Unfortunately, 8000 atoms is a huge number for DFT; you are typically limited to below 1000 atoms, although we are working hard to push this number up (and with 8 Gb you can probably go a bit higher, esp. with carbon which has a small basis).
With the Huckel model 8000 atoms might be doable with ATK 11.2, it too early to say, we're working on some improvements in the memory footprint. Our Huckel basis sets for carbon are excellent for graphene and nanotubes, we believe the results are as good as DFT (or even better sometimes). Now, our Huckel cannot yet do geometry optimizations since there are no forces, but soon ATK will have classical potentials too, in particular for carbon, and that is what I think you really need to find the energetic minimum you are looking for, in such large structures.
Note that the SingleZeta basis set is for testing only; it's not a basis set that can be trusted to deliver accurate results, even for carbon. You need at minimum SingleZetaPolarized or DoubleZeta.