Photopatterning, Etching, and Derivatization of Self-Assembled Monolayers of Phosphonic Acids on the Native Oxide of Titanium

Electron-hole pair formation at titania surfaces leads to the formation of reactive species that degrade organic materials. Here, we describe the degradation of self-assembled monolayers of alkylphosphonic acids on the native oxide of titanium following exposure to UV light. The rate of degradation was found to decrease as the length of the adsorbate molecule increased. Increasing order in the monolayer, resulting from the enhanced dispersion forces between longer adsorbates, impedes the progress of oxygen-containing molecules to the oxide surface and slows the rate of oxidation. Rates of degradation on titanium oxide are substantially greater than rates of degradation on aluminum oxide because of the photocatalytic effect of the titanium oxide substrate. Micrometer-scale patterns may be fabricated readily using a UV laser in conjunction with a mask, and nanometer-scale patterns may be fabricated using a scanning near-field optical microscope coupled to a UV laser. Photodegraded adsorbates may be replaced by contrasting molecules to yield chemical contrast. Such patterned materials have been utilized to fabricate patterns from polymer nanoparticles. The resist character is switchable--at lower exposures, the monolayer behaves as a positive tone resist, but at higher exposures, it exhibits negative tone behavior. Patterned samples may also be utilized as resists for solution-phase etching of the underlying substrate.