Some years ago we tried to use a 100Å thin Pd film with a Xe flashlamp. However, we decided to use Gold, instead, because its no oxidation properties. Otherwise, You could deposit a mono- or by-layer of Cs to reduce the Au work function.

At the very end, Au is the best compromise.

(see Rev. Sci. Instrum. vol. 57, p. 2234 (1986))

Hi Laura, the two problems are very much correlated, in that the larger the work-function, the less susceptible it is to oxidise and to do so irreversibly (rather than in a loosely bonded catalytic way.)

I understand actually trying to correlate a workfunction to a material is an extremely challenging experiment as it is so very sensitive to surface contamination, crystal face, texturing, preparation etc.

For reference the: Handbook of Chemistry and Physics book has a list of work functions in it.

You can tunnel electrons through the oxide coating of the more reactive metals, such as aluminium, [carbon?] zinc or indeed Magnesium (3.3ev and fine in air with oxide coating). But would need to look up the "native oxide thickness" and the energy needed to tunnel through it. I believe there is some suggestion that even silver does have a very thin oxide layer on it.

Alternatively to metals you can look at some of the highly (n-doped) doped semiconductors. For example some of the transparent conductors such a zinc aluminium oxide have workfunctions around 4 ev.

For semiconductors the "electron affinity" is the distance from the conduction band to the vacuum level, if the semiconductor is heavily n-doped then it has electrons free to travel in the conduction bad, so these are then sat at this energy, and the material behaves like a metal with a workfunction equal to that of the electron affinity. I suspect that these then however behave similarly to the metals and gain an "oxide" layer on the surface.

Finally you can make the electron emission more favourable by enhancing the local electric field by texturing the surface and ultimately having sharp points on the surface, to promote "field emission". Ultimately using nanowires or particles.

You can of course further "cheat" and apply a DC bias to the material to reduce the "apparent" work-function.

In the end however you will probably not do better for photoelectric emission than a material with a workfunction of about 5 ev in the presence of air (the energy needed to break the O=O bond), and experiments with larger workfunction materials will probably be more reproducible.

Hope that's some help!

Phil

Hi Phil,

many thanks for your reply.

1) You talk about "tunnelling electrons through the oxide coating". I read that Al2O3 forms mainly in the first weeks of exposure to air and that the growth is then logarithmic. According to what I have read, after one week of exposure to air, the oxide film is of ~20 A, reaching 50 A after one month. Any idea on the energy needed for tunnelling to occur given the thickness of the oxide layer?

2) You also mention nanowires or particles to enhance the electron emission, could you elaborate this further?

3) As for the semiconductors, from what you say I gather this has very little chance to work after air exposure, correct?

4) If you were to bet on any other metal rather than gold, would you then suggest silver?

I also did not mention in my question that such photocathode would have to be eventually used in liquid argon. To summarise, not only it would need to be initially exposed to air, but to warm gas argon, then cold vapours and finally liquid argon.

Looking forward to your reply,

Best,

Laura

Hi Armando,

many thanks for your reply and the reference to the paper.

Are you suggesting to deposit a mono-layer of Cs on top of the Au? If so, how thick? I gather photoelectron emission involves the first 20 nm of the surface, so I guess the Cs film should be thinner than that, correct?

Last but not least, do you think that such arrangement should be fine when the photocathode is exposed to air?

Looking forward to your reply,

Regards,

Laura

Hi Laura,

1 I don't have any figures to be exact but I would estimate it to be of the order of a couple of eV.

2 I'm thinking along the lines that sharp points produce very high local electic field strength so will be the site that preferentially emits the electrons... lightning conductors effectively!

This paper: Article Photosensitive field emission from silicon point arrays

3 I think it is very dependent on the semiconductor. Along these lines though, things which are used as electron emission surfaces, will also have been engineered to have a low work function so materials used in the old Thermionic valve: https://en.wikipedia.org/wiki/Hot_cathode

See section "types"

This can be used for something like this: Article Work function reduction by BaO: Growth of crystalline barium...

So what to use... i would actually er away from silver, as it can tarnish very easily especially for example if there is the slightest hint of sulphorous contamination. So for reproducibility i would look for a metal that keeps its lustre well (typical catalytic metals), so im thinking chrome may be a good cheap choice perhaps?! (or platinum.. rhodium... if your feeling wealthy!) But that's just a feeling!!

(Incidentally a useful link for energy conversion nm wavelength to ev for example: http://www.ilpi.com/msds/ref/energyunits.html )

Regarding the treatment.. for the air.. be aware of contamination as mentioned with the silver.. clean nitrogen would be better if possible, as ambient humidity, and traces of other gasses could really change the surface and vary during the experiment also. There should be no problem with argon, but with the cold vapours, surface water absorption will really mess with your results, as it tries to condense on the surface. Though with a good dose of UV you should be able to remove this layer, so you will probably find the results are better after a good blasting with the lamp once it has had time to settle.

We thought about trying to measure the workfunctions of some materials here, but a colleague advised us that when he had once asked an expert in the field he was told "What value would you like us to obtain for you.." -as you are so sensitive to the surface chemistry of the material. Although in your own case the high energy UV will certainly help to clean up the surface a little.

Regards,

Phil