  Si nanocrystals have been extensively studied because of their novel
properties and their potential applications in electronic, optoelectronic,
photovoltaic, thermoelectric and biological devices. These new properties are
achieved through the combination of the quantum confinement of carriers and the
strong influence of surface chemistry. As in the case of bulk Si the tuning of
the electronic, optical and transport properties is related to the possibility
of doping, in a controlled way, the nanocrystals. This is a big challenge since
several studies have revealed that doping in Si nanocrystals differs from the
one of the bulk. Theory and experiments have underlined that doping and
codoping are influenced by a large number of parameters such as size, shape,
passivation and chemical environment of the silicon nanocrystals. However, the
connection between these parameters and dopant localization as well as the
occurrence of self-purification effects are still not clear. In this review we
summarize the latest progress in this fascinating research field considering
free-standing and matrix-embedded Si nanocrystals both from the theoretical and
experimental point of view, with special attention given to the results
obtained by ab-initio calculations and to size-, surface- and interface-induced
effects.
