Surface recognition of biotin‐functionalized, chemisorbed thiol monolayers by streptavadin results in the formation of protein‐covered surfaces with no longrange order. This is due to the low lateral and vertical mobility of the solidbound thiols. On the other hand, protein surfaces transferred from a water surface reveal a good crystal structure under the scanning electron microscope, but the adhesion of these applied layers to the solid is unsatisfactory.
We have used scanning tunneling microscopy to study self-assembled monolayers of mercaptohexadecanol in ultrahigh vacuum. In addition to tunneling, the interaction force gradient acting between tip and sample was measured. Analysis of the force-gradient data shows that the tip is in mechanical contact with the surface of the monolayer which, in turn, is elastically compressed. The lateral dimensions of the mechanical contact are substantially (approximately five times) larger than the width of the tunneling-current filament. The results suggest that the compression of the monolayer constitutes an integral part of tunneling through the molecules. This view is further supported by spectroscopic measurements showing that the density of states at the Fermi level as seen by tunneling depends on the initial tunneling voltage used to define the gap width.
Poly(dimethylsiloxane) (PDMS) copolymers containing propanethiol side chain "stickers" for forming self-assembled films on gold surfaces have been studied by various experimental techniques including ellipsometry, contact angle measurements, FT-IR, angle-dependent XPS, and ToF-SIMS. The results show that the thiol-containing side chains do not all chemisorb to the gold surfaces. In addition, the concentration of these sticker units was found to have a dominant effect on the polymer film thickness, the surface wetting properties, and the orientation of the PDMS backbone. Ab initio calculations reveal that the IR-active PDMS Si-O stretching bands can be used as indications of chain orientation, and a comparison of the calculated spectra with grazing incidence reflection IR data shows that the self-assembled PDMS backbone is predominantly aligned parallel to the surface, plausibly as a result of the chemisorption.
Processible organic materials exhibiting high charge carrier mobilities are a step closer with the development of liquid crystalline photoconducting materials. The mobilities have been measured in various phase regions over the temperature range from – 100 to 165°C and the influence of different annealing conditions assessed, indicating that these materials could soon compete with amorphous semiconductors in electronics applications.
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