Reversal of the Charge Transfer between Host and Dopant Atoms in Semiconductor Nanocrystals

We present ab initio density functional calculations that show P (Al) dopant atoms in small hydrogen-terminated Si crystals to be negatively (positively) charged. These signs of the dopant charges are reversed relative to the same dopants in bulk Si. We find this novel reversal of the dopant charge to occur at crystal sizes on the order of 100 Si atoms and predict it to occur in nanocrystals of most semiconductors.