[3dem] (mean Inner potential) Re: 3dem Digest, Vol 142, Issue 38

Wed Jun 26 10:58:41 PDT 2019

The statement in Wanner et al. 2006 is very strange in my opinion.

Shouldn't "perpendicular" be replaced by "parallel"?

I don't see how the potential can be proportional to the distance between the dipole layers. If we think of a slab of neutral matter (as a thought experiment), which is covered first by a layer of positive charges and then an equal amount of negative charges on top of that, the following should happen: The negative charge curves the potential upwards and the potential increases, then the positive charge curves the potential down again by an equal amount. The total effect is that the potential is constant inside the slab and independent of the slab's thickness and therefore the phase shift is proportional to the thickness.
Maybe you meant that the potential is roughly proportional to the distance between the negative and positive layer (or more generally the dipole moment).

I can imagine a constant phase shift coming from multiple layers of charge, at least 3, that are balanced. A bit like - + + -, for example. That could lead to a potential that's confined to the surface and a phase shift contribution that's independent of the thickness of the solid.

All the best,

Philip

________________________________
From: 3dem <3dem-bounces at ncmir.ucsd.edu> on behalf of Vladan Lucic <vladan at biochem.mpg.de>
Sent: Wednesday, 26 June 2019 17:57:58
To: 3dem at ncmir.ucsd.edu
Subject: Re: [3dem] (mean Inner potential) Re: 3dem Digest, Vol 142, Issue 38

I agree that classically, the potential generated by two surface layers of dipoles oriented perpendicular to the surface is in the first approximation directly proportional to the distance between the layers, that is to the thickness, which argues against the thickness-independent phase shift. Perhaps that is the reason for Wanner et al 2006 (the paper recommended by Ben) to propose that dipoles perpendicular to the beam should be considered:

"Although only dipole moments with a component perpendicular to the electron beam would contribute to a phase shift the irregular and corrugated a-C surface will provide attachment sites for H2O molecules to fulfill this condition."

Interestingly, Hettler et al 2018, Charging of carbon thin films in scanning and phase-plate transmission electron microscopy ( https://doi.org/10.1016/j.ultramic.2017.09.009 ) argue that the high surface roughness of the Volta phase plate is needed to generate the phase shift (at a high temperature). Furthermore, in their picture, surface dipoles are absent from the region of the direct beam, which causes lateral (perpendicular to the beam) redistribution of electrons. Both of these could provide the "perpendicular dipoles" proposed by Wanner et al 2006.

Vladan

On 6/18/19 9:26 AM, Philip Köck wrote:

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

________________________________
From: 3dem <3dem-bounces at ncmir.ucsd.edu><mailto:3dem-bounces at ncmir.ucsd.edu> on behalf of Benjamin Himes <himes.benjamin at gmail.com><mailto:himes.benjamin at gmail.com>
Sent: Monday, 17 June 2019 20:45:48
To: 3dem at ncmir.ucsd.edu<mailto: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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cryoSTAC development @ emClarity<https://mail.hhmi.org/owa/redir.aspx?C=8yDzXj54yTidMevTB7q5m3liEVwqAZ9LxuXQ4iYOVvtl2i2eOXXVCA..&URL=https%3a%2f%2fgithub.com%2fbHimes%2femClarity%2fwiki>

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   1. mean inner potential of a solid (Philip K?ck)
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Date: Mon, 17 Jun 2019 08:27:35 +0000
From: Philip K?ck <philip.koeck at ki.se<mailto:philip.koeck at ki.se>>
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Subject: [3dem] mean inner potential of a solid
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 Hi all.

I've been wondering what the mean inner electrostatic potential of a solid (for example the 10 V of carbon) is actually due to.
Is it purely caused by the distribution of nuclei and electrons in the solid itself or could there be a contribution from adsorbed surface charges?

All the best,

Philip

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