Comment on "Self-Purification in Semiconductor Nanocrystals"

TL;DR

This paper challenges previous claims that smaller CdSe nanocrystals exhibit self-purification due to increased Mn impurity formation energies, showing that this size dependence is not supported across different computational methods.

Contribution

The authors demonstrate that the previously reported size dependence of impurity formation energies in CdSe nanocrystals is not consistent across various DFT calculations, questioning the self-purification concept.

Findings

Different DFT codes yield no size dependence of formation energies.

Formation energies are not relevant for doping in colloidal nanocrystals.

The claimed self-purification effect is not supported by broader computational evidence.

Abstract

In a recent Letter [PRL 96, 226802 (2006)], Dalpian and Chelikowsky claimed that formation energies of Mn impurities in CdSe nanocrystals increase as the size of the nanocrystal decreases, and argued that this size dependence leads to "self-purification" of small nanocrystals. They presented density-functional-theory (DFT) calculations showing a strong size dependence for Mn impurity formation energies, and proposed a general explanation. In this Comment we show that several different DFT codes, pseudopotentials, and exchange-correlation functionals give a markedly different result: We find no such size dependence. More generally, we argue that formation energies are not relevant to substitutional doping in most colloidally grown nanocrystals.