#1 Convergence of XPS core levels by Terry 11.10.2021 08:12

Hi folks

Somewhat inspired by the previous thread on simulating XPS, I am calculating the O 1s core levels for surface species of a metal oxide.

Mostly it's straight-forward, and the results converge quite quickly with respect to the thickness of the slab. However, when I decorate the surface oxides with hydrogen atoms, the convergence slows dramatically. Whenever I make a thicker slab and relax, the O 1s core levels of lattice oxygen in the centre of the slab increase. That makes it impossible to claim the surface oxygen levels are well-defined relative to the bulk lattice oxygen that I'd like to use as reference.

If I have bare oxide surfaces or surfaces decorated with water this does not happen.

I am using a consistent set of MT parameters and basis cutoffs somewhat increased from the default. What am I missing here?

Ciao
Terry

#2 RE: Convergence of XPS core levels by Gregor 11.10.2021 16:43

Could you provide an input file to get a better impression? Is this actually a film calculation or a periodic slab calculation with a large space in between the slabs? Is your film/slab setup symmetric or asymmetric? How thick is the slab, what kind of chemical element is involved. Depending on the quantity and the involved elements surface properties can either be converged with just a few layers or you may need several tens of atom layers.

In periodic slab calculations you also have the electrostatic interaction between the slabs that may require a very large vacuum region.

#3 RE: Convergence of XPS core levels by Terry 12.10.2021 05:55

Hi Gregor

These are 2D film calculations on ZnO.

I've attached two graphs and an input file. The graphs show the core levels for each O atom as a function of the z position. One is for ZnO slabs with attached H2O (some with a single surface decorated, some with both). The other is the problematic case with H atoms attached to surface oxygens. For the H2O case the lattice oxygen levels don't change much with position or thickness. For the surface OH they do. (I have many more of the latter slabs... I've just picked three to illustrate.) The input file is for one of the surface OH slabs, with the geometry quite close to being converged.

Getting the density to converge is quite tricky for the thicker slabs. I need tens or hundreds of iterations with small alphas and straight mixing to get the Anderson mixing to be stable.

Ciao
Terry

#4 RE: Convergence of XPS core levels by Gregor 12.10.2021 11:29

Dear Terry,

Could you please also paste the valenceDensity and coreStates sections for the last iteration of the largest setup from the out.xml file? It looks like you accumulate charge at the surfaces. Another thing that may become visible in this data is whether there are problems due to the very small MT radii you have to use due to the small bonding lengths. It may be that your observation has a physical origin and is not related to a problem of the calculation.

#5 RE: Convergence of XPS core levels by Terry 12.10.2021 12:32

Zitat von Gregor im Beitrag #4
It may be that your observation has a physical origin and is not related to a problem of the calculation.

I was afraid of that. As requested, the relevant chunk of out.xml is attached.

Looking there, I also note that the band gap is very small, which I guess is consistent with unhelpful things happening on the surface.

#6 RE: Convergence of XPS core levels by Gregor 12.10.2021 15:08

I think the bulk system of the surface you want to describe is an insulator. On the other hand the hydrogen atom only has a single s electron. The hydrogen layer on top of the bulk therefore may be similar to the situation in which you have an interface between an insulator and a metal. In such situations the metal can affect the electronic structure of the insulator. An example for this is a Schottky barrier. I think such effects can be rather extended. It may help a little to perform a structural optimization of the z positions of the different atoms, but this will not fundamentally cure the problem. I actually don't know how to deal with such situations in DFT calculations. Maybe there are tricks or there are extrapolation schemes. I think it may be a good idea to do some research on papers investigating metal-insulator or metal-semiconductor interfaces with DFT. Maybe one can find a good idea there.

EDIT: I just saw that you wrote that the geometry is already optimized. In this case the structural relaxation idea is nothing new.

#7 RE: Convergence of XPS core levels by Gregor 12.10.2021 15:18

With respect to the band gap I could imagine that a finite band gap actually is something artificial in the output here. Maybe with a finer k mesh it will just disappear. As mentioned I actually expect to see a conductive layer on the surface.

#8 RE: Convergence of XPS core levels by Gregor 12.10.2021 18:44

Just out of curiosity: Is this hydrogen termination actually observed for such surfaces in experiment? For example this could be the case if there are dangling bonds.

#9 RE: Convergence of XPS core levels by Terry 13.10.2021 00:48

As far as I can see, most of the info about what's happening on the ZnO surface comes from XPS

The added H is me trying to work out sensible terminations.

#10 RE: Convergence of XPS core levels by Terry 13.10.2021 03:16

I would have expected that the H must be covalently bound to O2-, putting (OH)- into the surface. Viewing ZnO as purely ionic it's not immediately apparent to me how that would make a conductive surface.

Nonetheless, I guess that is what I was missing was that even by my own argument the surface should be net charged.

#11 RE: Convergence of XPS core levels by Gregor 13.10.2021 13:38

Maybe you are right, I only had a short look at your input. Only a band structure calculation could tell us whether it is an insulator or a metal. But in the end, as you just mentioned, this is not really relevant. The charge at the surface stays and leads to a very slow convergence with respect to the number of layers.

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