MoS2/Al2O3

[MB]

Dear Sir,
I am trying to simulate band structure of monolayer MoS2 on Al2O3(cleaved along 0001). The problem is that the band structure of the system is not coming correctly. The bands are crossing the Fermi level, but we expect the system band structure to be similar to that of monolayer MoS2. However the band structure of bulk Al2O3(0001) is coming correct. Using the bulk passivated Al2O3(0001) I have created an interface with monolayer MoS2 and changed it to hexagonal super cell and then simulated this super cell to get the system band structure. Please suggest what the error can be.

[zh]

"The bands are crossing the Fermi level, but we expect the system band structure to be similar to that of monolayer MoS2."
It is not always as you expect, because the band structure of interface system depends strongly on the atomic structure of interface layer. If there are dangling bonds at interface layer, it may cause a metallic interface state. You had better check the atomic structure of your interface system of MoS2/Al2O3 carefully.

[MB]

I had already passivated the Al2O3 structure and then run the simulation and obtained the band structure with fermi level crossings. Another thing that I have observed is that , the band structure of this super cell keeping only the Al2O3 slab within it , is not producing the result as it should for an insulator. However the band structure for simple bulk Al2O3(cleaved along 0001) is showing a bandgap of ~5eV. Is there something wrong in super cell formation or some problem in the method of interface formation or something else. Please suggest.

[zh]

If possible, please show the script file for the problem you have mentioned.

[zh]

The band structure for the system with only Al2O3 (0001) in the supercell exhibits metallic character in your results. This is caused by the nonstoichiometric Al/O at the topmost layer and hence the surface states.  Indeed, you can calculate the DOS and obtain the projected  DOS  of the topmost atoms. The states just above the Fermi level mostly come from the topmost Al atoms.

Please check the following paper:
Takahiro Kurita, Kazuyuki Uchida, and Atsushi Oshiyama, Atomic and electronic structures of  alpha-Al2O3  surfaces, Phys. Rev. B 82, 155319 – Published 21 October 2010, http://journals.aps.org/prb/abstract/10.1103/PhysRevB.82.155319


Additional comment for your system with MoS2/Al2O3, the hydrogen atoms are added on the Al atoms. This is very unlikely. They should be on the oxygen atoms.  The vacuum layer in this system may be too small, resulting in the actually simulated system being a suplattice of MoS2/Al2O3, rather than a monolayer MoS2 adsorbed on Al2O3.

[MB]

Thank you for the reply. It was really helpful .  I would like to know
1. whether it is possible to get stoichiometric Al/O atoms in the Al2O3 arrangement inside the supercell, beause the moment I make an interface the arrangement of Al2O3 gets changed leading to nonstoichiometric Al/O atoms at the top layer.
2. And if there is any way to remove the surface states due to nonstoichiometric topmost Al/O layers. I have checked by increasing the vacuum space but there was no change in band structure  .

[zh]

For your system with only Al2O3(0001) surface in the supercell, it may correspond to the ideal Al-terminated Al2O3(0001) surface.  The ideal surface is usually unstable and the reconstruction could occur.

According to the atom stacking along the [0001] of Al2O3 [you take a look at Figure 1 in Phys. Rev. B 82, 155319(2010)], it may be unlikely to construct surface of Al2O3(0001) with a stoichiometric Al/O atoms at the topmost layer. If you optimize the structure of Al-terminate Al2O3(0001), the Al atoms at topmost layer may move down and such displacement can suppress the surface dipole to stabilize the Al-terminated Al2O3(0001). It may also lead to the suppression of surface states.   This is already discussed in a PRB paper:Bing Huang, Qiang Xu, and Su-Huai Wei , Theoretical study of corundum as an ideal gate dielectric material for graphene transistors,
Phys. Rev. B 84, 155406 – Published 7 October 2011, http://journals.aps.org/prb/abstract/10.1103/PhysRevB.84.155406

In the paper of PRB84,155406(2011), the authors considered three different substrates: Al-terminated Al2O(0001) surface after reconstruction, O-terminated Al2O3(0001) surface, and fully hydroxylated Al2O3(0001) surface.  You can follow them to conduct your study on the system of monolayer MoS2/Al2O3(0001).  Before putting monolayer MoS2 on Al2O3 substrate, you need to perform the geometry optimization for the substrates of Al2O3(0001) surface.  When the monolayer MoS2 is put on Al2O3 substrate, you have to choose proper surface area to make the lattice be matched as much as possible.  Finally, you need to do the geometry optimization for the whole system (i.e., the interface) of monolayer MoS2/Al2O3(0001).

The increase of vacuum space cannot modify the surface states.