The use of transition metal (TM) dopants to alter the electronic structures of colloidal semiconductor nanocrystals has stimulated broad interest in fields as diverse as solar energy conversion, 1–5 nanospintronics and spin-photonics, 6 and phosphors or optical labels. 7–24 Colloidal doped nanocrystals have shown efficient sensitized impurity luminescence, 7–17 leading to their potential application as biological labels 14,22,23 or as recombination centers in hybrid organic/inorganic electroluminescent devices. 18,24 The magneto-optical and magneto-electronic properties of many colloidal doped semiconductor nanocrystals have been explored. 6,11,25–35 Dopant-carrier magnetic exchange interactions in these so-called diluted magnetic semiconductors (DMSs) 36 give rise to “giant” Zeeman splittings of the semiconductor band structure, and very large magneto-optical effects have been observed. Interesting quantum size effects on impurity-carrier binding energies 30 and magnetic exchange energies 35 have also been reported.