transmission gap

[xiaolight]

in this tutorial   http://quantumwise.com/support/tutorials/item/505-transmission-spectrum-of-perfect-sheets-of-graphene-and-mos2
The transmission gap is due to the band gap of MoS2.I am confused why the band structure of the unit cell(hexagonal) can explain it instead of the band structure of the supercell, it says because of the zone  folding effect, I don't understand what that is.

[Jess Wellendorff]

Zone folding of bands means that electronic bands of the supercell are folded into the irreducible Brillouin Zone, which may obscure the visual representation of the band structure. Attached example: Primitive 1x1x1 silicon (blue) vs. 2x2x2 repeated silicon (green). Si is an indirect-gap semiconductor, but folding of the supercell bands into the IBZ makes it appear like it's almost a direct-gap semiconductor, which is not correct.

[Anders Blom]

folding of the supercell bands into the IBZ makes it appear like it's almost a direct-gap semiconductor, which is not correct.

Well, it's still correct, but you can just can't interpret the k-points in the same way as in the primitive cell. The gap itself still has the correct size, and the calculation as such is valid and all properties derived from it (like density of states etc) are also correct, as long as you remember the cell is larger and hence the Brillouin zone smaller than for the primitive representation.

[xiaolight]

Thank you so much.So if I am making band alignment of metal-semiconductor hetero-junction, I can just use the band structure of the unit cell instead of the super cell?

[Anders Blom]

No, because that kind of calculation is a completely different task than just calculating two band structures. See http://docs.quantumwise.com/casestudies/ag_si_interface