Download self-assembled monolayers lecture

Characterization:
• Among the most frequently used techniques are infrared spectroscopy,
ellipsometry, studies of wetting by different liquids, x-ray photoelectron
spectroscopy, electrochemistry, and scanning probe measurements. It has
been clearly shown that SAMs with an alkane chain length of 12 or more
methylene units form well-ordered and dense monolayers on Au(111)
surfaces. The thiols are believed to attach primarily to the threefold hollow
sites of the gold surface, losing the proton in the process and forming a
(sqrt(3)×sqrt(3))R30° overlayer structure (shown in Figure ). The distance
between pinning sites in this geometry is 5.0 Å, resulting in an available
area for each molecule of 21.4 Å2. Since the van der Waals diameter of
the alkane chain is somewhat too small (4.6 Å) for the chain to completely
occupy that area, the chains will tilt, forming an angle of approximately 30°
with the surface normal. Depending on chain length and chain-terminating
group, various superlattice structures are superimposed on the
(sqrt(3)×sqrt(3))R30° overlayer structure. The most commonly seen
superlattice is the c(4×2) reconstruction, where the four alkanethiolate
molecules of a unit cell display slightly different orientations when
compared with each other.