Atomic-resolution Studies of an Unusual Effect:
The Influence of Metal Adatoms in Chemisorption on Surfaces

John T. Yates, Jr.
U. of Virginia

           The formation of self-assembled monolayers on metals is an
area of intense research activity at present. These chemisorbed
layers are used in molecular electronics, in metal passivation, and
in the chemical functionalization of electrode surfaces to provide
specific chemical activity, as in sensors.

           Often, to make self assembled layers, organic molecules,
containing a reactive S-H functional group, are employed to anchor
the organic to the metal, and we have studied the molecule CH3SH
which is the simplest organo-sulfide. It is well established that the
S-H bond is broken upon chemisorption on Au at T > 200 K and the S
atom in the intact CH3S species then bonds to the metal. We have
found by STM measurements and DFT calculations that surprisingly, on
Au(111)-(22 x ?3), the bonding of the S atom also involves the
promotion of a Au atom from the bulk to above the surface, causing
each S atom in CH3S to be bonded to 2 Au atoms, one surface Au atom
and one Au adatom. The strained Au surface in the vicinity of the
adsorbed species then becomes relaxed forming the unstrained Au(111)
surface. Similar adatom bonding effects are also observed for various
chemisorbed species on Au(100) and Cu(110) [1].

           In the second part of the talk, I will show how we have
used low energy electron injection from the STM tip into self-
assembled chains of CH3SSCH3 species an Au(111) at 5K to promote S-S
bond scission. Injection of electrons into a single CH3SSCH3 molecule
within a chain results in the dissociation of as many as 10
contiguous CH3SSCH3 species, and the STM has witnessed, for the first
time, a propagating surface chain reaction resembling radical chain
reactions in the gas phase. It is likely that the initial excitation
step is due to dissociative electron attachment in the adsorbed
CH3SSCH3 species [2].