Recent advances achieved in the growth of films of π-conjugated systems based on supersonic free jets will be discussed. The advantages of this approach are based on the unprecedented control achievable by "tuning" kinetic energy Ek, state of aggregation, momentum and flux of the molecules. Growth process and interfaces are studied "In situ" by UPS, XPS, LEED, Auger and ex-situ by Optical, AFM and X-ray. Our major aim is understanding the role of the initial state of the molecule on the film formation. Ek is a critical factor for high quality crystalline films. We explored metallo-phthalocyanines, metallo-porphyrines and acenes, with a particular emphasis on Pentacene test case, the aim being to overcome limits in (opto-)electronic applications due to: lack of control at the solid state and at interfaces of molecular assembling. Growth on different substrates [1] shows that high Ek give unprecedented control on growth, reactivity, morphology and structure[2,3]. Thermal beams give rise to amorphous films. At Ek, over a few eV, a polycrystalline structure forms. When Ek is over 5-6 eV, the crystallites becomes very large, up to layer by layer growth with the properties of single crystals. The process will be discussed in terms of surface mobility increased by Ek and is coherent with generalized scaling lows where the critical cluster size depends on the kinetic energy. Pentacene FETs developed [4] demonstrate high performance achievable (including mobilities over 1 cm2/V sec) and interesting functional properties (i.e. gas-sensing FET) strongly correlated to Ek and the morphology and structure of the films.
References:
[1] S.Iannotta, T.Toccoli, Journal of Polymer Science: Part B: Polymer Physics, 41, 2501 (2003).
[2] L.Casalis, M.F.Danisman, B.Nickel, G.Bracco, T.Toccoli, S.Iannotta, G.Scoles, Phys. Rev. Lett. 90, 206101 (2003).
[3] Y. Wu, T.Toccoli, N.Koch, E.Iacob, A.Pallaoro, P.Rudolf and S.Iannotta Phys. Rev. Lett. (2007 in press.)
[4] T.Toccoli, A.Pallaoro, N.Coppedè, F.De Angelis, G.Fortunato and S.Iannotta, Applied Physics Letters 88 (2006)
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