Abstract
An octaphosphonic acid porphyrin was synthesized for the construction of Zr(IV) phosphonate porphyrin towers on modified silicon wafers.
Silicon wafers with phosphonic acid head groups served as substrate. These silicon wafers were then connected with isolated porphyrin towers by a repetitive, alternating self-assembly procedure of an aqueous solution of Zr(IV)chloride and a solution of octaphosphonic acid porphyrin. Initial attempts to bind the 2?10 nm porphyrin aggregates in water, which were found in electron micrographs, to the silicon surface from aqueous solution or from DMSO/water-mixtures, failed. We only observed statistically adsorbed 2-7,5 nm porphyrin crystallites with a width of up to 800 nm.
Using methanol solutions instead of water yielded isolated porphyrin hills with a height of up to 7,5 nm were found. The presence of tetrabutylammoniumhydroxide caused an attachment of the voluminous tetrabutylammonium cations at the deprotonated phosphonic acid groups of the porphyrins and diminish the aggregation of the porphyrin-molecules in the self-assembly solution. Besides 2,5 µm crystallites we observe isolated areas on the silicon surface with 2,5, 5, 7,5 and 10 nm porphyrin hills.
The introduction of acetonitrile and tetrabutylammoniumhydroxide as a solvent system caused a uniform covering of the modified silicon wafers with isolated porphyrin towers for the first time. After the first self-assembly cycle, porphyrin towers with heights of 2,5, 5 and 7,5 nm were found. After a 16-fold porphyrin self-assembly process, porphyrin towers with a height of up to 45 nm, emerged.
It was then to tried to limit the surface coverage by capping the Zr(IV) ions with phenyl- and tert.-butylphosphonic acid. These attemps failed however. A drastic increase of coverage was observed instead. Only alizarin s was successful as a capping-reagent in reducing the numbers of porphyrin towers. We thus succeeded not only for the first time to grow isolated porphyrin towers on a smooth surface, but also in adjusting their number and heights. The best capping reagent was alizarin s.
It was also found to be possible to move single porphyrin towers with the AFM-tip over a distance of 100 m on the silicon wafer surface without damaging it.
The width of the porphyrin towers was also determin by scanning electron microscopy (SEM). Independent of the number of the self-assembly cycles the width of the porphyrin hills was not uniform. Values from 8-21 nm were found. |
