Molecular orientation and film structure of gramicidin on highly oriented pyrolitic graphite.

Gramicidin molecules were deposited on HOPG surfaces to characterize molecular orientation and film structure as a function of surface coverage and temperature. At low coverage (0.35 ML), the molecules adopted a flat-lying orientation and formed dendritic islands. At higher coverage (0.86 ML), molecules adopted an upright orientation and circular holes formed in the films. The upright film exhibited higher adhesion in force spectroscopy measurements, supporting our molecular orientation assignments. At elevated deposition temperatures (50 °C) on the higher coverage films, the holes were still present, but partially filled in. At 60 °C the film structure was quite different, forming tall irregular islands without the circular holes observed at lower temperatures. These results demonstrate that gramicidin molecular orientation and film structure on HOPG can be controlled by tuning the surface coverage and deposition temperature.

Improved performance of aminopropylsilatrane over aminopropyltriethoxysilane as an adhesive film for anchoring gold nanoparticles on silicon surfaces.

Aminopropylsilatrane (AP-silatrane) was investigated as an adhesive layer for anchoring Au nanoparticles on silicon substrates. We compared the preparation procedure and film quality of the AP-silatrane films to those of 3-aminopropyltriethoxysilane (APTES), which is commonly used for nanoparticle attachment on silicon. The films were characterized by Fourier Transform infrared (FTIR) spectroscopy, contact angle measurements, and atomic force microscopy (AFM). The process for preparing the AP-silatrane films was much easier and resulted in more reproducibly high quality films due to its insensitivity to water. In surface roughness measurements, we observed a 39% increase (99 pm RMS) for the AP-silatrane film over that of a plasma-cleaned silicon sample (71 pm RMS). In contrast, we observed a 218% increase (226 pm RMS) for the APTES film. The much higher roughness observed for the APTES film was due to its sensitivity to water, which results in molecular aggregate formation and polymerization. Gold nanoparticles (7.5 nm) were deposited and firmly anchored to both types of film surfaces. The measured height of the particles on the films was less than the actual size of the particles, and plasma treatment was used to remove the organic layer, resulting in a corrected measurement of the particle size. AP-silatrane offers an easy preparation procedure that creates a smooth, aggregate-free adhesive layer for anchoring Au nanoparticles strongly to silicon substrates.