Using atomic force microscopy (AFM) to study adsorption of alkyltrimethylammonium bromide surfactants to mica, silica, and graphite from aqueous solution, we find that the sharp Krafft transition in bulk is not accompanied by a similar change in morphology at the interface. Instead, interactions between the solid substrate and the surfactant dictate an equilibrium morphology that is usually similar above and below the Krafft temperature (T K ). Mechanical properties, tested by pushing an AFM tip though the adsorbed film, do change near the T K . In general, the film is more resistant to passage of the AFM tip below T K , consistent with slower molecular motion. Depending on the temperature, the formation of the equilibrium structures on mica and silica proceeds by different paths. Above T K , where micelles are present in solution, adsorption proceeds via micelle-like structures, whereas below T K , adsorption occurs via growth of flat islands, which gradually coalesce. In some cases the adsorbed micelle intermediates were observed somewhat below T K , probably because the negative surface potential allows cationic micelles to form in the double layer or at the interface at monomer concentrations below the critical micelle concentration. We hypothesize that the absence of a distinct structural transition near T K at the surface of the solids is due to strong interactions that either suppress or enhance crystallization, pushing the surface transition point to lower or higher temperatures, respectively. Graphite suppresses crystallization of the bulk structure and enhances crystallization of a different structure, whereas mica and silica enhance formation of a structure that is similar to the bulk crystal. To test this hypothesis we modified the properties of one substrate, mica, through adsorption of KBr. When KBr is introduced to solution, we observed a temperature-dependent structural transition from a flat adsorbate to a cylindrical adsorbate. We propose that KBr weakens the ability of mica to template crystal formation at the interface in two ways: by adsorption of K + to mica in competition with alkyltrimethylammonium ions, and by interaction of Brwith the surfactant in competition with mica anions. The cylinder/flat transition occurs over a time scale of minutes, and we are able to monitor the growth of cylinder domains on increasing the temperature and the shrinkage of these domains on decreasing the temperature.

The adsorption of cationic polyelectrolytes and cationic surfactants from aqueous solution onto silica substrates was examined using atomic force microscopy (AFM) in surface force mode and in surface imaging mode. The polymers were poly(diallyldimethylammonium chloride) (PDADMAC), poly-L-lysine hydrobromide, and polyvinylbenzyltrimethylammonium chloride, and the surfactants were hexadecyltrimethylammonium chloride (CTACl) and hexadecyltrimethylammonium bromide. CTACl forms micelles at the interface between micellar CTACl solutions and silica. These micelles desorbed when the CTACl solution was replaced with water. An adsorbed layer of CTACl hindered adsorption of PDADMAC. This is because CTACl generates a surface change that has the same sign as the polymer. When PDADMAC adsorbed in the absence of CTACl, it formed a featureless, neutral layer. The PDADMAC did not desorb, even after extended rinsing with water. When an adsorbed layer of PDADMAC was exposed to CTACl solution above the critical micelle concentration, the AFM image and surface force are very similar to those observed when CTACl adsorbs to silica. This adsorbed layer is either spheres or hemispheres.

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A new atomic force microscopy (AFM)-based lithography method called field-assisted nanopatterning (FAN) has been demonstrated. Through the use of a conventional atomic force microscope with no alterations, FAN controllably patterns solid or liquid organic and inorganic molecules in the air under ambient conditions. In this manner, patterns can be produced with feature sizes that range from tens of microns to sub-20 nm. Examples include the high-resolution FAN of [60]fullerene, N-methylpyrrole, naphthalene, poly-3-octylthiophene, polyaniline, meso-tetraphenylporphyrin, and gold. These molecules have been patterned onto highly ordered pyrolytic graphite, indium−tin oxide, Au, and passivated Au. The molecules are first coated on a standard AFM tip and then are deposited onto the substrate when a threshold tip bias is achieved. The deposition process is a field-assisted transfer of the molecules from the tip to the substrate. Patterning is turned on or off by controlling tip bias, and the same tip is used for both patterning and imaging. Pattern dimensions are controlled by varying tip bias and fabrication (tip) speed.