Bonding self-assembled, compact organophosphonate monolayers to the native oxide surface of silicon

A new method is described to prepare strongly bonded, compact monolayer films of alkyl- or arylphosphonates on the native oxide surface of Si (SiO2/Si). This method is illustrated for octadecyl- and alpha-quarterthiophene-2-phosphonates. For both cases, AFM shows comprehensive coverage of the SiO2/Si surface. The thickness of the continuous film of 4TP/SiO2/Si was measured both by AFM and by X-ray reflectivity to be ca. 18 Angstrom. Direct gravimetric analysis shows surface coverage by alpha-quarterthiophene-2-phosphonate to be about 0.66 nmol/cm(2), which corresponds to molecular packing in the film close to that of crystalline alpha-quarterthiophene. Coverage by octadecylphosphonate was ca. 0.90 nmol/cm(2), corresponding to a cross-sectional area of about 18.5 Angstrom(2)/molecule, consistent with close-packed alkyl chains.