(\mathrm{K_2O})$, among others. However, in searching, I typically find their secondary electron emission coefficient instead, when checking sources such as CRC Handbook of Chemistry and Physics, or other fundamental texts and databases for material properties.

Where can reliable sources for work functions of compounds (primarily ceramics) be found?

Answer 1

The definition of work function that I am using is: the minimum thermodynamic work (i.e. energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface.

This could be viewed in the context of the photoelectric effect, the minimum energy photon required, incident to a surface, to liberate an electron from the material with zero kinetic energy.

This is the (first) ionization energy of your material. Try looking for the answer under that keyword.

Note that the concept of a work function is most commonly used where no charged material is left behind. This will most likely not describe the behavior of your isolating ceramics of interest. Hence the details, including the fact that at seperation the electron will need lots of kinetic energy to be counted as unbound (able to escape electrostatic attraction), will differ from what most people will associate with the term work function.

There is an argument to be made that "work function" is a historical term, used before engineers had a good understanding of the quantumchemistry of metals. Hence you will rarely find it outside of contexts where this concept proved good enough, such as cathodes in vacuum tubes, and where history tends to be retold, such as high school physics lessons. It's not well-suited for generalization because it ignores the internal structure of the material, misleading you to expect a uniform reservoir of quasi-free electrons (the electron gas that exists in the metallic conduction band) essentially only differing from free electrons outside the metal by an energy difference. That is simply not the case in materials that do not closely resemble metallic conductors.

Answer 2

I will comment that the initial question is a good one; the single answer is very, very incorrect. Work function is an exceedingly well defined term in electronic structure/solid state physics and can be found in all undergraduate and graduate tezt books.

However, it is also something which is quite hard to measure for insulators due as charging. Experimental work functions are also strongly changed by surface terminations, so can in fact vary by 1eV. (And, to make things worse, there can also be band bending.) Probably the closest one can get for large band-gap insulators such as Al2O3 and SiO2 is numbers that compare them to metals or (for SiO2) silicon. Even then there is the issue of the role of the interface (band bending, dipoles...)