# Computational Can you suggest books that will help understand DOS graphs?

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1. Jul 4, 2015

### physicsjn

Can you suggest books or any resource that will help understand DOS graph discussions in journal articles? I am reading DFT results. I try very hard to understand how their discussion is related to DOS graph presented, but I can't no matter how I reread it over and over again.

Basically, these are the things I want to know initially:
(1) How they identify that this certain peak is 4s, 2p, 3d, etc.
(2) How they know that this 4s peak iof one atom is strongly hybridized with 3d peak of another atom.
(3) What is the significance if the peak is above or below the Fermi level.
(4) What is the significance if the peak shifted above or below the dirac point.
(5) What is the significance of the fermi level? or the dirac point?

Thank you very much.

2. Jul 4, 2015

3. Jul 5, 2015

### physicsjn

Thank you Dr. Courtney for your reply. I need enough DOS knowledge to understand several DFT papers. I will use a paper by Chan et al. as an example to make my questions more concrete. Here is the link to the paper: https://www.fisica.uniroma2.it/~marsili/Grafene/Molecules_adsorbed/chan08_metal.pdf

You don't need to answer all these questions if you don't have time. All I need is a resource that would help me answer these questions. Some kind of easier to understand resource on DOS and PDOS, Dirac point, and Fermi level.

1. In the paper, they said, "Figure 4 shows the same K-graphene DOS near the EF along with the projection of the DOS onto graphene and the K s state. The spin-up 4s peak lies close to the Fermi level and is partially occupied, while the spin-down 4s peak lies approximately 0.5 eV above EF and is unoccupied." I pasted the picture below for reference.

Question:
-How did they know that this blue peak is the K 4s peak? I am thinking it is because the electronic configuration of potassium is [Ar] 4s1, but I am not sure.
2. It was also said in the paper that "There is no evidence for hybridization between the K 4s peaks and
the graphene states; the PDOS of the graphene shows no peaks near the K 4s levels. Furthermore, the broadening of the K 4s peaks due to interaction with the graphene is small.
Question:
-How do we say that there is hybridization between the K states and graphene states? My assumption is that there should be a green peak at same energy level (same x coordinate) as blue peak. Again, I am not sure about this.
-How do we qualify that the broadening of the K 4s peaks is small? (i.e. What does "small" mean?)

-What is the significance is there is broadening of a peak in DOS? What does it physically mean?

3. The paper said, "For K on graphene, the Fermi level is shifted higher in energy relative to ED, reflecting a greater occupation of graphene states."

Question:
-What does occupation of graphene states mean? Does it mean that there are more electrons in the graphene orbitals than in the K orbitals?
-What does it physically mean or what is the significance if the Fermi level is shifted higher in energy relative to the Dirac point ED?

4. The paper said, "Also, in an isolated atom, the spin-up 4s peak is fully occupied, while in the adatom-graphene system the spin-up 4s peak is only partially occupied. These observations suggest that the bonding is predominantly ionic and that close to one electron of charge (e) is transferred from the 4s state of the adatom to the graphene states."
Question:
-Does it mean that the peak crosses the Fermi level, it is partially occupied?
-How did the authors conclude that bonding is predominantly ionic? And how did they calculate the amount of charge (which is close to 1 e) transferred just from the DOS plots?

5. The paper said, "For Li, the 2s spin-up and spin-down states are degenerate and lie about 0.9 eV above EF, suggesting that more charge is transferred to graphene by Li than by Na or K."
Question:
-I am just guessing here. But I think that the higher the adatom peak is above the Fermi level, the more charge it contributed to the graphene. But why? What is the connection of the distance of the adatom DOS peak from the Fermi level to its charge contribution to the main substrate?

6. The paper said, "In the case of Ca, the isolated atom has a filled 4s shell with two electrons. However, when Ca is adsorbed onto the graphene H site, its spin-down 4s peak lies above EF and is unoccupied, resulting in a magnetic moment of about 1 μB/adatom."
Question:
-So, in relation with the previous statement above. If the peak is below EF, it is occupied. If it crosses EF, it is partially occupied. And if it above EF, it is unoccupied. Am I correct? I'm trying to connect this with my previous knowledge in bandstructure.

7. The paper said, "The 3s state of Al appears to split and hybridize with the graphene states at −8.6 and −3.3 eV relative to EF."

Question:
-Is the peak of graphene that is directly above the Al s peaks the reason why the author inferred that the there is hybridization?

8. The paper said, "The Al 3p peak lies 1.3 eV above EF and is noticeably broadened due to the Al-graphene interaction."
Question:
-Related to my question in item #2. How do we say that the peak is broadened? Is there a numeric criteria?

9. The paper said, "For Al, Ga, and In, the valence p peak remains above EF, so that in all cases, close to one electron per adatom is transferred to graphene. Since there is little occupation of the valence p peak, the up- and down-spin states are degenerate, and there is no net magnetic moment for the group III elements. "

Question:
-How did the authors know that close to one electron is transferred from Al to graphene? I am guessing that the line of reasoning goes something like this: The isolated Al atom has elec configuration of [Ne] 3s2 3p1. But in the graph, the PDOS of Al-graphene system shows the Al p peak above the Fermi level. Therefore, the Al p orbital is empty once Al is bonded to graphene. The reason why it is empty is because its electrons were transferred to graphene. Again, I am not sure with this.

10. The paper said, "Since there is little occupation of the valence p peak, the up- and down-spin states are degenerate, and there is no net magnetic moment for the group III elements."
Question:
-How are the occupation of p orbital and the degeneracy of its up- and down-spin states related to each other?
-How were the authors able to conclude that there will be no net magnetic moment, just from knowing that the up- and down-spin states are degenerate?

11. The paper said in relation to Fig. 6 which I pasted below, "Because the graphene states are strongly altered, the Dirac point is no longer evident. "

Question:

-I thought that the Dirac point is the point where the DOS is zero. From this graph, the Dirac point seems pretty evident to me, so why is the paper saying that the Dirac point is no longer evident? Am I missing something about the definition of Dirac point?

12. The paper said in relation with the figure above, "The DOS illustrates that approximately two electrons are shifted from the 4s states in atomic Fe to the spin-down 3d states in the Fe-graphene system"
Question:
-My understanding is that the two electrons lost by 4s states was inferred from the fact that the 4s peak is above Fermi level. What I do not understand is how the author figured out that the two electrons shifted to the 3d states and not to other states, such as the graphene states.

There are many graphs like this in Chan's paper and other papers that I am trying to understand and I feel that I lack enough background knowledge to be able to understand them.

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4. Jul 5, 2015

### Dr. Courtney

This is not an area in which I am sufficiently qualified to offer detailed answers. However, having entered several new research fields, jumping into the literature and figuring things out is a familiar process.

I take a couple of basic approaches when I'm reading a paper and realize I need to strengthen my background to understand what is going on. 1) Identify and read what seem to be the most important background papers referenced by the current paper. 2) Do google scholar searches on the terms and topics I don't understand and read the papers that come up and seem to give a broad overview. 3) Do google searches on the terms and topics and find background material more suited for the layman. 4) Find book chapters and review articles in the field that serve as more suitable introductions than primary research papers.

It usually takes me hundreds of hours to become well-versed and competent in a new field.

5. Jul 6, 2015

### physicsjn

Hi Dr. Courtney, thank you for your suggestions. I will do them as well. I am also reading other similar papers to see if there are some points which I could understand, although I aim to lessen the amount of time struggling with the text as soon as I can.

6. May 13, 2017

### Gogo

(After two years - maybe someone else finds it useful) You may also want to use resources from MIT OCW or NPTEL courses like solid state physics etc. Maybe that will help.

7. May 13, 2017