  Pentacenequinone (PnQ) impurities have been introduced into a pentacene
source material in a controlled manner to quantify the relative effects of the
impurity content on grain boundary structure and thin film nucleation. Atomic
force microscopy (AFM) has been employed to directly characterize films grown
using 0.0-7.5% PnQ by weight in the source material. Analysis of the
distribution of capture zones areas of submonolayer islands as a function of
impurity content shows that for large PnQ content the critical nucleus size for
forming a Pn island is smaller than for low PnQ content. This result indicates
a favorable energy for formation of Pn-PnQ complexes, which in turn suggests
that the primary effect of PnQ on Pn mobility may arise from homogeneous
distribution of PnQ defects.
