DISSERTATION

_____________________________________________________________ Results and Discussion
of SAM formation. All experiments were performed with a pulse duration of 10 ms and only
the intensities of the applied potentials within a pulse profile were varied. The capacitance
values are normalized by the real electrode surface area derived from CVs in sulfuric acid.
Using the ΔE = 400 mV pulse profile (0.3/-0.1 V vs. Ag/AgCl/3 M KCl) a tremendous
acceleration of the immobilization kinetics is observed as compared to SAM formation at OCP.
The monitored capacitance reaches a plateau after about 90 min implying that a fully SAM
covered electrode surface was obtained (Figure 3.34, blue curve). Increasing the potential
difference to 700 mV (0.5/-0.2 V pulse profile) an even faster immobilization kinetics is
achieved, and the capacitance plateau is reached within minutes (Figure 3.34, orange curve).
With a higher potential intensity within the applied pulse profile, the potential drop in the
vicinity of the electrode is faster generating a higher concentration gradient and thus a more
efficient ion stirring. This obviously results in a much faster immobilization kinetics of the
alkylthiol molecules. However, the efficiency of an applied pulse profile does not only depend
on the selected potential values relative to the pzc, but also on the pulse duration and the
migration of ions in solution as it was above described for the potential-assisted immobilization
of DNA. If the potential pulse is too short to allow the formation of a sufficient concentration
gradient, increasing of potential intensities while keeping the pulse time constant results in a
slower SAM immobilization kinetics. This is shown in Figure 3.34 (yellow curve) for the case
when increasing the potential difference to 900 mV (0.5/-0.4 V pulse profile) while keeping the
same pulse time of 10 ms.
In contrast to DNA immobilization, the evaluated alkanethiols are uncharged molecules, the
principle of potential pulse-assisted SAM formation is similar to the potential pulse-assisted
DNA immobilization. Namely, during a single potential pulse, a certain potential drop is
generated in the vicinity of the electrode surface. By switching to a potential with opposite sign
with respect to the pzc, the excess of ions in front of the electrode moves away from the surface
and exchanges with ions of opposite charge. Fast switching between positive and negative
potentials creates an ion stirring in the vicinity of the surface, moving along thiol molecules
that are within this layer. If a potential pulse is sufficiently long, a whole thiol molecule can be
pulled to the surface during a single pulse regardless of its orientation, facilitating the formation
of the Au-S bond. Thus, potential pulse-assisted thiol immobilization, like in the case of charged
molecules, is driven by the migration of ions in the vicinity of the electrode rather than the
diffusion of thiols, what is the case during of self-assembly at OCP 94 .
3.3 Importance of controlling the surface 72