STM of Self-Assembled Monolayers
Molecular self-assembly it a phenomenon ubiquitous in nature - it governs the
building of cell walls, and the folding of proteins. Alkanethiols mimic
biological molecular self-assembly by spontaneously forming organized
monolayer films on metal surfaces. The thickness of the films, their
dielectric constant, and the physical properties of the exposed surface can
be controlled by changing the length of the alkyl chain, its degree of
saturation, and its terminal functional group. Scanning tunneling microscopes (STMs) employ a sensitive current amplifier
that measures the flow of tunneling electrons when a biased conducting
sample is placed 5-10 � from a sharpened metal probe tip. This signal is
used in a negative feedback circuit that controls the flying height of the
tip while it is rastered over the surface by piezoelectric actuators. The
extreme proximity of the probe, combined with the extremely short
exponential decay length of ~0.5 �, affords the user of the technique the
ability to spatially resolve individual surface atoms.
The work described here utilized STM to study SAMs and is motivated by a
desire to understand, on the molecular level, the forces and mechanistic
pathways that govern molecular self-assembly.
This work was performed by Gregory E. Poirier, who died September 15, 2000.
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