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Doping of nanocrystals is an important and very difficult task. "Self-purification" mechanisms are often claimed to make this task even more difficult, as the distance a defect or impurity must move to reach the surface of a nanocrystal is very small. We show that self-purification can be explained through energetic arguments and is an intrinsic property of defects in semiconductor nanocrystals. We find the formation energies of defects increases as the size of the nanocrystal decreases. We analyze the case of Mn-doped CdSe nanocrystals and compare our results to experimental findings.

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Many-body perturbation theory in the GW approximation is a useful method for describing electronic properties associated with charged excitations. A hierarchy of GW methods exists, starting from non-self-consistent G 0 W 0 , through partial self-consistency in the eigenvalues (evscGW) and in the Green's function (scGW 0 ), to fully self-consistent GW (scGW). Here, we assess the performance of these methods for benzene, pyridine, and the diazines. The quasiparticle spectra are compared to photoemission spectroscopy (PES) experiments with respect to all measured particle removal energies and the ordering of the frontier orbitals. We find that the accuracy of the calculated spectra does not match the expectations based on their level of selfconsistency. In particular, for certain starting points G 0 W 0 and scGW 0 provide spectra in better agreement with the PES than scGW.

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