Diamond and bulk Si are both Group 4 elements found in their $sp^3$ hybridized states, having bandgaps of around 5.3eV and 1.1eV respectively, but Graphene and Silicene are both $sp^2$ hybridized allotropes of the said elements and are zero bandgap materials. What promotes this change in behaviour? Is it the change in hybridization? or the super-aromaticity??