DISSERTATION

_____________________________________________________________ Results and Discussion
The duration of a pulse has to allow the system to respond to the perturbation invoked by
applying a certain pulse profile. In order to achieve the highest SAM formation efficiency, the
duration of a given potential pulse should be long enough to allow for an appropriate
concentration gradient to form and a whole thiol molecule to be brought to the electrode surface.
By this the formation of the Au-S bond is achieved regardless of the orientation of a thiol
molecule. However, the pulse time should be also short enough to allow for a high number of
pulsing cycles. Figure 3.35 demonstrates the influence of the pulse duration on the SAM
formation kinetics. For the investigated system (MCU, ΔE = 400 mV) 10 ms pulse duration
manifests as the optimal pulse time. Prolonging the pulse time leads to a less efficient ion
stirring and with this a slower immobilization kinetics (Figure 3.35, yellow and green curves).
On the other hand, decreasing the pulse time prolongs the time necessary to obtain a compact
monolayer (Figure 3.35, blue curve) even though the number of stirring cycles during the
overall immobilization time is increased.
In order to demonstrate the influence of the length of the investigated thiol derivative on the
optimization of the potential-pulse profile for potential-assisted immobilization, Figure 3.36
compares alkylthiols of three different lengths: MCH (6 carbon atoms), MCU (11 carbon atoms)
and MCHD (mercaptohexadecanol, 16 carbon atoms). In all three cases, a more efficient ion
stirring and faster SAM formation kinetics are achieved using a higher pulse potential
difference, suggesting that the optimal potential difference does not depend on the molecule
length as long as the chosen pulse duration is appropriate. In contrast, the optimal pulse duration
depends on the molecule length, more precisely, on its diffusion coefficient. For the
immobilization of short and intermediately long molecules (Figure 3.36, a and b) the 10 ms
pulse duration is sufficiently long. Prolonging the pulse time leads to a lower number of pulse
cycles per time and slower adsorption kinetics. The optimal potential-pulse assisted
immobilization procedure for short and intermediate thiols employs the 0.5/-0.2 V pulse profile
with 10 ms pulse duration. In case of longer thiols such as MCHD (Figure 3.36, c) 10 ms is too
short to allow for the whole alkyl chain to be brought to the interface, which results in a slower
adsorption kinetics. By prolonging the pulse duration to 10 s a much faster SAM formation rate
is observed. Therefore, the optimal immobilization kinetics of longer thiols is achieved using
the 0.5/-0.2 V pulse profile with 10 s pulse duration. At these conditions the capacitance reaches
a plateau within only 5 min.
3.3 Importance of controlling the surface 74