ARTICLES nature materials | VOL 4 | JANUARY 2005 | www.nature.com/naturematerials 81 Supramolecular organization in ultra-thin films of α-sexithiophene on silicon dioxide MARIA ANTONIETTA LOI*, ENRICO DA COMO, FRANCO DINELLI, MAURO MURGIA, ROBERTO ZAMBONI, FABIO BISCARINI AND MICHELE MUCCINI* Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)- Sezione di Bologna, Consiglio Nazionale delle Ricerche, via P. Gobetti 101, I-40129 Bologna, Italy *e-mail: MA.Loi@ism.bo.cnr.it; M.Muccini@ism.bo.cnr.it Published online: 5 December 2004; doi:10.1038/nmat1279 The supramolecular organization of organic semiconductors on the dielectric layer of thin-film field- effect transistors is a crucial factor in achieving good device performance. Charge transport in these devices occurs near the interface with the gate dielectric. By confocal spectroscopy and microscopy we study the supramolecular organization in ultra-thin films of a prototype organic semiconductor, α-sexithiophene, on silicon dioxide, a widely used transistor gate dielectric. We demonstrate that in submonolayer films of sexithiophene (T6), regions where the molecules stand on their long molecular axis coexist with regions where the molecules lie flat on the substrate. When the first monolayer is completed, all T6 molecules stand on the substrate, and the flat molecules detected in the submonolayer films are no longer present. In films thicker than two monolayers, the photoluminescence spectra of standing molecules show a molecular H-like aggregation as in the single crystal. I n the past few decades, the materials science community has devoted great attention to organic electronics and optoelectronics 1–6 . Although organic semiconductors are not certainly expected to replace silicon, they can be considered for all the applications that can take advantage of their appealing properties in terms of processability (low temperature and solution processing), possibility of fabrication over large areas, integration with silicon technology 7 and compatibility with mechanically flexible substrates 8 . In this framework, organic thin-film transistors (TFTs) are of interest for low-cost large-area electronic applications like active matrix displays 9,10 , large-area flexible microelectronics 8 and hybrid electronics 7 . The physical properties of organic semiconductors, in particular the charge transport, which underlie electronic device operation, depend both on the molecular structure and on the supramolecular arrangement in the solid state 11–13 . Although it has been demonstrated that the molecular properties can be tuned by chemical tailoring, the solid–state supramolecular arrangement is generally more difficult to control, and appears to be one of the next challenges for materials science. Improvements in the charge-transport properties are observed when the supramolecular order is maximized, like in single crystals or in well-ordered thin films 14–16 . Field-effect mobilities up to 20 cm 2 V –1 s –1 have been obtained using molecular single crystals 17 , whereas for thin films the highest reported value is 3 cm 2 V –1 s –1 (ref. 15). For technological applications, thin films are much more desirable than single crystals, in particular, thin films with a well- defined supramolecular organization are the real wish. Among π-conjugated semiconductors, one of the most studied systems, both for the fundamental properties 18–20 and for the electronic applications is α-sexithiophene (T6) 21,22 . Since the first demonstration of oligothiophene-based TFTs 21,23 , the hole mobility of T6 has been improved from 10 –3 cm 2 V –1 s –1 up to 4 × 10 –2 cm 2 V –1 s –1 (ref. 24). In several crystallographic and morphological studies, attempts were made to correlate the transport properties of T6 thin films with morphology and molecular orientation 25 . Early X-ray and electron diffraction studies showed that T6 molecules are oriented perpendicularly with respect to silicon dioxide substrates 26,27 . Nature Publishing Group ©2005