  Tuning of the electronic properties of pre-synthesized colloidal
semiconductor nanocrystals (NCs) by doping plays a key role in the prospect of
implementing them in printed electronics devices such as transistors, and
photodetectors. While such impurity doping reactions have already been
introduced, the understanding of the doping process, the nature of interaction
between the impurity and host atoms, and the conditions affecting the
solubility limit of impurities in nanocrystals are still unclear. Here, we used
a post-synthesis diffusion based doping reaction to introduce Ag impurities
into InAs NCs. Optical absorption spectroscopy along with analytical
inductively coupled plasma mass-spectroscopy (ICP-MS) were used to present a
two stage doping model consisting of a "doping region" and a "growth region",
depending on the concentration of the impurities in the reaction vessel. X-ray
absorption fine-structure (XAFS) spectroscopy was employed to determine the
impurity location and correlate between the structural and electronic
properties for different sizes of InAs NCs and dopant concentrations. The
resulting structural model describes a heterogeneous system where the
impurities initially dope the NC, by substituting for In atoms near the surface
of the NC, until the "solubility limit" is reached, after which the rapid
growth and formation of metallic structures are identified.
