ok thank you i just want to make sure i understand correctly ..
i have another question . my understanding is that when making changes to atoms, such as i am doing with selective doping locations, .. you need to optimize the geometry in order to get a realistic structure .. my understanding is that although you can specify atoms in any position, that that is not realistic unless you let the optimizer relax the forces, etc .. in order to make a "Stable" configuration, for lack of a better word ... is this a correct understanding, and is it safe to say optimization has to be done always when making precise atomic level changes to structures like nanoribbons?
Usually, yes (with a wink to your question below).
i found this statement in the tutorial here: http://quantumwise.com/documents/tutorials/latest/ATKTutorialDevice/index.html/chap.relax.html
"Applying a finite bias will cause the position of atoms to shift slightly, so the geometry should in principle also be optimized under bias. However, the optimization under bias can be very time consuming and usually only has little effect on the transmission and is thus often omitted."
Now, i am concerned about the "usually" statement ... do you think it is possible the results i am seeing for current versus Vgs that i posted earlier could be beacause i am not optimizing after doping the electrodes?
No, I don't, because you are looking at an effect which is a function of Vgs, not as a function of the inserted doping. Optimizing might change the overall curve, slope/offset etc, but not its internal properties. At least I wouldn't expect that in such a system as yours.
and one more question ...
what i am doing is creating p-i-n tfet GNR structures, doped with N and B in the electrodes ... i am running bandstructure and transmission calcs ... now:
(1) should i optimize geometry after replaceing carbon atoms with N and B?
(2) when should I optimize? (before bandstructure calc? before trans calc? before any calc? ) I want to reduce time but be accurate
(3) as noted above, i see that it seems i would need to re-optimize under each bias ... does this include different gate voltages, or just Vds bias?
1. I think that would be wise, but you can probably do this "locally", i.e. check how the B-N bond compares to a C-C bond when embedded in a graphene matrix; probably the difference is small, and the primary effect on the current will be the fact that you now have a dopant present, rather than the small structural details.
2. The optimization is sort of step #1 always, after creating the starting guess geometry. Once you have it you don't need to reoptimize unless you change atoms (as for bias, see 3).
3. This is rarely necessary, the effects of this are usually very small, and typically only relevant if that's the specific effect you are investigating.
The burden of evidence is however always on you