The Role of Thermal Activation and Molecular Structure on the Reaction of Molecular Surfaces

Though
surface modifications of organic thin films dramatically
improve optoelectronic device performance, chemistry at organic surfaces
presents new challenges that are not seen in conventional inorganic
surfaces. This work demonstrates that the subsurface of pentacene
remains highly accessible, even to large adsorbates, and that three
distinct reaction regimes (surface, subsurface, and bulk) are accessed
within the narrow thermal range of 30–75 °C. Progression
of this transition is quantitatively measured via polarization modulation
infrared
reflection absorption spectroscopy, and atomic force microscopy is
used to measure the thin-film morphology. Together, they reveal the
close relationship between the extent of the reaction and the morphology
changes. Finally, the reaction kinetics of the pentacene thin film
is measured with a series of adsorbates that have different reactivity
and diffusivity in the thin film. The results suggest that reaction
kinetics in the thin film is controlled by both the reactivity and
the adsorbate diffusivity in the thin-film lattice, which is very
different than the traditional solution kinetics that is dominated
by the chemical activation barriers. Combined, these experiments guide
efforts toward rationally functionalizing the surfaces of organic
semiconductors to enable the next generation of flexible devices.