Over the past two decades, the use of scanning probe microscopy to directly visualize electrochemical processes in situ at the molecular and atomic levels has increased dramatically. To demonstrate the high resolution and utility of ECSPM techniques for interfacial investigations, we have presented a number of original experiments (please refer to the application notes posted on our website).

In one instance, we conducted an in situ ECSTM experiment to watch the order-disorder transition of 2,2´-bipyridine (22BPY) on Au(111) surface under potential control. Individual bipyridine molecules in the ordered phase and their orientations were resolved, helping to understand the polymerization and ordering process of 22BPY at the molecular level.

2,2´-bipyridine was dissolved in 100 mM NaClO₄ to a final concentration of 1 mM. Deionized water (18.0 MW cm) was used throughout the experiment. A small Teflon cell used had an exposed electrode area of 0.28 cm². Ag/Ag⁺ was the quasi-reference electrode. Apiezon-wax-coated Pt/Ir tips had typical leaking current of 10 pA or less. Before each experiment, the fluid cell and electrodes were cleaned with H₂SO₄/H₂O₂ mixture and thoroughly rinsed with deionized water. After a gold substrate was hydrogen flame annealed, it was immediately transferred to a sample stage and covered with the electrolyte. Typical bias and setpoint current used were 200 mV and 0.2 nA, respectively.

The ordering process of 22BPY molecules on Au(111) at different potentials was then demonstrated. At 0.0 V (versus Ag/Ag⁺), molecules tended to be loosely in contact with the surface, randomly orientated. STM did not resolve either molecular rows or single molecules. At slightly higher potential, molecules started to bind to the surface and became observable.

When the surface potential was changed to 0.15 V, the adsorbate began to form short, parallel rows. At 0.20 V, over half of the molecules on the surface appeared to be ordered. Domains formed by groups of the same orientated molecular rows began to appear. At 0.27 V, the adsorbate showed long-range ordering. Three distinct orientations perfectly fit the underlying atomic lattice of the Au(111) surface. Domains and domain boundaries were visible. The measured chain-chain spacing was around 9 Å. Individual bipyridine molecules closely packed along polymeric chains were clearly resolved with a period of 3.3 ± 0.3 Å. The disorder-to-order transition was reversible and images were stable over several hours.

We then used in situ ECAFM to repeat the experiment of Cu underpotential deposition (UPD) on Au(111) with both molecular and atomic resolution.

The electrolyte was 100 mM H₂SO₄ containing 5 mM CuSO₄. A piece of Cu wire was used as the quasi-reference electrode. The AFM fluid cell had an exposed electrode area of 0.57 cm². Si₃N₄ cantilevers used had a typical spring constant of 0.5 nN/nm. The cell and electrodes were thoroughly cleaned before the experiment. Similar to the STM experiment, a gold substrate was flame annealed right before being covered with the electrolyte.

A cyclic voltammogram of Au(111) in 100 mM H₂SO₄ containing 5 mM CuSO₄ showed very distinct UPD peaks at 0.275 V versus Cu/Cu₂⁺ (DER = 9 mV). AFM images were acquired both below and above the peaks. At high potential (prior to Cu deposition), the atomic lattice of bare Au(111) surface was repeatedly observed. The unreconstructed Au atomic structure on the (111) plane with the familiar threefold symmetry was clearly resolved, showing an atomic spacing of 2.9 ± 0.2 Å.

Further ramping up the potential (to 0.70 V) did not significantly change the atomic image. Ramping down, however, showed a new lattice after passing the UPD peak. The measured lattice constant was 5.0 ± 0.3 Å and the orientation was 30º ± 1º relative to Au(111) lattice. These parameters suggested that the new lattice was (√3 × √3)R30º. When the potential was ramped below 0.060 V, the lattice disappeared and a full monolayer of deposited Cu was formed. When the potential was returned, the lattice reappeared. The (√3 × √3)R30º structure was very stable at a constant potential, indicating a strongly bound layer of molecules.

With continued advances in AFM instrumentation and in situ technologies, ECSPM is being utilized in an increasingly broad range of application areas.

We invite you to share any AFM-related comments, queries, suggestions, and ideas with us, as well as with your fellow researchers, on this blog.

This entry was posted on Friday, March 16, 2007 at March 16, 2007 and is filed under Microscopy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.