[3dem] [Microscopy] mean inner potential

Fri Aug 16 05:12:16 PDT 2019

Hi again.

Some more little things I've come up with.

It seems that a MIP in the order of about 10 V can be explained without any surface effects or adsorbed molecules or ions.

That means a (non-existent) ideal crystal, where the surface unit cells are exactly the same as the unit cells inside the solid would have roughly this MIP. This is supported by estimations based on band theory and one can also get similar results using Poissons equation on a model charge density.

This is clearly in line with what Bill Tivol pointed out.

Obviously surface effects and adsorbed species will add to this value.

The thickness-independent contribution to the phase shift shown in the Wanner paper from 2006 is still a mystery to me, but there's one thing I'd like to point out: The smallest thickness they measured is only 1 nm, which is in the order of 5 atom layers I would say. In a C-film that thin I wouldn't even look for anything related to the bulk of the solid since a film that thin essentially consists of two surfaces and not much in between.

All the best,

Philip

________________________________
From: 3dem <3dem-bounces at ncmir.ucsd.edu> on behalf of Philip Köck <philip.koeck at ki.se>
Sent: Wednesday, 3 July 2019 10:57:31
To: microscopy at microscopy.com <microscopy at microscopy.com>; 3dem at ncmir.ucsd.edu <3dem at ncmir.ucsd.edu>
Subject: Re: [3dem] [Microscopy] mean inner potential

Hi again.

Thanks for all the input.

I've been looking at the papers suggested by Robert Keyse and Benjamin Himes.
What Spence seems to say is that the contribution to the MIP which is only due to "electrons spilling out a bit further at the surface of a solid" is only around 0.5 V.
The main contribution to the MIP is supposed to be due to adsorbed atoms and molecules.

Bill Tivol quite rightly said that then the MIP would not be a property of a specific material, but would depend a lot on what else is around in the vacuum or what the surface came in contact with.
Are there any experiments on that?

I've also heard the opinion that surface adsorbates contribute only little to the MIP and that it's mainly due to the solid itself.

Then there's still the question of a thickness independent contribution to the phase shift. A tripple surface charge layer like  - + -  with balanced charges could explain that, but where would that come from?

All the best,

Philip

From: Robert Keyse <rok210 at lehigh.edu>
 Sent: Wednesday, 19 June 2019 17:02:11
 To: Philip Köck; microscopy at microscopy.com
 Subject: Re: [Microscopy] mean inner potential

Hi Philip,

the Saldin & Spence paper: Ultramicroscopy 55 (1994) 397-406  suggests the surface atoms are less confined and spread their valence electrons out into the vacuum creating a surface dipole that differs from inside the material.
.  Electrons speed up slightly just as they enter a sample (increase in wave-vector) due to this inner potential.

Reference [12] O'Keefe & Spence: Acta Cryst. (1994) A50, 33-45 (J C H Spence would be a good reference to follow generally)
O'Keefe and Spence estimate the surface potential is weak and only about 0.5 volts (page 39: a back of an envelope estimate).

On Tue, Jun 18, 2019 at 3:40 AM <philip.koeck at ki.se> wrote:

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 Thanks for the reference.

 I can't make sense of a thickness-independent contribution to the phase shift either. The way I see it even a surface layer of dipoles would lead to a constant MIP and a phase shift proportional to the thickness.

 One can think of a simple model: A slab of completely neutral material (made of neutrons) covered in a layer of positive charge and outside that a layer of negative charge that balances the positive charge. The potential inside this slab will be constant and  independent of the thickness of the slab.

 I wonder if we can get a comment from someone who knows more.

 All the best,

 Philip

 X-from: 3dem <3dem-bounces at ncmir.ucsd.edu> on behalf of Benjamin Himes <himes.benjamin at gmail.com>
 Sent: Monday, 17 June 2019 20:45:48
 To:  3dem at ncmir.ucsd.edu
 Subject: [3dem] (mean Inner potential) Re: 3dem Digest, Vol 142, Issue 38

 Hi Philip,

 The mean inner potential (MIP) refers to a total "interaction" potential that is considered a material property. It consists of all the sources contributing to the potential well seen by an imaging electron, including those you suggest (nuclear and electronic  contributions.)

 Yes, physical changes to the surface via adsorbed matter will directly affect the MIP. I believe the working hypothesis for the source of the "Volta" potential is through heat/exposure related modification of surface adsorbates.

 It is also interesting to note that in addition to the electronic character of the object, the surface contributions of adsorbates and heating, there is another thickness independent phase shift (at least for carbon) the source of which I am not clear on. Happy  to hear an explanation from anyone in the know : )

 Please have a look at this paper where all of the non-Volta contributions are discussed and also measured.

 "Electron holography of thin amorphous carbon films: Measurement of the mean inner potential and a thickness-independent phase shift"

 doi: j.ultramic.2005.10.004

 HTH

 Ben

 ------------------------
 Benjamin Himes

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