  Robust control over the carrier type is fundamental for the fabrication of
nanocrystal-based optoelectronic devices, such as the p-n homojunction, but
effective incorporation of impurities in semiconductor nanocrystals and its
characterization is highly challenging due to their small size. Herein, InAs
nanocrystals, post-synthetically doped with Cd, serve as a model system for
successful p-type doping of originally n-type InAs nanocrystals, as
demonstrated in field-effect transistors (FETs). Advanced structural analysis,
using atomic resolution electron microscopy and synchrotron X-ray absorption
fine structure spectroscopy reveal that Cd impurities reside near and on the
nanocrystal surface acting as substitutional p-dopants replacing Indium.
Commensurately, Cd-doped InAs FETs exhibited remarkable stability of their hole
conduction, mobility, and hysteretic behavior over time when exposed to air,
while intrinsic InAs NCs FETs were easily oxidized and their performance
quickly declined. Therefore, Cd plays a dual role acting as a p-type dopant,
and also protects the nanocrystals from oxidation, as evidenced directly by
Xray photoelectron spectroscopy measurements of air-exposed samples of
intrinsic and Cd doped InAs NCs films. This study demonstrates robust p-type
doping of InAs nanocrystals, setting the stage for implementation of such doped
nanocrystal systems in printed electronic devices.
