Supercritical self-assembled monolayer growth

The growth of octadecyltrimethylammonium bromide (C(18)TAB) monolayers on mica was investigated using atomic force microscopy and infrared spectroscopy. A critical temperature was identified below which the monolayer formed via an "islanding" mechanism, that is, nucleation and growth of densely packed two-dimensional (2D) islands within a matrix of a disordered dilute phase. However, above the critical temperature, there was no coexistence of 2D phases during film formation. Instead, the monolayer gradually became better ordered, remaining laterally homogeneous throughout. We show that this corresponds to a critical point in a 2D phase diagram of the monolayer. Additional evidence is provided by the in situ observation of 2D phase separation upon cooling an incomplete monolayer from the one-phase to the two-phase region. The lack of coexisting domains (and domain boundaries) during growth above the critical point provides a possible route for the preparation of essentially defect-free monolayers.     

Effects of Adsorbed Polyaniline on Redox Processes on As(2)O(3) Surfaces

Polyaniline deposited on As(2)O(3) surface resulted in a new material, which was characterized by infrared spectoscopy, thermogravimetry, differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, and cyclic voltammetry. The mass percentage of polymer deposited on oxide surface is approximately 13%. The scanning electron microscopy images as well as the X-ray diffraction patterns provided conclusive evidence that the oxide surface is coated by the polymer. The cyclic voltammograms of the polyaniline adsorbed on As(2)O(3) surface showed that the adsorbate exerts remarkable effects on redox processes on this oxide. The pure oxide exhibited two oxidation/reduction peaks at 0.25/-0.06 and 0.47/-0.25 V attributed tentatively to the processes As(2)O(3)(s)+6H(+)+6e(-)=2As(s)+3H(2)O and As(s)+3H(+)+3e(-)=AsH(3)(g), respectively. The polyaniline-coated sample exhibited a better-defined voltammogram in which the first oxidation peak of the oxide had its intensity increased about four times. Copyright 2000 Academic Press.