Why do we modify values of primitive vectors when study a monolayer nanoribbon

[wot19920302]

         Dear QuantumWise Staffs,when I analyze band structures of a monolayer nanoribbon,I am told to expand vaccum around the unit cell before caculations. Does it mean that VNL only shows one visible unit cell, although there are many hidden unit cells around the particular one ? Increasing values of primitive vectors is to avoid effects of neighbor unit cells on the particular one, is that ture?
         By the way,is the result same by manipluting  primitive vectors  in Lattice parameters or  using sliders in Stretch Cell?
         Looking forward to your reply.
         Best regards

[Jess Wellendorff]

1) Your nanoribbon is a 1D system, meaning that the nanoribbon unit cell only repeats itself along one direction (the transport direction, C-direction). But your unit cell is always finite in all directions, so you need to make sure to have sufficient vacuum along the two non-periodic directions to allow the electron density to decay properly into vacuum.
2) Using the Stretch Cell tool gives basically the same result as the Lattice Parameters tool, but the latter has a few more options. Just make sure that cartesian coordinates of the atoms are constant when changing the unit cell size.

[Anurag Chaudhury]

Suppose
1) a nanoribbon (3 unit cells along B and 4 unit cells along C) is in BC place with C being the transport direction and vacuum space being added along A and B  direction; and
2) same 3x4 structure but vacuum padding only along the A direction such that it is a nanosheet.

Now, what do the 2 structures, in real, represent? In other words, if we take a  real graphene sheet which has 3 units along zigzag and 4 units along armchair, will it be called a nanosheet or nanoribbon; and to which case (1 or 2) will  it correspond to? I hope I am clear. Thanks.

[Jess Wellendorff]

I do not quite understand the question. The basic rules are these:
- 3D: must be periodic along both A and B (use k-point sampling for this).
- 2D sheets/layers: non-periodic along the direction perpendicular to the nanosheet plane (use vacuum padding and Gamma-point sampling for this).
- 1D tubes/ribbons: non-periodic along both A and B (use vacuum padding and Gamma-point sampling along both directions).