Noncontact Atomic Force Microscopy - Yale School of Engineering ...

Fr-1220
Redox-state Dependent Reversible Change of Molecular Ensembles in
Water Solution by Electrochemical FM-AFM
Ken-ichi Umeda 1† , Yasuyuki Yokota 2 , and Ken-ichi Fukui 2,3
1 Department of Chemistry, Tokyo Institute of Technology, Tokyo, Japan.
2 Department of Materials Engineering Science, Osaka University, Osaka, Japan.
3 PRESTO, Japan Science and Technology Agency, Saitama, Japan.
Electrochemisty can provide wide range of science and technology because of easy
control of electrochemical potentials of interfaces and adsorbed molecules, which initiate
electron transfer and chemical reactions hard to achieve in vacuum. Redox-active
molecules fixed on the electrode can reversibly change their charges depending on the
electrode potential, which makes it possible to introduce a point charge at the
electrode/solution interface. Following the recipe in literature to reduce the deflection
noise density in liquid-phase measurements, we have developed a frequency-modulation
AFM applicable in electrochemical environments with controlling the potential of the tip
and the sample. By using the EC-FM-AFM, we have studied how the charging of the
molecule fixed at the electrode/solution interface affect the local structure.
Ferrocenylundecanethiol (FcC11H22SH) molecules were embedded in an n-decanethiol
(C10H21SH) self-assembled monolayer (SAM) matrix as molecular islands In Figure 1(a),
the Fc-islands are shown as bright protrusions, whose apparent height differ depending
on their lateral size (number of involved molecules). By changing the charge of Fcterminal
groups from Fc 0 to Fc +1 , the apparent height and the lateral size of each Fcisland
has clearly increased as shown in Figure 1(b). The process was reversible against
the potential change, and originated from the change in the local charge. Simultaneously
obtained energy dissipation signal has decreased at the island when the Fc-islands had
positive charge. These results can be reasonably explained by the formation of ion
network from Fc + and ClO4 - counter anions supplied from the electrolyte water solution.
(a) Fc (b) 0
Fc +1
Figure 1: In situ EC-FM-AFM images (Δƒ = +528 Hz) of FcC11H22SH islands embedded in
C10H21SH SAM (200×160 nm 2 50 nm 50 nm
) at different electrochemical potentials: (a) substrate potential (Es)
= tip potential (Et) = -0.8 V, (b) Es = Et = -0.4 V vs. Au/AuOx, respectively. The amplitude of
cantilever vibration was maintained in a feedback loop to be 0.5 nm.
†
Present address: Department of Electronic Science and Engineering, Kyoto University, Japan.
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