Abstract
We investigated the effects of functionalization of dielectric surface on the growth of rubrene thin films by optical microscopy, atomic force microscopy (AFM), near-edge x-ray absorption fine structure (NEXAFS), and x-ray diffraction (XRD) techniques. We used self-assembly monolayer terminated by different functional groups, including alkyl chains with octadecyltrichlorosilane (OTS) and phenyl with 4-phenylbutyltrichlorosilane (4-PBTS) to study the influence of surface energy on the growth of vacumm-deposited rubrene thin films. Moreover, we used Langmuir-Blodgett (LB) films of poly(N-dodecylacrylamide) (pDDA), an aliphatic terminated polymer, to engineer the nanometer-structured suface by varying the layer thickness and the withdrawl speed of the substrate from the trough for the study of the influence of surface morphology on the growth of rubrene thin films. Optical microscopy and AFM images reveal the presence of rubrene spherulites composed of dendritic structure when deposited on the flatter dielectric surface bonded with different termination of SAMs. The diameters of rubrene spherulites mostly amorphous is found to increase with the decreasing surface energy of dielectric SAM-terminated surface. For nano-structured pDDA LB films, not only rubrene of pillar form but sheet form can be observed. By analyzing the chraracteristic x-ray absorption peak of NEXAFS spectra, one can assess the molecular orientation and the degree of oxidation of rubrene of three different forms. The XRD data further establish that sheet form is due to single crystal with thin-sheet appearance and the longest axis (26.86A) of rubrene unit cell is perpendicular to the substrate surface. The thin film transistor (TFT) fabricated from thin-sheet form exhibits the highest mobility (0.06 cm2/Vs). It is argued that the that the surface morphology seems to play more important role than surface energy in producing highly crystalline rubrene.