  Large surface to volume ratios of semiconductor nanocrystals cause
susceptibility to charge trapping, which can modify luminescence yields and
induce single-particle blinking. Optical spectroscopies cannot differentiate
between bulk and surface traps in contrast to spin-resonance techniques, which
in principle avail chemical information on such trap sites. Magnetic resonance
detection via spin-controlled photoluminescence enables the direct observation
of interactions between emissive excitons and trapped charges. This approach
allows the discrimination of two functionally different trap states in CdSe/CdS
nanocrystals underlying the fluorescence quenching and thus blinking
mechanisms: a spin-dependent Auger process in charged particles; and a
charge-separated state pair process, which leaves the particle neutral. The
paramagnetic trap centers offer control of energy transfer from the wide-gap
CdS to the narrow-gap CdSe, i.e. light harvesting within the heterostructure.
Coherent spin motion within the trap states of the CdS arms of nanocrystal
tetrapods is reflected by spatially remote luminescence from CdSe cores with
surprisingly long coherence times of >300 ns at 3.5 K.
