CONTROL OF STRUCTURAL ORDER IN SELF-ASSEMBLED ZIRCONIUM ALKYLPHOSPHONATE FILMS

Monolayer films of zirconium allrylphosphonates Zr(O3P-R) (R = C14H29, C18H37, C22H45) with different long-chain alkyl groups have been prep ared by adsorption from dilute solutions of the corresponding alkylpho sphonic acids (molecular self-assembling) onto different primer mono- and multilayers on gold and silicon substrates. The surface orientatio n of the film molecules and the film structure has been investigated a s a function of the hydrocarbon chain length of the adsorbate molecule s, the composition and structure of the primer layer, and the adsorpti on temperature, using ellipsometry and infrared spectroscopy in the ex ternal reflection mode. Structural order decreases with decreasing hyd rocarbon chain length and increasing temperature, spanning the range f rom a densely packed, highly ordered structure for the docosyl (C22) c ompound adsorbed at low temperatures (7 degrees C), exhibiting close-t o-vertical chain orientation and a surface coverage near the theoretic al maximum, to a liquidlike isotropic structure for the tetradecyl (C1 4) compound. Similarly, disorder in short-chain primer layers degrades the overlayer film structure. This effect is particularly noticeable in multilayer films of alpha,omega-biphosphonic acids used as a templa te for a terminal docosylphosphonate sample layer, where disorder incr eases rapidly with the number of layers in the template film. Compared to other self-assembling adsorbates, the structure of alkylphosphonat e monolayers is controlled by similar parameters, but more rigorous co nditions (long hydrocarbon chains, well-ordered primer layers, low ads orption temperatures) are required to achieve the same densely packed and highly ordered film structures.