AVS 59th Annual International Symposium and Exhibition
    In Situ Microscopy and Spectroscopy Focus Topic Tuesday Sessions
       Session IS+AS+BI+ET+GR+NS-TuA

Paper IS+AS+BI+ET+GR+NS-TuA2
In Situ Real Time Examination of the Thin Film Growth of Pentacene on Polymeric Dielectrics Using X-Ray Synchrotron Radiation: Unexpected Changes in the Evolution of Surface Morphology with Substrate

Tuesday, October 30, 2012, 2:20 pm, Room 007

Session: In Situ Studies of Organic and Soft Materials and In Situ Microscopy
Presenter: J.R. Engstrom, Cornell University
Authors: T.V. Desai, Cornell University
A.R. Woll, Cornell University
J.R. Engstrom, Cornell University
Correspondent: Click to Email
We have examined the thin film growth of pentacene on SiO2 and on three different polymeric dielectrics using in situ synchrotron x-ray scattering and ex situ atomic force microscopy (AFM). The polymeric dielectrics investigated spanned the range from a low surface energy hydrophobic surface (polystyrene, PS), to a medium surface energy hydrophobic surface (polymethylmethacrylate, PMMA), to a high surface energy hydrophilic surface [poly(ethylene imine), PEI]. We have also compared these results to pentacene growth on clean SiO2. On all surfaces, pentacene forms a polycrystalline thin film, whose structure is that of the previously identified “thin film” phase. From in situ real-time x-ray scattering, we find that pentacene exhibits layer-by-layer (LbL) growth on all surfaces investigated, but the extent of LbL growth is a strong function of the underlying substrate. This result is unexpected as the transition to more 3D-like growth occurs for thicknesses where the underlying substrate is effectively almost entirely covered by the growing pentacene thin film. Layer-by-layer growth is significantly more prolonged on PEI (up to ~6 MLs), followed by SiO2 and PMMA (up to ~4 MLs) and finally PS (up to ~ 3 MLs). This trend is also seen in the variation of both the roughness and the in-plane feature sizes of ~ 10 ML thick films, where the films are the smoothest, and the feature sizes are the largest for growth on PEI, whereas on PS, the films are roughest, and the feature sizes are the smallest. Concerning possible reasons for this behavior, we can exclude the effects of the structure of the crystalline thin film (they were the same in all cases), and the roughness of the polymeric dielectric (rms roughness differed by < 0.1 nm) as major contributing factors. Surface energy of the polymeric thin films, however, provided the best explanation for the observed behavior, suggesting that thermodynamic driving forces play an important role in the evolution of thin film structure. In terms of molecular scale phenomena, interlayer transport and step-edge crossing events may be influenced by the mobility of the near-surface polymeric layers in the underlying substrate, which can be quite different for the ultrathin PEI layers vs. the much thicker PMMA and PS thin films.