AFM and STM studies of the carbonization and graphitization of polyimide films

Abstract

Kapton polyimide and high-modulus polyimide (PPT) films were carbonized and graphitized at various temperatures from 600 to 3000 \ifmmode^\circ\else\textdegree\fi{}C. Their surface was studied by atomic-force microscopy and/or by scanning tunneling microscopy in order to follow the modification of the large-scale morphology and the atomic structure as a function of the heat-treatment temperature (HTT). On the pristine Kapton films, the local order of the molecules is brought to the fore. With increasing HTT (600 to 1000 \ifmmode^\circ\else\textdegree\fi{}C) the structure becomes more disordered while at larger scale a bumpy morphology appears. During graphitization, the bumpy morphology gradually disappears and is replaced by graphitized terraces whose size increases with HTT. At atomic scale, it is shown that the graphene layers progressively grow for HTT higher than 1800 \ifmmode^\circ\else\textdegree\fi{}C. On the films treated between 1800 and 2400 \ifmmode^\circ\else\textdegree\fi{}C, graphene layers containing point defects are imaged and (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R 30\ifmmode^\circ\else\textdegree\fi{} superstructures are observed near large defects. On the samples treated at 2400 and 2600 \ifmmode^\circ\else\textdegree\fi{}C, moir\'e patterns are observed and are attributed to stacking faults (turbostratic structure).